Speciation in Water Released from Mining and Milling Facilities

Abstract

Radionuclide concentrations have been found by gamma-ray spectrometry in rock samples taken from a 40-year-old uranium waste-rock pile. With greater depth, increased Ra-226/U-238 activity ratio was found. This can be explained by different migration rates of individual nuclides [1].

The uranium speciation in four uranium mining-related waters from Saxony in Germany was experimentally determined by laser spectroscopy. Furthermore, the influence of pH alterations on the speciation was measured in three of these waters. The obtained species distributions were compared with modelling predictions for the different U(VI) species calculated with the modelling software EQ3NR using the Nuclear Energy Agency (NEA) of the Organisation for Economic Co-operation and Development (OECD) database [2–3]. Depending on the chemical constituents of the individual water, the following different solution complexes characterise the uranium speciation in the investigated waters:

• 1. In carbonate- and calcium-containing mine and seepage water from the town of Schlema at pH 7.8 and 7.1, respectively, Ca₂[UO₂(CO₃)₃](_aq);

• 2. In carbonate-containing and calcium-poor tailing water from Helmsdorf at pH 9.8, UO₂(CO₃)₃ ^4-.

To validate the speciation code, we carried out a titration of the seepage and the tailing water. In the seepage water, the measured speciation as function of the pH was in good agreement with calculated data. In the tailing water we observed new complexes. These species we determined as uranyl arsenate. Using the derived complex formation constants for these uranyl arsenate species, we could show agreement between the speciation calculation and determination of uranyl species by laser-induced spectroscopic methods.