The formation of mixed-hydroxo complexes of Cm(III) and Am(III) with humic acid in the neutral pH range

Abstract

The complexation of humic acid with Cm(III) and Am(III) is studied in the neutral pH range from 6 to 10 in 0.1 M NaClO4under Ar-atmosphere. Cm(III) and Am(III) species are characterized and quantified by time-resolved laser fluorescence spectroscopy (TRLFS) and UV/Vis absorption spectroscopy, respectively. The formation of ternary humate complexes, i.e. hydroxo-humate species, is ascertained besides the well known binary complex AnHA(III) (An = Cm(III), Am(III)) by spectroscopic speciation in aid of radiometric quantification. The pH dependent change in concentrations of individual Cm(III) and Am(III) species corroborates ternary humate complex formation: An(OH)HA(II) and An(OH)2HA(I), with stability constants evaluated to be log β111= 12.82 ± 0.11 and log β121= 17.53 ± 0.13 for Cm(III) and log β111= 12.71 ± 0.17 and log β121= 17.40 ± 0.21 for Am(III). The formation of ternary humate complexes culminates in the strong interaction of trivalent actinide ions with humic acid in the neutral pH range as compared to the binary complexation alone. The results are compared with a previous study on the Cm(III) interaction with humic colloids in a real groundwater system.