Loughborough University
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Surface-modified minerals for radionuclide sequestration

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posted on 2018-07-06, 15:09 authored by Hayley S. Gillings
This thesis is concerned with experiments designed to identify materials to increase the utility of the Sellafield Ion Exchange Plant (SIXEP) process. Clinoptilolite, ZSM-5, vermiculite and kaolinite have been surface modified by grafting with APTES, TMSPE and TMSPETT ligands using varying grafting times (1-24 h), ligand concentrations (1-3 mmol) and solvents with different polarities (with dielectric constant between 1.9 80). Materials before and after grafting were analysed using long and short range techniques including elemental analysis (CHN), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), solid state nuclear magnetic resonance (SSNMR) and infra-red (FTIR) spectroscopy. The amount of graft placed on the surface increased with reaction time for all materials as the amount of carbon per unit mass of the support, determined by CHN analysis, increased with time. C-H stretches were consistently observed in the FTIR of the grafted materials but no change was observed to the long range order in the PXRD pattern. 13C SSNMR showed that small solvent molecules such as acetone can become trapped in the material supports during grafting and long drying times are required to ensure that trapped solvent molecules do not contaminate the CHN analyses of the materials after grafting. Clinoptilolite and vermiculite consistently achieved higher grafting levels than ZSM-5 or kaolinite, with clinoptilolite (NDA) showing the greatest carbon content of 56.16 mmol of APTES per gram of clinoptilolite using hexane as the solvent. Despite a similar molecular weight, TMSPE consistently showed lower levels of grafting than APTES. APTES grafting levels were increased by using citric, phosphoric and nitric acid (1-3 mol L-1) to pre-treat the surface of the materials over different time periods (1-24 h) to improve the level of APTES graft. While the carbon content was higher for acid treated clinoptilolite (NDA) after grafting, in the case of the citric acid treatment, this increase maybe due to the formation of iron (III) citrate from the dissolution of poorly crystalline iron impurities in the clinoptilolite (NDA). Ungrafted and grafted clinoptilolite, ZSM-5, vermiculite and kaolinite samples were investigated for their ability to sequester caesium, strontium, uranium and plutonium species. Absorption/desorption experiments were initially conducted on the ungrafted compounds using caesium and strontium cations to establish a baseline for the most abundant species in pond liquor. Caesium and strontium cations are trapped within the cages of clinoptilolite in an ion exchange reaction which places these ions on sites of the appropriate size and geometry for these species. Once ion exchanged, neither ion is desorbed from the clinoptilolite readily and they remain trapped inside the cage, irrespective of concentration of other species in solution. While vermiculite is also successful at removing both caesium and strontium cations, it is less successful in retaining them during desorption, with typically one third of the trapped ions being released back into solution. ZSM-5 and kaolinite have contrasting behaviour, due to few exchangeable cations, meaning caesium and strontium uptake is poor. Samples of clinoptilolite with different particle sizes and potassium content were compared and showed that smaller particles facilitated faster exchange. Grafting had a variable effect on the caesium/strontium cation uptake and release during absorption/desorption presumably as a result of the donors on grafted ligand interacting weakly with the large cations. Experiments using distilled water and synthetic liquor (carbonated sodium hydroxide), demonstrated uranium and plutonium species were extracted by all materials after grafting at concentrations in the SIXEP range. The most successful ligand/graft combination for plutonium at pH 7 was APTES grafted on clinoptilolite (10.79% carbon).


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© Hayley Sarah Gillings

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This work is made available according to the conditions of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) licence. Full details of this licence are available at: https://creativecommons.org/licenses/by-nc-nd/4.0/

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A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.


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