Tuning surface functionalizations of UiO-66 towards high adsorption capacity and selectivity eliminations for heavy metal ions

Zr-based metal-organic frameworks (MOFs), especially the UiO-66 family, present a huge application potential for the sorptive eliminations of heavy metal ions, still most of the studies often focus on only one type of adsorbent, thus lacking in some in-depth comparisons on the microstructure produced by the organic ligands. Herein, three kinds of Zr-based MOFs (UiO-66, UiO-66-(OH)2, and UiO-66-NH2) are prepared, and their adsorption capacities for Ni2+, Fe3+ and Cr6+ are systematically investigated and compared. Results suggest that, due to a higher specific surface area of 1569.2 m2 g-1 and a larger pore volume of 0.61 cm3 g-1, the UiO-66 depicts a better adsorption performance for Ni2+ and Fe3+ with larger capacities of 6.8 and 14.1 mg g-1, respectively. The UiO-66-(OH)2 presents a specific surface area of 785.5 m2 g-1, which is a little lower than UiO-66 and UiO-66-NH2, but shows the maximum adsorption capacity of 39.0 mg g-1 for Cr6+, obviously surpassing two other counterparts. In addition, for a multi-ion aqueous solution, UiO-66-(OH)2 still has an excellent Cr6+/Ni2+adsorption selectivity of 21.7. The adsorption isotherm of Cr6+ belongs to monomolecular layer Langmuir model, further verifying -OH2+ of the UiO-66-(OH)2 surface acted as the chemical adsorption sites and attracted electronegative Cr6+-containing ions. This study dramatically highlights the importance of the organic ligands for MOFs sorbents, which may facilitate achieving their large-scale applications in wastewater treatment.