Selenium contamination of surface and ground water poses a significant risk to human and wildlife health. As sorbents, common soil minerals such as goethite and hematite help naturally sorb and immobilize soluble selenite and selenate species. A wide array of sorption studies were carried out for goethite and hematite, and the sorption extents were generally considered to be dependent on solution as well as mineral surface conditions. However, a knowledge gap exists in the ligand exchange step between O in Se oxyanions and O in surface oxygen sites during Se sorption. To reveal the ligand exchange and macroscopic proton (H+) consumption reactions during Se complexation, batch sorption reactors were run using 18O-enriched (7 and 14 atom % 18O) and isotopically normal Fe oxides. After 24 hours of end-over-end mixing at constant pH, aqueous Se and 18O concentrations were measured using ion chromatograph (IC) and isotopic ratio mass spectrometry (IR-MS), respectively, and the Se loaded 18O oxides were examined by extended X-ray adsorption fine structure (EXAFS) for Se-O/Se-Fe coordination geometries. Sorption isotherm data were collected with isotopically normal Fe oxides to describe Se sorption and H+ consumption. Results show that sorption extent is highest at the lowest system pH (pH = 3), consistent with the highest observed zeta potential. Selenite sorption extent is always greater than selenate, and both can be best fitted with Langmuir model. Selenite sorption on both oxides was observed to form bridging bidentate inner-sphere complexes in EXAFS spectra. Together with H+ consumption data, this surface complex may release a H+ when the bond-forming O sites are doubly protonated or consume a H+ when singly protonated. Isotope tracing showed O release from oxide surface only at the most acidic pH, and likely from doubly protonated surface sites (=Fe-OH2+). For both oxides, approximately 22~23 % of the leaving O are broken off from surface Fe-O bonds at this pH. For selenate, EXAFS spectra suggested mainly outer-sphere complexes. The consumption of H+ in this case is most likely caused by hydrogen bond formation. Singly protonated surface O needs another H+ from solution to form bridging hydrogen bonds with a negatively charged pair of O from a selenate tetrahedron. The ratios of proton consumption to Se sorption under different pH for the isotherm experiments correspond to the reaction stoichiometries based on EXAFS conclusions.

Oxygen atom release during selenium oxyanion sorption on goethite and hematite

256th National Meeting (2018)

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