Experimental and modelling approaches to Am(III) and Np(V) adsorption on the Maoming kaolinite

- The titration study reveals that the kaolinite samples from different areas may have different surface property.
- The diffuse double layer model overestimates the effect of ionic strength on Np(V) adsorption on kaolinite.
- A non-electrostatic model is appropriate for interpreting Am(III) and Np(V) adsorption on kaolinite.

The goal of this paper is to study the adsorption of Am(III) and Np(V) on the Maoming kaolinite and to provide corresponding adsorption models. The surface properties of Maoming kaolinite were evaluated by continuous potentiometric titration. Comparisons with the titration curves of KGa-1, KGa-2, Zettlitz, St Austell and Twiggs County kaolinite samples indicate that the Maoming kaolinite may have different surface acid-base chemistry than these kaolinite samples. These titration curves of the Maoming kaolinite can be interpreted with both the diffuse double layer model (DDLM) and a non-electrostatic model (NEM). The adsorptions of Am(III) and Np(V) were studied as functions of contact time, solid-to-liquid ratios, pH, ionic strength, and concentration using a batch method. The adsorptions of Am(III) and Np(V) on kaolinite were rapid and equilibrated within 20 and 10 h, respectively. The adsorption of 6 × 10−10 mol/L Am(III) was shown to be sensitive to both pH and ionic strength, whereas the adsorption of 1 × 10−6 mol/L Np(V) was sensitive only to pH. The DDLM overestimates the effect of ionic strength on Np(V) adsorption. Finally, the adsorption of Am(III) and Np(V) was quantitatively interpreted using the NEM with the addition of one cation exchange reaction for Am(III). These adsorption data and corresponding models are useful for the safety assessment of an high-level nuclear waste (HLW) repository, which may be built in the clayey formation in China.