Electrochemical reduction of chromate ions from dilute artificial solutions in a GBC-reactor

Reduction of chromate in very low concentration (20 ppm) has been carried out in a GBC reactor. Final concentrations below 0.5 ppm have been achieved at reasonable reaction rates. Spontaneous reduction of chromate by hydrogen in a GBC reactor without an external power source requires careful selection of cathode material. Chemically stable materials such as titanium and RVC tend to passivate when in contact with chromate solution and are unsuitable for use as cathode materials. Graphite, active carbon and graphite felt show no obvious passivation. Reduction of oxidized groups and oxygen are the major side reactions. These occur significantly, especially when using an activated carbon bed as a cathode.     

Reduction of ferric ions in dilute solution in a semi-technical scale GBC reactor

The performance of a semitechnical gas diffusion electrode packed bed electrode cell (GBC) for the reduction of metal ions is investigated. Reduction of Fe3+ in sulphuric acid solution is described by first order kinetics and is found to be significantly influenced by temperature, H2SO4 concentration and the inlet concentration of Fe3+. The condition of packing has a major influence on the overall conversion in the cell. It is shown that a slight decrease in the amount of bed particles can cause a significant drop in the overall degree of conversion. The mass transfer coefficient km is found to be proportional to us0.48 at low flow velocities and us0.80 at high flow velocities, where us is the mean flow velocity. Partitioning of the overall reaction rate, ko,av, into an overall mass transfer coefficient km,av and an apparent overall electrochemical rate constant, kr,av, shows that, for H2SO4 concentration less than 1M, kr,av, can be described by an empirical relationship involving the effects of the inlet Fe3+ concentration, the solution conductivity and temperature.     

Reduction of metal ions in dilute solutions using a GBC-reactor: Part I: Experimental study on the laboratory scale reduction of ferric ions

To reduce metal ions in dilute solutions a new type of electrochemical reactor has been developed: the GBC-reactor. This reactor consists of a gas diffusion electrode coupled with a packed bed electrode. The working principle of the reactor is based upon two main reactions: the catalytic oxidation of hydrogen gas in the gas diffusion electrode and the simultaneous reduction of metal ions on the packed bed electrode. This process occurs spontaneously without the need for an external power supply when the Gibbs free energy of the total reaction is negative. To study the behaviour of the reactor the reduction of ferric ions was used as a model system. The experimental results, obtained from varying a number of key process parameters, could be described using a very simple macroscopic rate equation. It is concluded that the reduction of ferric ions in a GBC-reactor is controlled by both mass transfer and electrochemical kinetics. To carry out scale-up and optimization studies a reactor model incorporating the potential distribution in the packed bed electrode is, however, necessary.     

Electrochemical reduction of nickel ions from dilute solutions

Electrochemical reduction of nickel ions in dilute solution using a divided GBC-cell is of interest for purification of waste waters. A typical solution to be treated is the effluent from steel etching processes which contain low quantities of nickel, chromate and chromium ions. Reduction of chromate in a divided GBC-cell results in effluent containing Cr³⁺ ions. This paper examines the limitations to further treatment of the effluent by electrochemical reduction of nickel ions. Nickel current efficiency has been determined by varying temperature, pH, current density and concentration of supporting electrolyte. It was found that the presence of a complexing agent, e.g. NH₄ ⁺ is necessary. To deposit nickel, complete removal of Cr³⁺ ions from the solution is inevitable. Optimum nickel current efficiencies are obtained at pH 5 and current density 10 mA cm^–2.     

Electrochemical studies of the nickel electrode with cobalt modification

The electrochemical behaviour of the paste-type nickel hydroxide electrode with cobalt-modified nickel foam was investigated using galvanostatic charge–discharge, electrochemical impedance spectroscopy, cyclic voltammetry and current pulse relaxation methods. Experimental results showed that the performance of the nickel electrode with substrate deposition of a thin layer of cobalt was improved markedly. This improvement could be attributed to the enhanced electrical conduction between the substrate and the active material. The enhanced electrical conduction increases the charge efficiency and the discharge depth of the nickel electrodes and therefore increases the utilization of the active material. This suggests that the electrical conduction between the substrate and the active material is essential to the practical use of paste-type nickel hydroxide electrodes.     

Heat transfer to a moving packed bed of nickel pellets

Heat transfer between a bed of nickel pellets and a vertical section of electrically heated steel pipe has been measured, with the pellet bed inside the vertical pipe. Most of the data are for a 20.27 cm diameter pipe but some data were also obtained for a 10.23 cm diameter pipe. The effective thermal conductivity of the stationary pellet bed has been estimated approximately from the results of unsteady heating tests. Tests have been carried out with a downwardly moving bed, including the effect of air flowing upwards through the bed. Average values of the pellet‐side heat transfer coefficient are between 72 and 135 W/(m²°C) depending on the mass fluxes of air and pellets, and have been expressed as an empirical correlation.     

用电去离子过程从稀溶液中回收镍离子并制备纯水

利用基于EDI工艺的单个过程实现了低浓度废水中镍的回收并同时制备纯水.研究表明:当原水镍含量为55ppm时,镍去除率可达99.9%以上,淡水产水中镍的浓度低于0.05mg·L-1,其电阻率稳定在2.02～2.59MΩ•cm,浓水中镍浓度则可高达1263mg·L-1.该研究充分证明了EDI可用于回收低浓度含镍废水中的镍离子且同时产生优质纯水,从而可实现如电镀等行业的清洁生产和闭路循环。     

Electrodeionisation 3: The removal of nickel ions from dilute solutions

The removal of nickel ions from dilute solutions using a process that combines an ion-exchange bed with electrodialysis has been studied. The main aspects include: the concentration of nickel ions in the diluate, the voltage over the cell and the current density distribution along the ion-exchange bed. The current density distribution provides insight into the state of the bed as it is simultaneously loaded with Ni²⁺ and regenerated with an electric potential difference applied perpendicular to it. A simple model is used to describe the state of the bed and the quantity of nickel removed from it as a function of time. Under specific conditions the precipitation of metal hydroxides is observed in the compartment containing the ion-exchange bed. The results show that hydroxide precipitation is related to the nickel concentration in solution and the electric potential gradient across the bed.     

Electrochemical reduction of perchlorate ions on platinum-activated nickel

The electrochemical reduction of perchlorate anion has been studied in a cell with a nickel working electrode and a platinum counter electrode in concentrated solutions of HClO₄. Significant reduction of perchlorate to chloride ions occurs on the nickel cathode in the potential region where hydrogen evolution occurs. It is shown that the mechanism of this process involves platinum deposited in small amounts on the cathode as a result of oxidation of the platinum anode in the perchloric acid solution. The reduction of perchlorate is accompanied by oxidation of the nickel cathode, which is attributed to chlorate ions formed in the initial step of perchlorate reduction. Changes in the surface structure of the nickel electrode have been followed using atomic force microscopy.     

Mesoporous nickel electrodes plated with gold for the detection of glucose

Mesoporous nickel electrodes (MNEs) are efficient structural materials for advanced electrochemical sensors due to their extremely high surface areas. However, the poor oxidation resistance of nano-sized nickel in air severely limits their applications. Thus, the purpose of the present study was to improve the oxidation resistance of MNEs by applying relatively stable gold coatings via electroplating. The results revealed that the gold coatings provided excellent oxidation resistance to MNEs, and a loss in amperometric sensitivity for glucose sensing was not observed. The protection time increased with an increase as the thickness of the gold coating (passing charges of 0–25.49 C/cm²), and protection times ranging from a few hours to 2 weeks were obtained. With an alkaline electrolyte (NaOH, 4 M), MNEs exhibited excellent sensitivities. For instance, the sensitivity of the unmodified MNE and the Au coated-MNE was 3238 and 2978 μA mM⁻¹ cm⁻², respectively. Thus, coated and uncoated MNEs displayed remarkably higher linear sensitivities than conventional nickel-based electrodes (394 μA mM⁻¹ cm⁻²) at a wide range of glucose concentrations (0.001–3 M).     

Electrochemical reduction of nitrate in weakly alkaline solutions

The electrocatalytic activity of several materials for the nitrate reduction reaction was studied by cyclic voltammetry on a rotating ring disc electrode in solutions with different concentrations of sodium bicarbonate. Copper exhibited highest catalytic activity among the materials studied. Nitrate reduction on copper was characterized by two cathodic shoulders on the polarization curve in the potential region of the commencement of hydrogen evolution. In this potential range an anodic current response was observed on the Pt ring electrode identified as nitrite to nitrate oxidation. This indicates that nitrite is an intermediate product during nitrate reduction. These conclusions were verified by batch electrolysis using a plate electrode electrochemical cell. Copper and nickel, materials representing the opposite ends of the electrocatalytic activity spectra, were used in batch electrolysis testing.     

Simultaneous phenol removal and biological reduction of hexavalent chromium in a packed‐bed reactor

BACKGROUND: Phenol and hexavalent chromium are considered industrial pollutants that pose severe threats to human health and the environment. The two pollutants can be found together in aquatic environments originating from mixed discharges of many industrial processes, or from a single industry discharge. The main objective of this work was to study the feasibility of using phenol as an electron donor for Cr(VI) reduction, thus achieving the simultaneous biological removal/reduction of the two pollutants in a packed‐bed reactor. RESULTS: A pilot‐scale packed‐bed reactor was used to estimate phenol removal with simultaneous Cr(VI) reduction through biological mechanisms, using a new mixed bacterial culture originated from Cr(VI)‐reducing and phenol‐degrading bacteria, operated in draw–fill mode with recirculation. Experiments were performed for feed Cr(VI) concentration of about 5.5 mg L^?1, while phenol concentration ranged from 350 to 1500 mg L^?1. The maximum reduction/removal rates achieved were 0.062 g Cr(VI) L^?1 d^?1 and 3.574 g phenol L^?1 d^?1, for a phenol concentration of 500 mg L^?1. CONCLUSION: Phenol removal with simultaneous biological Cr(VI) reduction is feasible in a packed‐bed attached growth bioreactor. Phenol was found to inhibit Cr(VI) reduction, while phenol removal was rather unaffected by Cr(VI) concentration increase. However, the recorded removal rates of phenol and Cr(VI) were found to be much lower than those obtained from previous research, where the two pollutants were examined separately. Copyright © 2008 Society of Chemical Industry     

Electrocatalytic hydrogenation of phenol in aqueous solutions at a Raney nickel electrode in the presence of cationic surfactants

The electrocatalytic hydrogenation (ECH) of phenol at a Raney nickel cathode was studied in aqueous solutions. At 30 °C, without surfactants, cyclohexanol was obtained with low or medium yields. The best results were observed in alkaline solutions (pH 9). At pH 2 the efficiency of the hydrogenation reaction is significantly improved by low amounts of didodecyldimethylammonium bromide (DDAB). This surfactant effect is studied in relation to the substrate and hydrogen adsorption phenomena.     

壳聚糖对化学镀镍废液中Ni^2＋吸附

研究了用壳聚糖吸附化学镀镍废液中Ni^2＋的最佳条件及其脱附和再生方法，以及镀液中络合剂等其他组分对吸附率的影响。结果表明，壳聚糖对化学镀镍废液中的Ni^2＋具有较好的吸附能力，pH是影响吸附的主要因素。在室温、pH=5．0时，Ni^2＋质量浓度为4．6952g／L的化学镀镍废液被0．9390g壳聚糖吸附2h后，Ni^2＋的去除率达72．25％。     

Electrocatalytic hydrogenation of alkyl-substituted phenols in aqueous solutions at a Raney nickel electrode in the presence of a non-micelle-forming cationic surfactant

The electrocatalytic hydrogenation (ECH) of 2,6-dimethylphenol and 2-tert-butylphenol was performed at RaNi cathodes in aqueous buffers containing various amounts of didodecyldimethylammonium bromide (DDAB). Without surfactant, 2,6-dimethylcyclohexanol, 2-tert-butylcyclohexanone and 2-tert-butylcyclohexanol were obtained, at 65 °C, with very low yields. The surfactant effect on the yields, the selectivity of the reaction and the diastereoisomeric composition of alkylcyclohexanols produced was studied in acidic and basic solutions in relation to the substrate adsorption. At pH 9 the efficiency of the hydrogenation reaction was significantly improved by low amounts of DDAB, which led to an increase of the alkylcyclohexanols formation. In particular, cis-2-tert-butylcyclohexanol was obtained with a high diastereoselectivity.     

Electrochemical reduction of dilute chromate solutions on carbon felt electrodes

Carbon felt is a potential material for electrochemical reduction of chromates. Very dilute solutions may be efficiently treated due to its large specific surface area and high porosity. In this work, the up-scaling of this technology is investigated using a new type of separated cell and once-through flow of industrial rinse water. A significant enhancement of the process is obtained due to copper deposition during long-term operation. The co-deposition and re-solution of copper occurs depending on the inlet chromate concentration. When previously deposited copper is present a current-free reduction of chromate takes place resulting in current efficiencies apparently above 100%. Very high space time yields are obtained even for effluents at low concentration and optimised conditions (high flow rates and pH 2). The economic feasibility of the technology is also considered. Continuous, single-pass operation results in lower energy requirements than batch processing. The economic potential of the process is also evaluated in comparison with chemical detoxification of chromate. The operating costs for the electrochemical treatment of very dilute effluents on a carbon felt electrode are 30% lower than for the chemical method.     

填充床电极反应器中典型有机电合成反应系统的选择性分析

典型的有机电合成反应,常伴有相互竞争的电极副反应和均相反应,增加电极极化以提高反应速率会使选择性明显降低。填充床电极具有大的内表面,能在相对低的极化下,达到较高的表观电流密度,有利于缓解反应速率和选择性之间的突出矛盾。本文对填充床电极微分反应器（PBEDR）中典型有机电合成反应过程进行了理论分析,重点在于床层内超电势横向分布对选择性的影响。建立了描述超电势分布的普遍化数学模型,归纳出表征电极极化和副反应影响的量纲1数μ和ω,并用ADM（Adomian’s decomposition method）对该非线性微分方程模型求解;所获得的逼近解代数表达式,可方便地计算不同参数下超电势分布对平均选择性的影响,而毋需反复求解模型微分方程。最后给出了PBEDR硝基苯电还原制对氨基苯酚选择性分析实例,优化了反应器特征尺寸（填充床电极厚度）。结果证明：理论计算与实验数据令人满意地一致。     

两级泡沫镍填料旋转填充床压降及精馏研究

在两级逆流式旋转填充床(TSCC-RPB)的基础上,采用简单结构的整体泡沫镍填料替代其相对复杂的动静环结构填料,从而构建新型两级整体泡沫镍填料旋转填充床(TSNF-RPB)。采用空气-水体系,研究了TSNF-RPB的压降特性,并以甲醇-水为工作体系,对TSNF-RPB进行了连续精馏实验研究。结果表明:虽然TSNF-RPB的湿床压降比TSCC-RPB高出了20%—30%,但TSNF-RPB的分离效率提高了20%,且TSNF-RPB结构简单、安装方便,更利于工业推广应用。