【作者】 尤翔宇；

【导师】 柴立元；

【作者基本信息】 中南大学 ， 冶金工程， 2013， 博士

【摘要】 摘要：乙二胺四乙酸(Ethylenediaminetetraacetic acid, EDTA)是一种多元酸人造有机化合物,对重金属具有强烈的络合能力,在化工食品、电镀等行业应用广泛。EDTA进入环境后会导致水体富营养化程度升高,与重金属离子形成稳定的络合物使河流沉积物中的重金属释放出来,造成严重的环境污染。三维电极法具有传质速度快、电流效率高等优点,是电化学领域研究的热点,但在有机废水处理方面的应用较少,开展三维电极法处理典型有机废水的研究是环境领域的重要发展方向之一。采用自制的三维电极反应器处理EDTA模拟废水,研究三维电极法生成羟基自由基的电化学过程,优化三维电极法处理EDTA模拟废水的工艺,基于电化学催化氧化技术阐明三维电极法降解EDTA模拟废水的机理。主要研究成果如下：(1)提出并揭示了三维电极法生成羟基自由基的电化学反应机理,确定了控制步骤,为EDTA的高效降解创造了有利条件。证明了OH自由基是三维电极法降解有机物过程中产生的重要活性物种。基于电化学测试技术,研究了硫酸钠体系活性炭阳极上·OH自由基生成的电化学反应历程和控制步骤。通过计算确定·OH自由基吸附引起的弛豫时间常数为6.24×10-2s,是双电层充电所引起时间常数的104倍,吸附引起的等效电阻是电荷转移电阻的6倍,表明·OH自由基以较强的化学键吸附在活性炭电极上,不易发生析氧反应。(2)研究确定了EDTA在活性炭上的吸附行为,发现EDTA在活性炭上的吸附不具有催化活性,而是氢键吸附,建立了EDTA在活性炭上的氢键吸附模型。活性炭粒子电极对EDTA的吸附动力学研究表明吸附过程分为两个阶段：在初始阶段,浓度梯度是吸附推动力,吸附剂表面存在较多的活性吸附位点,吸附速度较快；随着反应的进行,伴随有化学吸附的发生,活性位点被逐渐占据,吸附速率逐渐下降,反应逐渐达到平衡。不同浓度和不同温度下EDTA在活性炭上的吸附过程均符合伪二级动力学方程。Freundlich方程更适于描述不同温度下EDTA在活性炭粒子电极上的吸附过程,吉布斯自由能的变化表明吸附是自发进行的,随温度升高吸附能力逐渐增大。吸附热表明EDTA在活性炭上的吸附主要是氢键力和配位基交换力为主导。红外光谱分析表明EDTA通过氢键缔合吸附在活性炭上,键能较弱,容易被降解。(3)优化了三维电极法处理EDTA模拟废水的工艺参数,阐明了活性炭粒子电极失活的原因和原位再生机理。在进水速度200mL/min、电导率为2.0mS/cm、电流强度为200mA、pH值为6、反应温度为25℃的情况下电解60min, EDTA的降解率在95%以上。探明了溶液中HCO3-、NO3-、Cl-、SO42-等阴离子存在时会与·OH自由基反应,影响其对有机物的降解,HC03-对EDTA降解过程影响最大,S042-的影响最小,HC03-浓度为5mM时,EDTA降解的动力学反应速率常数下降33.45%。活性炭粒子电极因吸附EDTA产生非催化活性缔合物并占据活性点而失活,电化学原位催化降解使活性位点再次产生空穴,恢复炭粒子电极活性,为EDTA废水三维电极处理工艺提供支撑。(4)基于电化学测试技术阐明了三维电极法降解EDTA的电化学催化作用机理。EDTA体系活性炭阳极极化曲线上·OH自由基生成反应的极限峰电流是硫酸钠体系的10倍以上,说明EDTA存在时,活性炭阳极上发生了催化反应。双电势阶跃实验表明·OH自由基生成后吸附在活性炭阳极的表面,在发生析氧反应前与溶液中的有机物发生了催化反应。单电势阶跃实验证明EDTA在活性炭阳极上不发生直接电荷转移反应,而是基于催化反应机理在0.70V到1.50V的电位范围内,EDTA与·OH自由基在活性炭粒子电极表面发生直接矿化反应：OH-→·H+e-·H+EDTA→nCO2+m H2O+zNO2更多还原

【Abstract】 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 (Δ 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更多还原