## 3 Results and Discussion
### Photocatalytic Efficiency of Pure and Palladium-Modified ZnO Films
The mechanism behind doxycycline degradation by pure and Pd-modified ZnO involves excitation of an electron (e\({}^{-}\)) to the conduction band, creating a hole (h\({}^{+}\)) in the valence band when photon energy exceeds the ZnO bandgap. The (e\({}^{-}\)/h\({}^{+}\)) pairs then produce reactive species, including hydroxyl radicals, superoxide radicals, and hydroperoxyl radicals. Hydroxyl radicals, a potent oxidant, degrade organic compounds on or near the photocatalyst surface [42]. Equations (5) through (10) describe the electron-hole reaction in Pd/ZnO sol-gel films that degrade antibiotics. \[\mathrm{Pd}/ZnO+\mathrm{h}\
u\rightarrow\mathrm{e}^{-}+\mathrm{h}^{+} \tag{5}\]  
\[\mathrm{e}^{-}+\mathrm{O}_{2}\rightarrow\mathrm{O}_{2}^{-\bullet} \tag{6}\]  
\[\mathrm{h}^{+}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{H}^{+}+\mathrm{OH} ^{\bullet} \tag{7}\]  
\[\mathrm{h}^{+}+\mathrm{OH}^{-}\rightarrow\mathrm{OH}^{\bullet} \tag{8}\]  
\[\mathrm{Doxyciline}+\mathrm{O}_{2}^{\bullet-}\rightarrow\mathrm{degradation\ products} \tag{9}\]  
\[\mathrm{Doxyciline}+\mathrm{OH}^{\bullet}\rightarrow\mathrm{degradation\ products} \tag{10}\]  
The co-catalytic photofixation of zinc oxide with palladium ions improves the semiconductor's photocatalytic activities by limiting e\({}^{-}\)/h\({}^{+}\) recombination via charge separation and promoting interfacial charge transfer between ZnO and Pd. The higher photocatalytic removal of DC in tap water, compared to distilled water, may be due to multiple factors. Tap water's slightly higher pH (7.39) than distilled water (6.5) indicates a marginally higher OH\({}^{-}\) concentration, which could produce more OH\({}^{\bullet}\) radicals as photogenerated hole acceptors, thereby enhancing pollutant degradation. Additionally, dissolved minerals such as sulfates and carbonates in tap water may generate additional reactive species, including carbonate and sulfate radicals, which could further enhance the overall DC degradation rate [43; 44]. Figure 9: Degradation percentage of doxycycline in distilled and tap water by pure and palladium-modified ZnO under UV light after 240 min. A radical test was used to confirm the presence of hydroxyl and superoxide radicals (Figure 10). Scavengers like ascorbic acid (AA) and isopropyl alcohol (IPA) were used to measure the role of superoxide and hydroxyl radicals in the degradation of doxycycline [45,46]. Figure 10 shows that when AA and IPA were added to the four photocatalyst systems, they all responded similarly. However, AA exhibited a more pronounced inhibition, indicating that superoxide radical generation plays a greater role in the DC degradation rate. Figure 11 shows the initial and final (at the fourth hour of photocatalytic removal) UV-vis spectra of the doxycycline liquid phase for all photocatalysts in distilled and tap water, and as can be seen from the spectra, the 275 and 370 nm bands gradually decrease with irradiation time, indicating efficient photocatalytic reactions. Figure 8:0 Initial and final (taken at the fourth hour) absorbance spectra of the degradation of doxycycline in (**a**) distilled and (**b**) tap water aqueous phase. Figure 8:1 Radical tests of scavengers on doxycycline decomposition using ZnO and Pd/ZnO films in (**a**) distilled water and (**b**) tap water. The UV-vis spectral evidence presented in Figure 11 suggests the possibility of longer-lived intermediates in the aqueous phase, though their identification and detailed analysis are beyond the scope of the current study. The photocatalytic degradation pathway of doxycycline has been extensively investigated in the previous literature [47; 48; 49; 50]. One of the most widely accepted reaction pathways, schematically illustrated in Figure 12, begins with the demethylation of doxycycline (I) to form intermediate II [50]. Intermediate II undergoes deamidation, hydroxylation, and a ring-opening reaction to produce intermediate III, which can subsequently degrade into intermediate IV via C-C bond cleavage [47].