Robin M. Sellers

The radiation chemistry of metal ions in aqueous solution

A. Introduction . . . . . . . . . . . . . . . . (i) The radiation chemistry of water . . . _ . . . (ii) Radiation chemical methods . . . _ _ . . . . (iii) Reactivity of e& OH and H with metal ions _ . . . (iv) Reactivity of the carboxyl and hydroxymethyl radicals R. Groups I and II (alkali and aikaiine earth metals) . . . . C. First row transition elements . . . . . . . . . . . (i) Scandium . . . . . . . . . . . . . . . (ii) Titanium . . . . . . . . . . . . . . . (iii) Vanadium _ _ _ . _ _ _ . . _ _ . . . . (iv) Chromium _ . . _ _ . _ . . . _ . . . (v) Manganese _ _ _ _ . . _ . . . . . . . . (vi) Iron . . _ _ . _ . _ _ . . . _ _ . . . D. Second and third :ow transition elements _ . . . . . . (i) Zirconium, niobium and hafnium . . . _ . . . (ii) Molybdenum . . . . . _ . _ . _ _ . . . (iii) Ruthenium _ _ . _ _ _ _ . . . . _ _ . _ (iv) Pafladium _ . . . _ . _ _ . _ _ _ . . . fv) Rhodium . _ _ . . _ _ _ _ _ _ _ _ . . (vi) Silver . . . . . . _ . . _ . . _ . . . (vii) Osmium. _ . _ . . . _ . _ _ _ _ . _ . (viii) Iridium . . . . . . . . . . . _ . . . . (ix) Platinum . . . . . _ _ . _ . . _ . . . (x)Goid _ _ _ _ _ . . _ _ . _ _ _ _ . . . (xi) Mercury _ _ _ . . _ _ _ . _ _ . _ . . E. Lanthanides _ _ . _ _ _ _ _ . _ . _ _ . . . (i) Reduction of the trivalent ions _ _ . _ _ . . . (ii)Cerium _ _ _ . . . . . . . . _ _ _ . . (iii) Praesodymium . . . . _ . . . . _ . . . F. Actinides (i) Gene& rem&s : : : : : : : : : : : : : (ii) Thorium . . . . . . . . . . . . . . . (iii) Uranium _ _ _ . _ _ _ . . . . . . . . . . . . . . . 196 . . . . . . _ * . . . * . . . . . 1 -

Kinetics of metal oxide dissolution. Reductive dissolution of nickel ferrite by tris(picolinato)vanadium(II)

A detailed study of the reductive dissolution of NiFe2O4 by V(pic)–3 is reported. The kinetics of the reaction follow a cubic rate law and exhibit a first-order dependence on [VII], indicating that the rate-determining step involves attack by V(pic)–3 on FeIII ions at the oxide surface. Dependences on [H+] and [free picolinate] are attributed to adsorption of these species at surface sites according to a simple model based on the Langmuir adsorption isotherm. Buffer and surfactant concentration had little effect. Some pitting etc. of the oxide surface occurs as the dissolution proceeds, although there appears also to be some general surface attack. The nature of the surface sites and subsequent steps in the dissolution process are discussed.

Radiation-induced redox reactions of iodine species in aqueous solution. Formation and characterisation of III, IIV, IVI and IVIII, the stability of hypoiodous acid and the chemistry of the interconversion of iodide and iodate

The unstable oxidation states of iodine have been investigated using pulse radiolysis.IIV is generated by one-electron reduction of IO–3 with e–eq, CO–2 and (CH3)2CO–; in the pH range 3–14 it exists as HIO–3 and IO2–3 with pKa= 13.3. IIV reacts rapidly with itself, (CH3)2ĊOH and CH2(CH3)2COH.IVI is generated by one-electron oxidation of IO3– or one-electron reduction of IVII. At pH 13 both methods produce species having similar spectra (λmax= 350 nm) where the oxidant is O–. At pH < 9 the spectrum of IVI formed by reduction is little changed, but that produced by oxidation of IO–3 by OH is extremely weak, showing that these species are dissimilar. In the presence of IVII, IVI decays by first-order kinetics at pH 6.3–9 and oxidises IVII to IVIII; at pH 13 it decays by second-order kinetics and does not oxidise IVII. At pH < 9 IVI also decays to form OH in an acid-catalysed reaction. A mechanism is proposed which accounts for these observations.IVIII results from oxidation of IVII by OH and IVI. Optical data show that the absorbing species is the dinuclear iodine species IVIIIVIII with λmax= 525 nm. The kinetics of formation of IVIII are pseudo-first order but show a complex dependence on [IVII]. A mechanism is presented which describes the data.The spectrum of IO is obtained by oxidation of IO– with O– at pH 13. It absorbs with λmax= 490 nm and Iµmax= 2.1 × 103 dm3 mol–1 cm–1; it decays by reaction with itself and I2– at diffusion-controlled rates.Hypoiodous acid is shown to be a significant product of the radiolysis of unbuffered dilute solutions of I– saturated with N2O. Under these conditions HOI decays slowly by second-order kinetics with k= 5.5 ± 1 dm3 mol–1 s–1. It is much less stable in solutions containing OH– or borate, where k=(5.6±1.2)+ 104[OH–] dm3 mol–1 s–1 and (7.2±3.3)+ 2.2 × 103[borate] dm3 mol–1 s–1, respectively.

Pulse radiolysis study of monovalent cadmium, cobalt, nickel and zinc in aqueous solution. Part 2.—Reactions of the monovalent ions

Reactions of the monovalent metal ions Cd+, Co+, Ni+ and Zn+ with electron acceptors in neutral aqueous solution have been studied and three types identified: (a) addition M++ O2→ MO+2(M = Ni) M++ olefin →(M—olefin)+(b) oxygen atom transfer M++ N2O → MO++ N2(c) electron transfer M++ S2O2–8→ M2++ SO–4+ SO2–4. In reaction (b) when M= Co, long lived CoIII species are produced.Evidence is also presented for the oxidation of Co2+ by OH (k= 8 × 105 dm3 mol–1 s–1) and by SO–4(k∼ 2 × 106 dm3 mol–1 s–1) in neutral solution.

The radiation chemistry of nuclear reactor decontaminating reagents

Abstract Processes involved in the radiation chemistry of some typical nuclear reactor decontaminating reagents including complexing, reducing and oxidising agents are described. It is concluded that radiation-induced decomposition is only likely to be a problem with dilute formulations, and/or with minor additives such as corrosion inhibitors which are not protected from attack by the other constituents. Addition of a “sacrificial” compound may be necessary to overcome this. The importance of considering loss of function, rather than the decomposition rate of the starting material, is emphasised. Reagents based on low oxidation state metal ions (LOMI) can be regenerated by the radiation field in the presence of formate ion.

Oxidation of copper(I)–olefin complexes in aqueous solution by oxygen and hydrogen peroxide

The absorption spectra and formation constants of the complexes of CuI with ethylene, isobutene, acrylamide. and allyl alcohol have been measured in aqueous solution. The oxidation of the allyl alcohol and ethylene complexes by O2 and H2O2 has been investigated. In the former case H2O2 is a major product, and in the latter epoxide is formed. Reaction mechanisms which account for the observed kinetics and stoicheiometries are discussed.

The effect of the low-oxidation-state metal ion reagent tris-picolinatovanadium(II) formate on the surface morphology and composition of crystalline iron oxides

The LOMI (low-oxidation-state metal ion) process is used widely in the decontamination of water reactor systems. An attempt has been made to elucidate the mechanism of dissolution by reference to a number of single crystals of iron oxides polished on specific crystallographic planes, e.g. magnetite, Franklinite and haematite. These have been studied by a combination of electron-spectroscopic techniques such as X-ray photoelectron spectroscopy, Auger electron spectroscopy and Mossbauer spectroscopy. Electron microscopy and chemical kinetic measurements have also been undertaken.

Pulse radiolysis study of monovalent cadmium, cobalt, nickel and zinc in aqueous solution. Part 1.—Formation and decay of the monovalent ions

The spectra and extinction coefficients of the hyper-reduced metal ions Cd+, Co+, Ni+ and Zn+ produced in the pulse radiolysis of aqueous solutions of the corresponding divalent ions (M2+) have been measured, and the reactions of these species have been investigated in the absence of oxidising solutes. In the absence of OH scavengers M+(except Ni+) decays by second order kinetics at approximately the diffusion controlled rate, and the decay is attributed to reoxidation of M+ by OH. Evidence for the oxidation of Co2+ by OH is presented. In the presence of formate ion, methanol, isopropanol or t-butanol as OH scavengers the decay of M+ is slower, varies with the scavenger used and is only approximately second order. Spectra of long lived products are presented. It is suggested that M+ can react with itself and with the radical formed from the organic solute.

Radiation chemistry of dilute aqueous solutions of thallous ion. Formation of colloidal thallium and its catalysis of the reduction of water by (CH3)2ĊOH and CH3ĊHOH radicals

In the absence of O2 relatively stable (several hours to several days) thallium metal colloids are formed when dilute aqueous solutions of thallous ion ([Tl+]0≈ 1.2 × 10–4 mol dm–3) are irradiated (dose rate ≈ 10 Gy min–1) under reducing conditions (10–1 mol dm–3 propan-2-ol or ethanol) in the pH range 6–12 in the presence of 10–3 mol dm–3 surfactant (sodium dodecyl sulphate or Triton-X-100). The colloid is also stable at pH 3, but it cannot be formed at this pH because H3O+ competes with Tl+ for e–aq. Once nucleation has occurred, Tl+ is reduced by (CH3)2ĊOH and CH3ĊHOH at the particle surface. Electrophoresis measurements showed that the particles are negatively charged, and kinetic analysis indicated that their mean diameter ranges from 30 nm at pH 3.4 to 24 nm at pH 11.8 under the experimental conditions specified above. Increasing the dose rate or [Tl+] resulted in smaller particles being formed.Colloidal thallium catalyses the reduction of water by (CH3)2ĊOH and (CH3)ĊHOH, and also catalyses the disproportionation of these radicals. A mechanism is proposed for these processes in which the rate-determining step for hydrogen production is the discharge of H3O+(low pH) or H2O (pH 7) at the metal surface, and radical disproportionation involving electron transfer to and from the particle. The rate constant for the discharge of H2O is estimated to be 820 s–1.

Radiation chemistry of colloidal haematite and magnetite in water. Reductive dissolution by (CH3)2ĊOH radicals and FeIIEDTA

The radiation-induced dissolution of colloidal α-Fe2O3 and Fe3O4 has been investigated in aqueous solution at ca. pH 2. Reductive dissolution is achieved with (CH3)2ĊOH radicals and FeIIEDTA by electron transfer to FeIII in the crystal lattice followed by dissolution of the product FeII. (CH3)2ĊOH dissolves α-Fe2O3 at the diffusion-controlled rate under the conditions used, but the fraction of the radicals that reach the particles is limited by competing radical–radical reactions. The rate of dissolution of FeIII in Fe3O4 is diffusion-controlled at low particle concentrations, but becomes slower at high particle concentrations. It is suggested that FeII in the original Fe3O4 lattice does not dissolve at the same rate as FeII resulting from reduction of FeIII.FeIIEDTA transfers an electron to FeIII in both colloidal oxides at a rate some 103-fold slower than diffusion-controlled, but a high rate of radiation-induced dissolution is achieved by reducing the product FeIIIEDTA with (CH3)2ĊOH. When excess EDTA is present to complex the dissolved Fe2+, the rate of dissolution increases in an autocatalytic manner as the irradiation proceeds. This increase is more marked for Fe3O4 and is also observed in the thermal dissolution of this oxide by FeIIEDTA because of the contribution from FeII in the lattice. It is also found that FeII is released more slowly than reduced FeIII from the Fe3O4 lattice during thermal dissolution by FeIIEDTA.The radiation chemistry of colloidal α-Fe2O3 solutions containing Fe3+ and Fe2+ has been used to probe the location of these ions. Results show that the ions are concentrated in the electrical double layer of the untreated sol prepared from FeCl3 in dilute HCl, but not when the colloid is washed and redispersed in dilute HClO4.