On the increasing competitiveness of UV/Cl to UV/H₂O₂ advanced oxidation as the organic carbon concentration...

UV/Cl and UV/H₂O₂ are advanced oxidation processes (AOPs) used for drinking water treatment and water reuse. This work explored the hypothesis that UV/Cl becomes more competitive to UV/H₂O₂ at neutral-to-high pH as the concentration of total organic carbon (TOC) increases. Lab experiments and kinetic modelling were used to compare initial pseudo first-order contaminant decay rate coefficients between the AOPs at various pH and TOC conditions. The relative effect of increasing TOC concentrations on UV/Cl vs. UV/H₂O₂ depended on the pH, contaminant, and organic matter reactivity towards radicals. For example, while the reaction rate coefficients during both AOPs generally decreased with increasing TOC, the UV/Cl reaction rate coefficients for the solely •OH-reactive sucralose decreased 41-138% less than the UV/H₂O₂ coefficients as the TOC concentration was increased from 0 to 5 mg-C L^-1. However, UV/Cl was more affected than UV/H₂O₂ when targeting caffeine (a contaminant reactive to chlorine radicals). The data were used to define TOC-pH conditions for which either AOP would be more energy-efficient, under a set of standard conditions. The results suggest that UV/Cl may be competitive to UV/H₂O₂ under a wider range of treatment scenarios than has been conventionally thought based on tests in pure water.