Studies of Metal Speciation and Nucleation in Complex Functional Liquid Systems

Abstract

This thesis describes studies of metal speciation in the context of a range of applications such as wastewater treatment, nuclear fuel separation, nanoparticle nucleation. For several systems it also explores the level of structural detail that can be obtained from full multiple scattering extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near edge structure (XANES) interpretation using electronic structure calculations. In the context of water treatment, inductively coupled plasma-optical emission spectroscopy (ICP-OES), pH monitoring and ultraviolet-visible spectroscopy (UV-Vis) were combined to examine 4-chlorophenol (4-CP) degradation over Fenton-like heterogeneous systems based on Fe and Cu oxides. Control experiments using homogeneous Fe3+ or Cu2+ systems revealed that even minute concentrations of dissolved metal ions (μM/sub-ppm) leached from the solid phases account for the observed 4-CP degradation rates, revaling that these systems are mechanistically homogeneous rather than heterogeneous. An XAFS study of Zr(IV) speciation in a three phase liquid system relevant as an analogue for nuclear fuel recycling by liquid extraction was conducted. The system contained Zr(IV), HNO3, H2O,-tributyl phosphate (TBP) and dodecane. The overall Zr(IV) stoichiometry in the aqueous HNO3 phase was detrmined as [Zr(OH)2(NO3)4.2]1.8, with Zr(NO3)4 monomers in equilibrium with OH-bridged Zr(IV) nitrate oligomers. The organic phase species was Zr(NO3)43TBP. Using quick EXAFS and energy dispersive EXAFS measurements of the liquid-liquid interfacial region of aqueous [PdCl4]2- in contact with an organic ferrocene solution, information on the Pd species formed during the early stages of interfacial nanoparticle nucleation was obtained. A stochastically fluctuating interfacial equilibrium in which Pd nuclei constantly formed and re-dissolved for >7 h was discovered. Open circuit potential measurements at the interface show that interfacial potential has an important role in controlling nucleation behaviour. Using XANES peak fitting on a series of Au L3-edge reference spectra, ionisation potentials (IP) and the energetic positions of electronic transitions to unoccupied valence states (white lines) were examined for a number of Au compounds. It was found that the oxidation state of Au does not correlate strongly with white line intensity and IPs, with the the ligand effect on electronic structure dominates over the formal oxidation state of the central metal atom. 2p3/2 ⟶ lowest unoccupied MO (LUMO) and LUMO+1 transitions were assigned to [AuCl4]- through molecular orbital (MO) and density-of-states (DOS) analysis. Based on this analysis, an automated XANES peak fitting procedure was explored, showing that simple algorithms can be built on a small number of absorption edge-specific trends in IP and white line positions, which capture the essential information in near edge spectra without requiring additional user input. One of the reference compound, tetrabutylammonium dichloroaurate(I), TBA[AuCl2] in 1,2-dichlorobenzene was found to be metastable and a reliable reference XA spectrum of [AuCl2]‾ ions was obtained by differential XAFS analysis of the solution composition, which also revealed that [AuCl2]‾ was primarily oxidised in the X-ray beam to an Au(III) species with an average formula of [AuCl3OH]‾. XANES revealed a strong solvation effect in aqueous [Au(CN)4]‾, with DOS analysis suggesting that the unoccupied MOs of [Au(CN)4]‾ were significantly perturbed by the presence of two axial H2O, resulting in an intense double white line. EXAFS and XANES separately quantified Au-OH2 distance to be ~2.9 A. EXAFS can be used to identify cis/trans stereoisomers. We determined that cis-[AuCl2(OH)2]‾ was formed when [AuCl4]‾ was gradually hydrolysed through an increase of pH, making use of the difference between EXAFS linear multiple scattering paths, i.e. Au-Cl-Cl-Au in the trans isomer vs. Au-Cl-OH-Au in the cis isomer

Date of Award	31 Dec 2016
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Roger Davey (Supervisor), Sven Schroeder (Supervisor) & Michael Greaney (Supervisor)