The atomic-scale structure of rare earth elements in deep-sea sediments facilitates their extractability

Deep-sea rare earth elements (REEs) have attracted global interest as terrestrial resources become less available and demand rises sharply. Seafloor REEs are hosted in authigenic carbonate fluorapatite (a-CFA), formed by the precipitation of calcium and phosphate, and in biogenic carbonate fluorapatite (b-CFA), derived from marine organisms’ skeletons. The ease with which REEs can be extracted from pelagic sediments via acid leaching challenges the common view that they are structurally incorporated into CFA, given fluorapatite’s low solubility. This apparent paradox was elucidated by investigating the nanostructure of samarium (Sm), used as a chemical REE probe. Using transmission electron microscopy and X-ray absorption spectroscopy, we found that the CFA crystals are nanosized and that Sm is not incorporated into the CFA lattice but instead primarily resides in an amorphous phase surrounding the a-CFA and b-CFA nanocrystals. At the atomic scale, Sm has a disordered apatitic bonding environment, and atomistic modeling suggests it clusters with other REEs. The facile extractability of REEs from pelagic sediments is attributed to the poor crystallinity of the host matrix at the atomic scale. This study demonstrates that understanding the location and atomic structure of critical elements enables quantitative prediction of their macroscopic properties. This knowledge can help improve recovery processes and inform the design of more efficient remediation strategies that protect the environment and public health. Rare earth elements accumulate at the fluorapatite-seawater interface in a poorly crystalline phase, making them both enriched in marine sediments and easy to extract by mild acid leaching, according to spectroscopy, geochemical and computational modeling analysis.