THE EFFECT OF IMPURITIES ON ABIOTIC PYRITE OXIDATION KINETICS: AN ELECTROCHEMICAL STUDY OF SYNTHETIC PYRITE DOPED WITH AS, CO AND NI

Natural pyrite almost always contains As, Co, or Ni in some combination, prompting a systematic study of the effect of these common minor elements on its oxidation rate. To determine if the impurity content of pyrite may affect its potential to produce acid drainage, the oxidation behavior of chemical-vapor-transport-grown synthetic pyrite, both undoped and doped with As, Co, or Ni, was investigated with cyclic and AC voltammetry in a standard three electrode electrochemical cell. Also investigated was pyrite from Leadville, Colorado containing primarily As and Pb impurity. A solution for the cyclic and AC voltammetry oxidation experiments was chosen to approximate water affected by acid drainage: pH 1.78 sulfuric acid containing 1 mM ferric iron, open to atmospheric oxygen. Results were compared to resistivity, carrier concentration and carrier mobility measured by the Hall and van der Pauw method. Pyrite with impurities generates more current from the reduction of ferric iron and the anodic dissolution of pyrite reactions, which are 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. Higher current from pyrite with impurities to the ferric iron reduction reaction in the AC and cyclic voltammetry experiments combined with evidence from solid state measurements suggests that increased reactivity may be due to bulk defect levels within the band gap, resulting in higher density of occupied surface states at the solid-solution interface. The increased reactivity of As-doped pyrite may be related to p-type conductivity resulting in corrosion by holes. The results of this study suggest that considering the impurity content of pyrite in mining waste may lead to more accurate risk assessment of acid producing potential.