Degradation kinetics and pathways of haloacetonitriles by the UV/persulfate process

Haloacetonitriles (HANs) are the most frequently found nitrogenous disinfection by-products (N-DBPs) in water. This study investigated the degradation of HANs, including dibromoacetonitrile (DBAN), monochloroacetonitrile (MCAN), dichloroacetonitrile (DCAN) and trichloroacetonitrile (TCAN) in the UV/persulfate process. DBAN was efficiently degraded by the direct UV photolysis, while chlorinated HANs (CANs) were more efficiently degraded by the UV/persulfate process than by the UV or UV/H2O2 process. The degradation kinetics of MCAN, DCAN and TCAN by SO4 center dot- followed second order kinetics at the rate constants of 7.48 (+/- 0.58) x 10(7) M-1.s(-1), 6.36 (+/- 0.16) x 10(7) M-1.s(-1) and 2.43 (+/- 0.15) x 10(7) M-1.s(-1), respectively, which were 10 times higher than those by (OH)-O-center dot. The degradation rates increased with increasing persulfate dosages and temperatures, but were suppressed by natural organic matter and bicarbonate. The degradation of CANs in the UV/persulfate process was initiated by hydrogen abstraction and C-C bond breakage by SO4 center dot- instead of C-Cl bond cleavage, and followed by subsequent oxidation and hydrolysis to produce Cl-, NO3-, CO2 and H2O. This study demonstrates that SO4 center dot- is more suitable than (OH)-O-center dot for the degradation of CANs, but the effective dehalogenation of aliphatic halogenated compounds by SO4 center dot- still requires at least one hydrogen or carbon atom connecting to an alpha carbon. (C) 2017 Elsevier B.V. All rights reserved.