COMPLEXATION OF NP(V) WITH HUMIC-ACID - INTERCOMPARISON OF RESULTS FROM DIFFERENT LABORATORIES

The humate complexation of the NpO2+ ion has been investigated at pH = 7-9 in 0.1 M NaClO4 with purified humic acid extracted from one of th e Gorleben groundwaters in northern Germany. The experiment is conduct ed under Ar atmosphere in a glove box by W-Vis spectroscopy with an op tical fibre connection. The Np(V) concentration is varied from 1.08x10 (-4) to 7.28 x 10(-4) mol L-1 while keeping the humic acid concentrati on constant at around 1 x 10(-3) mol L-1. The complexation constant, w hich is evaluated by taking into account the loading capacity (LC) of the humic acid under investigation for the NpO2+ ion at each given pH, is found to be log beta = 3.58+/-0.05. This value is in good agreemen t with the result of our previous experiment with lower Np(V) concentr ations and appears independent of pH and other experimental parameters . Our experimental results are compared with those from the literature , particularly with spectroscopic experiments, and the key uncertainti es involved in general in the humate complexation studies are discusse d.