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Abstract: This paper is focused on the in situ radiotracer and voltammetric studies of the induced HSO4 −/SO4 2− adsorption at Pt(poly) and Pt(111) surfaces in 0.1moldm−3 HClO4 solution in the course of Cr(VI) electroreduction. Besides this, the sorption behavior of HSO4 −/SO4 2− ions on bare Pt(poly) and Pt(111) electrodes is compared and discussed. From the experimental results it can be stated that: (i) although the extent of bisulfate/sulfate adsorption is strongly dependent upon the crystallographic orientation of Pt surfaces, the maximum coverage on the Pt(111) does not exceed 0.2 monolayer; (ii) the Cr(VI) electroreduction on both poly- and (111) oriented platinum proceeds via a ce (chemical-electron-transfer) mechanism to yield Pt surfaces covered with intermediate surface adlayers containing Cr(VI) particles (and reduced Cr-containing adspecies) and ‘strongly bonded’ HSO4 −/SO4 2− ions; (iii) while the coverage of platinum surfaces by the intermediate complexes formed in the course of Cr(VI) electroreduction at E >0.20V is basically independent of the crystallographic orientation of the Pt electrode, the onset for rapid Cr(VI) reduction is highly affected by the nature and crystallographic orientation of the electrode. [Copyright &y& Elsevier]

The electroreduction of Cr(VI) species to Cr(III) at noble metal electrodes in acidic solutions takes place via a ce (chemical-electron-transfer) mechanism where the heterogeneous chemical step which precedes the charge transfer involves anions (e.g. HSO4−/SO42−) leading to the formation of intermediate surface complexes. In this paper, we report some new findings obtained by in situ radiotracer and voltammetric studies of the simultaneous adsorption of anions (HSO4−/SO42− labeled with 35S, Cl− labeled with 36Cl) and Cr-containing species (labeled with 51Cr) on a gold electrode in the course of dichromate reduction in 1 mol dm−3 HClO4 supporting electrolyte. Moreover, special attention is paid to present a detection and calculation method elaborated for the quantitative evaluation of the surface excess of radiolabeled Cr species via measurement of the intensity of low energy X-rays (E=4.90 keV) emitted by 51Cr. From our experimental results it can be stated that: (i) at E<1.20 V the electroreduction of Cr(VI) particles presumably proceeds via a ce mechanism to yield a gold surface covered with an intermediate surface adlayer containing Cr(VI) species and added anions (HSO4−/SO42−, ClO4−); (ii) no data indicating the embedding of Cl− ions into the surface film could be detected; (iii) the maximum surface excess of Cr-containing species (Γmax=1.2×10−9 mol cm−2) as well as the molar ratio between the Cr species and HSO4−/SO42− ions (m&ap;6 at E=0.8 V) attests that the coverage of the gold surface with intermediate complexes does not exceed one monolayer. [Copyright &y& Elsevier]

Highlights: [•] Porous Pt layers were electroplated for radioelectrochemical measurements. [•] The theoretical relationships describing the porous electrode have been validated. [•] Surface excess of the chloride ions as a function of the potential could be measured. [•] Saturation time of the porous layers could be quantitatively estimated. [•] Mobility of the adsorbed chloride ions have been tested. [ABSTRACT FROM AUTHOR]

Highlights: [•] The basis of a new radielectrochemical method with porous electrode was described. [•] The relationship for the radiation intensity vs. surface excess function was given. [•] The parameters were optimized for the platinum black/36Cl chloride ion system. [•] The proper thickness of the infinitely thin porous layer was calculated. [ABSTRACT FROM AUTHOR]

Abstract: The so-called in situ thin gap method for the measurement of the surface excess of the radiolabelled species was originally designed for the application of noble metal electrodes with low surface roughness. Therefore, the observation of adsorbed species containing radioisotopes of high specific activity (i.e., 35S and 14C) was only possible. A recent extension of the original thin gap method to metals with a surface roughness factor higher than 10, however, made it possible to follow the adsorption of species with low specific activity (i.e., 36Cl). Based on the validation of the improved thin gap method (Horváth et al., 2013), this study aims at the comparison of the sulfate/bisulfate and chloride adsorption on both smooth (Pt and Au) and rough (platinized platinum) noble metal electrodes. It was found that the radiation signal of the adsorbed chloride ions can also be detected, and the surface excess of both radiolabelled bisulfate and chloride ions can be measured. The accumulation of the adsorbed species on rough electrodes took place at a much longer time scale than the removal of the same species. No such difference was found for smooth electrodes. The apparent asymmetry in the temporal behaviour of the adsorption and desorption processes on rough electrodes could be understood by the difference in the concentration gradient in the solution near or within the porous structures during the respective processes. [Copyright &y& Elsevier]

Abstract: The surface contamination by uranium in the primary circuit of PWR type nuclear reactors is a fairly complex problem as (i) different chemical forms (molecular, colloidal and/or disperse) of the uranium atoms can be present in the boric acid coolant, and (ii) only limited pieces of information about the extent, kinetics and mechanism of uranium accumulation on constructional materials are available in the literature. A comprehensive program has been initiated in order to gain fundamental information about the uranium accumulation onto the main constituents of the primary cooling circuit (i.e., onto austenitic stainless steel type 08X18H10T (GOSZT 5632-61) and Zr(1%Nb) alloy). In this paper, some experimental findings on the time and pH dependences of U accumulation obtained in a pilot plant model system are presented and discussed. The surface excess, oxidation state and chemical forms of uranium species sorbed on the inner surfaces of the stainless steel tubes of steam generators have been detected by radiotracer (alpha spectrometric), ICP-OES and XPS methods. In addition, the passivity, morphology and chemical composition of the oxide-layers formed on the studied surfaces of steel specimens have been analyzed by voltammetry and SEM-EDX. The experimental data imply that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. Some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 (Γ sample =1.22μgcm−2 U ≅4×10−9 molcm−2 UO2(OH)2) exceeds a monolayer coverage. [Copyright &y& Elsevier]