Understanding uranium behaviour in a natural rock–water system: leaching and adsorption tests on the Tufo Rosso a Scorie Nere ignimbrite (Viterbo area, central Italy)

Based on both chemical leaching and adsorption tests and a simple modelling using PHREEQC, geochemical behaviours of uranium during the ignimbrite–water interaction were evaluated mainly as a function of temperature, pH, and solution chemistry (esp., alkalinity). The main results of this work are: (1) uranium is more easily mobilized by slightly basic solution (pH 7.5) than by acidified water (pH 4.5) when relative concentrations of the main uranyl ion–calcium–carbonate species, Ca2UO2(CO3)3, increase from 0.6 to 90%; (2) the greatest leaching of uranium occurs at 50 °C (not at higher temperature) because the first dissociation constant of H2CO3 is directly correlated with temperature up to about 50 °C, but decreases from 50 to 80 °C. This directly influences the concentration of HCO3− which is mirrored by dissolved CO2 variations; (3) the presence of alkalinity, total C-species and calcium controls the saturation index of sorbate solution with respect to calcite, influencing also speciation, solubility, and sorption of dissolved U; and (4) higher adsorption of uranium is obtained in tests with deionized water (90.0 ± 0.7 mg/kg, at equilibrium) compared to those performed with the natural water (26.0 ± 1.5 mg/kg, at equilibrium), strengthening the role of complexes between uranyl, HCO3−, and Ca2+ ions in solution to explain the corresponding decrease in uranium adsorption.