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-NH 2 ) are prepared, and their adsorption capacities for Ni 2+ , Fe 3+ and Cr 6+ are systematically investigated and compared. Results suggest that, due to a higher specific surface area of 1569.2 m 2 /g and a larger pore volume of 0.61 cm 3 g −1 , the UiO-66 depicts a better adsorption performance for Ni 2+ and Fe 3+ 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 m 2 /g, which is a little lower than UiO-66 and UiO-66-NH 2 , but shows the maximum adsorption capacity of 39.0 mg g −1 for Cr 6+ , obviously surpassing two other counterparts. In addition, for a multi-ion aqueous solution, UiO-66-(OH) 2 still has an excellent Cr 6+ /Ni 2+ adsorption selectivity of 21.7. The adsorption isotherm of Cr 6+ belongs to monomolecular layer Langmuir model, further verifying –OH 2 + of the UiO-66-(OH) 2 surface acted as the chemical adsorption sites and attracted electronegative Cr 6+ -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.