Semiconductor-electrochemical aspects of bacterial leaching. Part 2. Survey of rate-controlling sulphide properties

Sixteen metal sulphides with widely differing solid state, chemical and electrochemical properties have been investigated with respect to their suitability as an energy source for Thiobacillus ferrooxidans. The most critical rate-determining parameter for bacterial oxidation (in the absence of any electron acceptors, e.g. Fe³⁺) was the solubility product of the sulphide. Deviations from the systematic dependence are, however, observed when a large concentration of the holes are present in the semiconducting sulphide (p-type conduction), or when holes are generated through electron extraction from the sulphide surface (e.g. by Fe³⁺). The experimental results are consistent with a mechanism in which Th. ferrooxidans utilises H⁺ and Fe³⁺ as catalytic agents which break surface bonds by chemical and electrochemical mechanisms, respectively. They are subsequently recycled while the bacterium oxidises the generated surface products, — SH^δ and S_o, etc., to sulphate. On the basis of the derived mechanism it should be possible to estimate and predict the suitability of sulphides for bacterial oxidation and to analyse the influence of crystalline quality and impurities, as well as the composition of the solution, on the rate of oxidation.     

Obtention de sulfures de cuivre non stoechiometriques,par voie electrolytique,analyse des solides par diffraction de rayons X

The anodic oxidation of cuprous sulphide has been studied in acidic medium using different methods: electrolyses of suspensions of cuprous sulphide and studies by means of a carbon paste electrode. At low current densities, the oxidation proceeds via the formation of four non stoichiometric copper sulphides: Cu_1.92S; Cu_1.77S; Cu_1.60S and Cu_1.31S.Some of these compounds have been isolated from controlled-potential electrolyses and characterized by X-ray diffraction measurements.     

Investigation of the surface oxidation of bornite by linear potential sweep voltammetry and X-ray photoelectron spectroscopy

The products of surface oxidation of a natural sample of the copper iron sulphide mineral, bornite, have been determined from analysis of linear potential sweep voltammograms and from X-ray photoelectron spectroscopic examination of the oxidized layer. Anodic oxidation of bornite in alkaline media results initially in the formation of an iron(III) oxide/hydroxide and an iron-free copper sulphide of stoichiometry Cu₅S₄. The latter species is oxidized further at higher potentials to form a copper sulphide of lower copper content and cupric hydroxide. Air oxidation involves the first of these two steps. In acid solution, anodic oxidation yields iron(II) ions rather than an iron oxide in the first stage and copper(II) ions in the second. Sulphate and elemental sulphur are not formed under the experimental conditions investigated.     

Microbial treatment of sour gases for the removal and oxidation of hydrogen sulphide

It has been demonstrated that the bacterium Thiobacillus denitrificans may be readily cultured aerobically and anaerobically in batch and continuous cultures on hydrogen sulphide (H₂S) gas under sulphide-limiting conditions. Under these conditions sulphide concentrations in the culture medium were less than 1 μM resulting in very low concentrations of H₂S in the reactor outlet gas. The stoichiometries of both the aerobic and anaerobic reactions were determined and stable reactor operation demonstrated for up to five months. Maximum loading of the biomass was determined to be 5.4–7.6 mmoles H₂S/h-g biomass under anaerobic conditions and 15.1–20.9 mmoles H₂S/h-g biomass under aerobic conditions. Indicators of reactor upset were determined and recovery from upset demonstrated. Heterotrophic contamination was shown to have negligible effect on reactor performance with respect to hydrogen sulphide oxidation. In fact, growth of T. denitrificans in the presence of floc-forming heterotrophs produced a hydrogen sulphide active floc with excellent settling characteristics. A case study of the application of this technology to the removal and oxidation of H₂S from biogas is presented.     

Monitoring and control of biogas desulphurization using oxidation reduction potential under denitrifiying conditions

BACKGROUND: Hydrogen sulphide (H₂S) present in biogas can be oxidized to elemental sulphur (S₀) or sulphate (SO₄^2?) using nitrate and nitrite. Both nitrate and nitrite are normally available in most wastewater treatment plants and could be used to oxidize H₂S depending on the molar loading ratio of wastewater and biogas. A control approach is required in order to minimize the fluctuations in inlet and outlet H₂S concentrations in biogas, and the oxidation potential of the wastewater used. RESULTS: A control scheme has been developed for biogas desulphurization using oxidation reduction potential under industrial conditions. The redox potential was maintained at about + 50 to + 100 mV in the activated sludge plant to monitor the performance of the nitrification process. The redox potential in the bioscrubber was related to sulphide removal from biogas. More than 90% of the hydrogen sulphide was removed from the biogas. CONCLUSION: The oxidation reduction potential can be used as a key parameter for monitoring and controlling biogas cleaning. Fluctuations of the inlet H₂S concentration in biogas can be compensated by manipulating the flowrates of wastewater used in order to achieve consistent and desired H₂S concentrations in treated biogas. Copyright © 2011 Society of Chemical Industry     

Contribution a l'etude des sulfures de cuivre non stoechiometriques

The anodic oxidation of cuprous sulphide has been studied using a carbon paste electrode. Typical voltammetric curves are obtained. At slow rates, the oxidation of Cu₂S in CuS and Cu²⁺ occurs via the formation of non stoichiometric compounds: Cu_1,92S, Cu_1,77S, Cu_1,60S and Cu_1,31S.     

Etude electrochimique de l'oxydation simultanee de la galene (PbS) et de l'ethylxanthate—condition de formation du xanthate de plomb en surface de la galene

The electrochemical oxidation of galena (PbS) has been investigated in the pH range 9–10, by controlled potential electrolysis using a special bed electrode. In the absence of xanthate, the oxidation of PbS leads to the formation of elemental sulphur, thiosulphate and lead species (Pb(OH)₂). Simultaneous oxidation of galena and xanthate has also been studied and the products characterized by their uv-visible spectrum. The oxidation produces lead xanthate and dixanthogen. It has been established that lead xanthate comes from an ion exchange reaction between excess xanthate in solution and lead species resulting from the lead sulphide oxidation.     

Oxidation of the sulphurised dolomite produced in the desulphurisation of the gasification gases

Dolomite reacts with H₂S to produce calcium sulphide and has been broadly investigated as a desulphurisation agent due to its low-cost and favourable properties.Because CaS reacts with water or water vapour in the environment to regenerate hydrogen sulphide and, therefore, disposal is problematic and the chemical cannot be uses as a landfill material. One of the methods used to make this material inert is oxidation to convert calcium sulphide into calcium sulphate or calcium oxide.In our study, tests were carried out using dolomite from Granada, Spain, that was previously calcined and sulphurised at high temperature with a gas similar to that produced in gasification facilities. To approximate real-scale results, a relatively large amount of substance was used for each sample (100–150 g) and the samples were used in a fixed-bed position.The influence of different conditions, such as grain size, composition of the oxidation gas, gas velocity, bed length and temperature, was them investigated. The final solid products were characterised by X-ray diffraction and chemical analysis and the CO₂, SO₂, H₂S and COS concentrations in the gases produced during oxidation were analysed by gas chromatography.The results showed that the most influential factor was grain size and that the best oxidant was O₂ mixed with nitrogen.The presence of water vapour increases the residual concentration of CaS in the end product, but increased the CaO contentThe higher the oxygen concentration and the higher the gas velocity, the lower the residual content of CaS. CO₂ used alone oxidises CaS to produce SO₂ and COS, but at very low rates. It also produces some CS₂. Water vapour used alone can also oxidise the CaS to produce H₂S and SO₂ but also at very low velocity.At higher oxidation temperature, between 700°C and 850 °C, lesser residual CaS is obtained in the oxidised product.     

Electrochemical and XPS studies of the surface oxidation of synthetic heazlewoodite (Ni3S2)

X-ray photoelectron spectroscopic (XPS) and electrochemical techniques have been applied to the investigation of the surface oxidation of synthetic heazlewoodite (Ni₃S₂). The XPS data showed that exposure of the sulphide to air resulted in nickel atoms migrating to the surface to form an overlayer of a hydrated nickel oxide and leave a sulphur-rich heazlewoodite. A hydrated nickel oxide was also produced on immersion of heazlewoodite in acetic acid solution in equilibrium with air, despite nickel being soluble under these conditions. After the acetic acid treatment, the S(2p) spectrum had a component at the binding energy of NiS and a small contribution due to sulphur-oxygen species. Voltammetry with bulk heazlewoodite electrodes, and the ground sulphide in a carbon paste electrode, indicated that, at pH 4.6, the initial anodic product was a sulphur-rich heazlewoodite and that oxidation was inhibited when NiS was formed on the surface. Further oxidation to higher nickel sulphides and elemental sulphur occurred at high potentials. In basic solutions, oxidation was restricted due to the formation of nickel oxide.     

Removal of Hydrogen Sulphide by Immobilized Thiobacillus sp. strain CH11 in a Biofilter

An autotropic Thiobacillus sp. CH11 was isolated from piggery wastewater containing hydrogen sulphide. The removal characteristics of hydrogen sulphide by Thiobacillus sp. CH11 were examined in the continuous system. The hydrogen sulphide removal capacity was elevated by the BDST (Bed Depth Service Time) method (physical adsorption) and an immobilized cell biofilter (biological conversion). The optimum pH to remove hydrogen sulphide ranged from 6 to 8. The average specific uptake rate of hydrogen sulphide was as 1·02×10⁻¹³ mol-S cell⁻¹ h⁻¹ in continuous systems. The maximum removal rate and saturation constant for hydrogen sulphide were calculated to be V_m = 30·1 mmol-S day⁻¹ (kg-dry bead)⁻¹ and K_s = 1·28 μmol dm⁻³, respectively. A criterion to design a scale-up biofilter was also studied. The maximum inlet loading in the linear region (95% removal) was 47 mmol-S day⁻¹ (kg-dry bead)⁻¹. Additionally, the biofilter exhibited high efficiency (>98·5%) in the removal of hydrogen sulphide at both low (<0·026 mg dm⁻³) and high (0·078 mg dm⁻³) concentrations. The results suggested that the Thiobacillus sp. CH11 immobilized with Ca-alginate is a potential method for the removal of hydrogen sulphide. © 1997 SCI.     

Nanofibrous carbon with herringbone structure as an effective catalyst of the H2S selective oxidation

Granular nanofibrous carbons (NFCs) with herringbone structure were synthesised by the decomposition of natural gas over Ni/Al₂O₃ catalysts, and their performance in the selective oxidation of hydrogen sulphide was studied. Samples obtained over pure Ni catalysts are the strongest mechanically and easiest to produce. However, they show low selectivity for sulphur and are unstable during operation. Boiling in nitric acid followed by annealing led to improvements in catalytic stability and a significant increase in the selectivity for sulphur in the direct oxidation of hydrogen sulphide. The addition of large amounts of water vapour to the reaction mixture dramatically improved the selectivity and stability of the NFCs.     

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.     

Interaction of galena with hydrosulphide ions under controlled potentials

The surface species produced by the anodic oxidation of hydrosulphide ions at galena surfaces have been determined by X-ray photoelectron spectroscopy. Components in the S(2p) spectrum appeared at binding energies typical of sulphur atoms in an oligosulphide. The relative intensities of these components suggested that lead oligosulphides with sulphur chain lengths of up to S₄ (or S₈ if a chelate) had been formed on the mineral surface. No evidence was found for the deposition of elemental sulphur. The oligosulphide is associated with a dispersed layer of lead sulphide formed by interaction of oxidized lead species and HS⁻. Investigations were also carried out on the surface oxidation of galena on exposure to air. Previous findings that the formation of lead hydroxide becomes evident from a shifted component in the Pb(4f) spectrum without any new sulphur environment being apparent from the S(2p) spectrum were confirmed. This behaviour is ascribed to diffusion of lead atoms from the bulk to maintain only a small metal deficiency in a lead-deficient sulphide layer formed concomitantly with lead hydroxide.     

Activated carbon catalyst for selective oxidation of hydrogen sulphide: On the influence of pore structure, surface characteristics, and catalytically-active nitrogen

The catalytic oxidation of hydrogen sulphide (H₂S) on various activated carbon materials was studied. The effects of pore structure, surface characteristics, and nitrogen content on the activity and selectivity of the carbons towards oxidation of H₂S were investigated. It was found that a high volume of both micropores and small mesopores, in combination with a relatively narrow pore size distribution, were crucial for the retention of sulphur dioxide (SO₂), a by-product of H₂S oxidation. For the retention of carbonyl sulphide (COS), another H₂S oxidation by-product, high surface reactivity with a significant amount of basic groups were found to be important. The only carbon with all these characteristics, and consequently the carbon that was able to retain both H₂S and COS for an extended period of time, was an experimental product, “WSC”. This carbon was found to be superior to the other carbons studied, exhibiting high activity and selectivity for oxidation of H₂S to sulphur. H₂S breakthrough capacities and selectivity values of the carbons were found to be dependent on the nitrogen content of the carbons. In a hydrogen stream, carbons possessing the highest nitrogen contents exhibited the greatest H₂S breakthrough capacities but, at the same time, the lowest selectivity with respect to sulphur formation. In reformate streams, the maximum breakthrough capacity and greatest selectivity were exhibited by carbons with a nitrogen content of about 1-1.5 wt%.     

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.     

On adsorption and electro-oxidation of some compounds on tungsten carbide; their effect on hydrogen electro-oxidation

Adsorption and electro-oxidation of carbon monoxide, ethylene, acetylene, and hydrogen sulphide on tungsten carbide, in solutions of these compounds in 1 N H₂SO₄, have been investigated. It was found that CO, C₂H₄, and C₂H₂ do not undergo adsorption and oxidation and do not affect adsorption and electro-oxidation of hydrogen. H₂S does not oxidise as well, and it does not displace adsorbed hydrogen in any measurable amounts, though it does inhibit electro-oxidation of molecular hydrogen. Methanol is inert on tungsten carbide like carbon monoxide and hydrocarbons. Electro-oxidation of formaldehyde and formic acid proceeds without apparent WC-surface coverage by the adsorbed compound.     

Oxidation Reactions over Multi-Component Catalysts: Low-Temperature CO Oxidation and the Total Oxidation of CH4

A series of NM/MO _x /Al₂O₃ (NM = Pd, Ag, Pt, and Au) catalysts were prepared and tested in the oxidation of CO and CH₄. The catalysts were characterized with X-ray diffraction and transmission electron microscopy. Where addition of MO _x generally does not seem to affect the catalyst activity in CH₄ oxidation, a large enhancement in CO oxidation was observed. Fourier transform infrared spectroscopy has been used to identify the role of MO _x as a promoter for low-temperature CO oxidation. The results were found to support a Mars and van Krevelen type model.     

Thermal stability of p-phenylene sulphide polymers

The thermal stability of a series of linear and branched p-phenylene sulphide polymers at 350° has been evaluated by differential thermal analysis, thermogravimetric analysis and weight-loss studies. It has been concluded that there is some indication that the thermal stability increases with the degree of branching present in the polymer and that the stability can be further improved by a preliminary heat treatment in the absence of air. The mechanism of ageing appears to involve a combination of crosslinking, chain scission and oxidation reactions.     

Composition and interfacial activity of hydrophobic sulphide-type extractants

The composition and interfacial activity of dihexyl sulphide (Elf), dodecyl methyl sulphide (two samples from Elf and Penwalt) and n-dodecyl hydroxyethyl sulphide (Elf) which are commercially available sulphide-type extractants have been investigated. Recrystallized n-dodecyl hydroxyethyl sulphide was found to be of high purity (i.e. impurities, if any, are below the detection level of a gas chromatograph equipped with an efficient capillary column), whereas it was shown that all the other sulphides contained many components which were separated and, for most of them, indentified by a GC/MS technique. Recrystallized n-dodecyl hydroxyethyl sulphide starts to adsorb at the 2 M HCI/dodecane interface at concentrations (typically 10^?3 M ) of about one order of magnitude lower than dodecyl methyl sulphide (Elf or Penwalt). Dihexyl sulphide exhibits a very weak interfacial activity and the observed decrease in the interfacial tension in the system containing about 0·4 M dihexyl sulphide cannot be attributed with certainty to its own interfacial activity.     

The activation of alumina spheres with platinum for the catalytic oxidation of organic air pollutants

The activation of alumina spheres with platinum to provide catalysts for the control of malodorous or toxic process emissions by destructive oxidation is examined with respect to the amount of platinum required, its location (determined by electron probe microanalysis) and dispersion (measured by selective chemisorption). The oxidation of dimethyl sulphide (8 volumes 10^?6 in air) or n-butanal (100 volumes 10^?6 in air) at realistic space-velocities were used as test reactions and the best spherical catalysts were compared with platinum-honeycombs.