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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
journal contribution
posted on 2020-05-21, 17:39 authored by Shikha Garg, Guowei Xing, T. David WaiteIn 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.
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pH dependenceSMIRAFO solubilityiron oxyhydroxide surfaceiron oxyhydroxide solubilityLMCT-mediated AFO photoreductive dissolutioniron oxides controls Fe availabilityNatural Organic Matterligand-to-metal charge transferAmorphous Iron OxyhydroxideSuwannee river fulvic acidinfluenceconcentrationiron oxyhydroxideSRFAAFO photoreductive dissolution
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