Influence of humic acid size on actinide complexation

The complexation of Am(III) and Eu(III) with humic acid of different molecular sizes was measured and the data for the greater than or equal to 300 kD and greater than or equal to 10 kD sized HA at p(c)H 4.0 and 6.0 was analyzed by the Charge Neutralization Model (CNM) and the Polyelectrolyte Model (PM). For the PM, solvent extraction was used to obtain the stability constants of Am-241 at tracer concentration. For the CNM, ultrafiltration was used to determine the loading capacity and the stability constants for Eu(III) at macro concentrations. There was a difference of 1-2 orders of magnitude in the stability constants from the two models. The D-5(0)-F-7(0) excitation spectra of Eu(III) as a function of loading of HA was found to have a shift in wavenumbers of the peak maximum, indicating that the coordination environment of Eu(III) varied with pH, presumably due to variation in the number of functional groups of the humate bound to the metal ion.