Construction Of Defective Ion-imprinted Zr-MOFs And Their Efficient And Selective Adsorption On Rare Earth Gd(Ⅲ)

Rare earth elements are highly valued as a strategic resource,referred to as the“Modern Industrial Cellulose”and the“Modern Industrial Gold”.Due to their distinctive electrical,optical,and magnetic properties,rare earth elements serve as crucial raw materials in the nuclear,pharmaceutical,and life science industries.However,rare-earth smelting wastewater,which represents an essential source of rare-earth resources,has garnered considerable attention from researchers due to its intricate composition,potential environmental toxicity,and high purity requirements of modern industry.Gadolinium（Gd）is the element with the largest number of unpaired electrons and the largest magnetic moment,making it an ideal choice for applications such as a neutron absorber and other high-tech materials.In recent years,the increasing demand for high-purity Gd has emphasized the importance of selectively separating Gd（Ⅲ）from rare earth smelting wastewater.Thus,the development of effective adsorbents for the selective recovery of Gd（Ⅲ）from wastewater is of significant environmental and economic significance.Metal organic frameworks（MOFs）represent a promising new class of adsorbents,featuring notable advantages such as high chemical stability,large specific surface area,controllable structure,and easy modification.However,the limited active area and poor selectivity of most MOFs materials currently hinder their effective use in Gd（Ⅲ）recovery.To address this issue,we proposed a novel strategy that combines surface ion imprinting and hybrid ligand construction defects to synthesize Gd（Ⅲ）ion imprinted MOFs（G-IIMs）based on phosphoric acid modified defective Ui O-66-NH₂ nanoparticles（PAMDUNs）,and to apply them to the selective adsorption of Gd（Ⅲ）in aqueous solutions.The successful synthesis of G-IIMs was confirmed through a range of characterizations,including XRD,FTIR,N₂ adsorption and desorption experiments,TGA,and SEM,which revealed the morphology,chemical composition,crystal structure,specific surface area,and pore structure of the adsorbents.In this study,2-Hydroxyphosphonoacetic acid（HPAA）has been used not only as a monodentate ligand to induce ligand vacancies in situ on Ui O-66-NH₂ and form hierarchical pores,but also as a functional monomer in the imprinted layer with a strong coordination ability to Gd（Ⅲ）.Our results indicate that UN-2-HPA-IIM3,the resulting material,exhibits a high adsorption capacity of 174.4 mg g^-1 at 298 K and p H=5.5 for Gd（Ⅲ）adsorption,as well as a short equilibration time of 30 min and a relative selectivity factor k_r>7.Furthermore,the material displayed good reusability,with an adsorption capacity of 156.5mg/g for Gd（Ⅲ）after the 5th cycle.The adsorption process followed the pseudo-second-order kinetic model and the Langmuir isotherm model,indicating that the adsorption of UN-2-HPA-IIM3 for Gd（Ⅲ）process was found to be monolayer adsorption,driven by chemical interactions between the adsorbent and the metal ion.The adsorption process was heat-absorbing and spontaneous.The adsorption mechanism of UN-2-HPA-IIM3 was investigated using FTIR and XPS before and after Gd（Ⅲ）adsorption,revealing that the material interact with Gd（Ⅲ）in wastewater by complexation.The adsorbent exhibited excellent Gd（Ⅲ）adsorption performance and selectivity.It was attributed to the hierarchical pores,and abundant binding sites and Gd（Ⅲ）imprinted cavity resulting from the combination of surface ion imprinting and mixed ligand construction defect strategies in the structure of UN-2-HPA-IIM3.These findings not only demonstrate the efficacy of UN-2-HPA-IIM3 in treating Gd-containing wastewater but also offer guidance for the development of novel MOFs materials.