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DIVISION S-2—SOIL CHEMISTRY

Kinetics of Arsenic Adsorption on Goethite in the Presence of Sorbed Silicic Acid

Dep. of Crop, Soil, and Environmental Science, 236 Smyth Hall, Virginia Tech, Blacksburg, VA 24061

* Corresponding author (eick{at}vt.edu)

	ABSTRACT

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The potential toxicity and availability of As in the environment is dependent on several factors including redox potential, pH, and the presence of ligands that can compete for adsorption sites on mineral surfaces. Silicic acid is a ligand ubiquitous in natural systems and strongly chemisorbs to Fe oxides. However, there are relatively few studies examining its influence on As adsorption on Fe oxides. This study examined the influence of silicic acid (0.10 and 1.0 mM) on the adsorption kinetics of arsenite and arsenate (0.10 mM) on goethite over a range of common soil pH values (4, 6, and 8). The rate of arsenic (III and V) and silicic acid adsorption was greatest at pH values near their pK₁ value. However, silicic acid sorption was characterized by biphasic kinetics with rapid adsorption followed by a much slower adsorption reaction. The rate and total quantity of arsenite adsorption decreased in the presence of silicic acid at all pH values and concentrations of silicic acid. Approximately 40% less arsenite was adsorbed in the presence of 1.0 mM silicic acid at all pH values. At 0.10 mM, silicic acid had less of an effect on arsenite adsorption. In contrast, only the rate and not the total quantity of arsenate was reduced in the presence of silicic acid. The rate of arsenate adsorption decreased as pH and silicic acid concentration increased. This was attributed to a decrease in the goethite's surface potential upon specific adsorption of silicic acid and deprotonation of the arsenate molecule creating an unfavorable electrostatic field. These results demonstrate the importance of evaluating As speciation, reaction kinetics, and the influence of naturally occurring ligands on the adsorption of As on variable charge surfaces.

Abbreviations: FESEM, field emission scanning electron microscopy • PZC, point of zero charge • TGA, thermal gravimetric analysis • XRD, x-ray diffraction

	INTRODUCTION

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ARSENIC IS A POTENTIALLY toxic trace element that is widely dispersed in the environment. Its high toxicity and increased appearance in the biosphere has triggered both public and political concern. For example, the public and political debate continues over the USEPA's current As drinking water standard of 50 ppb. Unusually high concentrations of As have been introduced into the biosphere through the past usage of arsenical pesticides [Pb₃(AsO₄)₂)], mining-related activities, atmospheric fallout, and through natural geologic processes. The two most commonly occurring forms of As in the environment are arsenate [AsO^3-₄, As(V)] and arsenite [AsO^3-₃, As(III)]. In general arsenite is considered to be the more mobile and more toxic species in the environment (Tamaki and Frankenberger, 1992). Adsorption of As onto mineral surfaces is an important process that will affect its potential bioavailability in natural systems. In particular, Fe oxides have been shown to have a high affinity for As since both arsenite and arsenate are strongly chemisorbed (Sun and Doner, 1996; Grossl et al., 1997; Fendorf et al., 1997). The potential bioavailability and toxicity of As in the environment depends upon its chemical form as well as numerous chemical, physical, and biological factors including pH, mineralogy, redox potential, microbial populations, and the presence of ligands that can compete for adsorption sites on mineral surfaces (Tamaki and Frankenberger, 1992; Grafe et al., 2001).

A great deal of research has been conducted examining the adsorption of As on Fe oxide surfaces using both spectroscopic and kinetic approaches (Waychunas et al., 1993; Sun and Doner, 1996; Waychunas et al., 1996; Grossl et al., 1997; Fendorf, 1997; Manning et al., 1998). Grossl et al. (1997) employed a pressure-jump relaxation kinetic technique to examine As(V) adsorption on goethite and proposed that the adsorption of arsenate to goethite involved a two-step ligand exchange reaction by which an inner-sphere bidentate surface complex is formed. Fendorf et al. (1997) examined As(V) adsorption on goethite using extended x-ray absorption fine structure spectroscopy (EXAFS) and determined that As(V) forms a mixture of bidentate and monodentate surface complexes which depended on surface loadings. Sun and Doner (1996) used Fourier transformed infrared spectroscopy (FTIR) and established that As(V) and As(III) replaced two singly coordinated surface OH groups on goethite to form binuclear bridging complexes. However, less research has examined the influence of competing ligands on As adsorption reactions (Xu et al., 1988; Manning and Goldberg, 1996a, b; Swedlund and Webster, 1999; Grafe et al. 2001; Liu et al., 2001). Natural systems are composed of a myriad of organic and inorganic ligands that strongly sorb to Fe oxide surfaces. Depending on their relative concentration in the environment and their affinity for Fe oxide surfaces, these ligands may have a profound effect on As adsorption. For example, Grafe et al. (2001)(2002) examined the adsorption of As(V) and As(III) on goethite and ferrihydrite in the presence of several naturally occurring organic ligands. They found that both As(V) and As(III) adsorption may be decreased which depended on the pH of the system, the type of Fe oxide, and the type of organic ligand.

Silicic acid is another ligand that is ubiquitous in natural systems and strongly chemisorbs to Fe oxides (Herbillon and Tran Vinh An, 1969; Vempati et al., 1990). Silicic acid concentrations in soils and natural waters range from 0.054 to 0.380 mM (5 to 35 ppm) with some as high as 0.814 mM (75 ppm) (Elgawhary and Lindsay, 1972; Iler, 1979). However, there is little research examining its influence on As adsorption to Fe oxides and these available studies only examine the influence of silicic acid from an equilibrium standpoint (Swendlund and Webster, 1999; Meng et al., 2000). To fully understand the dynamic interactions of ligands with mineral surfaces, a knowledge of reaction kinetics is important because soils and sediments are seldom, if ever, at equilibrium (Sparks, 1995). An accurate assessment of the potential mobility and toxicity of As in natural systems requires a thorough understanding of its adsorption behavior in the presence of common soil ligands. Accordingly, the objective of this study is to examine the influence of silicic acid on the adsorption kinetics of As on goethite over a range of pH values common in natural environments. Goethite was chosen because it is one of the most abundant Fe oxides found in the environment worldwide and has a high affinity for trace elements (Schwertmann and Cornell, 1996).

	MATERIALS AND METHODS

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Oxide Characterization
The goethite used in these experiments was synthesized based on the method described by Schwertmann and Cornell (1991) using reagent-grade Fe(NO₃)₃. The procedure was altered slightly by slowly adding 4 M NaOH to achieve a higher specific surface-area goethite. Excess salts were removed by electrodialysis until the conductivity of the wash solution was nearly equal to that of distilled deionized water. All solutions were prepared with double-deionized water and contact with glassware was avoided to prevent silica contamination. The goethite was subsequently washed with 0.01 M HNO₃ for 1 h to remove any remaining short-range ordered Fe phases, centrifuged to separate the colloidal goethite crystals, resuspended, and redialyzed. The clean goethite was freeze-dried for storage prior to adsorption experiments.

The identity and purity of the goethite sample was verified by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermal gravimetric analysis (TGA), and ammonium oxalate in the dark to total Fe (Fe_o/Fe_t) (Schwertmann and Cornell, 1991). X-ray diffraction and TGA patterns were consistent with those presented in Schwertmann and Cornell (1991) and no other crystalline phases were detected. Field emission scanning electron microscopy indicated euhedral acicular crystals similar to those found in natural environments (Schwertmann, and Cornell, 1996). The oxalate in the dark to total Fe ratio (Fe_o/Fe_t) was 0.23%, indicating essentially complete conversion to goethite (Schwertmann et al., 1985). Total Fe was determined using the citrate-bicarbonate-dithionite method (Loeppert and Inskeep, 1996). The specific surface area was 87 m² g^-1 as determined by a five-point N₂ Brunauer-Emmett-Teller (BET) gas adsorption-isotherm method.

Adsorption Kinetics
Adsorption kinetics of As(III,V) and silicic acid were examined as a function of pH (4, 6, and 8) and concentration (0.10 mM and 1.0 mM) at constant ionic strength (0.01 M NaNO₃) and adsorbent concentration (1.0 g L^-1). All experiments were conducted using a batch technique in a flat-bottomed, teflon-lined, water-jacketed reaction vessel (500 mL) covered with a glass lid containing ports for a stirrer, a pH electrode, N₂ gas, a burette tip, and a sample pipette. An appropriate quantity (0.40 g) of freeze-dried goethite was carefully weighed into a 500-mL teflon liner and 350 mL of 0.01 M NaNO₃ was added. The suspension was dispersed for 2 min using a sonic dismembrator. The teflon liner was placed in a jacketed reaction vessel and mixed at 300 rpm with a three-bladed impeller and sparged with N₂ gas to eliminate CO₂ effects throughout the experiment. The suspension was allowed to hydrate for a minimum of 24 h and adjusted to the appropriate pH value using a Brinkman 716 Stat-Titrino pH-stat (Brinkman Instuments, Westbury, NY). When the pH stabilized, the suspension volume was brought to 400 mL, less the quantity of silicic acid or As to be added. The silicic acid was added from a 0.0166 M sodium silicate stock solution in 0.10 M NaOH to prevent polymerization (Iler, 1979). The arsenite and arsenate were added from 0.1010 and 0.0714 M Na stock solutions, respectively. All stock solutions were made from reagent-grade Na salts. Sampling began immediately after addition of the stock solutions and continued until a steady-state was reached. A steady-state was determined to be the time when there was little change in the quantity of As or Si adsorbed to the goethite surface. For the adsorption kinetics of As(III,V) in the presence of silicic acid the same procedure was used, however, the arsenic was not added until the adsorption of silicic acid had reached a steady state (60 h). Arsenic and Si were analyzed using a SpectroFlame FTMOA85D inductively coupled plasma atomic emission spectrometer (ICP-AES) (Spectro Analytical Instruments, Fitchburg, MA). Various rate equations (e.g., parabolic, single first order, two first order) were applied to the kinetic data to obtain rate coefficients. However, none of these equations adequately described all our experimental data making rate comparisons between experiments difficult. Therefore, only the rate data are presented.

Zeta Potential Experiments
Zeta potential data was measured on selected samples from the adsorption experiments using a Malvern Zetasizer 3000 HS (Malvern Instruments, Southborough, MA). Samples were removed from the pH-stat with an electronic pipette and stored in N₂-purged 50-mL centrifuge tubes prior to analysis. The pH of each sample was measured prior to the Zeta potential measurements to account for any drift because of absorption of atmospheric CO₂. The sample cell was purged with 100 mL of ultra pure water between each sample and standards were run after every 10 samples.

	RESULTS AND DISCUSSION

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Adsorption Kinetics
Both arsenite and arsenate adsorption on goethite were rapid and essentially complete within 2 h (Fig. 1) . The rate of As(III) adsorption decreased (Fig. 1a) while the rate of As(V) adsorption increased (Fig. 1b) with a decrease in pH. These differences were related to the acid dissociation constant for the As species and their affinity for the goethite surface. In general, maximum adsorption of weak acid oxyanions occurs at their pK₁ values (Hingston et al., 1972). Arsenious acid has a pK₁ value of 9.29 while arsenic acid has a pK₁ value 2.24. Therefore, their greatest affinity for the goethite surface should be near these pH values. Similar to arsenite, silicic acid has a pK₁ value of 9.49 and the rate of adsorption also decreased with a decrease in pH (Fig. 2) . Furthermore, a smaller quantity of silicic acid was adsorbed to the goethite surface as pH was reduced while arsenite and arsenate adsorption was nearly constant over the pH range (Tables 1 and 2). The quantity of Si adsorbed increased as the adsorptive concentration increased from 0.10 to 1.0 mM and was similar to that observed by others (Hingston et al., 1972; Hansen et al., 1994). However, the rate of Si adsorption was similar at 0.10 and 1.0 mM. In contrast to As, silicic acid adsorption was characterized by biphasic kinetics; a rapid adsorption reaction followed by a much slower adsorption reaction. Similar results were obtained by others examining silicic acid adsorption on Fe oxides (Saito and Shoji, 1984; Hansen et al., 1994; Swedlund and Webster, 1999).

View larger version (14K): [in this window] [in a new window]   Fig. 1. Kinetics of As adsorption on goethite as a function of pH, (a) As(III), (b) As(V). As = 0.10 mM, ionic strength (I) = 0.01 M, and goethite suspension of 1.0 g L-1.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Kinetics of silicic acid adsorption on goethite as a function of pH. Ionic strength (I) = 0.01 M, and goethite suspension of 1.0 g L-1.

Zeta Potential
Zeta potentials were measured on selected samples to determine the influence of adsorbed silicic acid on goethite surface potential (Table 3). At a pH value of 4 the adsorption of silicic acid had little effect on the surface potential of goethite. At a pH value of 6 only 1.0 mM silicic acid affected the surface potential of goethite. In this case, the goethite surface potential was reduced from 44 to 19 mV. As pH was increased to 8, silicic acid adsorption substantially reduced the surface charge of the goethite. Silicic acid reduced the surface charge of goethite from 30 to 14 and -0.3 mV in the presence of 0.10 and 1.0 mM silicic acid, respectively. Similar results were obtained by Schwertmann and Fechter (1982) who examined the influence of silicic acid on the point of zero charge (PZC) of ferrihydrite. The specific adsorption of protolyzable anions on mineral surfaces results in new surface functional groups which may undergo protonation-deprotonation reactions (Anderson and Maltoky, 1979). The greater effect of silicic acid on the surface potential of goethite as pH increased suggests that Si surface species deprotonate to the greatest extent at higher pH values which is consistent with its pK₁ value of 9.49.

View larger version (17K): [in this window] [in a new window]   Fig. 3. Kinetics of As adsorption on goethite at pH = 4 in the presence and absence of silicic acid, (a) As(III) and silicic acid, (b) As(V) and silicic acid. As = 0.10 mM, ionic strength (I) = 0.01 M, and goethite suspension of 1.0 g L-1.

View larger version (17K): [in this window] [in a new window]   Fig. 5. Kinetics of As adsorption on goethite at pH = 8 in the presence and absence of silicic acid, (a) As(III) and silicic acid, (b) As(V) and silicic acid. As = 0.10 mM, ionic strength (I) = 0.01 M, and goethite suspension of 1.0 g L-1.

View larger version (17K): [in this window] [in a new window]   Fig. 4. Kinetics of As adsorption on goethite at pH = 6 in the presence and absence of silicic acid, (a) As(III) and silicic acid, (b) As(V) and silicic acid. As = 0.10 mM, ionic strength (I) = 0.01 M, and goethite suspension of 1.0 g L-1.

Based on the above experimental results, the rate and total quantity of arsenite adsorbed on goethite was inhibited by silicic acid. Oxyanion adsorption on oxide surfaces takes place via a ligand exchange reaction with either a water or hydroxyl ligand on the goethite surface. One would expect that the removal of a silicic acid ligand from the goethite surface would be slower than the removal of a water or hydroxyl ligand (Margerum et al., 1978). The kinetic data supports this as the rate of arsenite adsorption decreased as the quantity of adsorbed silicic acid increased. Additional kinetic and spectroscopic data such as ligand exchange rates of ferric complexes and energies of activation are necessary to determine the mechanism(s) involved with the reduced rate and quantity of arsenite adsorbed in the presence of silicic acid.

Kinetics of Arsenate in the Presence of Silicic Acid
Silicic acid reduced the rate of As(V) adsorption and this rate decreased further as pH and silicic acid concentration increased (bFig. 3b–5b and Table 2). Overall, the total quantity of As(V) adsorbed remained nearly constant from pH 4 to 8 at both silicic acid concentrations. These results differ from arsenite and those obtained by Swendlund and Webster (1999). At a Si_(T)/Fe ratio of 1.8, these authors observed a reduction in As(V) adsorption only at pH values >6. However, at a Si_(T)/Fe ratio of 0.10, they observed little effect of silicic acid on As(V) adsorption. Similar to their arsenite results they attributed the reduction in As(V) adsorption at the high Si_(T)/Fe ratio to the polymerization of silicic acid on the ferrihydrite surface which effectively blocked As(V) adsorption. However, these observed results may have been caused by the slow adsorption kinetics as pH and silicic acid surface coverages increased. Similar to arsenite, upon arsenate adsorption, a concurrent desorption of silicic acid from the goethite surface occurred (Table 2). In all experiments, the molar ratio of arsenate adsorbed to silicic acid desorbed is >1 (Table 2). In the case of 1.0 mM silicic acid at a pH value of 8, arsenate adsorption is increasing with time and the molar ratio is close to 1 (Fig. 5b and Table 1). Given enough time, it is anticipated that arsenate adsorption would reach the level achieved in the absence of silicic acid (molar ratio near 1). Similar to arsenite adsorption excess surface sites are available for arsenate adsorption in all experiments except at a pH value of 8 and 1.0 mM silicic acid.

The rate of As(V) adsorption followed a similar trend to the reduction in surface potential as determined by zeta potential measurements; both were reduced as pH and silicic acid concentrations increased. Overall As(V) had a greater affinity for the goethite surface relative to silicic acid, reaching the adsorption maxima observed in the experiments in the absence of silicic acid. It appears that As(V) has a stronger preference for the goethite surface compared with silicic acid and given enough time it can out compete silicic acid for goethite surface sites. We believe that the reduced rate of As(V) adsorption observed as pH and silicic acid concentration increased was because of electrostatic effects created by a reduction in the goethite surface potential and the deprotonation of arsenate molecules as pH increased. Arsenic acid is a polyprotic acid with pKa values of 2.24, 6.77, and 11.66. At pH values >7 the majority of the arsenate molecules in solution will be divalent anions. The increase in negative charge of the arsenate molecule, coupled with a decrease in surface potential upon adsorption of silicic acid, creates unfavorable electrostatics that reduced the rate of diffusion of arsenate to the goethite surface. In contrast arsenious acid has a pK₁ value of 9.29 and would remain neutrally charged at pH values 8 which is reflected in the smaller reduction in the adsorption rate in the presence of adsorbed silicic acid. Additionally, the removal of a silicic acid molecule from the goethite surface by arsenate would be expected to be slower than the removal of a water or hydroxyl ligand (Margerum et al., 1978). This would explain the reduced rate in arsenate adsorption at 0.10 mM silicic acid and pH values below 8 where silicic acid adsorption has little effect on the goethite surface charge.

We believe that a reduction in surface potential upon adsorption of silicic acid is responsible for the slow adsorption kinetics of As(V) at alkaline pH values. To test the hypothesis, we conducted additional As(V) adsorption kinetic experiments in the absence of silicic acid at pH values of 9 and 10. These pH values were chosen because the zeta potential of the goethite surface was similar to that in the presence of 0.10 and 1.0 mM silicic acid at pH 8. In contrast to the adsorption of arsenate at pH values 8, arsenate adsorption was markedly slower and had not reached a steady state even after 75 h (Fig. 6a) . In fact the adsorption kinetics of As(V) at pH 9 and 10 were very similar to the adsorption kinetics at pH 8 in the presence of 0.10 and 1.0 mM silicic acid (Fig. 6b). This demonstrates that As(V) has a greater affinity for the goethite surface compared with silicic acid and the reduction in the rate of adsorption was because of unfavorable electrostatics created by both a decrease in the surface potential of the goethite caused by silicic acid adsorption as well as an increase in the negative charge of the arsenate molecule because of deprotonation. This also demonstrates the importance of mineral surface potential in controlling the reaction kinetics of potentially toxic protolyzable anions.

View larger version (17K): [in this window] [in a new window]   Fig. 6. (a) Kinetics of As(V) adsorption on goethite as a function of pH, (b) Kinetics of As(V) adsorption alone at pH 9 and 10 and at pH 8 in the presence of 0.10 and 1.0 mM silicic acid. As(V) = 0.10 mM, ionic strength (I) = 0.01 M, and goethite suspension of 1.0 g L-1.

	CONCLUSIONS

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	ACKNOWLEDGMENTS

 
The authors would like to thank Marc Edwards of the Department of Civil and Environmental Engineering at VA Tech for the use of his Malvern Zetasizer and Paul Grossl of Utah State University and an anonymous reviewer whose comments significantly improved the manuscript.

Received for publication July 9, 2002.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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