Submitted to: Analytical Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 5, 1996
Publication Date: N/A
Interpretive Summary: The occurrence of toxic elements in irrigation drainage waters necessitates that these waters be properly managed to limit their impacts on the environment. Disposal of irrigation water from central California farms from 1978 to 1983 resulted in the deposition of 9 tons of the element selenium into a series of evaporation ponds and resulted in nearly all of the selenium becoming concentrated in the top of the soil profile in forms whose solubility and mineralization potential are unknown. Many questions in need of answers include: what are the selenium forms in these soils and will they become soluble and pose water quality problems in the future? The toxicity of higher than nutritionally required selenium amounts and the failure of present selenium extraction technology to identify the selenium forms in these contaminated soils has emphasized the importance of the development of a quantitative method for identifying selenium forms in natural systems. This developed method allows determination of soluble selenium, selenium of limited solubility and insoluble selenium forms. Understanding the transformations of selenium that occur in natural systems with the described methodology will allow development of drainage water management strategies that will limit the impact of these waste waters on our environment.
Technical Abstract: Understanding the speciation of the multioxidation states of selenium is vital to predicting the mineralization, mobilization and (or) toxicity of the trace element in natural systems. A sequential extraction scheme (SES) was developed for identification of Se speciation that first employed 0.1 M (pH 7.0) K2HPO4-KH2PO4 (P-buffer) to release soluble selenate (Se+VI) and organic Se (Se-II) and ligand-exchangeable selenite (Se+IV). The second step involved oxidation of organic materials with 0.1M K2S2O8 (90oC) to release recalcitrant organic Se and Se+IV associated or occluded with organic matter. The final step used HNO3 (90oC) to solubilize elemental (Seo) and metallic Se 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 persulfate precision ranged from 2.6 to 8.4%; and the nitric acid extraction precision ranged from 2.8 to 7.4% (n = 12) for three soils extracted at four different time periods. The method was then applied for analysis of Se speciation in seleniferous plant, soil and sediment samples.