Redox Chemistry of Iodine in Seawater

The thermal and photochemical conversion of iodide to iodate in seawater is examined by means of thermodynamic, kinetic, and frontier molecular orbital theory considerations. The reaction is not significant because of lack of available oxidants in the ocean and their slow or negligible reactivity. Oxidation of iodide by triplet oxygen (3O2), O3, and H2O2 leads to the reactive intermediates HOI and I2, which are electron acceptors. These intermediates re-form iodide on reaction with reduced material and will react with natural organic matter faster than most other reduced species in seawater. The conversion of iodide to iodate may occur in seawater by bacterial processes and at the microlayer, where stronger oxidants may be present. Oxidation occurs in the atmosphere when volatile iodine species are released from the sea surface. The chemistry of iodine in the ocean is coupled to organic carbon production because of the difficulty of oxidizing iodine to iodate. The analysis reinforces Stumm's idea that the kinetics of redox processes governs the chemistry of bioactive elements in the ocean.