EMPIRICAL OBSERVATIONS AND GEOCHEMICAL MODELING TO EVALUATE TREATMENT STRATEGIES FOR RECOVERY OF RARE-EARTH ELEMENTS FROM ACID MINE DRAINAGE (Invited Presentation)

The lanthanide rare-earth elements, yttrium, and scandium (REYs) generally occur at elevated concentrations in acid mine drainage (AMD) with low pH and elevated concentrations of Fe, Al, and SO₄. This study explains why concentrations of REYs and associated trace metals in AMD generally decrease as pH increases, particularly at pH > 5, and employs geochemical models to demonstrate the importance of key variables. Laboratory and field studies demonstrate that REYs are attenuated with metal-rich solids precipitated during neutralization of AMD, and that attenuation is enhanced by SO₄. Geochemical modeling indicates that observations can be explained by adsorption of REYs to hydrous ferric oxide (HFO), hydrous aluminum oxide (HAO), and/or hydrous manganese oxide (HMO). New, empirically derived adsorption reactions and associated equilibrium constants for binding of REYs cations (M⁺³) by HFO, HAO, and HMO plus sulfate complexes (MSO₄⁺) by HFO and HAO account for effects of SO₄. The PHREEQ-N-AMDTreat+REYs water-quality modeling tools (https://doi.org/10.5066/P9M5QVK0), which incorporate these and other speciation reactions, accurately simulate the precipitation of Fe, Al, and Mn solids during sequential AMD treatment steps, plus the precipitation and adsorption of REYs. Each model has a user interface to facilitate the input of water-quality data and adjustment to geochemical or treatment system variables. On-screen graphs display predicted changes in REYs and associated solute concentrations as functions of pH or retention time; details are summarized in output tables. A goal of such modeling is to identify strategies that could produce a concentrated REYs extract from AMD or mine waste leachate. For example, modeling supports the hypothesis that Fe can be removed at pH < 4 using conventional sequential oxidation and neutralization treatment processes without removing REYs and that further increasing pH can promote the adsorption of REYs by HFO, HAO, or HMO that forms thereafter. Alternatively, chemicals such as phosphate or oxalate may be added to precipitate REYs compounds following steps to decrease non-target metal concentrations. The PHREEQ-N-AMDTreat+REYs models will be incorporated as new tools in the AMDTreat 6.0 cost-analysis model by the Office of Surface Mining Reclamation and Enforcement.