Selenium speciation of soil/sediment determined with sequential extractions and hydride generation atomic absorption spectrophotometry

Understanding the speciation of the multioxidation states of selenium is vital to predicting the mineralization, mobilization, and toxicity of the trace element in natural systems. A sequential extraction scheme (SES) was developed for identification of Se oxidation states that first employed 0.1 M (pH 7.0) K2HPO4-KH2PO4 (P-buffer) to release soluble selenate (Se-+VI) and selenide (Se-+II) and ligand-exchangeable selenite (Se-+IV). The second step involved oxidation of organic materials with 0.1 M K2S2O8 (90 degrees C) to release Se--II and Se-+IV associated or occluded with organic matter. The final step used HNO3 (90 degrees C) to solubilize insoluble Se remaining in the sample. The solubilized Se compounds were speciated by a selective hydride generation atomic absorption spectrophotometry technique. Accuracy of the developed SES method (96-103% recovery) was verified by use of prepared Se compounds of known speciation, NIST standard reference materials, and existing seleniferous soils. The average precision (relative standard deviation) for the P-buffer extraction ranged from 5.5 to 7.7% (n = 12); the precision of the persulfate extraction ranged from 2.6 to 8.4% (n = 12); and the precision of the nitric acid extraction ranged from 2.8 to 7.4% (n = 12) for three soils extracted at four different time periods. The method was applied to analyze Se species in seleniferous plant, soil, and sediment samples.