2.3.4. Photocatalysis Coupled with Ozonation Process

Purification of wastewater, including phenolic compounds, has been realized by several treatment methods, such as photocatalytic degradation, electrochemical methods, adsorption, Fenton's reaction, and ozonation processes [104–106]. Ozone is a powerful oxidizing agent that destroys organic pollutants in wastewater by producing reactive oxygen species (ROS). Photocatalytic ozonation proved to be an efficient and promising advanced oxidation process available to remove widely spread organic contaminants in wastewater. The difference between photocatalytic ozonation and catalytic ozonation in an aqueous solution lies in the chain reactions initiation. The photochemical reaction is triggered by an electron transfer from a semiconductor to oxygen or ozone. The catalytic ozonation mechanism usually begins with the reaction between the hydroxyl anion with ozone [107]. In both processes, superoxide anion (O2 −) is primarily formed and subsequently reacts with ozone to give ozonide ion (O3•), consequently resulting in the formation of hydroxyl radical [108]. To a lesser extent, superoxide anion (O2 −) can also act as an oxidant, ultimately leading to the mineralization of the organic compounds. Photocatalytic ozonation shows a synergetic effect since it can decrease the electro-hole recombination due to the great electron trapping of ozone, together with the interaction of O3 with the superoxide radical. Both mechanisms are responsible for forming the ozonide radical, which further transforms into a hydroxyl radical, showing more powerful oxidant behavior [109]. Therefore, this technology can enhance the hydroxyl radical's generation, even at low pH, increasing the mineralization rate. The application of photocatalytic ozonation on wastewater treatment is expected to

be more effective than photocatalysis and/or ozonation technologies alone. During the ozonation process, the resulting hydroxyl radical in the water phase reacts with organic contaminants leading to their mineralization [110].

Very recently, Yu et al. [111] synthesized single-crystal WO3 nanosheets (NSs) by a hydrothermal method and checked their photocatalytic activity for phenol photomineralization under visible light. The excellent performances of WO3 NSs were attributed to their lamellar morphology with single-crystal microstructure and good dispersion, providing continuous interior channels for the charge carrier transportation from the bulk to the surface of WO3 nanosheets. The authors investigated the degradation efficiency (RD) and mineralization ratio (Rm) of phenol under different systems (Figure 14a), including ozonation alone (O3), ozonation combined with visible light exposure in the absence of photocatalyst (Vis/O3), catalytic ozonation in the presence of photocatalyst (WO3 NSs/O3 and WO3 NPs/O3), photocatalysis (WO3 NSs/Vis) and photocatalytic reaction conducted in the presence of ozone (WO3 NSs/Vis/O3 and WO3 NPs/Vis/O3). They found that the mineralization ratio for WO3 NSs/Vis/O3 reached 96% after 150 min, and it rose continuously to 98% at 240 min, while the Rm was 83% for WO3 NPs/Vis/O3 at 240 min. After WO3 nanoparticles (NPs) and nanosheets exposure to visible light, the electrons and holes were generated (Figure 14b). These photogenerated carriers were separated and transferred from the bulk to the surface of WO3 and reacted with O3 and HO−/H2O to produce hydroxyl radicals. The produced hydroxyl radicals attack the intermediates of phenol degradation. Subsequently, the complete mineralization of phenol occurred.

**Figure 14.** (**a**) Mineralization ratio (Rm) and degradation efficiency (RD) of phenol in various oxidation processes; (**b**) Proposed mechanism of phenol mineralization in WO3 nanosheets under visible light irradiation. Reproduced with permission from ref. [111]. Copyright 2022 Elsevier.

Similarly, Nishimoto and co-workers [112] demonstrated that the WO3 catalyst possesses excellent performance for the photocatalytic water treatment under visible-light irradiation combined with ozonation. The authors employed two different catalysts (e.g., WO3 and N-doped TiO2), comparing their capability for TOC removal. Bare WO3 exhibited a superior response for the photomineralization of phenol in the presence of ozone, which readily reacted with its photogenerated electrons in the conduction band. Tawabini and Zubair [113] presented a combined UV and ozone process for phenol removal while inhibiting the formation of bromate in water. Photolysis by UV partially degrades the pollutant. Although combining the UV/O3 techniques, total removal of 50 ppm of phenol in less than 5 min occurs. The authors observed that after the optimization of the operational parameters (e.g., continuous ozonation rate of 1 L/min, addition of 1.5 ppm ammonia for adjusting the pH), the bromate formation was diminished drastically to non-detected levels. In the same way, by coupling catalytic ozonation with photocatalysis, nearly 100% degradation performance for phenol removal within 2 min was obtained over MgO/g-C3N4 catalysts by An et al. [114]. The operational conditions were a visible-light source equipped with a

300 W xenon vertical irradiation, the concentration of the pollutant was 30 mg L−1, and a reactor volume of 250 mL. For the developed photocatalyst, the MgO played a dual role: (i) accelerating the photogenerated charges separation of g-C3N4 and (ii) facilitating the conversion of ozone into •OH, thus enhancing the catalytic ozonation process.
