# Adsorption of Methyl Orange on a Novel
## 3 Results and Discussion
### Dye Adsorption Evaluation  
The effect of contact time on the adsorption over the pure UiO-66 and Pal materials as well as the Pal/UiO-66 composite was first evaluated. The data are given in Figure 5a. Pure Pal had low adsorption efficiency over MO. This was expected because under the conditions of the adsorption experiments, both Pal and MO were negatively charged [15; 23; 50], resulting in an electrostatic repulsion force generated between the adsorbent and MO. On the contrary, both pure UiO-66 and Pal/UiO-66 presented significant adsorption efficiency against MO removal (Figure 5a). Considering that the adsorption efficiency of Pal was very low compared with that of pure UiO-66, the Pal content in the composite was kept low during the synthesis process. Taking into account the adsorption kinetics using UiO-66 and Pal/UiO-66 in Figure 5a, specifically the fast adsorption process that was completed within 5 min and the high removal efficiencies, both UiO-66 and Pal/UiO-66 could effectively and quickly remove MO from water. We also highlight that the high sorption rates that led to the almost immediate achievement of equilibrium using the materials containing UiO-66 in this study (i.e., -5 min) are not usual for this specific MOF family, which is used as adsorbents for similar substrates [35; 39]. The adsorption kinetic curves for pure UiO-66 and Pal/UiO-66 composite as a function of time, fitted using a pseudo-first-order and pseudo-second-order kinetic model, are shown in Figure 5b,c, respectively. As evidenced, the pseudo-second-order kinetic model better described the adsorption data for both materials. The correlation coefficient \(R^{2}\) for the pseudo-second-order kinetic model (\(R^{2}>0.999\)) was acceptable for both materials. In addition, the calculated adsorption at equilibrium (q\({}_{\text{e,cal}}\)) using the pseudo-second-order kinetic model matched the experimentally obtained values (q\({}_{\text{e,exp}}\)) for both materials. The correlation coefficient \(R^{2}\) for the pseudo-first-order model was not sufficient, and the model did not reproduce the experimental q\({}_{\text{e,exp}}\). Figure 4: N\({}_{2}\) adsorption–desorption isotherms for pure UiO-66 and Pal/UiO-66 composite. The adsorption isotherms of MO over UiO-66 and Pal/ UiO-66 are given in Figure 6a. The Pal/ UiO-66 composite presented superior adsorption capacity compared with pure UiO-66. The removal efficiency of Pal/ UiO-66 against MO was 94.7%, while pure UiO-66 removed only 86.7% at an initial MO concentration 90 mg L\({}^{-1}\). A similar increase in the removal efficiency was also observed at higher initial MO concentrations, i.e., 67.8% and 57.8% for Pal/ UiO-66 and pure UiO-66, respectively, using 180 mg L\({}^{-1}\) initial MO concentration. An impressive maximum equilibrium adsorption capacity (qmax,exp) was obtained for the composite material (ca. 340 mg g\({}^{-1}\)), which is among the highest reported in the literature using UiO-66-based materials [35,39,51,52]. This was more important considering that the MOF structure accounted for only 90 wt.% of the composite material. The maximum equilibrium adsorption capacity was further increased to 378 mg g\({}^{-1}\) considering only the mass of the UiO-66 in the composite. This corresponded to a 34.5% increase in qmax compared with that of pure UiO-66. For further insights regarding the adsorption behavior, the Langmuir model was used to simulate the isotherm data (Figure 6b,c) [35,39]. As evidenced by the \(R^{2}\) values (0.9983 and 0.9991 for pure UiO-66 and composite material, respectively), the Langmuir model described the isotherm data well. In addition, the calculated maximum equilibrium adsorption capacity (qmax,cal = 285.7 and 344.8 mg g\({}^{-1}\) for UiO-66 and composite material, respectively) matched the experimental values in both cases (qmax,exp = 281 and 340 mg g\({}^{-1}\) for UiO-66 and Pal/ UiO-66, respectively). The isotherms parameters are also given in Figure 6b,c. Stability is a critical parameter and a prerequisite for practical applications.