Neptunium-237 (²³⁷Np) is a component of spent nuclear fuel and other high level radioactive waste.  Of the radioactive material destined for disposal within a geological repository, ²³⁷Np is a radionuclide of significant concern.  This is due to its long half-life (2.14 x10⁶ years), biological toxicity, high chemical stability of the pentavalent oxidation state, and low sorption affinity of the neptunyl (NpO₂⁺) cation. 

A primary geochemical mechanism likely to limit the migration of ²³⁷Np away from underground nuclear waste repositories is the extent Np sorbs to solid surfaces.  Np(V) sorption to complex geomedia has been primarily investigated to determine distribution coefficients, Kd, over a narrow range of experimental conditions.  However, variations in geochemical conditions, such as pH, PCO₂ and mineral surface properties can significantly impact K_d values, resulting in inaccurate simulations of contaminant transport.  To address this inadequacy, surface complexation models (SCMs) have recently been incorporated into transport model simulations.  However, few SCMs have been developed to describe sorption to complex geomedia.

     The presentation will include experimental data for Np(V) sorption to an alluvial aquifer sediment over a range of system conditions.  Determination of the stability constant for Np(V) complexation with Suwannee River humic acid (SRHA) will be discussed.  Application of a generalized composite SCM to describe the Np(V) sorption data will be also be considered.  In addition, Np(V) and uranium(VI) sorption and complexation behavior will be compared.