A Thesis Proposal by

Katya Bazilevskaya

Department of Crop and Soil SciencesThe Pennsylvania State University

November 5, 2006

Advisor: Carmen Enid Martinez

Mixed Fe-Al hydroxide nano-particles: precipitation and transformation in

solution and on quartz surface

Center of Environmental Kinetic Analysis (CEKA):

What factors control the nucleation, growth, and transformation of mixed Fe-, Al-, and Si- oxides from solution (homogeneous) and on mineral surfaces (heterogeneous)?

Low-crystalline Fe and Al hydroxides exist as colloidal (<0.1 µm size) particles in natural waters or as coatings on the soil minerals

Have strong ability to adsorb contaminants due to their high surface area and pH-dependent surface charge.

Rationale

From Penn et al. (2001)

Better understanding of crystallization pathways is needed to formulate rate models for soil environments and to improve remediation techniquesThis research is developing new approaches and techniques to study composition and transformations of mixed Fe-Al hydroxide nano-colloids

Research Hypotheses

In nano-colloidal system:

Mixed Fe-Al hydroxides have slower crystallization rate than the pure phases

Two main factors that dictate the rate of crystallization of mixed Fe-Al hydroxides are time and chemical composition (Fe/Al ratio);

Crystallization pathways can produce a variety of intermediate metastable phases

Aluminum impurity in Fe-hydroxides results in lower degree of crystalinity with sluggish kinetics of crystallization of intermediate species which are more reactive towards contaminants

Transformation of mixed Fe-Al hydroxide nano-particles upon aging

Formation of Fe-Al coatings on quartz in the lab and in the field

Synthesis of Al-substituted hematite nano-particles

#1

#2

#3

XRD: crystal structure (cell parameters)EXAFS: hematite structure (Fe-O, Fe-Fe, Fe-Al distances)TEM: particle morphology of particular crystal phase

How Al-substitution influence the crystal structure and morphology of nano-hematite? Would these properties be different depending on the precipitation pathway?

What is the structure and composition of soil coatings? How coating structure and morphology change with time? How presence of Al modify the properties of coating?

XRD: qualitative mineral compositionATR-FTIR: quantitative composition

How to quantitatively resolve mineral composition in the mixture? What is kinetics of mineral crystallization in mixture?

GIXAS: structure of the coating (Fe-O, Fe-Fe, Fe-Al distances)GISAXS: particle morphology, distribution on the surface of particular crystal phase

Project Research Questions Methods

Project overview

Homogeneous precipitation and transformation of mixed Fe-Al nano-particles using ATR-FTIR and XRD

Research hypothesis: kinetics of crystallization of minerals (goethite and/or gibbsite) from mixed Fe-Al hydroxides is much slower than that of the pure phase due to the formation of intermediate phases that are indefinitely metastable in low-temperature soil environments.

Specific objectives:

(1) determine the mineral composition (percentage of goethite and gibbsite) in mixed Fe-Al hydroxides as a function of Al-substitution and reaction time, and

(2) determine the effect Al-substitution may have on the rate of crystallization of the primary (Fe) precipitate.

Project #1

[Fe+Al] = 10-2 M; %Al: 0, 10, 25, 30, 50, 75, 100; pH = 5Slow titration rate (0.1 ml/min) Dialysis to remove salts and excess AlTime: 0, 2, 9, 23, and 54 days, aged at 50 C

Experimental conditions:

Methods: nano-particle synthesisProject #1

Fe (+Al), pH ~2

0.1 M KOH

Methods: Infrared measurements

ATR Crysta

l

Evanescent wave0.1 µm

ATR Crystal

Sample

Incident radiation Reflected radiation

Mirror

The principle of ATR-FTIR

Infrared radiation is focused onto the edge of ATR crystal, reflected through the crystal and directed to the detector.Radiation penetrate up to 0.1 µm into solution, where it is absorbed and cause the vibrations of molecule bondThe infrared spectra is obtained with unique bands for each bond type

Project #1

Methods: XRD measurements

Project #1

Brookhaven National Laboratory beamline X-16C

Project #1

Molecular dynamic modeling: calculation of vibrational OH-frequencies

100% Fe 100% Al

(a) (b) (c) (d)

FeAlOH

Goethite Al-substituted goethite

Isomorphous substitution Al-clusters

Gibbsite

Build goethite and gibbsite models based on available experimental crystallographic data

Create Al-substituted goethite and calculate new atomic coordinates

Using Vienna Ab-initio Simulation Package (VASP) obtain theoretical OH- frequencies for goethite and Al-substituted goethite

Use these frequencies as a reference to interpret Infrared data

Changes in mineral composition of mixed Fe-Al nano-particles with increasing Al-substitution. Suspensions were aged for 2 days at 50 C

0% Al

Wavenumber, cm-1

700 900 1100 3000 4000

Inte

nsity

10% Al

100% Al

75% Al

50% Al

25% Al

30% Al

Preliminary work: infrared dataProject #1

Dotted and solid lines (perpendicular to x-axis) show characteristic goethite and gibbsite band positions, respectively

0%Al 25%Al

Transformation of Fe(-Al) nano-particles upon aging at 50 C.

Project #1 Preliminary work: infrared data

Project #1

Expected results

MCR analyses of high resolution ATR FTIR data will allow to de-convolute and quantify the mineral composition in complex Fe-Al hydroxides mixtures

XRD data will be in good agreement with infrared data

The main intermediate species that control the kinetics of crystallization will be identified

Project #2

The presence of aluminum in solution will change the mineral composition and coating thickness at any given time compared to aluminum free systems;Iron and aluminum migrate through the soil profile in the form of hydroxide nano-particles that accumulate in lower profiles to form coatings on sand grains;The composition and spatial distribution of different mineral phases in the coatings may reveal the mechanism of its formation.

Research hypotheses

Specific objectives

Formation of Fe-Al coatings on a quartz substrate: laboratory and field investigations

(1) Study the formation and development of coatings in situ by placing a quartz wafer into a Spodosol profile (field experiment);

(2) Determine structure of the coatings as a function of Al-substitution and reaction time (lab experiment)

Project #2Podzolization mechanisms

Organic acids

Organo-Al -(Fe) complexesSi, Al, Fe Al-Fe-Si

inorganic sols

Fe-oxide, allophane, imogolite

Microbial activity

supersaturation precipitation

Addition of Al and Fe

adsorption

Microbial degradation of organic ligands

Release of Fe and Al

Fe-oxide, allophane, imogolite

Flocuulation by cations: K+, Mg2+

O

E

Bh

Bs

C

pH = 4

pH = 5

Si

Adsorption/precipitation theoryProto-imogolite theoryBiodegradation theory

State College

Philipsburg

Milesburg

504

I-80

220322

BellefonteBlack Moshannon Lake X

100 cm 100 cm 100 cm 100 cm4 m

Time1=6 months Time 2 =12 months Time 3 =24 months

Time 4=36 months

4 m

5 cm

Project #2 Methods: field experiment

Field layout well-polished quartz wafersemi-polished quartz waferssilica gelsand

Project #2

Methods: lab experiment

[Fe2+] = 10-4 M pH ~ 5

[Al] = 0.2x10-4 M [Fe2+] = 10-4 M

pH ~ 5

Reaction time, hours

0.5 1 1.5 2

Methods: GIXAFS measurementsProject #2

< 0.18 degree

I1I0

Energy Dispersive Detector – 30 element Ge

X-ray fluorescence

Analysis within the first nanometers from the surface Polished surface is required 30 element Ge-detector in fluorescence mode Grazing incidence angle, θ = 0.18o

Info about oxide structure: Fe coordination number ID of Fe neighboring atoms (Fe, Al or C):differentiate among Fe-O-Fe, Fe-O-Al and Fe-O-C local bonding environments

Methods: GISAXS measurements

a non-destructive structural probe does not require a conducting surface or sample preparation (in-situ characterization possible) yields excellent sampling statistics (averages over macroscopic regions to provide information on nanometer scale) provides information on particle geometry, size distributions, spatial correlations

Grazing Incidence Small angle scattering of x-ray

Project #2

irradiate a sample with a well-collimated X-ray beam measure the resulting intensity as a function of angle between the incoming beam and scattered beamdetermine the structure that caused the observed pattern

Expected resultsProject #2

Statistically significant and steady increase in coating thickness in three years will allow to estimate the rate of coating formation in the field; Spectroscopic analysis (GIXAS) will give reasonable information of the coating structure, i.e. we should be able to distinguish between Fe-organic (Fe-O-C) bonding and Fe-O-Fe(Al) bonding in the organic-rich and organic-free horizons, respectively;Spectroscopic data will be similar for the laboratory and field samples (Bx horizon); We will be able to infer mineral composition on the coatings from our spectra; Coating structure and morphology will be different in the presence of aluminum compared to Fe-only experiments.

In the field: quartz wafer placed in spodosol Bx horizon and recovered after 1 year

AFM image

Fe K-edge

EXAFS spectrum (GIXAFS)

In the lab: fused silicon after 2.5 hours reaction with Fe(II) 10-4 M solution under oxidizing conditions

AFM image

Fe K-edge

EXAFS spectrum (GIXAFS)

Project #2 Preliminary work

Project #3

Synthesis and characterization of Al-substituted hematite nano-particles

The effect of Al-substitution in micron-size hematite particles (decrease in crystallinity and particle size) is also true for nano-size Al-substituted hematite particles;This effect is even more pronounced for nano-particles due to the higher surface to bulk ratios encountered in nano-sized particles,.The amount of aluminum incorporation into the hematite structure depends on the pathway of nano-hematite formation

Research hypotheses

Specific objectives

Synthesize Al-substituted nano-hematite particles (less than 30 nm) following different synthesis procedures

Compare particles properties (size, shape and structure) among different sets using XRD, TEM, EXAFS and chemical analyses.

a. Procedure #1

Fe Al Fe+Al Fe

Al

b. Procedure #2 c. Procedure #3

Methods: nano-hematite synthesisProject #3

mol%Al [Fe], M [Al], MTotal [Al+Fe],

M Fe:Al0 0.01 0 0.01 010 0.009 0.001 0.01 0.120 0.008 0.002 0.01 0.330 0.007 0.003 0.01 0.4

EXAFS: method

Fe:Al = 1:1

Fe-O Fe-FeFe-Al

Project #3

0

2

4

6

8

10

12

14

16

18

20

0 20 40 60 80 100Size, nm

Vol

ume

%

0% Al10% Al20% Al30% Al

Preliminary work

Project #3 Al incorporation in nano-hematite particles

Al/(Al+Fe) in starting solution, mol%0 5 10 15 20 25 30 35

Al(A

l+Fe

) in

hem

atite

, mol

%

0

2

4

6

8

10

nHm-30

nHm-20

nHm-10

nHm-0

Particle size of hematite suspensions with different initial aluminum mol %.

TEM dataXRD data

Project #3

Preliminary work

Expected results

Project #3

The percent of Al-substitution will be different depending on the synthesis procedure, and it will be, probably, the highest using procedure #3 (addition of Fe-chloride dropwise to Al-solution)

As Al-percentage in initial solution increases, hematite particle sizes are expected to decrease; particle morphology will change from round to elongated shapes

Research timetable