Arsenic oxidation and immobilization in acid mine drainage in karst areas

High concentrations of arsenic (As) occur in acid mine drainage (AMD), while the mechanisms governing its distribution along the flow of AMD are not fully understood. In this study, As species distribution was surveyed along the flow of an AMD in Jiaole coal mine in a typical kast area, in which length of creek is about 1100 m. AMD from the discharging source contained 1754.2 μg/L As (1570.0 μg/L in As (III)) and 644.1 mg/L Fe (all in Fe (II)) at pH 3.45. Both As and Fe concentrations decreased drastically to trace levels along the flow in the creek. As(III) oxidation to As(V) and Fe(II) oxidation to Fe(III) were discovered in a short distance from the discharging source. Lab experiments were performed to unveil the mechanisms governing As and Fe species distribution. Biological mechanism governed As(III) and Fe(II) oxidation in the AMD phase without contact with solid matrix, while different mechanisms governed the oxidation in the presence of solid matrix at different stages of AMD flow. At the beginning of AMD discharge, its contact with the soil matrix in rich of carbonate minerals in the karst area facilitated Fe(II) oxidation by O₂ due to pH rise, which generated reactive oxidants for As(III) oxidation and iron oxyhydroxides for As adsorption or co-precipitation. Along the AMD flow, bacteria in the underlying sediments profoundly accelerated the biological oxidation of As(III) and Fe(II) as well as the co-precipitation into the sediments. Findings of this study deepen the understanding of As transport and transformation along the AMD flow, particularly in karst areas.