Degradation of iohexol by UV/chlorine process and formation of iodinated trihalomethanes during post-chlorination

Abstract

Degradation kinetics of iohexol by UV/chlorine advanced oxidation process (AOP) and the formation of iodinated trihalomethanes (I-THMs) during post-chlorination were investigated in this study. Iohexol, a commonly detected iodinated contrast media in water, can be effectively removed during UV/chlorine process with pseudo-first-order reaction kinetics due to the combination of UV photolysis and oxidation of hydroxyl radicals. The second-order rate constant between iohexol and hydroxyl radicals was determined as 3.8×109M−1s−1 by competition kinetic experiment. Five intermediates were identified by ultra performance liquid chromatography–electrospray ionization-mass spectrometry analysis and degradation pathways of iohexol during UV/chlorine were proposed. Effects of chlorine dose, pH and bromide concentration on iohexol degradation and I-THM formation during post-chlorination were also studied. The results showed that iohexol degradation was accelerated with the increase of chlorine concentration as well as the decrease of pH and bromide concentration. On the other hand, I-THM formation from post-chlorination of UV/chlorine treated iohexol favored relatively low chlorine doses, high bromide concentrations at circumneutral conditions. Raw water experiments showed that I-THM formation after post-chlorination of UV/chlorine-treated iohexol was lower compared to that from UV irradiation, indicating that UV/chlorine is superior to UV alone in controlling I-THM formation.