Hybrid catalytic ozonation-membrane filtration process with CeOx and MnOx impregnated catalytic ceramic membranes for micropollutants degradation

Abstract

This study investigated the degradation of three micropollutants (MPs), namely bisphenol A (BPA), benzotriazole (BTZ) and clofibric acid (CA) by a novel hybrid catalytic ozonation-membrane filtration process. The catalytic ceramic membranes (CCMs) were fabricated via citrate sol-gel assisted wet impregnation method. The surface morphologies and elemental distributions across the CCMs were characterized by FESEM-EDX and AFM. The characterization results suggested that the metal oxides were successfully impregnated throughout the porous structure of the CCMs. The performance of the CCMs was evaluated by a custom-made Hybrid Oxidation Separation Technology (HOST) system. Results indicated that CeOx-impregnated CCM (Ce-CCM) exhibited higher MPs mineralization ability with a 2.1 mg O3(aq) mg−1 TOC removal, as compared to that of MnOx-impregnated CCM (Mn-CCM). The CeOx loading in Ce-CCM could be increased by multiple impregnation cycles, at the expense of decrease in its water permeability. The effect of hydraulic retention time (HRT) towards the hybrid process was further investigated, whereby a higher HRT of 13.7 s was able to achieve up to 38% TOC removal. The quantification of reactive oxygen species (ROS) revealed that the O3(aq) decomposition and ROS generation were enhanced by Ce-CCM, with 0.14 μmol HO• L−1 and 8.78 μmol O2•− L−1 detected. The intermediate by-products of the hybrid process were detected by LC-MS/MS and the MPs degradation pathways via catalytic ozonation were identified. The mechanism of heterogenous catalytic ozonation by Ce-CCM was proposed, based on the presence of surface hydroxyl groups as catalytic sites and involvement of Ce(III/IV) redox pair for electron transfer facilitation in ROS generation. The CCM pores could act as microreactors for effective MPs mineralization in the hybrid process.