Predicting iron speciation in coastal waters from the kinetics of sunlight-mediated iron redox cycling

Iron is a critical nutrient in marine systems, whose solubility is strongly influenced by the presence of natural organic ligands and sunlight. Because these parameters are never constant, and because they initiate processes that occur on a variety of timescales, equilibrium models are inadequate at predicting iron speciation in the presence of natural organic matter (NOM) and light. Instead, a dynamic redox cycle based on kinetically governed transformations of iron is proposed as a tool for predicting iron speciation. The presence of NOM is a critical inhibitor of precipitation of iron as insoluble oxyhydroxides, and dissociation of organic ferric complexes represents a limiting step in loss of iron from the soluble pool. Sunlight causes photochemical reduction of organic ferric complexes, producing transient amounts of ferrous iron. Although the process drives iron away from the ferric state, photo-oxidation of ligands allows some organically complexed iron to "escape" into the inorganic part of the cycle, and thus increases the rate at which soluble iron is lost from the system. Model predictions of spatial and temporal variations in ferrous iron and hydrogen peroxide concentrations are in excellent agreement with field measurements. These results indicate that detailed kinetic modelling of fundamental chemical processes is an extremely useful approach to prediction of iron speciation at an ecosystem scale.