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ABSTRACT
Batch experiments were carried out using three soils representing different geologic parts of the continental USA to identify processes influencing partitioning and interconversion of chromium species in soil porewaters. The partition coefficient (Kd) of each of the two main species, Cr(III) and Cr(VI), was calculated for each soil using adsorption isotherms measured after an equilibration time of 6 hours for the former species and 48 hours for the latter at the natural conditions of the soils. Cr(VI) in porewater was measured directly by ion chromatography, while total chromium was measured by inductively coupled plasma—atomic emission spectrophotometry (ICP—AES). Total chromium in the soil solids was measured by ICP—AES after microwave assisted digestion. The magnitude of the Kd values are much lower for Cr(VI), ranging from less than 1 to about 50 L kg−1. For Cr(III), the Kd values range from about 850 to 5,600 L kg−1. The effects of the initial porewater concentration of the chromium species, the equilibration time, and the pH, Eh and ion concentration of the porewater on the Kd values were investigated; the results are discussed relative to the differences in physical and chemical properties of the soils. Cr(III) concentration in porewater increases as pH decreases, but Cr(VI) increases as pH increases. The sorption of Cr(III) decreases with increased porewater concentration of cations, but increased concentration of common anions has slight effect on the sorption of Cr(VI) anions. The Kd values of both Cr(III) and Cr(VI) depend on equilibration time because of the kinetics of interconversion of species upon reaction with the soil and the resultant change in total chromium in the porewater with time. Cr(III) added to porewater reacts rapidly with the soil through adsorption and/or precipitation; some is then oxidized by Mn(IV)-containing minerals within a few hours, liberating both Cr(VI) and Mn(II) into the porewater. The yield of Cr(VI) can reach significant levels relative to the 100 μg L−1 drinking water standard, and the Cr(VI) disappears from porewater at a very slow rate compared to the disappearance of Cr(III).