## 3 Results
### Photocatalytic Properties of ZnO Powder  
The photodegradation of DNP and CIP was evaluated under simulated solar light. Figure 3a,b represents absorption spectra of unirradiated (0 min) and irradiated DNP (Figure 3a) and CIP (Figure 3b) for 2, 5, 10, 20, 30, 40, and 60 min. The peculiarities of irradiation absorption are studied in [23; 24]. As a result of irradiation of a semiconductor, surface photocatalytic reaction is initiated during which a photoelectron is promoted from the filled valence band to the empty conduction band of the semiconductor generating electron and hole pair (e\(-\)/h+). The photogenerated holes at the valence band can react with water to produce OH' radical. The electron in the conduction band is taken up by the oxygen in order to generate anionic superoxide radicals. These powerful oxidizing agents attack adsorbed organic molecules non-selectively causing their mineralization [25]. Degradation of most organic compounds by photocatalysis may be described using a pseudo-first-order kinetic model for reactions that occur at the solid-liquid interface [26]. The ln (C/C\({}_{0}\)) graph against time producing a linear line verifies that the reaction towards DNP and CIP follows the pseudo-first-order kinetic model. The apparent first-order rate constants (k\({}_{\text{app}}\)) were 0.036 and 0.051 min\({}^{-1}\) for DNP and CIP, respectively. During photocatalytic degradation of CIP antibiotic, complex organic molecules are converted into low molecular weight species. However, a tentative CIP photodegradation mechanism can include some pathways of the reaction: substitution of the F atom by the hydroxyl \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Zn** & **O** & **C** & **N** \\ \hline Initial & 49.6 & 40.8 & 9.6 & 0 \\ \hline After photocatalysis & 49.5 & 41.3 & 9.2 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Concentrations (in %) of the main components of ZnO powder before and after photocatalysis according to XPS data. group and the ring opening of the CIP molecule. The complete mineralization of CIP antibiotic occurs through the formation of CO\({}_{2}\), H\({}_{2}\)O, and NO\({}_{3}\)\({}^{-}\)[27]. Oxidation of 2,4-dinitrophenol may occur through the ring opening by the attack of the OH radical. The overall degradation of 2,4-dinitriphenol results in the formation of smaller byproducts which may lead to mineralization [28]. The results of this study are compared to those of similar papers that investigated the photodegradation of DNP and CIP (Table 3). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Photocatalyst**} & \multirow{2}{*}{**Pollutant**} & **Catalyst** & **Pollutant** & \multirow{2}{*}{**Light Source (g L\({}^{-1}\))**} & **k\({}_{\text{app}}\)** & **k\({}_{\text{app}}\)** \\  & & **Dosage** & **Concentration** & & **(10\({}^{-3}\) min\({}^{-1}\))** & **Reference** \\ \hline ZnO/g-C\({}_{3}\)N\({}_{4}\) & CIP & 0.5 & 5 mg L\({}^{-1}\) & \begin{tabular}{c} 32 W compact \\ fluorescent bulb \\ \end{tabular} & 24 & [29] \\ \hline ZnO nanoparticles & CIP & 0.6 & 12 mg L\({}^{-1}\) & \begin{tabular}{c} Natural sunlight, \\ 221 W/m\({}^{2}\) \\ \end{tabular} & 13 & [30] \\ \hline TiO\({}_{2}\)/SnO\({}_{2}\) nanocomposite & CIP & 0.5 & 5 mg L\({}^{-1}\) & \
\begin{tabular}{c} Three UVC lamps \\ with 35 W (253 nm) \\ \end{tabular} & 28.2 & [31] \\ \hline
\begin{tabular}{c} carbon-nano-onion- \\ functionalized \\ ZnO tetrapods \\ \end{tabular} & DNP & 2 & 0.1 mM & 60 W tungsten bulb & 18.34 & [32] \\ \hline Commercial P25 & DNP & 2 & 5 \(\mu\)M & 266 W/m\({}^{2}\) Hg lamp & 32 & [33] \\ \hline ZnO powder & CIP & 0.5 & 5 mg L\({}^{-1}\) & 500 W Xenon lamp & 51 & This work \\ (microparticles) & DNP & 0.5 & 5 mg L\({}^{-1}\) & 500 W Xenon lamp & 36 & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of some of the recent literature reports on the photocatalytic degradation of CIP and DNP by various photocatalysts with our study.