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

Abstract

The sensitized oxidation of Fe²⁺ 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 Fe³⁺ and I⁻. However, the I₃⁻-sensitized reaction was fast and complete only in the presence of excess Fe²⁺. Through a kinetics study, it becomes clear that I˙ and I₂⁻˙ are the main reactive species for the I₂ and I₃⁻- sensitized oxidation of Fe²⁺, respectively. Moreover, the quantum yields determined with the I₂ and I₃⁻-sensitized formation of Fe³⁺ 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 Fe²⁺ oxidation by I˙ is fast, while the rate-determining step is the formation and self-recombination of I˙ radicals.