## 3 Results and Discussion
### Photocatalytic Activity Evaluation  
The materials were further tested as photocatalysts for the degradation of MO. Before starting the catalytic process, the mixture containing 90 mg\(\cdot\)L\({}^{-1}\) MO and 20 mg of the material was stirred in dark conditions for 1.5 h, as indicated by the adsorption experiments. The shaded area in Figure 6a corresponds to the adsorption process. After adsorption/desorption equilibrium was achieved, the sample was irradiated with UV light. The kinetics of MO photocatalytic degradation was monitored for 2 h. The data are presented in Figure 6a. No obvious decrease in MO concentration was detected by the Pal, pure UiO-66, and PU composite. Clay minerals do not possess any photocatalytic activity. On the contrary, UiO-66 is photoactive [37; 53]. However, on the basis of the U and PU light absorption properties (Figure 4), light irradiation at wavelengths lower than 310 nm was required for their activation. Therefore, it is not surprising that U and PU were not active under the conditions of our experiment. MO concentration was reduced when materials containing TiO\({}_{2}\) were used. This implies that PUT is photoactive and able to degrade MO. UiO-66/TiO\({}_{2}\) composites were previously proven to be active in photocatalytic degradation of dyes [25; 60]. Pure TiO\({}_{2}\) removed approximately 50% of the initial MO. Comparing the materials presenting photoactivity, complete decolorization was achieved only in the PUT composite within less than 2 h irradiation. The composite bearing UiO-66 and TiO\({}_{2}\) (UT) also presented photoactivity, but the removal efficiency was lower than that of the PUT  
Figure 5: Time dependence of MO adsorption on the pure Pal, TiO\({}_{2}\), and UiO-66 and the prepared composites (PU and PUT) (**a**). Pseudo-second-order rate law for MO adsorption on materials containing UiO-66 (**b**). Experimental conditions: 90 mg\(\cdot\)L\({}^{-1}\) initial MO concentration, 0.4 g\(\cdot\)L\({}^{-1}\) of the adsorbent, pH = 5, volume = 50 mL, and T = 25 \({}^{\circ}\)C.  
composite. The high removal efficiency of PUT against MO was linked to both functions of the PUT composite (i.e., adsorbent and photocatalyst). Most of the MO (77.5%) was adsorbed, while the remainder was decomposed via photocatalysis. In order to further evaluate the photocatalytic efficiency of the prepared materials, the photocatalytic degradation rates were calculated. The data were well fitted with a fist-order kinetic model. Rate constants (k) were estimated by fitting the experimental data with the following equation:  
\[\ln\frac{\text{C}_{0}}{\text{C}}=\text{kt}, \tag{4}\]  
where k is the apparent first-order rate constant (min\({}^{-1}\)), t is the irradiation time (min), C\({}_{0}\) is the initial concentration (i.e., after the adsorption/desorption process reached equilibrium), and C is the concentration of MO at reaction time t. The data are presented in Figure 6b, and the photocatalytic degradation rates (k) are summarized in Table 1. The PUT composite presented higher degradation rates than the pristine TiO\({}_{2}\). This has previously been seen in UiO-66/TiO\({}_{2}\)[24, 60] composites, as well as in clay/TiO\({}_{2}\) or TiO\({}_{2}\)-based composites containing other natural porous minerals [37, 38, 39, 61]. The improved photoactivity in TiO\({}_{2}\) composites has been linked with synergistic effects related to the increase in charge availability [11, 25, 45], as well as the deposition of TiO\({}_{2}\) nanoparticles on the support, which decreases agglomeration phenomena [37, 39]. Both factors are important for photocatalytic reactions in the liquid phase [12]. An additional contributor to the improved photoactivity was the increase in the contact between TiO\({}_{2}\) and the substrate [17, 53]. This could be achieved through the increase in MO concentration close to TiO\({}_{2}\) due to the high adsorption efficiency of UiO-66.