CE(III) AND CE(IV) DISTRIBUTION AND FRACTIONATION IN ACID MINE DRAINAGE TREATMENT SOLIDS: IMPLICATIONS FOR RARE EARTH ELEMENT RECOVERY

Demand for rare earth elements (REE) is increasing due to clean energy and modern technological needs. Based on a survey of 140 acid mine drainage (AMD) discharges in Pennsylvania (PA), REE in AMD are present in concentrations up to 1,765 ug/L with loadings up to 7,364 kg/yr per stream. In PA alone, about 85 billion gallons/yr of AMD is treated to neutralize acidity and remove metals, generating about 18,000 tons/yr of AMD treatment solid wastes. These solids, primarily composed of Fe, Al, and Mn hydroxides, can contain up to 2,000 mg/kg REEs making them a potential feedstock for REE recovery. However, REE distributions and binding mechanisms are not well understood, potentially impacting REE extraction efficiencies from these solids. In this study, cerium (Ce), a redox sensitive REE, was used to provide insight into Ce distribution and fractionation in four geochemically diverse AMD solids (Al-rich, Mn-rich, and Al,Fe,Mn-rich solids) to inform better REE recovery. Ce oxidation states were measured via micro X-ray Absorption Near Edge Spectroscopy (µ-XANES) and REE distributions via micro X-ray Fluorescence at Stanford Synchrotron Radiation Lightsource. Ce distribution and REE extractability were further evaluated via 7-step sequential extraction and 1-step HCl extraction. Ce hotspots with detectable µ-XANES in our samples are mainly observed to co-localize with Al and Mn matrices and other light REEs (e.g., Sm), whereas heavy REEs (e.g., Gd, Yb) are co-localizing with Fe hotspots. Up to 58% of the Ce(IV) dominate hotspots were found to be more associated with Mn-rich solids, while up to 42% of Ce(III) dominate hotspots were found in Al-rich solids. For mixed matrix solids (e.g., Al, Mn, Fe rich-solid) Ce(III) and Ce(IV) were more evenly distributed in approximately 50:50 mixtures found in Mn-Al-rich hotspots, while high Ce(IV) content (up to 83%) was mainly in Mn only hotspots. Additionally, initial laboratory leaching experiments revealed that Ce fractionation impacted Ce extractability (i.e., Ce(IV) less mobile then Ce(III)) and light REE extractabilities (e.g., La, Pr) from AMD solids. Hence, exploring Ce speciation and fractionation will inform better REE recovery and provide innovative insight for extracting light and heavy REE from these AMD solids.