Bacterial Detoxification Of Chromium Slag Kinetics

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.