Mechanistic insights into the catalytic ozonation process using iron oxide-impregnated activated carbon

In the present study, radiolabelled formate was used as a probe compound in order to gain mechanistic insight into the catalytic ozonation process using a commercially available iron oxide-impregnated activated carbon catalyst. We simultaneously analysed the adsorptive and oxidative removal of formate in order to determine the contribution of the catalyst to adsorption and oxidant generation processes respectively. Our results show that the presence of the catalyst enhanced ozone decay as well as the rate and extent of formate oxidation at pH 3.0 compared to that observed in the presence of ozone alone as a result of oxidant generation via O₃-Fe oxide interaction. A reduction in rate and extent of formate oxidation on addition of t-butanol and Cl^- (known hydroxyl radical (^•OH) scavengers under acidic conditions) provides evidence that the oxidant generated during catalytic ozonation at pH 3.0 is ^•OH. Moreover, the oxidation of formate during catalytic ozonation mostly occurs at the solid-liquid interface and/or in bulk solution with adsorption playing no role in the overall oxidation process with this finding supported by the exceptionally high oxidation efficiency compared to the extent of adsorption observed when no O₃ was added. While catalytic ozonation was effective in formate oxidation at pH 3.0, the presence of the catalyst did not lead to an increase in either the rate or extent of formate oxidation at pH 7.3 and 8.5 suggesting that only protonated iron oxide surface sites generate strong oxidant(s) on interaction with O₃. Based on our understanding of the processes operating during the ozonation and catalytic ozonation processes, a mathematical model has been developed that adequately describes the experimental results obtained here. Overall, this study shows that systematic measurement of ozone decay, removal of the parent compound as well as formation of the oxidized products under well controlled conditions are required for unequivocal elucidation of the mechanism of catalytic ozonation.