Iron redox transformations in the presence of natural organic matter: effect of calcium.

The effects of calcium on iron redox transformations in acidic non-irradiated and irradiated Suwannee River Fulvic Acid (SRFA) solutions are investigated in this study. Our results reveal that, even though calcium is redox-inert, it affects the redox transformations of iron with increased Fe(III) reduction rates under irradiated conditions and decreased Fe(II) oxidation rates in the dark in the presence compared to the absence of calcium. While the exact mechanism via which the Fe(III) reduction rate under irradiated conditions is impacted by calcium addition is not clear, the formation of more photo-labile weakly complexed Fe(III)SRFA is most consistent with our experimental results. An observed decline in the Fe(II) oxidation rate in non-irradiated and previously-irradiated SRFA solutions on addition of calcium can be rationalized by formation of more weakly bound Fe(II) and Fe(III). The higher Fe(III) reduction rates and lower Fe(II) oxidation rates in the presence compared to the absence of calcium will help in maintaining higher concentrations of Fe(II) thereby increasing the bioavailability of iron in calcium-containing waters. Based on our experimental results we have developed a mathematical model that well describes the iron redox transformations mediated by SRFA in calcium-containing waters under irradiated and non-irradiated conditions.