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High-salinity organic wastewater refers to organic wastewater with a TDS content of ≥3.5% (salt content not less than 1%). It mainly comes from the collection and production of industrial products. It has the characteristics of high salt content, complex composition, difficult biodegradation, toxicity and harm. Direct discharge will cause soil compaction, water pollution, and ultimately affect environmental safety. At present, the main method for removing organic matter in sewage treatment is biological treatment. This method has high removal efficiency and low treatment cost, but the biological method is only suitable for treating biodegradable low-salinity organic wastewater, but it is powerless for high-salinity difficult-to-degrade organic wastewater. With the development of electrochemical catalytic technology based on coated titanium anodes, most organic compounds have been proven to undergo redox reactions, addition reactions or decomposition reactions on the electrode surface, which provides a theoretical basis for the electrocatalytic oxidation method to degrade organic pollutants in high-salinity wastewater. Electricity is the energy source of the electrocatalytic oxidation process. With the rapid development of the power industry, the power shortage has been effectively solved, and the large-scale application of the electrocatalytic oxidation process has good conditions. Relevant studies have shown that the hydroxyl radicals and high-valent metal oxides produced by the electrocatalytic oxidation anode can non-selectively oxidize organic matter in wastewater, and have extremely strong oxidation ability, which makes it possible to effectively treat high-salt organic wastewater.
1. At present, the DSA electrode coating commonly used in wastewater treatment is generally composed of one or more metal oxides of ruthenium, iridium, tantalum, lead, tin, and platinum.
2. The principle of treating high-salt organic wastewater with coated titanium electrodes
When treating high-salt wastewater, the electrocatalytic process based on coated titanium electrodes performs direct electrolysis and indirect electrolysis on the electrode. Direct electrolysis refers to the process in which organic matter in wastewater is directly oxidized or reduced on the surface of the coated titanium electrode, thereby reducing the concentration of organic matter in the wastewater. Direct electrolysis can be divided into cathode direct electrolysis and anode direct electrolysis. Anode direct electrolysis refers to the process in which organic pollutants obtain electrons on the surface of the coated titanium anode and are directly oxidized to biodegradable small molecular organic matter or directly converted into carbon dioxide and water; cathode direct electrolysis refers to the process in which organic matter loses electrons on the cathode surface and is reduced and degraded, which 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 oxidizing or reducing substances produced by coated titanium electrodes as oxidants, reducing agents or catalysts to convert organic matter in high-salinity wastewater into small molecules, easy to biodegrade, low-toxic and easy to treat organic matter. The removal of organic matter in high-salinity wastewater mainly occurs in the direct oxidation and indirect oxidation processes at the anode.
When the concentration of organic matter in high-salinity wastewater (COD, NH3-N, etc.) is high, direct anodic oxidation is mainly carried out, while indirect anodic oxidation is only carried out at low concentrations. Direct anodic oxidation is the discharge of water molecules on the anode surface through the current reaction to produce hydroxyl radicals. The oxidation potential of hydroxyl radicals is 2.8V. It is a strong oxidant in nature that is second only to fluorine in oxidation. It can oxidize organic matter in wastewater without selection. Then the organic matter near the anode will be directly oxidized and removed by hydroxyl radicals; indirect oxidation is the reduction of chloride in water by the action of current during the electro-oxidation process to produce strong oxidants, such as ClO-, high-valent metal ions, etc. These oxidants also have a strong ability to oxidize and remove organic matter, and can oxidize organic matter in high-salinity wastewater.
High-salt organic wastewater contains a large amount of salts, so the conductivity is high, the current utilization efficiency of the electrocatalytic system is high, and the coated titanium electrode has strong hydrophilicity. When it comes into contact with high-salt wastewater, a "surface hydroxylation" reaction will occur, and its surface will be wrapped by a layer of highly oxidizing hydroxyl radicals, which will oxidize and remove the organic matter adsorbed on the anode surface. 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.
3. Selection of coated titanium electrodes for treating high-salt wastewater
In the process of treating high-concentration organic wastewater by electrocatalytic oxidation process based on coated titanium electrodes, the electrode is not only a current conduction carrier, but also a catalyst for organic matter removal reaction. The selection of electrode coating materials directly affects the current conduction efficiency and catalytic performance of the electrode. The main competitive side reaction in the electrocatalytic oxidation process is the precipitation of oxygen or chlorine on the anode surface. The oxygen evolution potential of the anode coating is positively correlated with the electrode catalytic activity. The higher the oxygen evolution potential of the electrocoating, the higher the catalytic activity, and the higher the removal efficiency of organic matter. Therefore, the necessary condition for selecting the anode is that the coating material must have a high oxygen evolution potential.
At present, the coated titanium electrode anode materials commonly used for high-salt organic wastewater are Ti/SnO2.Sb2O3, Ti/PdO, Ti/RuO2.TiO2, Ti/RuO2.Ir2O3
Ti/SnO2.Sb2O3 coated electrode material has a higher oxygen evolution potential, so it must have a higher catalytic and removal efficiency in the process of degrading organic matter. Relevant studies have shown that the oxidation of organic matter by anode coating materials such as Pt, IrO2, and RuO2 tends to be electrochemically converted, that is, various fatty acids or other small molecular organic matter are the final products, and the current efficiency is low; while SnO2 and PbO2 are used as anode materials, and a large number of hydroxyl radicals are adsorbed on the surface of the metal oxide, which can completely oxidize organic matter into inorganic matter such as carbon dioxide and water, and the current efficiency is higher.
SnO2 and SbO2 metal oxide coated titanium electrodes have high oxygen evolution potential, and the hydroxyl radicals generated on the anode surface 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 coated electrodes have been developed. This type of electrode has an oxygen evolution potential of up to 1.77V, high catalytic activity, stable coating performance, long life, and high organic matter removal rate. It can be used as a preferred high-salt wastewater treatment coated titanium electrode for key research.
4. Application of coated titanium electrodes in high-salt organic wastewater treatment
Liang Zhenhai et al. used Ti/SnO2 electrodes prepared by thermal decomposition to treat high-salt phenol-containing wastewater, with a phenol conversion rate of 95.5% and a current efficiency of 73.5%.
Ti/PbO2 modified electrodes were prepared by thermal oxidation, and then Fe- and Ni-doped modified electrodes and undoped electrodes were used to treat acid fuchsin solutions. The experimental results showed that the removal rates of acid fuchsin by the three electrodes were all above 90%, and the removal rate of acid fuchsin by the nickel-modified electrode was as high as 93%.
Introducing a layer of IrO2 between the titanium substrate and SnO2-Sb2O5 helps to make TiO2 and SnO2 isomorphous and weaken the passivation effect of TiO2 on the electrode, which can effectively improve the electrode life. The modified electrode was used to conduct an electrocatalytic degradation test on high-salt chlorophenol wastewater. The results showed that when the mass ratio of the SnO2-Sb2O5 catalytic active layer and the IrO2 intermediate layer was 26, the removal effect was the best, and the TOC removal rate could reach 95%.
5. Precautions for the use of coated titanium electrodes in the treatment of high-salt organic wastewater
Fluoride ions have strong permeability and corrosiveness, which can corrode the titanium dioxide oxide film and other metal coating oxide films on the surface of the titanium substrate, causing the coating on the titanium electrode surface to fall off, greatly reducing the electrode life. Before using the coated titanium electrode, the concentration of fluoride ions in the wastewater should be measured. If the fluoride ion concentration 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 organic matter removal rate in the wastewater. The larger the current density, the higher the organic matter removal rate. However, excessive current density will cause severe heating of the electrode and easy shedding of the coating, which will significantly reduce the electrode life. In the treatment of high-salt organic wastewater, it is recommended that the current density be maintained at 500-1500A/m2.
The different waveform pulse voltages of the pulse power supply can significantly reduce the consumption of the coated titanium electrode. Selecting a suitable duty cycle can increase 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 electrode cost. At the same time, its irregular current conduction path distribution can also significantly reduce the possibility of electrode passivation.
6. Conclusion
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 for high-salt wastewater, research should be strengthened in the following aspects:
(1) Strengthen research on electrode coating types, use and maintenance methods, etc. to avoid electrode passivation.
(2) Strengthen 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, we should also strengthen the research on composite coatings of organic matter and metal oxides, so as to improve the electrode life while ensuring the electrode catalytic activity.
(4) Strengthen the theoretical research and processing technology research on coated titanium electrodes, realize the production standardization of electrodes, and promote the widespread application of engineering.
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