Factors influencing plutonium sorption in shale media

Abstract

The extent and factors influencing sorption of plutonium (Pu) in shale media have been studied in batch-type experiments. The equilibrium time and isotherm of Pu sorption have been firstly determined. The effect of shale grain size, aqueous pH, water/solid ratio, co-existing ions (cations and anions) and temperature were studied as the factors likely to influence Pu sorption. The experimental variables included the following: grain sizes in five ranges, 0.12 to 0.15 mm, 0.15 to 0.18 mm, 0.18 to 0.25 mm, 0.25 to 0.38 mm, and 0.38 to 0.83 mm; solution pH values of 4, 6, 8, and 10; water/solid ratios of 10:0.25, 10:0.5, 10:0.75, and 10:1; temperatures of 20 °C, 40 °C, and 60 °C); and selected co-existing conventional cations including Na⁺, Fe⁺, Al³⁺, Ca²⁺, and K⁺ and anions including SO₄^2-, CO₃^2-, NO₃^-, and Cl^- in each case in solutions of the same total ionic strength. The experimental results demonstrate that the equilibrium time of Pu sorption is 9 d, that the sorption behavior of Pu in the solution is well described by the Freundlich sorption isotherm (C_s=kC_eq^N), and that Pu is strongly sorbed on shale media. The extent of Pu sorption increased with decreasing grain size, apparently in response to the increase in specific surface area and total pore volume. The results also show that Pu sorption increases with increasing solution pH and with increasing water/solid ratio. The influence of co-existing anions on Pu sorption was much stronger, under the same conditions, than that of the cations; this was attributed to Pu complexation in solution, i.e., the complexing apparently weakened the sorption of Pu on shale particles in the solution. Finally, the influence of varying temperature on Pu sorption is found to be less important than that of other factors, but should never be ignored.