## 3 Results and Discussion
### Photocatalytic Activity of TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) Composite systems  
One of the main reasons for the functionalization of TiO\({}_{2}\) either by surface modification, doping, or by preparing composite systems is to alleviate its flaws, including very weak photoactivity in visible light or troublesome filtration after the remediation processes. By preparing TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) composites, these imperfections can be at least partially abolished. TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) composites possess a paramagnetic property that allows for the photocatalyst's filtration with a magnetic field after the photocatalytic reaction. Additionally, due to the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{3}{c}{**First Stage**} & \multicolumn{3}{c}{**Second Stage**} & **Residue** \\ \cline{2-7}  & **T\({}_{\text{p}}\) (\({}^{\circ}\)C)** & **T\({}_{\text{k}}\) (\({}^{\circ}\)C)** & **Am (\%)** & **T\({}_{\text{p}}\) (\({}^{\circ}\)C)** & **T\({}_{\text{k}}\) (\({}^{\circ}\)C)** & **Am (\%)** & **(\%)** \\ \hline TiO\({}_{2}\) & 26 & 384 & 2.51 & & & 97.49 \\ TiO\({}_{2}\)–Fe\({}_{3}\)O\({}_{4}\) & 26 & 187 & 2.24 & 188 & 635 & 1.45 & 96.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Thermogravimetric analysis of bare TiO\({}_{2}\) and TiO\({}_{2}\)–Fe\({}_{3}\)O\({}_{4}\) composite materials. Figure 4: Thermogravimetric analysis graphs of bare TiO\({}_{2}\) and TiO\({}_{2}\)–Fe\({}_{3}\)O\({}_{4}\) composite materials. band gap of Fe\({}_{3}\)O\({}_{4}\), TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) composites should reveal photoactivity after irradiation with visible light [22; 23]. Photocatalytic degradation tests of ibuprofen with bare TiO\({}_{2}\) and TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) composite material under irradiation with UV light (365 nm) and green light (523 nm) are presented in Figure 5. Photodegradation tests with UV light (Figure 5A) revealed impaired photoactivity of the composite material in comparison to bare TiO\({}_{2}\). The negative effect was observed both under UV and green light irradiations. After 2 h of irradiation with the UV light in the presence of bare TiO\({}_{2}\), a 44% reduction of initial ibuprofen concentration was noted, whereas in the presence of TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\), only a 19% reduction appeared. At the beginning of the experiment, both materials show comparable degradation rates. However, after 1 h of irradiation with TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\), a plateau is reached. After 2 h of irradiation with green light in the presence of bare TiO\({}_{2}\) only a mean 4% ibuprofen degradation rate was achieved (Figure 5B). The obtained result could be considered statistically insignificant and treated as a lack of activity. Interestingly, the composite showed only comparable photoactivity in green light as bare TiO\({}_{2}\). The result is contradictory to the expected effect as the Fe\({}_{3}\)O\({}_{4}\) band gap should allow the absorption of wavelengths from the green light region [22] and subsequent sensitization of TiO\({}_{2}\). Initially, TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) showed much better degradation rates of ibuprofen than bare TiO\({}_{2}\). However, after 1 h time, the ibuprofen concentration slightly increased, indicating that the photocatalytic reaction stopped and that ibuprofen desorbed from the photocatalyst's surface back to the reaction suspension. After 2 h of irradiation, only a 3% IBU degradation rate was noted with TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\). The disappearance of TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) photoactivity in both irradiation tests was accompanied by the browning of the TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) material during irradiation which is associated with the oxidation of Fe\({}_{3}\)O\({}_{4}\) within the photosensitizer to maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)).