Degradation of sulfamethazine in UV/monochloramine process: Kinetics, by-products formation, and toxicity evaluation

Ultraviolet combined monochloramine (UV/NH2Cl) process is a promising advanced oxidation process (AOP) to remove organic micropollutants effectively through the generation of various reactive species e.g., hydroxyl radicals (HO•), reactive chlorine species (RCS), and reactive nitrogen species (RNS). This study investigated the kinetics, transformation pathways, DBPs formation, and toxicity alternation in the degradation of sulfamethazine (SMT). Based on the calculation of degradation kinetic models, RNS was proved to be the most important reactive species in the degradation of SMT in UV/NH2Cl. The contribution of RNS, HO• and RCS were approximately 56.6%, 38.8%, and 4.5%, respectively. With the NH2Cl dosage increased from 100 to 800 µmol L-1, degradation rate increased from 37% to 73% in 10 min. Significantly alternation was not observed with the changing of pH. Natural organic matter (NOM) remarkably inhibited the degradation rate to 48.6% and 32.8% in the presence of 1 and 2 mg L-1 NOM. Degradation reactions of SMT in UV/NH2Cl system primarily comprised of hydroxylation, nitrosation, and ring opening reactions. The total theoretical toxicity of degradation system increased in the first 20 min and decreased eventually. Comparing with UV/free chlorine (UV/Cl2) process, less disinfection by-products (DBPs) generated in UV/NH2Cl treatment. Thus, the theoretical toxicity of DBPs formed in UV/NH2Cl was much lower than UV/Cl2. Within 40 min degradation, the theoretical toxicity value reached maximum at pH 7.1 due to the mass production of dicloroacetonitrile (DCAN). Bromide ions (10 µmol L-1) significantly promoted the formation of brominated DBPs especially dibromoacetonitrile (DBAN). This study provided insight into the important role of RNS in the SAs' transformation in UV/NH2Cl process and reference to benefits and disadvantages when selecting the process for disinfection in drinking water treatment.