Unexpected decrease of N-Nitrosodimethylamine formation during peroxymonosulfate oxidation: The other side of bromide

In this study, bromide ion (Br⁻) was found to accelerate the degradation of N,N-dimethylhydrazine compounds in peroxymonosulfate (PMS) oxidation and synchronously reduce their N-Nitrosodimethylamine (NDMA) formation. Hexamethylenebis (1,1-dimethylsemicarbazide) (HDMS) and daminozide (DMZ) were selected as the model compounds to clarify the mechanisms in the presence or absence of Br⁻ for the PMS batch oxidation experiments. In the absence of Br⁻, the degradation of the model compounds accompanied by the formation of NDMA was induced by the electron transfer between PMS/singlet oxygen (¹O₂) and the (CH₃)₂N-N- groups of the model compounds. In the presence of Br⁻, the generated hypobromous acid (HOBr) was verified to account for the acceleration of the reaction. The difference in NDMA formation in PMS oxidation and PMS/Br⁻ oxidation was related to the changes in degradation paths. The electron-withdrawing substituents Br or O of PMS or HOBr substituted the N atoms of the model compounds, generating brominated or oxidation products that were hard to be further oxidized, thus decreasing NDMA formation. Moreover, the toxicity of the products in PMS/Br⁻ oxidation was significantly lower than in alone PMS oxidation. This work provides new insight into the unique advantage of Br⁻ during PMS oxidation.