*3.3. Electrochemical Systems*

Degradation experiments were performed in a recirculating split-flow batch reactor divided into anolyte and catholyte compartments. The cell contains two parallel electrodes, boron-doped diamond (polycrystalline BDD film on monocrystalline *p*-type Si wafer, Adamant Technologies, B/C radio about 500 ppm) used as an anode, and stainless steel (SS) as a cathode. Both were flat (11.7 mm diameter × 2 mm) with an inert-electrode distance of 3.5 cm. Two peristaltic pumps were used to supply the cytostatic drug solution/mixture at constant volumetric flow rate. The total volume of both the anolyte and catholyte was 100 mL. In the divided cell, the anodic and cathodic compartments were separated into two equal spaces by a cation exchange membrane Nafion ® 424 to allow the passage of protons. The current was passed through the cell with a potentiostat/galvanostat. Experiments were performed at room temperature 25 ◦C ± 3 ◦C. The pH was monitored with the pH-meter. The reactor and glassware used were protected from light. The electrochemical system applied in the study is presented in Figure 6. All experiments were performed in duplicate. The optimal experimental conditions were previously investigated [20]. Electrochemical degradation of the mixtures (IF/CP and CP/5-FU) in the supported electrolyte solution (42 mM Na2SO4 or 42mM NaNO3) was carried out for the initial drug concentration of 25 mg·L−1. A comparison between the oxidative processes occurring in the mixtures of these medicaments and single-drug solutions was conducted. The initial concentrations of each drug in a single-drug solution were 25 and 50 mg·L−1. Subsequently, the degradation experiment was performed in the same operating conditions with the mixture of the drugs, adding 5 and 10 mg·L−<sup>1</sup> of PO4 3− or 10 and 100 mg·L−<sup>1</sup> of Cl<sup>−</sup>, or with a mixture of drugs added to the actual e ffluent from the wastewater treatment plan, as seen in Table 3. The following operating conditions were used in the assays: current density of 15 mA·cm<sup>−</sup><sup>2</sup> and flow rate of 13 L·h−1. For all kinetics experiments, the drug concentration was monitored by means of the HPLC-UV analysis. The mineralisation process was estimated based on the total organic carbon (TOC) removal and total nitrogen (TN) removal.

**Figure 6.** The electrochemical set-up.

The mineralization current efficiency (MCE) according to El-Ghenymy et al. [39] for mixtures of drugs was estimated from Equation (8):

$$\text{MCE}\% = \frac{n \text{FV} (\Delta \text{TOC})}{4.32 \times 10^7 \text{ mIt}} \times 100\% \tag{8}$$

where *n* is the number of electrons consumed per one molecule of drug assuming the total mineralization (based on the reactions Equation (9) for 5-FU and Equation (10) for IF, CF and molar ratio drugs in the mixture), F is the Faraday constant (96,487 C mol−1), V (dm3) is the solution volume, (ΔTOC) (mg <sup>L</sup>−1) is the difference between TOC before and after electrochemical process, 4.32 × 10<sup>7</sup> is a factor to homogenize units, m is the number of carbon atoms of drugs, I (A) is the applied current, and t (h) is electrolysis time.

$$\text{H}\_4\text{H}\_3\text{N}\_2\text{O}\_2\text{F} + 6\text{H}\_2\text{O} \rightarrow 4\text{CO}\_2 + 2\text{NH}\_4^+ + \text{F}^- + 7\text{H}^+ + 8\text{e}^- \tag{9}$$

$$\text{C}\_7\text{H}\_{15}\text{N}\_2\text{O}\_2\text{Cl}\_2\text{P} + 8\text{H}\_2\text{O} \rightarrow 7\text{CO}\_2 + 2\text{NH}\_4^+ + 2\text{Cl}^- + \text{PO}\_4^{3-} + 23\text{H}^+ + 20\text{e}^- \tag{10}$$

**Table 3.** Characteristics of effluents from municipal wastewater treatment plant (MWWTP) with mechanical and biological stages in Gda ´nsk in Poland.

