Photodegradation pathway of iodate and formation of I-THMs during subsequent chloramination in iodate-iodide-containing water.

This study investigated the mechanisms of mixed IO/I system under UV irradiation in drinking water and compared the iodinated trihalomethanes (I-THMs) formation of a mixed IO/I system to that of single I and IO systems during subsequent chloramination. The effects of initial I/IO molar ratio, pH, and UV intensity on a mixed IO/I system were studied. The introduction of I enhanced the conversion rate of IO to reactive iodine species (RIS). Besides, IO degradation rate increased with the increase of initial I concentration and UV intensity and the decrease of pH value. In a mixed IO/I system, IO could undergo direct photolysis and photoreduction by hydrated electron (e). Moreover, the enhancement of I-THM formation in a mixed IO/I system during subsequent chloramination was observed. The I-THM yields in a mixed IO/I system were higher than the sum of I-THMs produced in a single IO and I systems at all the evaluated initial I concentrations and pH values. The difference between I-THM formation in a mixed IO/I system and the sum of I-THMs in a single IO and I systems increased with the increase of initial I concentration. As the initial pH decreased from 9 to 5, the difference of I-THM yields enhanced, while the total I-THM yield of a mixed IO/I system and single I and IO systems decreased slightly. Besides, IO-I-containing water with DOC concentration of 2.5-4.5 mg-C/L, which mainly contained humic-acid substances, had a higher risk in I-THMs formation than individual I-containing and IO-containing water.