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/H 2 O 2 process. The degradation kinetics of MCAN, DCAN and TCAN by SO 4 − followed second order kinetics at the rate constants of 7.48 (±0.58)   ×   10 7   M −1 ·s −1 , 6.36 (±0.16)   ×   10 7   M −1 ·s −1 and 2.43 (±0.15)   ×   10 7   M −1 ·s −1 , respectively, which were 10 times higher than those by OH. 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 SO 4 − instead of C Cl bond cleavage, and followed by subsequent oxidation and hydrolysis to produce Cl − , NO 3 − , CO 2 and H 2 O. This study demonstrates that SO 4 − is more suitable than OH for the degradation of CANs, but the effective dehalogenation of aliphatic halogenated compounds by SO 4 − still requires at least one hydrogen or carbon atom connecting to an alpha carbon.