Influence of Oxic/Anoxic Conditions on the Mobility of Pb, Cu, and Fe from Acid Mine Drainage-Affected Sediments.

We assessed the effect of redox conditions on the mobility of lead (Pb), copper (Cu), and iron (Fe) from sediments affected by acid mine drainage (AMD). This was accomplished by integrating laboratory microcosm experiments, aqueous chemistry, diffraction, and electron microscopy. Microcosm experiments underwent 3 consecutive 5 day redox phases: oxic-anoxic-oxic. The sediments contained Fe (51,000 mg kg-1), Pb (307 mg kg-1), and Cu (30 mg kg-1), and minerals such as Illite, albite, and goethite. Microscopy analyses revealed that Pb and Cu are associated with Al-silicates and jarosite. Iron release peaked under anoxic conditions (~250 mg L-1), then decreased in the second oxic phase (<70 mg L-1). Extraction experiments confirmed that Pb and Cu are water-labile at pH 3.4 (Pb: 27 μg L-1-exceeding the United States Environmental Protection Agency drinking water action level of 15 μg L-1, Cu: 75 μg L-1), but less labile at pH 6.4 (Pb: 7 μg L-1, Cu: 3 μg L-1). DNA sequencing detected metal-tolerant fungal genera (Trichoderma, Fusarium, Penicillium, and Aspergillus) in the sediments. This study provides insights into the biogeochemical processes influencing the lability of metals in AMD-affected sites, which have relevant implications for risk assessment, remediation strategies, and recovery of critical minerals.