IDENTIFYING CRITICAL MINERAL BINDING MECHANISMS AND DISTRIBUTION IN ACID MINE DRAINAGE TREATMENT SOLIDS TO INFORM TARGETED RECOVERY METHODS

Acid mine drainage (AMD) is a long-standing environmental problem that impacts large portions of Appalachia and requires treatment to ensure the health of waterways throughout the region. Passive (limestone) remediation beds in Appalachia produce stable, metal-rich waste sludge (18,000 tons/year), with minimal engineering. These AMD solids are comprised of aluminum (Al)/manganese (Mn)/iron (Fe) (hydr)oxides and are high in critical minerals (up to 2,000 ppm rare earth elements (REE), 440 ppm lithium, and up to 1.2 wt.% cobalt (Co), nickel (Ni), zinc). The National Energy Technology Laboratory has led efforts in exploring critical minerals within unconventional feedstocks, such as AMD solids, to aid in the nation’s clean energy transition. In this study, micro-X-ray Fluorescence and micro/bulk X-ray Adsorption Near Edge Spectroscopy were collected at Stanford Synchrotron Radiation Lightsource and the Advanced Photon Source to: (1) gain a detailed characterization of critical minerals in Fe/Mn (hydr)oxide host phases; and (2) investigate the Co/Ni coordination and redox speciation associated with Fe/Mn (hydr)oxides phases in AMD solids with diverse composition (e.g., Al-rich, Mn-rich, Al/Mn/Fe-rich). Preliminary results show co-location of Co/Ni within Mn-rich hotspots, while Fe-rich hotspots were associated with heavy REEs. In Al-rich solids, Mn speciation was mostly comprised of Mn+3/+4 oxides, while the Mn-rich solids contain predominately Mn+4. These results suggest the AMD matrix plays an important role in how Co/Ni are coordinated, which aids in evaluating the efficacy to utilize AMD solids as a resource, and informs other novel sorbent and recovery technologies.