Neptunium(V) Sorption on Montmorillonite: An Experimental and Surface Complexation Modeling Study

Abstract

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 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"><mml:msub><mml:mi>P</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi>C</mml:mi><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:msub></mml:math> on Np(V) sorption on SAz-1 montmorillonite. The results show that Np(V) sorption on montmorillonite is strongly influenced by pH and <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"><mml:msub><mml:mi>P</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi>C</mml:mi><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:msub></mml:math> . 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 &lt; 6 to K D ∼ 1000 mL g −1 at a pH of 10.5. However, for experiments open to atmospheric CO 2 ( <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"><mml:msub><mml:mi>P</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi>C</mml:mi><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:msub><mml:mspace width="thickmathspace"/><mml:mo>=</mml:mo><mml:mspace width="thickmathspace"/><mml:msup><mml:mn>10</mml:mn><mml:mrow class="MJX-TeXAtom-ORD"><mml:mo>−</mml:mo><mml:mn>3.5</mml:mn></mml:mrow></mml:msup><mml:mspace width="thickmathspace"/><mml:mi>a</mml:mi><mml:mi>t</mml:mi><mml:mi>m</mml:mi></mml:math> ), 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 &gt; 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.