The evaluation of critical rare earth element (REE) enriched treatment solids from coal mine drainage passive treatment systems

Rare earth elements (REE; lanthanides and Y) are essential for the development of clean energy technologies. Global demand for REE is expected to increase sharply in the coming decades, especially for certain energy-critical REE (e.g., Y and Nd), spurring investigations into novel sources of REEs. Polluted mine drainage from eastern U.S. coalfields contains elevated concentrations of dissolved Fe, Al, and Mn and is also enriched in REE by up to three orders of magnitude over unpolluted groundwater. Mine drainage remediation systems, designed to precipitate dissolved metals, can sequester > 90% of dissolved REE into the precipitated solids (treatment solids). These solids, landfilled at cost to treatment system operators, are a promising REE source. Passive treatment systems with diverse geochemical environments were sampled to determine REE mobility in these systems and REE concentrations in treatment solids. Passive treatment systems were found to produce middle and heavy REE-enriched solids, relative to the North American Shale Composite, with up to 1950 ppm REE and 55% energy-critical REE. SEM-EDS and synchrotron mu-XRF analysis demonstrate the association of REE with Mn-oxide coatings on limestone from a passive treatment system. Calculated concentration factors indicate that passive treatment systems using natural processes (e.g., drainable limestone beds and vertical flow ponds) concentrate REE into treatment solids approximately three times more effectively compared to active treatment systems using caustic chemical additions (e.g., lime). This study suggests that passive treatment systems effectively concentrate REE into treatment solids and these solids could be an alternative and more environmentally friendly source of REE compared to conventional mining on land or the seafloor. Results of this study can also aid in the design of treatment systems engineered to remove and concentrate critical REE which could provide a financial incentive to treat polluted mine water.