Adsorption behavior of zirconium metal-organic frameworks in multicomponent metal-ion solutions

In this study, the adsorptive behavior of zirconium metal-organic frameworks (Zr-MOFs) was investigated for the adsorption of alkali and alkaline earth metal ions from multi-element aqueous solutions. Zr-MOFs exhibit high water stability, notable surface area, and a significant pore volume, all of which contribute to their enhanced adsorption capacity. Three Zr-MOFs, namely bare UiO-66, amine-functionalized NH2-UiO-66, and carboxyl-functionalized UiO-66-(COOH)(2), were tested for the adsorption of alkali and alkaline earth metal ions. Among the 3 MOFs that were tested, only the carboxyl group-functionalized Zr-MOF showed significant adsorption capacity toward divalent metal ions. Further, a thorough investigation was conducted to understand how the pH, initial concentration of the solution, and Zr-MOF dosage impact the adsorption properties of UiO-66-(COOH)(2). At the natural pH (6.5) of the solution, UiO-66-(COOH)(2) exhibited a superior adsorption capacity toward Sr2+ (15.3 mg g(-1)) and Ca2+ (7.9 mg g(-1)), which was attributable to the stronger electrostatic attraction of these ions relative to monovalent ions. The kinetic study results indicated that the preferred mechanism of adsorption was chemisorption for divalent metal ions. Additionally, the adsorption behavior of UiO-66-(COOH)(2) for 24 elements was evaluated and the MOF showed significant adsorption of Sr2+ and Ca2+ alongside other divalent and trivalent metal ions. The experimental findings of the present study suggest that carboxylic-functionalized Zr-MOF holds significant potential toward the preparation of suitable sorbents for the extraction of higher-valence metal ions.