COMPLEXATION OF NEPTUNIUM(V) WITH HUMIC ACIDS AT VERY-LOW METAL CONCENTRATIONS

The complexation of Np(V) with humic acids at metal concentrations bet ween 10(-13) and 10(-8) M was investigated by continuous electrophoret ic ion focusing and anion exchange chromatography combined with radiom etric detection. The influence of neptunium and humic acid concentrati ons, pH value, ionic strength, temperature and competing cations on th e complexation was studied. Formation of a 1:1 complex between Np(V) a nd humate is observed. After correction of the pH-dependent ionization of humic acid, a pH-independent complex stability constant log beta = 4.2+/-0.15 is obtained in 10(-13) M Np(V) solution for the Aldrich an d values between 3.6 and 4.4 for the Gorleben humic acid. The stabilit y constant changes by a factor of 2.5 when the neptunium concentration is varied between 10(-12) and 10(-8) M. No effect of ionic strength b etween 0.001 and 0.1 M is found. With increasing temperature the Np-hu mate complexation decreases. Calcium and magnesium act as competitive cations at concentrations higher than 10(-4) M. Under anaerobic condit ions and at very low metal concentrations, humic acid reduces Np(V) to Np(IV) at a pH-depending rate.