SummarySimple hydroxamic acids such as formo- and aceto-hydroxamic acids have been proposed as suitable reagents for the separation of either Pu and/or Np from U in modified or single cycle Purex based solvent extraction processes designed to meet the emerging requirements of advanced fuel cycles. The stability of these hydroxamic acids is dominated by their decomposition through acid hydrolysis. Kinetic studies of the acid hydrolysis of formo- and aceto-hydroxamic acids are reported in the absence and the presence of Pu(IV) ions. The slow reduction of these plutonium(IV) hydroxamate complexes to Pu(III) aquo-ions has been characterised by spectrophotometry and cyclic voltammetry. The reductions of Pu(IV) in the presence of FHA and AHA are consistent with a mechanism in which free hydroxamic acid in solution is hydrolysed whilst Pu(IV) ions remain fully complexed to hydroxamate ligands; then at some point close to a 1 : 1 Pu(IV) : XHA ratio, some free Pu

The design of a novel photoelectrochemical sensor, the micro-optical ring electrode (MORE), is described. Based on a thin-ring microelectrode and using a fibre-optic light guide as the insulating material interior to the ring, the MORE is capable of delivering light directly to the region of electrochemical measurement and can therefore be used to conduct microelectrochemical studies of systems with complex photochemistry. A novel fabrication procedure is described, involving the coating of commercially available fibre optics (radius 1.25 X 10-4 m) with a 600 nm layer of gold, so allowing exploitation of the electroanalytical advantages peculiar to thin-ring microelectrodes. The dark electrochemistry of the thin-ring microelectrode is characterized by use of cyclic voltammetry and chronoamperometry and found to agree with previously published theoretical results. Preliminary exploration of the photoelectrochemical response of the MORE is reported, achieved via the interrogation of the photoelectrochemically active phenothiazine dye methylene blue (MB+). Photocurrent signals obtained during cyclic voltammetric and chronoamperometric studies of MB+, conducted with the MORE under illuminated conditions and in the absence of any deliberately added reducing agent, are attributed to the formation and subsequent detection of 3MB+ within the diffusion layer of the microring electrode. The data demonstrate that the use of the MORE for the direct electrochemical detection of photogenerated species with lifetimes of c 9 x 10-5 s is possible. The electrochemistry of 3MB+ over the applied potential range from -0.4 to +1.0 V versus SCE is elucidated and discussed in the context of the behaviour of photoexcited MB+ in the presence of the deliberately added reducing agent Fe2+.

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Transparent mesoporous TiO2 (M-TiO2) thin films were prepared on quartz via a reverse micelle, sol-gel, spin-coating technique. Films were characterized by atomic force microscopy (AFM) and Raman and UV-vis spectroscopies and were found to be mostly anatase with low surface roughness (Rt approximately 5 nm). The time dependence of film photoinduced superhydrophilicity (PISH) was measured by observation of the spreading of a sessile water drop using a new, continuous measurement technique wherein the drop was first applied to the semiconductor surface and then was filmed while it and the underlying substrate were illuminated by 315 nm ultraband gap light. Results obtained at 100% relative humidity (RH) at 293 K showed that drops on M-TiO2 surfaces exhibited a photoinduced "stick-slip" behavior, the first time such an effect has been observed. The thermodynamic driving force for this photoinduced stick-slip was the departure of the system from capillary equilibrium as, with increasing illumination time, the concentration of surface Ti-OH groups increased and the equilibrium contact angle of the drop, theta0, decreased. A simple theoretical description of photoinduced stick-slip is derived and is used to calculate a value of the potential energy barrier associated with surface inhomogeneities that oppose onset of movement of the triple line, U = 6.63 x 10(-6) J m(-1). This is the first time that U has been quantified for a surface with photoinduced superhydrophilicity. Triple line retreat measurements on an evaporating drop on M-TiO2 in the dark, RH = 60%, T = 293 K, gave a value of U = 9.4 x 10(-6) J m(-1), indicating that U decreases upon UV illumination and that U in the light is primarily associated with inhomogeneities that are unaffected by an increase in the surface Ti-OH population, such as the physical roughness of the surface. In the dark evaporation experiment, the drop was found to retreat with an areal velocity of 1.48 x 10(-8) m2 s(-1). However, under UV illumination, the drop was found to spread at a substantially faster velocity of 2.33 x 10(-5) m2 s(-1), the latter being of the order of the velocities of 10(-4) m2 s(-1) observed in (dark) drop-spreading experiments conducted in the presence of trisiloxane surfactant superspreaders. This suggests that, once slip has started, the triple line processes over a thin precursor film of condensed water whose formation has been promoted by the photoinduced increase in the Ti-OH population at the semiconductor surface.

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The photocatalytic degradation of a herbicide derivative, glyphosate [(N-phosphonomethyl) glycine] has been investigated in aqueous suspensions of titanium dioxide at different pH values. This compound was found to degrade more efficiently under alkaline pH, where no adsorption takes place on the surface of the catalyst in the dark. The main degradation route involves the cleavage of the P-C bond giving rise to sarcosine and glycine as the intermediate products formed during the photooxidation process.

Hydroxamic acids are salt free, organic compounds with affinities for cations such as Fe 3+ , Np 4+ and Pu 4+ and have been identified as suitable reagents for the control of Pu and Np in advanced nuclear fuel reprocessing. The results of a UV-visible-near IR spectrophotometric study of the 1:1 and 2:1 complexes formed between formo-and aceto-hydroxamic acids (FHA, AHA) and Np(IV) ions are interpreted using speciation diagrams for the identification of the species present at diferent pH and ligand to metal ratios. A kinetic model that describes the instability of the complex due to the hydrolysis of the hydroxamate moeity, previously developed for the Fe(III)-AHA complexes [1], is tested here against experimental Np(IV)-FHA data. Consequently, the complexation constant for formation of the 1:1 Np(IV)-FHA complex in nitric acid is estimated at K 1 = 2715, and indications are that complexation protects the ligand against hydrolysis at 0.1 > pH >-0.1.