## 5 Titanium Dioxide as a Photocatalyst for Water Remediation Applications
Although Fe(III) achieved overall better degradation rates of tested substances, TiO\({}_{2}\) revealed better mineralization rates, which is an essential factor in photodegradation. The photocatalytic process often leaves still harmful intermediates that can accumulate in solutions. A good photocatalyst should also be able to remove them in the process [102]. [PERSON] et al., in their study, showed the effects of photocatalytic treatment of pharmaceutical wastes by TiO\({}_{2}\) nanopore array electrodes [103]. The MO was immobilized on electrodes to avoid problematic separation of TiO\({}_{2}\) powder from the treated solution after the procedure concluded. Next, the efficiency of such solution was evaluated in an experiment focusing on photocatalytic and photoelectrocatalytic degradation of substances belonging to the tetracycline (TCL) family, which is a group of antibiotics commonly used in agriculture and live stocking. The researchers used a three-electrode system, platinum, Ag/AgCl, and TiO\({}_{2}\) electrodes in the following conditions: 254 nm length UV light irradiation, 2.5 mW \(\times\) cm\({}^{-2}\) light intensity, 0.5 V current, pH 5.5, and 0.02 mol \(\times\) dm\({}^{-3}\) sodium sulfate as the electrolyte. The solution was stirred, and no air flow was applied in the photoelectrocatalytic and photocatalytic studies. In the latter case, no current was applied. TCL was applied in a 20 cm\({}^{3}\) solution at the 10 mg \(\times\) dm\({}^{-3}\) concentration. According to the results obtained for the nanopore array electrode, photoelectrocatalytic degradation allowed for the removal of 80% of TCL from the solution within 3 h. Nanofilm electrodes removed 55% of TCL content. The electrodes were tested under various conditions, allowing the occurrence of different mechanisms of degradation. Electrochemical, photocatalytic, and photoelectrocatalytic degradation rates were evaluated separately. Electrochemical degradation occurred at a negligible rate during the experimental time. In the sample under UV light exposure, 23% of TCL was degraded in 3 h. When both irradiation and current were applied, the removal ratio rose to 80%. During the course of the study, the electrodes proved to be effective for photodegradation; they had strong mechanical stability and were easy to separate from the solution. Therefore, TiO\({}_{2}\) nanopore array electrodes seem to constitute a promising material of photoelectrocatalytic potential for the removal of pharmaceutical pollutants from wastewater [103]. In a study by [PERSON] et al., carbamazepine and its derivates, carbamazepine epoxide (CBE), acridine (ACR), and acridone (ACO) were treated with TiO\({}_{2}\) (99% anatase) and ZnO NPs [104]. Additionally, the effects of environmental parameters (pH, ionic strength, natural organic matter content) on the efficiency of the reaction were evaluated. The catalytic efficiency varied by a large margin depending on the above-mentioned parameters as well as the treated derivate. The efficiency of TiO\({}_{2}\) was higher than that of ZnO in the degradation of CBZ and CBE, whereas no major changes in reaction efficiencies were observed in tests with ACR and ACO. The reaction parameters were tested, pH, and natural organic matter played a significant role in influencing the photodegradation of CBZ and CBE, which was not the case for the two other derivatives. The following materials were prepared for the tests: (a) NP suspensions--prepared by suspending 1 g of NPs in 1 dm\({}^{3}\) of MilliQ water; (b) 1 mg \(\times\) cm\({}^{-3}\) solutions of pharmaceuticals; and (c) formic, humic, and tannic acids--served as model substances for natural organic matter content. Experimental solutions consisted of a 400 \(\upmu\)L aliquot of each pharmaceutical stock solution in 40 cm\({}^{3}\) NP suspensions in 50 cm\({}^{3}\) polypropylene tubes. The final concentration for TiO\({}_{2}\) and ZnO NPs was 500 mg \(\times\) dm\({}^{-3}\) and 10 \(\upmu\)g \(\times\) cm\({}^{-3}\) for pharmaceuticals. Methanol content was measured to be under 1%.