Advanced oxidation of dodecyl dimethyl benzyl ammonium chloride by VUV/UV/chlorine: Synergistic effect, radicals, and degradation pathway

Quaternary ammonium compounds (QACs) are crucial ingredients in many cleaning agents, disinfectants and personal care products. They are widely detected in aquatic environments. In this study, vacuum ultraviolet (VUV)/UV/chlorine was used to degrade dodecyl dimethyl benzyl ammonium chloride (DDBAC, a typical QAC). The degradation efficiency of VUV/UV/chlorine on DDBAC was the highest among chlorine, UV, VUV/UV, UV/chlorine and VUV/UV/chlorine. During the VUV/UV/chlorine process, with chlorine concentration increasing from 0 to 700 μM, the degradation rate constant of DDBAC increased from 0.16 to 0.22 min −1 then decreased to 0.17 min −1 . When the chloride concentration was 10 mM and 50 mM, the degradation rate constant of DDBAC decreased by 31% and 55%, respectively. Inorganic anions and organic compounds in the reverse osmosis concentrate inhibited the degradation of DDBAC by quenching radicals. The degradation mechanisms of DDBAC degradation during the VUV/UV/chlorine process include UV photolysis and oxidation by OH, Cl and other reactive species. With the increase in pH from 5 to 10, the OH concentration decreased from 18.7 × 10 −14 M to 7.5 × 10 −14 M, and the Cl concentration decreased from 8.8 × 10 −14 M to 2.1 × 10 −14 M. At pH 5–10, contributions of OH and Cl to DDBAC degradation were 30%−44% and 18%−37%, respectively. Degradation mechanisms of DDBAC mainly include hydrogen abstraction, hydroxylation, and cleavage of the C-N bond. The concentrations of four chlorinated products (trichloromethane, monochloroacetic acid, dichloroacetic acid, and trichloroacetic acid) mostly initially increased and then decreased.