Effects of humic acid on the sorption of neptunium(V) on kaolinite

The sorption coefficient of Np(V), K-d, on kaolinite was measured at pH 6 to 11 and in humic acid concentrations of 0 to 40 mg dm(-3) at the ionic strength of 0.1 M. The K-d value increased with pH both with and without humic acid. The K-d value increased slightly with increasing concentration of humic acid in the pH range below 8 and decreased not more than an order of magnitude with increasing concentrations of humic acid in the pH region above 8. Below pH 8, Np(V) sorption is considered to be enhanced by the sorption of humic acid on the kaolinite to form Np(V)-humate complexes. Above 8, it is probable that the desorption of humic acid and formation of Np(V)-humate in solution result in the decrease of K-d. The behavior of Np(V) sorption on kaolinite with humic acid is described by a simple model. (C) 1997 Elsevier Science B.V.