Studies On The Degradation Of Biorefractory Organic Contaminants By Wet Electro-Catalytic Oxidation Process

It is of critical environmental importance to seek for sound technologies without second pollution for the treatment of highly concentrated organic wastewater. Cationic red X-GRL and p-nitrophenol (PNP) were selected as the model pollutants because they are not only the biorefractory pollutants, but also widely used in the textile, paint, varnish, paper and plastic industries. After a careful investigation of the processes of wet air oxidation (WAO) and electrochemical oxidation (EO) for organic wastewater treatment, a novel process—wet electrocatalytic oxidation (WEO) was invented. The effects of the parameters were systematic investigated. With activated carbon as heterogeneous catalyst, the effects of heterogeneous wet electrocatalytic oxidation were explored. Based on the message of the molecular pollutants and intermediates, possible mechanism and kinetics were discussed. All of the data and theory were carefully analyzed which would be helpful for the treatment of highly concentrated organic wastewater.Firstly, the decolorization of a strong colored azo dye solution of cationic red X-GRL was investigated by WAO under relatively mild conditions. Mono-factor experiments were carried out to investigate the effect of the operation factors and the relatively important parameters were selected for optimization using response surface methodology to explore the interactions of these relatively important parameters. Model regeneration analysis and the check experiments showed that the data of the general linear model agreed with the experiment results well. With multistage Monte-Carlo optimization, the best region of these variables could be predicted to dye color removal. At typical operational conditions, the intermediates of dye degradation were detected and confirmed by GC-MS system. Considering the intermediates and the structure analysis with the help of Gaussian 03W (version 6.0) and the theory of dye color, a possible degradation mechanism for the WAO of cationic red X-GRL was discussed and the probable degradation pathway was deduced.Secondly, electrochemical oxidation of cationic red X-GRL was investigated. Based on the properties of the electrode, effects of operational factors were carefully studied. With the addition of isopropanol, the degradation of pollutant was within 5%, which proved the free radical mechanism of electrochemical oxidation. When the temperature rose from 15℃to 120℃, the improvement was obvious that the degradation rose from 8.72% to 81.56%.Based on the studies of WAO and EO, wet electrocatalytic oxidation, which uses current to catalyze the oxidation of pollutants by oxygen, was invented. The effects of this new promising process were studied for cationic red X-GRL and p-nitrophenol abatement under moderate conditions. Systematic studies were carried out to explore the operational factors by Mono-factor experiments. At optimized operational conditions, the industrial organic wastewater was studied. Compared with the effect of WEO and those of the individual WAO and EO, the effect of WEO showed synergistic effect between 120-160℃for the organic wastewater treatment. After carefully studied the variations of synergistic factors, the mechanisms for current catalyzing oxygen oxidation of pollutants and the synergistic effect of WEO were discussed.To further reduce the treatment time, heterogeneous WEO was created with activated carbon as catalyst. The effects of heterogeneous WEO process showed that it could couple the advantages of adsorption-catalysis-oxidation. Excellent treatment effects could be abtained in a short time, which showed that it would be quite promising in application of industrial organic wastewater treatment.The mechanism and kinetics of WEO were carefully analyzed. The organics degradation by WEO is a free radical mechanism. The organics removal and mineralization is due to the oxidation of free radicals chain reactions, which shortened the radical initial time, greatly improve the treatment the wastewater. The elevated temperature strengthened the treatment effects. Based on the analysis of Hammatt theory and molecular message of pollutants with Gaussian 03W, the degradation pathway of organic pollutants by WEO was elucidated with the help of intermediates. The kinetcs studies showed that the pollutants degradation followed with multi-phase pseudo-first-order reaction, while the mineralization fit GLKM model well.