Iodine-sensitized oxidation of ferrous ions under UV and visible light: the influencing factors and reaction mechanism.

The sensitized oxidation of Fe(2+) ions by I2 or I3(-) in acidic aqueous solution has been examined. The reactions could occur either under UV or visible light, with the stoichiometric formation of Fe(3+) and I(-). However, the I3(-)-sensitized reaction was fast and complete only in the presence of excess Fe(2+). Through a kinetics study, it becomes clear that I˙ and I2(-)˙ are the main reactive species for the I2 and I3(-)- sensitized oxidation of Fe(2+), respectively. Moreover, the quantum yields determined with the I2 and I3(-)-sensitized formation of Fe(3+) at 456 nm were 0.119 and 0.118, respectively. The I2-sensitized reaction was first order only in I2, and had an Arrhenius activation energy of 29.3 kJ mol(-1). It is proposed that the process of Fe(2+) oxidation by I˙ is fast, while the rate-determining step is the formation and self-recombination of I˙ radicals.