Electrochemical behaviour of chalcopyrite in the presence of silver and Sulfolobus bacteria

The electrochemical behaviour of massive chalcopyrite electrodes has been studied in an acid medium (pH1.5) containing silver ions (0.02gdm–3Ag+) and thermophilic bacteria (68°C). Preliminary tests on particulate electrodes made from graphite, elemental sulfur and Ag2S were included to determine the electrochemical response of reactants (Ag+) and products (S° and Ag2S) associated with the dissolution of chalcopyrite in the presence of silver. Massive chalcopyrite electrodes under potential scan showed a dependence on the dissolution of the Ag2S film with both the time of contact with the silver solution and [Ag+]. As well as Ag2S, metallic silver was detected on the chalcopyrite surface. It has been demonstrated that Fe3+ and bacteria play an important role in the regeneration of the Ag2S film. The breakdown of this film is a requirement for the further dissolution of chalcopyrite. The bioleaching of chalcopyrite with thermophilic microorganisms in the presence of silver decreased the decomposition potential of the electrode and favoured its electrodissolution. Bioleaching treatment in the presence of silver ions for periods of time longer than two weeks did not improve the surface reactivity. However, in the initial stages of the process, the lower reactivity of the bioleached electrodes was probably related to a toxic effect of silver on the microorganisms.     

Electrochemical behaviour of silver and rhenium electrodes in molten alkali fluorides

Electrochemical oxidation of silver and rhenium electrodes in molten LiF–NaF–KF eutectic at 600 °C was investigated by cyclic and convolutional voltammetry. Simulated Cyclic voltammograms were calculated to reinforce understanding of the reaction mechanism. A theoretical relationship was proposed to determine standard potentials from voltammograms of metallic electrodes having an oxidation process governed by Butler–Volmer kinetics. The reversibility of silver oxidation was established and the number of exchanged electrons and the standard potential of the Ag/Ag(I) couple were determined. The rhenium electrode oxidation process was shown to be quasi-reversible and the Re/Re(x) standard potential was estimated.     

Electrochemical behaviour of aluminium in the presence of EDTA-containing chloride solutions

The electrochemical behaviour of pure aluminium in an EDTA-containing chloride solution was investigated using potentiodynamic and potentiostatic electrochemical techniques and electrochemical impedance spectroscopy. A pronounced EDTA-induced anodic dissolution was observed in alkaline solutions, but the presence of EDTA in solution had little activating effect on the passivation properties at pH values close to 4.0. This was attributed to the relative stability of the Al(EDTA)^– complex formed at the different pH values. This EDTA-induced activation had no deleterious effect on pitting attack, in fact, higher breakdown potentials were observed in the presence of EDTA. This was attributed to the buffering action of EDTA, which inhibited the attainment of a critical pit solution composition. But, in the presence of molybdate, EDTA had no observable effect on pitting attack. The impedance spectra recorded in the presence of EDTA showed evidence for the adsorption of EDTA species during the dissolution process, which in turn, form a large capacitive network, with capacitance values of the order of 3 mF cm^–2 in the low frequency region.     

Electrochemical behaviour of high strength steel wires in the presence of chlorides

The present work summarises results of the electrochemical behaviour of high strength steel wires in a high alkaline medium simulating the solution present in concrete. The aim is to expose the possible factors affecting the initiation period of stress corrosion cracking, the most frequent form of failure of pre-stressed concrete structures.Cold drawn steel wires were subjected to constant stress at 70% of their ultimate tensile strength and exposed to corrosive environments in order to study the stability of the passivating layer. The effects of chloride concentration, temperature, presence of oxygen, and cavitation induced by ultrasonic waves were studied. Two different techniques (cyclic voltammetry and impedance spectroscopy) were employed to study the effects of those parameters on the formation/breakdown of the passive layer. The results show that variations in the studied parameters can provoke breakdown of the passivating layer even at very low chloride concentrations. Chlorides, oxygen availability, temperature changes, and vibrations have been found to be important factors in the corrosion initiation of pre-/post-tensioned tendons.     

负载硅钨杂多酸纳米银修饰电极的电化学性质研究

以硅钨杂多酸作为光催化还原剂制备了纳米银(SiW12-NS)。用PVP作偶联剂,将SiW12-NS修饰到玻碳电极表面,用循环伏安法(CV)研究其电化学行为。结果表明,修饰电极具有良好的电化学响应和电极稳定性,对BrO3-,NO2-具有良好的电催化还原性能。     

黄铜矿物表面吸附细菌的研究

对黄铜矿表面吸附细菌的特性进行了研究。结果表明,氧化亚铁硫杆菌在黄铜矿表面的的吸附平衡符合Freundlich方程。在研究范围内,菌龄为对数生长后期、pH值1.5～3.0、离子强度50～100mg/mL和非离子表面活性剂质量分数在0.005%～0.1%时更有利于细菌在矿物表面的吸附,但温度对吸附影响不大。采用拟二阶反应模型考察了吸附动力学,并计算了此动力学模型的速率常数。进一步研究表明,二级反应模型和实验数据之间有较好的相关性。     

Influence of surface chemical structure of active carbon on its electrochemical behaviour in the presence of silver

Cyclic voltammetric studies of the influence of surface chemistry on the electrochemical behaviour of powdered active carbon electrodes were carried out in the presence of silver both in bulk solution and pre-adsorbed on carbon. Surfaces with a variety of chemical properties were obtained through the modification of carbon samples by heat treatment under vacuum, oxidation with concentrated nitric acid and annealing in an ammonia atmosphere. The surface area (BET), acid–base neutralization capacities and sorption capacity towards Ag+ ions were estimated in the samples obtained. The states and relative amounts of deposited Ag species were estimated using XPS. The influence of the surface chemistry of the powdered carbon material in terms of its electrochemical properties and the mechanism of silver adsorption and deposition is discussed.     

Electrochemical behaviour of the Cr(II)/Cr(III) couple on silver,silver amalgam and silver-based mercury film electrodes

The electrocatalytic activity of silver, mercury, silver-based mercury films, and silver amalgam electrodes for the reduction of Cr(III) ions in acidic solutions is analysed. An enhancement in the activity of the last two electrodes is observed when they are pretreated with high cathodic current polarization or ultrasonic waves. These effects can be explained by the formation of an unstable highly reactive amalgam. The electrocatalytic enhancement is not observed when the same electrode pretreatments are applied to sitting-drop mercury or silver electrodes.     

Electrochemical evaluation of the surface of chalcopyrite during dissolution in sulfuric acid solution

The dissolution of a massive chalcopyrite electrode (98.1% chalcopyrite, 1.9% siderite) was studied in 0.5 M sulfuric acid solution. Different anodic potentials were applied and the behavior of the electrode was observed by means of EIS, potentiodynamic, and Mott-Schottky techniques. Electrochemical impedance spectroscopy studies at open circuit potential (around −235 mV vs. MSE) proved the existence of a thin surface layer on the electrode. This layer was stable up to 100 mV vs. MSE and was assumed to be Cu_1−xFe_1−yS₂ (y?x) based on reports from previous studies. By increasing the potential to the range of 100-300 mV vs. MSE, the previously formed layer partially dissolved and a second layer (Cu_1−x−zS₂) formed on the surface. Both of the layers showed the characteristics of passive layers at low potentiodynamic scan rate (0.05 mV s⁻¹) while at high scan rates they acted like pseudo-passive layers. However, in the potential range of 300-420 mV vs. MSE, both of these surface layers dissolved and active dissolution of the electrode started. Further increase in potential caused the formation of a CuS layer which hindered the dissolution rate of the electrode. The formation of CuS is concomitant with Fe₂(SO₄)₃ formation and the latter may act as a nucleation precursor for jarosite at higher potentials (around 750 mV vs. MSE). Jarosite precipitation on the electrode surface hindered the dissolution of chalcopyrite at higher potentials. Different equivalent electrochemical circuits were modeled for each potential range and the model regression results compared with the experimental results of EIS to determine the proposed sequence of chalcopyrite dissolution.     

Electrochemical characterization of chemical species formed during the electrochemical treatment of chalcopyrite in sulfuric acid

The dissolution mechanism of chalcopyrite, and the potential range in which its passivation phenomenon takes place, were studied on carbon paste electrodes with chalcopyrite (99.46% purity, +300 mesh, 53 μm size) (CPE-CP) in 1.7 mol/dm³ H₂SO₄. A sequence of anodic potential pulses was applied to the CPE-CP to characterize its electrochemical behavior. Copper ions, dissolved by the potential pulses, were determined using a mercury film electrode (MFE) and the anodic stripping voltammetry (ASV) on a vitreous carbon disk. In addition, the modified surface of CPE-CP was characterized, before and after the potential pulses, by cyclic voltammetry (CV). The characterization of the final surface state of each electrochemically modified CPE-CP and the amount of dissolved copper showed five potential regions where the chalcopyrite dissolution mechanism changed. The initial dissolution occurs at 0.615 V ≤ E_anod < 1.015 V versus SHE forming a non-stoichiometric polysulfide (Cu_1−rFe_1−sS_2−t). The absence of copper ions in the solution indicates a passive sulfide. However, at 1.015 V ≤ E_anod < 1.085 V versus SHE, the passive product decomposes forming porous layers of non-stoichiometric polysulfide (Cu_1−xFe_1−yS_2−z) that allow the diffusional transport of charged species and the dissolution of the mineral. In the region of 1.085 V ≤ E_anod < 1.165 V versus SHE, formation covellite (CuS) was identified. At E > 1.165 V versus SHE, CuS is unstable and gives rise to complete dissolution of the chalcopyrite. Due to the experimental conditions, the mineral dissolution is inhibited by possible jarosite precipitation.     

Electrochemical behaviour of iron-complexes in presence of competitive ligands: A strategy for optimization of current density

Triethanolamine and D-gluconate both form alkali stable complexes with iron(III)- and iron(II)-ions. The redox behaviour of solutions containing one type of ligand is compared with solutions containing both ligands in excess compared with the iron(III)-concentration. Spectrophotometric investigations and experiments with cyclic voltammetry yield information about follow-up reactions proceeding during the cathodic reduction of the iron(III)-triethanolamine complex when D-gluconate is present in addition. The results indicate that the Fe(III)-triethanolamine complex is the predominant species as long as c(triethanolamine) exceeds c(Fe(III)) and thus the cathodic current density is mainly determined by the concentration of this complex. When the concentration of triethanolamine is lowered to 20% of the iron concentration then iron(III)–D-gluconate is the major species present in the solution. Then, besides ligand exchange reactions, an increase in cathodic current density indicates a redox reaction between Fe(II)-triethanolamine and Fe(III)–D-gluconate similar to the E_cat-type in cyclic voltammetry. Such conditions are of particular interest for the optimization of mediator systems with regard to the current density attainable at given iron(III)-concentration. The results show that a combination of different ligands can lead to an addition of the specific advantages of each metal-complex mediator.     

低温下非离子表面活性剂加速细菌浸出黄铜矿

为了提高中温菌氧化亚铁嗜酸硫杆菌(Acidithiobacillus ferrooxidans)在低温(8～10℃)下浸出黄铜矿的浸出速率和最终浸出率,研究了非离子表面活性剂(吐温-20、吐温.60、吐温-80)加速细菌浸出黄铜矿的作用.通过对浸矿溶液中铜的浸出率以及细菌浓度的测定来评价表面活性剂对氧化亚铁嗜酸硫杆菌浸出效果的影响.研究结果表明:3种吐温类表面活性剂在低温条件下对细菌浸出黄铜矿均有一定加速作用,其中吐温-20的加速浸出效果最好,其最佳添加量为0.01%,经过90天后最终浸出率可达49.87%.     

Comparative voltammetric behaviour of the silver/silver oxide electrode prepared on vitreous carbon and silver substrates

The voltammetric behaviour of vitreous carbon/chemically precipitated silver hydroxide layer and silver/electrochemically formed silver oxide layer electrodes are compared. As the former type of electrode is free from silver substrate contributions during the oxidation-reduction cycles (ORC) the voltammetric data indicate the occurrence of soluble Ag(I) species in the gel-like silver hydroxide matrix. Soluble species can be formed during the ORC as a consequence of AgOH formation at the early stages of the oxidation and reduction processes. Nucleation and growth of new phases produce a non-homogeneous layer structure and a decrease in the amount of active material participating in the ORC under preset conditions.     

Electrochemical properties of a new solid electrolyte in the system silver iodide — silver dichromate

A new silver solid electrolyte in the system AgI-Ag₂Cr₂O₇ has been characterized in terms of total and electronic conductivity, silver transport number and activation energy. The electrolyte undergoes a water-catalyzed decomposition reaction but, if kept in dry conditions, may be successfully used for the development of solid-state power sources capable of operating at room temperature.     

Anodic behaviour of silver in alkaline formaldehyde

Steady state potentiostatic polarization studies on the system Ag/HCHO, KOH reveal that the mechanism of anodic process is potential dependent. Direct oxidation of free formaldehyde (obtained as a result of the dissociation of hydrated formaldehyde) occurs at high anodic potentials range resulting in a limiting current plateau. Near the base potential region, on the other hand, the rate controlling electrochemical step appears to be the oxidation of the silver which is followed by a chemical reaction in which formaldehyde is oxidized. Results obtained from the accelerated Tafel plots on the mechanism in the base potential region are in agreement with Vielstich's earlier observations.     

Electrochemical oxidation of formaldehyde on gold and silver

The electrochemical oxidation of formaldehyde has been studied by cyclic voltammetry on gold and silver rotating disc electrodes varying the sweep rate, the angular velocity, formaldehyde concentration and pH of the supporting electrolyte. The Tafel parameters and reaction orders as well as the rate constants have been determined. A reaction path is proposed which involves the release and oxidation of the first hydrogen atom under an interaction of formaldehyde molecule with the metal surface and with the OH⁻-ion, followed by a non-electrochemical reaction or a second charge transfer reaction, depending upon the electrode potential.     

Electrochemical behaviour of the oxide covered antimony

The electrochemical behaviour of the system Sb-oxide layer-0.05 M H₂SO₄ was investigated by impedance, photopolarization and by recording voltammetric characteristics in the presence of the redox couple K₃Fe(CN)₆-K₄Fe(CN)₆ from 1.6 to ?0.86 V vs mercury mercurous sulphate electrode. Attention has been concentrated on potentiodynamically formed and stabilized films whose thickness enabled the development of the space charge. It will be shown that the mechanism of charge transfer in the Sb-oxide layer-electrolyte system having a given form of current potential characteristics, is dependent on the semiconducting properties of the oxide layer, which can be modified owing to the breaking of bonds and the formation of new ones. Impedance and photoelectric polarization have proved very sensitive and suitable for the investigation of structural and phase change of the solid phase in situ with potential changes in continuous polarization.     

Electrochemical behaviour of manganese in the aluminium electrorefining bath

The study of the mechanism of the electrode reaction Mn(II)/Mn in the aluminium electrorefining bath free of aluminium fluoride (1·48 NaF-1·05 BaCl₂) has been carried out by chronopotentiometry with an inert graphite electrode, a manganese electrode and a platinum electrode; this study was supplemented by utilizing anodic chronopotentiometry with current reversal from a stationary state. The tests carried out at 800° C show that the exchange mechanism of Mn(II)/Mn is reversible, involves the exchange of two electrons and that the diffusion coefficient D_Mn(II)=(1·9±0·5)×10^–5 cm² s^–1.