The reverse-reduction effect of dissolved organic matter on the degradation of micropollutants induced by halogen radicals (Cl2•- and Br2•-)

Reactive halogen radicals (e.g., Cl₂^•- and Br₂^•-) greatly impact the degradation of micropollutants in natural waters and engineered water treatment systems. The ubiquitous dissolved organic matter (DOM) in real waters is known to greatly inhibit the degradation of micropollutants by reducing micropollutant's intermediate (i.e., TC^•+/TC(-H)^•), however, such DOM's effects on the halogen-radical-induced system have not been understood yet. The present study focuses on investigating and quantifying such inhibitory effects of DOM during Cl₂^•-- and Br₂^•--mediated process. Guanosine (Gs) was selected as a model compound. The transient spectra show that Cl₂^•- and Br₂^•- react with Gs generating intermediates (i.e., Gs^•+/Gs(-H)^•) via single-electron transfer. In the presence of 1.0 mg_C L^-1 DOM, over 70% of this oxidized Gs was reduced back to Gs. Comparing the extent of reverse-reduction inhibitory among different reaction systems, this inhibitory in Br₂^•- system was slightly lower than that in Cl₂^•- and SO₄^•- system, corresponding the slightly difference of inhibition factor (IF) values as SO₄^•- < Cl₂^•- < Br₂^•-. The reverse-reduction effect of DOM was further quantified for 19 common micropollutants. It varied significantly with IF values of 0.21–1.26 and 0.28–1.40 in Cl₂^•-- and Br₂^•--mediated process, respectively. Purines and amines generally exhibited more pronounced inhibition than phenols in both systems. A good correlation of IF values with micropollutant's reduction potential was observed, which can be applied to predict the degradation of more unstudied micropollutants. This study highlights the important role of the reverse-reduction effect of DOM on micropollutant degradation. It can significantly improve the accuracy in predicting degradation rate in advanced oxidation processes for treating water containing halides.