An expected formation of TCNM from chlorination of bisphenol A with ultrasonic pretreatment: A new nitrogen source for N-DBP from N2 in air

Abstract

Ultrasonic irradiation has been widely adopted to remove emerging contaminants (ECs) in water, while little attention has been given to the impact of ultrasonic pretreatment on the downstream formation of disinfection byproducts (DBPs). In this study, bisphenol A (BPA), a typical EC that is frequently detected in water, was selected as a model contaminant to investigate the influence of ultrasonic pretreatment on DBP formation during the chlorination process. The ultrasonic degradation behaviors of BPA fit the heterogeneous Langmuir-Hinshelwood (L-H) model well, indicating that BPA undergoes decomposition by free radical reactions predominantly at the water-bubble interface. Interestingly, compared to direct chlorination, the ultrasonic pretreatment obviously enhanced trichloronitromethane (TCNM) formation from subsequent chlorination. The LC/MS/MS technique was applied to analyze the transformation products of BPA. In addition to hydroxyl radicals (OH) dominating hydroxylation of the aromatic ring and cleavage of the CC bond, an ultrasonic-induced nitration mechanism was also involved in BPA degradation, and the formed nitration products may be responsible for TCNM formation during subsequent chlorination. Factors affecting the ultrasonic degradation rate of BPA as well as DBP formation were also investigated. Generally, dissolved O2, higher ultrasonic frequency and higher power favored BPA degradation, whereas the presence of HCO3− and HA hindered BPA degradation, and Cl− showed a minor influence. TCNM formation was largely diminished by sparging with O2 or N2 alone. Higher ultrasonic frequency and power enhanced the TCM and TCNM formation. The presence of HA promoted THM formation, while HCO3− inhibited TCNM formation. Therefore, once ultrasonic treatment is adopted as an alternative water treatment, its impact on subsequent DBP formation requires further study.