Remediation Of Heavy Metal-contaminated Soil And Sediment Through Bioleaching Using Fermentation Liquid Of Acidithiobacillus Thiooxidans TS6 And Aspergillus Niger Y9

The rapid development of industry and agriculture improved people's living standard year by year.Meanwhile,the concomitant "three wastes" brought severely damages to the environment,among which heavy metal pollutions of soil and sediment became increasingly critical,which seriously threatened to terrestrial and marine ecosystem and even to health and life of human.Washing of soil or sediment is a very promising technique in treating with the heavy metal pollution.For the past few years,the biotic acids and various active substances have been proven to be efficient in removing heavy metals from environment medium.In view of the above,this paper used two kinds of acidic microbic fermentation liquid as soil(sediment)washing agents,including Acidithiobacillus thiooxidans TS6 and Aspergillus niger Y9 fermentation liquid from their pure culures,respectively.Meanwhile,EDTA solution was adopted as a contrast.The dissolution of heavy metals under various washing conditions were investigated through flask test and column test,and changes of heavy metal forms and other physical-chemical properties after washing were observed as well.The main results are as follows.(1)Multiple methods of contamination assessment were taken to evaluate the contaminating conditions of tested soil and sediment.As far as single factor contamination index and index of geo-accumulation were concerned,the contamination level of heavy metals in the soil and sediment showed Cd>Zn>Pb>Cu,yet Cr and Ni being barely contaminated.Nemero comprehensive contamination index indicated that the soil and sediment were in considerable contamination degree,sediment being worse.The potential ecological risk index suggested that Cd was the most heavily contaminated metal,which had the highest potential ecological risk coefficient,contributing the dominant proportion for the risk.The results of multiple assessment methods differed though,the overall conclusions were consistent:the level of heavy metal contamination in tested soil and sediment was extremely serious,contaminants were mainly Cd,Zn and Pb.(2)Fermentation liquid of TS6 and Y9 were applied in the flask test on soil and sediment.Results showed;extraction rate of heavy metals rised with the extraction time extended,and time within 2 hours was a fast-extracting-period.The pH of extracting system increased when the solid-to-liquid ratio increased,which lead to the decrease of heavy metal extraction rates.Fermentation liquid of TS6 and Y9 extracted Cd,Pb,Zn as highest as rates of 94.6%,18.3%,78.7%and 74.8%,48.5%,61.8%,respectively.When came to the sediment extraction,values turned out to be 100%,3.96%,82.3%and 68.3%,44.9%,67.8%.However,the increase of solid-to-liquid ratio would cause drastically decreases of heavy metal extraction rate.Took the extraction rates and operating costs into consideration,tested contaminated soil and sediment were suitable extracted by fermentation liquid of TS6 under 30%and 10%solid-to-liquid ratio,respectively.(3)Heavy metal extracting effects were studied using EDTA solution and EDTA-fermetation-liquid through flask tests.It was indicated that:the highest extraction rate of Cd,Pb,Zn in soil and sediment by EDTA were 88.9%,100%,55.9%and 76.6%,72.5%,51.6%,respectively.The extracting effect were all preferable under the pH ranged from 4.5 to 7.6(5%?30%solid-to-liquid ratio).The addition of EDTA solid into fermentation liquid could clearly increase the extraction rate of Cd,Pb,Zn by Y9 fermentation liquid,but it had little effect on TS6 fermentation liquid.(4)During soil column washing tests,the extraction rates of Cd,Pb,Zn were 89.5%,6.99%,68.1%and 73.7%,33.7%,58.4%,respectively,after washed with fermentation liquid of TS6 and Y9.The extraction rates of Cd,Pb,Zn were 64.6%,0.46%,48.8%and 36.2%,15.3%,50.7%,respectively when it came to sediment.The fractionation tests of heavy metals of soil and sediment before and after the column washing were carried out.It was suggested that the two kinds of fermentation liquid were able to remove all forms of Cd and Zn in soil,which made the remaining Cd and Zn existed mainly in the residual form,having reduced their toxicity.However,the exchangeable form of Pb content somewhat increased.Washing by two kinds of fermentation liquid changed Cd to more stable forms in the sediment and also lessened the Pb and Zn content in the forms bound to carbonates and Fe-Mn oxides,meanwhile exchangeable Pb and Zn content rised.By comparison,EDTA solution washing made a comparatively high removal rate of heavy metals,but resulted in more active forms in the soil and sediment than using fermentation liquid.Thus,considered the removal rate and forms remained of heavy metals,the contaminated soil was more suitable for washing with about 10 times PV fermentation liquid of TS6,however,the contaminated sediment was more suitable for washing with more than 20 times PV fermentation liquid of TS6.(5)After washing by the two kinds of fermentation liquid,CEC and content of available nitrogen,phosphorus and potassium of the soil and sediment increased somewhat.However,the pH of soil and sediment reduced to 2.0?4.8.Adding moderate amount of CaO was able to increase the pH to 6.6?7.0(weak acidic to neutral pH).The waste liquid produced during washing could be reused to culture new fermentation liquid after precipitating heavy metals by raising pH to 7.The results of this research could provide useful imformation to researchers and practitioners working on the remediation of heavy metal-contaminated soil and sediment,and promote the application of washing techonology by acidic microbic fermentation liquid.