Oxidative Dissolution of Sulfide Minerals in Porous Media Under Evaporative Conditions: Multiphase Experiments and Process-Based Modeling

The dissolution of sulfide minerals in subsurface porous media has important environmental implications. We investigate the oxidative dissolution of pyrite under evaporative conditions and advance a mechanistic understanding of the interactions between multiple physical processes and mineral/surface reactions. We performed a set of experiments in which initially water saturated and anoxic soil columns, containing a top layer of pyrite, are exposed to the atmosphere under no evaporation (single-phase) and natural evaporative (two-phase) conditions. The oxidative dissolution of pyrite was monitored by non-invasive high-resolution measurements of oxygen and pH. Additionally, we developed and applied a multiphase and multicomponent reactive transport model to quantitatively describe the experimental outcomes and elucidate the interplay between the physico-chemical mechanisms controlling the extent of pyrite dissolution. The results confirm that the extent of pyrite dissolution under single-phase conditions was constrained by the slow diffusive transport of oxygen in the liquid phase. In contrast, during evaporation, the evolution of fluid phases and interphase mass transfer processes imposed distinct physical constraints on the dynamics of pyrite oxidation. Initially, the invasion of the gaseous phase led to a fast delivery of high oxygen concentrations in the reactive zone and thus markedly increased pyrite oxidation and acidity/sulfate production. However, such enhanced release of reaction products was progressively limited over time as drying conditions prevailed in the reactive zone and inhibited pyrite oxidation. The transient phase displacement was also found to control the distribution of aqueous species and formation of secondary minerals by creating spatio-temporally variable redox conditions.