Byproduct formation in heterogeneous catalytic ozonation processes

Heterogeneous catalytic ozonation (HCO) is a promising advanced oxidation process (AOP) that can effectively degrade recalcitrant organic pollutants. While research efforts have been mainly devoted to the development of different catalysts to enhance the process efficiency, more studies are needed to investigate and address the other challenge faced by AOPs, i.e. generation of harmful byproducts. Bromate is the major inorganic byproduct of concern when ozone is involved. While most studies have reported less bromate formation in HCO than ozonation alone, the effects of catalysts depend on their interaction with O3 and the dominant bromate formation pathway (direct O3 oxidation vs. indirect ˙OH oxidation) in the system. Production of H2O2 and cyclic redox reactions on the catalyst surface can also reduce different Br species leading to a lower bromate yield. Generation of organic byproducts (OBPs; e.g. aldehydes, keto-acids, carboxylic acids) in HCO depends on the reactivity of precursors (e.g. dissolved organic matter/DOM) and OBPs with O3/˙OH, interactions between DOM/OBPs and catalysts, characteristics of DOM, and O3 dose. HCO generally increased the removal of dissolved organic carbon (DOC) and the biodegradability of the bulk organics. HCO treatment may also decrease the formation potential of some disinfection byproducts (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs) but may increase the brominated species of the DBPs and also the formation potential of haloacetonitrile (HAN) under certain conditions. This review discusses the current status of studies on both organic and inorganic byproduct formation in HCO as well as transformation of bulk organics and the effects on DBP formation in the downstream disinfection process, and further provides recommendations for future research and development. A standardized experimental protocol and rigorous experimental design is important to deepen our understanding and gain insights on the byproduct formation in HCO from different studies collectively.