Geochemical controls on uranium mobilization from open-pit wall rocks under environmentally relevant conditions: elemental and mineralogical constraints

Uranium (U) is a chemically toxic contaminant, and elevated concentrations in groundwater pose environmental and public health concerns in mining-impacted regions. U mobilization from mine-affected bedrock can contribute to groundwater contamination, yet the geochemical controls governing U release and transport under environmentally relevant conditions remain incompletely understood. This study investigates the roles of pH, complexing ligands, and mineralogy on U mobilization from pegmatite and trachyandesite rocks collected from Leveäniemi open pit, an active iron ore mine in Northern Sweden. Elevated U concentrations have been detected in groundwater entering the open pit through rock fractures. A systematic experimental approach combining batch leaching and dynamic flow through experiments was applied across a range of geochemical conditions, including varying acid concentrations, pH (acidic to alkaline), and ligand concentrations representative of groundwater (SO₄²⁻, NO₃⁻, Cl⁻, and HCO₃⁻) at environmentally relevant concentrations. The results demonstrate that U mineral dissolution and U mobilization are governed by distinct geochemical controls. Significant dissolution occurs under acidic and strongly alkaline conditions, with uraninite identified as the primary reactive U-bearing mineral, whereas other U-bearing minerals are comparatively resistant. Under neutral pH conditions representative of groundwater, U release is limited and controlled by surface-mediated processes rather than bulk mineral dissolution. However, the carbonate ligand promotes the formation of stable uranyl-carbonate and Ca-uranyl-carbonate complexes, enhancing U solubility and transport. These findings show that even limited mineral reactivity can sustain dissolved U concentrations over time and that groundwater composition plays a critical role in controlling U mobility. This is environmentally significant because such conditions are typical of groundwater systems, indicating that U can remain mobile and contribute to long-term contamination of water resources and downstream aquatic ecosystems. This study distinguishes the geochemical conditions that dissolve U-bearing minerals from those that sustain dissolved U transport under environmentally relevant groundwater conditions, providing new insight into mechanisms governing U transport in mine-impacted groundwater and supporting improved prediction, monitoring, and mitigation of U contamination of downstream recipients.