Fractionation of soil selenium by sequential partial dissolution

Sequential partial dissolution technique is developed to partition soil Se into association with operationally defined solid phases. The procedure extracts soil Se into five fractions: soluble (0.25 M KCl), ligand exchangeable (0.1 M KH2PO4), acid extractable (4 M HCl), oxidative acid decomposable (KClO3 + concentrated HCl), and strong mixed-acid digestable (HF + HNO3 + HClO4). The method for the extraction and determination of fractional Se and total Se is presented in detail. The technique has been shown to be satisfactory in terms of sensitivity, accuracy, and precision for the fractionation of soil Se. On the basis of the experimental data obtained on sample types of contrasting chemical properties, the distribution of Se in different fractions has been found to depend on the chemical and mineralogical differences as a result of chemical weathering. In soils developed through intensive leaching and weathering, Se tends to be associated with oxide minerals and a great proportion of the remaining Se is resistant to chemical dissolution. For the soils with high pH and low content of oxide minerals, Se is present as mobile selenate and can be easily extracted. Selenium associated with sulfide minerals may be detected by the sequential extraction as immobile and unavailable. The overall findings are consistent with the geochemical behavior of Se in a soil environment. The five-step sequential extraction may provide insight into the surficial geochemistry of Se and should serve as a useful tool for Se studies in soil and sediment chemistry, environmental geochemistry, and mineral exploration. The simplified two-step version involving extractions by 0.1 M KH2PO4 and 4 M HCl should indicate the immediate and potential impact of Se on the environment, both as a nutrient and as a pollutant.