| The chlorophenols are one kind of persistent organic pollutants which are hazardous and harmful to the environment. The removal of chlorophenols has attracted much attention. In this paper, the self-made Pd-Fe/graphene gas diffusion electrode was used as a cathode, Ti/RuO2/IrO2 electrode was used as an anode and the polyester filter cloth was used as diaphragm. The cathode and anode consisted of both three-electrode system and two-electrode system. Two systems were compared for the degradation of 4-chlorophenol in aid of the electrochemical simultaneous reducing and oxidizing process. The degradation kinetics and toxicity were studied in the process of degradation.Under the optimal degradation conditions, the degradation mechanism and generated intermediates of 4-chlorophenol were analyzed using total organic carbon, high performance liquid chromatography and ion chromatography. The results showed that after 120 min the degradation percentage of 4-chlorophenol reached 95.9%(cathode 1),95.4%(cathode 2), and 94.0%(anode), respectively. The degradation of single cathode system reached 91.0%(cathode) and 90.9%(anode). Intermediates of 4-chlorophenol of cathode and anode in three-electrode system were same as the two-electrode system. In anode, the 4-chlorophenol produced benzodiazepines intermediate, such as phenol, benzoquinone and hydroquinone. In cathode, the 4-chlorophenol produced phenol.Anodic and cathodic compartment produced the same organic acid, such as acetic acid, succinic acid, fumaric acid and formic acid. Based on the change of intermediates, a dynamic model was proposed. Select different concentrations of chlorophenol (25 mg/L,50 mg/L,100 mg/L,200 mg/L, and 500 mg/L) to verify this dynamics model with above 0.85 correlation coefficients. The results showed that the model could well fit the experimental data. The value of K2 (kinetic constants of aromatic organic generate organic acid) in two cathode chambers were greater than anode chamber. It indicated that the effect of 4-chlorophenol degradation to organic acids of Pd-Fe/graphene gas diffusion catalys is stronger than the effect of the anode.The texting toxicity during the degradation process was detected by using the luminescent bacteria. The theoretical toxicity was calculated according to the toxity of intermediate and concentration. The texting toxicity was compared with the theoretical calculation toxicity to make the analysis of the trend of two systems. Through the analysis of correlation, the result was correlated significantly with correlation coefficient 1. The toxicity of anode compartment increased firstly and then decreased, but the toxicity of cathode compartment decreased during the whole degradation for both the two systems. After the degradation, the theoretical calculation toxicity of anode (1.7 L/mg) of three-electrode system was lower than that of two-electrode system (2.0 L/mg). The three-electrode system was better than the two-electrode system.Under the optimal degradation conditions, the removal rate of 4-chlorinephenol, chemical oxygen demand and toxicity were analyzed. The results showed that after 120 min the degradation percentage of 4-chlorophenol reached 96.8%(cathode 1),96.4%(cathode 2) and 95.3%(anode), respectively. The removal rate of COD reached 80.7%(cathode 1),80.7%(cathode 2) and 78.2%(anode), respectively. The electrochemical system was applied to the papermaking wastewater treatment. Detoxicity of cathode 1 and cathode 2 were reached 100% and detoxicity of anode was 81.0%. The toxity of the final solution was lower than that of initial value (51.0%). It indicated that the three-electrode system showed good detoxification effect. |
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