In situ transformations of mineral particles in soils

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Doctoral Thesis

In situ transformations of mineral particles in soils

Author(s): Birkefeld, Andreas

Publication Date: 2005

Permanent Link: https://doi.org/10.3929/ethz-a-005035269

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Doctoral Thesis ETH No. 16036


Andreas Birkefeld

Zurich 2005

Doctoral Thesis ETH No. 16036


A dissertation submitted to the

SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH

for the degree of

DOCTOR OF NATURAL SCIENCES

presented by

ANDREAS BIRKEFELDDipl. Geol., University of Goettingen

born 3 November 1967citizen of Germany

accepted on recommendation ofProf. Dr. Rainer Schulin, examiner

PD Dr. Bernd Nowack, co-examinerProf. Dr. Ruben Kretzschmar, co-examiner

2005

I

Table of contents

Summary VII

Zusammenfassung IX

1 Introduction 1

1.1. Objectives of this study 3

2 In situ methods – Introduction, benefits and general use 9

2.1 What are in situ methods 9

2.2 Experimental in-situ methods 11

2.2.1 Polymer bags 11

2.2.2 Microcosms 11

2.2.3 Mineral into soil mixing 12

2.2.5 Diffusive gradients in thin films (DGT) 12

2.2.6 “Gellyfish” 13

2.2.7 Resin Bags 13

2.2.8 Lysimeters 13

2.2.9 Micro Suction Cups 14

2.3. Analytical in-situ methods 14

2.3.1 Field Based 14

2.3.1.1 Submersible electrochemically micro probe 14

2.3.1.3 Laser atmospheric analysis 15

II

2.3.2 Laboratory Based 15

2.3.2.1 X-ray absorption spectroscopy (XAS) 15

2.3.2.2 Scanning probe microscopy (SPM) 16

2.3.2.3 Fourier transform infrared micro spectrometry (micro-FTIR) 16

2.3.2.4 Raman micro spectrometry 17

2.3.2.5 Environmental scanning electron microscopy (ESEM) 18

2.3.2.6 Desorption Electrospray Ionization (DESI) 19

3 A new in situ method to analyze mineral particle reactions in soils 27

3.1 Introduction 28

3.2 Methods and Materials 30

3.2.1 Polymer supports 30

3.2.2 Epoxy Resin 30

3.2.3 Coating Process 31

3.2.4 Insertion into soil 33

3.2.5 Estimation of the exposed surface area of particles 34

3.2.6 Mechanical stability 35

3.2.7 Calibration of XRF 36

3.2.8 Soil experiments 36

3.3 Results & Discussion 37

3.3.1 Durability of supports 37

3.3.2 Coating Process 37

3.3.3 Mechanical stability 38

III

3.3.4 Exposed surface area of the attached particles 40

3.3.5 Quantification 40

3.3.6 Application 41

3.3.7 Further applications 44

4 In situ investigation of dissolution of heavy metal containing mineral particles in an acidic forest soil 49

4.1 Introduction 50


4.2.1 Sample preparation 53

4.2.2 Pre experimental analysis 53

4.2.3 Experiments 54

4.2.3.1 Field experiments 54

4.2.3.2 Laboratory experiments 55

4.2.4 Sample analysis 56

4.3 Results 56

4.3.1 Particle Characterization 56

4.3.2 Dissolution under field conditions 58

4.3.3 Dissolution under laboratory conditions 60

4.4 Discussion 62

4.4.1 Applicability of the new in situ technique 62

4.4.2 In situ dissolution rates 63

4.4.3 Comparison of in-situ field with laboratory dissolution rates 64

4.5 Conclusions 66

IV

5 In situ transformations of fine lead oxide particles in different soils 73

5.1 Introduction 74


5.2.1 Sample preparation 76

5.2.1.1 Pre experimental analysis 77

5.2.2 Soils and sample incubation 77

5.2.3 Sample analysis 78

5.2.4 Speciation calculations 79

5.3 Results 80

5.3.1 Field results 80

5.3.2 Identification of phases 82

5.3.4 Calculations 85

5.4 Discussion 86

5.5 Conclusions 88

6 Additional applications of the in situ method 93

6.1 Dissolution of PbO on the support 93

6.2 Extension of the method to particles with less than 20 µm diameter 94

6.3 Thin sections of polymer supports in soils 97

6.4 Biological activities on buried supports 98

6.5 Transformation of PbO in acid mine drainage 99

V

7 Conclusions 105

7.1 Development and evaluation of the new in situ method 105

7.2 Establishing dissolution behavior of mineral particles in acidic soils 106

7.3 Analysis and identification of mineral phase transformations 106

7.4 Outlook and further questions 107

Acknowledgements 109

Curriculum Vitae 111

VII

Summary

Transformations of particulate mineral phases are important natural, geological and pedological processes in rock weathering and soil formation. They can also be rate-limiting in the release of toxic metals from anthropogenic particles emissions distributed into the environment. The release of metal contaminants into soil solution caused by weathering of the emitted contaminant particles can seriously damage soil fertility, adversely affect the quality of surface and ground waters and create health risks for the consumers of crops produced on such soils. Several investigations were carried out using different in situ methods to study mineral behavior in the field. The in situ methods applied different technical approaches to study mineral reactions but they all had some drawbacks in common. First of all they disturb the soil matrix in a massive way during incubation and second the test minerals are surrounded by bags or bottles preventing direct particle-soil contact. The goal of this dissertation project was to develop, validate and apply a new in situ method to analyze mineral particle reactions under field conditions. In a first step a simple in situ method for studying (mineral) particle reactions under field conditions was developed. Fine mineral particles (20 µm – 200 µm Ø) were glued with a thin layer of epoxy resin onto small Plexiglas® plates (2 cm × 2 cm). The technique leaves the majority of the particle surface uncovered and thus provides a reactive surface to the soil system. The coated polymer supports were placed in soils and recorded after different times. After recovery the minerals can be analyzed for phase transformation reactions by non-destructive analysis methods e.g. scanning electron microscopy, Raman spectroscopy, x-ray fluorescence spectroscopy and x-ray diffraction.In a second step the newly developed technique was applied to investigate the dissolution behavior of selected mineral particles in an acidic forest soil. Lead oxide, copper concentrate and copper smelting slag were used as test particles. During the incubation time of up to 18 months, the lead oxide particles showed noticeable dissolution whereas the copper concentrate and slag particles did not show any significant dissolution signs. Comparing the field data with simultaneously carried out laboratory experiments showed a faster dissolution rate

VIII

in the field than in the laboratory probably by the high biological activity in the field compared to the pots in the lab. In a third step the method was used to investigate the phase transformations of lead oxide particles in three different soils. In calcareous soils the lead oxide particles showed significant formation of secondary mineral precipitates. During the complete incubation of 18 months no significant increase of mineral precipitates between the second and the eighteenth months of incubation could be recognized. Lead oxide particles incubated in the non-calcareous soil did not show significant formation of secondary precipitates. Using electron microscopy, x-ray probe and micro-Raman spectroscopy the newly formed mineral precipitates could be identified as predominately lead-hydroxy-carbonates (hydroxycerussite). Traces of lead phosphates could be identified on the particles buried in non-calcareous soil.The new in situ method has the potential to be used in other systems e.g. aquatic environments or sediments. The new method was able to provide comprehensive new data concerning the dissolution behavior of fine particles in field and laboratory experiments showing that field dissolution rates can be faster than laboratory dissolution rates.

IX

Zusammenfassung

Die Umwandlung von Mineralphasen ist ein wichtiger natürlicher, geologischer und pedologischer Prozess in der Gesteinsverwitterung und bei der Bodenbildung. Sie kann ebenfalls die Freisetzungsrate von toxischen Metallen aus anthropogen freigesetzten Partikelemissionen bestimmen. Die verwitterungsbedingte Freisetzung von gefährlichen Spurenmetallen in die Bodenlösung kann ernsthaft die Bodenfruchtbarkeit beeinträchtigen und Oberflächen- und Grundwässer verunreinigen. Eine gesundheitliche Gefahr für Konsumenten von landwirtschaftlichen Produkten die auf solchen belasteten Flächen angebaut wurden, kann nicht ausgeschlossen werden. Zahlreiche Untersuchungen zur Bestimmung des Verhaltens von Mineralen in Böden wurden mittels in situ Methoden ausgeführt. Diese Methoden haben verschiedene methodische Ansätze verfolgt um Mineralreaktionen in Böden zu untersuchen. Alle verwendeten Methoden hatten substantielle Schwachstellen aufzuweisen. Zum einen wurde die Bodenmatrix während des Probeneinbaus merklich gestört und zum anderen sind die zu untersuchenden Minerale durch kleine Gefässe oder Beutel von einem direkten Bodenkontakt ausgeschlossen. Das Ziel der vorliegenden Arbeit war es, eine neue in situ Methode zur Analyse von Mineralreaktionen unter Feldbedingungen zu entwickeln, auszutesten und anzuwenden. In einem ersten Schritt wurde eine einfach anzuwendende in situ Methode zur Untersuchung der Reaktionen von Mineralpartikeln unter natürlichen Bedingungen entwickelt. Mit Hilfe eines Epoxydharzfilmes wurden kleine Mineralpartikel (20 µm – 200 µm Ø) auf einer kleinen Plexiglasplatte (2 cm × 2 cm) fixiert. Diese Technik lässt den grössten Teil der Mineralpartikel unberührt von dem Epoxydharz und stellt damit eine relativ grosse reaktive Fläche zur Verfügung um mit dem Bodensystem zu reagieren. Diese beschichteten Plättchen werden dann in Böden eingebracht Nach der Reaktionszeit können die Proben einfach aus dem Boden entfernt werden und mit zerstörungsfreier Analytik, z.B. Elektronenmikroskopie, Mikroramanspektroskopie, Röntgenfluoresenzspek-

X

troskopie, Röntgendiffraktion auf Umwandlungsreaktionen untersucht werden.In einem zweiten Schritt wurde die neu entwickelte Methode angewandt um das Lösungsverhalten von ausgesuchten Mineralpartikeln in einem sauren Waldboden zu untersuchen. Bleioxid, Kupferkonzentrat und Kupferschlacke wurden als Testpartikel verwendet. Während der 18 monatigen Versuchszeit zeigten die Bleioxidpartikel merkliche Lösungserscheinungen wohingegen die Kupferkonzentrat- und Kuperschlackepartikel keine wahrnehmbaren Lösungserscheinungen aufwiesen. Ein Vergleich der Feldversuchsdaten mit Daten eines parallel durchgeführten Laborversuchs zeigten eine erhöhte Auflösungsgeschwindigkeit der Bleioxidpartikel im Feld. Dies wurde wahrscheinlich durch die stärkere biologische Aktivität im Feld verursacht, welche im Laborversuch nicht aufzeigbar war.In einem dritten Schritt wurde die Methode zur Untersuchung von Phasenumwandlungen von Bleioxidpartikeln in drei verschiedenen Böden angewandt. In einem Kalksand zeigten die Bleioxidpartikel deutliche Neubildungen von sekundären Mineralausfällungen bereits nach einer zweimonatigen Reaktionszeit. Während der 18 monatigen, gesamten Versuchszeit konnte keine weiter Zunahme der Ausfällungen beobachtet werden. Bleioxidpartikel die in einem kalkfreien Boden eingebracht wurden zeigten keine visuellen Spuren von Sekundärausfällungen. Durch den Einsatz von Elektronenmikroskopie, Röntgenmikrosonde und Ramanmikrospektroskopie konnten die neu gebildeten Sekundärmineralausfällungen als vorwiegend Bleihydroxycarbonate (Hydroxycerusit) identifiziert werden. Spuren von Bleiphosphaten (Pyromorphit) konnten an den Bleioxidproben aus dem kalkfreien Boden festgestellt werden.Die von uns neu entwickelte in situ Methode hat die Möglichkeit ihr Anwendungsgebiet von Böden auf Sedimente oder sogar aquatische Systeme auszuweiten. Die neue Methode konnte umfangreiche neue Daten bezüglich des Auflösungsverhaltens von feinen mineralischen Partikeln in Feld- und Laborversuchen liefern und dabei zeigen, dass Auflösungsraten im Feld schneller sein können als im Labor.

1

1 Introduction

“Human health and environmental quality are undergoing continuous degradation by the increasing amount of hazardous wastes being produced” (United Nations, 1992). This statement from the Earth Summit held 1992 in Rio de Janeiro shows the concern of society about environmental pollution as a major threat to ecosystems and global population. Worldwide economic growth and increasing global population are key factors for the environmental damage (Nriagu, 1990). Metal pollution of water, air and soils threatens human, animal and plant health. Already thirty years ago contamination was perceived as a predominant problem (Alloway and Ayres, 1997). This awareness led to environmental and pollution control regulations in industrial countries and recently also started to raise concern in Third World countries (United Nations, 1992). Regulations created an economic pressure on the industry and resulted in the development of efficient pollution control techniques (Sparks, 2003). They also spawned intensive environmental research and risk assessment activities (Pierzynski et al., 2000a). One area of particular concern is the increasing contamination of soils and sediments by heavy metals (Adriano, 2001). Although some metals are essential micronutrients for organisms in trace concentrations (Marschner, 1995), all metals become toxic at high concentrations (Alloway, 1995). The critical sources of soil-polluting heavy metals are (Alloway, 1995; Wild, 1993):

• Metalliferous mining and smelting• Agricultural and horticultural materials• Fossil fuel combustion• Metallurgical industries• Electronics• Chemical and other manufacturing industries• Waste disposal• Sports shooting and fishing• Military operations and training• Sewage sludge applications

Chapter 1

2

While soils often have a high capacity to bind and inactivate toxic metals, unfavorable conditions, e.g. with respect to pH, Corg, Eh, clay content, Fe oxides, colloids (Alloway and Ayres, 1997) result in high solubility, mobility and bioavailability. Moreover, if such soils are used for agriculture, metals may enter the food chain and pose a threat for human nutrition (McBride, 1994; Rieuwerts et al., 1998). The mobility and bioavailability of metals does not just depend on their total concentration but on their chemical speciation (Ure and Davidson, 1995). It is then essential to know in which form metals are present in soil. The uptake of metals by plants for example primarly depends on the free ion concentration in soil solution and how this pool is replenished from the solid phase (Wild, 1993). Figure 2.1 visualizes input, transformation and output paths of metals in soils.

Figure 2.1 Input and output paths of minerals and their behavior in soils. (Figure adopted from Pierzynski et al., 2000)

The composition of the soil solution depends on exchange reactions with adjacent mineral and organic phases and organisms, as well as on inputs and outputs through transport processes such as the migration of soluble species into the ground water. Primary minerals are dissolved in the course of in soil-forming processes. They can

Introduction

3

form secondary minerals which again can undergo dissolution and precipitation reactions. Mineral particles entering the soil through anthropogenic processes are transformed in similar ways. If the weathering of such particles leads to the release of toxic metals, this can play an important role for soil fertility and crop quality. The determination of the metal species is an important task in environmental analysis. An important factor for the concentration of available metals is the potential of the soil to maintain their actual concentration. This depends on the phase transformation potential (dissolution) of minerals in the soil matrix (Zhang et al., 1998). To get more insight in the behavior of heavy metal contaminated soils and sediments, methods are available or under development, which have the ability to analyze mineral-soil interactions in situ in the laboratory (Chapter 2 will introduce some of them). Table 2.1 gives a brief overview of modern molecular techniques applied in the environmental sciences. The synchrotron based techniques are limited to special facilities, whereas the other techniques can be seen as “standard” equipment in earth and environmental laboratories.

Analytical method Type of energy source

IR, FTIR and Raman (Laser) Infrared Radiation

Synchrotron XAS (XANES, EXAFS) Synchrotron X-Rays

Synchrotron microanalysis (µXRF, µXANES, µEXAFS, µXRD) Synchrotron X-Rays

SEM with EDX Electrons

SFM (AFM,) Electronic force

Table 2.1 Molecular techniques applied in environmental sciences [Source adapted from Sparks, 2003].

1.1. Objectives of this study

Techniques and methods available for the study of the behavior of particulate minerals in soils and sediments were predominately used in laboratory experiments and have the problem that the results are not easily transferred to real world situations because of the lack to simulate perfectly field conditions in the laboratory (White and Brantley, 2003). More recent, investigations of soil-forming processes have

Chapter 1

4

also adopted in situ field methods to gain results of primary mineral weathering in natural systems. (Maurice et al., 2002; Ranger et al., 1991). Unfortunately these in situ techniques are limited to relatively coarse mineral particles and they do not provide a direct particle contact with the environment. Moreover they substantially disturb the soil during the insertion and recovery process. To overcome these problems and expand the application towards potential hazardous fine mineral particles, the main objectives of this PhD project were:Develop an easy to use method to investigate the weathering of mineral particles in soils under in situ conditions with minimum disturbance of the natural processEvaluate the applicability of the method under field conditionsApply the method over an extended time period in field experiments under different soil and environmental conditions.Chapter 3 introduces the new in situ method in detail. It presents the applied materials as well as the preparation of the samples for different particulate phases and describes how the samples are inserted into the soil, recovered after the incubation and processed for analysis. Chapter 4 evaluates the applicability of the method under field conditions in studying the dissolution behavior of mineral samples in an acidic soil environment. Dissolution rates determined in the field are compared with dissolution rates obtained under laboratory conditions. Chapter 5 presents a study in which lead oxide particles are incubated under field conditions in soils with varying carbonate contents. It illustrates the advantages of the new method if used in combination with common accessible analytical devices such as micro Raman spectroscopy, environmental scanning electron microscopy and x-ray fluorescence spectroscopy for the analysis of mineral phase transformations processes occurring under field conditions. Chapter 6 gives an outlook on further application fields of the in situ technique. It introduces some alternative applications and briefly presents preliminary results.

Introduction

5

Bibliography

Adriano, D. C. 2001. Trace Elements in Terrestrial Environments. Springer.Alloway, B. J. 1995. Heavy Metals in Soils. Blackie.Alloway, B. J., Ayres, D. C. 1997. Chemical Principles of Environmental Pollution. Blackie.Fendorf, S. E., Sparks, D. L., Lamble, G. M., Kelley, M. J., 1994. Applications of X-ray Absorption Fine Structure Spectroscopy to Soils. Soil Science Society of America Journal 58, 1583-1595.Manceau, A., Lanson, B., Schlegel, M. L., Hargé, J. C., Musso, M., Eybert- Bérard, L., Hazemann, J.-L., Chateigner, D., Lamble, G. M., 2000. Quantitative Zn Speciation in Smelter-Contaminated Soils by EXAFS Spectroscopy. American Journal of Science 300, 289-343.Marschner, H. 1995. Introduction, Definition, and Classification of Mineral Nutrients. In: H. Marschner, Mineral Nutrition of Higher Plants. Academic Press, London, pp Maurice, P. A., McKnight, D. M., Leff, L., Fulghum, J. E., Gooseff, M., 2002. Direct observations of aluminosilicate weathering in the hyporheic zone of an Antarctic Dry Valley stream. Geochimica et Cosmochimica Acta 66, 1335-1347.McBride, M. B. 1994. Trace an Toxic Elements in Soils. In: M. B. McBride, Environmental Chemistry of Soils. Oxford University Press, Oxford, pp Nriagu, J. O., 1990. Global metal pollution - Poisioning the biosphere? Environment 32, 7-33.Pierzynski, G. M., Sims, T. J., Vance, G. F. 2000a. Soils and Environmental Quality. CRC Press.Pierzynski, G. M., Sims, T. J., Vance, G. F. 2000b. Trace elements. In: G. M. Pierzynski, T. J. Sims, and G. F. Vance, Soils and Environmetal Quality. CRC, Boca Raton, pp Ranger, J., Dambrine, E., Robert, M., Righi, D., Felix, C., 1991. Study of current soil-forming processes using bags of vermiculite and resins placed within soil horizons. Geoderma 48, 335-350.Rieuwerts, J. S., Thornton, I., Farago, M. E., Ashmore, M. R., 1998. Factors

Chapter 1

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influencing metal bioavailability in soils: preliminary investigations for the development of a critical loads approach for metals. Chemical Speciation and Bioavailability 10, 61-75.Schulze, D. G., Bertsch, P. M., 1995. Synchrotron X-ray techniques in soil, plant and environmental research. Advances in Agronomy 55.Sparks, D. L. 2003. Environmental Soil Chemistry. Academic Press. United Nations. 1992. Agenda 21.Ure, A. M., Davidson, C. M. 1995. Chemical Speciation in the Environment. Blackie.Welter, E., Calmano, W., Mangold, S., Tröger, L., 1999. Chemical speciation of heavy metals in soils by use of XAFS spectroscopy and electron microscopical techniques. Fresenius Journal of Analytical Chemistry 364, 238-244.White, A. F., Brantley, S. L., 2003. The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field? Chemical Geology 202, 479-506.Wild, A. 1993. Soils and the environment. Cambridge University Press.Zhang, H., Davison, W., Knight, B., McGrath, S., 1998. In Situ measurements of solution concentrations and fluxes of trace metals in soils using DGT. Environmental Science & Technology 32, 704-710.Zhang, H., Fang-Jie, Z., Sun, B., Davidson, W., McGrath, S. P., 2001. A New Method to Measure Effective Soil Solution Concentration Predicts Copper Availability to Plants. Environmental Science & Technology 35, 2602-2607.

9

2 In situ methods – Introduction, benefits and general use

This chapter gives an overview over approaches and applications of in situ methods used to study reactions under environmental conditions and to analyze reactions in the place where they occur.

2.1 What are in situ methods

The Latin term “in situ” means “in place” (Wikipedia, 2004). In science it refers to experiments which are conducted at the actual site and under the natural or normal conditions under which the phenomenon under study generally takes place (Encyclopedia Britannica, 2004). It can be distinguished between analytical and experimental in situ methods. Experimental in situ methods are procedures by which a reaction, trial or test, is carried out in situ (predominantly in field experiments). The reaction products may afterwards be analyzed in the laboratory. Analytical in situ methods are techniques by which samples are collected in the environment and then analyzed in the laboratory, or in which analysis are performed directly in the field, including measurements by remote sensing. In many cases they require special laboratory-based instrumentation. Figure 2.1 gives a brief classification of in situ methods.With the recent development of molecular and atomic scale techniques as well as in miniaturization of analytical and sampling devices, the possibility for the application of in situ methods has greatly been expanded. Also soil and environmental sciences have benefited from these developments (Davison and Zhang, 1994; Kuys and Roberts, 1987; Maurice et al., 2002; Nayar et al., 2003; Ranger et al., 1991; Reimers, 1987). The following introduction is not exhaustive, but presents the most important in situ methods which are available today. Figure 2.2 shows the working scales of these in situ methods. In situ methods can be applied from very large to extremely small scales. Remote sensing methods provide an example for the former, the atomic force microscope for the latter scale of application.

Chapter 2

10

Figure 2.1 Schematic classification of in situ methods

Figure 2.2 Arrangement of in situ methods in relation to their working scale

In situ methods

11

2.2 Experimental in situ methods

Experiments in the field are faced with the problem that conditions cannot be controlled to the same extent as in laboratories. The full complexity of environmental factors has to be taken in account. All the methods presented in the following have in common that samples are left to react in situ in the field and are then recovered and analyzed in the laboratory.

2.2.1 Polymer bags

Polymer bags have been used to study the weathering of primary minerals such as mica and feldspars in soils of different climatic zones (Augusto et al., 2001; Hatton et al., 1987; Righi et al., 1990). Polymer bags filled with test materials were buried in field soils and left to react for periods of several weeks to months or even years. The bag prevent the loss of unreacted mineral, while pores in their walls allow exchange with the soil solution. After incubation in the soil, the bags are recovered and the remaining test minerals are analyzed in the laboratory. Potential drawbacks are the outer layers of the mineral particles are in more intensive contact with the surrounding soil than the inner layers of the packing. The long term stability of the bag pores, regarding pore blocking due to clay dislocation or microbial activity is not guaranteed.

2.2.2 Microcosms

This method is a modification of the polymer bag method. The difference is that the walls of the containers which are used to hold the test materials in place are rigid. Most microcosms consist of perforated polyethylene bottles (Hiebert and Bennett, 1992). After filling these bottles with test materials they are inserted into groundwater aquifers or surface water sediments (Maurice et al., 2002) typically for several months to years (Bennett et al., 2001). The particles are limited to grain sizes above the diameter of the holes in the bottle walls in order to prevent losses. Furthermore the method requires water-saturated conditions and is then limited to water-logged soils, aquifers and sediments below surface waters.

Chapter 2

12

2.2.3 Mineral into soil mixing

Tsaplina (1996) studied the phase transformation of Pb, Zn and Cd oxides in acidic soils (pH 4.9) over periods of up to 8 years by simply mixing grains of these minerals into the topsoil, where they were left to react in situ. After the experimental time the particles were recovered via density fractionation using a

“heavy liquid” (Bromoform). The recovered metal oxide particles were analyzed by X-ray diffraction (XRD) in order to establish changes in their mineralogical composition. The problem of this method is that the soil structure is completely disturbed during the mineral mixing and the potential new mineral phases could be damaged due to the chemicals of the separation process.

2.2.4 Diffusive equilibration in thin films (DET)Diffusive equilibration in thin films is a micro in situ method to analyze pore water concentrations in fresh water and marine sediments with a spatial resolution down to 400 μm (Davison et al., 1991). The technique uses a ceramic plate with regular laser carved cavities (2 mm × 0.2 mm) which are filled with polyacrylamide. The plate is then inserted into a sediment until the gels have equilibrated with the surrounding pore water. After recovery the gels are analyzed for sorbents, in particular trace and ultra trace elements (ET-AAS; ICP-MS; PIXE). The concentrations of these components in the pore water solutions are calculated from known sorption or ion exchange isotherms. The method was applied to analyze Fe2+, Mn2+, Ca2+, Mg2+, Na+, K+, SO4

2-, Cl-, NO3- (Fones et al., 1998).

2.2.5 Diffusive gradients in thin films (DGT)

Diffusive gradients in thin films is an in situ technique designed to analyze the kinetics of trace metal fluxes from solid phases to pore waters in sediments and soils (Davison and Zhang, 1994). A metal capturing resin is acting as a sink and inducing a resupply flux of metal ions from the soil solid phase (Zhang et al., 1998). The resin is covered with a gel of known width and diffusivity. This allows to calculate the concentration at the boundary of the gel once the cumulative amount of metal sorbed to the resin within a given time has been determined. The captured metals are measured either by laser ablation inductively couples plasma mass spectrometry (LA ICP-MS) or by conventional wet dissolution and

In situ methods

13

further analysis with graphite furnace atomic absorption spectrometry (GF AAS) or ICP-MS. The DGT method has been extensively tested in laboratory studies (Zhang et al., 1998; Zhang et al., 1995; Zhang et al., 2001) and more recently also applied in situ under field conditions (Nowack et al., 2004). While this method is able to determine the amount of free metal ions, it is not analyzing the resupply/dissolution process at the source (mineral particles).

2.2.6 “Gellyfish”

This interestingly named method is similar to the DET method. It was developed by Senn et al. (2004). The “gellyfish” is a polyacrylamide gel which is mixed with ion exchange beads. The fully hydrated “gellyfish” (>95% water) had a dimension of 2 cm in diameter and a thickness of 2 mm. They were exposed to the solution of interest in Teflon bottles an left to equilibrate for approximately 16 hours. The recovered “gellyfish” gel were analyzed by ICP-MS, GF AAS. From the absorbed amount of metals, assuming full equilibrium free or labile metal concentrations of the surrounding solution were calculated.

2.2.7 Resin Bags

Augusto et al. (1998) used small polyamide bags filled with ion exchange resin to sample the soil solution below different tree species. The bags were buried in different depths in order to monitor changes in composition during percolation. Incubation times varied between days and years. The resin bag method was also used to study the bioavailability of various elements in mushroom cultivations (Beyer, 1998) as well as the bioavailability of Ni to various plant species (Becquer et al., 2002).

2.2.8 Lysimeters

Lysimeters are one of the most widely applied devices to collect soil solutions from. They can be used in two different specifications, zero-tension lysimeters and tension lysimeters (Berggren, 1998). Zero-tension lysimeters collect the soil solution that drains by gravity at the bottom of a soil into the free space of a collector under atmospheric pressure. This means that the soil must be saturated at this interface. In tension lysimeters a suction is applied to the soil through a

Chapter 2

14

porous plate or tube in order to get soil solution under unsaturated conditions, thus avoiding that water-logging results in anaerobic conditions. The soil is drained until the capillary forces holding the remaining soil solution are in equilibrium with the applied tension (Masarik et al., 2004).

2.2.9 Micro Suction Cups

Göttlein et al. (1996) developed micro suction cups in order to sample soil solution with high spatial resolution at a very small scale. The micro suction cups consisted of ceramic cells (1 mm diameter and 5 mm length) connected to a vacuum device through polyetheretherketone (PEEK) tubes forming a small lysimeter. In essence, these cups are small tension lysimeters. The method was also used by Wang et al. (2004) to analyze the dynamics of phosphorus in the rhizosphere. They installed micro lysimeters at distances of 5 mm from each other in a special growing box (rhizotron) to determine the influence of roots on the composition of the soil solution and the mobilization of phosphorous.

2.3. Analytical in situ methods

2.3.1 Field Based

2.3.1.1 Submersible electrochemically micro probe

Reimers (1987) used a submersible microprobe to measure oxygen profiles in deep-ocean sediments instead of taking sediment cores from the anoxic deep sea regions and analyze them in the laboratory environment. The coring technique implies the risk that the samples get in contact with ambient air and then are altered by oxidation. The micro profiling device of Reimers (1987) allowed to analyze the oxygen content in situ in the first few centimeters (0-10 cm) of the sediment-water interface. A small platform (lander) is placed at the sediment water interface on the sediment surface. After positioning, the micro profiler is inserted into the sediment. Müller et al. (1997) expanded the capabilities of the micro probe by developing also for measure in situ NH4

+, NO2-, NO3

-; Ca2+, CO32-, O2, pH and S2-.

Although this method may in principle also be applicable to water-saturated soils and sediments, this has not been done so far.

In situ methods

15

2.3.1.2 Open-Path Fourier Transform Infrared Spectroscopy (OP- FTIR)Op-FTIR is a modification of FTIR used to analyze the chemical composition of gases in free atmosphere. The method was recently applied to study gaseous lava fountains of volcanic eruptions from safe distances (~1000 m). The area of the analyzed region was about 8 m2 in the volcanic gas atmosphere and the analyzed gases were H2O, CO2, CO, HF, HCl, SO2 (Allard et al., 2005).

2.3.1.3 Laser atmospheric analysis

This large scale in situ technique uses laser light to analyse the concentrations and spatial distributions of components of the atmosphere (Patel, 1978; Roscoe and Clemitshaw, 1997; Uthe et al., 1982). This information is obtained from scattering of laser light pulses by atoms, molecules, particles and aerosols (Perry and Young, 1977). A laser impulse is emitted from a light detection and ranging (LIDAR) device into the atmosphere and the backscattered signals are recorded as a function of time (time of flight – TOF). There are various types of light scattering employed for this purpose, including fluorescence scattering, Raman scattering, Raleigh scattering and Mie scattering. Laser backscattering can be used for remote monitoring of the atmosphere from ground-based as well as mobile platforms such as airplanes and reach as far as the tropo- and stratosphere.

2.3.2 Laboratory Based

2.3.2.1 X-ray absorption spectroscopy (XAS)

X-ray absorption spectroscopy (XAS) is a powerful non-invasive technique which allows to identify the chemical and structural forms in which elements are present in a solid, liquid or gaseous sample (Schulze and Bertsch, 1995). This information is obtained from the changes of x-ray absorption of an incident high energetic monochromatic x-ray beam. The energy of the beam is varied according to the range of energy in which the specific element is absorbing X-rays. The resulting spectrum has two regions of intensive interest. These are the x-ray absorption near edge structure (XANES) and the extended x-ray absorption fine structure (EXAFS) regions. A peak of maximum absorption, also called absorption edge

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or edge region, appears when the incident x-ray beam energy reaches the specific element’s ionization energy. Beyond that maximum absorption peak the XANES region is located. This region gives information on the valency and coordination geometry of the element. The EXAFS region is the part of the spectrum that appears if energy is further increased. It gives information on bond length and type and also on the number of neighboring atoms. XAS has now an important place in fundamental research in the environmental sciences. The large number of recent studies employing XAS demonstrates this (Charlet and Manceau, 1992; Pattrick et al., 1997; Scheinost and Sparks, 2000; Voegelin et al., 2002; Welter et al., 1999). Because the high intensity x-ray radiation needed, XAS requires a synchrotron facility. It is not a technique of routine analysis, but as more and more synchrotron light sources become available, the technique becomes more and more accessible also for environmental laboratories.

2.3.2.2 Scanning probe microscopy (SPM)

The term “scanning probe microscopy” (SPM) denotes a whole group of atomic-scale methods based on the electron tunneling effect, which was discovered 1981 by Binning and Rohrer (1981) and got awarded with the Noble Prize in physics 1986 (Wiesendanger, 1994). For instance the scanning force microscopy (SFM), also known as atomic force microscopy (AFM), allows to analyze the nanometer-scale topography of solid matter (Binnig and Rohrer, 1999). It was used in environmental geosciences to study the dissolution of Hectorite mineral particles (Bosbach et al., 2000). Maurice et al. (2002) used it to determine dissolution rates of Muscovite minerals which had been incubated to weather under in situ conditions in the hyporheic zone of stream sediments. AFM does not need any special sample preparation and works in different environments. Although it can be applied to surfaces with a maximum variation in height of 10 µm (Magonov and Whangbo, 1996). AFM has great potential for the visualization and analysis of phase transformations of mineral particles in the environment (Louder and Parkinson, 1995).

2.3.2.3 Fourier transform infrared micro spectrometry (micro-FTIR)

Micro-FTIR is another non-destructive method requiring no special sample preparation steps. It is an extension of the Fourier transform infrared spectrometry

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17

(FTIR) to microscopic scales, made possible by the development of interferometer optics which allowed to use higher IR radiation power. Fourier transform signal analysis gives spectrometers a much better signal-to-noise ratio, better resolution and radiation power than conventional IR spectrometry (Skoog et al., 1998). As in other infrared spectroscopy it is based on the absorption of IR light, which depends on molecular bond oscillations. This IR spectrum is characteristic for a given compound and can be used to identify it like a “fingerprint”. Micro spectrometers are able to analyze samples of 10 to 500 µm size without extensive sample preparation. Combined with attenuated total reflectance (ATR), TFOR has been used to study reactions of inorganic and organic substances in situ (Johnston and Premachandra, 2001; Kuys and Roberts, 1987; Voegelin and Hug, 2003). IR spectroscopy, with all its enhancements, is a standard technique in environmental analysis.

2.3.2.4 Raman micro spectrometry

Raman micro spectrometry is closely related to IR spectrometry. It is also a non-destructive spectroscopic technique that does not need special sample preparation. The Raman spectrum of a compound often closely resembles its IR spectrum. Despite of that many substances are Raman active but not detectable with IR. Raman spectrometry therefore also could be seen as being complementary to IR spectrometry (Turrell and Corset, 1996). One important advantage of Raman spectrometry as compared to IR spectrometry is that no special optical devices or substances (halite salts) are required. Raman spectra can be used as “fingerprints” of specific compounds like IR spectra. Raman spectroscopy work with normal incident light and lenses made of glass. It is not affected by water interferences like IR spectroscopy (Skoog et al., 1998). Figure 2.3 shows the optical scheme of a modern laboratory Raman microscope. Infrared laser light is used to irradiate the sample, which has to be put in focus either using the microscope eyepiece or an attached camera. The Raman radiation emitted from the sample re-enters the microscope lens, passes some filters and is split on an optical grating, before it is detected by a Peltier-cooled charge-coupled device (CCD) detector.Recent advancements in laser light sources and spectrometer geometry resulted in a Fourier transform Raman microprobe (Sommer and Katon, 1991). It uses an infrared laser to attenuate disturbing fluorescence radiation emitted from

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the sample. Raman spectrometry does not need special sample preparation. In combination with the microscope it provides a powerful nondestructive micro

Figure 2.3 Schematic sketch of a Raman microscope[Figure adapted from (Ferraro et al., 2003; Turrell and Corset, 1996)]

analytical method suited for both inorganic and organic substances. It is routinely used in mineralogy and geology as well as environmental forensics (Sobanska et al., 1999). Raman microscopy was also applied in the present thesis to identify newly build mineral phases.

2.3.2.5 Environmental scanning electron microscopy (ESEM)

Environmental scanning electron microscopy (ESEM) is one of the latest developments in scanning electron microscopy (SEM) (Stokes, 2003). In contrast to the conventional SEM technique the sample does not need a conductive coating and the analysis does not need to be performed in an ultra high vacuum

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environment. ESEM is preferentially used in a water vapor atmosphere at pressure between 1.3 and 13 · 103 Pa (Stokes, 2003). This gives the method the unique capability to analyze moist samples (Romero-Gonzalez et al., 2003) and to monitor reactions over time (Devries and Wang, 2003; Stokes et al., 2003). Griffin (1997) used ESEM to study mineral crystallization, as well a weathering and dissolution processes (Sun et al., 2004). The limiting factor of this technique is the high acquisition cost for the instrument, declassifying it as a standard application. ESEM can be equipped with additional probes and detectors in a similar way as conventional SEM. Often it is combined with X-ray fluorescence detectors, either in the wavelength dispersive version (WDX) or the energy dispersive version (EDX). Thus, ESEM provides a powerful nondestructive tool to analyze sample surfaces and detect elemental compositions at µm scales.

2.3.2.6 Desorption Electrospray Ionization (DESI)

Takáts et al. (2004) recently developed a method of sampling ions under ambient conditions from surfaces and injecting them into a mass spectrometer (MS). Conventional mass spectrometry analysis needed sample introduction under vacuum conditions, restricting its application to liquid or gaseous samples. The technique of Takáts et al. (2004) uses electrospray (ES) to ionize the sample surface under ambient conditions, making the method an in situ analysis method. Charged solvent droplets act as projectiles and adsorb ions from the sample surface. These sample ions are then transferred via an atmospheric interface into the vacuum zone of a mass spectrometer.The DESI technique can be used with a variety of surfaces with a broad selection of analytes for spatial analysis under ambient conditions. Figure 2.4 shows the schematic layout of the DESI method.

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3 A new in situ method to analyze mineral particle reactions in soils

Andreas Birkefeld, Rainer Schulin and Bernd Nowackpublished in Environmental Science and Technology 2005, 39, 3302-3307

Abstract

We developed a simple method to monitor the transformation of particles in soils under in situ conditions. The particles were fixed on small polymer supports (2 × 2 cm) with a thin film of epoxy resin. Attached to these carriers, the particles could be put into close contact with soil at a chosen site and easily recovered after extended periods of time. The method was tested with lead oxide and copper concentrate in the field. Quartz and copper oxide particles were used in preliminary laboratory experiments. The used particles sizes ranged from 20 to 200 µm. Laboratory and field experiments with acidic and calcareous soils showed that the PbO and Cu concentrate coated polymer supports were stable under field conditions for at least one year. Non-destructive X-ray fluorescence spectroscopy was used to quantify the metals before and after exposure. Scanning electron microscopy combined with energy dispersive x-ray spectroscopy as well as µ-Raman spectroscopy was used to identify signs of dissolution and newly formed mineral phases. The mineral dissolution rate could be determined under field conditions. The new method has the potential to be used in other environmental media such as sediments or water to study the reactions of a variety of particles larger than 20 µm.

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3.1 Introduction

Soils represent an important sink for heavy metals released by human activities into the environment, e.g. through industrial processing, mining, agricultural application of metal-containing products and wastes, or traffic emissions (Alloway, 1995). Abandoned mine sites, tailing dumps or accidental spills (Cabrera et al., 1999) as well as metal smelters and waste incinerators are possible contamination sources of particulate metals. Large quantities of these particulate emissions end up in soils via atmospheric deposition (Adriano, 2001). There, they are dissolved and transformed into new phases with time. As this may strongly affect the mobility, reactivity and bioavailability of the polluting metals, knowledge about these phase transformation processes and their kinetics is crucial for the long-term site assessment of such particular pollution (Dudka and Adriano, 1997).The dissolution and transformation of solid heavy metal phases in soil have frequently been studied in laboratory systems (Jørgensen and Willems, 1987; Voegelin et al., 2002). However, reaction rates measured under laboratory conditions can be very different from rates determined in situ in soils and sediments (Maurice et al., 2002; Nugent et al., 1998; Sparks, 1989). The weathering kinetics of silica minerals for example were found to differ by some orders of magnitude between laboratory and field experiments (Hodson and Langan, 1999; Swoboda-Colberg and Drever, 1993; White and Brantley, 2003). Reasons for this discrepancy were reported to be e.g. differences in thermodynamic saturation conditions between column effluents and natural waters (White and Brantley, 2003) as well as problems to define and measure surface areas of natural porous media or decreasing reactivity with time (White et al., 1996). Some attempts have been made to study the reactions of heavy metal particles under field conditions. Test mineral particles were mixed into a top-soil horizon in the field (Tsaplina, 1996). This experimental approach disturbed the soil system in a massive way by means of mixing the test particles with the previously undisturbed soil. This is a major drawback of the method while it provides direct soil-particle contact. The following analysis steps of the in situ reacted particles can be carried out by synchrotron based XAS techniques like XANES or EXAFS and give detailed information about newly formed mineral phases in soils (Charlet and Manceau, 1992) as well as micro XRD techniques (Bilderback et al., 1994).

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Perforated (1 mm Ø) polypropylene bottles filled with minerals chips of millimeter to centimeter scale (Maurice et al., 2002; Rogers et al., 1998) have been used for mineral transformation studies in stream sediments and in ground waters (Bennett et al., 1996). This method needs saturated conditions and has therefore the drawback that it cannot be used in unsaturated soils. Furthermore the minerals are not in direct contact with the soil matrix. The in situ behavior of minerals has been studied in soils under environmental conditions by means of placing small porous (20 µm Ø) bags filled with test particles into soils (Ranger et al., 1991; Righi et al., 1990). This technique ensures that at the end of the incubation time the remaining minerals can be completely recovered and analyzed. The mineral grains, however, are not in direct contact with the surrounding soil. Microbial colonization, the interaction with roots and fungal hyphae are excluded by the bag material. Furthermore particles inside the bag are not in contact with soil solution but with solution that has been altered by the large amount of particles present inside the bag. These methods cannot be applied to fine particles and dusts because of the potential loss of particles through the sample container perforations. The sparse amount of available literature about the reactions of fine particles and non-defined phases indicates the methodical and analytical difficulties of determining the transformation of these mineral phases in soils under in situ conditions. To our understanding, an experimental in situ method applied to soils and sediments, will always have to disturb the systems in order to place samples for reaction into the system. However, a new method should at least minimize this disturbance while maximizing the soil-mineral contact. Our objective therefore was to develop and test a simple method to monitor such transformations of fine particles in terrestrial systems. The general concept of the new method was to fix particles on polymer plates so that they can be put into direct contact with soil or another matrix and subsequently be recovered without losses other than by the weathering process to be investigated.

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3.2 Methods and Materials

3.2.1 Polymer supports

Polymethylmethacrylate (PMMA) polymer (Plexiglas®/Acrylite®) was used as the base material for the supports. PMMA is highly resistant against weathering and has excellent mechanical and chemical properties (Degussa, 2003; Schwarz, 2002; Vieweg and Esser, 1975). PMMA polymer supports can remain for long times (years) in field experiments under environmental conditions without decomposition (Vieweg and Esser, 1975). The material is environmentally inert and does not release or accumulate any chemical substances (Degussa, 2003). PMMA is certified to be used for food and medical applications. According to the European standard EEC 90/128 (European Commission, 2002) it is physiologically harmless, which emphasized its suitability for environmental studies. The supports were pre-cut and fine moulded into squares of 2 × 2 cm. The size was chosen to fit the sample holder of the X-ray fluorescence spectrometer which was used for the analysis of the samples. Every support was cleaned with ethanol to remove any traces of the cooling fluid that had been used in the cutting process. Each plate was weighed with an analytical balance (Mettler AT 261, Switzerland) to an accuracy of 0.0001 g and the individual weight was carved into the reverse side. The carving did not change the support mass which was proved by a weighing test on N=10 supports performed on the same analytical balance. The thickness of the supports was chosen to be 2 mm in order to avoid torsion and bending of the material during further processing.

3.2.2 Epoxy Resin

In order to fix the particles a film of a two-component epoxy resin (Bisphenol A resin / Suter - Switzerland) was applied onto the polymer supports. A resin with relatively high dynamic viscosity (7000 mPa·s) was used to minimize creeping onto the particles due to capillarity effect. Hardened epoxy resin has excellent mechanical and chemical properties to withstand ambient conditions without loosing its adhering characteristics (Kajorncheappunngam et al., 2002; Suter, 2000). The resin is certified to be used for shipbuilding (Suter, 2000) which makes it suitable to be used under environmental conditions in the presence of water.


31

3.2.3 Coating Process

A set of ten weighed and marked supports was placed in two rows in a custom-made holder made of a PMMA plate (Plexiglas®/Acrylite®). The center part of the plate was countersunk to a depth of 2 mm make the polymer support surfaces level with the holder. The supports were pressed against the outer frame by means of a pressure plate of 2 mm thickness which pushed against the supports by means of two screws. Five milliliters of well-mixed two-component epoxy resin was put in a paint tray. A commercially available micro-foam paint roller was wetted with resin by touching the resin surface. Then, in order to enhance permeation of the resin the roller was rolled 10 times on the tray. This resulted in a homogenous distribution of the resin in the paint roller. The resin-loaded roller was scrolled four times over the polymer supports, resulting in a thin epoxy resin layer. After a pre-reaction time of 10 minutes, the frame with the epoxy prepared supports was placed in a spray chamber of 2 m length, 1 m width, and 1 m height. The chamber was shielded with polyethylene sheets and connected to the laboratory air extraction system. Selected particles/minerals were applied onto the support surfaces by using a dust spray gun from a commercial hardware store. It was found that that this dust spray gun had elevated particle consumption; so a down-sized version was constructed (Figure 3.1). Reducing the size of the spray device allowed us to handle much

Figure 3.1 Schematic sketch of the downsized spray gun

Spray nozzle Compressed air nozzle Particle reservoir holder Reservoir junction Parrticle reservoir (Plexiglas) Compressed air intake Air gun body (brass)

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smaller amounts of particles than with the original spray gun. Furthermore we mounted the particle reservoir on the top of the spray gun instead on the bottom. This allowed us to reduce the air flow as sufficient particles fell into the device by gravity. Particles were applied from a distance of 1.5 m to the supports holding the spray gun perpendicular to the frame with the supports. Gentle movement of the dust spray gun during application (30 s) resulted in a relatively homogenous coverage. The supply of compressed air was adjusted to a working pressure of 0.15 MPa providing a smooth air flow with homogenous particle distribution preventing particle damage upon epoxy resin impact. Directly after the application process the supports were removed from the frame and placed on a tray for hardening of the resin (hardening time according to the resin manufacturer) under ambient laboratory conditions. The particle mass on the supports was determined by subtracting the mass of applied resin and the mass of the support from the total weight of the coated support. Table 3.1 gives an overview of the selected particulate materials used in this study. Figure 3.2 shows a schematic sketch of the method. The drawing shows the polymer support as the carrier material with the thin film of epoxy resin. On the resin the particles are fixed, exposing their surfaces directly to the soil matrix. Unlike the previously presented methods this approach ensures a close and direct contact of the test particles with the surrounding soil. Recovery of the exposed particles after incubation allows the determination of dissolution and transformation rates and the examination of the changes which occurred during exposure to the soil environment by a broad range of analytical techniques.

Unit Lead Oxide (PbO)

Copper Oxide (CuO)

Copper Concentrate Quartz Sand

SupplierPennaroya

Oxide, Germany

Merck, Germany

Mining Industry,

ChileFluka,

Switzerland

Particle Size µm 20 – 100 80 - 200 20 – 100 100 - 200

N2-BET surface m2∙g-1 0.11 2.67 1.27 0.08a

Table 3.1 Origin, particle size ranges and surface areas of particles used in this study. a Data taken from reference (Scheidegger et al., 1993)


33

Figure 3.2 Schematic sketch of the particles attached to a polymer support through a thin resin layer. The particles´ surface area which are not covered by the epoxy resin, are in direct contact with the soil matrix and open for reactions.

3.2.4 Insertion into soil

Each polymer plate was analyzed by energy disperse X-ray fluorescence analysis (XRF) (Spectro X-Lab 2000, Germany) for the initial amount of metals. A thin thread of nylon (fishing line) was attached to the backside of the supports with a small drop of epoxy resin as glue. Then the polymer supports were implanted into field soils with the help of a small lancet made of stainless steel (Figure 3.3). The bottom of the lancet was flattened to facilitate penetration into soil. The implanting device was pushed into the soil to a depth of 20 cm and bent forward to open a small slit. The polymer support was inserted into this slit and its particle-coated side pressed against the soil. Then the slit was closed again by inserting the lancet 5 cm away from the slit and bending it towards it. The thin nylon thread attached to the support marked the location where the support was buried (Figure 3.4).

At the end of the chosen incubation time the plates were removed from the soil by digging them out with the adhering soil. The recovered supports were immersed for some minutes into a distilled water bath in order to loosen attached soil particles, this was supported by careful light strokes with a soft brush. Supports buried in sandy or Figure 3.3 Sketch of the implanting lancet

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loamy soils had only a small amount of attached particles that were very easy to remove. The samples were then left to dry at room temperature over night. Stereomicroscopy was used to get a first overview of dissolution signs or newly formed mineral phases on the particle surface. Scanning electron microscopy, SEM (Obducat CamScan CS44, Sweden) in combination with a microscope attached EDX probe (Edax ECON, USA) was used to analyze surface changes on the mineral particles as well as to identify the elemental composition of newly formed phases. Micro-RAMAN spectroscopy (Jobin Yvon Horiba LabRam HR, France) was used to analyze newly formed mineral phases.

3.2.5 Estimation of the exposed surface area of particles

The specific surface area of the mineral particles was determined in advance by BET analysis (Micromeritics Gemini 2360, Great Britain) using nitrogen gas as adsorbant. A set of PMMA supports coated with copper oxide (Merck Chemicals, Germany) as test substance was used to determine the free exposed surface of the particles. The exposed surface describes that part of the particle surface, which is not in contact/covered by epoxy resin, but in free contact with the soil (Figure 3.2). This surface part of the minerals can take place in potential phase transformation reactions. Phenylphosphonic acid (PPA) (Fluka Chemicals, Switzerland) was adsorbed on the copper oxide coated supports and on copper oxide in bulk suspension. PPA was used because phosphonic acids are known to strongly absorb onto almost all mineral surfaces (Nowack, 2003). For the copper oxide supports a concentration range of 5 µM to 60 µM PPA was used and for the copper oxide in suspension a range of 5 µM to 500 µM PPA. Pure PMMA supports and resin

Figure 3.4 Scheme of the sample insertion process. The insertion lancet is stuck into the soil and bend forward to open a small slit. The sample is inserted into the soil and the slit closed. The attached nylon thread helps to relocate the sample after the experiment.


35

only covered supports without particles were used as control samples. The PPA solutions were buffered at pH of 7.1 with a 2 mM morpholinoethanesulfonic acid (MES) (Fluka Chemicals, Switzerland). The concentration measurements were carried out by ion chromatography (Dionex DX100, USA).

3.2.6 Mechanical stability

To test the stability of the particle coating, two separate experiments were carried out. The first experiment intended to simulate the abrasion forces during the insertion process. Supports were coated with quartz sand to simulate particles and then a semi-hard paint brush was stroken about ten times over the surface. The loss of particles was determined as the difference between weights before and after the experiment. In addition we examine the supports for traces of detached particles by means of optical and electron microscopy. The lost particles can be clearly noticed because they leave behind empty cavities in the epoxy resin (Figure 3.5).

Figure 3.5 Support covered with PbO and buried in acidic soil for 18 months. Traces of a fallen off particle are shown in the top left picture. A partially dissolved particle is shown in the bottom picture.

The second test intended to simulate particle dissolution. Polymer supports were coated with lead oxide particles (approx. 50 mg support-1) and then submerged for 12 h in a stirred 0.1 M EDTA (Merck Chemicals, Germany) solution at pH 5. Micro photographs were taken before and after the incubation through a stereomicroscope (Zeiss Stemi 2000-C, Germany). In addition, SEM pictures were taken from parts of the incubated support where indications of particle dissolution were found in the stereoscope photographic pictures.

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3.2.7 Calibration of XRF

XRF was used to quantify the metal content of the supports before and after recovery. A custom made calibration was performed for all used particles. A set of 15 supports, coated with PbO were incubated for various times (1 - 240 min) in 0.1 M EDTA solution at pH 4.5 to obtain different concentrations of PbO and quantitatively analyzed by XRF. After the XRF analysis the remaining particles of the supports were completely dissolved by microwave-assisted dissolution (Milestone ETHOS, Italy; 0.1 M EDTA, pH 4.4, 150 W, 60 min) and the dissolved Pb was quantitatively analyzed by flame-AAS. With the acquired calibration it was then possible to get the actual elemental concentration on the individual support by means of the non-invasive XRF analysis.

3.2.8 Soil experiments

The method was tested in pot experiments under laboratory conditions. PMMA support samples were buried at a depth of 5 cm in polypropylene pots filled with weakly acid silty clay and acidic loam soil (Table 3.2). The pots were irrigated with synthetic “rain” (made from p.a. grade chemicals, Fluka Chemicals, Switzerland and Merck Chemicals, Germany) of the following composition: P 0.1 mg·l-1; Na 0.1 mg·l-1; K 0.3 mg·l-1; Ca 0.2 mg·l-1; Mg 0.03 mg·l-1; Zn 0.01 mg·l-1; Cl 0.6 mg·l-

1; SO4 0.3 mg·l-1; N 3.5 mg·l-1. The pH was adjusted to a value of 5.5 by HCl. The irrigation was applied with a multi channel peristaltic pump (Ismatec BVP-12, Switzerland) and distributed through perforated tubing, at a rate corresponding to the average precipitation of 1100 mm per year for Zurich. The same peristaltic pump was also used to collect the drainage water. The experiment was run for four months at 20°C (room temperature).

Soil Loam Silty clay Calcareous loam Calcareous sand

Origin forest agriculture pasture Pleistocene sand deposit

pH (in 0.01 M CaCl2) 3.8 6.6 7.5 8.1

Carbonate content (%) < 0.4 < 0.1 7.2 35.1

Organic matter content (%) 2.5 1.5 3.1 0.1

Table 3.2 Properties of the test soils


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In addition, field experiments were carried out where PbO and Cu concentrate-coated supports were placed for up to 18 months in a calcareous sandy soil (pH 8.1) and an acidic silty forest soil (pH 3.8). The supports were inserted at a depth of 10 cm. Soil properties are given in Table 2.

3.3 Results & Discussion

3.3.1 Durability of supports

In contact with acidic (pH 3.8) and near neutral (pH 6.6) soil (Table 3.2) as well as in acidified water (pH 3.0), the polymer supports did not show any visible changes over the incubation period of 4 months in the laboratory pot experiments. Also in the field, where supports were incubated for periods of up to 18 months, we observed no changes in the PMMA support material. The weight carving on the back side of the support was not affected and clearly readable after these time spans. Also, the epoxy resin did not show any signs of degradation in the tests. Neither neutral, nor acidic conditions affected its structure.

3.3.2 Coating Process

In first trials, the spray device was used as obtained by the supplier. However, the material consumption rate was found to be too high (100 g particles per 30 s spray time) making the use of small sample amounts impossible. Therefore, the device was downsized to a reasonable scale (Figure 3.1) with an optimal material consumption rate. The downsized dust spray gun enabled us to apply 15 g particles per 30 s spray time. To ensure a consistent, gravity enhanced particle flow the material reservoir was changed from the bottom to the top of the dust gun. The adjusted working air pressure for the spray device provided a gentle airflow transporting the particles homogenously towards the supports. The viscosity of the applied epoxy resin attenuates the impact forces of the particles on the support surface preventing them from potential harm. Particle damage could be excluded by comparison of pure bulk particles with particles applied with the spray device onto the supports.The average amount of epoxy resin on the support surface (N=30) was 0.011 g with a standard deviation of 6.0 %. The average mass of lead oxide particles fixed

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to the supports in one test experiment was 0.047 g (N=20), with a coefficient of variation of 11.5 %. Figure 3.6 shows that the particles were evenly distributed on the surface of the support in a monolayer. Only their rear side was covered by resin.

Figure 3.6 SEM microphotograph of initial PbO particles on the support.

Preliminary tests showed that the application of particles by simple sprinkling with a brush or fine meshed sieve did not provide a homogenous particle cover.The homogeneity of the coating can be increased by narrowing down the width of the size distribution of the particles. Tests with epoxy resins of various viscosities showed that resin with low viscosity tended to creep onto the particles and sometimes completely covered them. This effect was particularly pronounced for relatively small particles. The use of a high viscosity resin reduced this effect, but for very small particles it was still a problem. Tests with a variety of different materials showed that the particles should have a minimum size of 20 µm diameter.

3.3.3 Mechanical stability

A major concern of the method is that particles could be lost from the support during exposure and analysis. The effect of sample handling was analyzed by means of a abrasive brush test. The test results confirmed the stability of the test coating on the PMMA supports. Only 0.0022 % of the applied sand particle mass was lost on average (30 µg ± 14 µg from 0.048 g per support). Thus, particle losses due to abrasion during the handling, insertion and recovery of the coated


39

supports were considered to be negligible. Also potential weathering during exposure time did not weaken the adhesion of the particles to the resin. SEM pictures and stereomicroscope photographs showed only very few empty cavities in the resin surfaces or other signs which would have indicated that particles had fallen off. In Figure 3.5 both an empty cavity and a partially dissolved particle are shown and are distinguishable without problems. In all tests, particles were dissolved partially at their exposed surfaces, but remained attached to the epoxy resin surface. Particle counts on recovered supports (N=8) which had been buried up to 18 months in different soils, showed that a negligible amount of particles had fallen off the reacted support. Table 3.3 shows individual results for different soils from the particle counting. This confirmed that extended field incubation times (> one year) did not result in the loss of particles due to the insertion, recovery and cleaning process. Neither the soil pH, the soil texture or the changes between summer and winter with freezing and thawing and wet and dry periods affected the samples. The small percentage of observed PbO and Cu concentrate particles loss was considered to be within the acceptable experimental error.

Table 3.3 Loss of particles from supports after incubation times up to 18 months (field and laboratory data)

Material Laboratory / Field Soil % Particles

lossIncubation time

[months]PbO F calcareous sand < 0.2 18

PbO F acidic loam 0.5 18

PbO L calcareous sand 0.4 4

PbO F silty clay < 0.2 18

PbO F acidic loam 0.3 18

PbO L acidic loam 0.6 4

Cu concentrate F silty clay 0.2 18

Cu concentrate L acidic loam < 0.2 4

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3.3.4 Exposed surface area of the attached particles

The adsorption experiment with phenylphosphonic acid (PPA) on copper oxide coated supports and copper oxide in suspension resulted in two Langmuir adsorption isotherms. Figure 3.7 shows that adsorption of PPA to particles fixed on supports differed only slightly from sorption to particles in suspension. The conclusion is that only a small fraction of the surface area of the particles was covered by resin. This finding could be confirmed by SEM photographs of support attached particles showing that an area smaller than about 10 % of the particle surface is covered by resin. We have assumed that a spherical particle such as PbO is covered to a maximum 10 % of its height by resin. This equals to not more than 10 % of covered surface.

Figure 3.7 Adsorption isotherms of phenylphosphonic acid (PPA), buffered at pH 7.1 with MES) on polymer supports covered with copper oxide (filled squares) and on copper oxide in bulk suspension (empty circles). The dashed lines are the calculated Langmuir isotherms.

3.3.5 Quantification

In order to quantify the amount of mineral dissolution, particles remaining on the support after exposure were analyzed by XRF. The non-destructive XRF is able to quantify each coated support prior to the incubation and therefore to determine individual start and end concentrations of particles for the experiments. For each substance we made an individual calibration of the XRF. Figure 3.7


41

shows the calibration of PbO by XRF versus total dissolution AAS analysis. On the x-axis the measured detector counts acquired by XRF are plotted, on the y-axis the actual elemental concentrations per support, acquired by total dissolution are shown. With this calibration it is possible to determine the Pb concentration on the individual support by non-destructive XRF analysis. A 2nd order regression explained 95 % of the variance (R2 = 0.95). Repeated XRF measurements (N=10) with a rotation of the analyzed support by 45° turns after each run showed that the orientation of the support in the sample holder had no influence on the result. Repeated XRF control measurements (N=3) of one support over several months also showed that instrumental drift was no problem. The coefficient of variation was only 0.08 %.

Figure 3.8 Calibration of the XRF device for PbO-covered supports. The non-invasive XRF results (counts) can be converted directly into actual elemental concentrations per support (mg Pb·support-1) which was acquired by complete dissolution and AAS analysis. The different concentrations were achieved by partial dissolution of the particles. The lines represent the second order polynomial fit and the 95% confidence interval.

3.3.6 Application

The pot experiments in the laboratory as well as the field tests showed that the stainless steel lancet used for implanting the supports in the field worked without problems. It caused only a minor disturbance of the soil. However, each experimental method for studying particle reactions in soil has to disturb

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the system to be able to expose the particles. In contrast to the other methods, introduced in the beginning of this manuscript, our implanting technique caused only a small disturbance because it makes only a thin incision into the soil matrix. To maximize the contact of the particles with the soil, the coated support surface was gently pressed against the soil. After the supports had been implanted, no significant openings were left behind in the soil. The attached nylon thread always allowed relocating the implanting location at the field sites. In our study we implanted the supports vertically in the topsoil down to a depth of 20 cm. It is also possible to implant the supports horizontally from the wall of a soil pit.Cleaning the recovered supports with a soft brush in distilled water allowed removal of virtually all of the attached soil particles. Only under the electron microscope (SEM), very few small soil particles (10 – 15 µm size) could be observed which had been left on the supports. Energy dispersive X-ray spectra (EDX) of these particles showed clear elemental signs of soil minerals such as Al, Ti and Fe. Some of these particles can be seen in the bottom right under the particle of Figure 3.9.

Figure 3.9 PbO particle after recovery from an acidic soil (pH 3.8, soil 1) after 12 months. Clear signs of dissolution can be seen on the particle surface. The arrow indicates some residual soil particles left over from the cleaning process. The bottom right inset shows a unreacted PbO particle for reference.

One of the possible reactions of a mineral particle introduced into soil is the formation of a new phase. Figure 3.10 shows the surface of a PbO grain that had been buried in the calcareous sandy soil (soil 4) for 4 months. Comparison with


43

the original state (inset, Figure 3.10) shows that fine crystals had grown on the entire surface of the particle. The picture shows that even very fine structures and delicate mineral assemblages can be recovered and cleaned of adhering soil particles even after extended exposure in soil under field conditions. The EDX analysis showed that the newly formed metal phase contained lead, carbon, and oxygen. It was identified as hydrocerussite, Pb3(CO3)2(OH)2, by micro-Raman spectroscopy. Formation of lead carbonates in calcareous soils has been previously reported (Li and Thornton, 2001; Maskall and Thornton, 1998).

Figure 3.10 PbO particle after recovery from a sandy soil (pH 7, soil 4). A newly formed mineral phase can be clearly seen on the particle surface. This new phase has been identified by micro-Raman spectroscopy as hydrocerrusite. The bottom right inset shows a unreacted PbO particle for reference.

In addition to mineral transformation, dissolution of the initial phase can also be observed by our new method in field as well as in laboratory experiments. Figure 4 shows an example of PbO buried in an acidic loamy soil for 12 months. The comparison of this particle with the unreacted particle (inset Fig. 4) reveals that significant dissolution occurred during the incubation. PbO is very soluble at a soil pH of 3.8. Still, part of the mineral surface was hardly affected by dissolution. In addition to such qualitative information, the method can also be used to determine weathering rates. To give an example, the metal content of a support equivalent to that shown in Figure 4 decreased from 47 ± 18 mg support-1 to 36 ± 13 mg support-1 during 12 months of exposure in the acidic forest soil. This is equivalent to 24 % loss of Pb and corresponds to a dissolution rate of 3.2·10-10 mol m-2 s-1

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(normalized to the surface area of the PbO particles). One of the important advantages of the method is that it provides the ability to measure in a direct way the dissolution rate of fine mineral particulate phases in the field under natural conditions. No pre-concentration or refining process is needed prior to the analysis. This ability provides, as far as we know, for the first time direct information on dissolution rates in situ on a particle scale with fine minerals in environmental field experiments. This method will therefore be very useful in the future to refine the insights on the behavior of fine particles or dusts in environmental systems.

3.3.7 Further applications

The new method is a useful tool for the investigation of mineral transformation and dissolution in different environmental media. We believe that the method can also be used in “easier” environments such as sediments, and natural waters as it showed its applicability to heterogeneous soils. Its main advantage is the easy placement and recovery of the samples and the possibility to obtain both qualitative and quantitative information on mineralogical changes and dissolution rates. Apart from heavy metal phases as used in this study it may also be used to investigate transformation of other minor particles (e.g. silicates) in the environment.

Acknowledgments

We would like to thank René Saladin for cutting the polymer supports and helping with lab experiments, Hanspeter Läser for constructing the spray gun and the frame holder, Werner Attinger for his help in the field, Peter Wägli for making the SEM analysis at the ETH Institute of Solid State Physics possible, Eric Reusser for giving us access to the Raman spectrometer at the ETH Institute of Mineralogy and Andreas Wahl for providing us with mineral particles from Penarroya Oxide GmbH.


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Bibliography

Adriano D. C. (2001) Trace Elements in Terrestrial Environments. Springer.Alloway B. J. (1995) The origin of heavy metals in soils. In Heavy Metals in Soils (ed. B. J. Alloway). Blackie.Bennett P. C., Hiebert F. C., and Choi W. J. (1996) Microbial colonization and weathering of silicates in a petroleum-contaminated groundwater. Chem. Geol. 132, 45-53.Bilderback D. H., Hoffman S. A., and Thiel D. J. (1994) Nanometer spatial resolution achieved in hard x-ray imaging and Laue diffraction experiments. Science 263, 201-203.Cabrera F., Clemente L., Díaz Barrientos E., López R., and Murillo J. M. (1999) Heavy metal pollution of soils affected by the Guadiamar toxic flood. Sci. Total Environ. 242, 117-129.Charlet L. and Manceau A. (1992) In Situ Characterization of Heavy Metal Surface Reactions: The Chromium Case. Int. J. Environ. An. Ch. 46, 97-108.Degussa. (2003) Plexiglas - Product Description.Dudka S. and Adriano D. C. (1997) Environmental Impacts of Metal Ore Mining and Processing: A Review. J. Environ. Qual. 26(3), 590-602. European Commission. (2002) COMMISSION DIRECTIVE 2002/72/ EC of 6 August 2002 relating to plastic materials and articles intended to come into contact with foodstuffs. In 90/128/EEC. European Commission.Hodson M. E. and Langan S. J. (1999) The influence of soil age on calculated mineral weathering rates. Appl. Geochem. 14, 387-394.Jørgensen S. S. and Willems M. (1987) The fate of lead in soils: The transformation of lead pellets in shooting-range soils. Ambio 16(1), 11- 15.Kajorncheappunngam S., Gupta R. K., and GangaRao H. V. S. (2002) Effect of Aging Environment on Degradation of Glass-Reinforced Epoxy. J. Compos. Constr. 6(1), 61-69.Li X. and Thornton I. (2001) Chemical partitioning of trace and major elements in soils contaminated by mining and smelting activities. Appl.

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Geochem. 16, 1693-1706.Maskall J. E. and Thornton I. (1998) Chemical partitioning of heavy metals in soils, clays and rocks at historical lead smelting sites. Water Air Soil Poll. 108(3-4), 391-409.Maurice P. A., McKnight D. M., Leff L., Fulghum J. E., and Gooseff M. (2002) Direct observations of aluminosilicate weathering in the hyporheic zone of an Antarctic Dry Valley stream. Geochim. Cosmochim. Acta 66(8), 1335-1347.Nowack B. (2003) Environmental chemistry of phosphonates. Water Res. 37, 2533-2546.Nugent M. A., Brantley S. L., Pantano C. G., and Maurice P. A. (1998) The influence of natural mineral coatings on feldspar weathering. Nature 395, 588-590.Ranger J., Dambrine E., Robert M., Righi D., and Felix C. (1991) Study of current soil-forming processes using bags of vermiculite and resins placed within soil horizons. Geoderma 48, 335-350.Righi D., Bravard S., Chauvel A., Ranger J., and Robert M. (1990) In Situ Study of Soil Processes in Oxisol-Spodsol Sequence of Amazonia (Brazil). Soil Sci. 150(1), 438-445.Rogers J. R., Bennett P. C., and Choi W. J. (1998) Feldspars as a source of nutrients for microorganisms. Am. Mineral. 83, 1532-1540.Scheidegger A., Borkovec M., and Sticher H. (1993) Coating of silica sand with goethite: preparation and analytical identification. Geoderma 58, 43-65.Schwarz O. (2002) Polymethylmethacrylat [PMMA]. In Kunststoffkunde (ed. O. Schwarz). Vogel Verlag.Sparks D. L. (1989) Kinetics of Soil Chemical Processes. Academic Press.Suter. (2000) Epoxy resin L 135 LV low viscosity - Data Sheet. Suter.Swoboda-Colberg N. G. and Drever J. I. (1993) Mineral dissolution rates in plot- scale field and laboratory experiments. Chem. Geol. 105, 51-69.Tsaplina M. A. (1996) Transformation and transport of lead, cadmium and zinc oxides in a sod-podzolic soil. Eurasian Soil Sci. 28(1), 32-40.Vieweg R. and Esser F. (1975) Polymethacrylate. Hanser Verlag.Voegelin A., Scheinost A. C., Bühlmann K., Barmettler K., and Kretzschmar R. (2002) Slow Formation and Dissolution of Zn Precipitates in Soil: A


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Combined Column-Transport and XAFS Study. Environ. Sci. Technol. 36, 3749-3754.White A. F., Blum A. E., Schulz M. S., Bullen T. D., Harden J. W., and Peterson M. L. (1996) Chemical weathering rates of a soil chronosequence on granitic alluvium: I Quantification of mineralogical and surface area changes and calculation of primary silicate reaction rates. Geochim. Cosmochim. Acta 60(14), 2533-2550.White A. F. and Brantley S. L. (2003) The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field? Chem. Geol. 202, 479-506.

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4 In situ investigation of dissolution of heavy metal containing mineral particles in an acidic forest soil

Andreas Birkefeld, Rainer Schulin and Bernd Nowacksubmitted to Geochimica et Cosmochimica Acta

Abstract

We report the application of a new in situ method to obtain field dissolution rates of minerals in soils. Samples with different metal-containing mineral and slag particles (lead oxide, copper concentrate and copper slag) from the mining and smelting industry were buried in the topsoil of an acidic forest soil for up to 18 months. In addition we studied the dissolution of these particles in samples of the same soil also under constant temperature and moisture conditions in the laboratory and compared it with dissolution in a sand matrix and in acid solution. Under field conditions the lead oxide particles dissolved quite rapidly, whereas the copper concentrate and the copper slag particles proved to be persistent to weathering. The new method also allowed to study recovered samples by scanning electron microscopy. Weathering was indicated by distinct cavitation features on the particles. The rates found followed the order of: sand with acid percolation (pH 3.5; lab) < soil(lab) < soil(field) < acid solution (pH 3.5; lab). As commonly reported we also observed much faster dissolution rates in solution compared to soils. Dissolution rates in soil were found to be lower under laboratory than under field conditions. We attribute the faster field rates to the higher biological activity in the field soil compared to the same soil in the laboratory.

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4.1 Introduction

Considerable areas surrounding industrial and mining sites are polluted by metal-containing dusts emitted from these sources. The release of heavy metals into soil solution due to weathering of these particles can seriously damage soil fertility, adversely affect the quality of surface and ground waters and lead to health risks for the consumers of crops produced on such soils. Metal mines are one of the major industries producing metal-containing dusts and slags (USGS, 2002). In the process of crushing, milling and concentrating, large quantities of waste particles are generated (1999 approx. 8000 Mt solid waste) (Lottermoser, 2003). While they can vary considerably in composition, they usually contain high concentrations of hazardous metals. Over the centuries worldwide mining and smelting activities have led to the accumulation of enormous amounts of mining wastes deposited in the environment (Martin et al., 2002). In the past, several disastrous mining accidents have resulted in the release of large amounts of these wastes into the environment and contaminated wide areas (Penman, 2001; Riba et al., 2004). The possibility to study the transformation behavior of potential hazardous particles under field conditions in general contributes important data e.g. for environmental risk assessment studies in the mining and metallurgical industry.In order to assess the risks arising from such contamination it is important to know weathering kinetics of these particles in soils and sediments (Alloway and Ayres, 1997). The dissolution of (toxic) mineral particles has been investigated in many laboratory, lysimeter and field studies (Casey et al., 1993; Ettler et al., 2002; Schnoor, 1990; Strömberg and Banwart, 1999; Swoboda-Colberg and Drever, 1993; Velbel, 1990; Vidal et al., 1999; White and Brantley, 2003). These studies were primarily interested in weathering as a process of soil formation. Several of them reported a discrepancy between dissolution rates in the laboratory and in field experiments. Schnoor (1990) found that dissolution rates of aluminosilicate minerals were one to two orders of magnitude lower in field than in laboratory experiments. Velbel (1990) found that dissolution rates of feldspars differed between field and laboratory experiments by orders of magnitude in favor for the laboratory. Casey et al. (1993) investigated the weathering of olivine minerals and also reported differences by orders of magnitude between field and laboratory conditions. All of these authors attributed these discrepancies to differences

In situ study of dissolution of heavy metal containing mineral particles in an acidic soil

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in moisture and temperature conditions between laboratory and field. All the laboratory experiments were conducted under controlled conditions in batch reactors, fluidized bed reactors or saturated columns (lysimeters), whereas the field experiments have to deal with temporal fluctuations and spatial heterogeneities (mineralogy, soil types, biological activity) that are difficult to control. A difference in weathering conditions between laboratory and field, which is of particular importance, is the activity of organisms. Microorganisms, fungi and plants accelerate mineral weathering through biomechanical and biochemical attack (“bioweathering”) (Banfield et al., 1999; Burford et al., 2003). Mycorrhizal fungi are especially effective in dissolving soil minerals (Jongmans et al., 1997) and solubilizing heavy metals (Martino et al., 2003). The soil in the immediate vicinity of plant roots (rhizosphere) has chemical, physical and biological properties that are substantially different from those of the bulk soil (“rhizosphere effect”). For example, (Hinsinger et al., 1993) observed increased mobilization of metals and transformation of minerals with decreasing pH in the rhizosphere of Brassica napus. The exudation of organic substances is another mechanism by which plant roots can mobilize metal ions (Dakora and Phillips, 2002). In particular, organic ligands such as citrate, oxalate, phenolic acids and siderophores were all found to increase mineral weathering rates (Jones, 1998; Kraemer, 2004; Liermann et al., 2000; Pohlman and McColl, 1986; Zhang and Bloom, 1999). Also dissolved organic substances leached from plant litter have the ability to mobilize mineral-bound metals (Pohlman and McColl, 1988). Banks et al. (1994) found that leaching of zinc from soil contaminated by mine-tailing material was lowest in the absence of plants, intermediate in the presence of plants and microorganisms and highest if only plants and no microorganisms were present. Augusto (2000) observed that feldspar weathered at different rates under different tree species.The difficulty to reproduce field weathering conditions in the laboratory has led to an intensive search for methods to study the dissolution of mineral particles in situ under field conditions. Various approaches have been taken. One is the study of weathering in lysimeters experiments in which dissolution rates are determined from the analysis of solution and discharge samples (Swoboda-Colberg and Drever, 1993). Another approach uses bags filled with test minerals which are inserted into soils and recovered after some time and then analyzed by various techniques for differences in mineral composition (Augusto et al., 2000; Hatton et al., 1987).

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Bennet and coworkers (Bennett et al., 1996; Rogers et al., 1998) and Maurice et al. (2002) inserted perforated polyethylene bottles (“microcosms”) filled with silicate test minerals into groundwater wells and water saturated sediments and determined dissolution rates from microscopic analysis of recovered samples. One problem of field or lysimeter studies relying only on the analysis of percolating solution is that it is often difficult to ascertain that the chemistry of the samples solution is not influenced by other, uncontrolled factors. Bag methods have the disadvantage that the particles are not in direct contact with the matrix in which they are buried as they are separated from it by the envelope of the bag. In addition placing the bag into the soil causes considerable disturbance to the soil structure. Similar problems also apply to the microcosm methods of Bennett (1996) and Maurice et al. (2002), which consists in burying relatively large bottles (8-10 ml volume) in sediments. An advantage of the latter method is that also samples of the solution percolating the microcosms in situ can be obtained. However, the application of this method is limited to saturated sediments and soils with sufficient flow through the perforated containers.These problems inspired us to develop a new in situ technique which provides direct contact of test particles with the adjacent matrix (Birkefeld et al., 2005). In this method, the particles of interest are fixed by a thin film of epoxy resin to acrylic glass polymer supports. Apart from the area in contact with the epoxy resin, the particle surfaces are exposed to the surrounding soil without interference of any barrier. The support material and the fixation are very resistant to mechanical and chemical stress so that the sample can be incubated in soils under field conditions for periods of months to years. As they are flat and can be made very small in size, their placement into soil can be achieved with less disturbance than that of bags or microcosms. The supports can be recovered easily and the remaining metals can be quantified and the particles can be examined using microscopic and electron microscopic methods.In this study, we show the potential of the new method to determine dissolution rates of metal-containing mineral and slag particles in real soils under field conditions. It is the first investigation on the dissolution kinetics of heavy metal containing waste particles emitted from mining and smelting industries in acidic forest soils. In order to assess the influence of different conditions in field and lab experiments, we performed control experiments in which we incubated these


53

particles under constant temperature and moisture conditions in samples of the same soil as used in the field and for comparison also in acidic quartz sand and solution environments. Based on the results reported in the literature revised above, we expected to observe much higher dissolution rates in the laboratory than in the field.


4.2.1 Sample preparation

Particle supports were made from polymethylmetahcrylate (PMMA) polymer (Plexiglas®/Acrylite®). Pre-cut plates of (2 cm x 2 cm) were weighed with an accuracy of 0.1 mg by means of an analytical balance (Mettler Toledo AT 261, Switzerland). The weight of each plate was carved into its back side. Then the plates were covered with a thin adhesive film of a two-component epoxy resin (Bisphenol A resin - Suter, Switzerland) using a micro-foam paint roll. For this purpose, a set of ten weighed and marked supports was placed in two rows in a frame holder. Finally, the selected particles were applied onto the support surfaces by using a downsized copy of a commercial available dust spray gun. The spray gun was held perpendicular in a distance of 1.5 m to the supports. Gentle moving of the dust spray gun during application (30 s) produced a relatively homogenous distribution of the particle on the supports. For a more detailed description of method the reader is referred to Birkefeld et al. (2005)

4.2.2 Pre experimental analysis

For this study different sample plates were prepared with three types of particles: lead oxide which was obtained from a commercial manufacturer, copper concentrate which came from the mining industry (Chile) and copper smelting slag from a smelter facility in Chile. Lead oxide particles are a typical component of metal refining industry emissions. Copper concentrate and copper smelting slag particles are major components of mining industry emissions (including ore smelting). After hardening of the epoxy resin (12 h) the polymer plates were analyzed by energy dispersive X-ray fluorescence analysis (XRF) (Spectro X-Lab 2000, Germany) to determine the elemental composition of the mineral particles fixed to the supports. Stereo microscopy was used to assess the coating quality. A

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custom calibration was used for each particle type to convert the XRF counts into concentrations. For this purpose a sample set with different particle concentrations was first analyzed by XRF and afterwards completely dissolved and analyzed by AAS (Varian SpectrAA 220, Australia). The elemental composition, main mineralogical phases, and specific surface areas of the three types of particles are given in Table 4.1.

Unit Lead Oxide (PbO) Copper concentrate Copper slag

Origin Pennaroya Oxide Ltd Copper mine (Chile) Smelter (Chile)

Particle Size µm 20 – 100 20 – 100 20 – 100

BET surface m2·g-1 0.11 1.27 0.41

Table 4.1 Selected properties of applied particles from the study

4.2.3 Experiments

4.2.3.1 Field experiments

The topsoil of an acidic dystic Cambisol under a mixed oak-beech forest near Zurich (Switzerland) was chosen for the field test (pH0.01 M CaCl2 3.8, Corg 2.5%). The texture of the topsoil was a silty loam (clay/silt/sand 21/50/29 %). The site was situated at 520 m a.s.l. The total precipitation over the 18 months of the experiment was 1200 mm. The mean temperature was 8.9° C (Figure 4.1). In order to insert the particle coated polymer supports into the soil small slits were opened in the soil matrix from the surface by means of a sharpened stainless steel lancet (3 cm width; 0.5 cm thick; 40 cm length). A small nylon thread attached on the non-coated side of the support protruding from the soil surface marked the exact location for later recovery. Ten supports of each particle type were implanted at a depth of 15 cm in a row, separated by distances of 10 cm from each other. Samples were left up to 18 months in the soil, including two winters and one summer. The samples were inserted into the soil in the beginning of September 2002.Recovered samples were put into small labeled polyethylene bags to keep them


55

field-moist and transferred to the laboratory. Here they were immersed in a distilled water bath for about 5 min to soften residual soil particles, which were then removed from the support surface by mean of a small soft brush. Afterwards the samples were let to dry under ambient air over night.

Figure 4.1 Climate data (temperature and precipitation) during the experimental time – Data taken from weather recording station Hönggerberg of the Institute of Atmospheric and Climate Sciences (IACETH) of ETH Zurich

4.2.3.2 Laboratory experiments

For the laboratory experiments three dark polypropylene pots were filled with topsoil from the same site where the field experiment was carried out and two polypropylene pots were filled with acid washed clean quartz sand. Plates coated with lead oxide, copper concentrate and copper slag were inserted into the pots in the same way as in the field. The pots containing acidic forest soil were irrigated with synthetic rain solution (made from p.a. grade chemicals, Fluka Chemicals, Switzerland and Merck Chemicals, Germany) which had the following composition: N 3.5 mg·l-1; Cl 0.6 mg·l-1; Ca 0.2 mg·l-1; K 0.3 mg·l-1; SO4 0.3 mg·l-1; Na 0.1 mg·l-1; P 0.1 mg·l-1; Mg 0.03 mg·l-1; Zn 0.01 mg·l-1; pH 5.5. The pots with quartz sand were irrigated with a HNO3 solution of pH 3.5. Irrigation was provided from two glass bottles using a multi-channel peristaltic pump (Ismatec BVP-12, Switzerland) and distributed through perforated PE tubing at

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a rate of 75 ml per day and pot corresponding to a precipitation of 1100 mm per year. The same peristaltic pump was also used to collect the drainage water. The experiment was run for three months at room temperature (20°C). Samples were retrieved after regular time intervals and processed in the same way as the field samples. In an additional experiment, plates with lead oxide were incubated in a 5 l polyethylene tank filled with a HNO3 solution which had a pH of 3.5. The solution was slowly stirred with a magnetic stirrer (Heidolph MR 3004, Germany). As in the other experiments, samples were recovered at regular intervals and analyzed over a period of three months.

4.2.4 Sample analysis

Field and laboratory samples were analyzed in the same way. The cleaned and dried samples were inspected under a stereomicroscope for damages and then analyzed by XRF spectroscopy (Spectro X-Lab 2000, Germany) to establish the loss of concentration during the experiment. In addition, a selection of samples was sputter-coated with gold and analyzed by scanning electron microscopy (SEM) (Obducat CamScan CS44, Sweden) for morphological changes, in particular for features of dissolution on the particle surfaces and for growth of new phases. The SEM instrument was equipped with an energy dispersive x-ray detector (EDX) (EDAX Econ, USA), allowing to make a qualitative elemental analysis of small areas.Dissolution rates were calculated from the decrease in heavy metal contents of the samples (XRF before and after the experiment), divided by the BET surface area of the particles.

4.3 Results

4.3.1 Particle Characterization

The x-ray diffraction (XRD) analysis of the lead oxide particles revealed the presence of the two PbO polymorphs massicot and litharge (Table 4.2). Particle sizes ranged from 20 µm to 100 µm. The N2-BET surface area was 0.11 m2·g-1. X-ray fluorescence (XRF) analysis revealed Cu, Zn and As as minor constituents of the lead oxide particles (Table 4.3). The copper concentrate particles were a conglomerate of typical minerals from a porphyric deposit. Chalcopyrite


57

(CuFeS2) was the main mineral phase. Minor copper phases were roxbyite and nantokite (Table 4.2). The Cu concentrate had a copper content of 26% (Table 4.3) and a specific surface area of 1.27 m2·g-1. The mineral composition of the copper smelting slag was typical for smelter slags. Magnetite and fayalite were the main phases (Table 4.2). These iron bearing minerals are secondary minerals that are generated during the ore smelting process after iron oxide is added as a flux substance (Gee et al., 1997). The surface area of the slag particles was 0.41 m2·g-1 and the residual copper concentration was 0.6 % (Table 4.3). Such a low copper content is typical for slags cleaned in electric furnaces by the procedure described by Mechev (1997). According to Warczock et al. (2002) slags cleaned by this procedure also contain elevated magnetite contents. The occurrence of a fayalite phase was also reported for copper slags analyzed by Imris et al. (2000). We conclude from the XRF and XRD analysis that about half of the Fe in the slag material used in this study was present in the fayalite phase and the other half in the magnetite phase.

Phase Mineral name Chemical formulaLead oxide Massicot PbO

Litharge PbOCopper concentrate Chalcopyrite CuFeS2

Pyrite FeS2Nantokite CuClRoxbyite Cu7S4

Molybdenite MoS2

Copper slag Fayalite Fe2SiO4Magnetite Fe3O4



Element PbO Unit Cu concentrate Unit Cu slag UnitCu 100 mg kg-1 26.8 % 6000 mg kg-1

Zn 130 mg kg-1 1560 mg kg-1 2025 mg kg-1

Pb 80 % 120 mg kg-1 345 mg kg-1

As 2000 mg kg-1 375 mg kg-1 545 mg kg-1

Sb - mg kg-1 105 mg kg-1 235 mg kg-1

Mo < 50 mg kg-1 950 mg kg-1 2856 mg kg-1

Fe < 0.1 % 11.2 % 36.6 %Si < 0.1 % 0.8 % 4.5 %

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4.3.2 Dissolution under field conditions

Figure 4.2 shows the recovery of particles bound on the retrieved samples after various exposure times as a percentage of the initial metal mass on the respective sample. The lead oxide particles lost about 40% of the initial Pb content during the 18 months of the experiment. The SEM pictures show typical morphological features of the progressive dissolution of the lead oxide particles (Figure 4.3). First the top layer is dissolved, then cavities and holes appear which grow and form a cellular structure. The average dissolution rate, calculated from the slopes of the lines shown in Figure 4.2, was 2.4·10-10 mol Pb·m-2·s-1 (Table 4.4).

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

Am

ount

of P

bO [%

]

months

a

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

Am

ount

of C

u [%

]

months

b

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

months

Amou

nt o

f Cu

[%]

c

Figure 4.2 a, b, c Dissolution of lead oxide (a), copper concentrate (b) and copper slag (c) particles in acidic forest soil under field conditions for 18 months, expressed as mass of metal recovered in percent initial mass of metal.


59

Figure 4.3 a, b SEM pictures of PbO particles after different reaction times in an acidic soil under field conditions. a) Lead oxide particle appearance before the experiment, b) after 6 months.

Type of particles ExperimentDissolution Rate

(mol Pb resp. Cu m-2 s-1 )PbO forest (field site) 2.4·10-10

PbO forest soil (lab) 1.9·10-10

PbO acidic sand (lab) 7.7·10-11

PbO acid solution (pH 3.5) 9.5·10-9

Cu concentrate forest (field site) <1·10-11

Cu concentrate forest soil (lab) 1.1·10-11

Cu concentrate acidic sand (lab) 1·10-11

Cu slag forest (field site) 4.3·10-11

Cu slag forest soil (lab) 5.9·10-11

Cu slag acidic sand (lab) 1.1·10-11

Table 4.4 Dissolution rates in mol m-2 s-1 for the PbO, Cu concentrate and Cu slag in acidic forest soil in the field, in the same soil in the laboratory, in sand percolated with HNO3 solution (pH 3.5) and in stirred HNO3 solution of pH 3.5 (PbO only).

Figure 4.3 c, d SEM pictures of PbO particles after different reaction times in an acidic soil under field conditions. c) after 15 months, d) after 18 months.

c d

a b

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Figure 4.4 SEM microphotographs of copper concentrate particles before the experiment (a) and after 8 months (b) in the acidic forest soil under field conditions.

Figure 4.5 SEM microphotographs of copper slag particles before (a) and after 15 months exposure to the acidic forest soil (b). Arrows in (b) indicate minor dissolution features on the particle surface.

The Cu content of the copper concentrate varied in the samples recovered over the 18 months of the field experiment, but did not show a significant trend (Figure 4.2b). Also the SEM microphotographs do not show clear features of weathering (Figure 4.4). The dissolution rate was less than 1·10-11 mol Cu m-2 s-1. In the case of the copper smelting slag we found a slight decrease in the Cu content over time (Figure 4.2c). The SEM pictures show a few, but quite distinct minor dissolution features (Figure 4.5b). The average dissolution rate was 4.3·10-11 mol Cu·m-2·s-1.

4.3.3 Dissolution under laboratory conditions

Figure 6 shows the loss of metal content of the particles in the laboratory experiment. The lead oxide particles buried in pots with the acidic forest soil showed a dissolution comparable to the PbO particles in the field experiment.

a b

c d


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The average dissolution rate was 1.9·10-10 mol Pb·m-2·s-1. The lead oxide particles incubated in the acidified sand were dissolved at a rate of 7.7·10-11 mol Pb·m-2·s-

1. They also showed less morphological signs of dissolution than the particles incubated in the forest soil (Figure 4.7). The fastest dissolution was observed in the case of the lead oxide samples incubated in the stirred acidic solution at pH 3.5 (9.5·10-9 mol Pb·m-2·s-1). At the termination of the experiment almost all particles had been completely dissolved. All dissolution rates from the field and laboratory experiments are summarized in Table 4.4. The copper concentrate particles showed almost identical dissolution rates in the soil and in the sand. The dissolution rate was 1.1·10-10 mol Cu·m-2·s-1 in the soil and 9.5·10-11 mol Cu·m-2·s-1 in the sand. No distinct dissolution features were found in both cases in SEM pictures (Figure 4.4). The dissolution rates of copper slag particles differed between soil and sand. The dissolution rate of the slag particles was 5.9·10-11 mol Cu·m-2·s-1 in the soil pots and 1.1·10-11 mol Cu·m-2·s-1 in the sand pots.

0 1 2 30

20

40

60

80

100

PbO in sand (pH 3.5) PbO in soil (pH 3.6) PbO in water (pH 3.5)

Am

ount

of P

b [%

]

months

a

0 1 2 30

20

40

60

80

100

Cu concentrate in sand (pH 3.5) Cu concentrate in soil (pH 3.6)

Am

ount

of C

u [%

]

months

b

0 1 2 30

20

40

60

80

100

Cu slag in sand (pH 3.5) Cu slag in soil (pH 3.6)

Amou

nt o

f Cu

[%]

months

c

Figure 4.6 Dissolution of lead oxide (a), copper concentrate (b) and copper slag (c) particles under laboratory conditions in acidic forest soils, sand percolated by HNO3 solution with pH 3.5 and in stirred HNO3 solution with pH 3.4 (only for PbO).

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The field data (Figure 4.2) show much more scatter than the laboratory data (Figure 4.6), except for the copper slag particles. This indicates that conditions were much more heterogeneous in the field than in the lab, although the soil material was the same.

Figure 4.7 SEM microphotographs of lead oxide particle dissolution in a laboratory pot experiment. (a) dissolution in an acidic forest soil at pH 3.5 (b) dissolution in a sand irrigated with HNO3 solution (pH 3.5)

a b

4.4 Discussion

4.4.1 Applicability of the new in situ technique

The new in situ technique proved its general applicability for field and laboratory studies for extended times. The presented results are the first data on the dissolution of fine grained mineral particles under in situ conditions in soil. The new in situ polymer support method is not limited to particles in the cm-range, as other in situ methods (Maurice et al., 2002; Ranger et al., 1991) but can cope with particle sizes down to around 20 µm. The particles are well attached to the support and particle loss during the experiment is negligible. Only a small amount of the particle surface is in contact wit the polymer support which expose the majority of the particle surface for reactions with the surrounding soil. The detection limit of the quantification method is predominantly based on the analysis of the concentration difference of particles on the support before and after exposure to the soil. A major advantage of the method is that the dissolution can be observed by microscopic investigation of the particles as well as by quantification of the dissolution rate.


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4.4.2 In situ dissolution rates

Although dissolution rates had been measured for some of the components of the particles used in our study (e.g. fayalite, pyrite) (Parsons et al., 2001; Siever and Woodford, 1979), dissolution rates of composite mineral compounds consisting of these phases have never been determined before. The copper concentrate used in this study was composed of several different phases, in particular chalcopyrite and pyrite. We did not find data on dissolution rates for the composite particles, although these are the forms in which these minerals are actually released into the environment. However, dissolution rates have been measured for some of the components (pyrite, chalcopyrite) of these particles in acidic solutions (Abraitis et al., 2004). In a laboratory column experiment Nicholson et al. (1990) found significant surface alterations (cracks, surface spalling) on pyrite minerals after a period of several months. Davis et al. (1993) observed that sulfidic ore minerals in general begin to oxidize within less than one year. Williamson and Rimstidt (1994) observed dissolution rates of 8.8·10-9 mol Fe m-2 s-1 for pyrite in a laboratory batch experiment. Given these data, we expected a significant dissolution of the copper concentrate particles under the acidic soil conditions because pyrite was a major constituent of this material. However, we observed neither dissolution features nor a decrease in the Cu content of the material and the material proved to be stable for the investigated time period of 18 months.The copper slag was also a mixture of different phases, mainly fayalite and magnetite. The dissolution rates of copper concentrate and copper slag particles are given here referring to the release of Cu and not Fe. The slag had a molar Fe/Cu ratio of 68. If we assume that the ratio was the same in the component phases and that dissolution of fayalite and magnetite occurred at the same rate, then we can transform the dissolution rate to 2.9·10-9 mol Fe m-2 s-1. Ettler et al. (2002) and Kucha et al. (1996) studied the dissolution of polished slag sections in synthetic

“soil solutions” at pH 2 and 3. They reported preferential dissolution of the fayalite phase among the other phases (melilite, hedenbergite, bornite and wurtzite). Ettler et al. (2002) presented SEM pictures which showed obvious traces of dissolution on the polished slag surfaces. They found a dissolution rate of 5.7·10-16 mol Fe m-2 s-1 for the slag. Parsons et al. (2001) investigated the trace element release from base-metal slag’s with TCLP leaching tests (US EPA, 1992). For glassy slag they

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obtained a dissolution rate of 4.7·10-10 mol m-2 s-1 at pH 3.6. Wogelius and Walther (1992) studied the dissolution of fayalite in a fluidized bed experiment and found a rate of 3.9·10-9 mol Fe m-2 s-1 at pH 3.6. Siever and Woodford (1979) determined a dissolution rate for fayalite of 1.7·10-11 mol Fe m-2 s-1 in a batch experiment at pH 4.5. White et al. (1994) investigated the dissolution of magnetite minerals in stirred batch reactors and found a dissolution rate of 1.6·10-11 mol Fe m-2 s-1 for magnetite at pH 3.6. The observed in situ dissolution of the slag material in the forest soil is therefore in the upper range of rates reported in the literature for similar slags or slag components such as fayalite and magnetite.

4.4.3 Comparison of in situ field with laboratory dissolution rates

The lead oxide dissolution rates followed the order sand (pH 3.5) < soil (laboratory) < soil (field) < solution (pH 3.5).According to the literature dissolution usually occurs faster in the laboratory than in the field (White and Brantley (2003), Swoboda-Colberg and Drever (1993). Indeed, we also observed that dissolution of PbO was 40 times slower in the field than in the stirred solution. In most instances when dissolution rates have previously been compared between field and laboratory experiments, the laboratory rates originated from batch experiments, flow-through reactors, saturated columns or fluidized bed reactors (Strömberg and Banwart, 1999; White and Brantley, 2003; White et al., 1994; Wogelius and Walther, 1992). Under these conditions the particles are permanently flushed. Reaction rates are not limited by lack of moisture and by the build-up of diffusion gradients.The rates measured in the field were generally higher than the rates determined with soil or sand in the laboratory. Dissolution of PbO was 1.3 times faster in the field than in the laboratory and 3 times faster than in the acidified sand which contained no organic substances. The rate for Cu smelting slag was 4 times faster in the field than in the acidified sand, but only 73% of the rate measured in the soil in the laboratory. We used the identical test samples in all experiments and the same procedures in the field and in the laboratory to place them into the soil. The most obvious differences in the experimental conditions were the temperature, the moisture regime and the biological activity. The mean temperature in the field was 9° C (Ø 18 months) while the laboratory temperature was around 20° C. Therefore the higher temperature should have accelerated the dissolution in the laboratory.


65

Because most dissolution reactions are pH-dependent (Furrer and Stumm, 1986; Wieland et al., 1988), variations in soil solution pH between laboratory and field exposure may also lead to different dissolution rates. The pH value of the used soil was measured in a soil slurry in 0.01 M CaCl2. The pH in the quartz sand was established by percolating an acid solution with pH 3.5, resulting in a soil solution pH of about 3.5-3.9 at the depth where the supports were buried. Another important factor is the moisture content of the soils. Due to the large variability in moisture conditions of field soils, Swoboda-Colberg and Drever (1993) consider this factor a major source of uncertainty in field weathering experiments. This uncertainty includes the potential influence of preferential flow events. Hornberger et al. (1990) discussed the possibility that preferential flow in soil could prevent infiltrating water from passing along the mineral surfaces of the matrix and thus slow down particle dissolution. The pots in the laboratory were watered regularly and the soils thus kept moist all the time. In the field, however, dry periods interchanged with wet periods (Figure 4.1). It should also be mentioned here that the summer of 2003, which fell into the exposure time, was extremely hot and dry (Bosshard, 2004). Lower moisture should have reduced dissolution, however. Therefore, the difference in moisture regime does not explain the higher dissolution rates in the field.The larger rates measured in the field compared to the same soil in the laboratory point to a significant contribution of biological processes. The lowest dissolution rates for PbO and Cu slag were found in the sand, which had no biological activity. In the forest the particles were under the influence of vegetation, root processes, root-associated fungi, bacteria and leaf litter leaching. Prior to the laboratory experiments the soil had first been dried and sieved through a 2 mm mesh. Thus not only stones were removed, but also most roots, litter and macro fauna, before it was filled into the pots. Figure 4.8 shows fungal hyphae colonizing oxide particles recovered from the field. No such hyphae were found in the laboratory samples. The importance of fungi for mineral weathering has been reported frequently (Banfield et al., 1999; Burford et al., 2003; Hinsinger et al., 1993; Pohlman and McColl, 1988), but the SEM pictures which could be obtained with our in situ technique gave direct morphological evidence of the close interaction between fungi and the particles.The dissolution in the soil in the laboratory was much faster than the dissolution

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at the same solution pH in quartz sand. Francis and Dodge (1986) showed that microbial activity and organic acids increased the dissolution of PbO particles in comparison to pure inorganic acidified (HNO3) solutions. The capacity of the dissolved organic carbon to solubilize metals is well known (Hering, 1995). The faster rate in the soil compared to quartz sand can therefore be attributed to the presence of DOC in soil solution that accelerated the dissolution.

a b

Figure 4.8 Fungal hyphae on lead oxide particles recovered from acidic forest soil

4.5 Conclusions

Three types of particles were used to study mineral dissolution in an acidic forest soil under field and laboratory conditions over a period of 18 months. Lead oxide particles dissolved much more readily than copper concentrate particles and copper smelter slag particles. The new in situ method allowed to determine dissolution rates and micro-morphological weathering features on the particles. The laboratory experiments did not confirm the expectation of higher dissolution rates than in the field. We hypopthesize that the main factor to explain the higher dissolution rates in the field was a more intensive biological activity under field conditions due to the presence of active plant roots.The new method proved its potential for long term experimental applications. The samples showed no mechanical damage after recovery and cleaning and also no other signs of artifacts. In particular, the mineral particles are well fixed to the supports and do not fall off during sample handling and analysis. To our


67

knowledge it was the first time that dissolution of fine particles was analyzed quantitatively under in situ conditions in the field.

Acknowledgments – We thank Werner Attinger for his help in the field, Peter Wägli for making the SEM analysis at the ETH Institute of Solid State Physics possible. The authors thank Rosanna Ginocchio from the Chilean Center of Mining and Metallurgy (CIMM) for donating us the copper concentrate and copper slag particles and Andreas Wahl from Penarroya Oxide GmbH for providing us with the lead oxide particles.

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Nicholson R. V., Gillham R. W., and Reardon E. J. (1990) Pyrite oxidation in carbonate-buffered solution: 2. rate control by oxide coatings. Geochim. Cosmochim. Acta 54, 395-402.Parsons M. B., Bird D. K., Einaudi M. T., and Alpers C. N. (2001) Geochemical and mineralogical controls on trace element release from the Penn Mine base-metal slag dump, California. Appl. Geochem. 16, 1567-1593.Penman A. D. M. (2001) Tailing dams - risk of dangerous occurrences. In International Comission of Large Dams - Bulletins (ed. ICOLD- UNEP), pp. 98. United Nations Environmental Programme.Pohlman A. A. and McColl J. G. (1986) Kinetics of metal dissolution from forest soils by soluble organic acids. J. Environ. Qual. 15(1), 86-92.Pohlman A. A. and McColl J. G. (1988) Soluble organics from forest litter and their role in metal dissolution. Soil Sci. Soc. Am. J. 52, 265-271.Ranger J., Dambrine E., Robert M., Righi D., and Felix C. (1991) Study of current soil-forming processes using bags of vermiculite and resins placed within soil horizons. Geoderma 48, 335-350.Riba I., Conradi M., Forja J. M., and DelValls T. A. (2004) Sediment quality in the Guadalqivir estuary: lethal effects associated with the Aznalcóllar mining spill. Mar. Pollut. Bull. 48, 144-152.Rogers J. R., Bennett P. C., and Choi W. J. (1998) Feldspars as a source of nutrients for microorganisms. Am. Mineral. 83, 1532-1540.Schnoor J. L. (1990) Kinetics of chemical weathering: a comparison of laboratory and field weathering rates. In Aquatic chemical kinetics (ed. W. Stumm), pp. 475-504. Wiley.Siever R. and Woodford N. (1979) Dissolution kinetics and the weathering of mafic minerals. Geochim. Cosmochim. Acta 43, 717-724.Strömberg B. and Banwart S. (1999) Experimental study of acidity-consuming processes in mining waste rock: some influences of mineralogy and particle size. Appl. Geochem. 14, 1-16.Swoboda-Colberg N. G. and Drever J. I. (1993) Mineral dissolution rates in plot- scale field and laboratory experiments. Chem. Geol. 105, 51-69.US EPA. (1992) Toxicity Characteristic Leaching Procedure (TCLP). In SWA 486 - Method 1311, pp. 35. United States Environmental Protection Agency.


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USGS. (2002) Physical Aspects of Waste Storage from a Hypothetical Open Pit Porphyry Copper Operation, pp. 67. United States Geological Survey.Velbel M. A. (1990) Influence of temperature and mineral surface characteristics on feldspars weathering. Water Resour. Res. 26(12), 3049-3053.Vidal M., López-Sánchez J. F., Jiménez G., Dagnac T., Rubio R., and Rauret G. (1999) Prediction of the impact of the Aznalcóllar toxic spill on the trace element contamination of agricultural soils. Sci. Total Environ. 242, 131-148.Warczok A., Riveros G., Echeverria P., Díaz C. M., Schwarze H., and Sánchez G. (2002) Factors governing slag cleaning in an electric furnace. Can. Metall. Quart. 41(4), 465-474.White A. F. and Brantley S. L. (2003) The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field? Chem. Geol. 202, 479-506.White A. F., Peterson M. L., and Hochella M. F. j. (1994) Electrochemistry and dissolution kinetics of magnetite and ilmenite. Geochim. Cosmochim. Acta 58(8), 1859-1875.Wieland E., Wehrli B., and Stumm W. (1988) The coordination chemistry of weathering: III. A generalization on the dissolution rates of minerals. Geochim. Cosmochim. Acta 52, 1969-1981.Williamson M. A. and Rimstidt J. D. (1994) The kinetics and electrochemical rate-determining step of aqueous pyrite oxidation. Geochim. Cosmochim. Acta 58(24), 5443-5454.Wogelius R. A. and Walther V. (1992) Olivine dissolution kinetics at near-surface conditions. Chem. Geol. 97, 101-112.Zhang H. and Bloom P. R. (1999) Dissolution kinetics of hornblende in organic acid solutions. Soil Sci. Soc. Am. J. 63, 815-822.

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5 In situ transformations of fine lead oxide particles in different soils

Andreas Birkefeld, Rainer Schulin and Bernd Nowacksubmitted to Soil Science Society of America Journal

Abstract

We present the first field application of a new in situ technique to analyze phase transformations of small mineral particles. We applied this technique to investigate the transformation behavior of fine lead oxide particles (50-100 µm Ø) in different soils directly in the field. Mineral samples were left in the soils over periods of up to 18 months. After the first months of exposure to a calcareous sand we found newly precipitated secondary mineral phases on the lead oxide. The samples exposed to two loamy soils (dystric Cambisol and Luvisol) showed only very few traces of new phases. Scanning electron microscopy with attached energy dispersive x-ray probe revealed carbon, oxygen and lead as the main elements in the precipitates and some traces of phosphorus in the carbonate-free soil. Using micro-Raman spectroscopy and x-ray diffraction analysis we identified the new phases as mainly lead-hydroxy carbonates (hydrocerussite). Geochemical speciation calculations using Visual MINTEQ predicted the precipitation of cerussite in the calcareous sand and calcareous loam and of lead phosphates in the non-calcareous loam soil. The results confirmed reported observations that in situ mineral reactions in a real system can differ from model predictions, both in terms of reaction kinetics and formation of meta-stable phases. The use of experimental in situ methods is thus giving new information on contaminant mineral behavior under field conditions.

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5.1 Introduction

Transformations of mineral phases are important natural geological and pedological processes in rock weathering and soil formation. They can also be rate-limiting in the release of toxic metals from anthropogenic particles emissions distributed into the environment. Particulate contaminants (e.g. sulphide minerals) released from mining and tailing heaps are prone to phase transformations because of the different environmental conditions between the place they were formed (anaerobic) and the earth surface (aerobic) where they were moved to during mining (Callander, 2004) resulting in acid mine drainage (AMD) generation. Metal smelters can release significant amounts of metal oxide particles (Ripley et al., 1996) into the atmosphere resulting in widespread contamination (Sobanska et al., 1999). Soils and sediments generally are the ultimate sinks of such emissions. The pathways and rates of mineral transformation processes in the environment are subject to large uncertainties (Casey et al., 1993; Swoboda-Colberg and Drever, 1993), although a rich data base on thermodynamics of mineral phases and their transformation reactions is available. The reason is that due to the complexity of soil and sediment environments it is difficult to simulate field conditions in the laboratory and even more difficult to adequately capture all relevant processes and factors in model calculations (Strömberg and Banwart, 1994). Therefore it is crucial for the study of the kinetics of mineral transformations in the environment to have methods by which these processes can be studied under conditions as they occur in real soils and sediments (Schnoor, 1990; White and Brantley, 2003). Several types of in situ methods have been used for this purpose. The reactions of soil forming minerals were studied in situ by means of incorporating mineral particles into soils and recover them after selected reaction times (Augusto et al., 2001; Rogers et al., 1998). Ranger et al. (1991) examined the weathering of vermiculite phyllosilicates, incorporating small bags of polyamide with 20 µm mesh size filled with these minerals for various time periods into selected soil sites. This bag method was also applied to study the mineral behavior in mining affected soils over time (Monterroso and Maías, 1998) and to study the physico-chemical alteration of minerals in soils associated with changes in the vegetation (Hatton et al., 1987). Another in situ approach was to incorporate samples of minerals directly into

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soil and to recover them for analysis by means of a density fractionation (Cotter-Howells, 1993; Tsaplina, 1996). This technique has the disadvantage to disturb the soil structure as the test particles are mixed into the soil. Moreover the direct contact of the reacted test material with chemicals in the sample preparation step (Cotter-Howells, 1993) could alter, damage or remove potential new formed mineral phases. A new method to study the in situ reactions of particulate materials was developed and described recently (Birkefeld et al., 2005a; Birkefeld et al., 2005b). This method uses small polymer supports as carrier material onto which the minerals of interest are fixed with a thin layer of epoxy resin. The coated polymer plates are then inserted into soils, sediments or other environmental systems. Not only is it easy to insert these samples into soil and to achieve good soil contact with minimum disturbance, but it is also easy to recover the samples. In addition the method provides the possibility to identify individual mineral sample grains as well as analyze microscopic changes on the minerals. The method is suitable for particle sizes of down to 20 µm diameter. Previously the method has been used to investigate the dissolution of heavy metal containing particles in an acidic forest soil (Birkefeld et al., 2005c). It was possible to analyze the mineral behavior over time in this specific soil environment revealing obvious dissolution features on some of the mineral grains. The dissolution rates measured in situ in the forest soil with this method were compared to rates measured in the laboratory in different media using the same technique. Dissolution rates were found to be faster in the field than in the laboratory under similar conditions of exposure.Fore the study on mineral transformation lead oxide particles were selected because lead compounds are well characterized and several studies investigated the environmental behavior of lead phases in soils (Cao et al., 2003a; Corsi and Landim, 2002; Hardison et al., 2004; Jorgensen and Willems, 1987; Martinez and Motto, 2000). Davis et al. (1993) described the occurrence of lead oxide particles in mine wastes and mine waste contaminated soils and discussed the importance of the mineralogical form (species) of lead controlling the potential bioavailability of this heavy metal. Lead phases occur in different forms (e.g. PbO, PbSO4, PbS, PbCO3, Pb(OH)2, Pb5(PO4)3OH) in soil environments (Davis et al., 1993). Lead oxides and lead sulfides are typical primary species found in ore minerals (Davis

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et al., 1993), whereas metallic lead can be found in soils affected by mining and smelting activities (Lottermoser, 2003) or in shooting ranges (Lin et al., 1995). Under aerobic conditions these phases are subsequently transformed into secondary lead phases like lead carbonates, lead phosphates, lead sulfates or lead oxides (Ruby et al., 1994) depending on factors such as pH, anions, DOC, or water content (Bataillard et al., 2003; Essington et al., 2004; Lin et al., 1995). (2003) The objective of this study was to analyze phase transformation reactions of fine mineral particles under field conditions with the new in situ technique. Under the given physico-chemical soil properties we expected to see the formation of lead carbonates or lead phosphates directly on the mineral grains. We used small lead oxide particles incorporated in different circumneutral soils for a time span of up to 18 months applying the new in situ method of Birkefeld et al. (2005a; 2005b) which does not need special sample treatment prior to the analysis by micro analytical methods.


5.2.1 Sample preparation

Particle supports were made from polymethylmetahcrylate (PMMA) polymer (Plexiglas®/Acrylite®). Pre-cut plates of (2 cm x 2 cm) were weighed at 0.0001 g accuracy by means of an analytical balance (Mettler Toledo AT 261, Switzerland). The weight of each plate was carved into its back side. Then the plates were covered with a thin adhesive film of a two-component epoxy resin (Bisphenol A resin - Suter, Switzerland) using a micro-foam paint roll. For this purpose, a set of ten weighed and marked supports was placed in two rows in a frame holder. Lead oxide particles were obtained from a commercial manufacturer (Pennaroya Oxide, Germany). The lead oxide particles consisted of the two PbO polymorphs massicot and litharge. Particle sizes ranged from 20 to 100 µm in diameter. There were only minor elemental impurities of the lead oxide particles (Na 3.5 %, Mg 0.65 %, Al 0.14 %, K 0.05 %, Ca 0.03 %, Fe 0.02 %).The particles were applied onto the support surfaces by using a downsized copy of a commercial available dust spray gun. The spray gun was held perpendicular in a distance of 1.5 m to the supports. Gentle moving of the dust spray gun during


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application (30 s) provided a relatively homogenous material cover on the supports. For a more detailed description of sample preparation and field application of the method, the reader is referred to Birkefeld et al. (2005b)

5.2.1.1 Pre experimental analysis

After hardening of the epoxy resin (12 h) each polymer plate was analyzed by energy dispersive X-ray fluorescence analysis (XRF) (Spectro X-Lab 2000, Germany) to establish the elemental composition of the mineral particles fixed to the supports. Stereo microscopy was used to assess the coating quality. A custom calibration was used for the used mineral type to convert the XRF counts into concentrations. For the calibration a sample set with different particle concentrations was first analyzed by XRF and afterwards completely dissolved and analyzed by AAS (Varian SpectrAA 220, Australia). Figure 5.1 shows lead oxide particles glued onto the sample supports, ready for incubation.

Figure 5.1 SEM microphotograph of lead oxide particles attached to a polymer support. The minerals are arranged as a single layer on the support surface, ensuring optimal contact of each grain to the soil during incubation.

5.2.2 Soils and sample incubation

Three field sites in the vicinity of Zurich, Switzerland, were chosen for this study. They are characterized by different soils, a luvisol topsoil (non-calcareous loam; pH 6.4), a dystric Cambisol topsoil (loam; pH 7.5) and a pleistocene calcareous sand (pH 8.0). The carbonate content in the soils ranged between 0.3 % and 35 %

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(Table 5.1). The mean annual temperature of the field sites was 10.2°C and the mean annual precipitation was 1100 mm (Bosshard, 2004). The carbonate content of the soils was determined by gas-volumetric analysis (Page et al., 1982), the soil organic matter content by the Walkley-Black method (Page et al., 1982) and the soil solution phosphate concentration by ion chromatography from samples taken with suction cups at the field sites. The pH was measured in 0.01 M CaCl2 at a soil:solid ratio of 1:2.The polymer supports were inserted into the soils by means of a small lancet made of stainless steel (3 cm width; 0.5 cm thick; 40 cm length). The lancet was gently pushed into the top-soil down to a depth of 15 cm and bent forward to open a small slit. A coated sample was then introduced into this slit and the coated face pressed against the soil. A small nylon thread attached to the non-coated side of the support was left protruding from the buried sample to the soil surface in order to facilitate locating the sample for later recovery. For recovery the samples were dug out, put in small labeled polyethylene bags to preserve the soil moisture and transferred to the laboratory. Exposure times were varied from two up to eighteen months. In total, 18 samples were incubated.

Table 5.1 Properties of the used soils

Soil Dystric Cambisol Luvisol Pleistocene Sand

Soil lable loam non- calcareous loam calcareous sand

pH (in 0.01 M CaCl2) 7.5 6.4 8.0

Carbonate content (%) 7.2 >0.3 35.1

Organic matter content (%) 3.1 1.5 0.1

PO43- in soil solution (µM) 0.7 40 -

Grain size distribution (Clay - Silt - Sand) (%)

25 - 40 - 35 15 – 49 - 36 5 - 7 - 88

5.2.3 Sample analysis

In the laboratory the recovered samples were immersed in demineralized water for about five minutes in order to soften and detach adhering soil. Particles still remaining after this procedure were removed carefully using a soft brush. The


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cleaned samples were left to dry over night under ambient room conditions. After drying, all samples were checked for damages under a stereomicroscope (Zeiss Stemi 2000, Germany). Metal concentrations of the recovered samples were analyzed by XRF spectroscopy. Scanning electron microscopy (SEM) was used to identify changes in the surfaces of the mineral particles on the recovered samples using a CamScan CS44 instrument (Obducat CamScan, Sweden) with an attached energy dispersive x-ray (EDX) probe (EDAX Econ, USA) for elemental spot and small area analysis. One sample from the non-calcareous loam was analyzed using a Quanta 600 (FEI, USA) environmental scanning electron microscope (ESEM) with attached EDX probe (EDAX Econ, USA).Micro-Raman spectroscopy was carried out on a Jobin Yvon Horiba LabRam HR (Jobin Yvon, France) spectrometer with attached reflected light microscope. Spectra of expected lead phases (lead carbonate, -hydroxycarbonate, -nitrate, -sulfate, -phosphate) were recorded and added to the existing Raman spectra database. The reference compounds were obtained from commercial chemical suppliers (Merck, Switzerland; Fluka, Switzerland). Samples which showed newly formed mineral phases under the SEM were also analyzed by x-ray powder diffraction (XRD) analysis. Diffractograms were generated on an XRD system (D4 Endeavor Bruker, Germany) in spinning sample mode to increase the orientation randomness (Pecharsky and Zavalij, 2003). The PDF-2 database of the International Centre for Diffraction Data (ICDD) was used for the identification of spectra. All compounds, which were added to the Raman database were also analyzed by XRD and integrated into the search and match routine of the XRD evaluation software (Bruker Diffracplus, Germany).

5.2.4 Speciation calculations

The precipitation of solid phases under the conditions of the three field sites was modelled by speciation calculations with VisualMINTEQ (Gustaffsson, 2005). Precipitation of lead phases was calculated for the given pH and CO2 partial pressure of 3.510-3 atm, representative for conditions in topsoils (Scheffer and Schachtschabel, 1998). The measured phosphate concentration in soil solution determined the activity of phosphate in the system. Lead was initially present in the system as a finite phase of PbO (Litharge) at a concentration of 0.19 M (40 mg

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of PbO on a support ox 2×2 cm, distributed 1 cm into the soil with a water content of 25%) in an ionic strength of 0.005 at 12° C. In the calcareous soils Calcite was set as an infinte phase.

5.3 Results

5.3.1 Field results

As Figure 5.2 shows, the total mass of lead of the mineral samples did not change to any significant degree over the 18 months of incubation in non of the three soils. Each data-point represents an individual sample recorded after the respective incubation time. This means that transformation into new solid phases may have

Figure 5.2 Mass of lead oxide particles on the supports after incubation in calcareous sand (∆), loam (□) and non-calcareous loam (○) in percent of the initial amount.

occurred, but no detectable loss due to dissolution and leaching.The SEM microphotographs in Figure 5.3 to 5.6 indeed show obvious signs that new phases had formed during incubation in all three soils. Significant amounts of precipitates were detected already after 2 months on the supports and as well on the PbO mineral surfaces of the samples incubated in the calcareous sand. The lead oxide grains showed little change though, indicating that only a very small fraction had been transformed. The new mineral precipitates predominantly


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show a tabular hexagonal form and are mostly grouped in patches on the mineral surfaces, and on the calcareous sand samples, in the spaces between the grains. The spatial distribution of new mineral phases indicates that dissolution and phase transformation had occurred all over the sample surfaces.

Figure 5.3 SEM microphotograph of lead oxide particles after incubation in a calcareous sand for 2 months. Newly precipitated phases can be seen on and between the grains.

Figure 5.4 SEM microphotograph of an individual lead oxide particle after incubation in a calcareous sand for 10 months. Newly formed mineral phases can clearly be seen on the grain surface and on the support surface

Figure 5.5 SEM microphotograph of an lead oxide particle after incubation in the loam soil for 10 months. Small patches of new mineral phases can be seen in the enlarged cavity that resulted from weathering during that period.

Figure 5.6 SEM microphotograph of an PbO particle after 10 months reaction time in the non-calcareous loam soil. A slight precipitation of a secondary phase can be seen on the mineral surface. The visible cavity in the PbO grain on the left hand side picture resulted from the lead oxide manufacturing process.

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The samples incubated in the two loam soils showed much less formation of new phases on and between the sample particles than in the calcareous sand. Changes in surface morphology are limited to a few spots on particles incubated in the calcareous loam. These consisted in small cavities of the lead oxide grains in which precipitates of secondary phases were visible (Figure 5.5). These visible cavities are most probably generated by etching parts of the thin surface layer of the initial lead oxide particle which has a “cellular” structure caused by its manufacturing process. The remaining particle surface is partially covered by fine grained mineral phases. The samples incubated in the non calcareous loam showed the least signs of transformation (Figure 5.6).

5.3.2 Identification of phases

Energy dispersive point analysis in combination with SEM revealed that the new phases consisted of oxygen, carbon and lead. Figure 5.7 shows the example of an EDX scan taken from a sample in which a new phase had formed in an intergranular area on the support surface so that no background signal from the original PbO could interfere. The silicon and potassium peaks most likely originated from minor residues of the soil matrix.

Figure 5.7 SEM-EDX point-scan (white circle in the inset picture) of a precipitate next to an PbO particle after recovery from incubation in the calcareous sand.

Micro-Raman analysis clearly showed the difference between the initial mineral phase and the new phase. Figure 5.8 shows the disappearance of the initial lead


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oxide peak and the formation of a new peak in the 1050 cm-1 area in the Raman spectra taken from samples which had been incubated in the calcareous sand. The initial lead oxide phase shows only peaks in the 300 to 400 cm-1 region, whereas the new phases show peaks all over the scan, but with dominant peaks in the range from 1050 to 1100 cm-1, i.e. close to the sample peak. The peaks in the latter region are typical for lead carbonates (Frost et al., 2003). The inset in Figure 8 shows a magnification of the spectrum around 1050 cm-1 of the samples compared to the spectra of cerussite and hydrocerussite. The comparison suggests that the new mineral phase was predominantly hydrocerussite (Pb3(CO3)2(OH)2). Moreover, the peak in the 680 cm-1 area indicates the presence also of a minor cerussite (PbCO3) phase. Black et al. (1995) reported similar Raman spectra for cerussite and hydrocerussite prepared from pure chemicals in the laboratory. The formation of these lead carbonates must have occurred quite rapidly, because they are already formed in the first samples after 2 months of incubation. The Raman signals of the samples recovered from the two loam soils were not strong enough to analyze secondary phases.

Figure 5.8 Raman spectra of a lead oxide sample recovered from incubation in the calcareous sand. The graph shows the sample spectrum, the initial PbO mineral spectrum and two spectra of possible new lead phases. The inset graph is a magnification of the scan in the region from 1000 to 1100 cm-1.

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Comparing several XRD scans on the hydrocerussite main peak (34° 2 θ) of individual samples recovered from the test soils and normalizing them by the initial amount of mineral mass results in a plot showing the development of hydrocerussite phases over the different incubation times (Figure 5.9). Moreover, Figure 5.9 shows a rapid formation of hydrocerussite phases whose amount remains without significant changes over the whole experimental period of 18 months. Bulk XRD analysis on samples recovered form the calcareous soil revealed the

Figure 5.9 Development of the hydrocerussite main peak over time on lead oxide samples (XRD analysis). Circles indicating samples recovered from the calcareous soil and triangles representing samples from a carbonate free acidic control soil.

occurrence of hydrocerussite, cerussite, litharge and massicot. Litharge and massicot are the two polymorphs representing the primary mineral phases introduced into the soil with the samples, hydrocerussite and cerussite the secondary, new mineral phases that formed on the lead oxide particles during the incubation. Using XRD, the two lead carbonate phases were identified also on the samples recovered from the two loam soils, although they were present in traces only. An additional EDX scan on a sample recovered from the non-calcareous loam revealed the presence of carbon, oxygen and traces of phosphorus (Figure 5.10). The XRD data agree well with the results from the EDX and Raman analysis. The SEM analysis on the samples from the non-calcareous loam reveals almost no visible surface changes of the initial lead oxide surface. In Figure 5.6 only a very


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small area on the initial particle shows patches of secondary precipitates.

Figure 5.10 SEM-EDX analysis on the mineral surface of a PbO particle after recovery from the loam soil.

5.3.4 Calculations

We calculated the speciation of lead for the three test soils. PbO was completely dissolved in all soils. Hydroxylpyromorphite was predicted as the least soluble phase in the soils with appreciable dissolved phosphate. It had a saturation index (SI) of 22.1 in the clacareous loam and 27.9 in the non-calcareous loam. Hydrocerussite had a saturation index of 14.8 in the clacareous loam and 14.2 in the non-calcareous loam and 15.6 in the calcareous sand. Cerussite had saturation indices of around 5 in all three soils. All three lead phases are predicted to precipitate under the given conditions (Table 5.2). The variation of the CO2 partial pressure (from 0.035 to 0.00035 atm) did not show any significant changes in the precipitation of hydroxycerussite versus hydroxlpyromorphite. When the phosphate concentration was assumed to be controlled by apatite the predicted saturation index was 26.5, only slightly different to the original calculation.

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5.4 Discussion

The results show that our method allows to expose fine particles for up to 18 months to in situ weathering conditions in field soils and to recover them afterwards for analysis. A loss of particles due to detachment from the supports was negligible and the method gave quantitative information on the dissolution of the particles. Furthermore, we were able to obtain microanalytical information on the mineral phases that formed during incubation in soil. Because disturbance of the soils where the samples were incubated was minimal and because of the direct and close contact of the samples with the surrounding soil, we can assume the conditions of weathering were representative for undisturbed soils and do not represent an artificial situation.The great advantage of the method is that the reactions of fine mineral particles (20-100 µm Ø) can be directly studied. So far analysis of in situ reacted particles has been restricted to larger particles that can be separated form the soil matrix e.g. (White et al., 1996).The lead content of the samples remained relatively stable over the whole experimental time. PbO is very soluble and complete dissolution or phase transformation into more stable phases would be expected based on speciation calculations. Dissolution of PbO in an acidic forest soil (pH 3.6) has indeed been observed (Birkefeld et al., 2005c). The formation of a weathering crust of less soluble minerals is the most likely cause of the inhibited or slowed down dissolution in the studied soils.Cao et al. (2003b) reported a similar stability behavior of lead bullet weathering crusts in neutral and alkaline soils. The crusts predominantly consisted of lead carbonates. According to Cao et al.(2003b) they are stable in the range of pH 6

Table 5.2 Saturation indices of the precipitated phases in the three soils calculated with the geochemical model VisualMINTEQ

Hydrocerussite Cerussite Hydroxypyromorphite

Dystric Cambisol (loam) 14.8 5.0 22.1

Luvisol (non-calc. loam) 14.2 4.8 27.9

Pleistocene Sand (calc. sand) 15.6 5.3 -


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to pH 10. Souvent and Pirc (2001) investigated corrosion products of soils contaminated with metallic lead and found protective layers of lead carbonate covering the initial lead particles. Also Gamsjäger et al. (1984) observed cerussite precipitation on calcite mineral grain surfaces under similar conditions and concluded that the lead carbonate phase formed a protective layer over the more soluble lead oxide phase, controlling the kinetics of further dissolution. The SEM microphotographs of the recorded samples showed clear signs of newly formed precipitates already at the earliest sampling after 2 months incubation. The results of the various methods applied to analyze the precipitates all agree in the conclusion that the latter primarily consisted of hydrocerussite and traces of cerussite. These findings also are in good agreement with results of previous investigations of lead phases and lead oxide weathering in neutral and carbonatic soils (Graedel, 1994; Jorgensen and Willems, 1987; Martinetto et al., 2002; Ryan et al., 2001) and with the results of our geochemical model. The relatively fast kinetics of carbonate formation is also in agreement with the results of Hardison et al. (2004) who reported lead carbonate formation on lead or lead oxide in an alkaline soil within weeks. The samples exposed to the non-calcareous loam showed only minor traces of lead carbonate phases on an otherwise almost unaltered surface. Taking in account the geochemical characteristics of the soil matrix (0.3% carbonate; 40 µM phosphate in solution) and the results of the chemical speciation model, we would have expected secondary precipitates of lead phosphate on the particles. Conversely the SEM micro photographs are showing only very small lead carbonate phases and no visible lead phosphate precipitates, whereas it was the only sample where traces of phosphate have been found by EDX analysis on the particle surface. Essington et al. (2004) concluded according to their speciation calculations that in calcareous soils usually lead carbonate phases will control lead solubility in the long run, even in the presence of significant phosphate concentrations.We have assumed in our calculation that the phosphate concentration is sustained in solution and is controlled by the solid phase. However, due to the high amount of PbO on the support surface, phosphate has to be brought to the PbO by diffusion or water flow, a process that might be rate-limiting under topsoil conditions. Carbonate concentrations in solution are much higher and also the carbonate content of the matrix is much higher and therefore its concentration will be much

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higher close to the surface of the grains compared to phosphate. The lack of visible lead phosphate formation may have been due to interference of organic substances. Lang and Kaupenjohann (2003) found that lead phosphate precipitates did not increase in size beyond finley dispersed colloids in the presence of dissolved soil organic carbon. Also Buatier et al. (2001) found only poorly crystallized lead phosphate phases in soils under similar conditions. If lead phosphates precipitates did form they were to small to be visible in the SEM microphotographs. So it is possible that lead phosphates were formed in the non-calcareous loam soil, though to a much smaller extent than lead carbonates. This would follow the predictions of the speciation calculations and the observations with the EDX/ESEM on that sample. However, the fine layer of lead phosphate efficiently protected the soluble PbO from further dissolution.The results of our study could show that under in situ field reaction conditions the occurrence of lead carbonates seems to play an important role, even if geochemical models would predict their absence or favor other lead phases. As mentioned before, the lead carbonate phase is playing an important role in the solubility kinetics of the lead contaminant in neutral and alkaline soils. By using the new in situ reaction method under field conditions it could be shown that the mineral phase boundaries, as predicted by speciation calculations, are somehow partially blurred allowing the coexistence of mineral phases. The results using the new in situ method show that field experiments are needed to establish the phases formed under natural conditions. Model calculation can give an indication about the most stable phase but in situ studies are necessary to validate the calculations. Such experiments may help to improve our understanding of phase transformation reaction in soils leading to meta-stable phases or increase our knowledge about the kinetics of the transformation reactions.

5.5 Conclusions

The study shows the potential of the new incubation method to analyze the phase transformation of fine mineral particles under in situ conditions in the field, if combined with mineralogical and geochemical methods for the analysis of the recovered samples in the laboratory. Lead oxide particles incubated in a calcareous sand were covered by a crust of lead carbonate within two months. The crust protected the lead oxide particles efficiently against further weathering. Formation


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of lead carbonate phases was much slower in the two loam soils. Contrary to the results of speciation calculations no significant formation of lead phosphate was observed. Possibly this was due to interference with dissolved organic carbon and lack of chloride in the soil solution.

AcknowledgmentsWe thank Peter Wägli for making the SEM analysis at the ETH Institute of Solid State Physics possible, Eric Reusser for giving us access to the Raman spectrometer at the ETH Institute of Mineralogy, Pavel Trtik and Gabi Peschke to give us preferential access to the newly purchased ESEM of the ETH Institute of Building Materials, and Andreas Wahl for providing us with mineral particles from Penarroya Oxide GmbH.

Bibliography

Augusto, L., J. Ranger, M.P. Turpault, and P. Bonnaud. 2001. Experimental in situ transformation of vermiculites to study the weathering impact of tree species on the soil. Eur. J. Soil Sci. 52:81-92.Bataillard, P., P. Cambier, and C. Picot. 2003. Short-term transformations of lead and cadmium compounds in soil after contamination. Eur. J. Soil Sci. 54:365-376.Birkefeld, A., R. Schulin, and B. Nowack. 2005a. In situ method for analyzing the long-term behavior of particluate metal phases in soils, p. 3-11, In E. Lichtfouse, et al., Eds. Environmental Chemistry. Springer, Berlin.Birkefeld, A., R. Schulin, and B. Nowack. 2005b. In situ dissolution of heavy metal mineral particles in an acidic forest soil. Geochim. Cosmochim. Acta submitted.Birkefeld, A., R. Schulin, and B. Nowack. 2005c. A new in situ method to analyze mineral particle reactions in soils. Environ. Sci. Technol. 39:3302-3307.Black, L., G.C. Allen, and P.C. Frost. 1995. Quantification of Raman spectra for the primary atmospheric corrosion products of lead. Applied Spectroscopy 49:1299-1304.

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Bosshard, F. 2004. The weather in 2003. Statistical information of the canton Zuerich 2003:1-12.Buatier, M.D., S. Sobanska, and F. Elsass. 2001. TEM-EDX investigation on Zn- and Pb-contaminated soils. Appl. Geochem. 16:1165-1177.Callander, E. 2004. Heavy Metals in the Environment - Historical Trends, p. 67- 105, In H. E. Holland and K. K. Turekian, Eds. Treatise on Geochemistry, Vol. 9. Elsevier, New York.Cao, R.X., L.Q. Ma, M. Chen, S.P. Singh, and W.G. Harris. 2003a. Phosphate- induced metal immobilization in a contaminated site. Environ. Pollut. 122:19-28.Cao, X., L.Q. Ma, M. Chen, D.W. Hardison, and W.G. Harris. 2003b. Weathering of Lead bullets and their Environmental Effects at Outdoor Shooting Ranges. J. Environ. Qual. 32:526-534.Casey, W.H., J.F. Banfield, H.R. Westrich, and L. McLaughlin. 1993. What do dissolution experiments tell us about natural weathering? Chem. Geol. 105:1-15.Corsi, A.C., and P.M.B. Landim. 2002. Fluvial transport of lead, zinc and copper contents in polluted mining regions. Eng. Geol. 41:833-841.Cotter-Howells, J. 1993. Separation of high density minerals from soil. Sci. Total Environ. 132:93-98.Davis, A., J.W. Drexler, M.W. Ruby, and A. Nicholson. 1993. Micromineralogy of mine wastes in relation to lead bioavailability, Butte/Montana. Environ. Sci. Technol. 27:1415-1425.Essington, M.E., J.E. Foss, and Y. Roh. 2004. The soil mineralogy of lead at Horace´s Villa. Soil Sci. Soc. Am. J. 68:979-993.Frost, L.F., J.T. Kloprogge, and P.A. Williams. 2003. Raman spectroscopy of lead sulphate - carbonate minerals - implications for hydrogen bonding. Neues Jb. Miner. 12:529-542.Gamsjäger, H., A. Fluch, and J.H. Swinehart. 1984. The effect of potential aqueous pollutants on the solubility of Pb2+ in Cerussite -Calcite phases. Monatsh. Chem. 115:251-259.Graedel, T.E. 1994. Chemical mechanisms for the atmospheric corrosion of lead. J. Electrochem. Soc. 141:922-927.Gustaffsson, J.P. 2005. Visual MINTEQ, Stockholm. http://www.lwr.kth.se/


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English/OurSoftware/vminteqHardison, D.W., L.Q. Ma, T. Luongo, and W.G. Harris. 2004. Lead contamination in shooting range soils from abrasion of lead bullets and subsequent weathering. Sci. Total Environ. 328:175-183.Hatton, A., J. Ranger, M. Robert, C. Nys, and P. Bonnaud. 1987. Weathering of a mica introduced into four acidic forest soils. J. Soil. Sci. 38:179-190.Jorgensen, S.S., and M. Willems. 1987. The fate of lead in soils: The transformation of lead pellets in shooting-range soils. Ambio 16:11-15.Lang, F., and M. Kaupenjohann. 2003. Effect of dissolved organic matter on the precipitation and mobility of the lead compound chloropyromorphite in solution. Eur. J. Soil Sci. 54:139-147.Lin, Z., B. Comet, U. Qvarfort, and R. Herbert. 1995. The chemical and mineralogical behaviour of Pb in shooting range soils from central Sweden. Environ. Pollut. 89:303-309.Lottermoser, B. 2003. Mine Wastes. Springer, Heidelberg.Martinetto, P., M. Anne, E. Dooryhée, P. Walter, and G. Tsoucaris. 2002. Synthetic hydrocerussite, 2PbCO3·Pb(OH)2, by X-ray powder diffraction. Acta Crystallogr. C58:i82-i84.Martinez, C.E., and H.L. Motto. 2000. Solubility of lead, zinc and copper added to mineral soils. Environ. Pollut. 107:153-158.Monterroso, C., and F. Maías. 1998. Evaluation of the test-mineral method for studying minesoil geochemistry. Soil Sci. Soc. Am. J. 62:1741-1748.Page, A.L., R.H. Miller, and D.R. Keeney. 1982. Chemical and Microbiological Properties. 5 ed. American Society of Agronomy, Madison.Pecharsky, V.K., and P.Y. Zavalij. 2003. Experimental techniques (in XRD), p. 77, In V. K. Pecharsky and P. Y. Zavalij, Eds. Fundamentals of powder diffraction and structural characterization of materials, 1st ed. Kluwer, London.Ranger, J., E. Dambrine, M. Robert, D. Righi, and C. Felix. 1991. Study of current soil-forming processes using bags of vermiculite and resins placed within soil horizons. Geoderma 48:335-350.Ripley, E.A., R.E. Redmann, and A.A. Crowder. 1996. Environmental Effects of Mining. St. Lucie Press, Delray Beach.Rogers, J.R., P.C. Bennett, and W.J. Choi. 1998. Feldspars as a source of

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nutrients for microorganisms. Am. Mineral. 83:1532-1540.Ruby, M.V., A. Davis, and A. Nicholson. 1994. In situ formation of lead phosphates in soils as a method to immobilize lead. Environ. Sci. Technol. 28:646-654.Ryan, A.J., P. Zhang, D. Hesterberg, and D.E. Sayers. 2001. Formation of chlorpyromorphite in a lead-contaminated soil amended with hydroxyapatite. Environ. Sci. Technol. 35:3798-3803.Scheffer, F., and P. Schachtschabel. 1998. Lehrbuch der Bodenkunde. 14 ed. Enke, Stuttgart.Schnoor, J.L. 1990. Kinetics of chemical weathering: a comparison of laboratory and field weathering rates, p. 475-504, In W. Stumm, Ed. Aquatic chemical kinetics. Wiley, New York.Sobanska, S., N. Ricq, A. Laboudigue, R. Guillermo, C. Bremard, J. Laureynes, J.C. Merlin, and J.P. Wignacourt. 1999. Microchemical investigations of dust emitted by a lead smelter. Environ. Sci. Technol. 33:1334-1339.Souvent, P., and S. Pirc. 2001. Pollution caused by metallic fragments introduced into soils because of World War I activities. Environ. Geol. 40:317-323.Strömberg, B., and S. Banwart. 1994. Kinetic modelling of geochemical processes at the Aitik mining waste rock site in northern Sweden. Appl. Geochem. 9:583-595.Swoboda-Colberg, N.G., and J.I. Drever. 1993. Mineral dissolution rates in plot- scale field and laboratory experiments. Chemical Geology 105:51-69.Tsaplina, M.A. 1996. Transformation and transport of lead, cadmium and zinc oxides in a sod-podzolic soil. Eurasian Soil Sci. 28:32-40.White, A.F., and S.L. Brantley. 2003. The effect of time on weathering of silicate minerals: why do weathering rates differ in the laboratory and field? Chemical Geology 202:479-506.White, A.F., A.E. Blum, M.S. Schulz, T.D. Bullen, J.W. Harden, and M.L. Peterson. 1996. Chemical weathering rates of a soil chronosequence on granitic alluvium: I Quantification of mineralogical and surface area changes and calculation of primary silicate reaction rates. Geochim. Cosmochim. Acta 60:2533-2550.

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6 Additional applications of the in situ method

6.1 Dissolution of PbO on the support

The general approach of this experiment was to analyze the general dissolution behavior of particles, especially lead oxide, under idealized conditions. The SEM photographs are showing the dissolution signs of a PbO particle in a stirred HNO3 solution (pH 3.5). The first picture shows the fresh particle, the second picture illustrates a partially dissolved PbO particle and pictures three and four are showing the resulting “fingerprint” of the totally dissolved particle. The

“fingerprint” consist of epoxy resin membranes which migrated into particle cracks by capillary forces during the particle application procedure. This behavior is due to the special cellular particle “architecture” (Figure 6.2b) caused by the lead oxide production (supply of oxygen into molten metallic lead).In the following photographs the occurrence and behavior of several mineral particles is presented. Figure 6.1 shows lead oxide particles on a polymer support seen through a stereomicroscope before and after a solubility experiment with 0.01 M EDTA. This experiment was carried out to investigate the mechanical stability of the particles under influence of their partial dissolution. It could be shown that the particles remain stable on the supports and do not fall off when they are partially dissolved.

Figure 6.1 Stereomicroscopic photograph of lead oxide particles on a polymer support. a) Particles before and b) after partial dissolution in 0.01 M EDTA.

a b

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Figure 6.2 shows PbO particles under the electron microscope (SEM) after the same procedure with 0.01M EDTA. The appearance of the particles before and after the experiment are clearly visible.

Figure 6.2 Electron microscopic photographs of lead oxide particles on a polymer support. a) Particle occurrence before and b) after partial dissolution in 0.01 M EDTA. Photographs c and d are the remaining “footprints” of the lead oxide particles. Visible are nets of epoxy resin which creped into the particles during the coating process.

a b

c d

6.2 Extension of the method to particles with less than 20 µm diameter

During the development process of the new in situ method several particles sizes were evaluated. The sizes ranged from 5 µm to 500µm. Experiments with fine grained goethite mineral particles carried out a potential problem with small sized particles (< 20 µm). Under the electron microscope it was visible that the epoxy resin did cover the whole particles (Figures 6.3a and 6.3b). Additional experiments with fine filter dust particles (ZnO) from a metal refining factory showed further effects. The fine particles tend to form particle aggregations (Figure 6.3c)

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which can be completely coated as well (Figure 6.3d). Since the particles form aggregations, single mineral particles are difficult to analyze.

Figure 6.3 Electron microscopic photographs of very fine particles (approx. 5µm Ø) applied onto polymer supports. 3a and 3b are showing completely resin covered iron oxides. 3c shows partially covered filter dust particle conglomerates and 3d shows a completely covered filter dust conglomerate.

a b

c d

Test with epoxy resins of different elevated viscosities did improve the covering effect but could not solve the issue. The preliminary experiments carried out a minimum applicable particle size of 20 µm in diameter. This particle size limitation is excluding fine dust particles and other very small sized minerals. To overcome this drawback a special coating technique, previously developed and used by Scheidegger et al. (1993), was considered to be useful for this approach. Cristobalite minerals (70-100 µm Ø) were used as inert carrier particles. The coating method uses a suspension reaction where iron oxides (100 mg) are mixed with silicon oxide grains (2.5 g) at a fixed pH and ionic strength (pH 3; 0.01M NaNO3; 10 ml total volume) and mixed for 24 h in an overhead shaker. For our experiments iron oxides, co-precipitated with heavy metals (Cd, Cu, Zn), were coated onto the cristobalite particles following the adsorption procedure

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previously mentioned. Figure 6.4 is presenting the general preparation scheme. This adsorption step will produce coated silica mineral grains which can be used for the polymer support method introduced in chapter 3.

Figure 6.4 Principal setup to adsorb fine grained minerals onto silica sand carrier grains. Method setup adopted from Scheidegger et al. (1993). (a) fine grained minerals, (b) coarse silica carrier particles, (c) mixed in PE bottles at appropriate ionic strength and pH (0.01M NaNO3 @ pH 3), (d) shaking in over-head shaker; (e) mineral coated silica grains, ready to be applied on polymer supports.

The following two Figures are showing the uncoated cristobalite mineral grains (Figure 6.5a) and the iron oxide coated cristobalite grains (Figure 6.5b) after the adsorption procedure introduced in Figure 6.4. The yellowish color of the mineral grains is resulting from the adsorbed fine iron oxide minerals.

Figure 6.5 Photographs of a) uncoated cristobalite and b) iron oxide coated cristobalite minerals.

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Figure 6.6 is showing the occurrence of iron oxide coated cristobalite mineral particles visualized by scanning electron microscopy. The very fine coating of the silica mineral by the iron oxides can be noticed on Figure 6.6b.

Figure 6.6 Electron microscopic photographs of iron oxide coated cristobalite mineral particles on a polymer support. 6a shows the general occurrence of the cristobalite particles on the support, 6b shows more detailed the iron oxide coating.

a b

6.3 Thin sections of polymer supports in soils

In order evaluate the polymer support method to study the migration of heavy metals from the support into the surrounding soil matrix a preliminary experiment was carried out. By means of a soil corer for undisturbed soil sampling a polymer support with surrounding soil was recovered from an acidic forest soil (pH 3.5). The sample was dried in a dessicator under ambient laboratory conditions. After the drying procedure the sample was transferred into a polyethylene beaker and vacuum impregnated with epoxy resin. After hardening of the resin, the sample was cut into thin sections (1 mm) with a precision cut-off machine (Struers Accutom-50, Denmark). A selected thin section of the soil core was mounted on the sample stage of a micro x-ray fluorescence spectrometer (Edax µ-Eagle II, USA) and mapping of selected elements was carried out. Figure 6.7 shows the resulting elemental distribution maps. The copper concentrate particles can be seen as a thin band on the support surface and it is obvious that no copper migrated from the support surface into the surrounding soil matrix. The other element maps are showing typical elements of soil minerals (Si, Al, Fe) and Mn in distributed soil concretions. The SEM microphotograph is visualizing the setup of the sample and shows the perfect contact between the mineral grain and the soil matrix.

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6.4 Biological activities on buried supports

The following pictures (Figure 8) are showing biological activity on PbO samples recovered form an acidic forest soil. The structures which can be noticed on and around the mineral grains are presumably fungal hyphae (Egli, 2004).

Figure 6.7 Micro-XRF generated elemental distribution maps of a soil core thin section. The SEM photograph down right is showing the perfect contact between the mineral sample (Cu concentrate) and the surrounding soil. The size dimensions and the setup of the sample can be noticed as well from this photograph.


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6.5 Transformation of PbO in acid mine drainage

Another application of the polymer support method is to study the transformation behavior of mineral particles under the influence of acid mine drainage (AMD). In order to analyze the applicability of the polymer support method in acidified liquid media an experiment with lead oxide was carried out. Some polymer supports were prepared with lead oxide particles in the size range of 100-200 µm diameter. The PbO covered supports were immersed in a stirred synthetic acid mine drainage solution (pH 2.2; c FeSO4 500 mg/l) for 2 days. After the reaction period the initial PbO surface shows a cover with newly formed secondary mineral phases (Figure 6.9).

Figure 6.8 Biological activity on lead oxide particles. 6.8a and 6.8b shows fungal hyphae on PbO particles incubated in an acidic forest soil. 6.8c and 6.8d showing fungal hyphae on PbO particles incubated in a calcareous pasture soil.

a b

c d

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Additionally to the SEM/EDX analysis of the reacted lead oxide samples more detailed, phase sensitive techniques were applied. Micro Raman spectroscopy was carried out on the newly precipitated phases on the lead oxide surface. From Figure 6.10 can be denoted that the initial PbO phase peak is retracted and additional peaks are developed. Comparing the new peaks with a potential lead sulphate phase (anglesite) it can be seen that the peak profiles are matching. Frost

Figure 6.9 Lead oxide particle surface before the experiment (6.9a). Figures 6.9b-d are showing the particle surfaces after the reaction time of 2 days. New mineral phases attached to the initial smooth surface are observable. The SEM/EDX scan on Figure 6.9e shows a clear signal for sulfur and lead.

a b

c d

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et al. (2003) recorded a similar Raman spectrum of anglesite. They found similar characteristic Raman peaks for the PbSO4 phase as well as Alía et al. (2000) in their Raman study of orthorhombic sulphates.

Table 6.1 Listing of analyzed characteristic Raman peaks of Anglesite from our study and from Frost et al. (2003).

Anglesite (PbSO4) our study Anglesite (PbSO4) Frost et al. (2003)

Characteristic Raman Peaks Characteristic Raman Peaks1155 11561059 1060978 978640 642620 620606 608450 451439 440136 -

To confirm this finding a XRD scan was performed on the same sample after the Raman analysis. In Figure 6.11 the XRD results presents the occurrence of lead oxide (massicot) and lead sulphate (anglesite). From this experiment it can be concluded, that massicot (PbO) mineral particles develop secondary mineral precipitates on their surface in the presence of (synthetic) acid mine drainage at pH 2.2 and a sulphate (SO4

2-) concentration of 500 mg∙l-1 after a reaction period of 2 days. The origin of the iron and sulphate content in the AMD arises from the weathering (oxidation) of mill tailings from metal ore processing. This tailings consist predominantly of ferrous sulphide minerals which are discharged in tailing heaps or ponds (Blowes et al., 1991). Alpers et al. (2000) confirmed the occurrence of anglesite as a relatively insoluble phase detecting in their study about secondary minerals from acid mine drainage. Harris et al. (2003) found the same insoluble anglesite phase in abandoned mine tailings and pointed out that acid mine drainage generation occurs under almost all climatic conditions.

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Figure 6.10 Micro Raman scans of lead oxide (initial phase), lead sulfate (possible new phase) and the sample after the reaction of 2 days in synthetic AMD . It can be noticed that the initial PbO phase is retracting and a new phase is building up.

Figure 6.11 X-ray diffraction analysis of the reacted sample. Besides of the initial PbO phase (massicot = M) a new phase of PbSO4 (anglesite = A) appears. XRD scan was take after 2 days reaction of PbO particles in synthetic AMD.

A = Anglesite - Pb(SO4)

M = Massicot - PbO

Lin

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)

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2 10 20 30 40 50 60

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Bibliography

Alía, J. M., Edwards, H. G. M., López-Andrés, S., González-Martin, P., 2000. FT- Raman study of three synthetic solid solutions formed by orthorhombic sulfates: Celesite-Barytes, Barytes-Anglesite and Celesite-Anglesite. Spectroscopy Letters 33, 323-336.Alpers, C. N., Blowes, D. W., Nordstrom, D. K., Jambor, J. L. 2000. Secondary minerals and acid mine water chemistry. In: C. N. Alpers, Sulfate minerals: crystallography, geochemistry, and environmental significance. Mineralogical Society of America, Washington, pp Blowes, D. W., Reardon, E. J., Jambor, J. L., Cherry, J. A., 1991. Ther formation and potential importance of cemented layers in inactive sulfide mine tailings. Geochimica et Cosmochimica Acta 55, 965-978.Egli, S., 2004. Pilzmycelien (personal communication)Frost, L. F., Kloprogge, J. T., Williams, P. A., 2003. Raman spectroscopy of lead sulphate - carbonate minerals - implications for hydrogen bonding. Neues Jahrbuch für Mineralogie 12, 529-542.Harris, D. L., Lottermoser, B. G., Duchesne, J., 2003. Epehmeral acid mine drainage at the Montalbion silver mine, north Queensland. Australian Journal of Earth Sciences 50, 797-809.Scheidegger, A., Borkovec, M., Sticher, H., 1993. Coating of silica sand with goethite: preparation and analytical identification. Geoderma 58, 43-65.

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7 Conclusions

The prominent goals of the present PhD project were (A) to develop a new, straightforward to use method for in situ reactions of particulate material in soils which can be applied in field studies as well as laboratory studies, (B) to test and evaluate the new method under different physico-chemical parameters in different soil types and matrices, (C) to apply the method to study the dissolution behavior of different particulate heavy metal phases from the mining and smelting industry in acidic soil environments over extended time periods, and (D) to study the phase transformation behavior of heavy metal particles and the formation of secondary mineral phases in different soils over time using lead oxide as a test substance.

7.1 Development and evaluation of the new in situ method

The polymer material used to build the samples is a polymethylmethacrylate polymer (Plexiglas®) which is highly resistant to chemical and physical alteration (weathering). The thickness of the polymer material prevents the sample from bending and therefore potential damage. The adhesive substance to fix the particles is a two component epoxy resin with an elevated viscosity to suppress capillary creeping onto fine grained particles. The sample preparation step by means of a frame holder facilitated the coating of numerous samples in sets of ten each. The construction of a downsized copy of a sand blast gun made it possible to apply fine grained particles of limited amount onto the polymer supports. The insertion and recovery process of the individual samples in the field was facilitated with a special, custom made insertion device. In order to assess the behavior during insertion and recovery, preliminary test with the polymer supports proved an excellent mechanical stability without any significant particle loss. The design of the sample supports provided a unhindered direct contact of the particulate sample with its surrounding environment (soil matrix, sediment matrix, solution). Only a small surface area of the particles is covered by the adhesive resin while the majority is uncovered and free to react. This could be proven by an adsorption

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experiment with test particles. Particle specific calibration curves for the non destructive XRF analysis have to be obtained for each mineral type. This enabled the pre and post experimental quantification of the metal mass on the support which allows to detect potential concentration changes during the experimental time. The tested particle sizes for the method ranged between 20 µm and 200 µm diameter. Reducing the particle sizes below 20 µm increases the danger that the particle will be covered with epoxy resin due to capillary creeping of the adhesive resin. Extensive reaction periods of samples of up o 18 months in acidic, neutral and alkaline soils did not affect the polymer supports or the epoxy resin adhesive layer. Only the attached mineral particles did show the expected signs of alteration.

7.2. Establishing dissolution behavior of mineral particles in acidic soils

With the new in situ technique it was possible to analyze the long term behavior of different heavy metal particles under field conditions. The dissolution behavior of fine mineral phases could be analyzed without extensive disturbance of the soil matrix and it was possible to establish in situ dissolution rates for the mineral particles. Usually rates of dissolution were measured using laboratory experiments and then extrapolated to the field. The reported differences of orders of magnitude between laboratory and field rates could not be confirmed with the results of the in situ method. The method could show that several parameters (e.g. soil moisture, biological activity, temperature) are playing a role in the dissolution behavior of particles in the open field and that the in situ reaction approach takes account of these effects.

7.3. Analysis and identification of mineral phase transformations

The in situ method was used to investigate how mineral particles change their mineralogical phases under field conditions. The used PbO exhibited a precipitation of secondary phases on the initial mineral grain and in the intergranular spaces. It could be demonstrated that in a carbonate bearing soil matrix initial lead oxide particles were partially dissolved and formed secondary lead carbonate phases which could cover the whole initial mineral grain. The formation of this weathering crust is the dissolution controlling phase. The application of the non destructive micro-Raman spectroscopy made it possible to identify the new phase

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as hydrocerussite. The XRF quantification of the samples over the experimental time showed that no significant loss of lead and therefore dissolution occurred under the given soil conditions. Supporting analysis with SEM microscopy and EDX microprobe could document the precipitation of secondary mineral phases as well as their distribution on the sample and it could identify the elemental composition of the new phases . The use of a high pressure mode environmental SEM (ESESM) was able to show a minor phosphorous containing phase on the samples from the non-calcareous soil, indicating formation of a pyromorphite crust inhibiting further dissolution.

7.4. Outlook and further questions

The new in situ reaction is a valuable contribution to the environmental- and geo-sciences in analyzing the behavior of fine particulate mineralogical materials. It allows to analyze minerals under field conditions in a “real” system, avoiding to extrapolate laboratory results into the field and thus to eliminate the uncertainties of extrapolations.Although the new method shows obvious advantages in the field of in situ reaction examinations, the method is still new and might undergo further refinements and be extended to additional applications. The application in aquatic environments as well as in sediments could be an interesting new field for the method. The possibility to extend the method for very fine particles (< 20 µm) would be a useful advancement. This might require more detailed work on the applied adhesive substances (resin types). The use of an inert carrier particle for the very fine minerals was demonstrated with fine iron oxides (Goethite) adsorbed on cristobalite particles. This mineral-mineral compound can be further attached onto the supports in the normal way as introduced in chapter 3.The application of nondestructive analysis methods demonstrated its advantage, especially in the use of “before” and “after” comparative examinations. In the case of the scanning electron microscopy it will be a vast improvement to use high pressure mode, “environmental” microscopes (ESEM). This eliminates the need to cover the specimen with a conductive layer and thus the sample is kept untouched in its original state. A sample analysis on a recently acquired ESEM on ETH, which became accessible for us in the very end of the project time, showed very promising results (chapter 4).

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Additionally the method might be a useful addition for synchrotron based analysis methods, opening the ability to introduce in situ reacted samples to this high energetic analysis technique.

Acknowledgements

I would like to thank Rainer Schulin and Bernd Nowack for giving me the opportunity to carry out this interesting and exciting project, for their valuable and constructive advice and for the review of the manuscript. I am grateful for having many fruitful scientific discussions with them which helped me to conduct and analyze my experiments. Many thanks to Ruben Kretzschmar for the review of the manuscript and taking the co-examiners position.

I would like to thank Rosanna Ginocchio from the Chilean Center of Mining and Metallurgy (CIMM) for donating me the selection of samples from the copper mining industry and Andreas Wahl from Pennaroya Oxide GmbH for donating me the lead oxide particles. A special thank to René Saladin, Hanspi Läser and Werner Attinger who helped me in the workshop and on the field sites. A very special thank to Peter Wägli from the Insitute of Solid State Physics who gave me access to the electron microscope and taught me the basics of using the instrument. I also would like to thank Eric Reusser from the Insitute of Mineralogy and Pavel Trtik from the Institute of Building Materials for giving me access to the Raman spectrometer and the environmental scanning electron microscope.A special thank to Kurt Barmettler from the soil chemistry group, who always let me use their instruments. I also would like to thank all my colleagues at the Institute of Terrestrial Ecology for their support.

A very special thank to my fellow sufferers Ulla Wingenfelder, Susan Tandy and Marc Schumacher with whom I could exchange my problems and doubts and who knew exactly how I was feeling in difficult moments.

And of course a very special thank to my wife Paula who will have back my full attention from now on and to my parents who always supported me and understood where I wanted to go in life.

Curriculum Vitae

Surname Birkefeld

Given name Andreas

Date of Birth 03-November-1967

Place of Birth Göttingen, Germany

School Education 1974-1978 Bonifatiusschule, Göttingen, Germany 1978-1987 Hainberggynasium, Göttingen, Germany

Public Service 1987-1989 BG Emergency Hospital, Tübingen, Germany

Higher Education 1989-1992 Undergraduate Study of Chemistry, University of Göttingen, Germany 1992-1997 Study of Geology, MSc, University of Göttingen, Germany 2000-2005 PhD Study of Environmental Sciences, Swiss Federal Institute of Technology ETH, Zurich, Switzerland

Occupation 1993-1998 R.-U. Woode Engineering Consultancy, Hannover, Germany 1997-2000 Chilean Center for Mining and Metallurgy CIMM, Santiago de Chile, Chile

In situ transformations of mineral particles in soils

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