Neodymium isotopes track sources of rare earth elements in acidic mine waters

Abstract Acid mine drainage (AMD) has been proposed as a potential source of strategic rare earth elements (REEs) due to its high concentrations (>1,000 µg/L) of dissolved REEs, as well as the middle rare earth element (MREE) enriched pattern exhibited by both coal mine- and metal mine-derived AMD, in which some of the most energy-critical REEs are significantly concentrated. However, the source of REEs and the origin of the MREE enrichment observed in AMD discharges is poorly understood; suggestions include dissolution of MREE-enriched surface coatings, fractionation by colloids, solid-liquid exchange reactions, incongruent dissolution of pyrite, and selective leaching of MREE-enriched phases. Filtration experiments on acidic to circumneutral AMD (unfiltered to