Uranium Speciation - from mineral phases to mineral waters

Uranium in the environment is ubiquitous. Nevertheless human activities as uranium mining and milling, use of uranium in nuclear power production and as penetrating ammunitions as well as the distribution of uranium with fertilizers lead to an increase of the uranium content in the environment.
Especially the uranium ammunition can generate locally high concentrations of uranium in the environment.
Weathering processes of the uranium metal lead in a first step to the formation of uranium minerals1. Depending on the composition of the soil the formation of several types of minerals can be estimated. Especially the content of phosphate from fertilizers and the aluminium from soil components are involved in the mineral formation.
By use of time-resolved laser-induced fluorescence spectroscopy (TRLFS) the mineral type can be determined without any destruction. A large database of luminescence spectra, obtained from uranium minerals of the collection of the Technical University Mining Academy Freiberg, enables us to identify the formed uranium mineral2. In the experiment, described in (1) we found that mainly the mineral sabugalite was formed. Other experiments with pure calcium and phosphate containing solution lead to the formation of the mineral meta-autunite3.
In a second step the formed minerals than undergo further weathering processes, forming dissolved uranium species.
In the former uranium mining areas of eastern Germany we could discover a new dissolved uranium carbonate species4,5. However, the uranium concentration of about 2 mg/L in these mining related waters is relatively high. Nevertheless the carbonate and calcium concentration are high enough to form a very stable dicalcium-uranyl-tricarbonate species. This species is of great importance, as its existence explains the uranium migration at the Hanford site6.
In addition to the calcium species it can be stated that also the other alkaline earth elements form this type of alkaline earth uranyl carbonate species7,8,910.
Following the uranium migration in the soil we could detect in the experiments that mainly carbonate species are formed. The pure carbonate species do not show any luminescence properties at room temperature. Therefore the samples have to be frozen to temperatures below 220 K11, in order to minimize the dynamic quench effect of the carbonate anion. This increases also the luminescence intensity and the luminescence lifetime of all carbonate containing species.
Nevertheless, in one case of the soil experiment also hydroxo species were found. This may be connected to a non-equilibrium with atmospheric CO2 in this column.
Following the possible transport of uranium under environmental conditions we may start with the weathering of uranium compounds in the soil or in a mining waste rock pile. The seepage water contains about 2 mg/L uranium and the speciation is mainly influenced be the formation of the dicalcium-uranyl-tricarbonate species. The input of these seepage water leads to a dilution of the uranium by about three orders of magnitude. Using the cryogenic technique in TRLFS12 we could also determine the uranium speciation in the river water nearby the former uranium mining area. The uranium concentration was about 2 µg/L uranium and in the river water mainly uranyl-tricarbonate species are formed.