Identification of the uranium speciation in an underground acid mine drainage environment analysed by laser fluorescence spectroscopy

The subsurface acid mine drainage (AMD) environment of an abandoned underground uranium mine in Königstein/Saxony/Germany, currently in the process of remediation, is characterized by low pH, high sulfate concentrations and elevated concentrations of heavy metals, in particular uranium. Acid streamers thrive in the mine drainage channels and are heavily coated with iron precipitates identified as schwertmannite and jarosite. These precipitates are biomineralisations and related to the presence of Fe-oxidizing microorganisms. Such precipitates were also observed in stalactite-like dripstone, called snottites, growing on the gallery ceilings. The bacterial diversity was identified on basis of analysed 16S rDNA sequences and revealed that the beta-proteobacterium Ferrovum myxofaciens dominates. Colloidal uranium, neither as U(IV)- or as U(VI) eigencolloids nor uranium adsorbed on colloids was detected as photon correlation spectroscopy analyses and centrifugation experiments of drainage water and snottite water at different centrifugal accelerations between 500 g and 46000 g showed.
The uranium speciation in the underground AMD waters flowing in mine galleries as well as dripping from the ceiling and forming stalactite-like dripstones were studied by time resolved laser-induced fluorescence spectroscopy (TRLFS). The fluorescence lifetime of both waters were best described with a mono-exponential decay, indicating the presence of one species only. The detected positions of the emission bands and by comparing it in a fingerprinting procedure with spectra obtained for acid sulfate reference solutions, in particular Fe - SO42- - UO22+ reference solutions, indicated that the uranium speciation in the AMD environment of Königstein is dominated in the pH range of 2.5 to 3.0 by the highly mobile aquatic uranium sulfate species UO2SO4(aq) and formation of uranium precipitates is rather unlikely as is retardation by sorption processes. The presence of iron in the AMD reduces the fluorescence lifetime of the UO2SO4(aq) species from 4.3 µs found in iron-free uranium sulfate reference solutions to 0.7 µs in both AMD waters of Königstein and in iron containing uranium sulfate reference solutions, respectively. Thermodynamic calculations indicated the presence of two additional uranium species, i.e. UO2(SO4)2- and UO22+ in AMD of Königstein, however TRLFS provided no spectroscopic evidence for their existence.
TRLFS was directly applied to complex solutions of acid mine drainage (AMD) environments intimately associated with microbiology and Fe-precipitates and shows that it is a suitable spectroscopic technique to identify the uranium speciation in natural solutions containing a multitude of different complexing agents.