Phototransformation of iodate by UV irradiation: Kinetics and iodinated trihalomethane formation during subsequent chlor(am)ination

Abstract

The photodegradation of IO3− at 254nm and the formation of iodinated trihalomethanes (I-THMs) during subsequent chlorination or chloramination in the presence of natural organic matter (NOM) were investigated in this study. The thermodynamically stable IO3− can be degraded by UV irradiation with pseudo-first order kinetics and the quantum yield was calculated as 0.0591moleinstein−1. Solution pH posed no remarkable influence on the photolysis rate of IO3−. The UV phototransformation of IO3− was evidenced by the determination of iodide (I−) and hypoiodous acid (HOI) in solution. NOM sources not only enhanced the photodegradation rate of IO3− by photoejecting solvated electrons, but also greatly influenced the production I-THMs in subsequent chlor(am)ination processes. In UV irradiation and sequential oxidation processes by chlorine or chloramine, the I-THMs formation was susceptible to NOM sources, especially the two major fractions of aqueous humic substances (humic acid and fulvic acid). The toxicity of disinfected waters greatly increased in chloramination over chlorination of the UV photodecomposed IO3−, as far more I-THMs especially CHI3, were formed. As “the fourth iodine source” of iodinated disinfection by-products, the occurrence, transportation and fate of IO3− in aquatic environment should be of concern instead of being considered a desired iodine sink.