COMPLEXATION OF NEPTUNIUM(V) WITH HUMIC-ACID

The complexation of Np(V) with humic acid, which is extracted from one of the Gorleben groundwaters, has been investigated by spectrophotometry in the pH range from 6 to 9 in 0.1 M NaClO4. By the spectroscopic speciation, the complexed and uncomplexed species are precisely quantified and only 1:1 complex is observed under present experimental conditions. Constants determined at each pH are: log-beta = 2.28 +/- 0.06 at pH 6; log-beta = 2.45 +/- 0.03 at pH 7; log-beta = 2.71 +/- 0.09 at pH 8; log-beta = 3.10 +/- 0.07 at pH 9. By introducing the loading capacity of the humic acid for the NpO2+ ion at each pH, which is determined parallely in this experiment, a new constant is evaluated, i.e. log-beta* = 3.66 +/- 0.02, which remains constant independent of pH and ionic strength.