Study On Mechanism And Application Of Electro-catalytic Oxidation To Refractory Organic Pollutants

Chlorobenzene and tetracycline are two typical refractory organic pollutants, frequently detected in surface water, causing great harm to the ecological environment. The electrochemical technology is one of the promising water treatment technologies due to its simple operation, no need to add or rarely adding chemical reagents, environmental compatibility, amenability to automation. In this thesis, chlorobenzene and tetracycline are chosen as target pollutants, stainless steel is used as cathode and titanium-based ruthenium iridium coated electrode as anode to construct the two-dimensional and three-dimensional electrochemical reactors. Various process conditions affecting the removal of chlorobenzene and tetracycline in electrochemical reactor are investigated. Changes of active species and acute toxicity are examined in electrochemical process. The electrochemical oxidation mechanism is elucidated and the pilot is tested. The results can provide the theoretical basis, experimental support and engineering demonstration for electrochemical removal of organic pollutants.（1） The effects of initial concentration, current density, pH, humic acid and electrolyte type and concentration on the chlorobenzene degradation are systematically studied. The results indicate that the greater current density, the higher the chlorobenzene removal rate is, initial concentration, pH and the electrolyte concentration have little influence on chlorobenzene removal. A high concentration of OH is detected with salicylic acid as the probe, and the concentration of OH in sodium chloride electrolyte is much higher than that in sodium sulfate. The greater the current density is, the more the amount of OH is. The luminescent bacteria toxicity and daphnia magna toxicity tests show that acute toxicity is first increased and then decreased, which may be attributed to that some more highly toxic intermediates are produced and then further degraded into low-toxic products in the electrochemical process. The acute toxicity is much lower in sodium chloride electrolyte than that in sodium sulfate electrolyte, which is mainly caused by indirect oxidation of the effective chlorine produced at anode in sodium chloride electrolyte. GC-MS result combined with the determination of TOC and intermediates indicates that electrochemical degradation mechanism of chlorobenzene is that chlorobenzene first become chlorophenol by electrophilic addition reaction of OH, Cl substitution by OH generates hydroquinone and p-benzoquinone, then they are oxidized to open ring and form organic acids, eventually slowly be mineralized into CO₂ and H₂O.（2） The current density and pH have a great influence on the tetracycline removal. Decreasing the pH or increasing current density benefits generating OH and maintaining a high oxidation potential, which results in promoting electrochemical degradation of tetracycline. The electrolyte concentration in the range investigated has no significant influence on tetracycline removal, but high concentration of electrolyte is conducive to improve the conductivity, and thus can significantly reduce the energy consumption of the electro-catalytic oxidation process. Both sodium chloride and sodium sulfate as electrolyte, the concentration of TOC increases firstly and then decreases, which may be as a result of that the electrocatalytic process first produces small molecular and easy oxidation burning intermediates, and then further oxidized to mineralization. The treatment effect in sodium chloride electrolyte is better than that in sodium sulfate electrolyte, which is mainly due to that there is not only direct oxidation of anode and indirect oxidation of hydroxyl radical, but also indirect oxidation of chlorine and hypochlorite generated in sodium chloride electrolyte. Biological toxicity tests show that the luminescent bacteria toxicity initially decreases and then increases, Daphnia magna toxicity has been showing a downward trend. Sodium chloride as the electrolyte, two biological toxicities of the final effluent decrease significantly, reducing the risk of ecotoxicity. The intermediates capture and degradation process show that tetracycline experiences hydroxy radical electrophilic addition reaction and becomes hydroxylation products, carbonyl electron transfer reaction generates deamidation product, and continues to be oxidized and hydroxylated, and finally small organic acids are completely oxidized and mineralizatied into carbon dioxide, water and inorganic salt such as nitrate.（3） using stainless steel as the cathode, titanium as anode, filled with activated carbon particles, three-dimensional electrocatalysis reactor is constituted and used to treat simulated chlorobenzene wastewater with dynamic continuous operation mode. The results show that the current strength is the main factor to influence the electrical catalytic degradation of chlorobenzene. The stronger the current, the higher chlorobenzene removal rate is, but as the current is very large, energy consumption increased significantly. Extending the duration time, chlorobenzene removal rate first increases significantly and then flattens out. Initial pH has no significant impact on the chlorobenzene removal. As the reaction proceeds, chlorobenzene removal rate increases, TOC decreases and IC increases, while the pH decreases. The result indicats that the organics gradually degradate and produce small molecular organic acids, then gradually are mineralized to CO₂, and the toxicity showes decreasing trend with a similar mechanism to the two-dimensional electrochemical reactor. Comprehensive consideration of the the chlorobenzene removal and effluent toxicity, the three-dimensional electro-catalytic oxidation optimum conditions are as followed that chlorobenzene concentration is about 100 mg/L, current is 2.0A, and degradation time is 60 min. Under the optimum conditions more than 96% of the chlorobenzene is removed, and there are only non-toxic organic acids substances in the final effluent. The three-dimensional electrode is more conducive to practical application because of its high removal rate and one-fifth of the energy consumption compared with the two-dimensional electrode.（4） From several aspects such as the electrode material, multidimensional particle screening, equipment structure and automatic control, three-dimensional electric catalytic reaction device is improved. Porous titanium plate is used as anode instead of graphite, high specific-surface-area conductive particles coated with nanometer catalytic material are filled to form multiple dimensional anode, reaction zone structure and water-gas distribution mode are optimizated, the PLC control system and high performance pulse electrolysis power is supplied in the improved device, which reduces energy consumption, enhances the mass transfer efficiency and treatment efficiency. The pilot results show that the average removal rate of benzene and chlorobenzene is more than 96% under the conditions of chlorobenzene concentration of less than 200 mg/L, benzene concentration of 400 mg/L. Electrocatalytic oxidation device can be used as pretreatment unit of industrial wastewater with high concentration organic matter, reduce the harmful impact on biochemical systems, can also be used for removal of low concentration organics in biochemical effluent and the process cost is relatively low. The engineering application of electrochemical catalytic oxidation device in two refractory organic wastewaters containing chlorobenzene has achieved good results.