Oxidation byproducts from the degradation of dissolved organic matter by advanced oxidation processes - A critical...

Advanced oxidation processes (AOPs) have been increasingly used for the treatment of source waters and wastewaters. AOPs characteristically produce oxidation byproducts (OBPs) from the partial degradation of dissolved organic matter (DOM) and/or the transformation of inorganic ions (especially, halides) into highly toxic substances including bromate and halogenated organic OBPs (X-OBPs). However, despite the enormous health and environmental risks posed by X-OBPs, an integral understanding of the complex OBP formation mechanisms during AOPs is lacking, which limits the development of safe and effective AOP-based water treatment schemes. The present critical and comprehensive review was intended to fill in this important knowledge gap. The study shows, contrary to the hitherto prevailing opinion, that the direct incorporation of halide atoms (X^•) into DOM makes an insignificant contribution to the formation of organic X-OBPs. The principal halogenating agent is hypohalous acid/hypohalite (HOX/XO^-), whose control is, therefore, critical to the reduction of both organic and inorganic X-OBPs. Significant generation of X-OBPs has been observed during sulfate radical AOPs (SR-AOPs), which arises principally from the oxidizing effects of the unactivated oxidant and/or the applied catalytic activator rather than the sulfate radical as is commonly held. A high organic carbon/X^- molar ratio (>5), an effective non-catalytic activator such as UV or Fe²⁺, a low oxidant concentration, and short treatment time are suggested to limit the accumulation of HOX/XO^- and, thus, the generation of X-OBPs during SR-AOPs. At present, there are no established techniques to prevent the formation of X-OBPs during UV/chlor(am)ine AOPs because the maintenance of substantial amounts of active halogen is essential to these processes. The findings and conclusions reached in this review would advance the research and application of AOPs.