Electrochemistry at liquid/liquid interfaces for metal ion extraction
The aim of this project was to produce a selective extraction system for the separation of precious metal ions from waste electrolyte streams utilising liquid/liquid electrochemical techniques. During the course of this project the work partitioned into three different aspects of charge transfer: electron transfer, ion transfer and membrane studies.
Some fundamental physicochemical properties of liquid/liquid systems were explored. For electron transfer reactions the dependence of the rate of electron transfer on supporting electrolyte concentration in the organic phase for the water/1,2-DCE interface was measured using the technique of interfacial electrochemical impedance spectroscopy. No dependence of the rate of electron transfer on the concentration of the supporting electrolyte in the organic phase was observed. The effect of the solvent on the kinetics of electron transfer has also been examined. The results show that the rate does not follow the expected Marcusian dependence. Several solvents previously unused for liquid/liquid studies have been screened; one solvent in particular, 1,2-difluorobenzene, shows great promise for future work.
The ion transfer studies show that platinates can be separated under potential control across the water/1,2-DCE and the water/methyl isobutyl ketone interfaces. Ion transfer and separation of chloroaurate and chloroplatinates have been discussed on the basis of the Galvani potential of ion transfer. Evidence of ion pairing between the platinates and the organic supporting electrolyte cation was found. The thermodynamics of the platinate transfer have also been explored and, in collaboration with Dr Robert Deeth, preliminary density functional theory calculations have been performed to explain the experimental results in terms of a model accounting for the surface charge density of the platinates. This goes further than the classical Born approach of treating the ion as a hard sphere of charge in a dielectric continuum.
The ion transfer across membranes has been studied and the results were applied to single and dual membrane supported liquid/liquid interfaces for separating platinates from base metals. [PtCl6]2- transfer across a supported liquid membrane was demonstrated. The scale–up experiments were not successful, but if certain points can be addressed, it may be possible for future studies to build on what has been achieved here.
Additional to the three traditional sections of liquid/liquid work, a method of preparing gold nanoparticles has been found using ketones as the reducing agent at the water/ketone interface. The formation of gold mirrors has been observed and nanoparticles have been isolated and characterised.