Insights into the photodegradation of decamethylammonium bromide (DDAB) under UV/H2O2 system: Photodegradation kinetic modeling, degradation mechanism and toxicity evaluation

Decamethonium bromide (DDAB), as a class of quaternary ammonium-based disinfectants (QACs), was widely used during the COVID-19 outbreak. The abuse of DDAB is harmful to the health of aquatic organisms. The UV/H2O2 process is popularly utilized for the removal of trace contaminants from aqueous environments. Thus, in this paper, DDAB was selected as the model of QACs, and the degradation kinetics were simulated by Kintecus software. It was indicated that the effect of the degradation efficiency of DDAB was dominated by [·OH]ss. The degradation efficiency of DDAB was greater than 95 % when [·OH]ss was higher than 1.04×10−12 M. The high dosages of H2O2 (>100 μM) and the less concentration of pollutants (<10 μM) were more beneficial to the degradation of DDAB. The high dosages of NOM and CO32- in the system scavenged ·OH and thereby inhibited the degradation of DDAB. The interference of Cl- on the degradation efficiency was negligible. Additionally, the optimal pH value for removing DDAB was between 4 and 7. Besides, the degradation pathway of DDAB was inferred by Density Functional Theory (DFT) and Ultra-high Performance Liquid Chromatography-tandem Mass Spectrometry (UPLC-MS/MS). The results showed that the degradation of DDAB began with C-N bond breaking, then generated aldehydes, underwent demethylation reaction, and finally converted to low molecule weight molecules. The results of Ecological Structure Activity Relationships (ECOSAR) toxicity assessment demonstrated that the toxicity of all the degradation products was significantly low. Overall, this study provides theoretical basis for the removal of QACs in the aqueous environment.