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Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA 92507
* Corresponding author (sgoldberg{at}ussl.ars.usda.gov)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: EM, electrophoretic mobility • EXAFS, x-ray absorption fine structure • FTIR, Fourier transform infrared spectroscopy • PZC, point of zero charge
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Of the two inorganic redox states of As, the As(III) redox state is more acutely toxic than the As(V) redox state (Penrose, 1974). At natural pHs arsenite exists in solution as H3AsO3 and H2AsO-3, since the pKa values for arsenious acid are high, pK1a = 9.2 and pK2a = 12.7. Arsenate is present as H2AsO-4 and HAsO2-4 since the pKa values for arsenic acid are pK1a = 2.3, pK2a = 6.8, and pK3a = 11.6. Because the kinetics of As redox transformations are relatively slow, both oxidation states are often found in soils regardless of the redox conditions (Masscheleyn et al., 1991). The mobility of As is a function of its oxidation state with arsenite exhibiting greater mobility through sand columns than arsenate (Gulens et al., 1979). However, under high pH conditions arsenite is more strongly bound to soil components than arsenate (Manning and Goldberg, 1997a; Raven et al., 1998; Jain and Loeppert, 2000).
Arsenic adsorption is significantly positively correlated with Al and Fe oxide and clay content of soils (Elkhatib et al., 1984a, b; Sakata, 1987; Wauchope, 1975; Livesey and Huang, 1981). Adsorption studies of arsenite and arsenate have used a wide range of adsorbents including oxides, clay minerals, and whole soils. Inorganic constituents of soils that adsorb significant amounts of As are Al and Fe oxides, clay minerals, and carbonates. Arsenate adsorption on amorphous Fe hydroxide (Pierce and Moore, 1982), goethite, gibbsite (Hingston et al., 1971; Manning and Goldberg, 1996a), hematite (Xu et al., 1988), amorphous Al hydroxide (Anderson et al., 1976), alumina (Gupta and Chen, 1978), kaolinite, montmorillonite (Frost and Griffin, 1977; Goldberg and Glaubig, 1988; Xu et al., 1988), and illite (Manning and Goldberg, 1996b) increased at low pH, exhibited maxima in the pH range 3 to 7, and decreased at high pH. Somewhat similar behavior was found for arsenite adsorption on amorphous Fe hydroxide (Pierce and Moore, 1980, 1982), amorphous Al oxide (Manning and Goldberg, 1997b), and alumina (Gupta and Chen, 1978) where adsorption increased at low pH, exhibited a peak in the pH range 7 to 8, and decreased at high pH. Arsenite adsorption on the clay minerals kaolinite, montmorillonite, and illite increased up to pH 9 (Manning and Goldberg, 1997b). Arsenate adsorption was greater than arsenite adsorption on alumina (Gupta and Chen, 1978), amorphous Fe hydroxide (Pierce and Moore, 1982), kaolinite, and montmorillonite (Frost and Griffin, 1977). At pH values above 7, arsenite adsorption was often greater than arsenate adsorption, as observed on amorphous Fe oxide (Jain and Loeppert, 2000; Goldberg and Johnston, 2001).
Accurate description of As adsorption behavior in natural systems requires determination of the bonding mechanism of As anions on mineral surfaces. Insight into anion adsorption mechanisms can be provided by both macroscopic (point of zero charge [PZC] shifts and ionic strength effects) and microscopic (spectroscopic) experimental methods. Electrophoretic mobility (EM) measures movement of charged particles in response to an applied electric field such that zero EM indicates the PZC of the particle. Shifts in PZC and reversals of EM with increasing ion concentration can be used as evidence of strong specific ion adsorption and inner-sphere surface complex formation (Hunter, 1981).
Shifts in PZC were found upon arsenate (Hsia et al., 1994; Suarez et al., 1998; Goldberg and Johnston, 2001) and arsenite adsorption on amorphous Fe oxide (Pierce and Moore, 1980; Suarez et al., 1998) and arsenate adsorption on alumina (Ghosh and Yuan, 1987) and amorphous Al oxide (Anderson et al., 1976; Goldberg and Johnston, 2001). Arsenite adsorption on amorphous Al oxide did not produce shifts in PZC indicating the formation of an outer-sphere surface complex or an inner-sphere surface complex that did not produce a change in surface charge (Goldberg and Johnston, 2001).
Ions that form outer-sphere surface complexes show decreasing adsorption with increasing solution ionic strength, while ions that form inner-sphere surface complexes show little ionic strength dependence or show increasing adsorption with increasing solution ionic strength (McBride, 1997). Arsenate adsorption on amorphous Fe oxide (Hsia et al., 1994) and Al oxide (Goldberg and Johnston, 2001) and arsenite adsorption on amorphous Al oxide (Manning and Goldberg, 1997b) showed very little ionic strength dependence as a function of solution pH, suggesting an inner-sphere adsorption mechanism; while arsenite adsorption on amorphous Al and Fe oxide showed decreasing adsorption with increasing ionic strength indicative of an outer-sphere adsorption mechanism (Goldberg and Johnston, 2001). The apparent discrepancy between the studies can be explained. In the ionic strength range of 0.02 to 0.1 M there was little ionic strength dependence of As adsorption in both the study of Manning and Goldberg (1997b) and Goldberg and Johnston (2001). Ionic strength dependence became obvious in the range of 0.1 to 1.0 M (Goldberg and Johnston, 2001), a range not studied by Manning and Goldberg (1997b). Since ionic strength effects are more apparent in arsenite sorption studies, it is generally held that arsenite is more weakly bound than arsenate.
Spectroscopic techniques provide direct experimental observation of ion adsorption mechanisms. Arsenate was observed to form inner-sphere bidentate surface complexes on goethite using infrared (Lumsdon et al., 1984), Fourier Transform infrared (FTIR) (Sun and Doner, 1996), and x-ray absorption fine structure (EXAFS) spectroscopy (Waychunas et al., 1993; Fendorf et al., 1997). Inner-sphere surface complexes of arsenate were also observed on amorphous Fe oxide using FTIR (Suarez et al., 1998) and Raman spectroscopy (Goldberg and Johnston, 2001). Arsenite also formed inner-sphere surface complexes on goethite as observed with FTIR (Sun and Doner, 1996) and EXAFS spectroscopy (Manning et al., 1998). Adsorption of arsenate on amorphous Al oxide was found to occur as inner-sphere surface complexes using FTIR spectroscopy (Goldberg and Johnston, 2001). Arsenite surface species were observed on amorphous Al oxide using attenuated total reflectance (ATR)-FTIR spectroscopy (Suarez et al., 1998). Fourier Transform infrared analysis of KBr pellets found no discernible surface features that could be attributed to arsenite adsorbed onto amorphous Al oxide suggestive of an outer-sphere adsorption mechanism (Goldberg and Johnston, 2001).
The presence of competing anions reduced the amount of As adsorption on reference minerals. Arsenate adsorption on amorphous Fe oxide (Jain and Loeppert, 2000), goethite, gibbsite (Hingston et al., 1971; Manning and Goldberg, 1996a), kaolinite, montmorillonite, and illite (Manning and Goldberg, 1996b) was significantly reduced in the presence of competing phosphate concentrations. Arsenite adsorption by amorphous Fe oxide was also reduced in the presence of competing phosphate concentrations although some sites exhibited considerably higher selectivity for arsenite than for phosphate (Jain and Loeppert, 2000). Competitive effects of sulfate concentrations on arsenate adsorption on amorphous Fe oxide were minor (Wilkie and Hering, 1996; Jain and Loeppert, 2000). Arsenate adsorption on alumina (Xu et al., 1988) and arsenite adsorption on amorphous Fe oxide (Wilkie and Hering, 1996; Jain and Loeppert, 2000) were reduced by competing sulfate concentrations, although to a lesser degree than by competing phosphate concentrations. The competitive effect of molybdate on arsenate adsorption on goethite, gibbsite (Manning and Goldberg, 1996a), kaolinite, montmorillonite, and illite (Manning and Goldberg, 1996b) was only significant at pH values below 5. The competitive effect of arsenate on arsenite adsorption by amorphous Fe oxide at low As concentration was more pronounced than the effect of arsenite on arsenate adsorption (Jain and Loeppert, 2000). Competitive effects of As redox states on adsorption have not yet been studied on Al oxide and clay minerals.
Surface complexation models, such as the constant capacitance model, are chemical models used to describe ion adsorption on oxides and clay minerals. The constant capacitance model was able to describe arsenate and arsenite adsorption on amorphous Al and Fe oxide (Manning and Goldberg, 1997b; Goldberg and Johnston, 2001), goethite, gibbsite (Goldberg, 1986; Manning and Goldberg, 1996a), kaolinite, montmorillonite, and illite (Goldberg and Glaubig, 1988; Manning and Goldberg, 1996b). Prediction of binary competitive arsenate systems using the single ion surface complexation model parameters has met with limited success. Model predictions generally described the shape of the adsorption curves qualitatively but not quantitatively (Goldberg, 1985; Manning and Goldberg, 1996a, b). This study had the following objectives: (i) to determine arsenate and arsenite adsorption on amorphous Al and Fe oxide and the clay minerals kaolinite, montmorillonite, and illite as a function of solution pH in the presence or absence of the competing As redox state; (ii) to evaluate the ability of the constant capacitance model to fit arsenate and arsenite adsorption on these surfaces in single-ion systems; and (iii) to evaluate the ability of the model to predict arsenate and arsenite adsorption in competitive binary systems using the parameters obtained by fitting the single-ion systems.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Surface areas for the oxides were obtained from water vapor adsorption using a combination FTIR/gravimetric apparatus (Xu et al., 2000). Surface areas of the clay minerals were determined using single-point Brunauer-Emmett-Teller (BET) N2 adsorption isotherms obtained with a Quantasorb Jr. surface area analyzer (Quantachrome Corp., Syosset, NY). Extractable-Mn content of the clay minerals was determined using the method of Chao (1972) and found to be undetectable, 0.035, and 0.015% for kaolinite, montmorillonite, and illite, respectively.
Adsorption experiments were carried out in batch systems to determine As adsorption envelopes (amount of As adsorbed as a function of solution pH per fixed total As concentration). Samples of adsorbent were added to 250-ml centrifuge bottles or 50-ml polypropylene centrifuge tubes and equilibrated with aliquots of a 0.01 M NaCl solution by shaking for 2 h on a reciprocating shaker at 22.5 ± 0.08°C. Solid suspension density was 0.5 g L-1 for Fe oxide, 1.0 g L-1 for Al oxide, and 40 g L-1 for the clay minerals. The equilibrating solutions contained 20 µM As from Na2HAsO4·7H2O, NaAsO2, or both, and had been adjusted to the desired pH values using 0.49 M HCl or NaOH additions that changed the total volume by 
2%. Arsenic additions were chosen to not exceed As concentrations found in drainage and well waters of the San Joaquin Valley of California. After reaction the samples were centrifuged at a relative centrifugal force of 7800 x g for 20 min. The decantates were analyzed for pH, filtered through 0.45-µm cellulose nitrate membrane filters, and analyzed directly for As(V) and As(III) concentrations using high-performance liquid chromatography coupled with hydride generation atomic absorption spectrometry as described by Manning and Martens (1997).
The constant capacitance model (Stumm et al., 1980) was used to describe arsenate and arsenite adsorption behavior on the adsorbents as a function of solution pH. The computer program FITEQL, Version 3.2 (Herbelin and Westall, 1996) was used to fit intrinsic As surface complexation constants to the experimental adsorption data. In the constant capacitance model, the protonation and dissociation reactions for the surface functional group, XOH (where XOH represents a reactive surface hydroxyl bound to a metal ion, X [Al or Fe] in the oxide mineral or an aluminol group on the clay particle edge) are defined as:
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The constant capacitance model assumes that all surface complexes are inner-sphere. Therefore, the surface complexation reactions for arsenate adsorption:
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By convention surface complexation reactions in the constant capacitance model are written starting with the completely undissociated acids. The model application contains the speciation reactions for solution As. These surface configurations were chosen because they correspond to the dominant solution As species in the pH range investigated. The unprotonated arsenate species XH2AsO4 was considered and found to be insignificant during model optimization. Monodentate surface species were chosen, in agreement with the FTIR, PZC, and titration results of Suarez et al. (1998) for arsenate and arsenite adsorption on amorphous Al and Fe oxides.
The intrinsic equilibrium constants for the protonation and dissociation reactions of the surface functional group are:
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is the surface potential (v), R is the molar gas constant (J mol-1 K-1), T is the absolute temperature (K), and square brackets represent concentrations (mol L-1). The intrinsic equilibrium constants for the arsenate surface complexation are:
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The mass balance expression for the surface functional group is:
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The charge balance expression is:
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represents the surface charge (molc L-1). The relationship between surface charge and surface potential is:
 | [16] |
The surface-site density was set at a value of 2.31 sites nm-2 as recommended by Davis and Kent (1990) for natural materials. Use of a consistent site density value for all materials is critical to allow standardization of parameters for natural systems. Numerical values of the intrinsic protonation constant, K+(int), and the intrinsic dissociation constant, K-(int), were averages of experimental values compiled from the literature for Al and Fe oxides (Goldberg and Sposito, 1984). The intrinsic protonation-dissociation constants were fixed at log K+
= 7.31, log K- = -8.80 for amorphous Fe oxide and log K+ = 7.38, log K- = -9.09 for amorphous Al oxide and kaolinite and illite. To obtain an acceptable model fit to the montmorillonite adsorption data it was necessary to optimize the protonation-dissociation constants along with the As surface complexation constants. This is likely because of the preponderance of permanent negative charge sites and resultant low PZC of the montmorillonite mineral. The capacitance was fixed at C = 1.06 F m-2 for all materials as had been done in previous constant capacitance modeling of arsenate adsorption on amorphous Al and Fe oxide (Goldberg, 1986; Manning and Goldberg, 1997b; Goldberg and Johnston, 2001) and considered optimum for Al oxide by Westall and Hohl (1980).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Adsorption behavior on the clay minerals was very similar to that on the oxides with respect to the shape of the adsorption envelopes, showing adsorption maxima for arsenate around pH 5 (Fig. 3a, 4a, and 5a) and adsorption maxima for arsenite at pH 8 to 9 (Fig. 3b, 4b, and 5b). Contrary to its behavior on oxides (Fig. 1a and 2a), arsenate adsorption on the clays increased with increasing solution pH from pH 3 to 5 and decreased with increasing solution pH from pH 5 to 9 (Fig. 3a, 4a, and 5a). The renewed increase in arsenate adsorption observed when pH increased above pH 9 was most likely an artifact resulting from dissolution of the clay minerals at elevated pH. Adsorption affinity for As of the clay minerals was less than the adsorption affinity of the oxides. Adsorption of As at 100% was only reached on kaolinite by arsenate around pH 5. For all other clay systems adsorption was significantly below 100%, especially for the arsenite systems.
The competitive effect of the presence of equimolar concentrations of the both As redox states is also shown in Fig. 1 through 5. In the oxide systems there was no evidence of any competitive effect (Fig. 1 and 2). The discrepancy in arsenate adsorption on Al oxide between the binary and the single ion system was the result of differences in amount of total As added; both systems exhibited 100% adsorption from pH 3 to 9. There was no apparent competitive effect of arsenite on arsenate adsorption for the clay minerals (Fig. 3a, 4a, and 5a). Differences below pH 4 and above pH 10 should be ignored because of the likelihood of significant clay dissolution at these pH values. The data points for arsenate adsorption on montmorillonite below pH 3.5 could also be artificially low because of significant oxidation of arsenite occurring at these pH values. A competitive effect of the presence of arsenate on adsorption of arsenite was observed on kaolinite (Fig. 3b) and illite (Fig. 4b) in the intermediate pH range 6.5 to 9. No comparable competitive effect was observed for montmorillonite in the pH range 5 to 9 (Fig. 5b); the apparent increase in arsenite adsorption below pH 5 resulted because of the oxidation of arsenite to arsenate. The minor competitive effects in the systems studied were not surprising considering the small, environmentally realistic concentrations of As that were added, leading to surface concentrations that were far from site saturation (3.8 x 10-2 µmol m-2 for As(V) and 2.3 x 10-2 µmol m-2 for As(III) on amorphous Al oxide and 1.1 x 10-2 µmol m-2 for As(V) and 1.0 x 10-2 µmol m-2 for As(III) on amorphous Fe oxide). However, as has been shown by Benjamin and Leckie (1981) for metal adsorption on oxides, competitive effects can be observed at low surface coverage when site heterogeneity is present. Previous competitive adsorption studies of arsenate and phosphate on goethite and gibbsite have been conducted under conditions of site saturation (e.g., Hingston et al., 1971, 1.3 µmol P m-2). The only previous study of arsenate-arsenite competitive adsorption was carried out at sufficiently high loading rates that precipitation rather than adsorption may have been occurring (Jain and Loeppert, 2000; Jain et al., 1999a,b; Stanforth, 1999).
The ability of the constant capacitance model to describe arsenate and arsenite adsorption on oxides and clay minerals is depicted in Fig. 6 through 10 . Surface complexation model parameters used to obtain the model fits are indicated in Table 1. The fit of the model to arsenate adsorption on oxides and on montmorillonite below pH 8 was excellent (Fig. 6a, 7a, and 10a, respectively). For arsenate on kaolinite and illite the model fit overestimated adsorption below pH 4 and around pH 8 (Fig. 8a and 9a, respectively). The overprediction at low pH is not considered significant since dissolution of the clay mineral would be expected. It is not clear why the model overestimated adsorption at intermediate pH value.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Received for publication February 19, 2001.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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-FeOOH). J. Soil Sci. 35:381–386.This article has been cited by other articles:
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T = As
T = 20 µM. Suspension density: 1 g L-1.
