The dissociation kinetics of dissolved metal–humate complexes

The dissociation of Eu(iii)–HS (HS=fulvic or humic acid) complexes has been investigated using cation exchange resins to separate ‘free’ from ‘complexed’ Eu(iii). With relatively small amounts of resin, the amount of complexed Eu(iii) remaining after any resin contact time is calculated using four components to describe the Eu(iii) binding viz.: an easily exchangeable fraction of Eu (calculated using a single equilibrium constant) and three kinetically hindered fractions (calculated from three consecutive pseudo-first order rate expressions). With larger amounts of resin, the easily exchangeable Eu(iii) becomes negligible. The slowest dissociation step, which probably is the most important with regard to radionuclide transport, has a half life of ≈200 h. The fraction of Eu(iii) in the kinetically hindered sites increased with metal–HS contact time, pH, FA concentration and the degree of humification (HA>FA) but competing cations (Ca, Mg, Al or Fe) had little effect. Information on the effect of temperature on the rate constant for the slowest dissociation step was used to calculate the activation energy barrier.