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 (IO₃^-) is a two-step process, involving reactive iodine species, such as hypoiodous acid (HOI), as a key intermediate, and IO₃^- as the final product. In the first step, SO₄^•- 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 IO₃^-. The disproportionation of HOI and the oxidation by PDS alone contribute negligibly to IO₃^- formation. The IO₃^- yield increases to a maximum, before decreasing gradually, with increased PDS and Fe(II) dosages. The transformation of I^- into IO₃^- is favored by lower pH and higher temperature, due to the greater SO₄^•- 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 IO₃^-. In addition, the transformation of I^- into IO₃^- and iodinated disinfection byproducts by Fe(II) activated PDS was confirmed in the natural waters.