Electrochemical reduction of CI sulphur black 1—correlation between electrochemical parameters and colour depth in exhaust dyeing

Direct cathodic reduction of oxidised CI Sulphur Black 1 was achieved by means of a multi-cathode electrolyser at cell currents of 0.9–1.5 A. The redox potential in the catholyte decreased from initially −250 to −533 mV as a function of charge flow. The catholyte also served as dyebath for cotton fabric samples. Colour depth was characterised by Kubelka–Munk value K/S and CIELab-coordinates and was studied as function of charge flow and redox potential in the catholyte. Direct correlation between redox potential and colour depth of the dyed samples was observed. Electrochemical reduction permits steering of catholyte/dyebath potential by adjustment of cell current and thus permits direct control of the dyeing process by electrochemical methods.     

Transport phenomena in the electrodeionization of cesium from AMP-PAN

Electrodeionization (EDI) of cesium from cesium-sorbed ammonium molybdophosphate-polyacrylonitrile (AMP-PAN) was investigated by passing eluant through the packed bed of ion-exchange resin in an electrodialysis cell. The deionized cesium from the packed bed was recovered in catholyte by migration and in the eluant by convection. Recovery percentage of Cs by migration increased while the recovery by convection decreased with increase in current density from 20 to 40 mA/cm². Increased eluant concentration resulted in low migration percentage of cesium. Increased catholyte concentration had a negligible effect on total recovery. Apparent diffusion coefficients evaluated using the Nernst–Plank relation increased with increase in current density and catholyte concentration while a decreasing trend was observed with increase in eluant concentration.     

Analysis of molar flux and current density in the electrodialytic separation of sulfuric acid from spent liquor using an anion exchange membrane

Separation of sulfuric acid from a dilute solution involved a plate and frame type electrodialysis unit using a commercial anion exchange membrane. Experiments were conducted in batch with catholyte concentrations ranging from 1 to 5 wt%. Effect of applied current density, initial catholyte concentration and initial concentration difference of catholyte and anolyte on the molar flux was studied extensively. The maximum molar flux was estimated to be 10.52×10^-8 mol cm^-2s^-1 at 4.45 wt% catholyte concentration and applied current density of 30 mA cm^-2. Current efficiencies were observed to be 75 to 85% at lower current density, which rose to more than 100% at 20 and 30mA cm^-2, at equal initial concentration of catholyte and anolyte. Diffusive flux and flux due to membrane potential contributed very less compared to total flux in presence of applied electric current. An equation was developed to predict the practical molar fluxes, which fitted satisfactorily with minor standard deviation. Pristine and used membrane specimens were characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).     

Recovery of metals from complexed solutions by electrodeposition

Electrolytic recovery of metals from aqueous solutions containing complexing chelating agents such as EDTA, NTA, and citrate was studied in a two-chamber cell separating with a commercial cation-exchange membrane (CEM). Equimolar solutions of metal and a chelating agent as a catholyte and NaNO₃ as an anolyte were used; the effect of current densities, initial catholyte and anolyte pH, metal concentration and the type of the CEM, chelating agent and metal on the recovery of metals was determined. The recovery of metal increased with higher initial anolyte pH, concentration and current density, whereas it decreased with lower initial catholyte pH. The results show that electrodeposition seems to be an applicable method for the recovery of metals under appropriate conditions.     

Electrosynthesis of sodium dithionite in a trickle-bed reactor

Sodium dithionite was generated by electroreduction of sulphur dioxide on a graphite fibre mat cathode in a continuous ‘flow-by’ trickle-bed electrochemical reactor. Reactor performance was measured with respect to variation of the feed catholyte NaOH concentration [0.5-1.0 mol/L], feed gas SO₂ concentration [4-40 vol%], product catholyte pH [2.4-12.0] and applied current [10-50 A (0.4-2.0 kA/m²)]. Product catholyte temperature was in the range 18 to 30°C. With product catholyte pH between 2.4 and 6.9, dithionite was generated at current efficiency from 42 to 100%, concentration from 0.05 to 0.78 mol/L, yield from SO₂ up to 69% and specific energy from about 2 to 5 kWh/kg Na₂S₂O₄. Current efficiency fell with increasing current and rose with increasing gas load. High levels of thiosulphate [0.07-0.3 mol/L] and sulphide [0.01-0.13 mol/L] measured in the product catholyte would compromise the use of this process in commercial applications such as brightening wood pulp.     

Geochemical effects of electro-osmosis in clays

Geochemical effects of electro-osmosis in bentonite clay are studied in the laboratory, where a 6 mm thick bentonite layer is subjected to direct current. Acidification and alkalization near anode and cathode are expected, possibly causing mineral deterioration, ion mobilization and precipitation of new solids. Afterwards the clay is analysed by XRF and anolyte and catholyte are analysed by ICP-MS. In addition, as a preliminary experiment treated bentonite is analysed by high resolution μ-XRF. Electro-osmotic flow is observed. Due to its carbonate content the bentonite is pH-buffering. Alkalization in the catholyte is substantial. Ca, Na and Sr are significantly removed from the clay and accumulate in the catholyte. Recovery in the catholyte accounts for a small fraction of the element-loss from the clay. The rest will have precipitated in undetected solid phases. μ-XRF indicates the loss of Ca-content throughout the bentonite layer.     

硝基苯电还原制备对氨基苯酚工艺过程

采用固定床电解槽还原硝基苯制备对氨基苯酚,并对其工艺条件进行了优化。以铜网组成固定床电解槽阴极,镀铱钛网(DSA)作为阳极,在电流密度为1000 A·m^-2,阴极电解槽内流速为4.28 cm·s^-1,铜网厚度为10 mm,温度为85℃条件下,硝基苯还原的电流效率接近100%,对氨基苯酚的选择性可达到83%。电解液可循环套用5次,硫酸和氨水的消耗量降至原来的25%,硫酸铵和废水的排放量也减少了75%。采用扩散渗析法回收废弃电解液中的硫酸,以APS为阴离子交换膜,模拟液的流量为0.01 ml·min^-1、温度为20℃时,酸回收率达到61%,且对氨基苯酚和苯胺的透过率分别仅为1.4%和1.6%。     

Hydroxylamine production by electroreduction of nitric oxide in a trickle bed cell

Hydroxylamine was produced in a trickle bed cell by passing nitric oxide gas and sulphuric acid catholyte co-currently downward through a cathode bed of tungsten carbide particles. The dependence of hydroxylamine concentration and current efficiency on cathode activity and particle size, flow rate and composition of gas and catholyte, bed height, and reactor temperature and pressure are reported. Hydroxylamine concentrations of up to 0.18 mol/L were produced at 62% current efficiency in a single pass through a 0.375 m high cell operated at atmospheric pressure and a current density of 213 A/m². The hydroxylamine concentration increased with cell pressure, gas flow rate and decreases in catholyte flow and could be raised to 0.4 mol/L by recycling the catholyte. The process appears to be controlled by mass transfer at current densities over 400 A/m² and by electrochemical reaction below about 300 A/m².     

Modeling of a packed-bed electrochemical reactor for producing glyoxylic acid from oxalic acid

A two-dimensional reactor model was established for a packed-bed electrochemical reactor with cooled cathode (PERCC) for producing glyoxylic acid from oxalic acid based on the system's reaction kinetics, mass conservation equation, and the equation of charge conservation in terms of solution-cathode potential to describe the distributions of glyoxylic acid concentration and electrolyte potential in the cathode compartment of the PERCC. The equation for a circulating mixer was also presented to account for the accumulation of glyoxylic acid in the catholyte of a batch electroreduction process. Using the orthogonal collocation approach, the partial differential equations of the model could be converted into sets of algebraic equations and be numerically solved. The effects of operating temperature, conductivity of catholyte, operating cathode potential, and volumetric flow rate of the catholyte on the current efficiency and concentration of glyoxylic acid were simulated and discussed, with emphasis on the current densities generated from main and side reactions. The model was used in a batch operation process and a continuous operation process, with the predicted results being generally in good agreement with the experimental data for both the cases.     

离子交换膜的维护

介绍离子交换膜的工况。分析盐水中的杂质，阴、阳极液浓度变化，温度。压力对离子交换膜性能和寿命的影响原理。提出维护离子膜的方法有：平稳运行，减少开、停车次数和增加预防控制措施。修复离子膜的方法有停车时循环阳极液、用纯水清洗离子膜或根据实际情况改变运行状态，如离子交换膜受Ca^2 、Mg^2 轻微污染时，先停车再启动；受中度污染时可将阴极液质量分数从32％调整到33％，或将阳极液质量浓度从190g／L升至200g／L，或同时调整阴极液、阳极液浓度。     

Microbial fuel cells operated with iron-chelated air cathodes

The use of non-noble metal-based cathodes can enhance the sustainability of microbial fuel cells (MFCs). We demonstrated that an iron-chelated complex could effectively be used as an aerated catholyte or as an iron-chelated open air cathode to generate current with the use of MFCs. An aerated iron ethylenediaminetetraacetic acid (Fe-EDTA) catholyte generated a maximum current of 34.4 mA and a maximum power density of 22.9 W m⁻³ total anode compartment (TAC). Compared to a MFC with a hexacyanoferrate catholyte, the maximum current was similar but the maximum power was 50% lower. However, no replenishment of the Fe-EDTA catholyte was needed. The creation of an activated carbon cloth open air cathode with Fe-EDTA–polytetrafluoroethylene (PTFE) applied to it increased the maximum power density to 40.3 W m⁻³ TAC and generated a stable current of 12.9 mA (at 300 mV). It was observed that the ohmic loss of an open air cathode MFC was dependent on the type of membrane used. Moreover, increasing the anode electrode thickness of an open air cathode MFC from 1.5 to 7.5 cm, resulted in a lowering of the power and current density.     

Chromium hexacyanoferrate as a cathode material in microbial fuel cells

In this work, it has been demonstrated that the disadvantages associated with the use of the potassium ferricyanide solution as the catholyte in a microbial fuel cell (MFC) were overcome by using a graphite cathode electrochemically modified with chromium hexacyanoferrate (CrHCF) film. The existing use of potassium ferricyanide solution as the catholyte is limited by the need to replace the catholyte every week and it cannot be used in a sustained manner. The present work evaluates the suitability of the CrHCF modified film as a suitable cathode material in a prototype of a MFC wherein Hansenula anomala is used as the biocatalyst in the anode compartment. The CrHCF film was prepared in the presence of the dopant camphor sulphonic acid to improve the reversibility of the film in phosphate buffer.     

Ion transport in a two membrane electrowinning cell for the production of hydrochloric acid

A three-compartment electrowinning cell has been evaluated for the regeneration of HCl from metal chloride catholyte. By segregating the catholyte with an anion exchange membrane, and the anolyte with a cation exchange membrane from the middle electrolyte(ampholyte) compartment, HCl could be produced in the ampholyte electrochemically up to one molality. The anolyte consisted of sulphuric acid. Successful operation of such a double membrane cell depends on controlling ion transport through the membrane, especially chloride migration into the anolyte. Results of electrowinning and self-diffusion experiments for three types of cation exchange membrane are presented and discussed.     

The Application of Electrodialysis to Extend the Lifetime of Commercial Electroplating Baths

Abstract

Electrodialysis has been investigated as a method to extend the lifetime of industrial electroplating solutions via the selective removal of inert electrolyte salts that build up during electroplating operations.

The electrodialysis measurements were made using a commercially available plate-and frame-type cell and various combinations of Nafion cation exchange and either Tosflex or Neosepta anion exchange membranes.

Two commercial plating solutions were studied: a zinc-tin bath in which there is a buildup of excess potassium hydroxide and a nickel-tungsten bath characterized by a buildup of excess sodium sulfate. Potassium hydroxide was effectively removed from the zinc-tin bath with very little loss of the heavy metals. Two configurations were investigated: a three compartment configuration with potassium hydroxide in the anolyte strip and sulfuric acid in the catholyte strip, and a two compartment configuration with sulfuric acid in the catholyte strip and the anode placed directly in the plating solution. In both cases potassium hydroxide was stripped from the plating solution at greater than 94% current efficiency, but at a slightly greater voltage in the three compartment cell due to increased resistance caused by the extra membrane.

A three compartment configuration was used to remove sodium sulfate from the nickel-tungsten bath, with acid solution in the catholyte and alkaline solution in the anolyte. Current efficiencies for salt removal were high but with appreciable loss of tungsten and nickel to the strip solutions.     

Studies on promising cell performance with H2SO4 as the catholyte for electrogeneration of Ag2+ from Ag+ in HNO3 anolyte in mediated electrochemical oxidation process

Electrochemical performance of a divided cell with electrogeneration of Ag²⁺ from Ag⁺ in 6 M HNO₃ anolyte has been studied with 6 M HNO₃ or 3 M H₂SO₄ as the catholyte. This work arose because in mediated electrochemical oxidation (MEO) processes with Ag(II)/Ag(I) redox mediator, HNO₃ is generally used as catholyte, which, however, produces NO _x gases in the cathode compartment. The performance of the cell with 6 M HNO₃ or 3 M H₂SO₄ as the catholyte has been compared in terms of (i) the acid concentration in the cathode compartment, (ii) the Ag⁺ to Ag²⁺ conversion efficiency in the anolyte, (iii) the migration of Ag⁺ from anolyte to catholyte across the membrane separator, and (iv) the cell voltage. Studies with various concentrations of H₂SO₄ catholyte have been carried-out, and the cathode surfaces have been analyzed by SEM and EDXA; similarly, the precipitated material collected in the cathode compartment at higher H₂SO₄ concentrations has been analyzed by XRD to understand the underlying processes. The various beneficial effects in using H₂SO₄ as catholyte have been presented. A simple cathode surface renewal method relatively free from Ag deposit has been suggested.     

Separation and concentration of lactic acid by electro-electrodialysis

Reactive extraction combined with a modified two-phase electro-electrodialysis (MTPEED) process shows a strong potential in the recovery and concentration of lactic acid without generating a salt waste. In this paper, an electro-electrodialysis (EED) process was designed with mixed lactic acid and sodium lactate aqueous solution as catholyte to simulate this MTPEED process. The immediate factors affecting the current efficiency, cell voltage and energy consumption were investigated. The optimization of operating conditions was also discussed. The lowest cell voltage was reached when the sodium lactate concentration was about 1.7 mol L⁻¹. The current efficiency was kept near 100% before lactic acid in the catholyte was exhausted. When the current density was 160.7 A m⁻², a representative specific energy consumption of the MTPEED process was 1.404 kWh/kg lactic acid, far lower than 2.961 kWh/kg lactic acid of a comparable EED process with sodium lactate aqueous solution as catholyte. The cell voltage of the MTPEED process was also affected by the phase ratio of the organic phase to the water phase. The lower phase ratio is preferred.     

Dual solution-type HCHO fuel cell systems using an anion exchange resin membrane with electroless-plated Cu or Pd anode

Loading characteristics of a prototype HCHO fuel cell systems with an anion exchange membrane which separates the anolyte from the catholyte were investigated. Electrodes of Cu or Pd, deposited by an electroless-plating technique onto the membrane, showed high electrocatalytic activity to the anodic oxidation of HCHO in 1 M NaOH solution. The system with Cu anode and 1 M NaOH for both anolyte and catholyte showed high loading characteristics but poor durability, whereas that with 1 M K₂CO₃ showed low characteristics because of lowered pH of the anolyte. It was shown that a dual solution-type cell with 1 M K₂CO₃ anolyte and 1 M NaOH catholyte yielded improved characteristics as compared with the simple K₂CO₃ system. The output level was, however, at an unsatisfactory level owing to poor membrane conductance. The temperature dependence of the output performance was studied in the range 7–55°C.     

Separation of nickel from cobalt using electrodialysis in the presence of EDTA

Optimum conditions are determined for the removal of nickel from cobalt solutions by electrodialysis exploiting the greater stability of the EDTA complex with nickel. The Ni–(EDTA)^2– complex and hydrated Co²⁺ ions are transferred from the feed solution to the electrodialysis anolyte and catholyte chambers, respectively. A three compartment cell is required to prevent the transfer of hydrated Ni²⁺ from the anolyte chamber as the EDTA present is destroyed at the anode. Complete removal of nickel from cobalt can be achieved but there is a compromise between cobalt purity and the percentage of cobalt transferred to the catholyte chamber for recovery.     

Electrochemical membrane reactor: In situ separation and recovery of chromic acid and metal ions

An electrochemical membrane reactor with three compartments (anolyte, catholyte and central compartment) based on in-house-prepared cation- and anion-exchange membrane was developed to achieve in situ separation and recovery of chromic acid and metal ions. The physicochemical and electrochemical properties of the ion-exchange membrane under standard operating conditions reveal its suitability for the proposed reactor. Experiments using synthetic solutions of chromate and dichromate of different concentrations were carried out to study the feasibility of the process. Electrochemical reactions occurring at the cathode and anode under operating conditions are proposed. It was observed that metal ion migrated through the cation-exchange membrane from central compartment to catholyte and OH⁻ formation at the cathode leads to the formation of metal hydroxide. Simultaneously, chromate ion migrated through the anion-exchange membrane from central compartment to the anolyte and formed chromic acid by combining H⁺ produced their by oxidative water splitting. Thus a continuous decay in the concentration of chromate and metal ion was observed in the central compartment, which was recovered separately in the anolyte and catholyte, respectively, from their mixed solution. This process was completely optimized in terms of operating conditions such as initial concentration of chromate and metal ions in the central compartment, the applied cell voltage, chromate and metal ion flux, recovery percentage, energy consumption, and current efficiency. It was concluded that chromic acid and metal ions can be recovered efficiently from their mixed solution leaving behind the uncharged organics and can be reused as their corresponding acid and base apart from the purifying water for further applications.     

生物电Fenton系统对餐饮含油废水处理研究

利用生物电Fenton系统对某高校食堂的餐饮含油废水进行处理,探讨了系统对餐饮含油废水的处理性能,考察了温度、阴极液初始pH和阴极曝气量等因素对系统性能的影响,并进行了工艺条件优化。结果表明,系统运行的最佳工艺条件:温度35℃,阴极液初始pH=3,阴极曝气量0.2 L/min。在最佳工艺条件下,阳极COD去除率、阴极油去除率和输出电压分别达到91.1%、95.3%和431.9 m V。