## 5 Titanium Dioxide as a Photocatalyst for Water Remediation Applications
pH modifications were made by adjustments with either 0.1 M HCl or 0.1 M NaOH. Ionic strength was set by the addition of CaCl\({}_{2}\). Irradiation was performed with a 1500 W xenon lamp with irradiance reaching 735 W \(\times\) m\({}^{-2}\). The wavelength produced was within the range of 300-800 nm. The reactions were performed in the following conditions: (a) 10 mM H\({}_{2}\)O\({}_{2}\) was added to each sample; (b) irradiation time was 6 h; (c) temperature was kept at 35 \({}^{\circ}\)C; (d) three different pH values were used (3, 6.4, 11); (e) three different ionic strength parameters (0.01, 0.1 and 1M CaCl\({}_{2}\)); and (f) three concentrations of humic and tannic acids (1, 10, 50 mg \(\times\) dm\({}^{-3}\)). At pH 6.4,TiO\({}_{2}\) showed better photocatalytic rates than for ZnO in the photodegradation of CBZ. Transformation percentages for CBZ were as follows: 63% for TiO\({}_{2}\) and 35% for ZnO. For CBE, transformation rates reached even 80% for TiO\({}_{2}\) and 13% for ZnO. Following the authors' statement, the reason for the difference in degradation efficiency between TiO\({}_{2}\) and ZnO was unknown. At high pH, photodegradation of CBZ by TiO\({}_{2}\) was negatively impacted. This effect was also observed for ZnO. CBE photodegradation with ZnO occurred at higher rates both at low pH equal to 3 and at high pH equal to 11 than at pH 6.4. pH value revealed no effect on photodegradation of ACR. The model contaminants were also tested for degradation in dark conditions as well as in the presence of UV irradiation and in the absence of the photocatalyst, but no effect was noted. Ionic strength (IS) tests were made at pH 6.4. In the study, TiO\({}_{2}\) photodegradation rates increased with IS for CBZ, but this factor revealed no effect on the rates for CBE, ACR, and ACO. In the ZnO case, only in the sample where IS was the highest was the photodegradation rate of CBZ reduced. Similarly to TiO\({}_{2}\), IS had no effect on the photodegradation rates of CBE, ACR, and ACO. The effect of natural organic matter is difficult to evaluate because the reaction can be affected in two contrasting ways. Natural organic matter can either inhibit photodegradation by acting as a quencher or enhance it by acting as a photosensitizer [104]. Considering all the presented factors and their influence on the reaction environment, it is important to take into account all possible parameters when designing a photodegradation experiment [104]. Aside from pharmaceuticals, titania NPs have been applied for the UV-induced degradation of selected organic compounds including dyes. One of them was malachite green (MG), which is a synthetic dye commonly used in the leather and paper industries [105]. It was found that TiO\({}_{2}\) catalyzes step-wise N-demethylation of the MG dye, yielding N-demethylated MG species. In the study, it was possible to achieve up to 99.9% reduction in MG content with complete decolorization of the solution after 4 h of irradiation with 365 nm UV light. According to the study, TiO\({}_{2}\) was found to be an efficient photocatalyst in the removal of organic dyes in water solutions [105]. To sum up, titania has been applied for the degradation of synthetic dyes, APIs present in PPCPs as well as agricultural pollutants. Many photocatalytic systems have been studied where TiO\({}_{2}\) was used in different forms. In a number of examples, TiO\({}_{2}\) was modified to enhance its properties and overcome some of its shortcomings. However, simple modifications without the use of a dopant, co-catalyst, or a photosensitizer are limited in what they can offer. More complex systems based on titania, which are discussed in the next sections, are necessary to enrich titania in additional properties like photoactivity in visible light and improved charge transfer.