The synergistic trigger of the reductive dissolution of Schwertmannite-As(III) and the release of arsenic from citric acid and UV irradiation

This paper focused on understanding the effect of citric acid (CA) on the reductive dissolution of Schwertmannite loaded with arsenite (SCH-As(III)) under UV irradiation and the release of arsenic (As). Results demonstrated that CA could significantly promote the reductive dissolution of SCH and SCH-As(III) with the assistance of ultraviolet light. Initially, the dissolved total iron (Fe) and As exhibited a rapid increase (0 to 240 min), followed by a quick decline due to the depletion of CA. It was found that the dissolved total Fe and As were present mainly as the species of Fe(II) and As(V), respectively. This can be attributed to the electronic transfer from ligand to metal in the complex of Fe(III)-(CA) n occurring under ultraviolet light, leading to the production of Fe(II) and organic radicals. Moreover, As(V) was further generated via the interaction of As(III) with OH from a series of radical reactions. In the dark, CA could also enhance the total Fe and As released from the SCH-As(III). The lower the initial pH level, the greater the amount of total dissolved Fe and As. The presence of Al 3+ clearly suppressed the reductive dissolution of SCH-As(III) triggered by CA and ultraviolet light. SEM spectra indicated that the mineralogical phases of SCH-As(III) before and after the reaction were discrepant. Thus, this study provides an insight into the stability of SCH-As(III) and the migration of As in the natural environment.