Kinetics and mechanisms of degradation of chloroacetonitriles by the UV/H2O2 process

摘要

Haloacetonitriles (HANs) are emerging disinfection by-products (DBPs) that are more toxic than the regulated DBPs and widely found in the chlorinated/chloraminated water. This paper studied kinetics and mechanisms of the degradation of chloroacetonitriles (CANs) by the UV/H2O2 process at pH 6 and 7.5 and H2O2 concentrations of 1 x 10(-3) M, 5 x 10(-3) M and 1 x 10(-2) M. The degradation followed pseudo first-order degradation kinetics. The degradation rate of monochloroacetonitrile (MCAN) remained similar, while those of dichloroacetonitrile (DCAN) and trichloroacetonitrile (TCAN) increased with increasing pH and H2O2 concentrations. The different trends were attributed to the changing contributions of the two major mechanisms: the nucleophilic attack by hydroperoxide ions (HO2-) and the hydroxyl radical (center dot OH) oxidation. The second-order rate constants of HO2- towards MCAN, DCAN and TCAN were determined to be 11.8 (+/- 0.62), 4.83 (+/- 0.01) x 10(3), and 2.54 (+/- 0.23) x 10(5) M-1 s(-1), respectively, while that of center dot OH were 8.7 x 10(6), 4.4 x 10(6), and < 10(6) M-1 s(-1), respectively. The degradation of TCAN was mainly attributed to the HO2- nucleophilic attack, while that of MCAN was dominated by the center dot OH oxidation. DCAN was degraded by both mechanisms. The nucleophilic attack increased linearly with increasing HO2- concentration as a result of increasing H2O2 concentration and/or pH, while the center dot OH oxidation was less dependent on H2O2 concentrations and/or pH. The nucleophilic attack mainly transformed HANs to haloacetamides, while the center dot OH oxidation dechlorinated HANs. This paper firstly illustrated the importance of the HO2- nucleophilic attack, in addition to the center dot OH oxidation, on the CAN degradation in the UV/H2O2 process. (C) 2016 Elsevier Ltd. All rights reserved.