Published in J. Environ. Qual. 33:1703-1708 (2004).
© ASA, CSSA, SSSA
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TECHNICAL REPORTS
Heavy Metals in the Environment
Kinetics of Chromate Adsorption on Goethite in the Presence of Sorbed Silicic Acid
Stephanie M. Garman,
Todd P. Luxton and
Matthew J. Eick*
Department of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061
* Corresponding author (eick{at}vt.edu).
Received for publication February 26, 2004.
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ABSTRACT
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The adsorption of chromate on mineral surfaces has received much attention due to its toxicity in natural systems. Spectroscopic studies have demonstrated that chromate forms inner-sphere complexes on variable-charge surfaces. However, in natural systems chromate has been observed to be fairly mobile, which has been explained by the presence of naturally occurring ligands competing with chromate for mineral surface sites. Silicic acid is a ubiquitous ligand in soil and water environments and also sorbs strongly to variable-charge surfaces. Yet little research has examined its influence on chromate adsorption to variable-charge surfaces such as goethite. This study examined the influence of silicic acid (0.10 and 1.0 mM) on the adsorption kinetics of chromate (0.05 and 0.10 mM) on goethite over a range of common soil pH values (4, 6, and 8). The rate and total quantity of chromate adsorption decreased in all the experiments except at a pH value of 4 and a chromate concentration of 0.05 mM. The inhibition of chromate adsorption ranged from 3.1% (pH = 4, Si = 0.10 mM, chromate = 0.10 mM) to 83.3% (pH = 8, Si = 1.0 mM, chromate = 0.05 mM). The rate of chromate adsorption decreased with an increase in pH and silicic acid concentration. This was attributed to a reduction in the surface potential of goethite on silicic acid adsorption as well as a competition for surface sites. The presence of naturally occurring ligands such as silicic acid may be responsible for the enhanced mobility of chromate in natural systems and demonstrates the importance of competitive adsorption for evaluating the mobility of trace elements.
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INTRODUCTION
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IN NATURAL SYSTEMS, the bioavailability of trace element oxyanions is primarily controlled by adsorption–desorption reactions at the mineral–water interface (McBride, 1994). Numerous studies have investigated these reactions using both macroscopic and microscopic techniques. Recently, advanced spectroscopic techniques have been used to ascertain oxyanion adsorption mechanisms at the mineral–water interface (Hsia et al., 1993; Sun and Doner, 1996; Waychunas et al., 1996; Fendorf et al., 1997; Abdel-Samad and Watson, 1997; Manning et al., 1998; Winja and Schulthess, 2002). Most of these studies have focused on determining the type of surface complex formed by the oxyanion (inner- vs. outer-sphere complex). Inner-sphere complexes are expected to be much less bioavailable than outer-sphere complexes. However, spectroscopic evidence has shown that chromate forms inner-sphere complexes on goethite (Hsia et al., 1993; Fendorf et al., 1997) yet macroscopic studies have demonstrated that chromate is fairly mobile in the environment (Stollenwerk and Grove, 1985; Zachara et al., 1989; Selim et al., 1989). One explanation for this enhanced mobility is the presence of naturally occurring ligands (e.g., sulfate, dissolved organic and inorganic carbon) that can compete with chromate for sorption sites on mineral surfaces (Stollenwerk and Grove, 1985; Zachara et al., 1989; Mesuere and Fish, 1992). However, none of these studies examined the influence of silicic acid on the adsorption–desorption of chromate.
Silicic acid (H4SiO4) is ubiquitous in natural systems and has been shown to chemisorb to Fe-oxides (Herbillon and Tran Vinh An, 1969; Vempati et al., 1990; Hansen et al., 1994). Its concentration in soils and water ranges from 0.054 to 0.380 mM (5 to 35 ppm as H4SiO4) with levels as high as 0.814 mM (75 ppm) (Elgawhary and Lindsay, 1972; Iler, 1979). However, there is very little research examining its influence on oxyanion adsorption to Fe-oxides (Swedlund and Webster, 1999; Meng et al., 2000; Waltham and Eick, 2002). Waltham and Eick (2002) examined the influence of silicic acid on the adsorption kinetics of arsenite and arsenate on goethite. They found that silicic acid reduced the rate and total quantity of arsenite adsorbed to goethite. However, only the rate and not the total quantity of arsenate was reduced. It is hypothesized that silicic acid will have a similar but greater effect on the adsorption of chromate on goethite. Chromate has a smaller shared charge compared with arsenite and arsenate, creating a weaker bond on adsorption (McBride, 1994). Moreover, chromate will exhibit a steeper adsorption edge compared with arsenate (i.e., reduced adsorption at near-neutral pH values) (Grossl et al., 1997). To more fully understand the potential bioavailability and toxicity of chromate in natural systems it is necessary to evaluate its adsorption behavior in the presence of common soil ligands. Accordingly, the objective of this research is to examine the kinetics of chromate adsorption in the presence of adsorbed silicic acid over a range of pH values and silicic acid concentrations common in natural systems.
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MATERIALS AND METHODS
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Oxide Characterization
Goethite used in the adsorption experiments was synthesized by oxidation of ferric nitrate [Fe(NO3)3] (Fisher Scientific, Hampton, NH) using a method described by Schwertmann and Cornell (1991). The procedure was altered slightly by slowly adding 4 M NaOH (Fisher) during titration to pH 12, to achieve a higher specific surface area. Excess salts from the hydrolysis reaction were removed by dialysis until conductivity of the wash solution was near that of distilled, doubly deionized water (Millipore, Billerica, MA) exposed to the atmosphere. All solutions were prepared with distilled, doubly deionized water and reagent-grade chemicals, and contact with glass surfaces was avoided to prevent silica contamination. The goethite was washed with 0.40 M HNO3 (Fisher) for 1 h to remove short-ordered phases, centrifuged to separate colloidal goethite crystals, resuspended, and redialized. The clean goethite was freeze-dried for storage before adsorption experiments. The identity and purity of the goethite sample was confirmed by X-ray diffraction (XRD) (XDS 2000; Scintag, Sunnyvale, CA), differential scanning calorimetry (DSC) (DSC 2910; TA Instruments, New Castle, DE), thermogravimetric analysis (TGA) (Hi-Res TGA 2950; TA Instruments), field emission scanning electron microscopy (FESEM) (LEO 1550 FE-SEM; LEO Electron Microscopy, Thornwood, NY), and ammonium oxalate (Fisher) in the dark to total iron ratio (FeO/FeT) (Schwertmann and Cornell, 1991). Total Fe was determined using the citrate–bicarbonate–dithionite (Fisher) method (Loeppert and Inskeep, 1996). X-ray diffraction, TGA, and DSC patterns were consistent with those presented in Schwertmann and Cornell (1991) and a goethite standard from Bayer (Krefeld, Germany). Scanning electron microscopy images indicated that synthetic goethite colloids were of uniform shape and size, and that the crystals were similar to those found in natural environments (Schwertmann and Cornell, 1996). Crystals were euhedral acicular crystals approximately 200 nm in length and 25 nm in diameter. The ammonium oxalate in the dark to total iron ratio (0.28%) indicated minimal amounts of amorphous or short-ordered crystals. Specific surface area was 72.75 m2 g–1 as determined by a five-point N2 Brunauer–Emmett–Teller (BET) gas adsorption isotherm using an ASAP 2010 surface area analyzer (Micromeritics, Norcross, GA). The point of zero charge as determined by the isoelectric point (IEP) was 9.56.
Adsorption Kinetics
Adsorption kinetics of chromate (0.05 and 0.10 mM) and silicic acid (0.10 and 1.0 mM) were examined as a function of pH (4, 6, and 8) 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 approximately 2 min using a sonic dismembrator (ultrasonic disperser) (Fisher). 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 Instruments, Westbury, NY). When the pH stabilized, the suspension volume was brought to 400 mL, less the quantity of silicic acid or chromate to be added. The silicic acid was added from a 0.038 M sodium silicate stock solution in 0.10 M NaOH to prevent polymerization (Iler, 1979). The chromate was added from a 0.261 M sodium stock solution. All stock solutions were made from reagent-grade sodium salts (Fisher). 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 chromate or Si adsorbed to the goethite surface. For the adsorption kinetics of chromate in the presence of silicic acid the same procedure was used; however, the chromate was not added until the adsorption of silicic acid had reached a steady state (approximately 60 h). Chromate and Si were analyzed using a SpectroFlame FTMOA85D inductively coupled plasma atomic emission spectrometer (ICP–AES) (Spectro Instruments, Fitchburg, MA). Adsorbed chromate and Si were calculated by the difference between the sample solution concentration and the initial quantity added. 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 adsorption kinetic data are presented.
Zeta Potential Experiments
Zeta potential data were collected over the entire pH range (3–11) for the pure goethite surface and after silicic acid adsorption (0.10 and 1.0 mM) and for selected pH values (4, 6, and 8) for chromate adsorption (0.05 and 0.10 mM). The suspensions for zeta potential measurements were prepared using the methods previously described for the adsorption kinetic studies. Zeta potentials were determined from microelectrophoresis measurements on a Zetasizer 3000Hsa (Malvern Instruments, Southborough, MA). Based on preliminary data and particle size, the voltage applied to the capillary cell was set at 100 V and a Henry function [
(Ka)] of 1.5 was used in calculating the zeta potential. The pH of each 10-mL sample was measured before zeta potential measurement to account for any drift due to adsorption of atmospheric CO2. The electrophoretic capillary cell was rinsed with 50 mL of 0.001 M HNO3 and distilled, doubly deionized water before each analysis. Five independent zeta potential measurements were collected for each sample to ensure accurate and reproducible data.
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RESULTS AND DISCUSSION
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Adsorption Kinetics
The adsorption of chromate on goethite was rapid and essentially complete within 2 h (Fig. 1). The rate of chromate adsorption decreased with an increase in pH and this rate decrease was more pronounced at the higher initial chromate solution concentration (0.10 mM). Furthermore, the quantity of chromate adsorbed to the goethite surface decreased with increasing pH (Table 1). These results can be explained by the degree of protonation of the oxyanion molecule. Chromate is a diprotic acid and will exhibit a much steeper adsorption edge (adsorption as a function of pH) compared with a triprotic acid such as arsenate. This is because the adsorption of weak acid oxyanions is strongest at the pH values near their acid dissociation constants (chromate pK1 = 0.74; pK2 = 6.49) (McBride, 1994). These results are consistent with other studies examining oxyanion adsorption on goethite (Ainsworth et al., 1989; Manning and Goldberg, 1996; Grossl et al., 1997; Waltham and Eick, 2002).
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Fig. 1. Kinetics of chromate adsorption on goethite as a function of pH. Ionic strength (I) = 0.01 M and goethite suspension = 1.0 g L–1.
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Table 1. Quantity of 0.05 and 0.10 mM chromate [Cr(VI)] and silicic acid sorbed and desorbed as a function of pH.
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In contrast to chromate, silicic acid adsorption was characterized by biphasic kinetics: a rapid adsorption reaction followed by a slower adsorption reaction (Fig. 2). Additionally, the quantity of silicic acid adsorbed increased with an increase in pH, which is related to the relatively high pK1 value for silicic acid (pK1 = 9.2). Similar results have been observed by other researchers examining silicic acid adsorption on Fe-oxides (Hingston et al., 1972; Hansen et al., 1994; Swedlund and Webster, 1999; Waltham and Eick, 2002). Biphasic kinetics have been observed for phosphate as well as numerous trace metal cations and have been attributed to many processes including surface precipitation, intra- and interparticle diffusion, and change in the type of surface complex (Barrow, 1983; van Riemsdijk et al., 1984; McBride, 1994; Eick et al., 1999). Silicic acid surface-catalyzed polymerization on Fe-oxide surfaces may be responsible for the slow adsorption reaction (Swedlund and Webster, 1999). However, attempts to determine the presence of polymerized species using Fourier transform infrared spectroscopy (FTIR) were inconclusive.
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Fig. 2. Kinetics of silicic acid adsorption on goethite as a function of pH. Ionic strength (I) = 0.01 M and goethite suspension = 1.0 g L–1.
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Zeta Potential
Zeta potentials were measured on selected samples to determine the influence of adsorbed silicic acid and chromate on the goethite's surface potential (Fig. 3 and Table 2). At low pH values the adsorption of silicic acid had little effect on the surface potential of goethite. As pH increased above 6, adsorbed silicic acid reduced the surface potential of goethite and this reduction in surface potential increased with an increase in silicic acid concentration due to the increased deprotonation of the silicic acid molecule. In the absence of adsorbed silicic acid the goethite had a point of zero charge (PZC) of 9.56. The PZC was reduced to 8.80 and 7.45 in the presence of 0.10 and 1.0 mM silicic acid, respectively. Chromate adsorption had a similar effect; however, the goethite surface potential was reduced at all pH values investigated. Similar results were obtained by other researchers examining the adsorption of silicic acid and chromate on the PZC of Fe-oxides (Schwertmann and Fechter, 1982; Hsia et al., 1993; Waltham and Eick, 2002). Zeta potential data suggest that both chromate and silicic acid are specifically adsorbed to the goethite surface, which is consistent with spectroscopic results (Vempati et al., 1990; Fendorf et al., 1997). The specific adsorption of protolyzable anions on mineral surfaces can create new functional groups that can undergo protonation–deprotonation reactions, which alter the mineral's surface potential (Anderson and Malotky, 1979). The greater effect of chromate on the surface potential of goethite at low pH values compared with silicic acid can be related to the pK1 value of the oxyanion (pK1 = 0.74 chromate and pK1 = 9.49 silicic acid).
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Fig. 3. Zeta potential of goethite in the presence and absence of sorbed silicic acid. Ionic strength (I) = 0.01 M and goethite suspension = 1.0 g L–1.
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Kinetics of Chromate in the Presence of Silicic Acid
The adsorption of silicic acid reduced the rate and total quantity of chromate adsorbed at all pH values and silicic acid concentrations except 0.10 mM silicic acid at a pH of 4 (Table 1, Fig. 4 and 5). The inhibition of chromate adsorption ranged from 0 to 83.3 and 3.1 to 63.9% for 0.05 and 0.10 mM chromate, respectively (Table 1). Inhibition was less at pH 4 which is due to the strength of the chromic acid. Maximum adsorption of chromate and silicic acid occur near their respective pK1 values; hence, the inhibition of chromate adsorption by silicic acid would be expected to be greatest near the pK1 value for silicic acid. Furthermore, the rate of chromate adsorption decreased with increasing pH. This is due to the deprotonation of the chromate molecule and the reduction in the surface potential of goethite by the adsorption of silicic acid. At a pH of <6 the adsorption of silicic acid had a negligible effect on the goethite surface potential. As pH increases the adsorption of silicic acid reduces the goethite surface potential. This reduction in the goethite surface potential coupled with the deprotonation of the chromate molecule creates unfavorable electrostatics, which reduces the rate of chromate adsorption (Waltham and Eick, 2002).
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Fig. 4. Kinetics of chromate (0.05 mM) adsorption on goethite in the presence and absence of sorbed silicic acid. (a) pH 4, (b) pH 6, (c) pH 8. Ionic strength (I) = 0.01 M and goethite suspension = 1.0 g L–1.
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Fig. 5. Kinetics of chromate (0.10 mM) adsorption on goethite in the presence and absence of sorbed silicic acid. (a) pH 4, (b) pH 6, (c) pH 8. Ionic strength (I) = 0.01 M and goethite suspension = 1.0 g L–1.
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In all experiments the molar ratio of chromate adsorbed to silicic acid desorbed is >1 (Table 1). Therefore, a greater quantity of chromate is adsorbed compared with silicic acid desorbed demonstrating the presence of excess surface sites for oxyanion adsorption. This is consistent with the concentration of singly coordinated hydroxyls on goethite (5.73 µmol m–2) based on the predominant terminal planes of goethite (110 and 021) (Barron and Torrent, 1996). The singly coordinated hydroxyls are considered the reactive functional groups present on the goethite surface for oxyanion adsorption (Hiemstra and van Riemsdijk, 1996).
The above results are similar to those observed by Waltham and Eick (2002), who examined the kinetics of arsenite and arsenate adsorption on goethite in the presence of silicic acid. However, in the case of arsenate, silicic acid adsorption reduced only the rate and not the total quantity of arsenate adsorbed. Although both arsenate and chromate have been observed to form inner-sphere complexes with goethite, arsenate appears to form stronger associations with the goethite surface compared with chromate (Grossl et al., 1997). One would expect that naturally occurring ligands would reduce both the rate and quantity of chromate adsorbed to mineral surfaces, making it fairly mobile in natural systems. This is in agreement with the results of other researchers who found that chromate adsorption was reduced in the presence of sulfate, dissolved inorganic carbon, and dissolved organic carbon (Stollenwerk and Grove, 1985; Zachara et al., 1989; Mesuere and Fish, 1992). Stollenwerk and Grove (1985) examined the adsorption–desorption of chromate from an alluvial aquifer in the presence of naturally occurring ground water (pH = 6.8). Chromate adsorption was inhibited and desorption was rapid from the alluvial minerals. This was attributed to the presence of sulfate and inorganic carbon in the naturally occurring ground water. However, the researchers did not report silicic acid concentration of the ground water. Similar to chromate, sulfate is a diprotic acid whose adsorption on variable-charge surfaces decreases with an increase in pH (Zhang and Sparks, 1990; He et al., 1997). At a pH value of 6.8 one would expect little adsorption of sulfate on the Fe-oxide-coated alluvium. Therefore, silicic acid may have been a more important factor reducing chromate adsorption to the aquifer minerals than sulfate at the ambient pH of the ground water (6.8).
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CONCLUSIONS
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The results from the above experiments demonstrate that naturally occurring ligands such as silicic acid can reduce the rate and quantity of chromate adsorbed to variable-charge surfaces such as goethite. It is proposed that a reduction in the surface potential of goethite on silicic acid adsorption as well as a competition for surface sites are responsible for the observed results. Numerous studies have employed spectroscopic techniques to ascertain the type of surface complex (inner vs. outer sphere) formed by trace metal contaminants on mineral surfaces. While this information is important it is not sufficient for evaluating trace metal contaminants' potential bioavailability and toxicity in the biosphere. Numerous naturally occurring organic and inorganic ligands may reduce the adsorption of trace metal contaminants in natural systems thereby increasing their potential bioavailability. At near-neutral pH values naturally occurring ligands such as silicic acid may be responsible for the fairly mobile behavior of chromate in soil and subsurface environments observed by some researchers (Stollenwerk and Grove, 1985; Zachara et al., 1989). Further studies are necessary to thoroughly examine the influence of naturally occurring organic and inorganic ligands on the sorption behavior of toxic oxyanions such as chromate.
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ABSTRACT
INTRODUCTION
