A Comparative Study on the Stability and Adsorption Capacity of Cerium- and Zirconium-Based UiO-66-NH2 MOFs

In the current research, two different metal-organic frameworks (MOFs), zirconium (Zr)- and cerium (Ce)-based UiO-66-NH2, were synthesized to compare their adsorption capacities for the removal of the pharmaceutical pollutant ofloxacin (OFL). The stability of the adsorbents after adsorption and after adsorption-desorption cycles was examined by conducting XRD, FTIR, FESEM, and BET analyses. The XRD and BET analyses showed that Zr-MOF possessed higher crystallinity and a larger surface area compared to Ce-MOF. However, Ce-MOF exhibited better adsorption capacity than Zr-MOF, which could be attributed to surface defects resulting from the partial reduction of Ce(IV) to Ce(III) during the synthesis process, as well as electrostatic attraction under the study conditions. The experimental results demonstrated that OFL adsorption on both adsorbents followed the pseudo-second-order kinetic model. The Langmuir isotherm model accurately corresponded to the experimental adsorption isotherm of both adsorbents, exhibiting the maximum adsorption capacity of 114.94 and 136.99 mg/g for Zr-UiO-66-NH2 and Ce-UiO-66-NH2, respectively. Optimal adsorption occurred with a contact time of 240 min, an adsorbent dosage of 0.25 g/L, and a temperature of 298 K. Thermodynamic analysis confirmed that the adsorption process of both nanoparticles is spontaneous, and Zr-UiO-66-NH2 MOFs exhibited endothermic behavior, while Ce-UiO-66-NH2 MOFs showed exothermic nature. Additionally, the reusability test demonstrated that both adsorbents still have excellent adsorption performance after four cycles.