TRLFS study on the speciation of uranium in seepage water and pore water of heavy metal contaminated soil

In situ leaching of uranium ores with sulfuric acid during active uranium mining activity on the Gessenheap has caused longstanding environmental problems of acid mine drainage and elevated concentrations of uranium. To study there remediation measures the test site Gessenwiese, a recultivated former uranium mining heap near Ronnenburg/East Thuringia/Germany, was installed as a part of a research program of the Friedrich Schiller University Jena to study, amongs other techniques, the phytoremediation capacity of native and selected plants towards uranium. In a first step the uranium speciation in surface seepage and soil pore waters from the Gessenwiese, ranging in pH from 3.2 to 4.0, were studied by Time-resolved Laser-induced Fluorescence Spectroscopy (TRLFS). Both types of water samples showed a mono-exponential fluorescene decay, indicating the presence of only one major species. The detected emission bands were found at 477, 491, 513, 537, 562, and 591 nm (478 – 492 – 513 – 538 – 562 – 590). These characteristic peak maxima together with the observed mono-exponential decay revealed indicated that the uranium speciation in the seepage and soil pore waters is dominated by the uranium (VI) sulfate species UO2SO4(aq). Due to the presence of fluorescence quenchers in the natural water samples the measured fluorescence lifetimes of the UO2SO4(aq) species of 1.0 to 2.6 µs were reduced in comparison to pure uranium sulphate solutions, which show a fluorescence lifetime of 4.7µs.
This results convincingly show that in the pH range of 3.2 to 4.0 TRLFS is a suitable and very useful technique to study the uranium speciation in naturally occurring water samples.