RADIUM MINERAL ASSOCIATIONS WITHIN ABANDONED MINE DRAINAGE RELEVANT TO THE FUTURE OF CRITICAL MINERAL EXTRACTION

While coal mine drainage (CMD) poses a regional water-quality issue across Appalachia, these fluids also tend to have elevated concentrations of rare earth elements (REEs) and other critical minerals (elements), such as cobalt (Co) and nickel (Ni). These elements have been documented to co-precipitate with Fe, Mn, and Al oxyhydroxides within CMD treatment systems. In some cases, REE concentrations in solid phases are upwards of 2,500 mg/kg. Radium (Ra), a known carcinogen, has also been documented to sorb to Fe- and Mn- rich river sediments in a stream impacted by both upstream CMD and treated oil and gas wastewater in Pennsylvania. Despite potentially serious human and environmental impacts from Ra contamination, a baseline assessment of Ra in CMD and associated solids has never been conducted. Such a study is necessary in conjunction with evaluating the potential for extracting REEs from CMD treatment facilities. To test the hypothesis that Ra may accumulate with REE and other critical elements in CMD solids, we collected water and sediment samples from 4 bituminous and 9 anthracite CMD sites in Pennsylvania, representing gradients of pH, metals, and SO₄ concentrations. Our data suggest Ra concentrations were relatively low in CMD water and corresponding solid samples (<0.5 Bq/L and upwards of 850 Bq/kg, respectively) and positively correlated with increasing Mn content in solids (R² = 0.90). Sequential leaching of solids, largely composed of amorphous Fe, Al, and Mn oxyhydroxide and silicate minerals, indicate that Ra is largely retained in recalcitrant minerals less susceptible to dissolution. REEs are also largely retained in the solids throughout the leaching process. While retention of Ra throughout sequential leaching has environmental and human health benefits related to reduced mobility, associations with REEs in solids may lead to concentration of Ra in effluents or solids if REE extraction and subsequent solids manipulation is economically viable. These results provide an understanding of overall Ra activities and fate and transport behavior in CMD-impacted environs and are discussed with perspective toward guiding targeted REE and critical mineral extraction activities.