Summary:

Molecules of any substance are composed of atoms, and atoms are charged systems, so in any chemical reaction, when atoms interact with atoms, electrical phenomena occur. Electrochemical water treatment technology is to treat pollutants in water through the relationship between chemical phenomena and electrical phenomena of organic and inorganic substances in sewage.

Electrochemical technology is divided into two parts: one part is micro-electrolysis (galvanic cell reaction) technology, the process of converting chemical energy into electrical energy is regarded as a reversible process; the other part is electrolysis (electrolytic cell reaction) technology, the process of converting electrical energy into chemical energy , is an irreversible reaction battery, and its essence is the electrode reaction between the particles of the aqueous phase and the interface of the solid phase as the electrode material.

1. Introduction of electrocatalytic oxidation technology

1.1 Reaction mechanism of electrocatalytic oxidation reactor

The main process of organic wastewater treatment adopts the electrocatalytic oxidation method. Through electrocatalytic degradation on the surface of the electrode, the electrode can directly generate hydroxyl radicals with strong oxidizing ability and no selectivity to organic and inorganic substances [ OH ] (εo= 2.80v, p=569.3Kj) to deoxidize the pollutants in the water, so that the pollutants are completely mineralized, which is called the electrochemical combustion process; if the aqueous solution contains electrolytes such as CI- and SO42-, use: Sodium sulfate, chlorine, hypochlorite and chlorate, etc. The generated peroxides, such as S2O82-, H2O2, etc., catalyze the oxidation of pollutants in the water. This process is called an electrochemical conversion process, so that the electro-oxidation can effectively remove COD in the wastewater.

Some of the main reactions during electrolysis are as follows:

Anode reaction: OH— →1/ 2O2 + H2O + 2e—

Cathodic reaction: O2 + H2O + 2e—→ HO2— + OH—

Overall reaction: 1/ 2O2 + OH— → HO2—

Peroxide radicals [HO2—] produced by electrolysis, under alkaline conditions, cause secondary reactions in wastewater, so that pollutants are degraded or directly decomposed into CO2, or converted into other simple compounds.

The electrolysis process obeys Faraday's law. That is, the amount of substances precipitated on the electrode is proportional to the intensity of the current passing through the solution and the time it is energized. Likewise, the removal of organic pollutants in wastewater follows Faraday's law. In the process of removing organic pollutants in wastewater, it does not need to be completely oxidized into the final products, namely CO2 and H2O. It is only necessary to break the chain of organic pollutant molecules, tear them into charged organic "fragments", and settle them through electrolytic adsorption and flocculation. Remove organic contaminants.

1.2 Types of reactions in wastewater treatment by electrocatalytic oxidation reactors

1.2.1. Direct electrolysis

Direct electrolysis of electrochemical technology means that pollutants are directly oxidized or reduced on the electrodes to remove them from wastewater. Direct electrolysis can be divided into anodic electrode process and cathodic electrode process. The anode electrode process means that pollutants are oxidized on the anode surface and converted into smaller substances or easily biodegradable substances, and even inorganic organic matter occurs, so as to achieve the purpose of reducing and removing pollutants. The cathode electrode process is the reduction of pollutants on the cathode surface to remove them, which is mainly used for the reduction and dehalogenation of halogenated hydrocarbons and the recovery of heavy metals.

1.2.2 Indirect electrolysis

Indirect electrolysis of electrochemical technology refers to the use of electrochemically generated redox species as reactants or catalysts to convert pollutants into smaller substances. Indirect electrolysis is divided into reversible and irreversible processes. A reversible process (mediated electrochemical oxidation) means that the redox can be electrochemically regenerated and recycled during electrolysis. Irreversible process refers to the use of irreversible electrochemical reactions to produce substances, such as the process of oxidizing organic compounds such as chlorate, hypochlorite, H2O2 and O3 with strong oxidizing properties, and can also use electrochemical reactions to generate strong oxidizing intermediates. , including free radicals such as solvated electrons, •OH, •HO2, O2-, etc., undergo redox reactions to form new material particles for removal or mineralization.

2. Advantages of electrochemical water treatment technology

(1) High environmental compatibility

Electrochemical water treatment technology uses clean and effective electrons and does not need to add oxidants or reducing agents, flocculants and other chemicals during the electrolysis process. It is a "green" treatment technology that basically does not pollute the environment. Due to the extremely high potential gradient in the interfacial electric field, the electrodes are equivalent to catalysts for heterogeneous reactions, thus reducing the environmental pollution that may be caused by adding catalysts. It can directly react with organic pollutants in wastewater to degrade them into carbon dioxide, water and simple organic matter, with no or very little secondary pollution. At the same time, the electrochemical process is highly selective and can prevent the formation of side reaction products. reduce the occurrence of pollutants;

(2) Versatility

The electrochemical process has the functions of direct and indirect oxidation and reduction, phase separation, concentration and dilution, and biological killing, with high energy efficiency, and the electrochemical process can generally be carried out at normal temperature and pressure;

(3) Economical and applicable

The electrolysis equipment and its operation are generally relatively simple, the cost is low, the floor area is small, and it is generally a modular combination.

(4) Strong oxidizing ability

(1) The oxidation of hydroxyl radical (OH) is extremely strong, second only to fluorine (F2), and much stronger than ozone (O3);

(2) The chemical reaction of OH radical oxidative degradation of organic matter is a chain reaction, that is to say, once the oxidation reaction occurs, as long as no inhibitor is added, the reaction will continue to circulate continuously;

(3) The degradation of organic pollutants is thorough (sometimes it can be oxidized to carbon dioxide and water) and universal (any organic matter can be oxidized and degraded).

3. Factors Affecting the Effect of Electrocatalytic Oxidation

3.1 Influence of electrode materials on organic degradation

The change of electrocatalytic performance is not essentially caused by external conditions such as potential and current, but the influence of the electrode material itself. For the electrochemical degradation of refractory organic pollutants, the most important thing is the design and preparation of electrode materials.

Different electrode materials correspond to different transformation results and transformation mechanisms. The anode materials selected for anodic oxidation usually have high oxygen evolution overpotential, such as PbO2, graphite, TiO2/Ti and SnO2/Ti, etc. Pt electrodes are also used.

(1) Conventional electrodes

Carbon electrodes and graphite electrodes are the most widely used electrode materials in the electrochemical industry. Carbon and graphite electrodes have many advantages: good electrical and thermal conductivity; good corrosion resistance; easy to process into electrodes of different shapes; cheap, but when carbon and graphite are used as anode materials, especially in acidic solutions , its electrochemical oxidation will cause the loss of the electrode - not only to generate CO and CO2 from the outer carbon atoms, but also to cause the graphite to expand and exfoliate. The concentration and pH value of the solution both affect the anodic corrosion of carbon materials, and the polymers produced by the degradation are easily adsorbed on the electrode surface to hinder the reaction, which greatly reduces the electrocatalytic efficiency.

(2) Titanium-based coated electrode (DSA)

DSA is a metal titanium as the electrode ground state, and the surface is coated with an active coating with platinum group metal oxide as the main component, such as Ti/MnO2, Ti/PbO2, Ti/Pt, etc. The advantages of titanium electrodes are: the anode size is stable, the distance between electrodes does not change during the electrolysis process, which can ensure that the electrolysis operation is carried out under the condition of stable cell voltage; the working voltage is low, so the power consumption is small, which can save power consumption; the working life is long; Overcome the dissolution problem of graphite anode and lead anode, avoid contamination of electrolyte and cathode products; can improve current density; strong corrosion resistance; easy to make shape, high precision; base metal titanium can be used repeatedly. The modified metal oxides SnO2, TiO2, RuO2, etc. are modified on the surface of the titanium substrate to make an electrode with higher catalytic performance for the anode reaction, and the modification is composed of binary or ternary compounds, which can often improve the Electrocatalytic activity.

The basic principle of anodic catalytic oxidation degradation of organic matter is to use the catalytically active anode electrode to make the organic matter adsorbed on its surface undergo catalytic oxidation reaction, so that it can be degraded into harmless substances, or degraded into substances that are easily biodegradable. Further biodegradation treatment was carried out, and experiments showed that in the process of direct anode oxidation, pollutants were first adsorbed on the surface of the anode, and then the pollutants were oxidized and removed through the anode electron transfer process.

In the electrocatalytic oxidation process, the oxygen evolution reaction of the anode is often accompanied, which will greatly reduce the current efficiency and the service life of the electrode. Therefore, as an active coating material, it must have corrosion resistance and high oxygen evolution overpotential, etc. Features. The corrosion resistance, electrical conductivity, and oxygen evolution overpotential of platinum group element oxides are relatively satisfactory.

3.2 The effect of electrolyte concentration on the degradation of organic matter

The influence of electrolyte on the electrocatalytic oxidation process of organic matter is reflected in two aspects: one is that the increase of electrolyte concentration means that the conductivity increases, the cell voltage decreases, and the voltage efficiency increases; the other is that complex electrochemical reactions will occur in the electrochemical process. Electrolytes also play a different role. Some people believe that the removal of organic matter in the electrochemical treatment of Cl--containing organic wastewater is mainly achieved through indirect processes, that is, chloride is chemically oxidized to form hypochlorite, and OCl- is deoxidized to degrade organic matter. In the presence of Cl-, active chlorine will be generated, which will affect the electrolysis process; it plays an indirect oxidation role, but at the same time, it has been pointed out that Cl- does not contribute much to pollutants in the electrochemical oxidation process, indicating that Cl- - indirect oxidation only plays a role.

In addition, under the same conditions, the removal rate of p-nitrobenzene was higher when anhydrous sodium sulfate was used as the electrolyte than sodium chloride. The salinity of the reaction system provided a new way for the comprehensive utilization of actual wastewater. According to research reports, the COD removal rate increases with the increase of electrolyte concentration, but after increasing to a certain concentration, the COD removal rate begins to decrease. They believe that the increase of electrolyte concentration promotes the improvement of solution conductivity, the increase of electrode reaction rate, and the increase of reactive oxygen species generation rate. At the same time, the concentration of active oxygen on the surface of the filler adsorbed in the electrolytic cell increases, the oxidation rate increases, and the reduction rate increases. However, too much electrolyte concentration may cause a large number of anions to be adsorbed on the surface of the anode, hinder the electrode reaction of the positive electrode, and affect the generation rate of reactive oxygen species, so the COD removal rate decreases.

3.3 The effect of cell voltage on the degradation of organic matter

Some people have investigated sodium sulfate at a current density of 20 mA/cm2 and a plate spacing of 3 cm. The cell voltage is the driving force for the electrolysis reaction. For the same reaction system, increasing the cell voltage will increase the current intensity in the system. The cell voltage mainly depends on the distance between the plates and the conductivity of the sewage. The larger the distance between the plates, the higher the tank voltage, and the larger the power consumption. On the contrary, the power consumption is small. For the degradation of p-nitrobenzene, the removal rate of p-nitrobenzene was higher when the concentration of 10 g/L was higher than that of 17.75 g/L anhydrous sodium sulfate. At the same time, when the concentration of anhydrous sodium sulfate was higher than 15 g/L, the cell voltage was basically stable. At 6V, this is because according to Faraday's law, the increase in charge is proportional to the amount of actives produced by the system. When the applied cell voltage is small, the active substance produced is the decisive factor for the reaction of the system; when there is enough active substance in the system, the amount of active substance is no longer the limiting factor of the reaction system. However, it has been reported in the literature that before the cell voltage is 15V, the higher the load voltage, the higher the COD removal rate. When the voltage is increased after 15V, the degradation rate decreases. This is because as the voltage increases, the current density increases, the electrode reaction speed increases, and the active oxygen generation speed increases, so the degradation rate increases; The adsorbed excess active oxygen is quickly recombined into O2, the gas film separates the particles, increases the resistance, reduces the current density, and reduces the rate of active oxygen production, so the degradation rate decreases.

3.4 The effect of initial pH value on the degradation of organic matter

In the electrochemical treatment of wastewater, the initial pH value determines the reaction direction and main reaction of each chemical reaction in the electrochemical process, thus affecting the degradation effect of wastewater. Because under acidic conditions, it is conducive to the oxidative degradation of organic matter and the electrochemical reaction that produces H2O2. It is beneficial to the generation of active O and OH, and at the same time, under acidic conditions, the oxygen evolution potential is higher, and the oxygen evolution reaction is difficult to occur. Alkaline conditions containing chloride ions are conducive to the degradation of ammonia nitrogen. (break point method)

3.5 Influence of plate spacing on degradation of organic matter

The electrode spacing is a key influencing factor. The increase or decrease of the electrode spacing will increase or decrease the resistance of the reaction system, and the change in the resistance will cause the input energy to change. In this way, the effect of the electrode spacing on the removal rate of organic matter Like electrolyte concentration and cell voltage, it comes down to whether the input energy is the limit energy of the system. When the electrode spacing is 2 cm, the removal rate of nitrobenzene has the highest efficiency. When the reaction time is greater than 2 h, the removal rate of p-nitrobenzene decreases with the increase of the electrode spacing, which is mainly due to the increase of the electrode spacing. As a result, the resistance between the two plates increases, the current efficiency decreases, and the removal rate decreases.

3.6 Effect of aeration amount on the degradation of organic matter

Aeration generally has two functions. First, aeration plays the role of stirring the solution, so that the catalyst can be fully contacted with the solution to achieve higher reaction efficiency; secondly, aeration provides the oxygen required for the reaction. In addition, Chen Guowei investigated the influence of compressed air and N2 on the amount of H2O2 produced, and found that the amount of H2O2 produced when air was vented was more than the amount of H2O2 produced by passing N2, and the removal rate of organic matter increased with the increase. This is because When the air is vented, more dissolved oxygen can be obtained from the outside, which makes the system beneficial to the generation of H2O2.

4. Introduction of equipment structure of electrocatalytic oxidation reactor

(1). The anode plate of the electrolysis equipment adopts a combined or detachable connection, and the combination method of the anode plate is determined according to different sewage water quality conditions. The anode plate adopts high oxygen evolution potential, and the oxygen evolution overpotential can reach 1.93v (relative to the calomel electrode)

(2). A microporous aeration device is used at the bottom of the equipment. On the one hand, during the electrolysis process, the hydroxyl radicals (•OH radicals) generated by the anode plate can fully react with the organic matter in the sewage, and on the other hand, it can replenish the sewage in a timely manner. Reactive oxygen species degrade part of COD.

(3). There is a sludge discharge device at the bottom of the equipment, which can regularly discharge the complex precipitate formed by the sewage during the electrolysis process. Easy to operate and strong operability.

(4). The equipment has its own circulation and ventilation system. The flow rate can be adjusted according to the difficulty of sewage treatment, and the residence time and reaction time of sewage in the electrolytic cell can be controlled.

(5). Electrocatalytic oxidation equipment is generally used for highly difficult degradation of wastewater and biochemical terminal wastewater.

(6). It is generally formed into a modular type, and the electrolysis method usually adopts circulating electrolysis.

6. Coating selection

Since this pilot plant will process different wastewater (organic wastewater, high-chlorine wastewater, etc.), the coating selection for the above conditions is: ruthenium-iridium (chlorine evolution environment)).