pH-Responsive and Selective Adsorption of Divalent Metal Ions by Metal–Organic Frameworks with Subnanometer Pore Windows and Carboxylic Groups

Metal–organic frameworks (MOFs) are promising materials for ion adsorption due to their high surface area, structural tunability, and functional group versatility. While many studies have aimed to improve MOF performance, this work takes a comprehensive approach by investigating the pH-responsive adsorption behavior of MOFs with subnanometer pores, narrower than the hydrated diameters of common divalent metal ions. We synthesized two carboxyl-functionalized MOFs, UiO-66-COOH and UiO-66-(COOH)2, featuring 6 Å pore windows and accessible −COOH groups, and evaluated their ability to adsorb Mg2+, Ca2+, Zn2+, and Cu2+. Under single-ion conditions, both MOFs show the highest adsorption capacity for Ca2+, attributed to its smaller hydrated radius and favorable coordination with −COO– groups. In contrast, UiO-66-(COOH)2 exhibits the highest Cu2+ uptake in multi-ion systems due to its stronger binding affinity and competitive advantage, indicating selective adsorption. Furthermore, the MOFs demonstrate pH-responsive adsorption with lower uptake at acidic pH conditions and enhanced adsorption under alkaline conditions. This reversible behavior enables efficient regeneration and reusability, highlighting their potential for practical ion separation in water treatment applications.