Managed aquifer recharge (MAR) is a promising engineering solution to relieve the stress of limited groundwater supplies. However, the water injected during MAR can alter the groundwater geochemistry and promote the dissolution of arsenopyrite, a common arsenic-bearing mineral, releasing undesirable arsenic into groundwater. The oxidative dissolution of arsenopyrite can also cause the mobilization of iron(II) and its oxidation to form new iron(III) (hydr)oxides that can, in turn, adsorb on and incorporate arsenic. Bicarbonate commonly exists in both groundwater and injected water, but the effects of bicarbonate on the dissolution of arsenopyrite and the subsequent secondary mineral nucleation and growth under conditions relevant to MAR are still unclear. Here, we studied 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 nanoscale iron-containing secondary minerals. To mimic aquifer environments, dissolution experiments were conducted in both open and closed systems, with and without CO2 exchange. In the open system, the dissolution of arsenopyrite, quantified by aqueous arsenic concentrations, was inhibited by increasing bicarbonate concentrations within 7 days. This trend is related to the equilibrium pH at different bicarbonate concentrations, inhibiting the proton-promoted dissolution of arsenopyrite. Meanwhile, most dissolved arsenic was adsorbed on the surface of arsenopyrite and on newly formed iron(III) (hydr)oxides, highlighting the attenuation of arsenic mobility by secondary mineral precipitates. Atomic force microscopy images suggested that the formation and aggregation of secondary mineral was promoted in increased bicarbonate concentrations. Raman spectroscopy confirmed that, at 1 and 10 mM bicarbonate concentrations, phase transformations, initially from siderite and magnetite to maghemite, also occurred, and maghemite was the dominant secondary mineral phase after 7 days. In the closed system, the dissolution of arsenopyrite and the formation of secondary minerals were similar in all the bicarbonate concentrations (0.1 mM, 1 mM, and 10 mM) for the 7-day reaction because of their similar reaction pH values. These new findings about arsenopyrite dissolution and secondary mineral precipitates controlled by bicarbonate and CO2 exchange will help us to understand the evolution of groundwater geochemistry during MAR and to design more sustainable MAR.

Arsenopyrite dissolution and secondary iron(III) (Hydr)oxide nucleation and growth in the presence of bicarbonate and CO2 Exchange

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