Photochemically induced redox reactions in seawater, I. Cations

Experiments have been conducted to determine whether the speciation of the manganese and iron redox couples can be influenced by light and organic matter in seawater. The results indicate that photochemically induced reduction of Mn(IV) to Mn(II) occurs through the mediation of humic acids. The rate of the reaction is dependent on many variables, some of which are identified here. Stirring plays an important role in the reductive dissolution process, the effect of which is dependent on the organic loading. It is suggested that this results from an adsorptive competition between organic matter and photoproduced reductants such as H2O2. Photochemically induced reduction reaction, at least in part, controls the speciation and thus biological availability of manganese in surface seawater. The results obtained from studies of the iron redox system show generally poor reproducibility and no conclusive evidence for photochemically induced reduction of Fe(III) under seawater conditions. These results are perhaps understandable given the fast reoxidation reactions by hydrogen peroxide and oxygen at seawater pH and the use of membrane filtration to separate soluble and insoluble iron. Reproducible results are obtained in stirred seawater and show a gradual decrease in levels of 0.2 μm filterable iron over time. The light replicates show a slower rate of iron loss from solution than in the dark. The possible implications of these results for iron bioavailability in seawater are discussed.