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

Area of Science:

• Environmental Chemistry
• Soil Science
• Analytical Chemistry

Background:

• Selenium (Se) contamination in soils poses risks to livestock through plant ingestion.
• Understanding Se speciation and bioavailability is crucial for effective environmental mitigation strategies.
• Sequential extraction procedures (SEP) are commonly used to analyze Se in soils but may have limitations.

Purpose of the Study:

• To evaluate the accuracy of a sequential extraction procedure (SEP) for determining selenium (Se) speciation in contaminated soils.
• To compare SEP results with X-ray absorption spectroscopy (XAS) analyses of whole and sequentially extracted soil samples.

Main Methods:

• Utilized X-ray absorption spectroscopy (XAS), specifically X-ray absorption near-edge structure (XANES), to analyze Se speciation.
• Applied a sequential extraction procedure (SEP) to remove various Se fractions (soluble, PO4-extractable, carbonate, Fe-oxides, organic, residual).
• Compared XAS data from whole soils and fractions obtained through the SEP.

Main Results:

• XANES analysis of whole soils indicated predominant Se(0) and Se(-II) species, with Se(IV) associated with carbonates and Fe-oxides.
• SEP results underestimated oxidized Se species and overestimated residual/elemental Se compared to XAS.
• XAS analysis of extracted soil fractions showed incomplete removal of Se associated with carbonates, Fe-oxides, and organic matter, suggesting partial dissolution during extraction.

Conclusions:

• The validity of standard SEPs for Se speciation in contaminated soils is questionable due to incomplete extraction and potential artifact formation.
• Pre-analysis mineralogical and chemical characterization is essential for optimizing SEP effectiveness.
• Tailoring SEPs based on soil properties is necessary for reliable quantification of selenium speciation.