The formation of iodide in inshore waters from the photochemical decomposition of dissolved organic iodine

Dissolved organic iodine (DOI) in inshore waters was decomposed by solar irradiation. In the process, each mole of DOI was converted to about a mole of iodide. The disappearance of DOI and the formation of iodide were directly proportional to irradiation. The reaction could be expressed as a first-order reaction with respect to the concentration of DOI. The first-order rate constant ranged between −0.2 and −0.6 m2 kW−1 h−1. At the typical irradiance of 0.5–1 kW m−2 for a noon-day sun under a clear sky, the half-life of DOI relative to its photochemical decomposition was on the order of hours. This reaction provides an additional chemical pathway for the interconversions among the dissolved iodine species in the oceans. It also allows for alternative or complementary explanations for the observed photochemical production of iodide and methyl iodide in seawater.