Fate and transformation of iodine species during Mn(VII)/sulfite treatment in iodide-containing water

During oxidative treatment of iodide (I⁻)-containing waters, I⁻ is easy to be oxidized into hypoiodous acid (HOI) by various oxidants and the further reaction of HOI with organic compounds can lead to the formation of iodinated disinfection by-products (I-DBPs). Oxidation of HOI to iodate (IO₃⁻) or reduction of HOI to I⁻ has been proposed to reduce the formation of I-DBPs. Because the reaction of HOI with sulfite proceeds rapidly, this study examined the fate of iodine and the formation of I-DBPs in Mn(VII)/sulfite process. Results showed that I⁻ was oxidized to HOI but the further formation of IO₃⁻ was suppressed due to the fast reduction of HOI to I⁻ by sulfite. The reactions of HOI with SO₃²⁻ and IO⁻ with SO₃²⁻ are the major pathways with species-specific second-order rate constants determined to be 1.12 × 10⁵ M⁻¹ s⁻¹ and 9.43 × 10⁷ M⁻¹ s⁻¹, respectively. The rapid reaction of HOI with sulfite plays an essential role in minimizing the formation of iodinated products in HOI- and phenol-containing solutions. The toxic risk analysis showed that the toxicity of the generated DBPs from Mn(VII)/sulfite pre-oxidation followed by chlorination only changed slightly. Practitioner Points: The decay of I⁻ was negligible in Mn(VII)/sulfite process. The rapid reaction of HOI with SO₃²⁻ resulted in the negligible generation of IO₃⁻. Mn(VII)/sulfite process exerted slight influence on the formation of I-DBPs. Mn(VII)/sulfite process is promising for the pretreatment of I⁻-containing water.