Photochemical Activation of Free Chlorine to Reactive Oxygen Species for Enhanced Inactivation of Chlorine-Resistant Microorganisms

Recent findings by our group indicate that inactivation of chlorine-recalcitrant microorganisms may be substantially improved by harnessing sunlight-driven photolysis of free chlorine to yield various reactive oxygen species (ROS), including hydroxyl radical (HO•), atomic oxygen (O(3P)), and ozone (O3), during conventional chlorination. For example, our results indicate that a >2-fold increase in inactivation of chlorine-recalcitrant Bacillus subtilis spores can be achieved at pH 8 and 25° C in solutions dosed with 3-7 mg/L as Cl2, following exposure to simulated sunlight for as little as 12 minutes. No enhancement of inactivation rates was observed in the presence of 50- mM t-BuOH (a HO• and O(3P) scavenger), and no inactivation was observed under simulated sunlight alone, highlighting the key role of photochemically-generated ROS. Furthermore, the process was confirmed to function effectively under natural sunlight, as well as in the presence of DOC and alkalinity in natural water samples obtained from regional drinking water utilities.