Utilization of Eggshell Waste Calcite as a Sorbent for Rare Earth Element Recovery

The green energy transition requires rare earth elements (REE) for the permanent magnets used in electric cars and wind turbines. REE extraction and beneficiation are chemically intensive and highly damaging to the environment. We investigated the use of eggshell waste as a sustainable alternative sorbent for the capture and separation of REE from aqueous solutions. Hen eggshell calcite was placed in multi-REE (La, Nd, Dy) solutions at 25 to 205 °C for up to 3 months. A pervasive diffusion of the REE inside the eggshell calcite was observed along pathways formed by the intracrystalline organic matrix and calcite crystal boundaries. At 90 °C, kozoite (REECO3OH, orthorhombic) spherulites precipitate on the surface of the dissolving calcite. At 165 and 205 °C, an interface-coupled dissolution–precipitation mechanism is observed, resulting in the complete dissolution of the calcite shell and its pseudomorphic replacement by polycrystalline kozoite. At 205 °C, kozoite is slowly replaced by hydroxylbastnäsite (REECO3OH, hexagonal), the stable form of the rare earth hydroxycarbonate polymorphs. Our results demonstrate two potential applications of eggshell waste for the uptake of rare earth elements in solution: at low temperatures, as a mixed organic–inorganic adsorbent and absorbent, given sufficient sorption time; and at higher temperatures, as an efficient sacrificial template for the precipitation of rare earth hydroxycarbonates.