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Characterizing Kinetics of Sequential Selenium Transformation in Soil

Department of Environmental Sciences, Univ. of California, Riverside, CA 92521-0424.

^* Corresponding author (wajury{at}mail.ucr.edu).

ABSTRACT

The speciation of selenium (Se) in a Hanford sandy loam was measured as a function of time at two soil water contents under aerobic and anaerobic conditions to evaluate the kinetics of selenium transformation. The stepwise reaction coefficients describing sequential selenium transformation were determined using an inverse method based on a five-compartment model that included selenate, selenite, elemental Se, organic Se, and gaseous Se as the major Se species (or fractions). Transformation of Se was less affected by soil water content at 21.5%, but was distinctively faster under anaerobic conditions where O₂ was displaced with N₂ gas. During the incubation period of 49 to 52 d, between 27.9 and 74.9% of the total selenium, initially present as selenate, was reduced to elemental or organic Se species. Although volatilization of Se required a reduced precursor, the rate of volatilization was not correlated to the rate of reduction. No volatilization of Se was detected under anaerobic conditions, suggesting that O₂ was a critical factor controlling the methylation of Se, the reaction that converts nonvolatile Se species into gaseous forms.

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