Efficient reductive and oxidative decomposition of haloacetic acids by the vacuum-ultraviolet/sulfite system

Haloacetic acids (HAAs), as a representative category of halogenated disinfection byproducts, are widely detected in disinfected water. In this work, the vacuum ultraviolet (VUV)/sulfite process under N2 saturated conditions was proposed to eliminate a series of HAAs (i.e., monochloroacetic acid (MCAA), difluoroacetic acid (DFAA), trifluoroacetic acid (TFAA), dichloroacetic acid (DCAA), etc.). The in situ generated hydrated electron (eaq- ) demonstrated to be the main species to fulfill the initial degradation and dechlorination of MCAA, while hydroxyl radicals (OH) were in charge of the mineralization of MCAA. This means that the VUV/sulfite system is a combination of advanced reduction and oxidation processes (ARPs and AOPs). A significant enhancement of MCAA removal was observed with increasing pH values from 6.0 to 10.0, and surprisingly, kobs correlated well with the proportion of SO32- as the pH changed. This can be explained by the production of eaq- from VUV irradiation of SO32- rather than HSO3- and also due to eaq- being more stable under alkaline conditions. Increasing the sulfite dosage also elevated the degradation of MCAA. However, the addition of certain anions (i. e., chloride (Cl-), bicarbonate (HCO3- ), and nitrate (NO3- )) and dissolved organic matter (DOM) inhibited the removal of MCAA to varying degrees. The VUV/sulfite system was effective toward various types of halogenated disinfection byproducts, supporting its broad applicability. Nevertheless, even in real waters, the VUV/sulfite system was also promising for the simultaneous abatement of HAAs and other oxyanions.

Automated summary

This study proposes a VUV/sulfite system for the elimination of haloacetic acids (HAAs). The in situ generated hydrated electron (eaq-) is responsible for the initial degradation and dechlorination of HAAs, while hydroxyl radicals (OH) are responsible for their mineralization. Increasing pH values and sulfite dosage enhance the degradation of HAAs, but the presence of certain anions and dissolved organic matter inhibits the removal.