Chemical reaction of Np(V) with humic colloids in groundwater: influence of purification on the complexation behaviour

The complexation of neptunium(V) with humic colloids which are composed of humic and fulvic acids was investigated directly in natural gorleben groundwater (GoHy 2227). The results are compared with complexation studies of neptunium(V) with purified gorleben humic and fulvic acids to ascertain whether or not the isolation and purification influence the complexation behaviour of Np(V). The experiment was carried out with 237Np(V) in the concentration range from 1.5 x 10-5 to 1.2 x 10-4 mol/L at pH = 6 to 9 in a glove box under Ar atmosphere. The neptunium was speciated by UV-Vis spectroscopy with a subsequent numerical deconvolution of the spectra. The complexation involves a charge neutralisation reaction of the NpO2+ ion with one proton exchanging site of humic colloids. An average complexation constant is evaluated to be log β = 3.39 ± 0.11 which is in good agreement with the complexation constants on purified humic and fulvic acids. This corroboration infers that the complexation studies made on purified humic and fulvic acids are applicable to natural groundwaters containing humic and fulvic acids in colloidal form. (orig.)