| Abstract:Ethylenediaminetrtraacetic acid (EDTA) is polyprotic acid which can strongly chelate with heavy metal ions to generate comple, it is widely used in chemical industry, food industry and electroplating industry. Aquatic eutrophication and release of heavy metal ions from the sediment in rivers would occur accordingly when EDTA enters into the environment, causing serious environmental contamination. Three-dimensional electrode method has become the hot issue in elrctrochemistry due to its advantages of fast mass transfer, high current efficiency and so on. But the application of this method in organic wastewater treatment is seldom reported. Therefore, developing three-dimensional electrode method to dispose the typical organic wastewater is an important direction in field of environmental science and engineering.In the present study, a self-made three-dimensional eletrode reactor was used to treat EDTA-containing model wastewater. The electrochemical process of generation of hydroxyl radicals (· OH) by three-dimensional electrode method was investigated and the EDTA degradation process was optimized. Moreover, the relevant mechanism was studied based on electrochemical catalysis oxidation techniques. The main conclusions are as follows:(1) The generation mechanism of· OH by three-dimensional electrode method was systematically studied and determine the control step.· OH was the activity speicies in the process of EDTA-containing wastewater treatment by three-dimensional electrode method. The electrochemical reaction course of the· OH generation on activated carbon anode in NaSO4system was investigated through electrochemical measurement technique. According to calculation, the constant of relaxation time caused by the adsorbtion of· OH was6.24X10-2s and it was104times of that by double layer charging and the equivalent resistance induced by adsorption was6times of that by charge transfer resistance. It is found that the· OH was adsorbed on activated carbon through strong chemical bond, and the oxygen evolution was difficult to occur.(2) The adsorption behavior of EDTA on activated carbon was studied and the adsorption model was established. It was showed that adsorption of EDTA on activated carbon without catalysis activity, but through hydrogen bond. The adsorption can be divided into two stages according to the kinetics study:in the initial stage the driving forces are concentration gradient and the adsorption proceeds fast since many active adsorption sites exist in the system; then the chemical adsorption occurs as the reaction proceeds and the adsorption rate gradually slows down because of the occupation of active sites. Under this condition, the adsorption tends to be equilibrium. The adsorption kinetics of EDTA on the activated carbon at different concentrations and temperatures were well fitted by the pseudo-second-order kinetic model. The EDTA adsorption isotherms at various temperatures can be better described by Freundlich model. The calculation of Gibbs free energy showed the adsorption of EDTA was spontaneous (? G<0) and the adsorption capacity increased with increasing the temperature. Hydrogen bond and dentate exchange force were the main forces in the process of EDTA adsorption, based on the results of adsorption heat. EDTA adsorption on the activated carbon is achieved by hydrogen bond from the analysis of infrared spectroscopy.(3) The treatment process of EDTA-containing moel waste water by three-dimensional electrode reactor was optimized, and the inhibition effect of anion ions such as HCO3-,NO3-, Cl-, SO42-on degradation was observed. The degradation efficiency of total organic carbon (TOC) was above95%under current intensity of200mA, electric conductivity of 2.0ms/cm, electrolysis time of60min, temperature of25?, inflow rate of200mL/min, and pH of6.0. The HCO3-imposes the most significantly effects on EDTA degradation, while SO42-has the least effect. The kinetic constant of degradation drops by33.45%when the HCO3-concentration is increased to5mM. The regeneration technique of three-dimensional electrochemical method was proposed to destroy the complex without catalysis activity which occupies the active sites and therefore forcefully restore the reactivity.(4) The mechanism of EDTA degradation was revealed from the electrochemical method. Based on the anodic polarization curve of activated carbon, the limit peak current of the· OH formation reaction of EDTA system was10times of that for Na2SO4system, indicating that the catalysis reaction on activated carbon anode occured when EDTA exists. The· OH was adsorbed on the surface of activated carbon anode and subsequently reacted with organic compounds prior to the oxygen evolution, according to double potential step measurement. Instead of charge transfer reaction of EDTA, when the potential was in the range of0.7-1.5V the mineralization reaction between· OH and EDTA occurred: OH-?OH+e-·H+EDTA->nCO2+mH2O-zNO2... |
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