Mobility of Po and U-isotopes under acid mine drainage conditions: An experimental approach with samples from Río Tinto area (SW Spain)

Under acid mine drainage (AMD) conditions, the solubilities and mobilities of many elements are vastly different from conditions prevailing in most natural waters. Studies are underway in the Río Tinto area (Iberian Pyrite Belt), in order to understand the behavior and mobility of long-lived U-series radionuclides under AMD conditions. A set of leaching experiments utilizing typical country rocks from the Tinto River basin, waste rock pile composite materials, iron-rich riverbed sediments and gossan (weathered naturally rock) were performed towards this purpose. Initial leaching experiments using distilled water kept in contact with solid material for 300, 100, 50 and 1h resulted in very low concentrations of U with ²³⁴U/²³⁸U activity ratios close to equilibrium and activity concentrations of ²¹⁰Po<0.03mBq/g. Leaching experiments performed with sulfuric acid media (0.1 and 0.01M), and contact times between the solid and solution for 24h were conducted to quantify the amount of U-isotopes and ²¹⁰Po leached, and the radioactive disequilibria generated between the radionuclides in the leachate. These experiments show that Po mobility in acidic conditions (pH around 1-2) is very low, with ²¹⁰Po activity in the leachate tobe6% in average for the solid sample. By contrast, mobility of U-isotopes is higher than that of Po, around 1.2%.