The complexation of Np(V) with humic acid (HA) was studied under environmental conditions (pH≥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 constants were continuous electrophoretic ion focusing, anion exchange, and ultrafiltration in combination with radiometric measurements, and, as a direct speciation method, UV/Vis-spectroscopy. The interaction between Np(V) and humic acid has been described with the charge neutralization model including a correction for different loading capacities (LC) which makes possible the comparison of results obtained at various experimental conditions. Furthermore, the degree of deprotonation has also been used to calculate the complexation constants. The complexation constants calculated for Np(V) with the LC-concept vary with the metal ion concentration in the range between 10-14 and 10-4 mol/L. At metal concentrations below 10-11 mol/L, a complexation constant logβLC=5.0±0.3 was obtained. In the concentration range 10-7 to 5×10-4 mol/L, the logβLC values decrease continuously to 3.7±0.4 (10-5-5×10-4 mol/L). The latter is in agreement with direct measurements (UV/Vis-spectroscopy) at this concentration and data from the literature.
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