Free radical mechanisms of halogen coexistence promoting pollutants formation

Abstract

Halogens are common in industrial thermal processes and can induce formation of toxic organic pollutants. Currently, the specific effects of coexisting halogens on toxic pollutant formation are unclear. In this study, we found the boosting effects of halogens on organic pollutants formation during thermal processes. In situ electron paramagnetic resonance spectroscopy was used to distinguish the organic free radical intermediates of organic pollutants during thermal processes. The pure chlorine-containing reaction system dominantly induce the formation of highly chlorinated phenoxy radicals, whereas the pure bromine-containing reaction system induce the formation of semiquinone radicals. The results provide clear evidence for the molecular mechanisms by which halogen coexistence boosts pollutants formation. Coexisting halogens made the reactions much more complex. Bromine atom radicals (Br·) can enhance the level of active chlorine atoms radicals (Cl·), then (1) facilitating highly chlorinated organic radicals and toxic pollutants formation, (2) activating polycyclic aromatic hydrocarbons molecules for subsequent ring growth and (3) accelerating oxidation reactions. Combining the identification of organic free radical intermediates and characteristics of products, detailed mechanisms of halogens on the formations of organic pollutants during thermal-related processes were clarified, which can be helpful for their efficient control in the widespread bromines and chlorines co-existence reaction system.