Metal-organic framework composites for nuclear waste clean-up

Abstract

Radioactive pertechnetate (TcO₄^-) has been proven a troublesome fission product from the spent fuel of nuclear reactors. Current reprocessing is not effective at removing TcO₄^- from effluent resulting in discharges of the anion into watercourses through both authorised and accidental incidents. Metal-organic frameworks (MOFs) have been proposed as candidate adsorbents for TcO₄^- as a result of their high porosity and the ability to fine tune their properties towards particular applications. The processing of these materials into application specific configurations, such as composites, can enhance the physical properties and aid in recycling of the adsorbent. This project will involve the synthesis of both novel and existing MOFs followed by the processing of these materials into composites that can selectively adsorb TcO₄^-. We opt wherever possible to use benign reagents, and environmentally sustainable techniques for the synthesis of these materials. As such, we close this thesis by presenting our work in sustainable MOF syntheses. Chapter one introduces the key areas of this project. We discuss the nuclear industry and the need for nuclear waste remediating materials. Metal-organic frameworks and their composites are discussed as well as key literature. We close this chapter by highlighting current materials for pertechnetate remediation and lay out the project aims. In chapter two we present our work into ReO₄^- remediation, a nonradioactive surrogate of pertechnetate, with the MOF UiO-66-NH₂ ¬([Zr₆O₄(OH)₄(BDC-NH₂)₆]) (BDC-NH₂ =aminoterephthalic acid). The MOF undergoes post-synthetic modification (PSM)with excess ethyl isocyanate to generate the MOF UiO-66-(NH₂)_1.8(NHC(O)NHC₂H₅)_4.2. UiO-66-NH₂ is configured into two bio composites, alginate hydrogel beads and cellulose filter papers. The effectiveness of the PSM material and the bio-composites for perrhenate remediation is assessed. We turn our attention to crystal engineering of coordination polymers and MOFs in chapter three. Crystal engineering is applied to investigate the effect that structural isomerism of the organic linker N,N′-bis (pyridylmethyl)urea and the counter anion present in silver cationic coordination polymers plays on the resulting crystal structure and topology. Initial studies suggested that the ortho structural isomer could selectively crystallise ReO₄ ^- over competing anions such as nitrate and sulphate however this was ultimately found to not be the case. Silver coordination polymer stability in the presence of application concentration levels of chloride was found to be not sufficient to warrant ReO₄ ^- remediation studies. Three new zinc-based materials are also synthesised and characterised in chapter three. The urea-based organic linkers4,4′-(carbonylbis (azanediyl)) dibenzoic acid and N,N´-bis(4-pyridylmethyl)urea are used in these materials, two of which are mixed linker systems. This chapter presents a total of 14 crystal structures,11 of which have not been previously reported. In chapter four we investigate sustainable syntheses of calcium-based MOFs. Recycled chicken eggshells as well as polyethylene terephthalate (PET)bottles are used as sustainable sources of calcium and terephthalic acid respectively, for a one-pot synthesis of[Ca(BDC)(H₂O)₃]. Furthermore, eggshells were used to synthesise the MOFs[Ca(SQ)(H₂O)] and [Ca(FU)(H₂O)₃]. Mechanochemistry is applied to provided largely solventless and sustainable syntheses for the three listed MOFs. 

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