High-salt organic wastewater has the characteristics of high TDS content, complex composition, high content of refractory organic matter, high toxicity and environmental hazards. Traditional biochemical processes are difficult to treat effectively. Electrocatalytic oxidation process based on coated titanium electrodes can avoid the traditional treatment process. The disadvantage is that it provides the possibility for the treatment of high-salt wastewater. Coated titanium electrodes have extremely high electrocatalytic activity, which can significantly improve the current efficiency and play a crucial role in electrocatalytic processes.

1. Principle of Coated Titanium Electrode Treatment of High-Salinity Organic Wastewater

Electrocatalytic processes based on coated titanium electrodes perform direct electrolysis and indirect electrolysis on the electrodes when treating high-salt wastewater. Direct electrolysis refers to the oxidation or reduction reaction of organic matter in wastewater directly on the surface of coated titanium electrode, thereby reducing organic matter in wastewater.

process of concentration. Direct electrolysis can be divided into cathode direct electrolysis and anode direct electrolysis. Anode direct electrolysis means that organic pollutants get electrons on the surface of the coated titanium anode and are directly oxidized into easily biochemical small molecular organic compounds or directly converted into carbon dioxide and water; cathode direct electrolysis It refers to the process in which organic matter loses electrons on the surface of the cathode and is reduced and degraded. It can be applied to the dehalogenation of organic halides and the reduction and recovery of heavy metal ions. Indirect electrolysis of electrodes refers to the use of oxidized or reduced substances produced by coated titanium electrodes as oxidants, reductants or catalysts to convert organic substances in high-salt wastewater into small-molecule, easy-to-biochemical, low-toxicity, and easy-to-handle organics. The removal of organic matter in high-salt wastewater mainly occurs in the process of direct oxidation and indirect oxidation at the anode.

When the organic matter concentration (COD, NH3-N, etc.) in the high-salt wastewater is high, direct anodization is mainly carried out, while indirect anodization is only carried out at low concentrations. Anodic direct oxidation is the reaction of water molecules through current, and the discharge on the anode surface produces hydroxyl radicals. The oxidation potential of hydroxyl radicals is 2.8 V. It is a strong oxidant in nature, second only to fluorine, and can selectively oxidize organic matter in wastewater. , then the organic matter near the anode will be directly oxidized and removed by hydroxyl radicals; indirect oxidation is to reduce the chloride in the water through the action of electric current during the electro-oxidation process, and generate strong oxidants, such as ClO-, high-valent metal ions, etc. These oxidants also have Strong ability to oxidize and remove organic matter, can oxidize organic matter in high-salt wastewater.

High-salt organic wastewater contains a large amount of salts, resulting in high electrical conductivity and high current utilization efficiency in the electrocatalytic system, and the coated titanium electrode is strongly hydrophilic, and "surface hydroxylation" occurs when it comes into contact with high-salt wastewater After the reaction, the surface is covered by a layer of highly oxidizing hydroxyl radicals, which is oxidatively removed from the organic matter adsorbed on the surface of the anode. At the same time, high-salt wastewater contains a large amount of chloride, and indirect oxidation also produces a large amount of chlorate and hypochlorite. These strong oxidizing substances will effectively reduce the concentration of COD and ammonia nitrogen in high-salt wastewater.

2. Selection of Coated Titanium Electrodes for Treatment of High-Salinity Wastewater

In the process of treating high-concentration organic wastewater by electrocatalytic oxidation process based on coated titanium electrode, the electrode is not only a current conduction carrier, but also a catalyst for the removal of organic matter. The selection of electrode coating material directly affects the current conduction efficiency and catalysis of the electrode. performance. The main competitive side reaction in the electrocatalytic oxidation process is the evolution of oxygen or chlorine on the anode surface. The oxygen evolution potential of the anode coating is positively related to the catalytic activity of the electrode. The higher the oxygen evolution potential of the electrocoating, the higher the catalytic activity and the removal efficiency of organic matter The higher it is [4], so the necessary condition for choosing an anode is that the coating material must have a higher oxygen evolution potential.

At present, the commonly used coated titanium electrode anode materials for high-salt organic wastewater are Ti/SnO2.Sb2O3, Ti/PdO, Ti/RuO2.TiO2, Ti/RuO2.Ir2O3.

SnO2 and SbO2 metal oxide-coated titanium electrodes have high oxygen evolution potential, and hydroxyl radicals generated on the surface of the anode are extremely oxidizing to organic matter, so they are more suitable for the treatment of high-salt organic wastewater. In recent years, in order to take into account the catalytic activity and electrode life of the coated titanium electrode, Ti/IrO2·Ta2O5/SnO2 and Ti/IrO2·Ta2O5/SbO2 multi-dimensional coating electrodes have been developed. The oxygen evolution potential of this type of electrode is as high as 1.77V. It has high catalytic activity, stable coating performance, long life, and high removal rate of organic matter. It can be used as a preferred titanium electrode for high-salt wastewater treatment coating.

3. Application of coated titanium electrodes in high-salt organic wastewater treatment

The Ti/SnO2 electrode prepared by thermal decomposition was used to treat high-salt and phenol-containing wastewater, the phenol conversion rate was 95.5%, and the current efficiency was 73.5%. Hu Fengping and others prepared Ti/PbO2 modified electrodes by thermal oxidation, and then treated acid fuchsin solution with modified electrodes doped with Fe and Ni and undoped electrodes. All are above 90%, and the removal rate of acid fuchsin by nickel-modified electrode is as high as 93%. Correa-Lozano introduces a layer of IrO2 between the titanium substrate and SnO2-Sb2O5, which helps to make TiO2 and SnO2 into an isomorphic structure and weaken the passivation effect of TiO2 on the electrode, which can effectively improve the life of the electrode. The modified electrode was tested for electrocatalytic degradation of high-salt chlorophenol wastewater. The results showed that when the mass ratio of SnO2-Sb2O5 catalytic active layer and IrO2 intermediate layer was 26, the removal effect was the best, and the removal rate of TOC could reach 95%.

4. Precautions for the use of coated titanium electrodes in the treatment of high-salt organic wastewater

Fluoride ions are highly permeable and corrosive, which can corrode the titanium dioxide oxide film on the surface of the titanium substrate and the oxide film of other metal coatings, causing the surface coating of the titanium electrode to peel off and greatly reducing the life of the electrode. Before the coated titanium electrode is used, the concentration of fluoride ion in the wastewater should be measured. If the concentration of fluoride ion in the wastewater is greater than 10 mg/L, the electrocatalytic oxidation process based on the coated titanium electrode should not be used for treatment. The current density of the electrode is proportional to the removal rate of organic matter in the wastewater. The higher the current density, the higher the removal rate of organic matter. However, excessive current density will cause serious heating of the electrode, the coating will easily fall off, and the life of the electrode will be significantly reduced. In the treatment of high-salt organic wastewater, it is recommended to keep the current density at 500-1 500 A/m2. Different waveform pulse voltages of the pulse power supply can significantly reduce the consumption of the coated titanium electrode, and choosing an appropriate duty cycle can improve the electrode life and avoid electrode passivation. The mesh electrode has a larger specific surface area and lighter weight than the plate electrode, which can significantly reduce the cost of the electrode, and its irregular current conduction path distribution can also significantly reduce the possibility of electrode passivation.

The electrocatalytic process based on the application of coated titanium electrodes has the advantages of simple operation, short process flow, strong adaptability, rapid reaction, good treatment effect, and no secondary pollution. It has significant advantages in the treatment of high-salt wastewater and has broad application prospects. . However, there are also problems such as easy passivation of electrodes, expensive coating materials, short life, and low current efficiency. In order to ensure the industrialization of coated titanium electrodes applied to high-salt wastewater, research should be strengthened from the following aspects:

(1) Strengthen the research on electrode coating types, use and maintenance methods, etc., to avoid electrode passivation.

(2) Strengthen the research on rare earth element coatings, common transition element metal coatings, etc., and try to reduce the production cost of coated titanium electrodes.

(3) While strengthening the research on metal oxide coatings, the research on organic compounds and metal oxide composite coatings should be strengthened to ensure the catalytic activity of the electrode and improve the life of the electrode.

(4) Strengthen the theoretical research and processing technology research of coated titanium electrodes, realize the standardization of electrode production, and promote the widening of engineering applications.