Reversibility in radionuclide/bentonite bulk and colloidal ternary systems
Ternary systems of 152Eu(III), bulk bentonite and ethylenediaminetetraacetic acid (EDTA) ([Eu] = 7.9 ×
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 × 10−8 s−1 (values in the range 2.2 ×
10−8 – 1.0 × 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 bentonitewere 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 × 10−7 s−1 and a
range of 7.7 × 10−7–9.5 × 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.
RAGIAB ISSA(1-2021)
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