Catalytic ozonation has recently gained significant attention as an effective process used for the removal of organics from water. Unfortunately, despite increasing research efforts in the field of catalytic ozonation and the introduction of new catalysts, the mechanisms of catalytic processes are still largely unknown. An understanding of the mechanisms of catalytic ozonation is vital in order to introduce this technique in water treatment at an industrial scale. Therefore, the main aim of this paper is to provide a short overview of catalytic processes, their recent advances and to identify major directions taken to understand mechanisms governing catalytic processes utilised in water and wastewater treatment. Catalytic ozonation is considered to belong to Advanced Oxidation Processes. However, the results published by several research groups indicate that not all catalytic processes utilise the power of hydroxyl radicals.

The efficiency and mechanisms of catalytic ozonation

Centralized water treatment has dominated in developed urban areas over the past century, although increasing challenges with this model demand a shift to a more decentralized approach wherein advanced oxidation processes (AOPs) can be appealing treatment options. Efforts to overcome the fundamental obstacles that have thus far limited the practical use of traditional AOPs, such as reducing their chemical and energy input demands, target the utilization of heterogeneous catalysts. Specifically, recent advances in nanotechnology have stimulated extensive research investigating engineered nanomaterial (ENM) applications to AOPs. In this Perspective, we critically evaluate previously studied ENM catalysts and the next-generation treatment technologies they seek to enable. Opportunities for improvement exist at the intersection of materials science and treatment process engineering, as future research should aim to enhance catalyst properties while considering the unique roadblocks to practical ENM implementation in water treatment.

Challenges and prospects of advanced oxidation water treatment processes using catalytic nanomaterials.

Carbon nanotubes are promising materials for various applications. In recent years, progress in manufacturing and functionalizing carbon nanotubes has been made to achieve the control of bulk and surface properties including the wettability, acid–base properties, adsorption, electric conductivity and capacitance. In order to gain the optimal benefit of carbon nanotubes, comprehensive understanding on manufacturing and functionalizing carbon nanotubes ought to be systematically developed. This review summarizes methodologies of manufacturing carbon nanotubes via arc discharge, laser ablation and chemical vapor deposition and functionalizing carbon nanotubes through surface oxidation and activation, doping of heteroatoms, halogenation, sulfonation, grafting, polymer coating, noncovalent functionalization and nanoparticle attachment. The characterization techniques detecting the bulk nature and surface properties as well as the effects of various functionalization approaches on modifying the surface properties for specific applications in catalysis including heterogeneous catalysis, photocatalysis, photoelectrocatalysis and electrocatalysis are highlighted.

Carbon nanotube catalysts: recent advances in synthesis, characterization and applications.

Catalytic ozonation for water and wastewater treatment: Recent advances and perspective.

Nanocarbons have been demonstrated as promising environmentally benign catalysts for advanced oxidation processes (AOPs) upgrading metal-based materials. In this study, reduced graphene oxide (rGO) with a low level of structural defects was synthesized via a scalable method for catalytic ozonation of p-hydroxylbenzoic acid (PHBA). Metal-free rGO materials were found to exhibit a superior activity in activating ozone for catalytic oxidation of organic phenolics. The electron-rich carbonyl groups were identified as the active sites for the catalytic reaction. Electron spin resonance (ESR) and radical competition tests revealed that superoxide radical (•O2–) and singlet oxygen (1O2) were the reactive oxygen species (ROS) for PHBA degradation. The intermediates and the degradation pathways were illustrated from mass spectroscopy. It was interesting to observe that addition of NaCl could enhance both ozonation and catalytic ozonation efficiencies and make ·O2– as the dominant ROS. Stability of the catalysts wa...

Efficient Catalytic Ozonation over Reduced Graphene Oxide for p-Hydroxylbenzoic Acid (PHBA) Destruction: Active Site and Mechanism

Multi-walled carbon nanotube (MWCNT) has been used in the catalytic ozonation of oxalic acid in aqueous solution, which was pretreated using ozone and was characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption and Boehm titration. The effect of ozonation pretreatment on the catalytic activity of MWCNT and the ozonation mechanism of oxalic acid in the presence of MWCNT were investigated. The results indicate that with the increasing pretreatment time, the atom ratio of oxygen to carbon and the number of surface acid groups on MWCNT increase, while the catalytic activity of MWCNT on ozonation of oxalic acid decreases, suggesting that the chemical characteristics of MWCNT mainly determine its catalytic activity on oxalic acid mineralization. The inhibition of tert-butyl alcohol on the degradation of oxalic acid is slight for pristine MWCNT catalytic ozonation and this inhibition trend becomes more obvious when MWCNT was pretreated by ozone. A free radical mechanism was proposed which involves both surface reactions and bulk reactions between the oxalic acid and active species.

Effect of ozonation pretreatment on the surface properties and catalytic activity of multi-walled carbon nanotube

The influences of treatment conditions, gas atmosphere (N2 and H2) and temperature (450 °C and 950 °C), on the surface properties and catalytic performance of carbon nanotube (CNT) were investigated for the degradation of oxalic acid in aqueous solution by the process of catalytic ozonation. No obvious change on the textural properties of CNT is found after heat treatment, while the results of Boehm titration clearly indicate the reduction of acidic groups and the increase of basic groups on CNT surface after heat treatment under different conditions, and then a significant improvement on oxalic acid removal by CNT catalytic ozonation is obtained. The heat treatment of CNT under H2 atmosphere is more effective in eliminating acidic groups and increasing basic groups than the case of N2 atmosphere at the same temperature, which results in higher catalytic activity and lower total organic carbon (TOC) release in ozonation correspondingly. A linear relationship is observed between the pH at the point of zero charge (pHPZC) of CNT and the rate constant for ozonation of oxalic acid in the presence of CNT with different treatments. It is suggested that heat treatment can increase the basic property of CNT, which can enhance the contribution of surface reactions to the whole reactions for ozonation of oxalic acid.

Bei-Ping Zhang

Papers

Effect of ozonation pretreatment on the surface properties and catalytic activity of multi-walled carbon nanotube

Influence of different heat treatments on the surface properties and catalytic performance of carbon nanotube in ozonation

Factors affecting the catalytic activity of multi-walled carbon nanotube for ozonation of oxalic acid