Sulfite enhanced transformation of iopamidol by UV photolysis in the presence of oxygen: Role of oxysulfur radicals

UV/sulfite process in the absence of oxygen was previously applied as an advanced reduction process for the removal of many halogenated organics and inorganics in water and wastewater. Here, it was found that UV/sulfite process in the presence of oxygen could act as an advanced oxidation process. Specifically, the oxysulfur radicals (including sulfate radical (SO4·−) and sulfite/peroxomonosulfate radicals (SO3·−/SO5·−)) played important roles on the degradation of iopamidol (IPM) as a typical iodinated contrast media (ICM). Furthermore, the contribution of SO4·− on IPM removal gradually increased as pH increased from 5 to 7 and that of SO3·−/SO5·− decreased. Besides, all water quality parameters (i.e., chloride (Cl−), iodide (I−) and natural organic matter (NOM)) investigated here exhibited inhibitory effect on IPM removal. Three inorganic iodine species (i.e., I−, reactive iodine species and iodate (IO3−)) were detected in UV/sulfite process in the presence of oxygen, while only I− was detected in that without oxygen. During UV/sulfite/ethanol, UV photolysis and UV/peroxydisulfate (PDS)/tert-butyl alcohol (TBA) processes, thirteen transformation products including eleven deiodinated products of IPM were identified by ultra HPLC quadrupole time of flight-mass spectrometry (UPLC-Q-TOF-MS). Besides, these products generated by direct UV photolysis, SO4·− and SO3·−/SO5·− were further distinguished. The acute toxicity assay of Vibrio fischeri indicated that transformation products by UV/sulfite under aerobic conditions were less toxic than that by direct UV photolysis.