Surface passivation model explains pyrite oxidation kinetics in column experiments with up to 11 bars p(O2)

Despite decades of research in numerous experimental and field studies, the reaction kinetics of pyrite oxidation is still not characterized for high partial pressures of oxygen and near-neutral pH-levels. These conditions potentially exist in aquifers where oxidative site remediation, temporary water storage, or a leakage from a compressed air energy storage facility is present. For planning and monitoring of such field operations, their potential side effects on protected natural resources like groundwater have to be characterized. Thereby, site-scale assessments of such side effects of subsurface use by numerically modeling geochemical changes caused by the presence of oxygen need parametrization. Also, a function transferring results from simple, low pressure experiments to high pressure environments requires experimental bases. Pyrite oxidation can be the main consequence of oxygen intruding reduced aquifers. In this study, pyrite oxidation kinetics was examined at oxygen partial pressures from 0 to 11 bars, corresponding to an air intrusion in up to 500 m depth, at neutral pH-levels in high and low pressure flow-through column experiments representing aquifer conditions. A reaction rate equation was developed and evaluated with 1D PHREEQC numerical reactive transport models using experimental data as transfer function between high pressure and low pressure experiments. This model development included an improvement of established rate laws with a passivation term, which is, in contrast to previously published functions, dependent on the partial pressure of oxygen. The resulting model on passivated oxidation kinetics of pyrite at high oxygen partial pressures was able to reproduce independent experimental results acquired using different experimental set-ups. This assessment found the passivation to overcome the theoretical increase in pyrite oxidation kinetics caused by elevating oxygen partial pressure. These findings contribute to future experimental and modeling efforts for risk assessment and monitoring of oxygen-rich plumes in the subsurface.