Complexation behaviour of neptunium with humic acid

The complexation of Np(V) with humic acid (HA) was studied under environmen tal conditions (pH greater than or equal to 6, concentration of HA 1-150 mg /L) over a wide range of metal ion concentrations (10(-14) to 10(-4) mol/L. The experimental methods used for the determination of the complexation co nstants were continuous electrophoretic ion focusing, anion exchange, and u ltrafiltration in combination with radiometric measurements, and, as a dire ct speciation method, UV/Vis-spectroscopy. The interaction between Np(V) an d humic acid has been described with the charge neutralization model includ ing a correction for different loading capacities (LQ which makes possible the comparison of results obtained at various experimental conditions. Furt hermore, the degree of deprotonation has also been used to calculate the co mplexation constants. The complexation constants calculated for Np(V) with the LC-concept vary with the metal ion concentration in the range between 1 0(-14) and 10(-4) mol/L. At metal concentrations below 10(-11) mol/L, a com plexation constant log beta (LC) = 5.0 +/- 0.3 was obtained. In the concent ration range 10(-7) to 5 x 10(-1) mol/L, the log beta (LC) values decrease continuously to 3.7 +/- 0.4 (10(-1)- 5 x 10(-4) mol/L). The latter is in ag reement with direct measurements (UV/Vis-spectroseopy) at this concentratio n and data from the literature.