The application of colloidal photocatalysis in actinide photoredox chemistry

Colloidal semiconductor particles may act as efficient photocatalysts for a range of environmentally usellil reactions such as pollution abatement and nitrogen fixation. The primary step in these reactions is the absorption of ultra-band gap energy photons by the particles, which generates electron-hole (eB,htB) pairs within the semiconductor lattice. The valence band holes/conduction band electrons may react with oxidisable/reducible species in solution or particle lattice sites. Photocatalytic treatment of metal ion species by such particles has a number of important commercial applications in precious metal recovery and in the removal of heavy elements from effluent streams. The selective manipulation of actinide metal oxidation states plays an important role in the purification/separation of target metal ions from process/waste streams during nuclear reprocessing. Thus, the technological objective of this project is to explore the use of photocatalysis in metal ion redox chemistry leading to an assessment of its potential for application within nuclear reprocessing. Consequent to this, the scientific objective of this project is to identi& factors contributive to efficient photocatalytic metal ion valence control. Nuclear reprocessing streams typically have pFls of 1 or less. Thus, the photocatalyst employed must be both stable toward acid dissolution at low p1-I and be possessed of charge carrier thermodynamics which may be exploited in actinide metal valence control experiments. Equilibrium thermodynamic calculations have indicated that Sn0 2 flilfils both conditions, and atomic spectrometric studies of acid photodissolution of Sn02 have confirmed material stability under illuminated conditions. The Ce4 VCe3 system is widely used as a non-radioactive, thermodynamic analogue of the Pu4 fPu3 system. Thus, valence control experiments have concentrated on studying the light-on transient behaviour of the Ce 4 VCe3 couple using experimental techniques such as photopotentiometry and UV-Visible spectrophotometry, the former being wholly developed within these laboratories. From these experiments, the effect of particle thermodynamics on reaction yield has been identified and the rate determining step in the valence control process elucidated. The roles played by static and dynamic quenching reactions of the photogenerated holes and electrons respectively have also been determined. A model has been developed that allows for the reliable assessment/prediction of whether efficient valence control is occurring in any one candidate system