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Irradiation by UV-254 nm is a popular and efficient approach for disinfection in water and/or wastewater treatment plants. With the addition of oxidants such as hydrogen peroxide, persulfate, or chlorine into UV irradiation, the removal efficiency can be improved and synergistic degradation can be achieved through the formation of reactive radicals, such as hydroxyl radical, sulfate radical, reactive chlorine species, etc. Some of the radicals are non-selective (e.g., hydroxyl radical), while others are solely reactive to specific organic compounds. UV-based advanced oxidation processes (AOPs) for the decomposition of chemicals of emerging concern (CECs) were investigated in this dissertation. The utilization of UV/peroxide and UV/persulfate for iodinated pharmaceuticals (e.g., thyroxine and diatrizoate) and dibutyl phthalate (DBP) elimination in wastewater, and UV/chlorine for cyanotoxin microcystin–LR (MC-LR) degradation in drinking water, with the UV dose in the range applied in typical UV disinfection treatment units, were discussed. Both kinetic and mechanistic studies on the degradation of those chemicals were conducted in this study.UV/chlorine was evaluated as a potentially practical technique for the degradation of MC-LR. Compared to sole UV irradiation or chlorination, the combined process significantly lowered the chemical and energy demand, which was attributed to the generation of reactive radical species such as hydroxyl radicals, as demonstrated using various radical probes. The enhanced performance was also achieved at different pH conditions, with different oxidant dosages, and by using various natural water sources as background matrix. Via mass spectrometry analysis, products of hydroxylation, chlorination, isomerization, photohydration, as well as bond cleavage were identified during the decomposition, with fewer chlorinated MC-LR products by UV/chlorine than chlorine alone. The trends of disinfection byproducts formation were different immediately following treatment when compared after 24 hours of dark reaction when sand filtered natural water was used as a background matrix. The UV/chlorine treated samples also showed microscopically and biochemically less cytotoxicity in vitro in HepaRG human liver cell line tests than the samples treated by chlorine alone. Similarly, UV/persulfate achieved better degradation of iodinated pharmaceuticals and phthalate esters than UV/peroxide. When biologically treated wastewater samples were used as reaction matrices, the destruction of the contaminants was still efficient in the presence of radical scavengers. Hydroxylation, and C-I and C-O bond cleavage were the major pathways in the decomposition of thyroxine; while hydroxylation, dealkylation, quinone formation, hydrolysis, and decarboxylation occurred in plasticizer DBP degradation.Besides conventional UV lamps, newly invented multi-wavelength UV-LEDs were introduced and evaluated to reduce energy consumption, and to provide a more cost-effective strategy for the destruction of CECs. The activation of persulfate or chlorine can be achieved not only by UV-254 nm but also by higher wavelengths, indicating that medium pressure UV, which is more commonly installed in water treatment utilities, can remove CECs when coupled with photoactivatable oxidants. The study is anticipated to provide options for water/wastewater treatment facilities equipped with UV units to make slight changes to the current treatment processes to achieve satisfactory removal of CECs.