The potential applications of the TiO2 photocatalytic process strongly depend on future development in photocatalytic engineering. As we discussed earlier, many scientific hurdles including visible light activation and enhancement of catalytic activity, and technical and practical challenges including immobilization of TiO2 and fabrication of TiO2 membrane reactors have been currently solved by introducing nanotechnological material synthesis approaches and new reactor design and concepts. The TiO2-based AONs seem promising for water and wastewater treatment due to the following aspects:
• Chemical stability of TiO2 (insoluble in water) in all pH range
• Relatively low cost of TiO2
• No chemical additives required
• System applicable at low and high pollutant concentration
• Absence of inhibition or low inhibition by ions present in water
• Nonselective radical attack by the generated oxidizing species
• Complete mineralization for almost all organic pollutants
• Removal of toxic anions, harmful metals, and nonbiodegradable organics
• Disinfection and detoxification of drinking water
• Antibiofouling properties
• Possibility of using sustainable solar energy
• Practical for combination and integration with other treatment methods
• Decreasing overall costs
However, because of low quantum yield, TiO2-based AONs are used in limited devices with optimum treatment capacity. In order for TiO2 photocatalysis to become a competitive process for full-scale applications, many efforts and research studies should be focused on easy scale-up, effective reactor design, UV fouling reduction, and TiO2 fouling prevention. It is believed that the catalyst activity should be increased by at least one order of magnitude [2,3,7-9]. The synthesis of more efficient TiO2, which can be activated under visible light irradiation, has tremendous impacts on the development of sustainable solar energy-based water treatment systems. Meanwhile, it should also be emphasized that the unique properties of nanostructured TiO2 with high catalytic activity, which has recently developed with the help of nanoscience and nanotechnology, could potentially lead to unexpected threat to the environment and thus research studies on its environmental impacts and risks should be pursed in parallel.
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