Uranium – species trace analytics in the nanomolar concentration range

Dresden
Uranium is a ubiquitous element. Besides this depleted uranium amunition as well as uranium mining and milling and manifold other use of uranium leads to an increase of uranium contamination in the environment.
Application of laser-induced and time-resolved methods allow the direct determination of uranium speciation at extremely low concentrations. This behaviour can be directly observed due to the properties extraorbitant luminescence properties of uranium-(VI).
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 minerals. 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 mineral. 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 species. 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 site.
The pure carbonate species do not show any luminescence properties at room temperature. Therefore the samples have to be frozen to temperatures below 220 K, 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.
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 TRLFS 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.
In this case uranium may come back to the food chain by the production of mineral waters. We have studied the uranium speciation in several German mineral waters with uranium concentrations between 50 ng/L and 5 µg/L.
Contact of dissolved uranium with living cells at ambient conditions changes dramatically the uranium speciation. Some examples fluorescence properties of uranium species relevant to the environment are shown. The change of this speciation can be observed then due to a change in luminescence properties. Besides of several organic phosphate binding forms although other uranium species were found as uranium bond to phenolic and thiol groups. Some of them do not emit any luminescence at room temperature. Nevertheless low temperature measurements allow the assignment of species not fluorescing at room temperature, due to strong dynamic quench effects of H2O molecules and COO- groups.