Electrochemical Activation: a New Trend in Applied Electrochemistry

Published in “Zhizn & Bezopasnost” Magazine (Life and Safety, rus.), N 3, 2002, p. 302 – 307.

A .P. Tomilov

Background. In 1802, 30 years before M. Faraday discovered the electrolysis laws, Russian academician V. V. Petrov discovered, through using a high-voltage galvanic battery he had developed, that emission of electrolysis gases near the electrodes is accompanied by acidification of water near the anode and alkalization of water near the cathode. V. V. Petrov divided the space between the anode and the cathode with a porous diaphragm, and for the first time ever obtained water rich with products of mostly cathodic or chiefly anodic electrochemical reactions: catholyte and anolyte, respectively.

In 1807-1808, G. Davy, an English researcher, obtained novel, previously unknown metals sodium and potassium through electrolysis. Later electrolysis was used to obtain magnesium and aluminum.

In 1837, a member of the Russian Academy of Sciences, academician B. S. Jacobi published his report on the galvanoplasty method he had developed: production of metal copies of relief items via electrolysis. This discovery became the basis for commercial copper refining.

First electrochemical copper refineries were built in the 1870s after the dynamo had been invented. In 1886 – 1888, factories emerged to electrolytically produce aluminum and hypochlorous acid salts. In 1890, plants were put into operation, to electrolytically produce chlorine, alkali, and metal sodium, and then for hydrogen and oxygen production via water electrolysis, for production of electrolytic nickel, copper and zinc.

Currently, electrolysis of aqueous salt solutions, melts, manufacture of chargeable elements and galvanic batteries is one of the largest industries: the electrochemical industry. The scope of the electrochemical industry is versatile. The most important deliverables are:

  • refining of nonferrous and noble metals;
  • production of nonferrous metals from ores;
  • production of alkali, alkali-earth and other light metals;
  • generation of hydrogen, oxygen, chlorine and alkalis;
  • electrolytic synthesis of inorganic and organic substances;
  • ornamental and corrosion-resistant coatings/plating for metals;
  • manufacture of electric accumulators, galvanic batteries and other chemical electricity sources;
  • dimensional electrochemical machining of metals and alloys.

Virtually all chlorine (the global production is about 50 mln tpa), almost all caustic soda, metals such as magnesium and aluminum are produced using electrochemical processes which also play an important role in the metallurgy of copper, zinc, cadmium, nickel, tin, sodium, beryllium, zirconium, indium, and in production of some noble metals, including gold.

The majority of the most important modern electrochemical sites comprise large-scale electrolyzers or electroplating bathes spread over extensive areas of thousands of square meters, interconnected into a single production complex with a network of piping and electric lines. The operation of this complex is maintained by a broad range of auxiliary facilities: water treatment, preparation of initial solutions, conditioning of electrolysis products, waste regeneration and neutralization etc.

What impeded the discovery of electrochemical activation? Virtually all electrolytic solutions used in applied electrochemical processes have high concentrations and low specific electrical resistance, which is stipulated by the requirements to minimize the electricity consumption per unit of the product generated.

Since fresh water or low-mineralized initial solutions did not find any practical use as electrolytic solutions in applied electrochemistry, an opinion have been formed in more than a century of its development, that fresh water cannot be electrolyzed due to its low content of ions. This notion was based on the traditionally established approaches to commercial electrochemical processes, where the range of used voltages did not usually exceed 6 Volts at a current strength of several hundreds of Amperes on the electrodes of a single element.

In fact, fresh, ultra-fresh and even distilled water can be electrolyzed, but it requires high voltage between the electrodes, while the water electrolytic decomposition goes on at a low current density i.e. at a very high wasteful (in term of industrial electrolysis) electricity consumption.

How the phenomenon of electrochemical activation was discovered? In 1972, engineer V. M. Bakhir first mentioned a previously unknown fact: the anolyte and catholyte generated from low-mineralized water in a diaphragm electrochemical reactor, have physical and chemical parameters and reactivity which differ quite strongly from those of model catholyte and anolyte prepared through dissolution of chemicals in water (with the type and the quantity of the chemicals being determined according to the classical electrolysis laws). Further research showed that the differences between the properties of freshly prepared catholyte and anolyte of diluted aqueous salt solutions and their chemical model counterparts (solutions of stable alkalis or acids) are not constant and stable over time. Upon some time (the relaxation time) the qualities and the reactivity of anolyte and catholyte through spontaneous changes become identical to the corresponding parameters of their chemical models, i.e. in the end the electrolysis laws are strictly met, even though not immediately but after quite a long period: generally from tens of minutes to tens or even hundreds of hours.

These discovered significant differences in reactivity and physical and chemical parameters allowed V. M. Bakhir to name the anolyte and catholyte during their relaxation period as "activated", or, in other words, electrochemically activated solutions (water) and to formulate the basic principles of the electrochemical activation technology.

Electrochemical activation: the phenomenon and the technology. Electrochemical activation as a physical & chemical process is a complex of electrochemical and electrophysical impacts acting on water and the ions and molecules of the dissolved substances, under conditions of the minimum heat emission, in the area of volume charge near the electrode (either anode or cathode) surface of an electrochemical system, when the charge transfer by electrons across the 'electrode-electrolyte' border is non-equilibrium.

The electrochemical activation causes water to transform into a metastable (activated) state characterized by unusual values of chemical and physical parameters including the oxidation-reduction potential (connected with the activity of electrons in water, electric conductivity, pH and other parameters and properties). The characteristics and qualities of water that have been disturbed by a previous external exposure change spontaneously over time and gradually reach equilibrium values through relaxation.

The process of generation of electrochemically-activated water and solutions is classified as extremely non-equilibrium and is the subject of research of a new intensively developing area in chemistry: the synergetics in chemical processes and chemical technology. While the major goal of the applied electrochemistry is to find parameters of the optimum approach to equilibrium for an electrochemical process, for electrochemical activation it is important to determine parameters of the optimum removal from the electrochemical reaction equilibrium conditions.

Electrochemical activation as a technology is the production and further utilization of electrochemically activated water either in the process of its water treatment and purification, or as a medium or a reagent in versatile processes with a view to controlling complicated physical & chemical reactions; saving of energy, time, and materials; final product quality improvement; and less waste generation.

It is to be explained that the term "water", from the viewpoint of the electrochemical activation processes, means a diluted aqueous solution of electrolytes of simple or complex composition, with a total concentration from several milligrams to several grams per liter. Generally, this is both distilled, and ultra-fresh, and fresh, including drinking, and low-mineralized (process or service) water, i.e. aqueous electrolytic solutions with the specific electric conductivity changing greatly with a comparatively small change of concentration. Fig.1 shows a generalized dependence of the specific electric conductivity on the concentration for most of inorganic electrolytes: acids, alkalis, and salts. Electrochemical activation effects are most prominent for aqueous solutions with the electrolyte concentration below 0.1 mol/l; these effects are much weaker in solutions with the electrolyte concentration above 0.1 mol/l.

It is to be noted, that the reaction products obtained from activated solutions, generally do not change their properties and conditions with time, i.e. they are not prone to relaxation processes. For example, if water with a high iron-ion content reacts with an activated catholyte, and thrice more iron ions turn into insoluble compounds than in the case of the same water reacting with a chemical model of the catholyte, the reaction results are irreversible in both cases.

Technical systems for electrochemical activation. Generally, when direct electric current flows through water, chemical reactions always take place on the electrodes, and the chemical composition of water near the anode and cathode changes. The objective of electrochemical activation is to subject the entire volume of liquid to the action of an electric field with the maximum possible intensity at the maximum possible chemical exposure and the minimum heat emission. This task is quite difficult, because the maximum intensity of the electrophysical effect can only be ensured in the intimate vicinity of the electrode surface, i.e. in the electric double layer (EDL) area, for any electrochemical systems consisting of two electrodes immersed in liquid.

Hence, special electrochemical reactors are necessary to perform electrochemical activation processes because conventional electrolyzers (both laboratory and industrial) are designed to optimize the performance of conventional applied electrochemical processes, and are not suitable for operation with fresh water or diluted aqueous solutions. In fact, water only gets activated in the immediate proximity to the electrode surface where the electric field intensity in the electric double layer (EDL) reaches hundreds of thousands Volts per cm.

EDL is very thin: it is about 0.1 m m in weak solutions and fresh water, and much thinner in concentrated solutions. To understand clearer how difficult it is to ensure the contact between all the micro-volumes of the water surrounding the electrode and the electrode surface, let us assume that an electrode (a metal rod with roughly a pen's diameter) is immersed into a beaker. If we also assume that the high-intensity electric field area around the electrode (EDL) would suddenly have expanded to 1 mm, the beaker diameter should have increased from 7 cm to 700 m, to retain the proportions of the system. Of course, all the water in this 'lake' cannot be processed near the electrode surface, if special means are not applied.

First special technical devices for carrying out electrochemical activation (ECA), diaphragm electrochemical reactors, were developed in 1974 – 1975 by V. M. Bakhir and his colleague Yu. G. Zadorozhnii. Strenuous work to create an optimum reactor design for electrochemical treatment of fresh water and diluted aqueous solutions had been going on for almost 20 years, and resulted in the creation of a basically new design, the flow-through electrochemical modular FEM-1 cell, in the end of the 1980s – beginning of the 1990s. However, the broad commercial application of the processes using electrochemically activated solutions and water only became possible in the recent 7-8 years due to the advent of a new type of industrial electrochemical systems based on flow-through electrochemical modular cells of the 3rd generation (FEM-3, see Fig. 2) and RFE reactors (units of FEM-3 cells of various configurations), which were also created by the above-mentioned inventors.

Fig. 2. Section of the FEM cell: 1 - cathode; 2- anode; 3 - diaphragm

The FEM cell differs from the known electrochemical reactors in the following respects:

  • The FEM cell is modular, has small outline dimensions, low weight, high productivity and economic efficiency; the combination of these features allows a FEM-cell to be used both in industrial and household technical electrochemical systems.
  • The FEM cell diaphragm is made of zirconium- and aluminum-based ceramics. It features a very high strength, a low filtration capacity, which prevents cathode and anode water volumes from physical mixing, can withstand the trans-membrane pressure gradient up to 1 at, with the electrode chamber sizes kept unchanged.
  • The FEM cell diaphragm can adsorb positively-charged particles on the surface facing the anode, and negatively charged particles on the surface facing the cathode. This causes a reduction in its electrical resistance in diluted aqueous solutions and fresh water, and hence decreases the power consumption as well as makes durable work possible when there is pressure gradients of opposite charges between the electrode chambers. Due to this the diaphragm may be used as an ion-selective partition in an electrochemical reactor.

  • The FEM cell electrode chambers are annular elongated spaces between the cylindrical surfaces of the electrodes and the diaphragm. The ratio of their dimensions ensures an equal average movement velocity for liquid microvolumes in each cross-section, and facilitates contact of the maximum possible quantity of water microvolumes and the electrode surface, i.e. the EDL area.
  • The optimal length to width ratio in the electrode chambers in the FEM cell ensures that saturation of solutions with gas in electrode chambers will have no detrimental effect on the energetic and functional characteristics of the FEM cell at a high current density and a low liquid flowrate.
  • FEM cells in electrochemical water treatment devices can be hydraulically connected into a single hydraulic chain without flow interruption not only in parallel, but in series, too. This is not possible when using conventional electrochemical reactors. Both anode and cathode chambers of various FEM cells can be connected into the circuit arbitrarily and in any order, and auxiliary devices (flotation, catalytic reactors; flow, pressure, velocity, temperature control systems, etc.) can be cut in between the elements as required.
  • FEM cells can be electrically connected in parallel, in series, or in series-parallel. This allows the circuit to be easily switched when required from bipolar electrolyzer to monopolar or bipolar-monopolar electrolyzer, without changing the hydraulic configuration.

Conditions are created in the FEM cells for the major part of water microvolumes to get treated within split seconds in a high-intensity field in the EDL. This enables to obtain water with pronounced electron-acceptor properties near the anode (oxidant water), whereas water with electron-donor properties is generated near the cathode (antioxidant water). Both anolyte and catholyte are kinds of water with an increased electric activity, which becomes apparent in the following physical & chemical or biochemical reactions not only as an independent factor, but as the catalyst increasing the activity of the small amount of the products of anodic and cathodic reactions synthesized during the electrochemical treatment process.

Where is the electrochemical activation currently used? Since the beginning of the 1990s, OAO “NPO EKRAN” and VNIIIMT (the Russian Scientific and Research Institute for Medical Engineering) of the RF Ministry of Health have been the leaders in the area of researches into electrochemical activation and development of various processes based on electrochemically activated media. A number of major scientific centers in Russia is also involved in this problem, in co-operation with the above-indicated organizations. Currently, the ECA phenomenon and technologies are studied throughout the world: in the USA, the UK, Germany and some other countries.

STEL devices have become well recognized both in Russia and abroad: they provide environment-friendly germicide and disinfectant solutions (ANK-type anolyte) for medical and childcare outfits, public utilities, food industry, swimming pools.

Table 1 compares the characteristics of the electrochemically activated ANK anolyte produced in STEL devices and conventional disinfectant and germicide chemicals.

Table 1

ANK anolyte and various disinfectant solutions: comparison of characteristics

The results of the research completed in the Memorial Battelle Institute (USA) provided convincing evidence of the advantages of electrochemically activated ANK anolyte over conventional disinfectant solutions. Anthrax spores were established to perish in the ANK anolyte within several seconds, while the same result was obtained only after 30 min in a sodium hypochlorite solution with the concentration of the active substance 12 times higher than in the ANK anolyte.

Various modifications of EMERALD-type devices are well known: they provide disinfected water with an improved composition to individual users, childcare outfits, food industry and public catering facilities: this water is just as useful as spring and best mineral waters. Table 2 shows the technical characteristics of different designs/types of household medical EMERALD devices commercially manufactured by OAO "NPO EKRAN". The dimensions of these devices are 230´ 50´ 300 mm, the weight is about 1.9 kg, the power consumption is 30 to 60 W.

Table 2.

Water treatment in EMERALD devices: quality indicators.

Pollutant description

Maximum permissible concentration (MPC) according to WHO

Relative pollutant concentrations in source water (related to MPC)

Purification degree, %

Total microbe number, CFU/ml

50 – 100

2 - 10

> 99.9

Coli index, CFU/l

3

~ 105

> 99.99

Poliomyelitis virus, units/ml

-

up to 100 units/ml

> 99.99

Trichloroethylene, mg/l

0.005

Up to 20

70-80

Tetrachloroethylene, mg/l

0.005

10

89

Benzene, mg/l

0.01

200

74

Surfactants, mg/l

0.5

7 - 8

94

Pesticides (DDT), mg/l

-

up to 10 mg/l

90

Aluminum, mg/l

0.5

2

8

> 10

63

75

97

Chromium, mg/l

0.05

1.5

10

> 100

70

95

> 99

Iron, mg/l

0.3

3

5

10

70 – 80

85

95

Copper, mg/l

1.0

3 – 4

20 - 30

84

80 to >99

Zinc, mg/l

5.0

5

10

97

> 99

Arsenic, mg/l

0.05

3 – 3.5

90

Lead, mg/l

0.03

2.5

3.5

> 10

65

78

> 95

Phenol, mg/l

0.001

6

10 - 20

> 100

85

50 – 70

80 - 96

Nitrates, mg/l

10

2

3 -3.5

19 – 43

up to 10

Nitrites, mg/l

1.0

1.0

80 - 99

Trihalomethanes, mg/l

0.1

0.8

2

100

95

Additional resources of technical electrochemical activation systems. Applying the flow-through electrochemical modular FEM cells to solve problems of conventional applied electrochemical technologies, we were able to create a basically new technology for on-site chlorine production. This technology was implemented in AQUACHLOR devices, which have no analogs in the world. The philosophy of the process realized in the AQUACHLOR devices is to feed 10¸ 20% NaCl solution slowly, commensurable to the electrochemical reaction rate, into the FEM cell anode chambers under the pressure of 0.5¸ 0.7 kgf/cm2. Evolved gaseous chlorine is removed through a pressure regulator and sent to users, for example, it is fed to an ejector mixer and gets dissolved to give chlorine water. This chlorine water can further be used to disinfect natural fresh water in water treatment facilities or to treat wastewater, while the catholyte generated during pressure gradient-caused filtration of water and sodium ions from the anodic chamber into the cathodic one through the porous oxide-zirconium diaphragm is used as a detergent/cleaning agent or as a pH-controlling agent in coagulation water treatment processes. It is to be emphasized that energy and NaCl consumption in the AQUACHLOR device is quite close to the theoretical limits. This makes this technology attractive not only for local chlorine users, but for Cl commercial manufacturers, too.

References:

  1. Bakhir V. M., Atadzhanov A. R., Mamadzhanov U. D., Alyokhin S. A., Mariampolsky N. A., Nadzhmitdinov A. Kh. Activated Substances. Some Theoretical and Practical Issues // Bulletin of the Uzbek Academy of Sciences. Series of technical sciences, 1981, No. 5. - p. 68-72.
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  3. Bakhir V. M., Zadorozhnii Yu. G. Electrochemical RFE Reactors. -M.: Giperox, 1991. - 35 pp; - figures.
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  5. Bakhir V. M. Electrochemical Activation. - M.: VNIIIMT, 1992. – 2 parts - 657 pp; - figures
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  7. Bakhir V. M. Medical Technical Systems and Technologies for Synthesis of Electrochemically Activated Solutions. - M., VNIIIMT , 1998. - 66 pp; - figures
  8. Panicheva S. A. New Technologies for Sterilization and Disinfection of Complicated Items Designed for Medical Use. - M., VNIIIMT , 1998. - 122 pp; - figures
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  10. Bakhir V. M. Modern Technical Electrochemical Systems for Water Decontamination, Purification and Activation. – M.: VNIIIMT , 1999. – 84 pp; - figures
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