Complexation of Np(V) with fulvic acid

A spectroscopic study is presented for the complexation of the pentavalent neptunyl ion with fulvic acid, extracted from the Gorleben aquifer system, at pH 6 to 8 in 0.1 M NaClO4. The speciation is made by UV-VIS spectroscopy in the wavelength range of 950-1020 nm. The complexation reaction is described according to the metal ion charge neutralisation model by introducing the loading capacity (LC) of given fulvic acid for the NpO2+ ion at each pH under consideration. Only 1:1 humate complex is found as prevalent without formation of higher order complexes. The complexation constant is found to be log beta = 3.53 +/- 0.04, which appears independent of the metal ion concentration and pH, and comparable to the value found for its complexation with humic acid. Discussion is extended to different methods of calculation for the free fulvic acid concentration and their importance to the evaluated constants.