Reactivity of U and co-occurring metals from mine deposits in a wetland under oxidizing conditions

Between 1948 and 2001, the extraction of Uranium (U) ores in France have produced a large quantity of waste (tailings, waste rock, etc.), still containing U-rich minerals and other trace elements (TE), that were associated to the initial mineralization. Some of those TE can be relatively mobile and redistributed with U in the various reservoirs of the critical zone, contaminating environmental compartments such as soils and sediments.

Wetlands correspond to areas where the physical and chemical conditions may greatly vary according to the variations of the water table. When these areas are contaminated by mining inputs, the presence of particulate organic matter (POM) in large quantity may influence the mobility of U and many various TE. Moreover, in recent years, the multiplicity of drought events can lead to the modification of the ability of such wetlands to sequestrate U and its co-occurring metals. The consequences of these events in terms of speciation and mobility of contaminants under oxidizing conditions need thus to be better understood.

For this purpose, wetland soils impacted by former U mining and milling activities in a site of the Massif Central (Rophin, France) were studied. Various analytical approaches (SEM, EPMA analyses and BCR chemical extractions) were deployed to determine the speciation of TE and understand their stability (leaching tests) under oxidizing conditions.

The pollution index Igeo, calculated along a soil core of 48 cm, highlights significant anthropogenic contributions in U, Pb and Cu the highest contamination levels in those elements being linked to a white layer, probably inherited from the former mining activities. For Cu and U, the solid speciation in this mining deposits is mainly governed by adsorption on surface particles and to a lesser extent, by precipitation of authigenic/inherited phases such as oxides for U (e.g. uranium dioxide - UO₂) and sulfides for Cu (e.g. chalcopyrite - CuFeS₂). Sorption processes onto the POM seems to mainly govern their speciation in ancient and recent POM-rich layers. Along the soil core Pb solid speciation seems mostly associated to stable phosphates e.g. plumbogummite (PbAl₃(PO₄)(PO₃OH)(OH)₆) inherited from the regional granite and U-Pb-rich mineralization. Again, mining deposits show differences with Pb adsorbed on surface particles and linked to other mineral phases such as sulphates e.g. anglesite (PbSO₄) and hokutolite ((Ba,Pb)SO₄). Additionally, refractory granite minerals contain U and Pb in variable quantities all along the soil core, (e.g. titanium oxides, REE phosphates, zircons). Eventually, minute amounts of Pb and U phosphates containing traces of Cu and As, with a stoichiometry close to parsonsite (Pb₂(UO₂)(PO₄)₂ 2H₂O), the prevailing U mineralization of the Rophin deposit, are also identified. The various U and TE-phases identified in this wetland outline different behavior and mobility according to environmental parameters modifications. Finally, leaching tests and chemical extractions highlighted a certain mobility for some elements, such as Cu and U, that may be hazardous for the environment.