Humic colloid-borne Np migration: influence of the oxidation state

The migration behavior of Np(IV/V) is investigated by column experiments us ing a sandy sediment with groundwater rich in humic substances. NIR spectro scopy, redox speciation by TTA extraction and ultrafiltration are used for the Np speciation in the groundwater prior to its introduction into-the col umn and after elution. Np is found to be transported as humic colloid-bound species which are eluted slightly faster than the water flow velocity. The fraction of humic colloid-borne Np increases with the reduction progress o f Np(V) to Np(TV), because the interaction of Np(IV) with humic substances is much stronger than Np(V). The results demonstrate the importance of the speciation of redox sensitive actinides in natural aquifers for the assessm ent of the humic colloid facilitated migration.