Kinetic models and pathways of ronidazole degradation by chlorination, UV irradiation and UV/chlorine processes

Abstract

Degradation kinetics and pathways of ronidazole (RNZ) by chlorination (Cl2), UV irradiation and combined UV/chlorine processes were investigated in this paper. The degradation kinetics of RNZ chlorination followed a second-order behavior with the rate constants calculated as (2.13 ± 0.15) × 102 M−2 s−1, (0.82 ± 0.52) × 10−2 M−1 s−1 and (2.06 ± 0.09) × 10−1 M−1 s−1 for the acid-catalyzed reaction, as well as the reactions of RNZ with HOCl and OCl−, respectively. Although UV irradiation degraded RNZ more effectively than chlorination did, very low quantum yield of RNZ at 254 nm was obtained as 1.02 × 10−3 mol E−1. RNZ could be efficiently degraded and mineralized in the UV/chlorine process due to the generation of hydroxyl radicals. The second-order rate constant between RNZ and hydroxyl radical was determined as (2.92 ± 0.05) × 109 M−1 s−1. The degradation intermediates of RNZ during the three processes were identified with Ultra Performance Liquid Chromatography – Electrospray Ionization - mass spectrometry and the degradation pathways were then proposed. Moreover, the variation of chloropicrin (TCNM) and chloroform (CF) formation after the three processes were further evaluated. Enhanced formation of CF and TCNM precursors during UV/chlorine process deserves extensive attention in drinking water treatment.