Sustainable rare earth capture using seashell carbonates: Mineralogical pathways and comparative uptake behaviour of...

This study investigates the use of waste seashells as a sustainable sorbent for rare earth elements. Grounded shells of oysters, mussels, and cockles interacted with solutions with high concentration of La, Nd, and Dy, at 25 to 205 °C, for up to 3 months. The reaction resulted in the crystallisation of rare earth carbonates, following the crystallisation pathway Calcium carbonate → Lanthanite → Kozoite → Hydroxylbastnäsite. Kozoite was the most common phase observed in our experimental conditions. The replacement of the calcium carbonate substrate by kozoite occurred via a pseudomorphic interface coupled dissolution-precipitation reaction, forming a crust at the surface of the grains. The crust is made of two layers, an internal one made initially of spindle-shaped kozoite crystals, and an exterior one made of prismatic kozoite. In the case of cockles and mussels, the crust becomes rapidly impermeable and stops the reaction, preventing >50 wt% of the calcium carbonate to be dissolved. In oyster shells, due to unique grain morphology and intercrystalline porosity, full replacement occurs, making it the most efficient sorbent out of the three tested shells with a sorption efficiency for the tested experimental conditions of up to 66.6 mol% and an uptake of up to 10 mmol_REE g^-1_{oyster shell}. Partitioning of REE was observed in kozoite, with preferential uptake of Nd over La and Dy, especially in the early stage of crystallisation, and minor preferential uptake of La over Dy throughout the reaction. Our results point towards the potential of oyster shells to be used as sustainable and environmentally friendly sorbents to remove rare earth elements from high concentration aqueous solutions, such as industrial spills or in the final stages of REE processing.