Degradation of iohexol in the Co(II)/peracetic acid system under neutral conditions: Influencing factors, degradation pathways and toxicity

Advanced oxidation processes based on peracetic acid (PAA) activation have received more attention in water treatment recently. In this study, the degradation of iohexol (IOH, a classical iodinated X-ray contrast media) in the Co(II)/PAA system under neutral conditions, which mainly produced carbon-centric radicals, was investigated. Results showed that IOH decomposition follows the pseudo-first-order kinetic with a rate constant of 0.283 min−1. Appropriately increasing concentrations of PAA or Co(II) could promote the removal of IOH, while elevating the dosage of H2O2 inhibits the elimination of IOH in the Co(II)/PAA system. The IOH elimination was affected by pH, with the best response in pH between 5 and 7 in the Co(II)/PAA system. Additionally, low Cl− doses have negligible effects whereas high Cl− impeded the breakdown of IOH. Br−, I−, HCO3−, H2PO4− and humic acid suppressed the degradation of IOH. Removal of IOH was significantly inhibited in three selected real water samples. Furthermore, six IOH degradation pathways, including H-abstraction, amide hydrolysis, amino oxidation, deiodination reaction, hydroxylate and elimination reaction, were proposed on the basis of the liquid chromatography-tandem mass spectrometry, ion chromatography and DFT theoretical calculations (Frontier electron densities and fukui functions) results. Finally, the ECOSAR assessment showed that some degradation byproducts were much more toxic than IOH parent, but were much more biodegradable. Overall, this study indicates a potential risk when applying the carbon-centric radicals involved in PAA-based AOPs to remediate halogenated organic pollutants in water.