Effects of Phosphate, Silicate, and Bicarbonate on Arsenopyrite Dissolution and Secondary Mineral Precipitation

Managed aquifer recharge (MAR) has been applied to meet quickly growing water demands. However, during MAR operations, the injected water can induce dissolution of local minerals and result in the release of toxic metalloids, such as arsenic. To alleviate this concern, it is pivotal to understand the effects of injected water chemistry on arsenic mobilization during MAR. In this bench-scale study with geochemical conditions relevant to MAR operations, we investigated the impacts of three environmentally abundant oxyanions (i.e., phosphate, silicate, and bicarbonate) on arsenic mobilization from arsenopyrite (FeAsS) and secondary mineral precipitation. Phosphate showed time-dependent reversed effects on arsenic mobility. In short term (6 h), phosphate promoted the dissolution of FeAsS through monodentate mononuclear surface complexation. However, over a longer experimental time (7 days), the enhanced formation of secondary minerals, such as iron(III) (hydr)oxide (maghemite, γ-Fe₂O₃) and iron(III) phosphate (phosphosiderite, FePO₄·2H₂O), helped to decrease arsenic mobility through readsorption. Silicate increased arsenic mobility and bicarbonate decreased arsenic mobility during the entire 7 day reaction. The phosphate system showed the highest amount and largest sizes of secondary precipitates among the three oxyanions. These new observations provide a useful mechanistic understanding of the impacts of different oxyanions on arsenic mobilization and secondary mineral formation during the geochemical transformation of arsenic-containing sulfide minerals in MAR and also offer useful insight into water chemistry factors during pretreatment for MAR source water.