Influence of Cadmium Sorption on FeS₂ Oxidation

Abstract

Pyrite oxidation leads to the formation of acid-mine drainage and the release of associated trace metals. A better understanding of the processes that influence pyrite oxidation will help to determine the rate of acid and cation evolution from pyritic mine tailings. The oxidation process is surface-controlled and may be influenced by cadmium sorption, which forms surface precipitates and complexes that may limit pyrite oxidation. The purpose of this research is to investigate the effect of cadmium sorption on the rate of FeS 2 oxidation by molecular oxygen. Raman spectroscopy was used to track the evolution of oxidized sulfur products, and X-ray absorption spectroscopy was used to quantify iron oxidation rates. Cadmium concentrations as low as 50 μM depressed pyrite oxidation rates. Oxygenation of amorphous FeS 2 (1 g/L) is described by a pseudo-first-order reaction with a rate constant of 6.85 × 10 - 5 s - 1; FeS 2 sorbed with 500 μmol of Cd g - 1 prior to oxygenation exhibited an oxidation rate 5.15-fold lower with a rate constant of 1.33 × 10 - 5 s - 1 . Raman and XANES data indicate that cadmium sorption influences FeS 2 oxidation. In natural systems, suppression of FeS 2 oxidation by Cd and other soft Lewis acids may retard the release of acidity and trace metals to the environment.