Abstract
AbstractTernary 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 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 × 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.
Highlights
IntroductionSTUDIES suggest that for colloidal transport to be significant, colloids must bind radionuclides irreversibily, as any reversibly bound radionuclides
The aim of this work is to measure directly the dissociation rate constants for bulk and colloidal bentonite using sinks that do not interact with bentonite: such data are useful in the prediction of radionuclide migration in environments where bentonite exists, such as deep geological disposal
Powder X-ray diffraction was used to show that the ethylenediaminetetraacetic acid (EDTA) had no effect on the bentonite
Summary
STUDIES suggest that for colloidal transport to be significant, colloids must bind radionuclides irreversibily, as any reversibly bound radionuclides. The interactions of radionuclides with bulk and colloidal clays are important, because they have been suggested as a potential backfill material for a radioactive waste repository (Mori et al, 2003), and so the sorption of radionuclides by bulk bentonite, its constituents and other bulk clays have been studied extensively (Wold, 2010). Geckeis et al (2004) found that Am(III) and Pu(IV) transport through fractures could only be explained by slow dissociation from colloids, the interactions were eventually reversible on a time scale of months Slow dissociation from the colloids was observed, and there was some evidence that the system took 7500 hours (313 days) to reach equilibrium: for Am(III), the rate constants were in the range 1‒2.5 × 10−6 s−1, whilst for Pu(IV), the range was 3.9 × 10−7 ‒ 2.4 × 10−6 s−1. Wold (2010) estimated representative first-order dissociation rate constants from sorption rate constants and Kd values for: Pu(IV) 1.2 × 10−6 s−1; Am(III) 5.6 × 10−7 s−1; Np(IV) 1.2 × 10−10 s−1; Cm(III) 1.7 × 10−6 s−1; U(VI) 8.3 × 10−7 s−1; Tc(IV) 1.75 × 10−4‒4.2 × 10−3 s−1. Geckeis et al (2004) found that Am(III) and Pu(IV) transport through fractures could only be explained by slow dissociation from colloids, the interactions were eventually reversible on a time scale of months
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