## 4 Conclusions  
The degradation and elimination of pharmaceutical compounds from aquatic environments is an urgent environmental challenge that requires sustainable and efficient remediation strategies. In this study, we investigate the photocatalytic degradation of trimethoprim (TMP), a common pharmaceutical, using a nanostructured TiO\({}_{2}\) film on a glass ring under simulated solar irradiation. The results showed that the degradation process is predominantly driven by hydroxyl radicals and positive holes as the primary reactive species, with isopropanol and triethanolamine found to strongly inhibit the photocatalytic degradation of TMP. However, the presence of ammonium oxalate indicates that superoxide radicals do not play a dominant role in the process. The complete degradation of TMP was achieved in less than 3 h under simulated solar radiation, and the specific energy consumption for the mineralization of TMP was calculated to be a 3261 kWh/g TOC reduction. The electrical energy per order (\(E_{\text{EO}}\)) analysis revealed that using natural solar radiation as a source of UV-A radiation in reactor configurations based on this approach could greatly reduce the cost of pharmaceutical degradation treatment, as evidenced by the results obtained under simulated solar radiation with an irradiation intensity of \(33\pm 2\) W/m\({}^{2}\) in the UV-A region. This study provides valuable insights into the photocatalytic degradation mechanism of TMP and highlights the potential of utilizing natural solar radiation to address the challenges of pharmaceutical contamination in aquatic environments.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13095681/s1](https://www.mdpi.com/article/10.3390/app13095681/s1)]([https://www.mdpi.com/article/10.3390/app13095681/s1](https://www.mdpi.com/article/10.3390/app13095681/s1)), Figure S1: The irradiation spectrum of the lamp model SOL 500 by Honle AG, (Gilching, Germany) taken by certified company Metron Instruments, Zagreb, Croatia, that used wide-range spectroradiometer of internal serial number SM240-IM0P2239-EU, with H1 filter at the distance of 17 cm above the reactor; Figure S2: HPLC-PDA chromatograms of reaction solution during photolytic degradation (TMP--trimethoprim, DP1 and DP2--degradation products 1 and 2); Figure S3: Mass spectra of trimethoprim and its photolytic and photocatalytic degradation products.  
**Author Contributions:** Conceptualization, S.B., D.L. and H.J.; methodology, S.B., D.L., H.J. and M.B.; formal analysis, A.B., M.B. and S.B.; investigation, S.B., D.L., H.J., M.B. and A.B.; resources, D.L. and S.B.; data curation, H.J., S.B., M.B. and A.B.; writing--original draft preparation, S.B., D.L. and H.J.; writing--review and editing, H.J., S.B. and D.L.; visualization, S.B. and H.J.; supervision, H.J., D.L. and S.B.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Croatian Science Foundation under the project Fate of pharmaceuticals in the environment and during advanced wastewater treatment (PharmaFate) (IP-09-2014-2353).  
Not applicable.