TRANSFORMATION OF VOLATILE METHYLATED SELENIUM GASES IN SOIL

Authors: MARTENS, D - NAT'L SOIL TILTH LAB,IA; Suarez, Donald

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: The cycling of selenium (Se) in soil has inadvertently caused toxicity problems for wildlife in irrigation waste water evaporation ponds. This contamination is generally thought to occur by formation of organic Se compounds similar to sulfur amino acids. These Se- containing amino acids can occur in soils and at higher levels in evaporation pond sediments. A portion of the cycling of Se amino acids occurs by biological metabolism to the gaseous products, dimethylselenide (DMSe) and dimethyldiselenide (DMDSe). Very little information is currently available documenting the fate of DMSe and DMDSe gases when released in the environment. This manuscript details the fate of neat DMSe and DMDSe and naturally-generated Se gases in the presence of soils, clay minerals and soil humic substances.

Technical Abstract: Microbial volatilization of selenium (Se) as dimethylselenide (DMSe) and dimethyldiselenide (DMDSe) from soil is an important part of the Se cycle in nature, but little is known about the stability and transformations of these gases during residence in the soil environment before dissipation to the atmosphere. Experiments monitored by gas chromatography and atomic absorption spectroscopy were conducted with various clay mineral standards, charcoal, commercial humic substances and soils to determine the sorption and transformations of DMSe and DMDSe injected into the headspace or passed through soil materials. Batch experiments conducted with 2 to 5 g materials placed into 40 mL Teflon centrifuge tubes equipped with MininertTM gas sampling valves showed that DMSe was slowly sorbed by soil materials and the majority of the DMSe deficit in the headspace was recovered as SeO3= and SeO4=. In contrast, DMDSe was rapidly partitioned from the gas phase and resulted in an increased recovery of less soluble elemental and selenide-Se forms. These results were confirmed during flow-through soil column studies with both little DMSe sorption and sorption of the majority of DMDSe addition. Additions of selenomethionine (SeMet) to soil to produce DMSe and DMDSe in sealed flasks resulted in an increased partitioning of Se into inorganic Se when compared with a flow-through system designed to limit the contact of Se gases with soil. These results suggest that soil Se volatilization as DMSe and DMDSe results in Se loss to the atmosphere as DMSe with concomitant soil Se immobilization due to the instability of DMDSe.