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 (IO 3 − ) 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 IO 3 − was suppressed due to the fast reduction of HOI to I − by sulfite. The reactions of HOI with SO 3 2− and IO − with SO 3 2− are the major pathways with species-specific second-order rate constants determined to be 1.12 × 10 5 M −1 s −1 and 9.43 × 10 7 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 SO 3 2− resulted in the negligible generation of IO 3 − . 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.
The transformation of iodide (I − ) and hypoiodous acid (IO − ) by Fe (II)-activated peroxydisulfate (PDS) in the presence of natural organic matter (NOM) was investigated. In the water of a reservoir in Shenzhen, four kinds of organic matter were detected: iodoform > diiodo-bromomethane > dibromo-iodomethane > dichloromethane. As pH increased from 3 to 10, I − conversion rate decreased from 31.0% to 11.0%. There is basically no iodic acid ( ) generated under alkaline conditions, probably due to these organic matters. When PDS concentration is low, PDS can oxidize part of I − to IO − , and the oxidation of I − into reaction and continued oxidation reaction exist at the same time, so some is generated in the system. As the proportion of Fe (II) increases, the generation rate of gradually increases. When Fe 2+ /PDS is 1:5, 2:5, and 3:5, the conversion rates of are 4.7%, 6.5%, and 8.4%, respectively. There are two main reasons for this: (i) the reaction of IO − produced in the system with humic acid (HA) inhibits the further conversion of IO − to ; (ii) both HA and I- in the system can react with PDS, resulting in a reduction in the oxidation rate of I − . Formation of I-DBPs by Fe (II)/PDS process in real water was evaluated. I-DBPs generation in various reaction conditions was examined. Existence of HA showed apparent inhibition on IO 3 − generation.
Zijun Dong,Chengchun Jiang,Jingxin Yang,Xi Zhang,Weili Dai,Pengwei Cai
This work investigates the transformation of iodide (I − ) by Fe(II)-activated peroxydisulfate (PDS). The transformation of I − into iodate (IO 3 − ) is a two-step process, involving reactive iodine species, such as hypoiodous acid (HOI), as a key intermediate, and IO 3 − as the final product. In the first step, SO 4 •- produced by Fe(II)-activated PDS is mainly responsible for the transformation of I − into HOI. In the second step, Fe(IV) formed by the reaction of Fe(III) with PDS is required, to facilitate the further oxidation of HOI to IO 3 − . The disproportionation of HOI and the oxidation by PDS alone contribute negligibly to IO 3 − formation. The IO 3 − yield increases to a maximum, before decreasing gradually, with increased PDS and Fe(II) dosages. The transformation of I − into IO 3 − is favored by lower pH and higher temperature, due to the greater SO 4 •- production by the reaction of Fe(II) with PDS under these conditions. Humic acid , as a representative natural organic matter, scavenges the formed HOI to form iodinated disinfection byproducts , which largely inhibit the transformation of I − into IO 3 − . In addition, the transformation of I − into IO 3 − and iodinated disinfection byproducts by Fe(II) activated PDS was confirmed in the natural waters.
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