Neptunium(V) sorption to goethite at attomolar to micromolar concentrations

Sorption of 10−18–10−5 M neptunium (Np) to goethite was examined using liquid scintillation counting and gamma spectroscopy. A combination approach using 239Np and long lived 237Np was employed to span this wide concentration range. 239Np detection limits were determined to be 2 × 10−18 M and 3 × 10−17 M for liquid scintillation counting and gamma spectroscopy, respectively. Sorption was found to be linear below 10−11 M, in contrast to the non-linear behavior observed at higher concentrations both here and in the literature. 2-site and 3-site Langmuir models were used to simulate sorption behavior over the entire 10−18–10−5 M range. The 3-site model fit yielded Type I and II site densities of 3.56 sites/nm2 (99.6%) and 0.014 ± 0.007 sites/nm2 (0.4 ± 0.1%), consistent with typical “high affinity” and “low affinity” sites reported in the literature [21]. Modeling results for both models suggest that sorption below ∼10−11 M is controlled by a third (Type III) site with a density on the order of ∼7 × 10−5 sites/nm2 (∼0.002%). While the nature of this “site” cannot be determined from isotherm data alone, the sorption data at ultra-low Np concentrations indicate that Np(V) sorption to goethite at environmentally relevant concentrations will be (1) linear and (2) higher than previous (high concentration) laboratory experiments suggest.

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► 239Np tracers enable Np sorption studies at ultra-low concentrations.
► Change to linear Np–goethite sorption behavior below 10−11 M.
► Multi-site Langmuir models require trace, Type III site to describe sorption behavior.