A Comparative Study on Adsorption Behavior of Heavy Metal Elements Onto Soil Minerals- Illite,...

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Journal of KoSES Vol. 4, No. 1 57~68, 1999

Original Articles

A Comparative Study on Adsorption Behavior of Heavy Metal Elements onto Soil Minerals: Illite, Halloysite, Zeolite, and Goethite

Chang-Oh Choo · lg-Hwan Sung*

Research Center for Groundwater Environments, Korea Institute of Geology, Mining and Materials

ABSTRACT

Adsorption behavior of metal elements onto soil minerals such as illite, halloysite, zeolite (clinoptilolite), and goethite was comparatively investigated at 25°C using pollutant water collected from a gold-bearing metal mine. Speciation of solutions reacted was determined by WATEQ4F program, indicating that most of metal ions exist as free ions and that there is little difference in chemical species and their relative abundances between initial solution and reacted solutions. The experimental data exhibit that the adsorption extent of elements varies depending on the type of mineral and the reaction time. The adsorption process practically takes place within one hour, with Fe and As significantly removed from solutions. On the whole, halloysite is regarded as the most effective adsorbent among minerals used in the experiment. Adsorption properties of alkali elements are not consistent with a manner predicted from hydrated ionic radii.

Key words: adsorption, illite, halloysite, zeolite, goethite

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ABSTRACT

The adsorption behavior of heavy metal elements present in an acid solution was comparatively investigated using illite which is one of the common soil minerals, halloysite which is a kind of kaolin, zeolite (clinoptilolite), and goethite as adsorbents. The adsorption reactions of metal elements exhibit different characteristics depending on the type of mineral and the reaction time. There is hardly any difference in the chemical species present in the solution after the reaction took place for 1 week or 2 weeks and the initial solution, which indicates that the reaction proceeds at a similar rate to the original condition without experiencing a change in the chemical species of individual elements. Fe and As turned out to have the highest removal percent among all minerals, and the adsorption reaction mostly takes place within one hour although it could vary slightly depending on the type of mineral and the type of heavy metal element. On the whole, halloysite is regarded as the most effective adsorbent among minerals used in the heavy metal adsorption. Meanwhile, adsorption properties of alkali elements are not consistent with a manner predicted from hydrated ionic radii.

Key words: adsorption, illite, halloysite, zeolite, goethite

1. Introduction Heavy metal contamination in soil and water

has drawn a lot of attention these days not only because it destroys the food chain of the ecological system, but because of its toxicity and danger to human life and the environment. As soil and groundwater environments are closely connected to each other, heavy metal contamination in soil is very likely to develop groundwater contamination. Current studies on contaminants are mostly relevant to heavy metal contaminations in soil and water caused by a variety of artificial contamination sources such as industrial waste, mine waste, agricultural activities, industrial activities, and so on 1).2).3). The fluidity and solubility of heavy metal elements largely depend on how they are bound to solid phase elements, and when the heavy metal elements are leached out under certain appropriate conditions the soil and water environments are directly affected thereby4).5).6). Especially the soils formed on the surface of the earth or the sediments in water

serve as a reservoir of heavy metal elements, and among other things metal elements which form complexes with ligands are the most harmful ones that can contaminate the groundwater recharging zone when leached out7).

The natural environment is composed of many kinds of minerals, and such minerals are capable of removing metal elements from an aqueous solution in different environments such as soil, groundwater, river, and so on. Examples of effective adsorbents among common components of soil, sediment and aquifer may include clay minerals, zeolites, metal hydroxides, and so on. Their large specific surface, and high surface charge and cation exchange capacity make it possible to adsorb a substantial amount of metal ions. Adsorption reactions that take place between the heavy metal elements in an aqueous solution and the solid-phase interface include the adsorption of metal ions, the ion exchange in the structure of a clay mineral, the formation of a metal-ligand complex, and so on8). The adsorption mechanism of heavy metal elements in an actual soil, sediment or aquifer is

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very complicated, but the cation exchange reaction and the adsorption reaction are its basic schemes. In particular, the adsorption selectivity is greatly influenced by the characteristics of cations, pH, the concentration of an adsorption material, and the type of adsorbent9). Typically, the adsorption of metal elements is weak in an acidic environment, and an increase in the pH leads to a sharp increase in the adsorption rate10).11). However, since the solubility of metal elements is very high under an acidic environment, the fluidity is increased to make the drinking groundwater toxic, or nutrients are leached out of the soil to impoverish the soil. Meanwhile, heavy metal elements in the natural world actually become bigger problems in the acidic condition; the adsorption behavior of soil minerals in such an environment provides information that is required to reduce the concentration of heavy metals in the soil, groundwater or river.

In this study, we used illite which is a main mineral component of the soil, halloysite which is a kind of kaolin, zeolite (clinoptilolite), and goethite (α-FeOOH) as adsorbents, and observed how heavy metal elements present in an aqueous solution are adsorbed and removed to and from these minerals to comparatively investigate the adsorption capacity of each mineral. Especially, we used waste water of a metal mine in order to investigate adsorption capacity of minerals under an acidic environment, as well as adsorption selectivity of different elements that are present at various ratios in the natural system.

2. Materials for the Experiment and Experimental Methods

2.1 Materials for the experiment

Among the properties of minerals for use in an adsorption experiment, inter alia, the structural characteristics and purity contribute greatly to the extent of adsorption. It is difficult to analyze the adsorption behavior of minerals if a mineral to be analyzed is a mixture consisting of at least two kinds of minerals, or there is not enough information about the crystallinity of the mineral of interest. Therefore, mineralogical understanding on them needs to be undertaken sufficiently before executing an adsorption experiment. In this

study, prior to the adsorption reaction, each mineral was first evaluated and analyzed structurally by an X-ray diffractometer (XRD). Kα radiation of a Cu target was filtered with Ni under 40 kV/30 mA and scanned onto a sample powder of a mineral at 5oC/min in the range from 3o to 60o 2θ. The XRD analysis result revealed that those minerals used in the adsorption experiment were highly pure single mineral species. As the size of particles and the particle size distribution are important factors, just like other factors, which influence the adsorption capacity, the particle size of minerals that were used in the experiment was analyzed by a laser scattering analyzer. A brief explanation of the properties of each mineral used in the experiment is given below. Illite is usually formed by weathering, hydrothermal alteration, or denaturation of mica or feldspars, and has a structure composed of tetrahedral layers and octahedral layers at a ratio of 2:1, with each unit structure layer being interconnected with K. For this experiment, illite that is produced in a dike form from the Yukwang mine (Kyeongsangnam-do, Korea) was used. When an expansion test was conducted using ethylene glycol, no mixed layers were observed in the structure of illite, but a 2M1-type multiform having a high crystallinity was shown. The sum of interlayer cations per unit cell, which is calculated on the basis of the chemical composition, was equal to 0.96, and the average particle size was 7.6±2.1 µm.

Halloysite is a kind of kaolin, and has a structure composed of tetrahedral layers and octahedral layers at a ratio of 1:1. It is often formed at the initial phase of weathering of feldspars, and then gradually converts to kaolinite and dockyete12). This explains why halloysite is one of the most commonly found minerals among the decomposed granite or granite gneiss soils. The halloysite used in the experiment was obtained from Okdong Myeon (Kyeongsangnam-do, Korea) and its XRD pattern is characterized by distinct reflection at 10Å and 7Å. Halloysite has an average particle size in the range of 8.2±2.7 µm.

Zeolite is characterized by its porous structure of channels and cages having different volumes. Zeolite used here was formed by denaturation of volcanic products of the tertiary deposit of the Cenozoic era, and identified as clinoptilolite by the XRD evaluation. Zeolite has an average particle size in the range of 10.5±3.6 µm.

Goethite is a kind of iron oxide and it, together

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with limonite, remains in a stable form under varying soil environments. Because of excellent reactivity and a large surface area, goethite has a key role in adsorbing trace elements on the surface of the ground and in the groundwater13).14). In particular, iron oxides tend to be dispersed as very fine particles, making the iron oxides good adsorbents in many different environments and function as a coating over other materials15). Hence, iron oxides have an important role in controlling the movement of metals in an aqueous environment. Although iron oxides constitute only several percent of the soil composition, they can greatly contribute to the total reactive surface area of the soil. Goethite was selected for the experiment because it can be synthesized within a short period of time in vitro. Goethite has an average diameter in the range of 5.4±1.6 µm, and is characterized by its needle-like crystal form. The reflection pattern confirms a slightly high crystallinity of goethite.

2.2 Adsorption experiment In this study, an adsorption reaction of metal elements against each mineral was carried out at different reaction times. Wastewater flowing out of a gold-silver-fluorite mine located in the Geumsan area (Chungchungnamdo, Korea) was collected for use as a reaction solution. After measuring the pH of mine wastewater in situ, the mine wastewater was filtered with a 0.45 µm filter, sealed and transferred to a laboratory for the analysis of cations and anions. The elemental analysis for this reaction solution was conducted using an atomic absorption spectrophotometer (AAS), an inductively coupled plasma mass spectrometer (ICP-MS), and ion chromatography (IC) on 27 cations and 6 anions, including a main element and trace elements.

0.2g of a mineral sample that had previously been subjected to the particle size analysis was mixed with 30 ml of the reaction solution in a polyethylene container (50 ml) to have an adsorption reaction at ambient temperature. To promote a continuous reaction, the mixture was stirred at 150 rpm with a thermostatic shaker.

The reaction time was set to 10 min, 30 min, 1 hour, 12 hours, 24 hours, 48 hours, 1 week, and 2 weeks, except for goethite where the reaction time is set on a regular basis as described above with 1 week for the maximum reaction time. Upon the completion of the reaction, the samples were separated into solid and liquid fractions by means of a high-speed centrifuge and a 0.45 µm filter, and then the pH of the solution before and after the adsorption reaction were measured. Any change in the concentration of the solution was detected by performing an analysis of the cations and anions using the same analysis method described above. The solid fractions were subjected to a structural analysis by Fourier transform infrared (FTIR) spectroscopy to find out a change in the coupling state between molecules that are involved in the heavy metal adsorption.

3. Consideration and Conclusion 3.1 Existing forms of elements

When the concentration of a reaction solution is too high, the adsorption behavior sensitive to ionic strength may vary substantially by the initial concentration of each element. Usually contaminants that are actually discharged from a mine or a contaminated aquifer have a low concentration. Even if the hourly adsorption conducted under the constant ionic strength and pH as in this experiment explains about a reaction at a very specific condition, it can still provide valuable information associated with the treatment of contaminated discharges16). The mine wastewater that is used as the reaction solution in this experiment is a strong acid having a pH of 3.19 and looks like an orange opaque solution when observed in situ. The chemical composition of the aqueous solution includes, in descending order, 1090 ppm of SO4

2-, 237.5 ppm of Ca, 72 ppm of F, 49.6 ppm of Mg, 42.1 ppm of Al, 32.2 ppm of Fe, 22.3 ppm of Mn, 16.1 ppm of Si, 9.6 ppm of Na, 4 ppm of Cl-, 3.2 ppm of K, and 2.4 ppm of Zn. As, Ba, Cd, Co, Cr, Cu, Cs, Li, Ni, Pb, Se, Ti, PO4 and NO3 fall within the range of a few hundred ppb.

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The pH of a heavy metal aqueous solution slightly changes (Fig. 1) after the reaction with minerals. For example, after 30 minutes of the reaction, illite has a pH of 3.3, halloysite 4.0, zeolite 3.58, and goethite 3.3.

Fig. 1. pH variations of solutions after

reaction with minerals as a function of time (initial solution pH=3.19)

After 2 weeks of the reaction, illite had a pH of 3.1, halloysite 4.2, and zeolite 4.0. After 1 week of the reaction, the pH of goethite changed to 3.4, respectively. Halloysite showed the largest change in the pH of the reaction solution by time, and zeolite showed a small change in the pH. However, there is hardly any change in the cases of illite and goethite. This phenomenon occurs probably because hydrogen ions at the adsorption site on the surface of a mineral such as halloysite and zeolite compete relatively more with other metal ions, but the competition is weak in the cases of illite and goethite.

Metals in the natural water or soil water typically exist as free ions or diverse complexes7). Types of individual chemical species that exist in the aqueous solution and their ratios can be easily obtained using a program for balance model calculations. In this study, WATEQ4F17) was used to obtain the existing forms of chemical species and their relative ratios. Table 1 illustrates the chemical species that are present in the initial solution as well as in the residual solution after 1 or 2 weeks of the reaction, and the calculations of their content ratios, respectively. In the initial

solution, Al is present predominantly as AlF2+

and AlF3, while As is present primarily as H2AsO4

-. Most of the other components are present, in part, as sulfate compounds. In this case, because the concentration of sulfate ions in the initial reaction solution is very high, it seems easy to produce compounds in such form. In addition, there is no significant difference between the initial solution and the solution after 1 or 2 weeks of the reaction in terms of a change in chemical species. This indicates that there is hardly any change in the stability of the chemical species under varying conditions of pH as well as in the speciation of the chemical species that are substantially involved in the adsorption. That is to say, the adsorption reaction at a low pH as in this experiment is performed at a similar rate to that of the initial condition, with hardly changing the chemical species of elements. 3.2 Adsorption capacity 3.2.1 Adsorption results of Fe and As

The concentration of the reaction solution after the adsorption reaction changes in proportion to the adsorption rate which can be expressed as the ratio of removal of a particular element. Among all minerals, Fe and As are the elements that have the highest rate of removal, and their adsorption rate after 1 hour of the reaction is at least 40% (Fig. 2). In the case of Fe, it showed the lowest adsorption rate onto illite in the early stage of the reaction, while other elements show high adsorption rates as high as about 70%. As the reaction time goes by, the adsorption rate onto zeolite no longer increases, while the adsorption rates onto other minerals reach almost 90% or higher. After 1 week of the reaction, the adsorption rates were as follows: goethite>illite>halloysite>zeolite in descending order (Fig. 2a). Especially halloysite and goethite had a similar adsorption removal percent. Meanwhile, in the case of As, it showed the lowest adsorption rate (about 40%) onto zeolite after 1 hour of the reaction, and the adsorption rates for others were as follows: goethite>illite>halloysite>zeolite in descending order (Fig. 2b). After 1 week of the reaction, the

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adsorption rates were as follows: goethite>zeolite>halloysite>illite in descending order. After 2 weeks of the reaction, the adsorption rates onto halloysite and illite reached almost 95% or higher, but the adsorption rate onto zeolite went down to about 50%.

Fig. 2. Removal percent of heavy elements with time (a) Fe removal, (b) As removal In this experiment, although halloysite and zeolite having relatively larger particles seem to have a higher extent of the adsorption reaction than illite and goethite having smaller particles, it is understood that there is no evident correlation with the particle size. Among the minerals used for this experiment, halloysite had the cation exchange capacity (CEC) in the range from 10 to 40 meq/100g, illite 15 to 40 meq/100g, iron oxides such as goethite 160 meq/100g, and clinoptilolite 225 meq/100g. Also, it was reported that clinoptilolite has a specific surface of 10 to 45 m2/g, and muscovite similar to illite has a specific surface of 60 to 100 m2/g18).19). Thus, zeolite and goethite are believed to have the highest apparent adsorption capacity, and illite and halloysite have the lowest apparent adsorption capacity. In addition, illite having a 2:1 structure is more advantageous for the adsorption than kaolinite having a 1:1 structure, and its charge imbalance enables ion exchange to some extent. However, as shown in this experiment, the adsorption capacity of halloysite was higher than expected. In view of such a phenomenon, halloysite seems to have a

very low crystallinity on account of its particular configuration, and a very high reactive potential because of its complicated tube shape.

The cation exchange capacity of zeolite depends on the degree of substitution of Si with Al or Fe3+ at a tetrahedral site. A higher degree of substitution requires alkali or alkaline earth metal cations to achieve the charge balance. In particular, a mineral such as clinoptilolite that has a high Si/Al ratio has fewer charge sites per cage; it exhibits the selectivity for monovalent cations over divalent cations. The degree of ion exchange is mainly influenced by the channel and layout of a structure, the size and polarity of an ion, the charge density within a channel and a cage, the valence and charge density of an ion, and the electrolyte composition and concentration of an external solution19).20). Moreover, in the case of goethite, similar to zeolite, its porous structure facilitates the cation exchange within the structure.

3.2.2 Adsorption results of Cu, Pb, Zn and Co Each component exhibits a different degree of

adsorption as a function of the adsorption reaction time, but the overall adsorption rate onto them is not that high, except for some components. In a heavy metal solution, Cu, Pb, Zn, and Co have the highest adsorption rate onto halloysite. In the case of Cu, after 1 hour of the reaction, the adsorption rates were in the order of halloysite>illite>goethite>zeolite, with zeolite showing little or no adsorption rate (Fig. 3a). However, after 1 week of the reaction, the adsorption rates increased up to 40% or higher, with the adsorption rates being in the order of halloysite>zeolite>illite>goethite. After 2 weeks of the reaction, however, zeolite and illite had an adsorption rate very close to 0, undergoing a reversible reaction and being separated. Meanwhile, halloysite had an adsorption rate of about 95% or higher. In the case of Pb, after 1 hour of the reaction, it had an adsorption removal percent close to about 75% or higher, similar to that of halloysite and zeolite. Illite and goethite had a similar adsorption rate of 50% or lower (Fig. 3b).

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Fig. 3. Removal percent of some heavy elements with time (a) Cu, (b) Pb, (c) Zn, and (d) Co.

The real adsorptions of heavy metal elements such as Cu, Pb, and Zn onto the surface of goethite at low pH (3) are usually as low as 10% or below13), but the removal percent is high as in this experiment because they are likely to be substituted with Fe within the structure21). After 1 week of the reaction, the adsorption order was as follows: halloysite>zeolite>goethile>illite. However, after 2 weeks, the adsorption rate onto zeolite gradually increased, while the adsorption rate onto halloysite sharply decreased. Especially, the adsorption rate onto illite increased from the second day of the reaction even higher than the concentration of the initial solution. In the case of Zn, after 1 hour, halloysite had an adsorption rate of 60% or higher (Fig. 3c). The adsorption rates, although being low, were in the order of: zeolite>illite, goethite. As time goes by, the adsorption rate onto halloysite gradually increased continuously up to 90% or higher, while after one week the adsorption rate onto zeolite started decreasing. Meanwhile, the adsorption rates onto illite and goethite, although being low, tended to increase.

In the case of Co, after 1 hour, no minerals showed any adsorption, but the solution

concentration thereof increased (Fig. 3d). However, after 2 days, the adsorption rate onto halloysite continuously increased up to about 80% at the end of 2 weeks. However, other minerals were hardly adsorbed.

3.2.3 Adsorption results of alkali elements (Li,

Na, K)

Alkali elements such as Li and K usually have a high adsorption rate, but Na is not readily adsorbed. After 1 hour of the reaction, the adsorption rate of Li was in the order of goethite>illite>halloysite>zeolite, with about 70% adsorption rates onto goethite and illite (Fig. 4a). After 2 days, the adsorption rate was in the order of goethite>zeolite>halloysite>illite. The same adsorption order was shown 1 week later, and the adsorption rates onto other minerals except for illite were 70% or higher. After 2 weeks, the adsorption rates onto halloysite and illite were about 95%, and the adsorption rate onto zeolite sharply went down to about 50%. In the case of Na, it is hardly adsorbed onto illite and halloysite (Fig. 4b). Goethite rather got separated a lot, and zeolite showed a high adsorption rate only after 2 days and 1 week of the reaction although zeolite also got separated a lot 1 hour and 2 weeks later. Despite the low hydration energy, Na has a low degree of adsorption because it has a lower affinity, compared with other general metal elements22).

In the case of K, it showed a high adsorption rate, except onto zeolite (Fig. 4c). K was readily adsorbed in the order of halloysite>goethite>illite>zeolite. This order remained unchanged with time. Zeolite had a low adsorption rate and large variation, and other minerals maintained almost a constant adsorption rate after 1 hour and the adsorption no longer occurred. In the case of a monovalent cation, the relative selectivity of absorption is generally more approximate to the surface with a smaller hydration radius so as to obtain a stronger bond, but this adsorption selectivity predicted by the hydration radius was not observed in the experiment. This is probably

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because, under a strong acidic environment, it should compete with different elements for the same adsorption site. Also, it is known that the above-stated order is not well applied when oxides are concerned14). The adsorption of K onto illite is poorer than the adsorption onto halloysite or goethite. It is understood that a very low imbalance of layer charge of illite from the aspect of crystal chemistry no longer requires additional cations (especially K) that are necessary to achieve the charge balance.

Fig. 4. Removal percent of alkali metals with time (a) Li, (b) Na, and (c) K. 3.3 Comparison of the adsorption behavior of each mineral

On the whole, halloysite has the highest

adsorption rate, followed by zeolite>illite, goethite. The degree of adsorption of illite after 1 hour was in the order of As>K>Pb>Fe>Cu>Na>Zn>Cd, and the concentration of Co, Ni, Cs, Li, Cr or the like increased in the reaction solution. Except for Fe, As, Zn, Na, K, Ca or the like, the minerals have irregular adsorption rates with time and the rates are somewhat poor.

Although there are few reports about the study of the adsorption behavior of halloysite, since halloysite has a similar crystal structure to that of kaolinite, permanent charge is likely to occur mainly at the tetrahedral site. However, since a permanent charge that is generated by isomorphous substitution of Al with Si is very small, the adsorption by such a permanent charge is expected to be insignificant. Rather, it is understood that such a high adsorption rate is obtained by the crystallographical features described above. However, for a more quantitative analysis, an adsorption modeling calculation technique might need to be applied later, based on measurement data of the surface charge, specific surface and cation exchange capacity. In the case of halloysite, most elements have a steadily increasing adsorption rate onto halloysite with an increase in the reaction time. After 1 hour of the reaction, the adsorption order was as follows: Cu>K>Pb>Li>Fe>Cr>Zn>As>Cd, with Co, Ca Na being separated instead.

In the case of zeolite, the degree of adsorption of illite after 1 hour was in the order of Pb>Cs>Fe>Cr>Ge>As>K>Se>Zn>Mn>Cd>Ca>Ni, and the concentration of Cu, Co, Mg, Li or the like, not having been adsorbed onto minerals, increased in a solution. Afterwards, the adsorption rate continuously varies with the reaction time. As, Cu, Cr, Fe, Ge, Pb and the like are elements that have a high adsorption rate overall throughout the entire reaction time, and Ba, Al, Cd, Co, Li, Mg, Mn and the like are elements that have a low adsorption rate. Elements with large variations in the adsorption and separation rates are Ba, Cu, Cs, K, Na and the like. The selectivity of adsorption onto zeolite differs by experimental condition. It was reported in Ames23) that the selectivity follows

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the order of Cs>Rb>K>NH4>Ba, Mg>Sr, Li>Na>Ca>Fe>Al. Meanwhile, according to the experiment results on clinoptilolite, the selectivity of adsorption within 24 hours was in the order of Pb>Cd>Zn>Cu>Ni>Fe>Cr24). It was found out that the selectivity of adsorption described above remains unchanged even in a solution where different metals are mixed at an equal ratio. This phenomenon can be explained by the selectivity of adsorption according to the hydration energy. That is to say, in the case of adsorption selectivity of Pb>Cd>Zn>Cu, Cu has the highest hydration energy, thereby preferring a solution state22). In the experiment, the order was similar to the adsorption selectivity in Loizidou24). Zeolite is an element that determines whether a cation is suitable for a particular structure having a certain ion size and channel dimensions. However, in the experiment, the adsorption effect by the hydration radius of a channel and ion was not evident. This is probably because even the activities of different elements that were originally present in a solution are also affected. Zeolite is also known to cause reversible reactions often. According to Loizidou24), because of the hysteresis effect, the ion exchange reaction of zeolite is reversible although it looks non-reversible. For example, the substitution with a transitional metal ion in a small cage occurs readily with a longer reaction time and at a higher temperature. Meanwhile, the reversible reaction occurs at a lower temperature and with a shorter reaction time, eventually creating the hysteresis effect24). Therefore, the behavior of some elements that are separated with time seems to be similar to this phenomenon.

In the case of goethite, the degree of adsorption of illite after 1 hour was in the order of Fe>As>K>Ge>Pb>Se>Cu>Zn>Ca, and the concentration of Al, Ba, Co, Li, Mg, Mn, Na, Ni or the like increased in a solution instead. As, Fe, Pb, Se, Ge, K, Zn and Cu are elements that have a high adsorption rate overall, and Ba, Cs, Mg, Mn, Ca, Cd and the like are elements that have a low adsorption rate. Elements with large variations in the adsorption and

separation rates are Cr, Al and the like. In the meantime, Ag, Al, Co, Cr, Li, NA, Ni are not adsorbed at all, but separated. In the case of goethite, its adsorption order having been reported varies slightly by experiment. For example, Cu>Pb>Zn>Co>Ni>Mn 25), and Cu>Zn>Co, Cd 26) have been observed. Such adsorption behavior does not necessarily coincide with the adsorption order that is obtained by the experiment, and it is believed that the adsorption order is a phenomenon consistently occurring from a strong acidic area to a weak alkali area.

In this study, the selectivity of adsorption of elements is substantially similar to the order of affinity that is predicted based on the ion charge. In the case of a monovalent cation, the relative selectivity of adsorption is generally more approximate to the surface with a smaller hydration radius so as to obtain a stronger bond, but this adsorption selectivity predicted by the hydration radius is not applicable, especially in the case of oxides14). The inconsistency in the adsorption selectivity is probably because the electric properties are not predominant in an adsorbed metal bond27) and because the dissociation constant of each metal element and the initial concentration of each element in a solution control the adsorption behavior in the case of using a multi-component aqueous solution is used as a reaction solution as in this experiment.

4. Conclusion 1. An adsorption reaction with a metal element onto main minerals forming the soil shows diverse behaviors for each mineral. On the whole, halloysite had the most effective adsorption capacity. 2. The adsorption reaction proceeds at a rate similar to that of the initial state, without causing any change in the chemical species of individual elements. 3. Fe and As have the highest adsorption rate onto every mineral species.

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Address of Thanks

We would like to acknowledge that part of

this study could be possible with the help of the post doctoral research fellowship program of Korea Science and Engineering Foundation (1997) for Chang-Oh Choo who is a senior author of this study, and we deeply appreciate all help and advice from committee members to make this perfect.

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References

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