Washing Removal Of Heavy Metals In Soil Based On The Citric Acid Fermentation Broth

The key point in the development and application of the soil washing remediation technology is that an economical,environmentally friendly and suitable washing reagent can be found.The development of new type of washing reagent and the improvement of existing washing reagent are very necessary in the study of washing technology.The washing remediation were carried out for heavy metals contaminated soil with the prepared citric acid fermentation broth as eluent in this subject,and the research work was performanced using a heavy metals contaminated plateau red-soil and a mined soil polluted by Pb as action objects.The washing performance of citric acid fermentation broth and the effect of compound washing system consisted by it and saponin were mainly explored.Specific research contents are as follows:1.Analysis of soil physicochemical properties and heavy metals speciation: The soil basic physicochemical parameters such as PH,water content,organic matter,cation exchange capacity and its particle size distribution were determined with Pb,Cu,Cr and Cd contaminated plateau red-soil as an example.In addition,analysis method of heavy metals content was established and the standard recovery experiment was performanced.The speciation analysis of heavy metals in soil sample was carried out with a modi-fied BCR-sequential extraction method.2.Preparation and washing performance research of citric acid fermentation broth: The citric acid fermentation broth was prepared by microbial fermentation technology with Aspergillus niger as strain and its washing effect was investigated by oscillation washing experiment,the removal percentages of heavy metals are that48.19% for Pb,30.56% for Cu,43.65% for Cr and 58.43% for Cd and higher than that using citric acid solution.The effect of concentration,liquid-solid ratio,pH and washing time on the removal rate of heavy metals in soil were investigated:respectively,the kinetic fitting indicates that desorption of Pb,Cu,Cr and Cd can be described by the double constant equation and Elovich equation: the latter is better than the former,and is a heterogeneous diffusion process.The speciation analysis ofheavy metals in soils before and after washing was carried out with a BCR method,the removal effect of acid-soluble fraction Pb,Cu,Cr and Cd is the best in the four forms,and the second is the reducible fraction,only partial oxidizable fraction and a small amount of residual fraction heavy metals can be removed.Spectroscopy characteristics analysis of soils before and after washing were carried out with SEM,XRD and FTIR,and analysis results suggest that the citric acid fermentation broth has only a small effect on the mineral composition and does not destroy the framework of soil system.3.The construction and application of compound washing system based on the citric acid fermentation broth: A composite washing system was established using citric acid fermentation broth and saponin and it possesses a good washing effect for Pb,Cu,Cr and Cd from plateau red-soil,and a higher removal rate for Pb was obtained and compared to using citric acid fermentation broth or saponin alone.Composite eluent was applied to washing remediation of Pb contaminated mined soil in the follow-up study,the desorption process of Pb can be fitted by the double constant equation and Elovich equation.Removal effect of compound washing solution on Pb in different particle size soil was investigated,and result shows that Pb in larger particle size is relatively easy to remove.Effects of saponin concentration,liquid-solid ratio and washing time on removal rate was investigated by the Response Surface Methodology based on single factor experiments,the influence degree of 3factors were: liquid-solid ratio>washing time>saponin concentration,and the interaction between washing time and liquid-solid ratio is also significant,moreover,56.20% Pb can be removed under the optimized conditions and it is close to the predicted value.The speciation analysis of Pb in soils before and after washing shows that the compound eluent can effectively remove the active Pb.