Investigation on the optimal UV-LED photolysis of chlorine, inorganic chloramines and chlorine dioxide for advanced water treatment

The effective control of emerging contaminants is becoming a priority in securing safe potable water and it is particularly true in the countries and regions where water shortage and quality deterioration necessitate the consideration of less pristine water as a crucial part of the water supply formula. As many of these contaminants are recalcitrant to conventional water treatment processes, advanced abatement processes need to be added. This thesis work developed new advanced oxidation processes (AOPs) by integrating the UV light emitting diodes (UV-LEDs) that are considered as the next-generation UV radiation sources with free chlorine/inorganic chloramines/chlorine dioxide. This thesis work overcame challenges in the design and operation of the UV-LED-based AOPs: understanding the unique complexity of the photochemistry and radical chemistry of UV-LED photolysis of free chlorine, inorganic chloramines, and chlorine dioxide under potable water treatment conditions. The UV photolysis of free chlorine, inorganic chloramines, and chlorine dioxide were all highly wavelength dependent. At neutral and slightly alkaline pHs, the photolysis of chlorine and the consequential radical formation increased with increasing UV wavelength from 255 to 300 nm, and the radical yields at 265, 285 and 300 nm were higher than that at 254 nm (i.e., the emission wavelength of the conventional low-pressure UV lamps). The photolysis of monochloramine and subsequent radical generation decreased with increasing UV wavelength from 255 to 300 nm, while the photolysis of dichloramine exhibited an opposite wavelength-dependence. The photolysis of chlorine dioxide and subsequent radical production increased with increasing UV wavelength from 255 to 365 nm. The wavelength-dependences on the photolysis of free chlorine, inorganic chloramines and chlorine dioxide were more strongly affected by the molar absorptivity than the apparent/innate quantum yields of these oxidants. Selected micropollutants were destructed by the UV-LED-based AOPs via several mechanistic pathways, e.g., chlorination alone, direct UV photolysis, and radical oxidation, depending on their molecular structures. The micropollutants were partially mineralized and transformed to undesired (by)products in the UV-LED-based AOPs, and the remaining micropollutants and (by)products could be removed by post-adsorption process. The UV-LED-based AOPs also produced higher concentrations of the halogenated by-products from natural organic matter, compared to the oxidation by the chlorine/inorganic chloramines/chlorine dioxide alone. Reactive chlorine species (e.g., Cl•) were responsible for the enhanced (by)product formation. The newly established “wavelength-pH-photodecay-radical nexus” in this thesis work provides references and implications for water engineers and participants in selecting sources of UV radiation for chlorine/inorganic chloramines/chlorine dioxide photodecay and micropollutant abatement by the UV-LED-based AOPs in different water settings. The findings also offer a new direction in designing the UV-LED-based point-of-use or point-of-entry water purifiers in treating chlorine-/inorganic chloramine-/chlorine dioxide-containing potable water.