Structural insights into amino-modified defective UiO-67: Enhancing phosphate adsorption through multiple active sites synergistic interactions

Abstract

Zirconium-based Metal-Organic Frameworks (Zr-MOFs) have attracted significant attention for phosphate removal due to their large specific surface area and abundant active sites. In this study, amino-modified Zr-MOFs were synthesized and used for phosphate removal, with a focus on investigating the relationship between the physicochemical structure of the amino-modified Zr-MOFs and phosphate adsorption, as well as the role of the amino groups. A series of amino-modified Zr-MOFs (UiO-67) were synthesized and characterized, with particular emphasis on the variant with one amino group functionalization on a defect-rich framework, designated as DUN. The results reveal that amino modification of defective UiO-67 (DUN) increases the number of phosphate adsorption sites per unit mass and specific surface area, thereby enhancing phosphate adsorption capacity, which is significantly higher than that of other synthesized UiO-67 variants. The role of amino group not only enhances the affinity for phosphate and the adsorption efficiency at low phosphate concentrations but also accelerates the phosphate adsorption rate. Through comprehensive characterization and DFT simulations, the superior performance of DUN is primarily attributed to the synergistic effects of its structural defects, carboxyl groups, and introduced amino functionalities. The defects at Zr nodes in DUN contribute significantly to phosphate adsorption by providing Zr-OH groups and oxygen vacancies. Concurrently, the carboxyl groups present in the DUN's ligand linkers engage in hydrogen bonding interactions with phosphate. The amino groups, a novel addition to the DUN structure, not only provide additional adsorption sites but also significantly enhance the strength of the existing hydrogen bonds and the Zr-O-P coordination interactions. This study sheds light on the intricate relationship between the physicochemical structure of Zr-MOFs and their phosphate adsorption capacity, providing a theoretical basis and practical approach for developing efficient Zr-MOF-based adsorbents for water purification.