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

- The kinetics and mechanisms of the degradation of chloroacetonitriles (CANs) by UV/persulfate are elucidated.
- The simulated rate constants for CANs reacting with SO4− are faster than those with OH by an order of magnitude.
- The degradation is suppressed by alkalinity and natural organic matter, but not by chloride.
- CANs initially undergo hydrogen abstraction and CC bond breakage then dechlorination.

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− followed second order kinetics at the rate constants of 7.48 (±0.58) × 107 M−1·s−1, 6.36 (±0.16) × 107 M−1·s−1 and 2.43 (±0.15) × 107 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 CC bond breakage by SO4− instead of CCl bond cleavage, and followed by subsequent oxidation and hydrolysis to produce Cl−, NO3−, CO2 and H2O. This study demonstrates that SO4− is more suitable than OH for the degradation of CANs, but the effective dehalogenation of aliphatic halogenated compounds by SO4− still requires at least one hydrogen or carbon atom connecting to an alpha carbon.

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