Transformation of iodide by Fe(II) activated peroxydisulfate.

This work investigates the transformation of iodide (I) by Fe(II)-activated peroxydisulfate (PDS). The transformation of I into iodate (IO) is a two-step process, involving reactive iodine species, such as hypoiodous acid (HOI), as a key intermediate, and IO as the final product. In the first step, SO 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. The disproportionation of HOI and the oxidation by PDS alone contribute negligibly to IO formation. The IO yield increases to a maximum, before decreasing gradually, with increased PDS and Fe(II) dosages. The transformation of I into IO is favored by lower pH and higher temperature, due to the greater SO 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. In addition, the transformation of I into IO and iodinated disinfection byproducts by Fe(II) activated PDS was confirmed in the natural waters.