Electrochemical Behavior Research Of Refractory Organics In Wastewater On Insoluble Electrode Of Stainless Steel

As one of advanced oxidation processes, electrocatalysis treatment was developing quickly in refractory organic wastewater treatment. Our reaearch team developed stainless steel insoluble electrodes and electrocatalysis reactors, which had been applied successfully in pharmaceutical wastewater treatment and reuse. For expanding the application range of electrocatalysis in wastewater treatment and improving the current efficiency, it was necessary to further study electrochemical behavior of refractory organics on stainless steel insoluble electrodes.Using stainless steel as the working electrode, the electro-catalysis process of organic compound in electrolyte solution was studied through the potentiodynamic scanning, and a characteristic electrode potential and current maximum was observed below oxygen evolution. The concentration and the choosed species of the organic compound, the scanning speed and the temperature did not affect the form of the characteristic electrode potential, which moved towards positive with the raising of the scanning speed and the falling of the temperature. At 28℃and the scanning speed of 2 mV/s, the characteristic electrode potential exists between 600 to 1050 mV, the maximum of the current appears at the electrode potential of 920 mV. The current maximum at the characteristic electrode potential became higher with the concentration of the organic compound, and the current maximum disappeared when the solution did not contain organics. Since the characteristic electrode potential was independent to the concentration and the choosed species of the organic compound in the wastewater, the characteristic electrode potential should be the synthesis of the hydroxyl radical. The characteristic electrode potential had great significance for the rapid detection of organics in wastewater, real time trace of the hydroxyl radical, and the reduction of electrochemical side reactions in wastewater treatment.However, a yellow brown polyphenol coating was generated on stainless steel electrode surface, when anodic oxidation treatment of phenol wastewater was carried out using stainless steel based insoluble electrode. The coating formation was discussed by linear scanning, cyclic voltammetry and constant current process. The anodic electrode potential of 1.45V was the best condition for good reaction efficiency, while the bath voltage between anode and cathode was about 2.5V. Coating product was analyzed by IR, and the chemical structure was proved to be polyphenol. Polyphenol film contained linear chain and side chain, in which large number of substitutine hydroxyl existed. Making use of part solubility of polyphenol in tetrahydrofuran, the micro structure of polyphenol film was analyzed and compared between original and tetrahydrofuran cleaned polymer coating by SEM micrograph, by which linear mode and plane mode of polyphenol growth was summarized. In neutral media major part of polyphenol grew following the flake-layer mode, layer by layer, parallel to the electrode surface. Polyphenol growth began at some active sites on electrode or polymer, and formed nearly round flakes with 50nm diameter parallel to the anode plane. Flakes connected each other through low polymer and unpolymerized phenol, and formed a layer.The electrochemical process of coating covered electrode was studied, and the polyphenol coating notablely enhanced the pit corrosion resistance of 304 stainless steel in 3.5% sodium chloride solution. Keeping the sum of phenol and aniline concentration, the phenol-aniline copolymer coating was synthesized at the same electrochemical condition on 304 stainless steel anodes. When the solution contained 0.09 mol/L phenol and 0.01 mol/L aniline, the copolymer coating achieved the best pit corrosion resistance, while the pit corrosion potential in 3.5% sodium chloride increased to 0.382 V. Through the infrared spectrum comparison of phenol polymer and phenol-aniline copolymer coatings, the polyaniline structure was proved in the copolymer, and the reasons of the corrosion resistance enhancement were discussed. There was more 1,2,4-ring substitution occurring in phenol-aniline copolymer than polyphenol, which caused more side chains generated during the electropolymerization and a much more compact coating product. Taking advantage of the part solubility of polyphenol and polyaniline in THF, SEM was used to analyze the microstructure of phenol-aniline copolymer coating, with the comparison of polyphenol coating. In the copolymer synthesized in the solution contained 0.09 mol/L phenol and 0.01 mol/L aniline, the bifurcate network structure was observed, which was mainly composed from polyaniline, with other copolymer, mainly polyphenol, filling the interspaces. The network structure in copolymer coating restrained the growth of cracks, enhanced the connection between layers, synthesied the copolymer coating, which raised the pit corrosion resistance against chloride ions.In further research, rapid electropolymerization rate was observed in higher phenol concentration. However, the phenol removel effect became lower in much higher phenol concentration, so the concentration of phenol wastewater in electropolymerization treatment was fixed at 0.002 mol/L. The COD removal efficiency was better between the bath voltages of 2.9 V and 3.1 V,2.9 V was chosen for the less oxygen evolution and energy consumption. After the 12-hour electropolymerization treatment, the polymer coating cracked and wholly dropped from the stainless steel base, which meant the exhausting of phenol in wastewater. The phenol concentration became 0.087mmol/L from 0.002 mol/L, with the removal efficiency of 95.6%, and the COD became 68 mg/L from 500 mg/L, with the removal efficiency of 86.5%. During the treatment the average current efficiency was 60.36% and power consumption was 6.96 kwh/t, which showed a great decline in power consumption and treatment cost comparing with electrocatalysis treatment.