Use of Dissolved Sulfur Species to Measure Pyrite Dissolution in Water at pH 3 and 6

The oxidative dissolution of pyrite (FeS₂ ) is difficult to study because of redox transformations of the iron and sulfur, and adsorptive and precipitative loss of products from solution. Quantitative measurement of pyrite dissolution can perhaps be done most accurately with electrochemical techniques, but monitoring of dissolution products is of interest for interpretation of water-quality data at mine drainage and other field sites where pyrite dissolution is occurring. This l aboratory study was undertaken to assess the usefulness of the sum of sulfate, sulfite, and thiosulfate in solution as a reaction progress variable, or RPV, for abiotic pyrite dissolution. Sulfate was the only sulfur species detected in solution, and thus was chosen as the RPV. Previous investigators have demonstrated that ferrous iron is not reliable as an RPV because of adsorption to the pyrite surface. Results indicated that sulfate is also not a highly accurate RPV. There was a sulfate loss from s olution, perhaps by adsorption, with the extent of loss varying with solution conditions. While not a highly accurate RPV for kinetics modeling, the sulfate concentration provided a measure by which relative rates of abiotic pyrite dissolution could be assessed for pH 3 and 6, and with oxygen and ferric iron as oxidants. Sulfate production was greatest at pH 6 in the presence of oxygen, but no sulfate was produced under anoxic conditions (Fe³⁺ added as oxidant) at the same pH. The s ulfate release under oxic conditions at this circumneutral pH was an order of magnitude greater than the release at pH 3 under both oxic and anoxic conditions.