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 NaC10₄. 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: logβ = 2.28±0.06 at pH 6; logβ = 2.45 ±0.03 at pH 7; logβ = 2.71 ±0.09 at pH 8; logβ = 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. logβ = 3.66 ±0.02, which remains constant independent of pH and ionic strength.