Study On Synergistic Reductive Dechlorination Of2,4-dichlorophenoxyacetic Acid By Electrocatalytic And NZVI

In this study, considering the organo chlorine pesticides’non-volatile, non-biodegradable and the toxic effects on plants and animals through bioconcentration and food chain in the environment, typical organochlorine pesticide2,4-dichlorophenoxyacetic acid (2,4D) was selected as the target pollutant. And systematic studies were carried out to investigate the catalytic reductive dechlorination behavior of2,4-D in the traditional electrochemical system and the synergistic system combined electrochemistry and nZVI.Pd/Ni catalytic electrode was prepared by chemical deposition method. X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectrometer were applied to characterize the changes in micro structure and element composition of the electrodes before and after the reaction. The results proved the successful deposition of catalyst Pd on nickel foam substrate. Part of Pd particles were lost after reaction in the electrochemical system, while characteristic signal of Fe element was detected in the electrode structure after reaction in the nZVI-electrochemical synergistic system. nZVI was characterized by X-ray diffraction and transmission electron microscopy, to observe the morphology and composition.The synergistic effect between nZVI technology and electrochemical method on2,4-D catalytic reductive dechlorination was explored. In a separate electrochemical system, the removal rate of2,4-D after4h reaction was87%, while no changing of2,4-D concentration in the reaction solution was observed in a separate0.2g/L of nZVI system. However, if the0.2g/L of nZVI was added into the cathode electrolysis chamber without changing the reaction conditions of the original basic electrochemical system,86.3%of2,4-D were removed in the first1h and the final removal rate reached99.7%. This result proved the obvious synergistic effect existed between nZVI and electrochemical on2,4-D dechlorination.The paper studied the influences of supporting electrolyte NaCl concentration, initial2,4-D concentration, current density and Pd loading on2,4-D degradation in the basic electrochemical system. The optimal reaction conditions were:2g/L of NaCl, 50mg/L of2,4-D,1.7mA/cm2of current density and1.78mg/cm2of Pd loading. In this case,71%of2,4-D was removed. The mass balance of the total carbon during the whole reaction was maintained at99.5%or more. The N2condition was likely to take away the active hydrogen, which was participated in2,4-D dechlorination, and even overflow the cathode chamber. By optimizing the ventilation condition, the removal rate of2,4-D reached87%under magnetic stirring condition. Studying on the other electrolyte, obvious toxic effects of Na2SO4and NaNO3were observed, resulting in the decrease of2,4-D dechlorination efficiency. In the light of actual wastewater, follow-up treatment efficiency and emission standards, it was reasonable to choose NaCl as the electrolyte in this study. The life test of Pd/Ni electrode showed87,84and78%of2,4-D removal rates. The data of2,4-D electrochemical reductive dechlorination fitted the first order kinetic equation. The value of reaction rate k1was significantly influenced by Pd loading and NaCl concentration. The fastest degradation reaction was obtained under the optimal reaction conditions and the magnetic stirring condition.The effects of reaction conditions on2,4-D dechlorination in the nZVI-electrochemical synergistic system were studied. The results showed that both Pd loading and current density have a greater effect on2,4-D dechlorination. And the reaction was less affected by nZVI dosage. But in any case, the removal rates of2,4-D and the current efficiencies were greatly enhanced in nZVI-electrochemical synergistic system. When the Pd loading was0.044mg/cm2(1/40of Pd loading of original electrode), nZVI dosage was0.2g/L and the current density was0.17mA/cm2(1/10of current density of original electrochemical system), the final removal rate of2,4-D was87.6%and the maximum current efficiency was79.9%. The results fully illustrated that the small dosage of cheap nZVI into the electrochemical system not only reduced both of expensive catalyst Pd loading and the value of voltage and current, but also increased the removal rate of2,4-D and the current efficiency. Therefore, nZVI-electrochemical synergistic system reduced the sewage treatment cost and saved energy, while improving the removal rate and energy utilization. The paper also investigated the dechlorination mechanism of the synergistic system. nZVI would gradually migrated to the surface of Pd/Ni electrode during the reaction. So, the active hydrogen produced by electrolysis of water and the electron released by oxidation processes of nZVI, Fe(Ⅱ) and Fe2+(formed by the oxidation of nZVI) together promoted the2,4-D reductive dechlorination reaction under the catalysis of Pd on the surface of electrode.