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DIVISION S-2-SOIL CHEMISTRY

Selenate and Selenite Sorption on Iron Oxides

An Infrared and Electrophoretic Study

^a ManTech Environmental Research Services Corp., R.S. Kerr Environmental Research Center, 919 Kerr Research Drive, Ada, OK 74821-1198 USA
^b USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Road, Riverside, CA 92507-4617 USA

su.chunming{at}epa.gov

We studied selenate and selenite sorption by amorphous Fe oxide [am-Fe(OH)₃] and goethite (-FeOOH) as a function of time (25 min–96 h), pH (3–12), ionic strength (0.01–1.0 M NaCl), and total Se concentration (0.0001–1.0 M). We examined sorbed selenate and selenite by in situ attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and electrophoresis to deduce sorption mechanisms. Sorption of both selenate and selenite reached equilibrium in <25 min and the sorption isotherm was not reversible. Increasing ionic strength decreased selenate sorption but did not affect selenite sorption. The presence of either selenate or selenite lowered the electrophoretic mobility (EM) and decreased the point of zero charge (PZC) of both sorbents, suggesting inner-sphere complexation for both selenate and selenite species. Both in situ ATR–FTIR and DRIFT difference spectra showed bidentate complexes of selenate with am-Fe(OH)₃. The structure of selenite complexes in am-Fe(OH)₃ –solution interface was uncertain due to insensitivity of the in situ ATR–FTIR technique. The DRIFT spectra of selenite on am-Fe(OH)₃ showed ₃ splitting as evidence of complexation. The DRIFT spectra of selenite on goethite showed bridging bidentate complex of selenite. We conclude that the influence of ionic strength on Se sorption cannot be used as a criterion for distinguishing outer- vs. inner-sphere complex formation.

Abbreviations: ATR–FTIR, attenuated total reflectance–Fourier transform infrared • DRIFT, diffuse reflectance infrared Fourier transform • EM, electrophoretic mobility • EXAFS, extended x-ray absorption fine structure • ICP-AES, inductively coupled plasma-atomic emission spectrophotometry • IR, infrared • PZC, point of zero charge

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