**4. Conclusions**

In the present work, the electrochemical oxidation of three cytostatic drugs and their mixtures in a recirculating split-flow batch reactor equipped with a BDD anode was investigated. The oxidation of cytostatic drugs was performed in an anodic compartment separated from the cathodic compartment by a cation-exchanged membrane. The values of kapp for the single-drug solution and for mixtures of cytostatic drugs in Na2SO4 as electrolyte were in the following order: kapp 5-FU < kapp CP < kapp

IF. The degradation of drugs in their mixtures showed that the PO4<sup>3</sup>− has no significant effect on the drugs' degradation, while Cl− mainly accelerated this process. Based on the TOC removal and TN conversion to gaseous products, it was found that the degradation pathway of IF was different than that of CP. The organic intermediates found after 6 h of electrolysis were also different for these drugs.

Comparing the results in NaNO3 and Na2SO4 electrolytes, it was suggested that the different oxidising species with different levels of reactiveness participated in cytostatic drugs degradation. In Na2SO4, 5-FU was mainly oxidized by OH• and SO4−• radicals, while chlorine active species simultaneously with OH• and SO4−• entities participated in the CP and IF degradation and nitrogen conversion to gaseous products. Given that Cl− is abundant in natural waters, the involvement of chlorine active species in the electro-oxidation fate of drugs should not be neglected. This was confirmed by the high mineralization and efficiency of drugs removal in the effluent from WWTP, and this is beneficial from the practical point of view. This study is helpful in understanding the fundamental reaction mechanism, as well as the effects of natural water constituents on the kinetics and mechanisms of electrochemical oxidation of cytostatic drugs in their mixtures. Further studies should address the effects of composition of contaminated water (by natural organic matter, carbonates and other anions and cations) on the transformation of cytostatic drugs and the examination of effluents toxicity obtained from AOPs.

**Supplementary Materials:** The following are available online. Figure S1: Cyclic voltammograms of IF, CP and 5-FU (25 mg/L) in 42 mM Na2SO4 (pH = 6.6), BDD as a working electrode, counter electrode (CE)—Pt, scan rate = 100 mV·s<sup>−</sup>1, T = 20 ± 2 ◦C.

**Author Contributions:** E.M.S. conceptualized and designed the experiments; S.A.O.-C., A.P., A.B.-G. and P.W. performed the experiments and analyzed the data; E.M.S wrote the paper; and polished the paper; E.M.S. and A.P. acquired funding for the research. All authors read and approved the final manuscript.

**Funding:** This research was supported by the Polish Ministry of Research and Higher Education, Poland under the Grant DS530-8626-D596-18 and DS530-8626-D596-19.

**Acknowledgments:** Thank you very much for the help in lab Katarzyna Bachli ´nska and Agnieszka Fiszka Borzyszkowska.

**Conflicts of Interest:** The authors declare no conflicts of interest.
