Competitive Adsorption of Arsenate and Arsenite on Oxides and Clay Minerals

Masscheleyn et al., 1991). The mobility of As is a function of its oxidation state with arsenite exhibiting greater Arsenic adsorption on amorphous Al and Fe oxides and the clay mobility through sand columns than arsenate (Gulens et minerals, kaolinite, montmorillonite, and illite was investigated as a function of solution pH and As redox state, i.e., arsenite [As(III)] al., 1979). However, under high pH conditions arsenite is and arsenate [As(V)]. Arsenic adsorption experiments were carried more strongly bound to soil components than arsenate out in batch systems to determine adsorption envelopes, amount of (Manning and Goldberg, 1997a; Raven et al., 1998; Jain As(III), As(V), or both adsorbed as a function of solution pH per and Loeppert, 2000). fixed total As concentration of 20 M As. Arsenate adsorption on Arsenic adsorption is significantly positively correoxides and clays was maximal at low pH and decreased with increasing lated with Al and Fe oxide and clay content of soils pH above pH 9 for Al oxide, pH 7 for Fe oxide and pH 5 for clays. (Elkhatib et al., 1984a,b; Sakata, 1987; Wauchope, 1975; Arsenite adsorption exhibited parabolic behavior with an adsorption Livesey and Huang, 1981). Adsorption studies of arsemaximum around pH 8.5 for all materials. There was no competitive nite and arsenate have used a wide range of adsorbents effect of the presence of equimolar arsenite on arsenate adsorption. including oxides, clay minerals, and whole soils. InThe competitive effect of equimolar arsenate on arsenite adsorption was small and apparent only on kaolinite and illite in the pH range organic constituents of soils that adsorb significant 6.5 to 9. The constant capacitance model was able to fit the arsenate amounts of As are Al and Fe oxides, clay minerals, and arsenite adsorption envelopes to obtain values of the intrinsic As and carbonates. Arsenate adsorption on amorphous Fe surface complexation constants. These intrinsic surface complexation hydroxide (Pierce and Moore, 1982), goethite, gibbsite constants were then used in the model to predict competitive arsenate (Hingston et al., 1971; Manning and Goldberg, 1996a), and arsenite adsorption from solutions containing equimolar As(III) hematite (Xu et al., 1988), amorphous Al hydroxide and As(V) concentrations. The constant capacitance model was able (Anderson et al., 1976), alumina (Gupta and Chen, to predict As adsorption from mixed As(III)-As(V) solutions in sys1978), kaolinite, montmorillonite (Frost and Griffin, tems where there was no competitive effect. 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 deA is a toxic trace element for animals including creased at high pH. Somewhat similar behavior was humans. Mineral dissolution, use of arsenical pestfound for arsenite adsorption on amorphous Fe hydroxicides, disposal of fly ash, mine drainage, and geothermal ide (Pierce and Moore, 1980, 1982), amorphous Al oxide discharge elevate concentrations of As in soils and wa(Manning and Goldberg, 1997b), and alumina (Gupta ters. Agricultural drainage waters from some soils espeand Chen, 1978) where adsorption increased at low pH, cially in arid regions are elevated in As concentration. exhibited a peak in the pH range 7 to 8, and decreased Adsorption reactions on soil mineral surfaces potenat high pH. Arsenite adsorption on the clay minerals tially attenuate toxic soil solution As concentrations kaolinite, montmorillonite, and illite increased up to pH reducing contamination to groundwaters. Federal water 9 (Manning and Goldberg, 1997b). Arsenate adsorption quality standards had considered As concentrations in was greater than arsenite adsorption on alumina (Gupta excess of 0.667 mol L 1 (50 ppb) to be hazardous to and Chen, 1978), amorphous Fe hydroxide (Pierce and the welfare of humans and domestic animals (USEPA, Moore, 1982), kaolinite, and montmorillonite (Frost and 1991). The U.S. Environmental Protection Agency Griffin, 1977). At pH values above 7, arsenite adsorption (USEPA) issued a new standard for As in drinking was often greater than arsenate adsorption, as observed water of 0.133. mol L 1 (10 ppb) on 31 Oct. 2001 on amorphous Fe oxide (Jain and Loeppert, 2000; Gold(USEPA, 2001). berg and Johnston, 2001). Of the two inorganic redox states of As, the As(III) Accurate description of As adsorption behavior in redox state is more acutely toxic than the As(V) redox natural systems requires determination of the bonding state (Penrose, 1974). At natural pHs arsenite exists in mechanism of As anions on mineral surfaces. Insight solution as H3AsO3 and H2AsO 3 , since the pKa values into anion adsorption mechanisms can be provided by for arsenious acid are high, pKa 9.2 and pKa 12.7. both macroscopic (point of zero charge [PZC] shifts and Arsenate is present as H2AsO 4 and HAsO2 4 since the ionic strength effects) and microscopic (spectroscopic) pKa values for arsenic acid are pKa 2.3, pKa 6.8, experimental methods. Electrophoretic mobility (EM) and pKa 11.6. Because the kinetics of As redox transmeasures movement of charged particles in response to formations are relatively slow, both oxidation states are an applied electric field such that zero EM indicates the often found in soils regardless of the redox conditions PZC of the particle. Shifts in PZC and reversals of EM with increasing ion concentration can be used as S. Goldberg, George E. Brown Jr., Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA 92507. Contribution from the George E. Brown Jr., Salinity Laboratory. Received 19 Feb. 2001. *CorreAbbreviations: EM, electrophoretic mobility; EXAFS, x-ray absorpsponding author ([email protected]). tion fine structure; FTIR, Fourier transform infrared spectroscopy; PZC, point of zero charge. Published in Soil Sci. Soc. Am. J. 66:413–421 (2002).