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DIVISION S-2-SOIL CHEMISTRY

Modeling Arsenic(III) Adsorption and Heterogeneous Oxidation Kinetics in Soils

^a Dep. of Chem. and Biochem., San Francisco State Univ., 1600 Holloway Ave., San Francisco, CA 94132 USA
^b USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507-4617 USA

bmanning{at}sfsu.edu

Arsenite [As(III)] is a soluble and toxic species of arsenic that can be introduced into soil by geothermal waters, mining activities, irrigation practices, and disposal of industrial wastes. We determined the rates of As(III) adsorption, and subsequent oxidation to arsenate [As(V)] in aerobic soil–water suspensions using four California soils. The rate of As(III) adsorption on the soils was closely dependent on soil properties that reflect the reactivity of mineral surfaces including citrate–dithionite (CD) extractable metals, soil texture, specific surface area, and pH. Heterogeneous oxidation of As(III) to As(V) was observed in all soils studied. The recovery of As(V) from As(III)-treated soils was dependent on levels of oxalate-extractable Mn and soil texture. After derivation of rate equations to describe the changes in soluble and recoverable As(III) and As(V) in soil suspensions, soil property measurements were used to normalize the empirically derived rate constants for three soils. The fourth soil, which had substantially different soil properties from the other three soils, was used to independently test the derived soil property–normalized model. The soil property–normalized consecutive reaction model gave a satisfactory description of the trends seen in the experimental data for all four soils. Understanding the effects of soil properties on the kinetics of chemical reactions of As(III) and As(V) in soils will be essential to development of quantitative models for predicting the mobility of As in the field.

Abbreviations: CD, citrate–dithionite • DI, deionized • HPLC–HGAAS, high performance liquid chromatography–hydride generation atomic absorption spectrometry • Me_T, total citrate–dithionite extractable metals (Al + Fe + Mn) • Mn_OX, oxalate-extractable manganese • SA, specific surface area • XAS, x-ray absorption spectroscopy • XRD, x-ray diffraction