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 'complex ed' Eu(iii). With relatively small amounts of resin, the amount of complexe d Eu(iii) remaining after any resin contact time is calculated using four c omponents to describe the Eu(iii) binding viz.: an easily exchangeable frac tion of Eu (calculated using a single equilibrium constant) and three kinet ically hindered fractions (calculated from three consecutive pseudo-first o rder rate expressions). With larger amounts of resin, the easily exchangeab le Eu(iii) becomes negligible. The slowest dissociation step, which probabl y is the most important with regard to radionuclide transport, has a half l ife of approximate to 200 h. The fraction of Eu(iii) in the kinetically hin dered sites increased with metal-HS contact time, pH, FA concentration and the degree of humification (HA > FA) but competing cations (Ca, Mg, Al or F e) had little effect. Information on the effect of temperature on the rate constant for the slowest dissociation step was used to calculate the activa tion energy barrier.