Development of a Quartz Crystal Microbalance sensor for the detection of Technetium in groundwater

Papageorgiou, Athanasios and Andrieux, Fabrice and Evans, Nicholas (2022) Development of a Quartz Crystal Microbalance sensor for the detection of Technetium in groundwater. PhD thesis, Lancaster University.

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Abstract

Technetium-99, a pure β-emitter with Emax =294 keV, is relatively long-lived radio isotope (t1/2 =2.1×105 y) found in nuclear waste, produced by the fission of U with a yield of 6%. Under normal environmental conditions, it is mostly encountered as the pertechnetate ion, TcO4-, which is highly soluble in water and consequently mobile in natural environment. For this reason, and because it often is the first radioisotope that can be detected in contamination scenarios, its monitoring into groundwater is a statutory requirement for every nuclear license site. Current determination methods of 99Tc involve several steps, such as chemical separation from the matrix, purification and source preparation, prior to radiometric (e.g. liquid scintillation counting) or mass spectroscopic (e.g. inductively coupled plasma mass spectroscopy) determination, due to its relatively low concentration in environmental samples, even in contamination scenarios. The detection of 99Tc may take up to several days, thus making these techniques inappropriate for an emergency situation, where real-time monitoring is ideally required. Hence, in this work the development of a sensor for the real time monitoring of 99Tc in groundwater based on the Quartz Crystal Microbalance (QCM) is presented. The QCM is a piezoelectric resonator, which oscillates in a resonant frequency fs when an electric potential is applied across its body. It is capable of measuring very small changes in mass at its surface, through the change of the resonant frequency. The QCM is modified to respond exclusively to the presence of TcO4-, by application of materials reported in the literature for the effective take-up of Tc, such as TREN ligand, TEVA resin and a Ag-4,4bipyridine metal-organic framework. These materials are chemically modified in order to create effective coatings on the surface of the QCM, thus developing novel ligands and films. The structures of these novel materials are characterised by several techniques such as and NMR, IR, MS, XRD and SEM and their response and selectivity is evaluated by monitoring the changes in the resonant frequency of the QCM as a function of the concentration of ReO4 -, which is a non-radioactive chemical surrogate for TcO4-. Finally, the adsorption of ReO4- and other interferences is modelled with adsorption isotherms such as Langmuir, Freundlich and Sips. Adsorption experiments were conducted with NaReO4, NaCl, Na2SO4 and Na2CO3 and it was found that self-assembled monolayers of the TREN ligand and the active component of TEVA resin, Aliquat-336, did not yield any adsorption results, possibly because ReO4- is too light to be detected by a single monolayer. On the other hand, a multi-layered thin film of the Ag-4,4bipyridine metal-organic framework yielded adequate adsorption results; however, through the analysis with adsorption isotherms, it was found that Na+ cations where adsorbed in the thin film, rather than ReO4- anions, possibly through cation-π interactions.