Multifunctional base-modified copper/zinc bimetallic organic framework enables highly efficient and selective separation of the rare earth element yttrium(III)

In this work, trihydroxyl-and phosphate-functionalized Cu/Zn bimetallic organic frameworks (CZOH-2 and CZP-2) were synthesized via a solvothermal method, and applied for the extraction of low-concentration and high-value rare earth yttrium ions (Y(Ⅲ)) from wastewater. Batch adsorption experiments demonstrated that both materials exhibited outstanding adsorption capacity, salt tolerance and adsorption selectivity at pH = 6. In the presence of coexisting interfering ions including La(Ⅲ), Ni(Ⅱ), Co(Ⅱ), Mg(Ⅱ) and Mn(Ⅱ), the distribution coefficients of Y(Ⅲ) on CZOH-2 and CZP-2 reached 10170.45 mL·g −1 and 17716.09 mL·g −1 , respectively. The efficiency for Y (Ⅲ) uptake stayed higher than 85 % after five cycles, revealing excellent reusability. Fitting results of adsorption isotherms and kinetic models indicated that the theoretical maximum capture efficiency of CZOH-2 and CZP-2 toward Y(Ⅲ) were 139.70 mg·g −1 and 361.30 mg·g −1 , respectively. The adsorption behavior of Y(Ⅲ) on the two materials conformed to the characteristics of monolayer chemisorption. Analysis based on the Array of Adsorption Occupied Sites and External Diffusion Resistance-Internal Diffusion Resistance model showed that the rate constants of chemisorption were remarkably higher than that of physisorption, confirming that chemisorption dominated the entire adsorption process. According to the Approximate Site Energy Distribution theory, the adsorption sites on material surfaces presented obvious energetic heterogeneity. Additionally, increasing temperature significantly enhanced the adsorption affinity of CZP-2 toward Y(Ⅲ). Combined with comprehensive characterizations and DFT calculations, the microscopic interaction mechanisms for Y(Ⅲ) capture by CZOH-2 and CZP-2 were systematically elucidated. The adsorption process was synergistically driven by electrostatic attraction, ion exchange and chelation. Specifically, the coordination interaction between Y(Ⅲ) and surface functional groups (–OH, −P-OH and P=O) played a key role in the selective capture of Y(Ⅲ). CZOH-2 and CZP-2 possess great application prospects for the efficient enrichment and resource recovery of yttrium from low-concentration rare earth-containing wastewater.