Influence of Humic Acid on the Am(III) Sorption onto Kaolinite

The influence of humic acid (HA) on the sorption of Am(III) onto kaolinite KGa-1b (Clay Minerals Society Source Repository) as a model mineral was studied by batch experiments. The sorption experiments were performed under ambient atmosphere (pCO2 = 10-3.5 atm) in a glove box, for a Am(III) concentration of 110-6 M and a HA concentration of 10 mg/L in 0.01 M NaClO4 solution. The solid/solution ratio was 4 g/L and the pH values were varied between pH 3 and 10.
In the absence of HA, Am(III) exhibits a very strong sorption onto kaolinite, which is almost independent on the pH value. The percentage of sorbed Am(III) amounts to 98% at pH 3 and to almost 100% at higher pH values. Presumably the high solid/solution ratio used in the sorption experiments represents the reason for no observation of a sorption edge at lower pH values. In the presence of HA, there are small changes in the Am(III) sorption in comparison to the system without HA. At pH < 5, HA enhances very slightly the sorption of Am(III). At pH ≥ 5.5, the sorption of Am(III) decreases in comparison to the HA-free system due to the formation of dissolved Am(III)-humate complexes. The formation of such binary and ternary Am(III)-humate complexes was reported by Kim et al. [1] and Morgenstern et al. [2]. However, the drop of the Am(III) sorption onto kaolinite observed in this study is very small. Generally, the trend in the sorption behavior of Am(III) in the presence of HA is consistent with the results reported in literature for trivalent actinides. At low pH values the sorption is enhanced by the presence of HA, while at high pH the presence of HA lowers the sorption. At pH > 8.5, HA has no influence on the Am(III) sorption onto kaolinite. Further batch experiments with varying experimental conditions will be performed.
First laser-induced fluorescence spectroscopic measurements were carried out in the systems Am(III)-kaolinite and Am(III)-HA-kaolinite to study the surface complexes of Am(III) on kaolinite. HA decreases the fluorescence intensity of the samples. The shape of the measured fluorescence spectra in both systems, binary and ternary, differs from the spectrum observed in the solution.

[1] J.I. Kim et al., Radiochim. Acta 48, 135 (1989).
[2] M. Morgenstern et al., Radiochim. Acta 88, 7 (2000).