Evaluation of the Effect of Catalysts on Ozone Mass Transfer and Pollutant Abatement during Laboratory Catalytic Ozonation Experiments: Implications for Practical Water and Wastewater Treatment

Abstract

Adding catalysts can usually substantially enhance pollutant removal during laboratory semi-batch ozonation experiments with continuous ozone gas bubbling. However, it is unclear whether the enhanced pollutant abatement is mainly caused by the higher •OH yields (moles of •OH formed per mole of O3 consumed) or the higher O3 utilization efficiency in the presence of the catalysts─two effects that can or cannot be effectively transferred to large-scale applications, respectively. To distinguish these two effects during the laboratory experiments, this study compared oxalate removal by conventional ozonation (in the absence of catalysts) and catalytic ozonation with three carbon nanotube catalysts based on the consumed O3 doses during the different processes. In addition, the results of several previous studies that had evaluated varying catalysts (e.g., NiCo2O4, MnFe2O4, and MnO2) for pollutant removal by catalytic ozonation were revisited based on the consumed O3 doses measured in these studies. The results show that while all the catalysts considerably accelerated O3 transfer/consumption and pollutant removal during catalytic ozonation than during conventional ozonation, many of them did not enhance the extent of pollutant removal when the same O3 doses were consumed during the two processes. This finding suggests that in the laboratory experiments, these catalysts only enhanced the O3 utilization efficiency but did not significantly enhance the •OH yield from O3 decomposition during catalytic ozonation relative to conventional ozonation. Therefore, they may not be able to enhance pollutant removal during large-scale applications to the same extent as has been shown in the laboratory experiments. These results indicate that to more realistically predict the performance of catalysts for large-scale applications, the extent of pollutant removal should be evaluated based on the transferred/consumed O3 doses during the laboratory experiments.