Ultraviolet (UV)-based advanced oxidation processes for micropollutant abatement in water treatment: Gains and problems

Ultraviolet (UV)-based advanced oxidation processes (UV-AOPs) are broadly used methods for the removal of organic contaminants from drinking water. Degradation kinetics, reaction mechanisms, radical formation, effects of water matrixes, and the formation of by-products in UV/Cl2, UV/NH2Cl, UV/ClO2, UV/peracetic acid (PAA) are reviewed in this paper. UV-AOPs mainly rely on the formation of a large number of free radicals through UV photolysis to degrade pollutants. The molar absorption coefficients of the major oxidants in these processes range from 8 to 371 M-1 cm-1, while the quantum yields range from 0.2 to about 0.88 mol Einstein-1. HO• is the most common radical among these processes. A rough order of comparison in terms of degradation efficiency is UV/Cl2 ＞ UV/NH2Cl ＞ UV/ClO2 ＞ UV/PAA. The effect of different substances in the water will also have an effect on the rate and pathway of the reaction and the formation of by-products. NOM and carbonates can inhibit degradation efficiency while halogens and ammonium will influence reaction pathway and have impact on by-product formation and toxicity. A rough order of comparison in terms of by-product formation is: UV/Cl2 ＞ UV/NH2Cl >> UV/ClO2 ≈ UV/PAA. UV/Cl2 remains the most efficient process for contaminant removal, although it has the highest risk of disinfection by-product formation. UV/NH2Cl can lower the possibility of producing by-products but may lead to increased water toxicity and nitrogen contamination. UV/ClO2 has an advantage of the lowest by-product formation among three chlorine-containing UV-AOPs, while maintaining relatively considerable removal efficiency. UV/PAA, as an alternative to UV/H2O2, remains minimal by-product formation potential, while the effect on organic content in water should also be noted. Selection of suitable UV process and reaction conditions according to the characteristics of pollutants as well as reduce or dispose of the generated by-products will be the future research direction.