Sustainable recovery of rare earth elements with eggshell waste calcite

In November 2023, the European Union reached a provisional agreement on an European Critical Raw Materials Act, which encourages the local production, processing, and recycling of critical elements, notably the rare earth elements (REE). While indispensable for the green energy transition, their production is notoriously environmentally damaging, and efforts are being made to reduce this environmental footprint, for example via the use of secondary REE sources from waste or by the application of green chemistry and circular economy principles.

In this study, we investigated the potential of hen eggshell calcite waste to be recycled for the uptake of REE from industrial and waste streams. We interacted commercial eggshells with 50 mM multi-REE (La, Nd, Dy) solutions at 25 to 205 °C between three hours and three months. The resulting products were characterized by powder XRD and Rietveld refinement for quantitative phase identification; SEM electron imaging for structural characterization; SEM energy dispersive spectroscopy for elemental mapping and quantification of major and minor elements; and laser ablation ICP-MS for trace element mapping.

We observe a pervasive diffusion of the REE inside the eggshell calcite, along pathways formed by the intracrystalline organic matrix and calcite crystal boundaries, and without any partitioning of La, Nd and Dy. At 90 °C, calcite is observed dissolving and being replaced by kozoite spherulites, reminiscent of natural kozoite crystals. At 165 °C and 205 °C, an interface coupled dissolution-precipitation mechanism is observed, resulting in the complete dissolution of the calcite and its pseudomorphic replacement by polycrystalline kozoite. At 205 °C, kozoite itself is slowly replaced by hydroxylbastnäsite, the stable form of rare earth hydroxycarbonate, following a crystallization pathway previously established with inorganic calcite. Minor REE zoning at the eggshell grain scale is also observed, hinting at a potential use for REE separation.

Our results demonstrate two potential applications of eggshell waste for the sustainable recovery of REE from aqueous solutions: at low temperatures, as a mixed organic-inorganic adsorbent and absorbent; and at higher temperatures as an efficient sacrificial template for the precipitation of rare earth hydroxycarbonates.