Photochemical redox cycling of iron in coastal seawater

Reduced iron, Fe(II), and hydrogen peroxide, H2O2, were measured in numerous laboratory solar simulation experiments. Under constant irradiation of freshly collected, 0.2 μm filtered water from Narragansett Bay (NBSW), H2O2 accumulates in a linear fashion for at least 2 h at rates from 30 to 59 nM h−1. Our measurements of Fe(II) accumulation in this study provide some of the first direct measurements of photochemical iron reduction in natural seawater at pH 8 in a controlled laboratory setting. At ambient pH, the peak concentration of Fe(II) ranged from 4 to 8% of the total iron concentration. Lowering the pH resulted in increased Fe(II) concentrations over the entire course of the experiment. Measured H2O2 accumulation, published iron oxide solubility products, and published reaction rates for iron oxidation and reduction by oxygen, hydrogen peroxide, and oxygen radicals were used to develop a kinetic model which accounts for the general magnitude and timing of Fe(II) concentration variations observed in our experiments. Our model shows that measured steady state Fe(II) concentrations in irradiated seawater are controlled largely by pH dependent oxidation by oxygen and hydrogen peroxide, and by both oxidation and reduction of iron by photochemically produced Superoxide.