Evaluation of Selenium Speciation and a Sequential Extraction Procedure Using X-Ray Absorption Spectroscopy.

Soil sequential extraction procedures (SEP) are employed to identify target elements in various phases through a series of progressively stronger extractions. While they are cost-effective, quick techniques that are easily accomplished, many researchers question the validity of their results because SEPs are operationally defined. There are also a number of potential limitations, such as lack of reagent specificity for target phases, partial dissolution of solid phases, and redistribution of solubilized elements. The objectives of this work were to identify Se speciation and geochemical phases in phosphate mine soils and to assess an SEP formulated for selenium (Se) in calcareous soils using synchrotron-sourced X-ray absorption spectroscopy (XAS). Selenium was extracted from soils in a step-wise approach using a SEP for phases that are soluble, exchangeable, associated with carbonated, associated with iron oxides, organic, and elemental. Following each step of the extraction, soils were subsequently analyzed using XAS. Highest Se relative abundances were attributed to elemental and organic Se. Additional phases present included selenite (SeO₃^2-) sorbed and/or co-precipitated with carbonate and iron oxide minerals. However, XAS results indicated estimation errors related to the SEP and possible incomplete dissolution of both sorbed and organic Se phases. Because of this, our results suggest that caution should be exerted prior to SEP implementation.