Effects from As, Co, and Ni impurities on pyrite oxidation kinetics: studies of charge transfer at a semiconductor/electrolyte interface
Lehner, Stephen W
Pyrite crystals were synthesized with no impurities and doped with As, Co, or Ni to be investigated along with natural samples, using electrochemical techniques, wet chemical kinetic experiments, and solid-state measurements of semiconducting properties to determine the effect of impurity content on pyrite’s oxidation behavior. Potential step experiments, cyclic voltammetry, AC voltammetry, and electrochemical impedance spectroscopy (EIS) were performed along with mixed-flow-through and batch reactor experiments to measure rates of pyrite oxidation and propose charge transfer mechanisms. A solution near pH 2 containing 1 mM ferric iron, open to atmospheric oxygen, was chosen for all experiments to approximate water affected by acid mine drainage. For the electrochemical experiments van der Pauw/Hall effect measurements determined resistivity, carrier concentration and carrier mobility of pyrite electrodes. Concentration of As, Co, and Ni in pyrite electrodes was determined with laser ablation inductively coupled plasma mass spectroscopy. The anodic dissolution of pyrite and the reduction of ferric iron half-reactions were taken as proxies for natural pyrite oxidation. Pyrite containing no impurities is least reactive. Pyrite with As is more reactive than pyrite with either Ni or Co despite lower dopant concentration. Solid state measurements, AC voltammetry, and EIS data agree with previous studies reporting As, Co and Ni impurity defect states residing at different energy levels within the band gap. The current density generated from potential step experiments increased with increasing As concentration indicating the higher reactivity of As-doped pyrite may be related to p-type conductivity and corrosion by holes. The mixed-flow-through and batch experiments also showed higher rates of oxidation for pyrite with impurities as determined by inductively coupled plasma atomic emission spectroscopy. However, the Co-doped and As-doped pyrite rates were statistically equal. The results indicate that Co and Ni are released to solution as the oxidation reaction progresses while As is not. Evidence from the AC voltammetry and EIS suggests that the higher reactivity of doped pyrite results from introduced defect levels which lead to higher density of occupied surface states at the solid-solution interface and increased metallic behavior. Based on the EIS equivalent circuit model, it is hypothesized that charge transfer occurs directly from the pyrite conduction band but also in a two step recombination process mediated by intra-bandgap defect surface states. .