A Mechanistic Study of the Formation of Peroxide in Solutions Containing the Uranyl(VI) Ion

Abstract

This body of work concerns the formation of peroxide in solutions containing the uranyl(VI) ion and a ligand proposed for use in next generation nuclear fuel reprocessing technologies, CyMe4-BTPhen.1 The uranyl(VI) ion is known to readily form insoluble complexes in the presence of the peroxide anion and hence the formation of peroxide under process conditions could lead to increased rates of fouling of plant equipment.2 It is therefore of importance to determine the mechanisms by which peroxide can form in such solutions so these processes may be controlled. To this end this report initially focusses on determining the speciation of actinyl(VI) ions in solutions containing CyMe4-BTPhen in order to establish a baseline computational model against which the formation of peroxide may be probed. The synthetic research presented in this thesis predominantly focusses on the coordination chemistry of the plutonyl(VI) ion in solutions containing CyMe4-BTPhen. The analogous speciation in solutions containing the uranyl(VI) ion is covered explicitly briefly, although much of the chemistry of this specific species is known and hence is inferred from the studies of a collaborator, D.M. Whittaker.3 The limiting species formed in both solutions is a complex with 1:1 stoichiometry and this complex is observed to not dissociate readily, as determined by competition experiments in the presence of the chloride anion. With reference to kinetic studies carried out by the likes of Bakac et al.4 and Burrows et al.5 in the period between the 1970s and 1990s two mechanistic routes to form peroxide in solutions containing the uranyl(VI) ion, the ligand CyMe4-BTPhen and methanol are identified as feasible processes in RT solutions when exposed to sunlight. These mechanisms proceed via the initial formation of an excited state uranyl(VI) species which subsequently is able to abstract a hydrogen atom from the methanol solvent in order to generate a formally uranyl(V) species. This species subsequently reacts with molecular oxygen via an inner-sphere electron transfer reaction to form a solvated uranyl(VI)-superoxide complex. Following this point the favoured mechanisms diverge, where one follows a path to the peroxide bound product that proceeds via a second iteration of the inner-sphere electron transfer process whilst in the other peroxide is formed via an outer-sphere electron transfer between the aquated uranyl(V) ion a a solvated hydroperoxyl radical.

Date of Award	1 Aug 2015
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Neil Burton (Supervisor), Clint Sharrad (Supervisor) & Louise Natrajan (Supervisor)