Electrochemical and spectroscopic investigations of redox interactions between aqueous selenium species and galena surfaces in acidic conditions

Abstract

As a redox sensitive element, the speciation and, therefore, the behavior of selenium in the geochemical environment can be affected by interactions with semiconducting minerals that can mediate redox reactions on the mineral surface. To determine the capacity of the sulfide mineral galena (PbS) to mediate such reactions, voltammetry was performed with powdered galena as working electrode in powder microelectrode (PME) experiments. Current/voltage (I/V) curves are generated using this setup where the voltage of a current peak can be related to the potential (Eh) of a specific redox transition and the current is indicative of the intensity and thus the kinetics of the reaction. At pH 4.6, three anodic (oxidizing) and cathodic (reducing) peaks in current were identified comprising three different redox couples assigned to (1) HSeO3- + 7H+ + 6e- → H2Se + 3H2O at 0.0 V, (2) HSeO3- + H2Se + 2e- + 2H+ → 2Se0 + 3H2O at −430 mV, and (3) Se0 + 2e- + 2H+ → H2Se at −560 mV. Reversibility of all three couples was affected by the loss of cathodically produced H2Se to the solution phase. The oxidation of Se4+ to some higher oxidation state, presumably Se6+, was identified at 400 mV; however, assignment of the anodic feature could not be made due to the lack of a corresponding cathodic feature likely due to the electrochemical inactivity of Se6+. XPS analysis revealed the formation of Se0, likely of the black variety, after polarization at −125 mV attributed to the conproportionation of Se4+ and Se2- species in solution. In contrast, red Se0 was formed after polarization at −490 mV which was attributed to conproportionation mediated by the PbS surface. The latter process produces a voltammetric signal whereas the former process is not be detectable by voltammetry. Pb 4f and S 2p XPS spectra exhibited no correlation with the Se 3d spectra suggesting the lack of participation of PbS constituents in the redox transitions of selenium. In other words, PbS merely acts as a semiconductor mediating electron transfer and not as a source of electron for the reduction of selenite. This study also proposes a novel technique for assessing reaction kinetics strictly using voltammetric data for estimating reaction rates, rate laws, and the number of participating PbS 2 surface sites occupied by selenium atoms during forward reactions. For example, for the reduction of bi-selenite to selenide at pH 4.6, the reaction equation can be written as: HSeO3- + 7H+ + 6e- → H2Se + 3H2O Eo = 387 mV The rate equation can be written as: r = k [HSeO3-] [e−]6 [H+]7 with a rate constant of about 8.8 1020 s-1mol-13. While redox reaction can produce or consume hydronium in solution, this process can change the pH and produce pH gradients in a certain volume around the PME. This study proposes a mechanism to quantify this pH change (from 4.6 to 4.8 for the reaction shown above) within a reaction volume which is on the order of 1-2 mm around the PME which is 0.1 mm in diameter. This information is important to evaluate diffusion kinetics about the reactive area during the redox process, which may control the overall reaction kinetics and to semi-quantify pH gradients that occur and are close-to impossible to measure in the pores of soils under environmental conditions.