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Model for Biomethylation and Volatilization of Selenium from Agricultural Evaporation Ponds

Dep. of Land, Air, and Water Resources, Hydrologic Science Program, Univ. of California, Davis, CA 95616.

^* Corresponding author (fztanji{at}ucdais.edu).

ABSTRACT

Selenium has evapoconcentrated to hazardous levels in agricultural evaporation ponds in the San Joaquin Valley of California. Microbial methylation and volatilization is one of the pathways by which high Se concentrations may be dissipated from these ponds. To obtain a more complete understanding of this potential remediation process, kinetic models are developed to evaluate the factors affecting Se biomethylation such as temperature and organic C sources (carbohydrates and proteins) using experimental data from the literature. By assuming that dimethylselenide (DMSe, (CH₃)₂Se) formation is irreversible and follows first-order kinetics, the experimental data are described reasonably well using temperature- and organic matter-characteristics, except deviations occurred at a higher temperature (35°C). Proteins are known to stimulate Se volatilization dramatically as compared with carbohydrates as the energy source. This stimulatory property appears to depend on the ability of microorganisms to absorb protein hydrolysis products, a source of methyl groups. A coupled reaction mechanism is proposed in which proteins provide methyl groups for Se methylation to form volatile DMSe. The model fitted experimental data successfully. The volatility of DMSe through the air-water interface was evaluated using a two-layer film model. The predicted half-life of DMSe in water ranged from 1 to 3 d when assuming that wind speed was <3 m s^–1 and the water body was 1 m deep. An opportunity exists to enhance Se volatilization by fungi, bacteria, microalgae, and other indigenous microbes found in evaporation ponds as a remediation technique. Additional knowledge on the fate of DMSe both in the water and in the atmosphere, however, is needed to evaluate this dissipation strategy in high Se water bodies.

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