Speciation of Soluble Selenium in Agricultural Drainage Waters and Aqueous Soil-Sediment Extracts Using Hydride Generation Atomic Absorption Spectrometry

There are few methods to effectively measure organic selenium [Se(−II)] in natural water and soil-sediment extracts. A method has been developed to determine organic Se(−II) in soil-sediment extracts and agricultural drainage water by using persulfate to oxidize organic Se(−II) and using manganese oxide as an indicator for oxidation completion. This method was used to determine Se speciation in eleven soil-sediments and four agricultural drainage water samples collected from the western United States. Results showed that organic Se(−II) can be quantitatively oxidized to selenite without changing the selenate concentration in the soil-sediment extract and agricultural drainage water and then quantified by hydride generation atomic absorption spectrometry. Recoveries of spiked organic Se(−II) and selenite were 96−105% in the soil-sediment extracts and 96−103% in the agricultural drainage water. Concentrations of soluble Se in the soil-sediment extracts were 0.0534−2.45 μg/g, of which organic Se(−II) accounted for 4.5−59.1%. Selenate is the dominant form of Se in agricultural drainage water, accounting for about 90% of the total Se. In contrast, organic Se(−II) was an important form of Se in the wetlands. These results showed that wetland sediments are more active in reducing selenate compared to evaporation pond sediments.