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.
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