Studies On Electro-reduction Of Cr～(6+) In Alkaline Media

With the rapid development of industrial production, chrome has become a main environmental pollutant in the recent years. Chromium residues are the main wastes in the manufacturing of chromium salts and many of them are piled up in the open air without being dealt with in which there are water-soluble and acid-soluble Cr6+ and these soluble chrome can run off with the rain then pollute the soil, river and underground water source of the environment badly. In general, the chrome-included solution with the rain shows alkalescent, so it shows significant practical value to electrolyze and reduct Cr6+ in alkalescent media directly to dispose the extraction of chromium residues to disposal the chromium waste pollutant.In the paper, some factors influencing the effect of reduction including the anodic material, the cathodal material, the area ratio of cathode to anode and the kind of electrolyte have been investigated and screened out. The result showed that under the same reduction conditions, the reduction of Cr6+ has better effect to use lead-antimony alloy as anode, iron plate as cathode, which area ratio is 7:1 and potassium sulphate as electrolyte. At the same time, using lead-antimony alloy as anode, through orthogonal tests, the other factors influencing the reduction of chrome waste water have been investigated including the original concentration of Cr6+, the space between polar plates, the addition of electrolyte and stirring to decide the major influencing factor. Based on the studies, the best electrochemical reducting conditions have been gained after single factor optimizing tests of some important factors, that is: using certain potassium sulphate (2.5g·L-1 electrolyte) as electrolyte, space (4mm) between the polar plate and the right original concentration of Cr6+( ≤ 20mg·L-1) under these conditions, chrome(VI)-included waste water can be reduced to less than 0.5mg/L which reaches national emission standard; At last, the research of the influence of the temperature on the electrochemical reaction rate and the kinetics of reaction has been carried on and the result showed that the reaction rate will increase with temperature increment, also showed that the electro-reducting reaction of Cr6+ in alkalescent media is the same first order reaction as in acid media.In addition, the reduction of Cr6+ included waste water indirectly in alkalescent media has been researched which suggested that the waste water can be coped with in which the concentration of Cr6+can be reduced from less than 50mg·L-1 to 0.5mg/L. The research on the factors influencing the indirect reduction of Cr6+ showed that it is advantageous for increasing the percent reduction of Cr6+ to use certain space between polar plate (4mm), add certain potassium sulphate (2.5g·L-1 electrolyte) and control lower pH value (pH=7-9).By comparing direct and indirect ways of electrolyzing, conclusions have been gotten that the percent reduction of indirect way is higher than that of direct way, but there will be plentiful flocculent precipitates in the indirect reduction.It is found in experiments that there are certain erosive problems in not only the direct way but also the indirect way. If the anode is iron, in which the temperature is less than 298.15K, the voltage is less than 2.5V and the concentration of SO42- is less than 25g L-1, the erosive degree can be reduced effectively; If the cathode is iron, we can reduce the current density to reduce the erode; And if lead-antimony is used as anode, taking out the electrode after the electroanalysis and using the distilled water to rinse it, the erosive degree can be reduced effectively.