Arsenic release and transport during oxidative dissolution of spatially-distributed sulfide minerals

The oxidative dissolution of sulfide minerals, naturally present in the subsurface, is one of the major pathways of arsenic mobilization. This study investigates the release and fate of arsenic from arsenopyrite and löllingite oxidation under dynamic redox conditions. We performed multidimensional flow-through experiments focusing on the impact of chemical heterogeneity on arsenic mobilization and reactive transport. In the experimental setups the As-bearing sulfide minerals were embedded, with different concentrations and spatial distributions, into a sandy matrix under anoxic conditions. Oxic water flushed in the flow-through setups triggered the oxidative dissolution of the reactive minerals, the release of arsenic, as well as changes in pore water chemistry, surface-solution interactions and mineral precipitation. We developed a reactive transport model to quantitatively interpret the experimental results. The simulation outcomes showed that 40% of the arsenic released was reincorporated into a freshly precipitated iron-arsenate phase that created a coating on the mineral surface limiting the dissolution reactions. The faster dissolution rate of löllingite compared to arsenopyrite was responsible for sustaining the continuous release of As-contaminated plumes. The model also allowed shedding light on the spatial distribution, on the temporal dynamics, and on the interactions between arsenic sources (As-bearing minerals) and sinks (freshly formed secondary phases) in flow-through systems.