Semiconductor-electrochemical aspects of bacterial leaching. I. Oxidation of metal sulphides with large energy gaps

Thiobacillus ferrooxidans has been cultivated successfully on synthetic metal sulphides with large energy gaps (CdS, ZnS) as the only energy source for many culture generations during a period of 4 years. The results obtained, which were quantitatively evaluated by calculations of electron transfer probabilities, show that a direct electron transfer from the metal sulphide valence band to the bacterial metabolic system (hole injection into the valence band of the sulphide) has to be excluded as the cause for the enhanced oxidative dissolution for energetic reasons. A detailed analysis of the dynamics of the metal sulphide aqueous electrolyte interface reveals that protons are involved in the mechanism on which bacterial activity is based. By reacting chemically with the metal sulphide surface they break chemical bonds and shift electronic states energetically into the forbidden energy gap to produce surface states which can be chemically described as ?SH? groups. These control the rate of dissolution of the metal sulphide and are removed by bacterial activity. In this way the proton is recycled and its action can be considered catalytic. For sulphides in which the valence band of the semiconductor is derived from metal orbitals instead of from sulphur orbitals, this mechanism is bound to fail. MoS₂ and WS₂ are discussed as examples of such metal sulphides which are not a suitable energy source for bacteria. Some kinetic aspects of the bacterial surface reaction are discussed.     

Metal sulfide semiconductor electrochemical mechanisms induced by bacterial activity

Various bacterial species (e.g. Thiobacilli, Leptospirillum) have adapted to utilize solid inorganic semiconducting sulfides (e.g. FeS₂, ZnS, CuFeS₂) as their energy source for carbon dioxide fixation. The interfacial electrochemical mechanisms which they apply have practical relevance for bioleaching of minerals, acid mine pollution, for the biocorrosion of steel, and for solar powered chemosynthesis based on the bacterial energy cycle. The bacterial attack on the sulfide surface is based on the use of recyclable chemical species (H⁺, Fe²⁺, thiol-compound) which disrupt chemical bonds in the sulfide interface and thereby induce disintegration. Model experiments performed with 100-nm thin synthetic FeS₂ layers allowed a detailed study of the interaction between bacterial cells and the pyrite interface. Thiobacillus ferrooxidans uses an organic polysaccharide layer to extract sulfur in the form of colloids via a cysteine-based carrier molecule. The mechanism which leads to corrosion pit formation of bacterial size could be analyzed in great detail. Addition of a surface active agent was found to induce increasingly localized leaching activity of bacterial cells. Addition of cysteine stimulates bacterial activity and acts as a sulfide dissolving agent even in the absence of bacteria. Mechanisms to block bacterial attack were identified on the basis of thiol chemistry. Leptospirillum ferrooxidans was found to apply a different strategy. It can only exist on Fe²⁺ oxidation and dissolves FeS₂ by pushing — via a very positive Fe^2+/3+ redox potential, generated within the organic capsula — the semiconductor towards the electrochemical dissolution potential. The FeS₂ interface thus disintegrates into small fragments from which free energy of electrons for Fe³⁺ reduction is utilized. Two isostructural materials with analogous electronic structure, FeS₂ which serves as energy source for T. ferrooxidans and RuS₂ which cannot at all be oxidized, are compared in detail to understand the molecular aspects of bacteria-induced semiconductor electrochemistry.     

Nitrate and nitrite reduction of a sulphide-rich environment

A sulphide-rich anaerobic sludge acclimated with a molasses wastewater was used to carry out studies on nitrate and nitrite reductions in continuously stirred batch reactors. It was shown that a COD/N-NO_x ratio as high as 65·6 mg mg⁻¹ did not promote dissimilatory reduction of nitrogen oxides to ammonia. Denitrification was characterized by a probable accumulation of gaseous intermediates, nitric oxide (NO) and nitrous oxide (N₂O), by sulphide consumption with concomitant elemental sulphur production and by an increase of the redox potential. In addition, sulphate reducers were completely inhibited by nitrogenous oxides. Cultures performed without any carbon source proved that denitrifiers were able to use sulphides as electron donors. Furthermore, while a lag phase preceded nitrate denitrification, nitrite was consumed immediately. Chemical reduction of nitrite by ferrous iron (Fe²⁺) was considered to be responsible for this difference. Evidence of such a chemodenitrification has been presented by using a sterilized sludge which kept its ability to reduce nitrite while it lost its capacity to use nitrate. Moreover, this chemical activity was favoured by Fe²⁺ addition. Finally, it has been suggested that during the cultures performed with non-sterilized sludge, a biological reduction of the ferric ions (Fe³⁺) would be coupled to nitrite chemodenitrification and would allow a regeneration of Fe²⁺. © 1998 SCI     

Studies on chemically precipitated Mn(IV) oxides. IV. Effect of dopants/impurities on the discharge behaviour of chemical manganese dioxide in alkaline medium and the applicability of Atlung-Jacobsen model to the (MnO2)1-r(MnOOH)r system

Manganese (IV) oxides are synthesized by chlorate oxidation of Mn(II) salts containing varying amounts of Fe³⁺ ions. An Mn(IV) oxide containing MoO₃ is also synthesized. All the oxides are characterized by chemical analyses, X-ray, infrared and magnetic susceptibility studies. It is observed that increasing the amount of Fe³⁺ in the Mn(II) solution favours the formation of the gamma-phase. The catalytic and electrochemical activity of the dioxides are evaluated and discussedvis-à-vis their structural factors. The structure and activity are further discussed in the light of the Atlung-Jacobsen model and the recent electrochemical reduction mechanism of Maskell, Shaw and Tye.     

Oxydation anodique des ions Mn2+ et Fe2+ en milieu marin reconstitue

The anodic oxidation of Mn²⁺ and Fe²⁺ ions in a reconstituted marine solution has been studied under atmospheric pressure and 500 atm. The study concerning the Mn²⁺ ion showed that the electrode process is a monoelectronic step which is followed by a chemical reaction of disproportionation; the obtained deposit corresponds to manganese dioxide of variety γ. For the electrochemical oxidation of Fe²⁺ ion, a passivation reaction has been observed by stationnary voltammetry; a qualitative study by cyclic voltammetry showed that the slow monoelectronic step is followed by a slow chemical reaction in solution which has no influence on the transfer of electron exchange. The resulting compound is a hydrated iron (III) sulphate.     

Effect of Fe3+ and F− on black micro-arc oxidation ceramic coating of magnesium alloy

This study focuses on a black micro-arc oxidation ceramic coating prepared on the surface of magnesium alloy by the technology of micro-arc oxidation in the electrolyte containing F^– and Fe³⁺ as well as its mechanism of F^– and Fe³⁺. It needs coatings to experience detail analyses on their thickness, roughness, corrosion resistance, thermal control property, valence states of elements, phase composition, and morphology of coatings, respectively, through coating thickness gauge, roughness tester, electrochemical workstation, AE radiometer, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS). Results showed that with the help of F^– in electrolyte, Fe³⁺ can be complexed and MgF₂ can be obtained in the coating, which reduces the pores on the surface of micro-arc oxidation coating. In addition, Fe³⁺ in the electrolyte contributes to the preparation of Fe₂O₃ and Fe₃O₄ in the coating, which can blacken the surface of the coating. Both F^– and Fe³⁺ benefit to improve the corrosion resistance and thermal control performance of micro-arc oxidation coating. There is higher iron oxide in the outer layer but higher fluoride in the inner layer of the coating.     

In Situ Soft X‐ray Microscopy Study of Fe Interconnect Corrosion in Ionic Liquid‐Based Nano‐PEMFC Half‐Cells

This in situ soft X‐ray scanning microscopy electrochemical study of model proton exchange cathodic and anodic nano‐fuel cells is exploring the evolving structure and chemical composition of key cell components represented by Au and Fe electrodes in contact with Nafion‐ionic liquid composite electrolyte containing Pt black catalyst particles. Morphological and chemical changes of the electrodes as well as the chemical state and fate of the Fe species released into the electrolyte are monitored in short circuit and with applied cathodic or anodic polarization. The in situ X‐ray absorption images of the cathodic cell fed with 2.5 × 10^–5 mbar O₂ have revealed corrosion‐induced morphology changes in the Fe electrode, being more pronounced in the vicinity of Pt‐black particles, and deposition of the Fe species released into the electrolyte, onto the intact Au counter electrode upon cathodic polarization. The Fe electrodes of the anodic cell containing NaBH₄ in the electrolyte appear relatively more corrosion resistant. The Fe L₃ absorption spectra taken in different locations within the Fe electrode have shown lateral variations in the relative ratio between Fe²⁺ and Fe^3&4+ oxidation states, whereas the Fe species released into the RTIL electrolyte are only in the high Fe^3&4+ oxidation states.     

Electrochemical Deposition of Highly Dispersed Palladium Nanoparticles on Nafion‐Graphene Film in Presence of Ferrous Ions for Ethanol Electrooxidation

An efficient method was developed to produce highly dispersed Pd nano particles (NPs), supported on Nafion‐graphene film by electrochemical deposition at constant potential in presence of ferrous ions. The Fe²⁺ ions govern the size, shape and morphology of Pd NPs. The as‐prepared catalyst was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X‐ray diffraction (XRD). It was obeserved from TEM that the mean diameter of electrodeposited Pd NPs was 6.4 ± 1.3 nm with narrow diameter range from 4 to 10 nm. The electrocatalytic performance of the Pd NPs deposited on Nafion‐graphene (Nf‐G) catalyst was studied by cyclic voltametry (CV) and chronoamperometric measurements. The highly dispersed Pd NPs on Nf‐G film were obtained in presence of Fe²⁺ ions. This alters electrochemical active surface area and hence catalytic activity of Pd NPs. The prepared Pd/Nf‐G catalyst exhibit highest tolerance to the intermediate poisoning species (ratio I_f/I_b = 2.2). The as‐obtained catalyst shows an efficient electrocatalytic activity and good stability for ethanol oxidation in alkaline medium.     

Effects of tin doping on physicochemical and electrochemical performances of LiFe1−xSnxPO4/C (0 ≤ x ≤ 0.07) composite cathode materials

Nanocrystalline LiFe_1−xSn_xPO₄ (0 ≤ x ≤ 0.07) samples are synthesized using SnCl₄·5H₂O as dopant via an inorganic-based sol–gel method. The dependency of the physicochemical and electrochemical properties on the doping amount of tin are systemically worked out and regular changes are revealed. In the whole concentration range, the chemical valence of Fe²⁺ is not basically changed whereas tin is found in two different oxidation states, namely +2 and +4. The replacement of Fe²⁺ by supervalent Sn⁴⁺ would lead to electron compensation. Under the synergetic effects between the charge compensation and the crystal distortion, the electrical conductivities for the bulk samples first increase and then decrease with the increasing amount of Sn doping. Upon the doping amount, the apparent lithium-ion diffusion coefficient and the electrochemical performance also display the similar trends. The doping is beneficial to refine the particle size and narrow down the size distribution, however optimizing the doping amount is necessary. Compared with other samples, the sample with a doping amount of about 3 mol% delivers the highest capacities at all C-rates and exhibits the excellent rate capability due to the high electrical conductivity and the fast lithium-ion diffusion velocity.     

Kinetics of bio-oxidation of metal sulphides

The microbiological oxidation of cadmium, cobalt, nickel and zinc sulphides has been investigated using a pure strain of Thiobacillus ferrooxidans. Kinetic parameters (V_m and K) have been derived regarding the effect of initial total surface area of these substrates. A relationship is suggested to exist between the rate of sulphide oxidation and the solubility product of the respective metal sulphide.     

Cyclic voltammetric studies of methylene blue in presence of Fe3+: catalytic currents

The electrochemical reduction of methylene blue has been examined in presence of Fe³⁺. Catalytic currents generated due to a chemical reaction between leucomethylene blue and Fe³⁺ have been used to estimate the rate constant of the reaction.     

Formation of structural defects and strain in electrodegraded Fe‐doped SrTiO3 crystals due to oxygen vacancy migration

We report on our investigation of structural defect and strain formation in electrodegraded reduced and oxidized, Fe‐doped SrTiO₃ (Fe:STO) single crystals using optical second harmonic generation (SHG) and confocal Raman spectroscopy. SHG and Raman spectra reveal structural and electrochemical inhomogeneity resulting from the formation of Fe⁴⁺/oxygen ion and Fe³⁺/oxygen vacancy aggregation sites along the degraded anode and cathode interfaces, respectively. We show that mixed Fe³⁺/Fe⁴⁺ states and structural strain gradients are generated across the color fronts. These results, as well as oxygen sublattice differences between the anodic and cathodic bulk, present the color front as an interface between two dominant oxygen bonding distortions. The strain near the color front shows a strong dependence on oxygen vacancy concentration and diffusion within the crystals. Our characterization of structural and electrochemical changes due to electric field‐induced strain and oxygen vacancy migration advances knowledge of electrodegradation in perovskite‐based titanate single crystals.     

The influence of different surface conditions in the corrosion process of carbon steel in alkaline sour media

Different electrochemical methodologies were established to induce general corrosion and blistering on homogeneous and heterogeneous carbon steel surfaces similar to the corrosion damage in a catalytic oil refinery plant. In one case, the film porosity and the iron sulphide stoichiometry were modified and in other case, the surface conditions were changed with sulphur films and microblisters. Additionally, we studied the influence of 1018 carbon steel surface conditions on the corrosion process in a medium simulating the average composition of sour waters in catalytic plants of PEMEX Mexico (0.1 M (NH₄)₂S, 10 ppm CN^– as NaCN, pH 8.8). Using the impedance spectra, from 10 kHz to 0.01 Hz, it was possible to qualitatively identify the carbon steel surface condition in an alkaline sour environment and to suggest the same corrosion process steps for this system, despite different surface conditions: charge transfer resistance of steel oxidation in the metal/corrosion product film interface and Fe²⁺ ion and H° diffusion through the corrosion product film. Finally, scanning electron microscopy of a freshly polished surface showed the formation of a homogeneous film immediately after introducing the carbon steel into the sour media. The other surface changes depended on the induced corrosion process and corroborated the electrochemical impedance predictions.     

Sibunit-based catalytic materials for the deep oxidation of organic ecotoxicants in aqueous solutions. II: Wet peroxide oxidation over oxidized carbon catalysts

The influence of the surface chemical composition of carbon catalysts prepared by oxidative treatment on the basis of the Sibunit-4 carbon material of the Sibunit family on their catalytic properties in the liquid-phase oxidation of formic acid by hydrogen peroxide was studied for the first time. Pure carbon samples were found to be active in the destruction of hydrogen peroxide and the oxidation of an organic substrate, and their activity decreased with an increase in the number of carboxyl and lactone groups on the surface of a carbon catalyst. Nevertheless, the rates of such processes on carbon catalysts are lower than in the presence of even small amounts of homogeneous Fe³⁺. It was shown that carbon samples accelerate or (to the contrary) inhibit the Fe³⁺-catalyzed peroxide oxidation of an organic substance, depending on the quantitative ratio of surface carboxyl and lactone groups and Fe³⁺ ions in the reaction solution. Possible acceleration and inhibition mechanisms for peroxide oxidation on carbon catalysts are discussed. The established influence of the surface chemical properties of carbon catalysts must be taken into account in the development of catalysts and processes for the oxidative purification of industrial wastewater.     

Engineering Acidithiobacillus ferrooxidans growth media for enhanced electrochemical processing

The chemolithoautotroph Acidithiobacillus ferrooxidans has been proposed as a potential electrofuel synthetic platform, and its growth medium is engineered to increase its conductivity and energy density, thereby improving viability of the process. The ion V³⁺ is used as an indirect electron supplier together with Fe²⁺ to grow A. ferrooxidans to increase the energy density of the medium, overcoming the Fe³⁺ solubility limit. A medium containing 10 mM Fe²⁺ with 60 mM V³⁺ was able to support cell growth to a final cell concentration very similar to medium of 70 mM Fe²⁺. Integration of the biological process with an electrochemical reactor requires, for economical operation, a medium with high ionic conductivity. This is achieved by the addition of salt, and Mg²⁺ was found to be least toxic to the bacterium. A concentration of 500 mM Mg²⁺ is optimal considering constraints on bacterial growth and electrochemistry. © 2014 American Institute of Chemical Engineers AIChE J 60: 4008–4013, 2014     

Research progress in the electrochemical synthesis of ferrate(VI)

There is renewed interest in the +6 oxidation state of iron, ferrate (VI) (Fe^VIO₄²⁻), because of its potential as a benign oxidant for organic synthesis, as a chemical in developing cleaner (“greener”) technology for remediation processes, and as an alternative for environment-friendly battery cathodes. This interest has led many researchers to focus their attention on the synthesis of ferrate(VI). Of the three synthesis methods, electrochemical, wet chemical and thermal, electrochemical synthesis has received the most attention due to its ease and the high purity of the product. Moreover, electrochemical processes use an electron as a so-called clean chemical, thus avoiding the use of any harmful chemicals to oxidize iron to the +6 oxidation state. This paper reviews the development of electrochemical methods to synthesize ferrate(VI). The approaches chosen by different laboratories to overcome some of the difficulties associated with the electrochemical synthesis of ferrate(VI) are summarized. Special attention is paid to parameters such as temperature, anolyte, and anode material composition. Spectroscopic work to understand the mechanism of ferrate(VI) synthesis is included. Recent advances in two new approaches, the use of an inert electrode and molten hydroxide salts, in the synthesis of ferrate(VI) are also reviewed. Progress made in the commercialization of ferrate(VI) continuous production is briefly discussed as well.     

Anodic dissolution of n-type and p-type chalcopyrite

The anodic dissolution of n- and p-type chalcopyrite (CuFeS₂) was studied in both acidic sulphate and acidic chloride media under conditions relevant to chemical leaching, i.e., at temperatures >70° C and over the potential region 0.2–0.6 V versus SCE. Double potential pulse chronoamperometry was used to probe the surface of the chalcopyrite anodes to determine the activation currents at various applied overpotentials. Analysis of the data obtained in both systems indicated the formation of a surface layer, a solid electrolyte interphase (SEI), which slows the rate of electron transfer. The electron transfer between various redox couples, including Cu²⁺/Cu²⁺, Fe²⁺/Fe²⁺ and I ₃ ^– /I^– on n- and p-type chalcopyrite and on Pt, was compared using cyclic voltammetry. In the potential region of interest, the Fe²⁺/Fe²⁺ couple is much less reversible on chalcopyrite than are the Cu²⁺/Cu²⁺ and I ₃ ^– /I^– redox couples. Chemical leaching of chalcopyrite in the presence of various oxidants was also carried out, and the results of the chemical experiments were discussed in terms of the electrochemical properties of the systems.     

The effects of iron sulphide surface area variations on coal liquefaction

The purpose of this work was to determine the effects of surface area variations of iron sulphides on coal liquefaction. Several iron sulphides were synthesized including pyrites (FeS₂) with 46.6 wt% Fe, pyrrhotites (Fe_1?xS) with ~ 60 wt% Fe and iron-sulphur compounds of unknown composition. Surface areas of the synthetic pyrites varied from 2 to ? 10 m² g^?1, pyrrhotite surface areas were 6 and 10 m² g^?1, and the surface areas of the iron-sulphur compounds were 40 and 80 m² g^?1. These iron sulphides were tested for catalytic activity in tubing reactor runs with West Virginia Blacksville no. 2 coal and SRC-II heavy distillate. All these sulphides showed catalytic effects as compared to runs with only coal and solvent, although the effects were not as large as those obtained with a cobalt-molybdenum on alumina catalyst. Large differences in surface areas before reactor testing did not cause any significant differences in conversion. The results from an additional series of tubing reactor runs, which was carried out to determine how iron sulphide surface areas change during liquefaction, showed that the surface areas were drastically changed during the two-minute heat-up of the reactor. Robena pyrite with a surface area of 2.0 m² g^?1 and the iron-sulphur compound with a surface area of 80 m² g^?1 yielded iron sulphides with surface areas of 5.2 and 10.8 m² g^?1 after a two-minute heat-up to 425°C and subsequent one-minute quench.     

Electrochemical properties of N-doped hydrogenated amorphous carbon films fabricated by plasma-enhanced chemical vapor deposition methods

Nitrogen-doped hydrogenated amorphous carbon thin films (a-C:N:H, N-doped DLC) were synthesized with microwave-assisted plasma-enhanced chemical vapor deposition widely used for DLC coating such as the inner surface of PET bottles. The electrochemical properties of N-doped DLC surfaces that can be useful in the application as an electrochemical sensor were investigated. N-doped DLC was easily fabricated using the vapor of nitrogen contained hydrocarbon as carbon and nitrogen source. A N/C ratio of resulting N-doped DLC films was 0.08 and atomic ratio of sp³/sp²-bonded carbons was 25/75. The electrical resistivity and optical gap were 0.695 Ω cm and 0.38 eV, respectively. N-doped DLC thin film was found to be an ideal polarizable electrode material with physical stability and chemical inertness. The film has a wide working potential range over 3 V, low double-layer capacitance, and high resistance to electrochemically induced corrosion in strong acid media, which were the same level as those for boron-doped diamond (BDD). The charge transfer rates for the inorganic redox species, Fe^2+/3+ and Fe(CN)₆^4−/3− at N-doped DLC were sufficiently high. The redox reaction of Ce^2+/3+ with standard potential higher than H₂O/O₂ were observed due to the wider potential window. At N-doped DLC, the change of the kinetics of Fe(CN)₆^3−/4− by surface oxidation is different from that at BDD. The rate of Fe(CN)₆^3−/4− was not varied before and after oxidative treatment on N-doped DLC includes sp² carbons, which indicates high durability of the electrochemical activity against surface oxidation.     

An investigation of the deposition and reactions of sulphur on gold electrodes

The oxidation of sulphide species in solution has been investigated on a gold electrode in borate solutions of pH 6.8 and 9.2 and in 1 mol dm^?3 NaOH. Sub-monolayer coverage by sulphur occurs at underpotentials. At more positive potentials, multilayers of sulphur can be deposited. The reaction producing sulphur, and the reverse process, involve the formation of poly sulphides as intermediates. The predominant polysulphide intermediate is S ₂ ^2? . Oxidation of sulphide to sulphate occurs to some extent in parallel with sulphur deposition. Both reactions are inhibited by the presence of a surface layer of sulphur. At high potentials, sulphur is oxidized to sulphate.