Sunlight-driven Photolysis of Free Available Chlorine to Reactive Oxygen Species for Enhanced Inactivation of Chlorine-resistant Microbial Pathogens
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Free available chlorine (FAC) is the sum of hypochlorous acid, HOCl, and hypochlorite, OCl-, in aqueous solution. As the most widely utilized single drinking water disinfectant in the United States and worldwide, it is inexpensive, portable, and highly effective in inactivating most waterborne pathogens. However, FAC is not effective toward some important pathogens, such as Cryptosporidium parvum oocysts, Mycobacterium avium cells, and Giardia lamblia cysts. The Cryptosporidiosis outbreak in Milwaukee, WI, in 1993, which caused over 400,000 illnesses and greater than 100 deaths, led to substantial shifts in treatment process designs to incorporate alternative disinfectants, such as ultraviolet (UV) light and ozone (O3), to increase the effectiveness of water treatment processes in the inactivation of chlorine-resistant pathogens. Although alternative disinfectants reinforce water facilities’ ability to treat microbial pathogens, they can significantly increase capital and operational costs and carbon footprints of water treatment plants. Thus, an innovative water treatment approach that could simultaneously increase the effectiveness and sustainability of chlorine-based disinfection strategies in existing centralized water treatment facilities would be very beneficial. Sunlight-driven photolysis of chlorine can produce extremely reactive oxygen species (ROS), namely hydroxyl radical (HO•) and O3 (from atomic oxygen (O(3P)), the latter of which is known to be much more effective as a disinfectant than FAC. Such photochemical reactions dramatically enhance the effectiveness of chlorine-based disinfection processes towards chlorine-resistant microorganisms, via in situ generation of O3 and/or HO•. The results from this study show that the FAC exposure ( ) required for 99% inactivation of Bacillus subtilis endospores can be lowered by 71% during solar photolysis of 8 mg L-1 FAC in 10 mM phosphate buffer at pH 8 and 10 °C. Similar conditions yield up to 3 log inactivation of C. parvum oocysts and M. avium cells, while no inactivation of oocysts and <0.5 log inactivation of M. avium are observed without light. Enhancements were also observed in natural surface water samples obtained from public utilities around the Seattle area. A photochemical model developed to enable prediction of O3 and HO• formation during FAC photolysis – validated using experimental measurements obtained with the O3 and HO• probes cinnamic acid and para-chlorobenzoic acid – indicates that the O3 and HO• exposures to which microorganisms are subjected can exceed 7 (mg min) L-1 and 6 × 10-8 (mg min) L-1, respectively, under these conditions. At these levels, B. subtilis endospores are inactivated by both FAC and O3, C. parvum oocysts are inactivated by O3 and sunlight (with >75% oocyst inactivation due to O3), and M. avium cells are inactivated by FAC and O3 (with >95% cell inactivation due to O3). For B. subtilis endospores, its sensitivity to FAC increases with increasing exposure to O3, while its sensitivity to O3 increases with increasing exposure to HO•, leading to synergistic enhancements in overall inactivation kinetics. A composite Chick-Watson model accounting for simultaneous exposure to FAC, O3, and/or HO• was developed on the basis of these findings and – coupled with the photochemical model described above – utilized to accurately predict B. subtilis endospore, C. parvum oocyst, and M. avium cell inactivation during FAC photolysis. For Coxsackievirus B5 (CVB5), the CTFAC required for 3 log inactivation can be lowered by 38% during solar photolysis of 1.5 mg L-1 FAC at pH 8 and 10 °C. Practical implications of this work include potential applications in small, decentralized water systems, point-of-use treatment, solar water disinfection (SODIS), outdoor swimming pool decontamination, etc.
- Civil engineering