Predicting Arsenate Adsorption by Soils using Soil Chemical Parameters in the Constant Capacitance Model

dynamically stable redox state under oxidizing conditions in soils. The constant capacitance model, a chemical surface complexation Arsenate adsorption on soils and soil minerals has model, was applied to arsenate, As(V), adsorption on 49 soils selected been described using various modeling approaches. Such for variation in soil properties. The constant capacitance model was able to fit arsenate adsorption on all soils by optimizing either three models include the empirical distribution coefficient, Kd monodentate or two bidentate As(V) surface complexation constants. (de Brouwere et al., 2004), Freundlich and Langmuir A general regression model was developed for predicting soil As(V) isotherm equations (Chakravarty et al., 2002), and sursurface complexation constants from easily measured soil chemical face complexation models: constant capacitance model characteristics. These chemical properties were cation exchange ca(Goldberg, 1986, 2002; Goldberg and Glaubig, 1988; pacity (CEC), inorganic C (IOC) content, organic C (OC) content, Manning and Goldberg, 1996a, 1996b; Goldberg and iron oxide content, and surface area (SA). The prediction equations Johnston, 2001; Gao and Mucci, 2001, 2003), diffuse were used to obtain values for the As(V) surface complexation conlayer model (Dzombak and Morel, 1990; Hering et al., stants for five additional soils, thereby providing a completely indepen1990; Swedlund and Webster, 1999; Lumsdon et al., dent evaluation of the ability of the constant capacitance model to 2001), triple layer model (Hsia et al., 1992; Khaodhiar describe As(V) adsorption. The model’s ability to predict As(V) adsorption was quantitative on three soils, semi-quantitative on one soil, et al., 2000; Arai et al., 2004), and CD-MUSIC model and poor on another soil. Incorporation of these prediction equations (Hiemstra and van Riemsdijk, 1999; Gusstafsson, 2001). into chemical speciation-transport models will allow simulation of soil Empirical model parameters are only valid for the consolution As(V) concentrations under diverse agricultural and environditions under which the experiment was conducted. Surmental conditions without the requirement of soil specific adsorption face complexation models are chemical models because data and subsequent parameter optimization. they define surface species, chemical reactions, mass balances, and charge balance and contain molecular features that can be given thermodynamic significance A is a trace element that is toxic to both plants (Sposito, 1983). and animals. Concentrations of As in soils and Arsenate has been observed spectroscopically to adwaters can become elevated as a result of application sorb specifically on the oxide minerals, goethite, amorof arsenical pesticides, disposal of fly ash, mineral dissophous iron oxide, and amorphous aluminum oxide, formlution, mine drainage, and geothermal discharge. Eleing strong inner-sphere surface complexes containing no vated concentrations of As are found in agricultural water between the adsorbing arsenate ion and the surdrainage waters from some soils in arid regions. In recface functional group (Waychunas et al., 1993; Fendorf ognition of the hazards As poses to the welfare of huet al., 1997; Goldberg and Johnston, 2001). A mixture of mans and domestic animals, the U.S. Environmental monodentate and primarily bidentate arsenate surface Protection Agency (USEPA) recently lowered the As complexes was observed on goethite and ferrihydrite drinking water standard from 0.667 mol L 1 (50 ppb) (Waychunas et al., 1993). to 0.133 mol L 1 (10 ppb) effective 23 Jan. 2006. All surface complexation-modeling approaches indiAdsorption reactions on soil mineral surfaces can atcated above postulated monodentate inner-sphere surtenuate elevated soil solution As concentrations reducface complexes for arsenate adsorption with the exceping As contamination to ground waters. Arsenic adsorption of the studies of Manning and Goldberg (1996a), tion is significantly positively correlated with clay and Al Hiemstra and van Riemsdijk (1999), and Gusstaffsson and Fe oxide content of soils (Wauchope, 1975; Livesey (2001), which considered a combination of monoand and Huang, 1981; Yang et al., 2002). Arsenic adsorption bidentate surface complexes. Arai et al. (2004) found studies have been performed on a wide range of adsorcomparable model fits for monodentate and bidentate bents including oxides, clay minerals, organic matter, arsenate surface complexes on hematite. They recomcarbonates, and whole soils. Arsenic (V) is the thermomended use of bidentate species to provide closer agreement with the spectroscopic results found on goethite by Waychunas et al. (1993). S. Goldberg, S.M. Lesch, and D.L. Suarez, USDA-ARS, George E. Chemical modeling of arsenate adsorption has been Brown Jr., Salinity Lab., 450 W. Big Springs Road, Riverside, CA 92507. Contribution from the George E. Brown Jr., Salinity Laboraperformed on natural materials for the clay minerals: tory. N.T. Basta, School of Natural Resources, Ohio State Univ., 2021 kaolinite, montmorillonite, and illite (Manning and GoldCoffey Road, Columbus, OH 43210-1085. Received 16 Dec. 2004. berg, 1996b) and a soil (Goldberg and Glaubig, 1988) *Corresponding author ([email protected]). Abbreviations: ARMSE, average root mean squared error; CEC, Published in Soil Sci. Soc. Am. J. 69:1389–1398 (2005). Soil Chemistry cation exchange capacity; Cip, coefficient of imprecision; DF, degrees of freedom; ICP, inductively coupled plasma; IOC, inorganic carbon; doi:10.2136/sssaj2004.0393 © Soil Science Society of America MANOCOVA, multivariate analysis of covariance; MSE, mean square error; OC, organic carbon; SA, surface area. 677 S. Segoe Rd., Madison, WI 53711 USA 1389 Published online August 4, 2005