Paper No. 8
Presentation Time: 3:10 PM


VOORHIS, James C., Earth Sciences, Dartmouth College, HB 6105, Dartmouth College, Hanover, NH 03755 and RENOCK, Devon, Department of Earth Sciences, Dartmouth College, Fairchild Science Center, HB 6105, Hanover, NH 03755,

The mobility and availability of arsenic (As) in anoxic environments is controlled by abiotic and biologically-mediated redox reactions, sorption to mineral surfaces, and dissolution/precipitation reactions. Pyrite has been shown to attenuate dissolved arsenite, As(III), concentrations via a surface-catalyzed reduction of As to arsenopyrite- or orpiment-like solids (Bostick and Fendorf, 2003). In this study, we investigate the energetics, reaction mechanisms, and kinetics of two redox half-reactions occurring on pyrite: 1) the reduction of As(III), and 2) the oxidation of reduced As phases on the mineral surface. The redox reactions were investigated by electrochemical methods using a three electrode voltammetric cell with a pyrite powder microelectrode (PME) as the working electrode.

Two redox peaks were identified under anoxic conditions at pH 2.5. First, a broad peak appears in the voltammogram from +0.1 V to -0.5 V vs Ag/AgCl that is not present for pyrite alone. This peak occurs in the same potential range as the known reduction of As(III) to As(0) on Pt electrodes at low pH, suggesting that a similar reduction mechanism occurs on pyrite (Cabelka et al, 1994). This peak is absent in voltammograms obtained at pH >7, indicating that pyrite surface chemistry and/or As speciation is suppressing As reduction at high pH. Second, an oxidation peak at +0.2 V corresponds to the direct oxidation of reduced As species on pyrite. The area of this peak increases with respect to the time the pyrite electrode is held at negative potentials prior to the positive-going sweep. This result indicates that more As is reduced on the surface and subsequently stripped away by oxidation. In addition, the amount of electric charge transferred in the reduction half-reaction is ~1/3 the amount transferred in the oxidation half-reaction. The asymmetry of the redox couple suggests two reaction mechanisms are occurring on the surface: 1) direct reduction of As(III) by pyrite, and 2) a surface-mediated process involving a sorbed reductant.

This is the first study that uses a PME to evaluate As redox reactions on the pyrite surface. The results of this study describe the reaction mechanisms for arsenic reduction and oxidation on pyrite, and will provide an improved understanding of As behavior in anoxic environments.