Transformation of Iodide by Carbon Nanotube Activated Peroxydisulfate and Formation of Iodoorganic Compounds in the Presence of Natural Organic Matter

In this study, we interestingly found that peroxydisulfate (PDS) could be activated by a commercial multiwalled carbon nanotube (CNT) material via a nonradical pathway. Iodide (I^-) was quickly and almost completely oxidized to hypoiodous acid (HOI) in the PDS/CNT system over the pH range of 5-9, but the further transformation to iodate (IO₃^-) was negligible. A kinetic model was proposed, which involved the formation of reactive PDS-CNT complexes, and then their decomposition into sulfate anion (SO₄^2-) via inner electron transfer within the complexes or by competitively reacting with I^-. Several influencing factors (e.g., PDS and CNT dosages, and solution pH) on I^- oxidation kinetics by this system were evaluated. Humic acid (HA) decreased the oxidation kinetics of I^-, probably resulting from its inhibitory effect on the interaction between PDS and CNT to form the reactive complexes. Moreover, adsordable organic iodine compounds (AOI) as well as specific iodoform and iodoacetic acid were appreciably produced in the PDS/CNT/I^- system with HA. These results demonstrate the potential risk of producing toxic iodinated organic compounds in the novel PDS/CNT oxidation process developed very recently, which should be taken into consideration before its practical application in water treatment.