Reversibility in radionuclide/bentonite bulk and colloidal ternary systems

Ternary systems of Eu-152(III), bulk bentonite and ethylenediaminetetraacetic acid (EDTA) ([Eu] = 7.9 x 10(-10) M; pH = 6.0-7.0) have been studied. Without EDTA, there was slow uptake in a two-stage process, with initial rapid sorption of Eu(III) (96%), followed by slower uptake of a much smaller fraction (3.0% over a period of one month). The reversibility of Eu(III) binding was tested by allowing Eu(III) to sorb to bentonite for 1-322 days. EDTA was added to the pre-equilibrated Eu bentonite systems at 0.01 M, a concentration that was sufficient to suppress sorption in a system where EDTA was present prior to the contact of Eu(III) with bentonite. A fraction of the Eu was released instantaneously (30-50%), but a significant amount remained bound. With time, the amount of Eu(III) retained by the bentonite reduced, with a slow fraction dissociation rate constant of approximately 4.3 x 10(-8) s(-1) (values in the range 2.2 x 10(-8) -1.0 x 10(-7) s(-1)) for pre-equilibration times >= 7 days. Eventually, the amount of Eu(III) remaining bound to the bentonite was within error of that when EDTA was present prior to contact (4.5% +/- 0.6), although in systems with pre-equilibration times >100 days, full release took up to 500 days. Europium interactions with colloidal bentonite were also studied, and the dissociation rate constant measured by a resin competition method. For the colloids, more Eu was found in the slowly dissociating fraction (60-70%), but the first-order dissociation rate constant was faster, with an average rate constant of 8.8 x 10(-7) s(-1) and a range of 7.7 x 10(-7)-9.5 x 10(-7) s(-1). For both bulk and colloidal bentonite, although slow dissociation was observed for Eu(III), there was no convincing evidence for 'irreversible' binding.