Electrochemical purification

G. D. Hitchens, O. J. Murphy, L. Kaba and C. E. Verotsko, 20th Int. Conference, Environmental Systems (1991). Electrochemical purification of waste water. 

Huang, K.L., Holsen, T.M., Chou, T.C., and Selman, ).R. (2003) Comparing nafion and ceramic separators used in electrochemical purification of spent... 

Refs. [i] Tran TD, Farmer JC, Murguia L (2001) Method and apparatus for capacitive deionization, electrochemical purification, and regeneration of electrodes. US 5,954,937 [ii] Farmer JC, FixDV, Mack GV, Pekala RW, Poco JF (1996) J Electrochem Soc 143 159... 

One of the problems in electrocatalysis is that electrochemical reactions are generally carried out in aqueous or nonaqueous solution. Thus, the solvent may intervene in the over-all reaction. In addition, it is necessary to carry out the reaction under highly purified conditions. Otherwise, impurities in the solution may affect the kinetics of the reaction concerned, so that mechanism studies become difficult. For gas phase reactions, though impurity concentrations are generally lower than in electrochemical reactions, one uses high-vacuum techniques for purification. Electrochemical purification techniques— pre-electrolysis or adsorption of impurities near the potential of maximum adsorption—are often simpler. The activation of a poissoned catalyst is often difficult or impossible. An electrocatalyst can often be reactivated in situ, by pulse techniques (cf. Section VII,D). 

S. Sarangapani, D. Gage, and T.V. Bommaraju, Electrochemical Purification of Chlorine from Chlor-Alkali Tail Gas, 2002 NSF Design Service and Manufacturing Grantees and Research Cortference, San Juan, PR (2002). 

Bao, S., Newman, P.J., Voelkel, A., Zhou, Z., MacFarlane, DJt., 1995. Electrochemical purification and GFAAS analysis of heavy metal fluoride glass. J. Non-Cryst. Solids 184, 194-199. 

Farmer, J.C., Method and apparatus for capacitive deionization, electrochemical purification and regeneration of electrodes, US Patent 5,425,859 (1995). 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Electrolytic Reductions. Both nitro compounds and nitriles can be reduced electrochemically. One advantage of electrochemical reduction is the cleanness of the operation. Since there are a minimum of by-products, both waste disposal and purification of the product are greatiy simplified. However, unless very cheap electricity is available, these processes are generally too expensive to compete with the traditional chemical methods. 

A sample of this compound was obtained from Hooker Electrochemical Co. and used without further purification. 

Purification of solvents and salts is essential for reliable electrochemical studies and measurements. A water content of 20ppm already corresponds to a 10 3molL solution. This is in the concentration range of dilute solutions used in conductivity studies for the determination of association constants (see Sec.7.3.2). Traces of water may affect chemical equilibria and therefore act on specific conductivities and limiting ion conductivities. For example, addition of 30 ppm water to a 2xl0-4 mol LT1 solution of LiBF4 in THF at 15 °C increases its conductivity by 4.4 percent (precision of measurements about 0.02 percent) 380 ppm water causes an increase by 51.7 percent see Fig. 3 [20J. 

H. Christensen, J. Dinesen, H.H. Engell, and K.K. Hansen, Electrochemical Reactor for Exhaust Gas Purification, SAE paper 1999-01-0472, Diesel Exhaust Aftertreatment (SP-1414)225-229 (1999). 

Electrochemical cells play important roles in both the purification and the preservation of metallic materials. Redox reactions are used throughout the chemical industry to extract metals from their ores. However, redox reactions also corrode the artifacts that industry produces. What redox reactions achieve, redox reactions can destroy. 

Attempts to bnild electrochemical systems for oxidative purification of effluents have met a nnmber of problems ... 

Among electrochemical methods of water purification, one can also list the various electromembrane technologies, electrodialysis in particular. The simplest elec-trodialyzer consists of three compartments separated by semipermeable membranes (usually, cation- and anion-exchange membranes). The water to be purified is supplied to the central (desalination) compartment. In the outer (concentration) compartments, electrodes are set up between which a certain potential difference is applied. Under the effect of the electric field, ions pass througfi the membranes so that the concentration of ionic contaminants in the central compartment decreases. 

In Section 24.3, use of electrosorption for effluent purification was mentioned. The same principle of an electrochemically controlled hemosorption (sorptive blood purification) is used in modem toxicology to extract toxins from blood. By appropriate potential control of the carbon sorbent, particular toxins can be removed selectively without traumatizing the blood, that is, without removing essential blood components such as the thrombocytes. 

The metallic impurities present in an impure metal can be broadly divided into two groups those nobler (less electronegative) and those less noble or baser (more electronegative) as compared to the metal to be purified. Purification with respect to these two classes of impurities occurs due to the chemical and the electrochemical reactions that take place at the anode and at the cathode. At the anode, the impurities which are baser than the metal to be purified would go into solution by chemical displacement and by electrochemical reactions whereas the nobler impurities would remain behind as sludges. At the cathode, the baser impurities would not get electrolytically deposited because of the unfavorable electrode potential and the concentration of these impurities would build up in the electrolyte. If, however, the baser impurities enter the cell via the electrolyte or from the construction materials of the cell, there would be no accumulation or build up because these would readily co-deposit at the cathode and contaminate the metal. It is for this reason that it is extremely important to select the electrolyte and the construction materials of the cell carefully. In actual practice, some of the baser impurities do get transferred to the cathode due to chemical reactions. As an example, let the case of the electrorefining of vanadium in a molten electrolyte composed of sodium chloride-potassium chloride-vanadium dichloride be considered. Aluminum and iron are typically considered as baser and nobler impurities in the metal. When the impure metal is brought into contact with the molten electrolyte, the following reaction occurs... 

Solid metal electrodes are usually polished mechanically and are sometimes etched with nitric acid or aqua regia. Purification of platinum group metal electrodes is effectively achieved also by means of high-frequency plasma treatment. However, electrochemical preparation of the electrode immediately prior to the measurement is generally most effective. The simplest procedure is to polarize the electrode with a series of cyclic voltammetric pulses in the potential range from the formation of the oxide layer (or from the evolution of molecular oxygen) to the potential of hydrogen evolution (Fig. 5.18F). 

Chen, G.H., Electrochemical technologies in wastewater treatment, Separation and Purification Technology, 38 (1), 11-41, 2004. 

Drogui, P., Asselin, M., Brar, S.K., Benmoussa, H., and Blais, J.F., Electrochemical removal of pollutants from agro-industry wastewaters, Separation and Purification Technology, 61, 301-310, 2008. 

Despite the technical success of electrochemical H2 purification, it has not found commercial use. The reason is economic at a total voltage of 200 mV and an electricity cost of 0.05/kWh, the electric cost alone amounts to 2.36 per million BTU, an unacceptable price for commercial-grade H2. The real value in this process is the high purity of the product, and if a need for this purity arises the process will be attractive. At present, electrolytically-produced by-product H2 is often discarded. 

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