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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 O 3 and the dominant bromate formation pathway (direct O 3 oxidation vs. indirect ˙OH oxidation) in the system. Production of H 2 O 2 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 O 3 /˙OH, interactions between DOM/OBPs and catalysts, characteristics of DOM, and O 3 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. 

Heterogeneous catalytic ozonation has been increasingly studied for the degradation and mineralization of refractory organic water pollutants in recent years. Compared with homogeneous catalysts, an important advantage of heterogeneous catalysts is that they can be easily separated from the treated water, making the process economically viable. While many studies have focused on the development and evaluation of metal oxide-based catalytic ozonation, possible leaching of metal ions and the subsequent effect on the contaminants' degradation are sometimes overlooked. Here, we examined metal leaching from several solid catalysts and further investigated the influence of the leached metal ions on the mineralization of two model compounds (oxalate and nitrobenzene) during continuous ozonation. Metallic ion leaching was observed from both commercially-available catalysts and catalysts prepared via wet-chemistry methods in the lab. The water matrix has been demonstrated to play an important role in metal leaching. The homogeneous catalytic effect resulting from the leached metal ions was found to be significant. A mechanism involving the formation of an unstable Cu( iii )/oxalate complex through the reaction between ˙OH and Cu( ii )/oxalate was proposed to explain the experimental observations. Our results indicate that the stability of the solid catalysts and the effects of the leached ions must be carefully examined during the catalytic ozonation of organic contaminants. Through this study we highlight the importance of rigorous, accepted protocols for evaluating and reporting heterogeneous catalyst performance in water/wastewater treatment within the research community.