Sorption of actinides on magnetite and goethite under reducing conditions

The present study investigated the sorption of 237Np, 239Pu and 241Am onto magnetite and goethite as a typical iron corrosion product under reducing conditions as a function of pH in the range 2-7, both in the presence and absence of CO2. The solid/liquid ratio in the batch experiments was 4 g/dm3 and the specific surface areas of magnetite and goethite were 4.7 and 40 m2/g, respectively. The initial concentrations of 237Np, 239Pu, and 241Am were 0.015, 1.6 and 0.87 Bq/mL, respectively. The sorption period was 13 days at the temperature of 25 deg C. After the experiments, alpha-spectrometry of the liquid phase was performed to determine the equilibrium concentrations of the actinides. The sorption experiments were carried out for each actinide independently and for the simultaneous presence of the three actinides. Reducing conditions was achieved to purge the solutions containing the actinides with a mixture of 5 % H2 in N2 atmosphere and add 100 ppm hydrazine. The experiments in the presence of CO2 were done by purging the solution with a mixture of CO2 in N2. The sorption was strongly dependent on pH. At low pH, the sorption was close to zero, and at high pH, it was close to 100 %. The pH of sorption edges of 237Np and 239Pu sorption onto magnetite were lower in the presence of CO2, indicating the enhancement of sorption of those actinides by CO2. Similar tendency to this was slightly observed in the sorption of each 237Np and 239Pu onto goethite. In other cases, the presence of CO2 prevents the actinides from sorption onto the minerals. The mixture of the actinides resulted in higher pH of the sorption edges in some cases, showing the possibility of the competition in occupying of sorption sites of the iron minerals. There seems no simple tendency in the observed data under the conditions examined probably because of effects of aqueous and surface complexation reactions with carbonate ions, and changes in the valence state of the actinides on the mineral surfaces. (P.A.)