Electrochemical, Spectroscopic, and Computational Investigations on Redox Reactions of Selenium Species on Galena Surfaces.

Despite previous studies investigating selenium (Se) redox reactions in the presence of semiconducting minerals, Se redox reactions mediated by galena (PbS) are poorly understood. In this study, the redox chemistry of Se on galena is investigated over a range of environmentally relevant Eh and pH conditions (+0.3 to −0.6 V vs. standard hydrogen electrode, SHE; pH 4.6) using a combination of electrochemical, spectroscopic, and computational approaches. Cyclic voltammetry (CV) measurements reveal one anodic/cathodic peak pair at a midpoint potential of +30 mV (vs. SHE) that represents reduction and oxidation between HSeO₃⁻ and H₂Se/HSe⁻. Two peak pairs with midpoint potentials of −400 and −520 mV represent the redox transformation from Se(0) to HSe⁻ and H₂Se species, respectively. The changes in Gibbs free energies of adsorption of Se species on galena surfaces as a function of Se oxidation state were modeled using quantum-mechanical calculations and the resulting electrochemical peak shifts are (−0.17 eV for HSeO³⁻/H₂Se, −0.07 eV for HSeO³⁻/HSe⁻, 0.15 eV for Se(0)/HSe⁻, and −0.15 eV for Se(0)/H₂Se). These shifts explain deviation between Nernstian equilibrium redox potentials and observed midpoint potentials. X-ray photoelectron spectroscopy (XPS) analysis reveals the formation of Se(0) potentials below −100 mV and Se(0) and Se(−II) species at potentials below −400 mV. [ABSTRACT FROM AUTHOR]