Water scarcity has been exacerbated by environmental pollution caused by increasing global industrialization and urbanization. This has led to continued research for more ecologically safe and cleaner technologies for eliminating pollutants. In the recent past, nano-engineered metal oxides such as titanium oxide (TiO2) have shown promising photocatalytic effect of completely mineralizing organic pollutants to form harmless byproducts. TiO2 is the most frequently used photocatalyst, however its effectiveness is limited to light absorption within the ultra violet region due to its wide band-gap. A redshift by TiO2 has been achieved through tuning of the anatase-rutile phase transitions, doping, surface modifications and creation of multi component heterojunctions. This chapter discusses the synthesis of TiO2–based photocatalysts and photocatalytic activity enhancement of TiO2 through doping and formation of hybrids. Their application in the photo-degradation of dyes and other organic pollutants as well as their limitations and future prospects are also addressed.
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Availability of clean drinking water in the world today is increasingly becoming a challenge due to the pollution of water resources by effluents emanating from industries and run-offs from agricultural fields. Among the pollutants released to the aquatic systems are heavy metals, dyes, pesticides among other organic pollutants which pause potential harm to both humans and aquatic life. Several techniques have been employed in the removal of these pollutants from water including reverse osmosis, coagulation, precipitation, electrochemical processes, membrane filtration, adsorption and irradiation or advanced oxidation processes (Ngeno et al., 2022; Tome et al., 2023). However, collectively these technologies have diverse drawbacks such as need for expensive equipment, trained personnel and production of secondary pollutants that require further treatment. Moreover, non-biodegradable organic compounds are difficult to remove by the conventional water treatment techniques such as coagulation and precipitation (Poole, 2004).
Amongst these water treatment techniques, advanced oxidation process (AOP) has demonstrated to be the most effective alternative route for removal of these organic pollutants from aqueous medium (Shikuku & Nyairo, 2019). AOP is a chemical process that involves the use of highly reactive hydroxyl (⦁OH) radicals to completely destroy a wide range of organic pollutants to mineralization with no selectivity. Various oxidation processes used to produce the hydroxyl species by use of UV light include Fenton’s reaction, H2O2/UV, O3/UV, H2O2/O3/UV, TiO2/UV among others (Xiang, Yu, & Wong, 2011). Fenton oxidation utilises H2O2 and Fe2+ ion source to produce ⦁OH radicals to decompose organic molecules. Fenton’s approach is considered to be fast and easy to apply. However, it has the disadvantages of high maintenance costs and the need for iron sequestration from the effluent system after use (Bello, Raman, & Asghar, 2019). On the other hand, coupling of TiO2, a heterogeneous catalyst, with UV for the production of ⦁OH offers the advantage of integrating it into the water treatment. Furthermore, TiO2/UV photocatalysis can be carried out at room temperature and it makes use of atmospheric oxygen as the oxidant for complete mineralization of organic pollutants. As a result, TiO2/UV has proved to be the most effective AOP and in the recent past, therefore, the photocatalytic process using TiO2 has received immense attention (Lee & Park, 2013). The merits and demerits of various oxidation processes and photocatalysis are enumerated in Table 1.
Table 1. The merits and demerits of various oxidation processes and photocatalysis.
| AOP | Advantages | Disadvantages | Ref. |
| Fenton reaction | Wide coverage.
Simple and easy to use.
No off gas treatment required. | Requirement for iron recovery from the system.
Relatively high maintenance costs involved.
Works only at low pH. | (Bello, Raman, & Asghar, 2019) |
| H2O2/UV | No off gas treatment required.
More effective than UV or H2O2 used independently. | Gives turbidity which inhibits UV light penetration.
Contamination caused by UV lamp failure. | (Poole, 2004) |
| O3/UV | Produces more ⦁OH than H2O2/UV. | Relatively costly.
Contamination due to failure of UV lamp. | (Lim, Shi, von Gunten, & McCurry, 2022) |
| Photocatalysis | Use of solar irradiation.
Low operational and installation costs.
Low energy requirement compared to other AOPs.
Unmanned operation is possible. | It is pH sensitive.
Reduction of catalytic activity with time. | (Kwon, Fan, Cooper, & Yang, 2008) |