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.     

Oxidation of petroleum sulphides with nitrogen dioxide

• 1.1. Petroleum sulphides (obtained as concentrates from appropriate distillates) were oxidized with a mixture of air and nitrogen dioxide to sulphoxides, with yields of 75 to 80% for sulphoxide sulphur, but the process was complicated by secondary processes: the formation of up to 30% of nitrosulphoxides of variable structure; sulphones and carbonyl compounds were present as traces. The target product was separated from most impurities by an alkaline wash, followed by chromatography.
• 2.2. Oxidation of individual sulphides under the conditions adopted gave sulphoxides, which contained only traces of compounds with a nitro-group, easily removed by recrystallization or elution chromatography.

     

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.     

High-throughput syntheses of nano-scaled mixed metal sulphides

The use of metal and mixed metal sulphides as heterogeneous catalysts is often hampered by an undefined composition formed by in situ sulphidation of oxides. Controlled synthesis methods are serial and thus slow, because high-throughput (HT) methods for the well-directed syntheses of exactly defined metal sulphides and especially mixed metal sulphides are still lacking. The present contribution presents the development of HT synthesis methods for a broad range of nano-scaled sulphides for various applications, such as heterogeneous catalysts or photocatalysts. The nano-scale regime for the particle sizes offers the possibilities of high surface area, appearance of quantum size effects and the possibility of modification of their properties as well as stabilisation of the particles by surface coating. Several precursor molecules have been tested, which hydrolyse under specific conditions and release sulphide ions for metal and mixed metal sulphide precipitation. Particle size confinement was achieved by w/o microemulsions using non-ionic surfactants. In a first step for the parallel synthesis of sulphides a 9-fold reactor assembly was developed, which offers the possibility of synthesis by precipitation and separation of the resulting materials by filtration in one setup.     

Oxidation of thiols in gas-liquid systems. I. Reaction in the absence of added metal catalysts

Studies have been made of the kinetics of the reaction with oxygen of several simple aliphatic and aromatic thiols in alkaline solution and in the absence of added metal catalysts. The results have been shown to be markedly affected by trace amounts of impurities, and the kinetics of the reaction have been measured under conditions of the minimum impurity levels obtainable. Under these conditions in the early stages of reaction, the rate law with, for example, ethanethiol has been found to be: which after ≈ 10–30% of reaction changes approximately to: Values of the energies of activation and pre-exponential factors for the oxidation of ethanethiol have been determined and investigations have been made of the effect on the reaction kinetics of the efficient removal, and of the addition of trace quantities of metals.     

Functional groups in liquid polypropylene sulphides prepared with ammonia/metal carbonate initiators

Liquid polymers of propylene sulphide made in aqueous ammonia with metal carbonate initiators may have both amine and thiol end-groups. Analysis of these materials to determine the number and types of end-groups was carried out to establish the structure of the products and also to indicate which analytical methods were suitable for this task.     

Precipitation of metal sulphides using gaseous hydrogen sulphide: mathematical modelling

A mathematical model has been developed that describes the precipitation of metal sulffides in an aqueous solution containing two different heavy metal ions. The solution is assumed to consist of a well-mixed bulk and a boundary layer that is contacted with hydrogen sulphide gas. The model makes use of Higbie's penetration model to calculate the transfer of gaseous hydrogen sulphide to this boundary layer. The conditions that have been used in the simulations resemble those of industrial wastewater from a zinc factory. The model predicts the rate of H₂S absorption, the size distribution of the metal sulphide crystals and the selectivity of precipitation. Higher precipitation rates are predicted at higher pH values and higher H₂S concentrations. In all cases considered, the rate of precipitation is fully controlled by mass transfer of H₂S, higher H₂S concentrations and higher specific surface areas yielding higher precipitation rates. The size of the obtained crystals is predicted to increase with H₂S concentration, but to decrease with specific surface area and liquid side mass transfer. These results illustrate the importance of reactor layout and operating conditions on the process of gas-liquid precipitation.     

Oxidation of ferrous hydroxide suspensions. I

Ferrous hydroxide suspensions, obtained by the addition of calcium hydroxide to ferrous sulphate solution have been oxidised at room temperature (28 ± 3 °C). The extent of oxidation and the rate of oxidation were studied as a function of time by varying the air flow rate, concentration of calcium hydroxide (stoichiometry) and the concentration of ferrous sulphate. Expressions have been obtained giving the relation between velocity constant, k, and the air-flow rate as well as between k and concentration of ferrous sulphate.     

Oxidation of ketones over metal oxide catalysts: I. Catalytic synthesis of biacetyl from methyl ethyl ketone

The gas-phase oxidation of methyl ethyl ketone was studied on metal oxide catalysts in the presence of water vapor. Two types of competitive partial oxidations, i.e., biacetyl formation and oxidative scission reaction leading to acetaldehyde and acetic acid, took place on every oxide studied at 400–500 K. An approximate linear relationship was observed between the selectivity of each reaction and the acid-base property of the oxides; the former reaction was accelerated on the basic oxides such as Co₃O₄, while the latter reaction became predominant on the acidic oxides. As Co₃O₄ was the most effective biacetyl former of single-component oxides, modification of Co₃O₄ was examined to develop more effective catalysts for biacetyl synthesis. Scission reaction took the place of biacetyl formation over a catalyst where Co²⁺ ions were located in Y zeolite by an ion-exchanged method. Scission reaction was suppressed when Co oxide was supported on basic oxides such as MgO or CaO; however, the selectivity to biacetyl was slightly decreased due to the occurrence of a new reaction, acetone formation. The addition of Na₂O or Li₂O to Co₃O₄ was found to improve the selectivity to biacetyl without loss of catalytic activity. The maximum efficiency (13%) in biacetyl formation was attained at a Li content of ca. 7 at.%.     

Hydrolytic aging of polycarbonate. I. Physical aspects

The hydrolytic aging of an unstabilized industrial sample of polycarbonate was studied at 40, 70, 80, and 90°C, 100% RH. The water absorption characteristics show that, at equilibrium, the polymer absorbs about 0.04 mol water per ester group and that the equilibrium is reached after about 10–40 h exposure, i.e., far before irreversible changes of physical properties are observed. Differential scanning calorimetry reveals the combined effects of hydrolytic chain scission and physical aging on glass transition temperature. Investigations on tensile yield properties showed that hydrolytic chain scission leads to a significant decrease of the apparent Eyring's activation volume. These observations strongly support the hypothesis that there are preexisting or hydrolysis-induced defects responsible for an heterogeneous distribution of chain scissions that would be concentrated into localized microdomains. © 1995 John Wiley & Sons, Inc.     

Percolation bacterial leaching of low-grade chalcopyrite using acidophilic microorganisms

Bioleaching studies were carried out in percolation columns using low-grade copper containing rock (granite). The lixiviant consisted of acidified ferric sulfate containing acidophilic microorganisms. The iron oxidizing strain was isolated from Malanjkhand mine water and after adaptation the iron oxidation observed to be 500 mg/L/h. Leaching parameters studied were lixiviant flow rate, particle size and bed height. It was observed that leaching efficiency increased with decrease of particle size and lixiviant flow rate. The precipitation of iron during leaching observed to be low as pH was maintained at 2. Based on the leaching kinetics, a unified rate equation was developed and shown as rά(d _p )^−0.61 (F)^−0.76 (H _b )^0.97.     

过氧化氢氧化法合成C.I.酸性紫17

在催化剂存在下，以N-乙基-N-苄基苯胺磺化物与对-（N，N-二乙基）氨基苯甲醛在酸性条件下于95℃-100℃进行缩合。在pH=5-6时，用30%过氧化氢氧化，合成了C.I.酸性紫17,氧化收率86.5%。成品中铬含量为10.8mg/kg。     

Blends of polyamide 6 with bisphenol-A polycarbonate. I. Thermal properties and compatibility aspects

Blends of polyamide 6 (PA6) and polycarbonate (PC) have been investigated, over a full range of composition, to check interactions between them. SEM observations show that the mixtures are characterized by domains of clearly segregated homophases and voids between the two polymers. DSC and DMTA data indicate the presence of two T_g' s, corresponding to two separate phases, with the T_g of the PC phase decreasing on increasing the PA6 amount. Moreover, the crystallization kinetics of PA6 is slightly showed down by the PC. Chemical reactions between the two polymers are supposed to give rise to low molar mass compounds, as shown by GPC; these species plasticize the PC and partially dissolve into the molten polyamide, causing decrease of PC T_g and reduction of overall crystallization rate of PA6. Apparent influence of PC on melting temperature and enthalpy of PA6 is also discussed.     

Catalytic Partial Oxidation. Part I. Catalytic Processes to Convert Methane: Partial or Total Oxidation

The presence of large reserves of natural gas has stimulated research to utilize methane, its principal component, as an alternative energy source and to convert it to other fuels and industrially important chemicals. The reserves of natural gas in the world are estimated to be 1.4 × 10¹⁴ Nm³, while new gas fields are being discovered every year. Although this natural gas is available under pressure for piping and transport, extensive research efforts have been directed to develop gas-to-liquid (GTL) technology for the conversion of remote natural gas reserves into high-added-value liquid products, such as methanol and synthetic fuels, that can be more easily transported. A further incentive for natural gas utilization originates from environmental concerns that drive the search for cleaner energy sources. Catalytic combustion of methane offers an attractive alternative to gas-phase homogeneous combustion since it can stabilize flames at lower fuel-to-air ratios, thereby lowering flame temperatures and reducing NO_x emission. Another alternative can be found in the conversion of natural gas into hydrogen, which can be used to generate electricity in fuel cells. Fuel cells have a much higher energy efficiency compared to current combustion-based power plants. Also, hydrogen is a much cleaner fuel than hydrocarbon feedstocks since the only product from hydrogen fuel cells is water.     

Anodic oxidation of aryl sulphides—IV. Thianthrene

Anodic oxidation of thianthrene in 80% (v/v) acetic acid-water mixtures containing HClO₄ at 1·15 V vs Ag/AgCl sat'd. KCl (approx. 1·10 V vs sce) gives the monoxide in near quantitative yield. At 1·6 V, thianthrene and its monoxide give a mixture of products consisting of 44% cis-, 28% trans-dioxide, 13% sulphone, 10% trioxide and 5% tetroxide.     

Studies on the oxidation of enamines with molecular oxygen. I. Oxidation of Some Piperidino Alkenes

In the autoxidation of 1-piperidinocyclohex-1-ene ( 1 ), 1-piperidinobut-1-ene ( 2 ), 3-piperidinopent-2-ene ( 3 ), and 1-piperidino-2-methyl-prop-1-ene( 4 ) only products of an oxidative attack at the CC-double bond are formed. Both α-aminoketones, the products of the rearrangement of primarily formed epoxides, and products of the oxidative session of the CC double bond are obtained. As a side reaction some hydrolysis of the starting enamines takes place. This hydrolysis proves that some water must be formed during the enamine oxidation.     

Materials aspects of photoelectrochemical energy conversion

Stabilization of the light-harvesting semiconductor electrode is a key factor in the design of a photoelectrochemical (PEC) system for solar energy conversion. Approaches to circumvent the problem of PEC corrosion of these electrodes involve four major categories of materials: (a) the electrode materials themselves; (b) protective coatings; (c) catalysts; and (d) electrolytes. Wide band-gap semiconductors are candidates for electrode materials; however, their use hinges on devising effective means of sensitizing their photoresponse to wavelengths comprising the solar spectrum. These and related aspects in the design of PEC systems are reviewed in this article. Devices both of the regenerative type as well as photoelectrolysis approaches are considered.