Sorption and Redox Transformation of Arsenite and Arsenate in Two Flooded Soils

The fate of As in soils is regulated mostly by Its participation in sorption reactions and redox transformations. Few studies haveft examined the rate of arsenite and arsenate reduction or the cxtentjoi^ which these redox transformations may be affected by sorption rest-/ tions. The objective of this study was to examine changes in solution concentrations of H^AsOj and HjAsO<~_in two soils subjected to prolonged flooding. The soils, which differed in HjAsOj and HjAsOf sorption capacities, were flooded by suspending 1 g of soil in 25 mL of a solution containing 0.01 M CaClj and 1 g D-glucose kg". The suspensions were amended with NaAsOi or Na2HAsO4-7HiO and were incubated for 0.5 h to 20 d. Changes in solution chemistry (electrode potential [Eh], pH, and dissolved Fe, Mn, HjAsO! , and HiAsO4~) were observed with time. Sorption processes controlled the dissolved concentrations of HjAsOj and HjAsOf during initial stages of flooding. When anaerobic conditions were achieved, dissolution of Fe and Mn oxyhydroxides occurred, causing desorption of HjAsOS and HjAsOf. In NaAsOi-amended suspensions, desorbed HjAsOS disappeared from solution within 10 d. In NajHaSOr amended suspensions, desorbed HjAsOr also disappeared within 10 d. Concurrent with the disappearance of HjAsO4~ was the appearance of HjAsO! , indicating that HjAsOr was rapidly reduced to HjAsOS. First-order plots of HjAsO] and HiAsOi" disappearance had a linear relationship. Rates of desorption and disappearance of HjAsOS and HiAsOf were slower in the soil with higher adsorption capacity, suggesting that sorption processes may influence redox transformations of As oxyanlons. 'T'HE Bi6~AVAiLA||UTY, tqxicity, and mobility of Aj A_yL_sQil-water systems is determinedjargely by~ jtj^speciation. Metallic As [As(6)], ajsjne [As(-in)]? affl9~methyIaTed^forms of As an£ thermotfynamicliflyf stable in reduced systems, wh«rea¥H2Asp HsSsD? [As(III)] predominatfTin oxidized systems gusonand Gavis. 1972). Theoxyanions of As also exhibllZ various degrees of protonation and valence charge, des pending onj>H. "" The solution concentration of r^AsO^ in soil is controlled primarily by adsorption reactions on oxides and S.L. McGeehan and D.V. Naylor, Div. of Soil Science, Univ. of Idaho, Moscow, ID 83843. Idaho Agric. Exp. Sin. Manuscript no. 92723. Received 23 Apr. 1992. Corresponding author. Published in Soil Sci. Soc. Am. J. 58:337-342 (1994). hydroxides of Al (Anderson etal., 1975; Huang, 1975), Fe (Pierce and Moore, 1982; Belzile and Tessier, 1990), and Mn (Oscarson et al., 1983). Arsenite undergoes similar adsorption reactions, although usually of lower magnitude, to those of FhAsO^ (Pierce and Moore, 1982; Oscarson et al., 1983). Adsorption of both oxyanions occurs primarily via a specific adsorption (ligand exchange) mechanism (Parfitt, 1978; Huang, 1980). Nonspecific adsorption (electrostatic attraction) of As also occurs but is limited to pH-dependent charged surfaces at pH values below the zero-point-of-charge for a given adsorbent (Huang, 1980). Transformation between the various oxidation states and species of As may occur as a result of biotic or abiotic processes. Bacterial oxidation of HjAsO? to H2AsO.T has been observed in mine waters, arsenical cattle-dipping fluids, and raw sewage (Turner, 1954; Phillips and Taylor, 1976; Wakao et al., 1988). H3AsO? was readily oxidized to HjAsOi" in aquatic sediments via an abiotic mechanism (Oscarson et al., 1980). It was later shown that this oxidation was catalyzed by Mn dioxides present in the sediments (Oscarson et al., 1983; Moore et al., 1990). Biotic reduction of H2AsO^~ has been observed in groundwater (Cherry et al., 1979), aquatic sediments (Aggett and OTBrien, 1985; Andreae, 1979), activated sewage sludge (Myers et al., 1973), and soils (Cheng and Focht, 1979). H3AsO§ and H2AsOT may also undergo biomethylation to form cacodylic acid [(CHOzHAsOz], which can be reduced to dimethylarsine [(CH3)2HAs] or trimethylarsine [(CH3)3As] (Braman, 1975; Woolson, 1977). Few studies have examined the rate of H3AsO-j and H2AsO,T reduction or the extent to which these redox transformations may be affected by sorption reactions. The objeclive of this siudy was to examine the changes in solution concentrations of H3AsO§ and HjAsO* in two soils subjected to prolonged flooding. MATERIALS AND METHODS The surface mineral horizons of the Santa (coarse-silty, mixed, frigid Typic Fragiochrept) and Huckleberry (media! 338 SOIL SCI. SOC. AM. I. , VOL. 58, MARCH-APRIL 1994 Table 1. Selected chemical properties of Santa and Huckleberry silt loam soils.