HG-AFS Speciation Analysis for 5 Species of Selenium in Se-Rich Soil with Separation by Sequential Extraction

The sequential extraction was modified and applied to the speciation analysis of 5 species of selenium in samples of Se-rich soil by HG-AFS. For separation of the 5 species (i.e., the water-soluble species, the exchangeable species, the organic-bound species, the acid-soluble species and the residual species), 0.500 g of the soil sample was taken and extracted in succession by swirling with 20 mL of water, 10 mL of 0.1 mol·L^-1 K₂HPO₄-KH₂PO₄ buffer solution (pH 7.0), 20 mL of 0.1 mol·L^-1 NaOH solution and 10 mL of 3 mol·L^-1 HCl solution. After each extraction, the mixtures were centrifuged, the supernatants containing respectively the former 4 species were reserved for HG-AFS determinations, and the residues were collected for the next extraction. The residue, remained after the 4th extraction, was dried and transferred into a PTFE vessel, and digested with 1 mL of HNO₃, 0.5 mL of HF and 0.5 mL of HClO₄ under tight closing and pressure. After digestion, the PTFE vessel was cooled and taken out, and the solution in it was evaporated to dryness. The residue was taken up with 1 mL of 6 mol·L^-1 HCl solution and let Se(Ⅵ) be reduced to Se(Ⅳ). The solution was diluted to 10.0 mL with water, and residual species of Se were determined by HG-AFS. The supernants, obtained in the former 3 extractions, should be treated separately to convert into solutions of 10 mL with concentration of 0.6 mol·L^-1 HCl, before used for HG-AFS analysis. Amounts of Se found in the 5 solutions, represented the contents of 5 species of Se in the soil sample. Value of detection limit (3s/k) found was 0.21 μg·L^-1, and RSDs (n=6) found were all less than 11%. Recovery found by standard addition were ranged from 91.7% to 107%.