Selenium speciation in phosphate mine soils and evaluation of a sequential extraction procedure using XAFS

• Published in: Environmental pollution (1987) 2017-10, Vol.229 (C), p.911-921
• Main Authors: Favorito, Jessica E., Luxton, Todd P., Eick, Matthew J., Grossl, Paul R.
• Format: Article
• Language: English
• Subjects: Biological Availability; Environmental Monitoring - methods; ENVIRONMENTAL SCIENCES; Oxidation-Reduction; Phosphates - analysis; Selenium; Selenium - analysis; Sequential extraction procedure; Soil - chemistry; Soil Pollutants - analysis; Speciation; Trace Elements - analysis; X-Ray Absorption Spectroscopy; XANES
• ISSN: 0269-7491; 1873-6424
• DOI: 10.1016/j.envpol.2017.07.071

Selenium is a trace element found in western US soils, where ingestion of Se-accumulating plants has resulted in livestock fatalities. Therefore, a reliable understanding of Se speciation and bioavailability is critical for effective mitigation. Sequential extraction procedures (SEP) are often employed to examine Se phases and speciation in contaminated soils but may be limited by experimental conditions. We examined the validity of a SEP using X-ray absorption spectroscopy (XAS) for both whole and a sequence of extracted soils. The sequence included removal of soluble, PO4-extractable, carbonate, amorphous Fe-oxide, crystalline Fe-oxide, organic, and residual Se forms. For whole soils, XANES analyses indicated Se(0) and Se(-II) predominated, with lower amounts of Se(IV) present, related to carbonates and Fe-oxides. Oxidized Se species were more elevated and residual/elemental Se was lower than previous SEP results from ICP-AES suggested. For soils from the SEP sequence, XANES results indicated only partial recovery of carbonate, Fe-oxide and organic Se. This suggests Se was incompletely removed during designated extractions, possibly due to lack of mineral solubilization or reagent specificity. Selenium fractions associated with Fe-oxides were reduced in amount or removed after using hydroxylamine HCl for most soils examined. XANES results indicate partial dissolution of solid-phases may occur during extraction processes. This study demonstrates why precautions should be taken to improve the validity of SEPs. Mineralogical and chemical characterizations should be completed prior to SEP implementation to identify extractable phases or mineral components that may influence extraction effectiveness. Sequential extraction procedures can be appropriately tailored for reliable quantification of speciation in contaminated soils. [Display omitted] •XANES spectra indicated whole soils consisted of mostly elemental and organic Se and lower amounts of sorbed oxidized Se.•Sorbed Se(IV) was higher and elemental Se was lower than ICP-AES SEP results suggested.•A possible site and concentration dependence on SEP accuracy was also identified.•For extracted soils analyzed by XANES, only partial recovery of oxidized and organic Se was noted.•Lack of full recovery of Se indicates researchers should exert caution while employing SEPs. XANES results indicate incomplete recovery of selenium from a sequential extraction procedure due to lack of target phase dissolution.