The Impact of Dissolved Organic Matter on Arsenic Mobilization from Goethite in the Presence of Silicic Acid and Phosphate under Reducing Conditions

The release of arsenic (As) adsorbed onto iron oxide (Fe-oxide) surfaces is affected by dissolved organic matter (DOM), phosphate (hereafter referred to as PO₄), and silicic acid (H₄SiO₄). Further, the reductive dissolution of As from Fe-oxide phases is also affected in reduced soils and sediments. Thus, the aim of this study was to understand the adsorption competition and redox-related mechanisms by which DOM affects As mobilization from Fe-oxide in a complex system containing both H₄SiO₄ and PO₄. The results demonstrated that the DOM-driven, microbially mediated As biotransformation, and, thus, mobilization of As significantly increased when both dissolved inorganic H₄SiO₄ and PO₄ were present, as the co-presence of H₄SiO₄ and PO₄ decreased As adsorption sites on Fe-oxides. The availability of DOM in the co-presence of H₄SiO₄ and PO₄ increased the microbial activity in the system by providing more substrates for microbial metabolism, which also decreased the redox potential (reducing conditions) and consumed acidity, causing the pH to increase from 4 to 6.8. In addition, DOM, H₄SiO₄, and PO₄ competed with As for sorption sites on Fe-oxides. The effects of DOM on As mobility by DOM-mediated or -triggered redox reactions were apparently stronger in the co-presence of H₄SiO₄ and PO₄ than DOM competition with arsenate for sorption sites on Fe-oxide alone. These findings advance our understanding of As mobilization processes in natural systems and can provide information for soil As management.