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Title: Radionuclide dissociation from bentonite colloid systems
Author: Sherriff, Nicholas Kevin
ISNI:       0000 0004 5357 9861
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2015
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Deep geological disposal is a method of managing high level, long-­‐lived nuclear waste. It is a concept that many countries are exploring for the possibility of managing nuclear waste generated from power production. For deep geological disposal to be viable then areas where problems may surface have to be explored. Bentonite clay has been proposed as the material to be used for the backfill of the repositories. Its swelling properties ensure that it will expand to plug the bore holes that will be made for the waste, its impermeable nature restricts contact between groundwater and the waste package and its stability on a geological timescale all make it desirable as a backfill material. This project looks at the role that colloids formed from the bentonite clay could have in facilitating radionuclide transport away from a nuclear waste repository. Several radionuclides (Eu(III), U(VI), Th(IV) and Am(III)) have been considered in this research, and information from these studies will be used in the BELBaR project’s outputs, which will eventually support a disposal safety case. Ternary systems of 152Eu(III), bulk bentonite and 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 smaller fraction (3.0 % over a period of 1 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 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 was measured. Eventually, the amount of Eu(III) remaining bound to the bentonite was within error of that when EDTA was also present prior to contact (4.5 % ± 0.6). Eu interactions with colloidal bentonite were studied, and the dissociation rate constant measured by a resin competition method. A dissociation rate of 8.8 x 10-­‐7 s-­‐1 and a range of 7.7 x 10-­‐7 – 9.5 x 10-­‐7 s-­‐1 were measured. For both bulk and colloidal bentonite slow dissociation was observed for Eu(III), but there was no evidence for ‘irreversible’ binding. The interactions of 232U(VI) with bentonite colloids ([U] = 5.43 x 10-­‐10 M; pH = 8.8 ± 0.2) have been studied using a resin ion exchange competition technique. The reversibility of the interaction was studied by allowing U(VI) to sorb to bentonite colloids for periods from 1 – 35 days. A fraction of the U(VI) was removed from the solution instantaneously (28-­‐50 %), and after 3 days, the amount of U(VI) remaining on the bentonite colloids was 17-­‐ 25%. With time, the amount of U(VI) retained by the bentonite colloid is reduced further, with a first order dissociation rate constant of 5.6 x 10-­‐7 s-­‐1. Whilst the dissociating fraction was small (24% (+34; -­‐12 %)), complete dissociation was not observed. Although slow dissociation was observed for U(VI), there was no convincing evidence for ‘irreversible binding’ of the radionuclide by the colloid. The interactions of 228Th(IV) ([Th] = 3.79 x 10-­‐12 M; pH = 8.8 ± 0.2) and 241Am(III) ([Am] = 3.27 x 10-­‐9 M; pH = 8.8 ± 0.2), with bentonite colloids have been studied using an ion exchange competition technique. Th(IV) was not fully associated with the bentonite colloids, and filtration showed that the uptake after 1 week was 78.3% (± 2.7%). Am(III) was weakly associated to the bentonite colloids, the uptake after 1 week was 20.1 % (± 5.2 %). Cellulose phosphate was added to the radionuclide/bentonite colloid systems (1 g for Th(IV), 0.2 g for Am(III)), an amount that was sufficient to retain the radionuclide when no bentonite colloids are present. A fraction of the Th(IV) is initially removed by the Cellphos (75-­‐93 %), and after 7 days the amount of Th(IV) remaining on the colloids is 1 -­‐ 3 %. Over the time of the experiment, the amount of Th(IV) retained by the bentonite colloid appears to remain level and the amount bound to the bentonite colloid at the end of the experiment is 2.1 % ± 0.88 % which is within experimental error of the steady state equilibrium of the system. A fraction (48-­‐94 %) of the Am(III) is also initially removed by the Cellphos, after 7 days the8amount of Am(III) remaining on the colloids is 1.2 – 9.3 %. However, after 35 days of contact time with the cellulose phosphate it appears that Am(III) is released back into the system, preventing dissociation rates from being calculated in this case. Studies of the association of Eu(III) to the clay colloids and its subsequent dissociation in this thesis follow similar trends to those described elsewhere in the literature (Missana et al. (2008), Bouby et al. (2011)). The Eu/bentonite colloid dissociation rate calculated here (8.8 x 10-­‐7 s-­‐1 (± 9.1 x 10-­‐7 s-­‐1)) is within error of the dissociation rates for trivalent ions estimated by Wold (2010) (Am(III) 5.6 x 10-­‐7 s-­‐1 Cm(III) 1.7 x 10-­‐6 s-­‐1). The U(VI) studies in this thesis show a dissociation rate of 5.6 x 10-­‐7 s-­‐1 (± 4.2 × 10-­‐7) which is within error of the U(VI) dissociation rate estimated by Wold (2010) (8.3 X 10-­‐7 s-­‐1). Reliable dissociation rates could not be obtained from the Am(III) and the Th(IV) studies in this thesis, other studies (e.g. Bouby et al. (2011) showed signs of irreversible binding of Th(IV) to bentonite colloids, however, no irreversible binding was observed in this thesis. Am(III) did not appear to be a close analogue of Eu(III) in these systems. All of the isotopes studied in this thesis showed no evidence of irreversible binding to bentonite or bentonite colloids. As such, the role that bentonite colloids will have in the facilitated transport of radioisotopes away from a repository is likely to have only a limited impact, at most, on the environmental safety case.
Supervisor: Livens, Francis ; Morris, Katherine Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: bentonite ; colloid ; dissociation ; radionuclide ; kinetics