Uranium speciation in two Freital mine tailing samples: EXAFS, µ-XRD, and µ-XRF results

Forty years of uranium mining in the German state of Saxony have left a legacy of uranium-contaminated pits, waste piles, mine tailings and surrounding soils. Since 1963, and more extensively since 1989, contaminated sites were covered in order to protect people and environment. Little is known on the further fate of uranium at these buried sites. Therefore, we investigated two mine tailing samples from hydrochloric-acid ore-extraction, which were buried for 30 years under several meters of mine and communal waste.
The two samples were collected at depths of 5 m (sample 1) and 12 m (sample 2) below the surface. Due to the neutralizing influence of the waste cover, the upper sample 1 had a pH of 8, while the lower sample 2 had a pH of 4. Both samples were retrieved from oxic redox conditions. Chemical extractions showed that U is predominantly water soluble and/or ion exchangeable in sample 1, while U is predominantly bound in weakly soluble solid phases in sample 2. To further identify the uranium species, we applied a combination of Synchrotron-based methods, namely Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy of bulk samples, micro X-Ray Diffraction (µ-XRD) and micro X-Ray Fluorescence (µ-XRF) spectroscopy. In sample 1, uranium predominates homogeneously distributed at concentrations in the mg/kg range in aggregates with a diameter of tens to hundreds of µm. The aggregates consist of layer silicates (muscovite, kaolinite, illite), jarosite and gypsum. Chemical extractability, EXAFS, µ-XRD, and µ-XRF strongly suggest that U(VI) is adsorbed on edge sites of the layer silicates. In sample 2, U is heterogeneously distributed among single crystals and small aggregates with very high U concentrations (g/kg) and variable elemental composition. Besides the matrix minerals muscovite, kaolinite, illite and quartz, we identified pitchblende and coffinite, and found evidence for other uranyl hydroxide and vanadate solids. In addition, a smaller amount of uranium seems to be adsorbed to mineral surfaces as in sample 1.
The results suggest that a substantial amount of uranium remained in the buried tailings as relatively mobile, adsorbed U(VI) species. No clear evidence for secondary uranium mineral precipitates was found.