Toshiaki Ohtsuka

Confirmation of a sulfur-rich layer on pyrite after oxidative dissolution by Fe(lIl) ions around pH2

Abstract The stoichiometry of pyrite dissolution by Fe(II) ions was studied in chloride media around pH 2. Pyrite was found to dissolve nonstoichiometrically during the initial tens of hours and a S-rich layer was formed on the pyrite due to preferential dissolution of iron. The major constituent of the layer was elemental S, identified by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.

The influence of the growth rate on the semiconductive properties of titanium anodic oxide films

Abstract The semiconductive property of the anodic oxide films on titanium formed by potential sweep oxidation at various sweep rates is investigated by AC impedance in 0.1 mol dm−3 sulfuric acid solution. The dielectric constant and donor density of the oxide films change with the sweep rate. The dielectric constant increases and the donor density decreases with increase of the sweep rate. The dependence may be interpreted by the degree of hydration of the film. The oxide film formed by the higher sweep rate is more hydrated. High content of the hydrating water or an OH− bridge in the film may introduce a relatively high dielectric constant and low donor density.

The role of sulfur-oxidizing bacteria Thiobacillus thiooxidans in pyrite weathering

Abstract The role of the sulfur-oxidizing bacteria Thiobacillus thiooxidans in pyrite weathering was investigated to clarify the effects of the bacteria on the dissolution behavior of pyrite and the formation of secondary minerals using Raman spectroscopy and powder X-ray diffraction (XRD) in addition to solution analysis. It was found that T. thiooxidans, when present with the iron-oxidizing bacteria Thiobacillus ferrooxidans, enhanced the dissolution of Fe and S species from pyrite, whereas T. thiooxidans alone did not oxidize pyrite. Enhancement of the consumption of elemental sulfur and regeneration of Fe(II) ions were also observed with T. thiooxidans together with T. ferrooxidans, while this did not occur with T. ferrooxidans alone. It may be assumed that the contribution of T. thiooxidans to the formation of acid mine drainage on a geochemical scale should not be overlooked, though pyrite weathering is predominantly caused by iron-oxidizing activity in T. ferrooxidans.

The dependence of the optical property of Ti anodic oxide film on its growth rate by ellipsometry

The film growth of the anodic oxide film on titanium during potential sweep has been investigated in 0.1 mol dm−3 sulfuric acid by three-parameter ellipsometry. The complex refractive index and thickness-topotential ratio of the oxide film depends on the sweep rate of potential increase: the refractive index decreases and the thickness-to-potential ratio increases with increase of the potential sweep rate. The smaller refractive index is, therefore, estimated for the oxide film more rapidly growing. The dependence of the complex refractive index on the growth rate may be explained by a relatively slow process of dehydration following the initial formation of a hydrated titanium oxide.

Effect of ferrous ion in solution on the formation of anodic oxide film on iron

The effect of Fe2+ ion in the solution on the anodic formation of the passive oxide film onto iron was investigated by the ellipsometric trace of the film growth in a borate solution at pH 8.4. In addition to a compact inner passive film, formed by initial direct oxidation, the Fe2+ ion in the solution is oxidized to build up an outer hydrous layer at the latter stage. The inner layer behaves similarly as the usual passive film, in which the film growth follows the ionic migration mechanism under high electric field. The outer layer growth may, however, be determined by a surface reaction between the film and the electrolyte solution, i.e., the growth rate is independent of potential and a linear function of the Fe2+ ion concentration in the solution.

Electrochemical quartz crystal microbalance study of polypyrrole and tungstate polyanion composite films

Abstract A composite film comprising polypyrrole (PPy) and isopoly tungstate (PW) anion was prepared by electropolymerization of pyrrole in an aqueous solution of polytungstic acid. The formation of the PPy-PW film was traced using an electrochemical quartz crystal microbalance (EQCM). The oxidation-reduction behavior of the film was also investigated by the EQCM and compared with that of PPy film prepared in acidic KCl solution. From the EQCM measurements, the growth rate of the PPy-PW film under constant current oxidation at 0.186 mA cm −2 was evaluated as 0.97 μg mC −1 , which was 2.8 times as high as that of PPy-Cl film prepared in acidic KCl solution. From emission spectroscopic analysis, the composite PPy-PW film was found to contain 51 wt.% tungsten, or 67 wt.% meta-polytungstate anion, H 2 W 12 O 40 6− The PPy-PW film behaved like a redox polymer film in which the W(V)/W(VI) couple of the doped polytungstate anion functions as a reversible redox center. The doped PW changes the ionic permeating property of the polymer film; the PPy-PW film reveals protonic permeability, although the ordinary chloride ion doped PPy has anionic permeability.

Optical characterization of polypyrrole-polytungstate anion composite films

Abstract The polypyrrole (PPy)-polytungstate anion (H2W12O406-, PW) composite film formed by electrochemical polymerization was studied by two in situ optical methods: absorption spectroscopy and Raman spectroscopy. The PPy doped by chloride anions was also studied as a comparison. The potential dependence of the absorption band and the Raman shift indicates that the PPy-Cl film reduces in turn from the bipolaron-loaded PPy to the polaron-loaded PPy, and further to the neutral PPy with decrease of potential. The PPy-PW film does not, however, reach the neutral PPy even at a relatively low potential, where the reduction of tungsten ions of W(V1) to W(V) in the PW preferentially takes place.

The composition of Fe(III) oxy-hydroxide film anodically deposited in borate solution

Abstract The anodic formation of Fe(III) oxy-hydroxide film from Fe(II) ion has been investigated in neutral borate solutions by electrochemical quartz microbalance (EQCM), ellipsometry, and Raman spectroscopy. The comparison between the film weight from the in-situ EQCM and the film thickness from the in-situ ellipsometry provides the density of the film. The density obtained is consistent with that of partially hydrated FeOOH. A ratio of the film weight to the electricity passed during the film formation also indicates the formation of the hydrated FeOOH. The Raman spectra measured under the ex-situ condition shows that the film in pH 7.50 and 8.48 borate solutions is an amorphous FeOOH, while the film in pH 6.45 borate solution includes γ-FeOOH.

The effect of the isopoly tungstate anion on the composition and conductivity of polypyrrole composite films

The polypyrrole composite films were prepared by the electropolymerization of pyrrole from the aqueous solution containing isopoly tungstate (poly-W) and the chloride anion (Cl−) at various ratios. The film formation was traced by electrochemical quartz crystal microbalance, and the tungsten concentrations in the films were measured by the inductively coupled plasma spectroscopy. The electric conductivities of the films were measured by the four-point probe method. In the ratio of poly-W concentration to Cl− concentration, ((poly-W)/Cl−)), higher than 1.26 × 10−2 in the solution the concentration of tungsten in the films showed a constant value at 51 wt.% and the chloride anion was not included in the film. The conductivities of these films showed the constant value about 40 S cm−1. In the ratio lower than (poly-W)/(Cl−) = 1.26 × 10−2, the conductivity decreased with the decrease of poly-W concentration and the composite films consisted of two layers; the inner layer initially formed contained larger amount of tungsten than that of the outer layer.

Growth of a passive film on iron in a neutral borate solution by three-parameter ellipsometry

Abstract The growth of passive film on iron has been traced during potential sweep at various sweep rates in a neutral borate solution at pH 8.4. The transient thickness increase from a stationary state at 0.0 V (vs. AgAgCl) was measured by automated ellipsometry by which the ellipsometric parameters Ψ and Δ, and the reflectivity change ΔR/R, are simultaneously obtained. The transient thickness change and the complex refractive index, N = n − jk, of the film were calculated by the three parameters. The complex refractive index of the film during the transient growth is estimated to be N = 2.18 – 0.352j for light of 632.8 nm wave length. The linear potential increase initially causes an increase of the potential difference at the film/solution interface, where the film thickness remains constant and the observed current increases gradually. In the latter stage, where the interfacial potential difference reaches a constant value, steady growth of the film thickness takes place and the current reveals an almost constant value. The interfacial potential difference is a function of the potential sweep rate, and accordingly the current at the steady film growth stage is also a function of the sweep rate. From the transient change of the film thickness and current during the potential sweep at various sweep rates, the interfacial ionic transfer reactions and the mechanism of the film growth are discussed.