Advancements in functional adsorbents for sustainable recovery of rare earth elements from wastewater: A comprehensive review of performance, mechanisms, and applications.

Rare earth elements (REEs) are crucial metallic resources that play an essential role in national economies and industrial production. The reclaimation of REEs from wastewater stands as a significant supplementary strategy to bolster the REEs supply. Adsorption techniques are widely recognized as environmentally friendly and sustainable methods for the separation of REEs from wastewater. Despite the growing interest in adsorption-based REEs separation, comprehensive reviews of both traditional and novel adsorbents toward REEs recovery remain limited. This review aims to provide a thorough analysis of various adsorbents for the recovery of REEs. The types of adsorbents examined include activated carbons, functionalized silica nanoparticles, and microbial synthetic adsorbents, with a detailed evaluation of their adsorption capacities, selectivity, and regeneration potential. This study focuses on the mechanisms of REEs adsorption, including electrostatic interactions, ion exchange, surface complexation, and surface precipitation, highlighting how surface modifications can enhance REEs recovery efficiency. Future efforts in designing high-performance adsorbents should prioritize the optimization of the density of functional groups to enhance both selectivity and adsorption capacity, while also maintaining a balance between overall capacity, cost, and reusability. The incorporation of covalently bonded functional groups onto mechanically robust adsorbents can significantly strengthen chemical interactions with REEs and improve the structural stability of the adsorbents during reuse. Additionally, the development of materials with high specific surface areas and well-defined porous structures is benifitial to facilitating mass transfer of REEs and maximizing adsorption efficiency. Ultimately, the advancement of the design of efficient, highly selective and recyclable adsorbents is critical for addressing the growing demand for REEs across diverse industrial applications.