Influence of pH on the Kinetics and Mechanism of Photoreductive Dissolution of Amorphous Iron Oxyhydroxide in the Presence of Natural Organic Matter: Implications to Iron Bioavailability in Surface Waters

In this study, we investigate the influence of pH on the kinetics and mechanism of photoreductive dissolution of amorphous iron oxyhydroxide (AFO) in view of the recognition that the light-mediated dissolution of iron oxides controls Fe availability in many natural waters. Our results show that both ligand-to-metal charge transfer (LMCT) and photogenerated superoxide (O₂^•–) play an important role in AFO photoreductive dissolution in the presence of the chosen surrogate of natural organic matter, Suwannee river fulvic acid (SRFA). The pH dependence of LMCT-mediated AFO photoreductive dissolution is mainly controlled by the influence of pH on AFO solubility. A decrease in pH increases the concentration of the dissolved and more photolabile Fe­(III)–SRFA complex present in equilibrium with AFO, a complex in which Fe­(III) is readily reduced by LMCT. The pH dependence of superoxide-mediated Fe­(III) reduction (SMIR) is also controlled by the influence of pH on AFO solubility with an increase in the dissolved inorganic Fe­(III) concentration with the decrease in pH resulting in an increased rate of SMIR. No influence of pH was observed on the steady-state O₂^•– concentration generated on SRFA irradiation as well as the O₂^•– decay rate in the presence of SRFA, suggesting that the concentration and lifetime of O₂^•– are not important factors in controlling the pH dependence of O₂^•–-mediated AFO dissolution. Overall, the results of this study show that the impact of acidification of natural waters on Fe availability will be much more pronounced when Fe is present as iron oxyhydroxide compared to that observed when organically bound Fe dominates with this effect because of the strong dependency of iron oxyhydroxide solubility on pH. The increased rate and extent of dissolution of iron oxyhydroxides on the acidification of natural waters will also have implications to the fate of other contaminants (such as heavy metals and organic compounds) that may be present on the iron oxyhydroxide surface.