细菌解毒铬渣过程动力学研究

【作者】 赵堃；

【导师】 柴立元；

【作者基本信息】 中南大学 ， 环境工程， 2007， 硕士

【摘要】 铬渣是铬盐及铁合金等行业在生产过程中排放的有毒废渣，铬渣中的Cr（Ⅵ）被列为对人体危害最大的8种化学物质之一，是国际公认的3种致癌金属物之一，同时也是美国EPA公认的129种重点污染物之一。我们的前期研究已经成功分离出一株还原碱性介质高浓度Cr（Ⅵ）的短杆状细菌，经鉴定为无色细菌属杆状菌（Achromobacter sp．），将其命名为Ch-1菌，并研发了细菌还原Cr（Ⅵ）的新工艺，建立了扩试工程，解毒后达到国家危险废物浸出毒性标准（GB5085-1996）。然而对于Ch-1菌还原Cr（Ⅵ）的反应动力学过程，培养基成分之一的NaCl溶解铬酸钙的动力学过程，Cr（Ⅵ）在酸性环境下的选择性溶出过程，以及细菌浸出铬渣前后Cr（Ⅵ）扩散系数D的变化过程还缺乏深刻认识。因此，本研究通过对上述过程作以全面考察，建立了细菌解毒铬渣各个过程的动力学方程，确定了各因素对细菌解毒铬渣速度的影响程度，并采用电化学方法测定出细菌解毒铬渣前后Cr（Ⅵ）在铬渣球团中的扩散系数，从理论上证明了本工艺的先进性，并为实施铬污染治理工程提供重要的理论依据。研究获得了如下创新性成果：（1）Ch-1菌还原Cr（Ⅵ）的反应为零级反应，初始pH值为8～10时，Ch-1菌还原Cr（Ⅵ）的反应速率基本相同，pH＜8和pH＞10时，反应速率逐渐减小。随细菌接种量减少，反应速率减小，反应表观活化能增大。细菌接种量分别为50％、30％、20％、10％、5％时对应的动力学方程式分别为：v=57.5211，v=51.6306，v=45.9764，v=24.0025，v=7.3265，其表观活化能分别为6.89kJ／mol、14.75kJ／mol、19.24kJ／mol、33.54 kJ／mol、72.52 kJ／mol。得到了按细菌接种量划分的Ch-1菌还原Cr（Ⅵ）的反应控制区域图，Cr（Ⅵ）的细菌还原反应逐渐由内扩散控制区转入化学反应控制区。优化了Ch-1菌还原Cr（Ⅵ）的最佳工艺条件：温度30℃，pH值10，细菌接种量20％。（2）铬渣NaCl浸出的动力学方程为：v=3.38×10^-7c_NaCl^0.26。pH值增大，k不断增大，CaCrO₄溶解率增加，溶解速率加快。振荡速度增大，k不断增大，CaCrO₄溶解速率增大。铬渣NaCl浸出反应表观活化能为34.24kJ／mol，Cr（Ⅵ）浸出速率受温度影响较大，k与温度关系式为：k（T）=exp（13.71-4117.9／T）。（3）铬渣HCl浸出过程的数学模型为：v=1.8676t^-0.54。浸出速率v随时间t呈-0.54级指数衰减，即随浸出时间增加，浸出速度逐渐减小；浸出过程数学模型表明：要提高浸出率及浸出速率，流速和温度应尽量大，铬渣粒度应小。HCl选择性浸出铬渣所确定的最佳实验条件为：pH=3，液固比5：1，流速180mL／min，温度40℃。在最佳条件下，浸出率较大，HCl消耗量较小。（4）六价铬在铅电极上还原为三价铬的电位约为-0.5V（vs．SCE）。细菌解毒铬渣后Cr（Ⅵ）的扩散系数为2.62×10^-8m²／s，大于细菌解毒铬渣前Cr（Ⅵ）的扩散系数4.4×10^-9m²／s，本细菌解毒铬渣工艺过程中，随细菌解毒时间增加，扩散系数增大，更利于铬渣内部的六价铬溶出。更多还原

【Abstract】 Chromium-containing slag is a kind of poisonous pollutionsgenerated from chromium industry, in which Cr（Ⅵ） contained isconsidered as one of the 8 most harmful chemicals to human beings,one of the 3 carcinogenic metals recognized internationally, and oneof the 129 main pollutions recognized in EPA of USA. In thepreliminary studies a strain of high effective bacteria identified asAchromobacter sp., naming Ch-1 bacteria has been successfullyseparated and obtained, which can reduce Cr（Ⅵ） of highconcentration in alkaline media. And a new novel technique ofchromium-containing slag detoxification by bacteria has been alsodeveloped. The Cr（Ⅵ） concentration of the residues afterdetoxification meets the national standards of hazard contaminations’leaching toxicity （GB5085-1996）.However, it lacks of profound understand about the reactionkinetics process of Cr（Ⅵ） reduction by Ch-1 bacteria, the kineticsprocess of Cr（Ⅵ） leaching by NaCl, the process of selective leachingof Cr（Ⅵ） by HCl, and the Cr（Ⅵ） diffuse coefficient of the leachingprocess. Therefore, these processes were investigated in this study.The kinetics of every step of the process were established and theeffects of each factor on the Cr（Ⅵ） reduction and leaching rate wereinvestigated. The diffuse coefficient D of Cr（Ⅵ） was also deteminedby electrochemical method. And the technics parameters were furtheroptimized. It proved the progress of the new novel techniquethroretically and provided important theoretical proofs for Cr（Ⅵ）pollution treatment.The conclusions obtained are listed as follows:（1） The reaction rates are approximately the same under pH valueof 8 to 10 and decrease gradually when pH＜8 and pH＞10. Thereaction order of Cr（Ⅵ） reduction in alkaline media by bacteria iszero. The reaction rate decreases and the apparent activation energy increases along with the decrease of bacteria inoculation amounts.The kinetic equations in bacteria inoculation of50%,30%,20%,10%,5% are respectively listed as follows: v=57.5211,v=51.6306, v=45.9764, v=24.0025, v=7.3265, and the apparentactivation energys are respectively 6.89kJ/mol, 14.75 kJ/mol,19.24kJ/mol, 33.54 kJ/mol, 72.52 kJ/mol. Meanwhile, control areafigure of Cr（Ⅵ） reduction by Ch-1 bacteria with the changing of thebacteria inoculation amounts is obtained. The controlling step forreduction of Cr（Ⅵ） by bacteria is varied from inner diffusion tochemical reaction with the inoculation amounts being changed from50% to 5%. The optimized technological conditions is obtained, i.e.,temperature 30℃, pH=10 and 20% bacteria inoculation amounts.（2） The reaction order of slag leaching process by NaCl is 0.26,and the kinetics equation is v=3.38×10^-7c_NaCl^0.26. The reaction rateconstant k increases with increasing pH value and shaking speed. Anddissolution rate of CaCrO₄ increases too. The apparent activationenergy is 34.24kJ/mol and relationship between temperature and k isk（T）=exp（13.71—4117.9/T）.（3） The mathematic model of the leaching process by HCl is:v=1.8676t^0.54. The leaching rate of Cr（Ⅵ） v decays with theexponential of -0.54. The mathematic model indicates that highleaching rate and leaching speed can be obtained under the conditionof high flowing speed, tempetrue and small slag granularity. Theoptimized conditions are pH=3, liquid to solid ratio of 5:1, flowvelocity of 180mL/min and temperature of 40℃. Under the optimizedconditions, the highest leaching rate and the least HCl consumptioncould be obtained.（4） The electric potential of Cr（Ⅵ） reduced to Cr（Ⅲ） under Pbelectrode pole is -0.5V（vs.SCE）. The diffuse coefficient D after theCr（Ⅵ） reduction by Ch-1 bacteria(2.62×10^-8m²/s) is bigger than thatof the original slags(4.4×10^-9m²/s). The diffuse coefficient Dincreases during the detoxification process and it is good for theCr（Ⅵ） in the inner of the slags to dissolve out.更多还原