## 3 Results and Discussion
### COD and TOC Removals  
Figure 1A shows the COD and TOC values measured in pure water (filled symbols, irradiated and unirradiated) and also the calculated ones for the unirradiated solution (empty symbols). The calculation was based on the molecular formula and concentration using the method described in the paper of [PERSON] et al. [31]. The measured and calculated COD values differ considerably: in pure water, 43.1 and 59.2 mg dm\({}^{-3}\), respectively. In the case of TOC, there is a reasonable agreement between the two values, 22.2 and 21.6 mg dm\({}^{-3}\), respectively. Because of the good agreement in the TOC values, it is probable that the observed difference in COD is not due to inaccuracy or impurities [32]. The disagreement between the measured and calculated COD values was observed very often in different laboratories for many chemicals. [PERSON] et al. suggested that during the boiling process some small organic molecules may evaporate and leave the reaction vessel; this loss may cause the mentioned deviation [31]. In Figure 1A, both COD and TOC decrease with the absorbed dose. At doses 0.5, 1, 2, and 4 kGy, 10, 11, 15, and 40% smaller COD values, respectively, were measured than in the untreated sample; the values for TOC were 4, 5, 11, and 25%, respectively. In the experiments of [PERSON] et al. at 3 kGy absorbed dose, 29.3% TOC reduction was observed at the same ofloxacin concentration as we used here (0.1 mmol dm\({}^{-3}\)) [12]. At the beginning of the treatment, the percentage decrease of COD, measured here, and in other AOP systems was higher than that of TOC (e.g., [13; 33]. As Figure 1A shows, at higher doses the COD/TOC ratio slightly shifted toward lower values, showing that with the increase in dose, more and more oxidized products are in the solution. AOP experiments with a large number of organic molecules have shown that with the progress of degradation, small, strongly oxidized molecules, first of all organic acids (acetic, propionic, formic, oxalic), dominate the product spectrum [32; 34], resulting in a decrease in pH [3; 17]. These molecules have high oxygen-to-carbon ratios. They degrade very slowly in \({}^{\bullet}\)OH reactions [35]; this is why the COD/TOC ratio decreases with increasing dose. The predominance of small acids in the reaction mixture at high doses is also supported by the average oxidation state (AOS) values calculated using the measured COD and TOC data (here both are expressed in mol dm\({}^{-3}\) units): AOS = 4 \(\times\) (1-COD\({}_{\text{mol}}\)/TOC\({}_{\text{mol}}\)) [33]. By definition, the AOS values vary between \(-\)4 (methane) and +4 (carbon dioxide). In our experiments, these values were around 1-1.5 in irradiated solutions (Figure 1A-C). We mention as examples some AOS values: in acetic and lactic acid solutions, they are calculated to be 0, and in malonic, formic, and oxalic acid solutions, they are 1.33, 2, and 3, respectively. Figure 1: COD, TOC, and AOS values as a function of absorbed dose in 0.1 mmol dm\({}^{-3}\) ofloxacin solutions in pure water (**A**), tap water (**B**), and purified wastewater (**C**) matrices. The decrease in both COD and TOC is observed even at the smallest dose, 0.5 kGy, used in our experiments, indicating that oxidation and mineralization occur simultaneously. During degradation, the antibiotic is finally mineralized, forming CO\({}_{2}\), H\({}_{2}\)O, and inorganic ions. Two types of mineralization kinetics can be suggested. 1: In one of them, the molecules are oxidized step-by-step, and the degradation occurs through several stable products before final mineralization (the oxidized molecules in the last step transform into inorganic species). 2: The simultaneously observed decrease in both COD (oxidation) and TOC (mineralization) at the beginning of the treatment, i.e., building oxygen atoms into the organic substrates and also transformation of organic carbon atoms to CO\({}_{2}\) may reflect another mechanism.