Transformations of volatile methylated selenium in soil

Microbial volatilization of selenium as dimethylselenide (DMSe) and dimethy ldiselenide (DMDSe) from soil is an important part of the Se cycle in natur e, but little is known about the stability and transformations of these gas es during residence in the soil environment before dissipation to the atmos phere. Experiments monitored by gas chromatography and atomic absorption sp ectroscopy were made with various clay mineral standards, charcoal, commerc ial humic substances and soils to determine the sorption and transformation s of DMSe and DMDSe injected into the headspace or passed through soil mate rials. Batch experiments conducted with 2-5 g materials placed into 40 mL T eflon centrifuge tubes equipped with Mininert(R) gas sampling valves showed that DMSe was slowly sorbed by soil materials and most of the DMSe deficit in the headspace was recovered as SeO3= and Se-4(=). In contrast, DMDSe wa s rapidly partitioned from the gas phase and resulted in an increased recov ery of less soluble elemental and selenide-Se forms. These results were con firmed during flow-through soil column studies with both little DMSe sorpti on and sorption of the majority of DMDSe addition. Additions of selenomethi onine (SeMet) to soil to produce DMSe and DMDSe in sealed flasks resulted i n an increased partitioning of Se into inorganic Se when compared with a fl ow-through system designed to limit the contact of Se gases with soil. Thes e results suggest that soil Se volatilization as DMSe and DMDSe results in Se loss to the atmosphere as DMSe with concomitant soil Se immobilization d ue to the instability of DMDSe. (C) 1999 Elsevier Science Ltd. All rights r eserved.