Managed aquifer recharge (MAR) is viable method for water storage, recycling, and reuse, alleviating the stress of groundwater over-extraction. However, water injected during MAR can trigger unexpected oxidative dissolution of arsenic-bearing iron sulfide minerals, releasing arsenic into groundwater. In particular, the effect of bicarbonate on sulfide mineral dissolution and subsequent secondary mineral precipitation under conditions relevant to groundwater is still unclear. Using arsenopyrite, one of the most common arsenic-bearing minerals in subsurface environments, here we examined the effects of bicarbonate concentrations (0.01 mM, 0.1 mM, 1 mM, and 10 mM) on the dissolution of arsenopyrite and the precipitation of iron-containing secondary minerals. Furthermore, to mimic aquifer environments with and without CO<sub>2</sub> exchange, the dissolution experiments were conducted in both open and closed systems. In the open system, the mobilization of arsenic in the short term (&lt; 6 hours) was similar for all bicarbonate concentrations, while in long term (7 days), the aqueous arsenic concentrations were reduced by increasing bicarbonate concentrations. This long-term arsenic dissolution trend is attributed to the equilibrium pH at different bicarbonate concentrations, because a high bicarbonate concentration will result in high pH and will slow the proton-promoted dissolution of arsenopyrite. We also measured the total arsenic concentrations dissolved from arsenopyrite. When we delineated aqueous and adsorbed arsenic concentrations, we found that most of the dissolved arsenic was adsorbed on the surface of arsenopyrite and iron(III) oxide minerals. In addition, higher bicarbonate concentrations reduced both the absorbed and total dissolved arsenic. Based on nanoscale observations using atomic force microscopy, the increased bicarbonate concentrations promoted secondary mineral precipitation. Raman spectroscopy confirmed that newly formed mineral phases were maghemite and magnetite. The results in the closed system revealed that, due to similar equilibrium pH values for the bicarbonate concentrations of 0.1 mM, 1 mM, and 10 mM, arsenic mobilization in all the bicarbonate concentration systems was similar during the entire 7-day reaction. These new observations of bicarbonate’s effects on arsenopyrite reactions illuminate the role of bicarbonate in arsenic mobilization in groundwater systems and can aid in designing safe and sustainable MAR operations.

Roles of bicarbonate in arsenopyrite dissolution and nanoscale secondary iron mineral precipitation in managed aquifer recharge systems

ACS Fall 2021

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