Pyrite oxidation in unsaturated aquifer sediments. Reaction stoichiometry and rate of oxidation

The oxidation of pyrite (FeS2) contained in unsaturated aquifer sediment wa s studied by sediment incubation in gas impermeable polymer laminate bags. Reaction progress was followed over a period of nearly 2 months by monitori ng the gas composition within the laminate bag. The gas phase in the incuba tion bags became depleted in Q(2) and enriched in CO2 and N-2 and was inter preted as due to pyrite oxidation in combination with calcite dissolution. Sediment incubation provides a new method to estimate low rates of pyrite o xidation in unsaturated zone aquifer sediments. Oxidation rates of up to 9. 4.10(-10) mol FeS2/g.s are measured, and the rates are only weakly correlat ed with the sediment pyrite content. The reactivity of pyrite, including th e inhibition by FeOOH layers formed on its surface, apparently has a major effect on the rate of oxidation. The code PHREEQC 2.0 was used to calculate the reaction stoichiometry and partitioning of gases between the solution and the gas phase. Pyrite oxidation with concurrent calcite dissolution was found to be consistent with the experimental data while organic carbon oxi dation was not. The reaction involves changes in the total volume of the ga s phase. The reaction scheme predicts the volume Of O-2 gas consumed to be larger than Of CO2 produced. In addition the solubility Of CO2 in water is about 30 times larger than Of O-2 causing a further decrease in total gas v olume. The change in total gas volume therefore also depends on the gas/wat er volume ratio and the lower the ratio the more pronounced the loss of vol ume will be. Under field conditions the change in total volume may amount u p to 20% in the absence of calcite and over 10% in the presence of calcite. Such changes in gas volume during the oxidation of pyrite are expected to result in pressure gradients causing advective transport of gaseous oxygen.