Selenium is a concern for human and wildlife health due to exposure to toxic levels within surface waters, plants after bioaccumulation, and groundwater affected by industrial and mining activities. Sorption of selenium oxyanions (selenite and selenate) on soil minerals greatly reduces its bioavailability, and may be an important process in the biogeochemical cycling of selenium. In our study, batch sorption experiments with goethite and hematite, two important iron oxides commonly found in soil, were performed to investigate Se surface complexation under both acidic and circumneutral pH along with stable oxygen isotope tracing and spectroscopic techniques. Sorption isotherms showed selenite sorption extent was always greater than selenate sorption extent and could best be modeled with the Langmuir model. Maximum sorption extent for both selenite and selenate was highest at lowest pH, consistent with observed higher positive surface charge evident in zeta potential measurements. The sorption step was observed to result in the release of hydroxide ions within batch experiments utilizing acid titration to maintain constant pH. The amount of hydroxide ions released per sorbed selenium oxyanion (-0.1 to 1.0) could be explained by the surface charge measured by zeta potential and the Se-O-Fe bonding coordination measured by Se EXAFS. For selenite, sorption onto goethite and hematite was observed to form a bridging bidentate inner sphere surface complex. This surface complex may release O as OH- or H2O depending on surface charge. To test the source of OH- release during sorption, 18O-isotope tracing experiments were performed using 18O-enriched (7 and 14 atom % 18O) goethite and hematite and isotopically normal Se oxyanions and water. The results suggest that O release from the surface of iron oxides may occur only at acidic conditions, likely from doubly protonated surface sites (=Fe-OH2+). For selenate, EXAFS spectra suggested mainly outer sphere complexes, despite significant OH- release observed in titration experiments, which requires further exploration. These results suggest iron oxide surface O atoms can be more readily exchanged during sorption processes in acidic environments.

Oxygen atom release during selenium oxyanion sorption on goethite and hematite

254th National Meeting (2017)

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