Sorption of Np(V) and Np(IV) onto kaolinite: Effects of pH, ionic strength, carbonate and humic acid

The sorption of Np(V) and Np(IV) onto kaolinite has been studied in the absence and presence of humic acid (HA) in a series of batch equilibrium experiments under different experimental conditions: [Np]0: 1.0x10-6 or 1.0x10-5 M, [HA]0: 0 or 50 mg/L, I: 0.01 or 0.1 M NaClO4, solid to liquid ratio: 4 g/L, pH: 6-11, anaerobic or aerobic conditions.
The results showed that the Np(V) sorption onto kaolinite is effected by solution pH, ionic strength, Np concentration, presence of carbonate and HA. In the absence of carbonate, the Np(V) uptake increased with pH up to ~ 96% at pH 11. HA further increased the Np(V) sorption between pH 6 and 9 but decreased the Np(V) sorption between pH 9 and 11. In the presence of carbonate, the Np(V) sorption increased with pH and reached a maximum of 54% between pH 8.5 and 9. At higher pH values, the Np(V) sorption decreased due to the presence of dissolved neptunyl carbonate species with a higher negative charge that were not sorbed onto the kaolinite surface which is negatively charged in this pH range. HA again decreased the Np(V) uptake in the near-neutral to alkaline pH range due to formation of aqueous neptunyl humate complexes. The decrease of the initial Np(V) concentration from 1.0x10-5 M to 1.0x10-6 M led to a shift of the Np(V) adsorption edge to lower pH values. A higher ionic strength increased the Np(V) uptake onto kaolinite in the presence of carbonate but had no effect on Np(V) uptake in the absence of carbonate.
To the best of the author`s knowledge, the sorption of Np(IV) onto kaolinite in the presence of HA was studied the first time. For this, a synthetic HA with pronounced reducing properties was applied. This HA effectively reduced Np(V) to Np(IV) and stabilized the tetravalent oxidation state during sorption experiments over a wide pH range. The Np(IV) uptake onto kaolinite is strongly effected by HA. Especially in the near-neutral pH range the Np(IV) uptake was found to be very low in the presence of HA which was attributed to the strong Np(IV) humate complexation in solution.
Thus, depending on the prevailing geochemical conditions, HA has an immobilizing as well as a mobilizing effect on Np(V). In case of Np(IV), the mobilizing effect predominates.