Influence of humic acid on the Am(III) sorption onto kaolinite

The sorption of americium(III), (Am(III)), onto kaolinite was studied in batch experiments in the absence and presence of humic acid (HA) ([Am(III)](0) = 1 x 10(-6) M, [HA](0) = 0 or 10 mg/L, I = 0.01 M NaClO4, pH = 3-10, p(CO2) = 10(-3.5) atm, solid-to-liquid ratio (S/L) = 1 or 4 g/L). The results show that the Am(III) sorption onto kaolinite is influenced by S/L, the presence of HA and the pH value. In the absence of HA, Am(III) exhibits a very strong and almost pH independent sorption onto kaolinite at the S/L ratio of 4 g/L. In the presence of HA, there are small differences in the Am(III) sorption compared to the HA free system. At pH values <5, HA very slightly enhances the sorption of Am(III). Conversely, at pH values >= 5.5, the presence of HA decreases the sorption of Am(III) due to the formation of dissolved Am(III) humate complexes. The decrease of S/L from 4 to 1 g/L has a significant effect on the Am(III) sorption onto kaolinite. A sorption edge occurs at pH 6 and the influence of carbonate on the Am(III) sorption at higher pH values becomes evident. Furthermore, the influence of HA on the Am(III) sorption onto kaolinite is more pronounced. The Am(III) sorption results are compared to literature data and to those of U(VI) sorption onto kaolinite obtained under the same experimental conditions.