Oxidative Dissolution of Arsenic-Bearing Sulfide Minerals in Groundwater: Impact of Hydrochemical and Hydrodynamic Conditions on Arsenic Release and Surface Evolution

The
dissolution of sulfide minerals can lead to hazardous arsenic
levels in groundwater. This study investigates the oxidative dissolution
of natural As-bearing sulfide minerals and the related release of
arsenic under flow-through conditions. Column experiments were performed
using reactive As-bearing sulfide minerals (arsenopyrite and löllingite)
embedded in a sandy matrix and injecting 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 dissolution fronts, the mineral dissolution progress, and
the change in mineral surface composition. Process-based reactive
transport simulations were 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
of secondary mineral phases causing the sequestration of arsenic and
the passivation of the reactive mineral surfaces. The impact of surface
passivation strongly depends on the flow velocity and on the spatial
distribution of the reactive minerals. These results highlight the
fundamental interplay of reactive mineral distribution and hydrochemical
and hydrodynamic conditions on the mobilization of arsenic from sulfide
minerals in flow-through systems.