Advanced oxidation processes (AOPs) combining chlorine-based oxidants (e.g., free chlorine, monochloramine, and chlorine dioxide (ClO₂)) with UVC irradiation at 254 nm are increasingly used for micropollutant degradation in water and wastewater. However, these AOPs suffer from the use of energy-intensive and mercury-based UVC lamps, chloro-organic byproduct formation, and the low yields of reactive species in the UVC/ClO₂ AOP. This thesis work reports a novel UVA/ClO₂ AOP based on the photolysis of ClO₂ using energy-efficient UV radiation sources in the UVA range (e.g., UVA-LEDs) by taking advantage of the high UV absorbance of ClO₂ in the UVA range. At a ClO₂ dosage of 74 μM (5.0 mg L^–1 as ClO₂) and a UV fluence at 47.5 mJ cm^–2, the UVA₃₆₅/ClO₂ AOP generated a spectrum of reactive spec...[ Read more ]

Advanced oxidation processes (AOPs) combining chlorine-based oxidants (e.g., free chlorine, monochloramine, and chlorine dioxide (ClO₂)) with UVC irradiation at 254 nm are increasingly used for micropollutant degradation in water and wastewater. However, these AOPs suffer from the use of energy-intensive and mercury-based UVC lamps, chloro-organic byproduct formation, and the low yields of reactive species in the UVC/ClO₂ AOP. This thesis work reports a novel UVA/ClO₂ AOP based on the photolysis of ClO₂ using energy-efficient UV radiation sources in the UVA range (e.g., UVA-LEDs) by taking advantage of the high UV absorbance of ClO₂ in the UVA range. At a ClO₂ dosage of 74 μM (5.0 mg L^–1 as ClO₂) and a UV fluence at 47.5 mJ cm^–2, the UVA₃₆₅/ClO₂ AOP generated a spectrum of reactive species, including chlorine oxide radicals (ClO^•), chlorine atoms (Cl^•), hydroxyl radicals (HO^•), and ozone at a concentration of ~10^–13, ~10^–15, ~10^–14, and ~10^–7 M, respectively. The concentrations of the generated reactive species were barely affected by the changes in pH, with changes of less than 20% as pH increased from 6.0 to 8.0. A kinetic model to simulate the reactive species generation in the UVA₃₆₅/ClO₂ AOP was established, validated against the experimental results, and used to predict the pseudo-first-order rate constants and relative contributions of different reactive species to the degradation of 19 micropollutants in the UVA₃₆₅/ClO₂ AOP. The presence of NOM increased chlorite but decreased chlorate formation in the UVA₃₆₅/ClO₂ AOP. The molar yields of chlorite and chlorate, together with the second-order rate constants of NOM towards ClO₂ and reactive species, were determined by experiments and kinetic modelling, which enables accurate prediction of chlorite and chlorate formation in the UVA₃₆₅/ClO₂ AOP in the presence of NOM. Compared to the well-documented UVC₂₅₄/chlorine AOP, the UVA₃₆₅/ClO₂ AOP produced similar levels of reactive species at similar oxidant dosages but was much less pH-dependent and required much lower energy input, with a much lower formation of chloro-organic byproducts and marginal formation of chlorite and chlorate.