Research advance of low concentration rare earth ions adsorption materials

In recent years, with the constantly increasing demand and production of rare earths, the efficient and green extraction and separation of rare earth resources have put forward higher requirements. In particular, the recovery of rare earth elements (REEs) from low concentration rare earth solutions such as rare earth minerals processing wastewater, refining wastewater, seawater and hot springs has become a research hotspot. Compared with traditional extraction technologies including chemical precipitation and solvent extraction, adsorption is a potential low concentration rare earth recovery method due to its advantages of simple operation, low cost, large treatment capacity and strong adaptability. This paper summarizes in detail the research progress on rare earth ion adsorption materials in recent years, and introduces the design ideas, microscopic morphology (specific surface area, pore size and particle geometric dimension, etc.), adsorption behaviors (adsorption kinetics, adsorption isotherm, etc.), material performances (the maximum adsorption capacity, adsorption-desorption cycles, etc.), and potential application potentials (pH, dosage, coexisting competing ions etc.) of mineral-based, carbon-based, metal-organic framework-based, and polymer-based adsorbents. Mineral-based adsorbent materials are characterized by clay minerals of the layered silicate type, carbon-based adsorbent materials include biochar, graphene, carbon nanotubes, etc., metal-organic frameworks materials include the series of ZIF, UIO, MIL, and HKUST, etc., and polymer-based materials include natural polymers, hydrogels, and artificial polymers. Single material adsorbents usually have the disadvantages of low adsorption capacity, poor selectivity, weak mechanical strength, and unstable under acidic conditions. Preparation of composite materials that fully utilize the advantages of single materials is the main method to overcome the above disadvantages. For example, the use of polymer hydrogels loaded with fine-grained mineral materials to avoid agglomeration, the in-situ growth of metal-organic frameworks on the surface of graphene oxide to enhance its stability under acidic conditions, the use of polymers with specific functional groups containing O, N, and P to modify porous materials to enhance adsorption capacity and the magnetization modification of carbon-based materials and polymers to facilitate the subsequent recycling. Finally, the future development trend of rare earth adsorbents is proposed: 1) development of green adsorbent materials including green raw materials and no new pollution in the using process, 2) development of highly selective adsorbent materials that can extract REEs from competing impurity ions as well as realize the separation of a single rare earth among the REEs, 3) development of high-efficient adsorbent materials including extraction of REEs from low concentration rare earth solutions and fast adsorption kinetics, and their applications in the field of REEs wastewater treatment.