While many studies have demonstrated that both conventional ozonation and heterogeneous catalytic ozonation (HCO) processes possess at least some capacity to remove organic contaminants from solution, the mechanisms underlying these ozone-based technologies remain unclear due to the contradictory results that have often been presented. We hypothesize that part of the inconsistency among different studies may be due to the different buffering solutions used. In this work, we investigated the influence of two commonly applied buffers, phosphate and carbonate, on ozone decay as well as the rate and extent of degradation of particular target organic compounds (formate and oxalate) in both conventional ozonation and HCO processes. Our results reveal that the rate of ozone self-decay was considerably faster in phosphate buffer compared to carbonate buffer, with this effect resulting from the differing • OH scavenging capacities of the buffering ions. Interestingly, while the nature of the buffer used affected the rate of organic oxidation in conventional ozonation, there was minimal effect on the overall extent of oxidation of the target organic compounds. The results obtained also indicate that the carbonate radicals generated as a result of carbonate− • OH reaction are capable of oxidizing oxalate and formate, albeit slowly; however, the oxidation of these organics by phosphate radicals appears to be minimal. The presence of phosphate ions also affects the surface chemistry of the two Cu-based catalysts used in the HCO studies with phosphate inhibiting catalyst-mediated O 3 decay and sorption of the target organic compounds on the catalyst surface. These results suggest that the influence of the buffering ions on the efficacy of organic oxidation is not only dependent on the nature of the organics but also on the mechanism of the catalytic ozonation process. Overall, caution should be exercised when selecting the buffer that will be used in investigations of the conventional ozonation and catalytic ozonation processes.

Heterogeneous catalytic ozonation (HCO) has gained increasing attention as an effective process to remove refractory organic pollutants from industrial effluents. However, widespread application of HCO is still limited due to the typically low efficacy of catalysts used and matrix passivation effects. To this end, we prepared an Al 2 O 3 -supported Fe catalyst with high reactivity via a facile urea-based heterogeneous precipitation method. Due to the nonsintering nature of the preparation method, a heterogeneous catalytic layer comprised of γ-FeOOH and α-Fe 2 O 3 is formed on the Al 2 O 3 support (termed NS-Fe-Al 2 O 3 ). On treatment of a real industrial effluent by HCO, the presence of NS-Fe-Al 2 O 3 increased the removal of organics by ∼100% compared to that achieved with a control catalyst (i.e., α-Fe 2 O 3 /Al 2 O 3 or γ-FeOOH/Al 2 O 3 ) that was prepared by a conventional impregnation and calcination method. Furthermore, our results confirmed that the novel NS-Fe-Al 2 O 3 catalyst demonstrated resistance to the inhibitory effect of high concentration of chloride and sulfate ions usually present in industrial effluent. A mathematical kinetic model was developed that adequately describes the mechanism of HCO process in the presence of NS-Fe-Al 2 O 3 . Overall, the results presented here provide valuable guidance for the synthesis of effective and robust catalysts that will facilitate the wider industrial application of HCO.

In this study, we enunciate the limitations associated with the use of tert-butanol (TBA) to deduce the contribution of hydroxyl radicals ( • OH) in organic oxidation during conventional ozonation and/or heterogeneous catalytic ozonation (HCO). Our results show that TBA is unable to access surface-located • OH formed during HCO. Furthermore, TBA may also interfere with the adsorption of organic compounds on the catalyst surface and decrease the adsorptive as well as concomitant oxidative removal of organics via nonradical-mediated pathways (if important). Our results also demonstrate that TBA scavenging results are inconclusive for mildly ozone reactive compounds due to switching from O 3 / • OH-mediated oxidation in the absence of TBA to O 3 -driven oxidation in the presence of TBA. The presence of TBA may also decrease the rate of ozone decay with the increased stability of O 3 in the presence of TBA, facilitating (i) direct oxidation of ozone-reactive organics in the bulk solution and/or (ii) diffusion of O 3 to the catalyst surface and subsequent surface-mediated oxidation of organics. Overall, we conclude that caution needs to be exercised when interpreting the observations made in the presence of TBA and also describe the further experimentation required to confirm/reject the role of • OH in organic oxidation. The findings of this work should assist in preventing researchers from reaching erroneous conclusions regarding the involvement of • OH in catalytic or conventional ozonation processes.

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