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Sorption coefficients of Am(III) and Cm(III) on kaolinite were determined at pH from 3.5 to 10 and humic acid (HA) concentrations of 0 to 20 ppm at ionic strength of 0.1 M (NaClO 4 ) at 298 K with N 2 bubbling. The sorption ratio of HA on kaolinite was also measured under the same experimental conditions. The sorption coefficients of Am(III) and Cm(III) in the absence of HA increased with pH over the whole pH range. The presence of HA enhanced sorption coefficients at pH values up to around 5, above which the sorption coefficients were lowered by introducing HA. The effects of HA on sorption coefficients were interpreted by the use of a linear additive model, modified for metal-humate complex formation in the aqueous phase. The model was found to be in fairly good agreement with the experimental data.

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Sorption coefficients of Eu(III) on kaolinite were measured at pH from 3.5 to 8.5 and humic acid (HA) concentrations of 0 to 20 ppm at ionic strength of 0.1 (NaC10 4 ). The Eu(III)-humate stability constants were also measured at the same ionic strength at pH from 3.3 to 5.5. In the absence of HA, the sorption coefficients of Eu(III) ions increased with pH over the whole pH range. By introducing HA, the sorption coefficients were enhanced at pH up to around 6, above which there was a considerable decrease in the coefficients. Sorption of HA-bound Eu(III) onto kaolinite results in higher sorption coefficients at low pH, while sorption of the HA-bound Eu(III) at higher pH decreases. The effects of humic acid were interpreted by a model based on the stability constant of Eu(III)-humate complexes, fractional sorption of HA on kaolinite, and sorption coefficients of Eu(III) on kaolinite in the absence of HA. The model was found to be in fairly good agreement with the experimental data.