Oxidative Dissolution of Arsenic-Bearing Sulfide Minerals inGroundwater: Impact of Hydrochemical and HydrodynamicConditions on Arsenic Release and Surface Evolution

The dissolution of sulfide minerals can lead to hazardous arsenic levels in groundwater. This study investigates theoxidative dissolution of natural As-bearing sulfide minerals and the related release of arsenic underflow-through conditions. Columnexperiments were performed using reactive As-bearing sulfide minerals (arsenopyrite and lo''llingite) embedded in a sandy matrix andinjecting oxic solutions into the initially anoxic porous media to trigger the mineral dissolution. Noninvasive oxygen measurements,analyses of ionic species at the outlet, and scanning electron microscopy allowed tracking the propagation of the oxidative dissolutionfronts, the mineral dissolution progress, and the change in mineral surface composition. Process-based reactive transport simulationswere performed to quantitatively interpret the geochemical processes. The experimental and modeling outcomes show that pore-water acidity exerts a key control on the dissolution of sulfide minerals and arsenic release since it determines the precipitation ofsecondary mineral phases causing the sequestration of arsenic and the passivation of the reactive mineral surfaces. The impact ofsurface passivation strongly depends on theflow velocity and on the spatial distribution of the reactive minerals. These resultshighlight the fundamental interplay of reactive mineral distribution and hydrochemical and hydrodynamic conditions on themobilization of arsenic from sulfide minerals inflow-through systems.