A comparative study of free chlorine and peroxymonosulfate activated by Fe(II) in the degradation of iopamidol: Mechanisms, density functional theory (DFT) calculatitons and formation of iodinated disinfection by-products

Abstract

Both Fe(II)/free chlorine (Fe(II)/FC) and Fe(II)/peroxymonosulfate (Fe(II)/PMS) have a faster reactive species formation rate than the classic Fenton process, so their efficiencies of removing pollutants are better than that of the Fenton process. In this study, we compared the differences in the mechanisms and kinetics of iopamidol (IPM) degradation by Fe(II)/FC and Fe(II)/PMS. Although the production rate of reactive species of the two processes is at the same level (104 M−1 s−1), the Fe(II)/PMS process performed better regarding IPM removal. Reactive chlorine species (RCS) were the major contributor to the degradation of IPM (73.8%) by Fe(II)/FC, and hydroxyl radicals (HO•) and sulfate radicals (SO4•–) contributed 50.7% and 49.3% to the degradation of IPM by Fe(II)/PMS, respectively. Fe(IV) showed low contributions (<0.1%) to the degradation of IPM during the Fe(II)/FC and Fe(II)/PMS processes because of the low values of kFe(IV), IPM ((25 ± 5) M−1 s−1) and the low steady-state concentration of Fe(IV) in the Fe(II)/FC ((5.1 ± 0.7) × 10−8 M) and Fe(II)/PMS processes ((2.2 ± 0.09) × 10−8 M) at pH 3. Based on density functional theory (DFT) calculations and the MS results, the transformation pathway of IPM has been confirmed to mainly occur on chains B and B’ and the iodine group in between. The formation of disinfection by-products (DBPs) indicates that the Fe(II)/PMS process has lower environmental hazards. In general, we should pay more attention to the type of active species to evaluate the Fenton-like process. This study provides a basis for choosing a suitable alternative to the Fenton process and understanding the process in the Fenton-like procedure.