Speciation of selenium in soil

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Abstract
A method for quantitative speciation of selenium (Se) in soil was developed. Se in soil was divided into two fractions after repeated extractions with a 0.1 mol L-1 sodium hydroxide solution: alkali-soluble and alkali-insoluble. Then the alkali extract was fractionated into humic and fulvic acid fractions at pH 1.5. Since the inorganic Se (IV and VI) in the alkali extract was present in the fulvic acid fraction at this step, Se contained in the humic acid fraction was considered to be organically-bound. Furthermore, Se in the fulvic acid was divided into four fractions: organic Se with high molecular weight, organic Se with low molecular weight, Se(IV) and Se(VI). Selenium in the alkali insoluble fraction was considered to be contained in the soil mineral particles and/or nonextractable organic fraction. By the application of this method, quantitative speciation of inorganic Se and organic Se in soil could be achieved, and the Se content in each fraction could be measured with fair accuracy. Six soils were examined using this method. In 5 soils which contained 16–150 g kg-1 of carbon, more than 60% of the total Se was organically-bound. On the other hand, in one soil with a low carbon content (1.6 g kg-1), inorganic Se (VI and VI) accounted for 65.8% of the total Se content.