When a direct electric current is passed through a conductor, a magnetic field appears around it. It is detected by the location of the steel filings on it. Electrolysis. Electrolysis is a set of redox processes that occur during
Electrolysis is a set of redox processes that occur when passing a constant electric current through a solution or melt of electrolyte with electrodes immersed in it.
The device in which electrolysis is carried out is called an electrolyzer.
The electrode on which oxidation processes occur is called an anode. In the electrolyzer it is positively charged (connected to the positive pole of an external DC source).
The electrode on which the reduction processes take place is called the cathode. In the electrolyzer it is negatively charged (connected to the negative pole of an external DC source).
When voltage is applied, cations (positively charged particles) move towards the cathode, anions (negatively charged particles) move towards the anode, and there they are discharged. At the anode, ions give up electrons and oxidation occurs. At the cathode, ions accept electrons and their reduction occurs.
Only cations and anions of the electrolyte do not always participate in electrode processes; solvent molecules, in particular water, compete with them if electrolysis of an aqueous solution is carried out.
In addition, the participation of water in electrochemical processes during electrolysis can lead to a different result. Free radicals OH ( due to oxidation of hydroxide ions at the anode) and N ( due to the reduction of hydrogen ions at the cathode) have a high reactivity and strong oxidative and restorative properties. At the surface of the electrode they are able to interact with substances dissolved in water. In such cases, they speak of oxidation in the anodic and reduction in the cathodic spaces.
Features of electrical flow chemical processes in aqueous solutions are due to the ability of water molecules to undergo both oxidation (at the anode) and reduction (at the cathode).
Anode (+) pH=0 pH=7 pH=14
2H 2 O – 4e = 2O + 2H + 4OH – – 4e = 4OH 4OH – – 4e = 4OH
2O = O 2 4OH = O 2 + 2H 2 O 4OH = O 2 + 2H 2 O
2H 2 O – 4e = 2O + 2H +
Cathode (–) pH=0 pH=7 pH=14
2H + + 2e = 2H 2H 2 O + 2e = 2H + 2OH – 2H 2 O + 2e = 2H + 2OH –
2H = H 2 or 2H = H 2
Distinguish primary And secondary electrode processes. Primary ones are of an electrochemical nature, secondary ones are non-electrochemical. As a result of electrolysis, the corresponding reduction and oxidation products (primary processes) are released at the electrodes (cathode and anode), which, depending on the conditions, can react with the solvent, the electrode material, with each other (atomic recombination), etc. (secondary processes). In some cases, it is not possible to clearly separate the primary and secondary processes. In the example above, the free radicals OH (at the anode) and H (at the cathode) were formed as a result of primary processes, and the oxidation of manganate ions and the reduction of nitric acid were secondary processes. Let's look at another example.
In some cases, the main processes during electrolysis are affected by side reactions: interaction between electrolysis products or reactions of products with water. To prevent secondary reactions between electrolysis products, diaphragms (partitions between the anode and cathode) are used to prevent the diffusion of certain ions. For example, in the given example with the electrolysis of a sodium chloride solution, to prevent interaction between chlorine and hydroxide ions, the cathode is surrounded by a diaphragm that prevents the diffusion of sodium and chlorine ions. As a result, alkali (NaOH) is concentrated in the cathode space. Therefore, in most cases, one should expect a slight difference in the composition of the products when electrolysis of the same solution with and without a diaphragm.
E diff = E A – E K
For each electrolyte, there is a certain minimum voltage value (from an external current source) that must be applied to the electrodes for electrolysis to occur. It is called decomposition voltage (E decomposition).
The decomposition voltage is the difference between the electrode potentials of the anodic and cathodic processes.
E diff = E A – E K
At the cathode, the reduction of ions or molecules that are part of the redox system with the most positive potential (being the reduced form in redox systems with the most positive potential) occurs first.
1) If subjected to electrolysis melt containing several different cations metals, then in this case the sequence of reduction is determined by the electrode potentials of the metals under given conditions ( in this melt!). In this case, metal cations with a high electrode potential are reduced first (from the end of the voltage series for a given melt).
2) Restorative processes at the cathode in aqueous solutions:
· metal cations located in the voltage range after hydrogen (with a standard electrode potential greater than that of hydrogen): Cu 2+, Hg 2 2+, Ag +, Hg 2+, Pt 2+ ... Pt 4+. During electrolysis, they are almost completely reduced at the cathode and released as metal.
· metal cations located at the beginning of the row (with a standard electrode potential less than that of aluminum): Li +, Na +, K + ... Al 3+. During electrolysis, they are not reduced; instead, water molecules are reduced.
· metal cations located in the series after aluminum and before hydrogen (with a standard electrode potential greater than that of aluminum, but less than that of hydrogen): Mn 2+, Zn 2+, Cr 3+, Fe 2+ ... H During electrolysis, these cations are reduced at the cathode simultaneously with water molecules.
3) If a gradually increasing voltage is applied to a solution containing several cations, electrolysis begins when the decomposition potential of the cation with the most positive potential is reached. Thus, during the electrolysis of a solution containing Cu 2+ ions (E 0 Cu 2+/ Cu = 0.35 V) and Zn 2+ (E 0 Zn 2+/ Zn = – 0.76 V), copper is first released at the cathode, and only after Once almost all copper ions are discharged, zinc will begin to be released.
It would seem that, based on the values of the electrode potentials, only metals in the voltage series after hydrogen could be deposited in an aqueous solution. However, thanks to hydrogen overvoltage, it is possible to precipitate from aqueous solutions many metals that, according to their standard potentials, should not be precipitated (for example, Zn). In addition, the nature of the environment (acidic, neutral, alkaline) affects the nature of the discharged metal. This is due to the fact that, as shown above, the electrode potential depends on the reaction of the medium.
3.4.2 Electrochemical production
Electrolysis is a redox reaction that occurs when a direct electric current is passed through a melt or electrolyte solution.
The essence of electrolysis is as follows: when an electric current is passed through a melt or electrolyte solution, positive ions of the electrolyte (metal or hydrogen ions) are attracted by the cathode, and negative ions (acid residues or hydroxyl groups) by the anode. The electrons brought to the cathode from the current source join the positive ions of the electrolyte, reducing them. At the same time, the negative ions of the electrolyte give up their electrons to the anode, from which they move to the current source. Losing their electrons, they are oxidized into neutral atoms or groups of atoms. Thus, a reduction process occurs at the cathode, and an oxidation process occurs at the anode.
A (+): nA n - - ne - → nA p -
K (-): nB n + + ne - → nB p +
Both processes form a single redox reaction. But unlike conventional redox reactions, electrons do not pass from the reducing agent to the oxidizing agent directly, but through an electric current. The cathode, which brings in electrons, is a reducing agent, and the anode, which carries them away, is an oxidizing agent.
The main indicators of electrochemical production are current efficiency and the degree of energy use. Energy consumption coefficient, voltage applied to the electrolyzer, etc. Most calculations are based on Faraday’s law, according to which the mass of a substance released during electrolysis is proportional to the current strength I, electrolysis time t and the electrochemical equivalent of this substance E E
The mass of a substance is calculated using the formula
where, I - current strength, F - Faraday constant (96500 C)
(g-eq) (1.3.2)
Mr – relative molecular mass substances,
n is the charge of the ion (absolute value) in the form of which the substance is in solution or in the melt (i.e., the number of electrons given or received).
The current output is determined by the ratio of the mass of the substance released during electrolysis to the mass of the substance that theoretically should be released according to Faraday's law, and is expressed as a percentage:
(1.3.3)
The mass m theor is found by the formula
The energy yield is determined by the equation
where, E theor and E pr are the theoretical and practical decomposition voltage during electrolysis, respectively, V; η - energy yield,%.
Energy output can also be calculated by the amount of energy expended:
(1.3.6)
where w theoretical and w pr are the amount of energy theoretically required and practically spent to obtain a unit of product.
(1.3.7)
where 1000 is the conversion factor from Wh to kWh;
1*10 -6 is the number used to convert grams to tons.
The theoretical energy consumption is related to
(1.3.8)
where φ decomposition is the decomposition voltage.
Examples of problem solving
1. What processes occur during the electrolysis of molten sodium hydroxide?
The caustic soda melt contains Na + and OH ions. The OH ions oxidizing at the anode in the next stage decompose to form water and oxygen. The process can be depicted as follows:
K(-): 2Na + + 2е - = 2Na;
A(+): 2OH - 2e - = H 2 O + O 2
Two oxygen atoms combine with each other to form an oxygen molecule O2. Thus, summary equation
4NaOH = 4Na + 2H 2 O + O 2
During the electrolysis of molten salts of oxygen acids, the oxidizing ions of acidic residues immediately decompose into oxygen and the corresponding oxides.
Electrolysis occurs in an aqueous solution in a unique way. The fact is that water itself is an electrolyte, albeit a very weak one. Thus, an aqueous solution actually contains two electrolytes - a solvent and a solute and, accordingly, two types of both positive and negative ions. Which of them will be discharged depends on a number of conditions. As a rule, you can be guided by the following. If the positive ions of the electrolyte are ions of very active metals, such as Na + or K -, then during electrolysis it is not the ions of these metals that are discharged, but hydrogen ions from water with the release of free hydrogen and the release of hydroxyl ions, which can be expressed by the following electron-ion equation :
2H+OH+ 2e - = H 2 + 2OH
If negative ions electrolyte are acidic residues of oxygen acids, then during electrolysis it is not the acidic residues of these acids that are discharged, but OH ions from water with the release of oxygen, which can be expressed by the equation:
4H 2 O - 4e - = 4H + + 4OH
4OH - 2H 2 O+O 2
Adding both equations, we get:
2H 2 O - 4e - = 4H + + O 2
2. Determine the current output (in%), if within 24 hours 4200 liters of electrolytic alkali with a NaOH concentration of 125 kg/m 3 were obtained in an electrolyzer of a sodium chloride solution at a current of 15500 A.
According to equation (1.3.4), the mass of sodium hydroxide should theoretically be
almost received
Therefore, the current output according to formula (1.3.3) will be equal to
Answer: current efficiency 94.6%.
3. Determine the actual energy consumption (in kilowatt-hours) for the production of chlorine weighing 1 ton and the energy yield (%) if the average voltage on the electrolyzer is 3.35 V, the current yield is 96%, and the electrochemical equivalent of chlorine is 1.323 g/ A*h.
Using formula (1.3.7), we determine the actual energy consumption
If we take the current efficiency as 100%, then with a theoretical voltage of NaCl decomposition equal to 2.17 V, the theoretical energy consumption per 1 ton of chlorine will be
In this case, the energy output
Answer: energy efficiency 62.2%; 2637 kW/h
Tasks for independent decision
1. One way industrial production calcium – electrolysis of molten calcium chloride. What mass of metal will be obtained if it is known that as a result of electrolysis, chlorine with a volume of 896 l (n.s.) was released?
2. During the electrolysis of a sodium chloride solution in an electrolysis that operated for 24 hours at a current of 30,000 A, 8.5 m 3 of electrolytic alkali with a NaOH concentration of 120 kg/m 3 was obtained. Calculate the current output (for alkali)
3. Determine the current required to produce 100% sodium hydroxide weighing 1720 kg per day in an electrolyzer with iron roll during continuous operation, if the current efficiency is 96%
4. Calculate the mass of chlorine produced per year by a plant that installed 5 series of 150 electrolyzers with iron cathodes with continuous operation for 350 days, a current of 34,000 A and a current efficiency of 95%. Determine the power of the power plant’s alternating current generator to meet the plant’s needs for electrical energy at a bottom series voltage of 550 V, if the efficiency of the current rectifier is 95%.
5. Calculate the theoretical and practical energy consumption per 1 ton of 100% NaOH for the electrolysis of a sodium chloride solution with a mercury cathode. The theoretical decomposition voltage is 3.168 V. Determine the energy yield if the practical decomposition voltage is 4.4 V and the current yield is 92.5%.
6. What substances and in what quantities are released on the carbon electrodes if the solution composition is 0.1 mol HgCl 2 and 0.2 mol CuCl 2 and a current of 10 A is passed through it for 1 hour?
7. When an electric current passes through a dilute solution of sulfuric acid for 10 minutes, 100 ml of hydrogen is released at 18C and pressure
755 mmHg Art. Calculate the current.
8. In the electrolytic production of magnesium, molten magnesium chloride can serve as an electrolyte. Calculate the current output if 72.6 kg of magnesium is released in a bath operating at a current of 40,000 A for 5 hours.
9. Determine the amount of electricity required to release 1 m 3 of hydrogen and 0.5 m 3 of oxygen obtained from the electrolysis of water. The theoretical water voltage is 1.23 V, and the actual voltage is 1.5 - 2 times higher. Calculate the actual consumption of electrical energy.
10. During the electrolysis of a solution containing 2.895 g of a mixture of FeCl 2 and FeCl 3, 1.12 g of metal was released at the cathode. Calculate mass fraction each of the components of the initial mixture, if electrolysis was carried out until complete precipitation of iron.
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And the didactic foundations of teaching organization make it possible to more clearly explain the material being studied in physics lessons when studying the topic “Fundamentals of Electrodynamics”. The analysis of various technologies made it possible to create the author’s technology for developing students’ focus on dialogical communication in a group form of learning. How correctly the learning process will be structured when using...
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The formation of an insoluble substance as a result of a chemical reaction is only one of the conditions for obtaining a colloidal solution. Another equally important condition is the inequality of the starting materials taken into the reaction. The consequence of this inequality is the limitation of the growth of particle size in colloidal solutions, which would lead to the formation of a coarsely dispersed system.
Let us consider the mechanism of formation of a colloidal particle using the example of the formation of a silver iodide sol, which is obtained by the interaction of dilute solutions of silver nitrate and potassium iodide.
AgNO 3 +KI = AgI + KNO 3
Ag + + NO 3 ¯ +K + + I ¯ = AgI ↓ + NO 3 ¯ + K +
Insoluble neutral silver iodide molecules form the core of the colloidal particle.
At first, these molecules combine in disorder to form an amorphous, loose structure, which gradually develops into a highly ordered crystalline core structure. In the example we are considering, the core is a crystal of silver iodide, consisting of a large number (m) of AgI molecules:
m - core of colloidal particle
An adsorption process occurs on the surface of the core. According to the Peskov-Fajans rule, on the surface of the nuclei colloidal particles ions that are part of the particle core are adsorbed, i.e. silver ions (Ag +) or iodine ions (I –) are adsorbed. Of these two types of ions, those that are in excess are adsorbed.
Thus, if you obtain a colloidal solution in an excess of potassium iodide, then iodine ions will be adsorbed on the particles (nuclei), which complete the crystal lattice of the nucleus, naturally and firmly entering its structure. In this case, an adsorption layer is formed, which gives the core a negative charge:
Ions that are adsorbed on the surface of the nucleus, giving it a corresponding charge, are called potential-forming ions.
At the same time, there are also oppositely charged ions in the solution, they are called counterions. In our case, these are potassium ions (K +), which are electrostatically attracted to the charged nucleus (the charge value can reach I in). Some of the K+ counterions are firmly bound by electrical and adsorption forces and enter the adsorption layer. The core with a double adsorption layer of ions formed on it is called a granule.
(m . nI – . (n-x) K + ) x – (granule structure)
The remaining part of the counterions (we denote them by the number “x K +”) forms a diffuse layer of ions.
The core with adsorption and diffuse layers is called a micelle :
(m . nI –. (n-x) K + ) x – . x K + (micelle structure)
When a direct electric current is passed through a colloidal solution, the granules and counterions will move towards the oppositely charged electrodes, respectively.
The presence of the same charge on the surface of sols particles is important factor of its stability. The charge prevents particles from sticking together and becoming larger. In a stable disperse system, particles are kept in suspension, i.e. There is no precipitation of colloidal substance. This property of sols is called kinetics chemical stability.
The structure of micelles of silver iodide sol obtained in excess AgNO 3 is shown in Fig. 1a, in excess KCI - 1b .
Fig.1.5. The structure of micelles of silver iodide sol obtained in excess:
a) silver nitrate; b) potassium chloride.
Electroactivated water solutions - catholytes and anolytes can be used in agriculture, to increase plant productivity, in animal husbandry, medicine, for water disinfection and for domestic purposes. Electrochemical treatment of water includes several electrochemical processes associated with the transfer of electrons, ions and other particles in a constant electric field (electrolysis, electrophoresis, electroflotation, electrocoagulation), the main of which is water electrolysis. This article introduces the reader to the basic processes underlying the electrolysis of water.
Introduction
The phenomenon of electrochemical activation of water (ECAW) is a combination of electrochemical and electrophysical effects on water in double electrical layer(DES) electrodes (anode and cathode) with nonequilibrium charge transfer through the DES by electrons and under conditions of intense dispersion of the resulting gaseous products of electrochemical reactions in the liquid. During the ECHA process, four main processes occur:
— electrolytic decomposition of water (electrolysis) due to redox reactions at the electrodes caused by an external constant electric field;
- electrophoresis - movement in an electric field of positively charged particles and ions to the cathode, and negatively charged particles and ions to the anode;
— electroflotation — the formation of gas flocculi and aggregates consisting of finely dispersed gas bubbles (hydrogen at the cathode and oxygen at the anode) and coarse water impurities;
- electrocoagulation - the formation of colloidal aggregates of particles of the deposited dispersed phase due to the process of anodic dissolution of the metal and the formation of metal cations Al 3+, Fe 2+, Fe 3+ under the influence of a constant electric field.
As a result of treating water with a direct electric current, at potentials equal to or exceeding the decomposition potential of water (1.25 V), water passes into a metastable state, characterized by anomalous values of electron activity and other physicochemical parameters (pH, Eh, ORP, electrical conductivity). The passage of a direct electric current through a volume of water is accompanied by electrochemical processes, as a result of which redox reactions occur, leading to the destruction (destruction) of water contaminants, coagulation of colloids, flocculation of coarse impurities and their subsequent flotation.
The phenomenon of electrochemical activation of water is a combination of electrochemical and electrophysical effects on water in a double electric layer of electrodes during nonequilibrium charge transfer.
Electrochemical processing is used for lightening and bleaching natural waters, their softening, removal of heavy metals (Cu, Co, Cd, Pb, Hg), chlorine, fluorine and their derivatives, for cleaning Wastewater containing petroleum products, organic and organochlorine compounds, dyes, surfactants, phenol. The advantages of electrochemical water purification are that it allows you to adjust the values pH value pH and redox potential E h, on which the possibility of various chemical processes in water depends; increases the enzymatic activity of activated sludge in aeration tanks; reduces resistivity and improves conditions for coagulation and sedimentation of organic sediments.
In 1985, ECHA was officially recognized as a new class of physicochemical phenomena. By order of the Government of the Russian Federation dated January 15, 1998 No. VCh-P1201044, recommendations were given to ministries and departments to use this technology in medicine, agriculture, and industry.
Electrolysis of water
The main stage of electrochemical water treatment is water electrolysis. When a direct electric current is passed through water, the entry of electrons into the water at the cathode, as well as the removal of electrons from the water at the anode, is accompanied by a series of redox reactions on the surface of the cathode and anode. As a result, new substances are formed, the system of intermolecular interactions, the composition of water, including the structure of water, changes. A typical installation for electrochemical water treatment consists of a water preparation unit 1, an electrolyzer 2, a water treatment unit after electrochemical purification 3 (Fig. 1).
Some installations for electrochemical water treatment provide for preliminary mechanical purification of water, which reduces the risk of clogging the electrolytic cell with coarse impurities with a large hydraulic resistance. A block for mechanical water purification is necessary if, as a result of electrochemical treatment, the water is saturated with coarse impurities, for example, flakes of metal hydroxides (Al(OH) 3, Fe(OH) 3, Mg(OH) 2) after electrocoagulation. The main element of the installation is an electrolyzer, consisting of one or several electrolysis cells (Fig. 2).
An electrolysis cell is formed by two electrodes - a positively charged anode and a negatively charged cathode, connected to different poles of a direct current source. The interelectrode space is filled with water, which is an electrolyte capable of conducting electric current. As a result of the operation of the device, transfer occurs electric charges through a layer of water - electrophoresis, that is, the migration of polar particles, charge carriers - ions, to electrodes having the opposite sign.
When a direct electric current is passed through water, the entry of electrons into the water at the cathode, as well as the removal of electrons from the water at the anode, is accompanied by a series of redox reactions on the surface of the cathode and anode.
In this case, negatively charged anions move to the anode, and positively charged cations move to the cathode. At the electrodes, charged ions lose their charge, depolarize, turning into decay products. In addition to charged ions, polar particles of various dispersions participate in electrophoresis, including coarse particles (emulsified particles, gas bubbles, etc.), but main role Charged ions with the greatest mobility play a role in the transfer of electrochemical charges. Polar particles include polar particles from aqueous impurities and water molecules, which is explained by their special structure.
The central oxygen atom, which is part of the water molecule, has a higher electronegativity than hydrogen atoms, attracts electrons to itself, giving the molecule asymmetricity. As a result, a redistribution of electron density occurs: the water molecule is polarized, taking on the properties of an electric dipole having a dipole moment of 1.85 D (Debye), with positive and negative charges at the poles (Fig. 3).
The products of electrode reactions are neutralized aqueous impurities, hydrogen and oxygen gases formed during the electrolytic destruction of water molecules, metal cations (Al 3+, Fe 2+, Fe 3+) in the case of using metal anodes made of aluminum and steel, molecular chlorine, etc. In this case, hydrogen gas is generated at the cathode, and oxygen at the anode. The water contains a certain amount of hydronium ion H 3 O +, which depolarizes on the surface of the cathode to form atomic hydrogen H:
H 3 O + + e - → H + H 2 O.
IN alkaline environment H 3 O + is absent, but the destruction of water molecules occurs, accompanied by the formation of atomic hydrogen H— and hydroxydione OH -:
H 2 O + e - → H + OH - .
Reactive hydrogen atoms are adsorbed on the cathode surfaces and, after recombination, form molecular hydrogen H2, which is released from water in gaseous form:
N + N → N 2.
At the same time, atomic oxygen is released at the anodes. IN acidic environment this process is accompanied by the destruction of water molecules:
2H 2 O - 4e - →O 2 +4H +.
In an alkaline environment, the source of oxygen formation is always the hydroxide ions OH -, which move under the action of electrophoresis on the electrodes, from the cathode to the anode:
4 OH - → O 2 + 2 H 2 O + 4 e - .
The normal redox potentials of these reactions are +1.23 and +0.403 V, respectively, but the process occurs under conditions of some
overvoltage. The electrolysis cell can be considered as a generator of the above products, some of which, entering chemical reaction between each other and with water contaminants in the interelectrode space, provide additional chemical cleaning water (electroflotation, electrocoagulation). These secondary processes do not occur on the surface of the electrodes, but in the volume of water. Therefore, in contrast to electrode processes, they are designated volumetric. They are initiated by an increase in water temperature during electrolysis and an increase in pH during the cathodic destruction of water molecules.
A distinction is made between cathodic and anodic oxidation. During cathodic oxidation, molecules of organic substances, sorbed on cathodes, accept free electrons, are reduced, transforming into compounds that are not pollutants. In some cases, the recovery process takes place in one stage:
R + H + + e - → RH, where R is an organic compound; RH is the hydrated form of the compound and is not a contaminant.
In other cases, cathodic reduction takes place in two stages: in the first stage (I), the organic molecule is converted into an anion, in the second (II), the anion is hydrated, interacting with a water proton:
R + e - → R - , (I) R - + H + → RH. (II)
A distinction is made between cathodic and anodic oxidation. During cathodic oxidation, molecules of organic substances, sorbed on cathodes, accept free electrons and are reduced.
Cathodes made of materials that require high overvoltage (lead, cadmium) make it possible, with a large expenditure of electricity, to destroy organic molecules and generate reactive free radicals - particles that have free unpaired electrons in the outer orbits of atoms or molecules (Cl*, O*, OH* , BUT*2, etc.). The latter circumstance gives free radicals the property of reactivity, that is, the ability to enter into chemical reactions with water impurities and destroy them.
RH → R + H + + e - .
Anodic oxidation organic compounds often leads to the formation free radicals, the further transformations of which are determined by their reactivity. Anodic oxidation processes are multistage and occur with the formation of intermediate products. Anodic oxidation reduces the chemical stability of organic compounds and facilitates their subsequent destruction during bulk processes.
In volumetric oxidative processes, a special role is played by the products of water electrolysis - oxygen (O 2), hydrogen peroxide (H 2 O 2) and oxygen-containing chlorine compounds (HClO). During the electrolysis process, an extremely reactive compound is formed - H 2 O 2, the formation of molecules of which occurs due to hydroxyl radicals (OH*), which are the products of discharge of hydroxyl ions (OH-) at the anode:
2OH - → 2OH* → H 2 O 2 + 2e - , where OH* is the hydroxyl radical.
Reactions between organic substances and oxidizing agents occur over a certain period of time, the duration of which depends on the value of the redox potential of the element and the concentration of the reacting substances. As purification and contaminant concentrations decrease, the oxidation process decreases.
The rate of the oxidation process during electrochemical treatment depends on the temperature of the water being treated and the pH. During the oxidation of organic compounds, intermediate products are formed that differ from the original one both in their resistance to further transformations and in their toxicity.
The source of active chlorine and its oxygen-containing compounds generated in the electrolyzer are chlorides found in the treated water and sodium chloride (NaCl), which is introduced into the treated water before electrolysis. As a result of anodic oxidation of Cl— anions, chlorine gas Cl 2 is generated. Depending on the pH of the water, it either hydrolyzes to form hypochlorous acid HOCl, or forms hypochlorite ions ClO - . The equilibrium of the reaction depends on the pH value.
At pH = 4-5, all chlorine is in the form of hypochlorous acid (HClO), and at pH = 7, half of the chlorine is in the form of hypochlorite ion (OCl -) and half is in the form of hypochlorous acid (HClO) (Fig. 4). The mechanism of interaction of the hypochlorite ion (ClO -) with the oxidized substance is described the following equation:
ClO - + A = C + Cl, where A is the oxidizable substance; C is an oxidation product.
Electrochemical oxidation of organic compounds with hypochlorithione (ClO -) is accompanied by an increase in the redox potential Eh, which indicates the predominance of oxidative processes. The increase in Eh depends on the ratio of the concentration of active chlorine in the interelectrode space to the content of organic impurities in water. As the amount of pollution is cleaned and the amount of pollution decreases, this ratio increases, which leads to an increase in Eh, but then this indicator stabilizes.
The amount of substance that reacted on the electrodes when passing a direct electric current according to Faraday’s law is directly proportional to the current strength and processing time:
G = AI cur τ, (1)
where A is the electrochemical equivalent of the element, g/(A⋅h); I cur—current strength, A; τ — processing time, hours. The electrochemical equivalent of an element is determined by the formula:
A = M/26.8z, (2)
where M is the atomic mass of the element, g; z is its valence. The values of electrochemical equivalents of some elements are given in table. 1.
The actual amount of substance generated during electrolysis is less than the theoretical one, calculated using formula (1), since part of the electricity is spent on heating water and electrodes. Therefore, the calculations take into account the current utilization factor η< 1, величина которого определяется экспериментально.
During electrode processes, charged particles and ions are exchanged between the electrode and the electrolyte - water. To do this, under established equilibrium conditions, it is necessary to create an electrical potential, the minimum value of which depends on the type of redox reaction and on the water temperature at 25 °C (Table 2).
The main parameters of water electrolysis include current strength and density, voltage within the electrode cell, as well as the speed and duration of water residence between the electrodes.
The voltages generated in the electrode cell must be sufficient to cause redox reactions to occur on the electrodes. The voltage value depends on the ionic composition of water, the presence of impurities in water, for example surfactants, current density (its strength per unit area of the electrode), electrode material, etc. All other things being equal, the task of choosing an electrode material is to reduction reactions on the electrodes, the required voltage was minimal, since this reduces the cost of electrical energy.
Some redox reactions are competitive - they occur simultaneously and mutually inhibit each other. Their flow can be regulated by changing the voltage in the electrolytic cell. Thus, the normal potential of the reaction of formation molecular oxygen is +0.401 V or +1.23 V; when the voltage increases to +1.36 V (the normal potential of the reaction for the formation of molecular chlorine), only oxygen will be released at the anode, and with a further increase in the potential, both oxygen and chlorine will be released simultaneously, and the release of chlorine will occur with insufficient intensity. At a voltage of about 4-5 V, the evolution of oxygen will practically stop, and the electrolytic cell will generate only chlorine.
Calculation of the main parameters of the water electrolysis process
The main parameters of water electrolysis include current strength and density, voltage within the electrode cell, as well as the speed and duration of residence of water in the interelectrode space.
The current strength I cur is a value determined depending on the required performance of the generated product [A], determined by the formula:
I cur = G/A tη, (3)
This formula is obtained by transforming formula (1) taking into account the current utilization factor η. Current density is its strength per unit area of the electrode [A/m 2 ], for example, the anode, is determined from the following expression:
i an = I cur / F an, (4)
where Fan is the anode area, m2. Current density has the most decisive influence on the electrolysis process: that is, with an increase in current density, electrode processes intensify and the surface area of the electrodes decreases, but at the same time the voltage in the electrolysis cell and, as a consequence, the entire energy intensity of the process increase. An increased increase in current density intensifies the release of electrolysis gases, leading to bubbling and dispersion of insoluble products of electrical water treatment.
As the current density increases, passivation of the electrodes also increases, which consists in blocking incoming electrons by surface deposits of the anode and cathode, which increases the electrical resistance in the electrode cells and inhibits the redox reactions occurring on the electrodes.
Anodes are passivated as a result of the formation of thin oxide films on their surfaces as a result of sorption of oxygen and other components on the anodes, which, in turn, sorb particles of aqueous impurities. Carbonate deposits are mainly formed on the cathodes, especially in the case of treating water with increased hardness. For these reasons, the current density during water electrolysis should be set to the minimum under the conditions for the stable occurrence of the necessary redox reactions during the technological process.
The length of time water remains in the interelectrode space of the electrolyzer is limited by the time required to generate the required amount of electrolysis products.
The voltage in the electrode cell [V] is determined by the formula:
V i = i an ΔK g / χ R , (5)
where i an is the current density, A/m 2 ; D—distance between electrodes (width of the interelectrode channel), m; χ R—specific electrical conductivity of water, 1/(Ohm⋅m); K g is the gas filling coefficient of the interelectrode space, usually taken K g = 1.05-1.2.
Formula (5) does not take into account the electrical resistances of the electrode due to their low values, but during passivation these resistances turn out to be significant. The width of the interelectrode channel is assumed to be minimal (3-20 mm) to ensure that it is not clogged with impurities.
The specific electrical conductivity of water χ R depends on a number of factors, among which the most significant are temperature, pH, ionic composition and ion concentration (Fig. 5). With increasing temperature, electrical conductivity χ R increases and voltage decreases (Fig. 6). The minimum value of electrical conductivity corresponds to pH = 7. In addition, during the electrolysis process, the temperature and pH of the water increase. If pH > 7, then we can expect a decrease in the specific electrical conductivity of water χ R, and at pH values< 7 удельная электропроводность воды χ R , наоборот, возрастает (рис. 5).
The specific electrical conductivity of natural waters of medium mineralization is 0.001-0.005 1/(Ohm⋅m), urban waste water is 10-0.01 1/(Ohm⋅m). During electrolysis, the specific electrical conductivity should be in the range of 0.1-1.0 1/ (Ohm⋅m). If the source water has insufficient electrical conductivity, the salt content should be increased (Fig. 7). Typically, sodium chloride (NaCl) is used for this, the doses of which are determined experimentally and most often amount to 500-1500 mg/l (8-25 mEq/l). Sodium chloride is not only convenient in terms of use and safety (storage, solution preparation, etc.), but in the presence of NaCl the passivation of the electrodes slows down. By dissociating in water, NaCl saturates the water with chlorine anions Cl - and sodium cations Na +. Chlorine ions Cl - are small in size and, penetrating through passivating deposits to the anode surface, destroy these deposits. In the presence of other anions, especially sulfate ions (SO 2-4), the depassivating effect of chlorine ions (Cl -) decreases. Stable operation of the electrolyzer is possible if the ions - Cl - make up at least 30% of the total number of anions. Sodium cations Na + as a result of electrophoresis move to the cathodes, on which hydroxide ions OH - are generated, and, interacting with the latter, form sodium hydroxide (NaOH), which dissolves carbonate deposits on the cathodes.
The power consumption [W] of the electrolyzer is determined by the following relationship:
N consumption = η e I cur V e, (6)
where η e is the efficiency of the electrolyzer, usually taken η e = 0.7-0.8; I cur—current strength, A; V e is the voltage on the electrolyzer, V.
The residence time of water in the interelectrode space of the electrolyzer is limited by the time required to generate the required amount of electrolysis products, as well as the duration of the corresponding volume reactions, and is determined experimentally.
The speed of water movement in the interelectrode space is set taking into account the conditions for the removal of electrolysis products and other impurities from the electrolyzer; In addition, turbulent mixing depends on the speed of water movement, which affects the course of volumetric reactions. Like the residence time of the water, the water speed is selected based on experimental data.
To be continued.
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