Chlorine-based process has been investigated for micropollutant degradation and disinfection. In an effort to contain the spread of COVID-19, China has been applying chlorine disinfectants to both indoor and outdoor spaces. These chemicals can get into sewage systems, and the direct run-off and indirect sewage effluents will eventually end up in lakes and rivers. What's more, the sunlight could induce free chlorine to converse into different existential patterns and NOM in surface water which could allow the synthesis of disinfection by-products and put aquatic ecosystems at risk. Therefore, deeper researches on free chlorine dosages, environmental considerations to select the most appropriate solar/chlorine technology are needed. In this study, we investigated the process ability to degrade the typical NSAIDs aspirin (ASA) and flunixin meglumine (FMME), which quantified by competition kinetics with probing compounds of nitrobenzene (NB), benzoate acid (BA). All of these four pollutants are resistant to direct solar photolysis or chlorination alone (except FMME), but they can be degraded by the solar/chlorine process effectively. The rate of ASA and FMME degradation was dramatically enhanced (8.3×10-2 min-1 and 2.3×10-1 min-1, respectively) with a free chlorine dose of 0.2 mM. The enhancement was due to the presence of reactive chlorine species , hydroxyl radicals and ozone during free chlorine photolysis. The highest ASA and FMME degradation rate were observed at pH 7.0 with almost 88% and 70% elimination. It has been determined that the contribution of O3 to degradation of ASA and FMME was less than 0.05% for all pH. Reactive chlorine species and hydroxyl radicals accounted for more than 50% and 30% elimination for two pollutants. The kobs decreased with increasing pH. The inhibition of the degradation depended on the concentration of NOM and temperature of the system. Solar/chlorine system increased the formation of trihalomethanes as well as the calculated cytotoxicity, compared with chlorine alone. Overall, this study provided new insights into the dosage of chlorine and other environmental impact factors during the solar/chlorine process. Allowing us to pay attention to the photochemical formation of free radicals and DBPs. The natural sunlight irradiation of groundwater with increasing chlorine could actually improve the removal rates of micropollutants but also put aquatic ecosystems at risk during the pandemic.

Degradation of organic pollutants by solar/chlorine: Roles of reactive species and formation of disinfection by-products

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