Porous materials for the recovery of rare earth elements, platinum group metals, and other valuable metals: a review

The demand for valuable metals such as rare earth elements and platinum group metals is rising fast in the context of the depletion of natural resources and international conflicts. Moreover, the future circular economy requires that raw material be recycled from waste by advanced methods such as adsorption by innovative porous materials. Here, we review the recovery of metals using porous materials with focus on adsorbent properties, factors governing the performance, and adsorption mechanisms. Porous materials include carbon-based, oxygen-containing, organic polymer-based, nanoparticle-based, ionic liquid-based, and composite material-based adsorbents. Both soft and hard templating methods yield mesoporous porous materials, yet enhanced metal recovery is achieved by cross-linking and metal-doping to improve electrostatic interaction and complexation. Compared to other porous materials, metal–organic and covalent organic frameworks are effective for metal recovery under a wide range of operating conditions, e.g., pH, but the pollution of effluents should be prevented. The major adsorption mechanisms are understood, but mechanisms of spatial nanoconfinement are poorly known.