Abstract Batch sorption experiments at fixed initial Np(V) concentration (∼1 × 10 −6 M 237 Np), M/V ratio (4 g L −1 ), and ionic strength (0.1 molal NaNO 3 ) were conducted to determine the effects of varying pH and PCO2 on Np(V) sorption on SAz-1 montmorillonite. The results show that Np(V) sorption on montmorillonite is strongly influenced by pH and PCO2 . In the absence of CO 2 , Np(V) sorption increases over the entire pH range examined (∼3 to ∼10), with measured sorption coefficients ( K D ) of about 10 mL g −1 at pH < 6 to K D ∼ 1000 mL g −1 at a pH of 10.5. However, for experiments open to atmospheric CO 2 ( PCO2=10−3.5atm ), Np(V) sorption peaks at K D ∼ 100 mL g −1 at pH of 8 to 8.5 and decreases at higher or lower pH. A comparison of the pH-dependence of Np(V) sorption with that of Np(V) aqueous speciation indicates a close correlation between Np(V) sorption and the stability field of the Np(V)-hydroxy complex NpO 2 OH 0 (aq). In the presence of CO 2 and aqueous carbonate, sorption is inhibited at pH > 8 due to formation of aqueous Np(V)-carbonate complexes. A relatively simple 2-site Diffuse-Layer Model (DLM) with a single Np(V) surface complexation reaction per site effectively simulates the complex sorption behavior observed in the Np(V)-H 2 O-CO 2 -montmorillonite system. The good agreement between measured and DLM-predicted sorption values suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling and could provide a better alternative to the current use of constant K D values in performance assessment transport calculations.

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