Formation of Dimethylselenonium Compounds in Soil

Abstract

Selenium (Se) methylation/volatilization from Se-contaminated environments has been extensively studied over the last 15 years, but the methylation pathway of Se still remains largely unknown. We developed a sequential purge-and-trap system to collect volatile dimethylselenide (DMSe) and dimethyldiselenide (DMDSe) and to extract dimethylselenonium compounds. These dimethylselenonium compounds included dissolved DMSe and DMDSe, dimethylselenoxide (DMSeO), methylselenomethionine (MSemet), and dimethylselenoniopropionate (DMSeP). We monitored volatile DMSe and DMDSe and soluble dimethylselenonium compounds in a soil spiked with selenomethionine (Semet) at levels ranging from 16 to 80 μg of Se/g. In a moist soil spiked with Semet (80 μg of Se/g), concentrations of volatile DMSe and DMDSe were much higher than those of dissolved DMSe and DMDSe, DMSeO, MSemet, and DMSeP, although all of these compounds followed a similar trend in concentration over the course of the experiment. Concentration of dissolved DMSe and DMDSe, DMSeO, MSemet, and DMSeP increased with soil moisture and Semet concentration. After a 15-day experiment with a moist soil, about 65% of the spiked Semet was transformed to volatile DMSe and DMDSe, 26% was still in soluble forms of Se, and only 9% accumulated in the soil. This study suggests that Semet can undergo a methylation pathway to DMSe (Semet → MSemet → DMSeP → DMSe), and oxidation of dissolved DMSe to DMSeO may occur in soil.