Remediation Of Highly Contaminated Soil With Multi-heavy Metals By Complex Leaching Agents

With the adjustment of industrial structure and the advancement of urbanization,a large number of contaminated sites with mullti-heavy metals have been left in the center of the city,which brings serious threat to the safety of surrounding soil and groundwater environment and health of local residents,so the remediation of heavy metal contaminated site is imperative.The chemical leaching technology has attracted more attention because it can rapidly and efficiently remove heavy metals from soil with simple operation,and is suitable for remediation of soil highly contaminated with heavy metals.Generally single leaching agent can not reach the ideal remediation effect for contaminated soil with multi-heavy metals,and because of their synergistic effect,leaching with complex agents is one of focus in this research field of remediation for contaminated soil.Therefore,this thesis took the soil highly contaminated with multi-heavy metals in a certain site as research objects,aimed to select an appropriate complex leaching agent and optimized its leaching conditions to remediate the contaminated soil with multi-heavy metals,then studied the removal of heavy metals by leaching with sieveing pretreatment,at last did the leaching with complex agent in the reactor.The main conclusions were as follows:(1)The soil was highly contaminated with Cd,Cr,Pb and Zn and most of heavy metals were unstable form(carbonate bound form,exchangeable form,ferromanganese oxides bound form).Through comparison,CA and FeCl3 in a ratio of 5:5 was chosen to leach the highly contaminated soil and with the concentration of 0.5 mol/L,the removal rate of Cd,Cr,Pb,and Zn were 73.1%?41.3%?98.0%and 71.4%respectively.(2)The leaching condition with CA and FeCl3 was optimized:0.86 mol/L CA and 0.86 mol/L FeCl3 in a ratio of 5:5 and soil-water ratio 1:10 for 10 h.The leaching desorption process of heavy metals could be well described with Elovich equation and double constant equation,which was mainly controlled by the diffusion factor,and the desorption of Cd,Cr,Pb,and Zn was supposed to be heterogeneous diffusion.(3)The adsorption of heavy metals on soil was closely related to mineral elements in soil microaggregate.The content of Cd and Zn in contaminated soil increases with the decrease of particle size,and the highest content of Cr and Pb were found in fine sand.The content of unstable form of Cd,Pb and Zn increased with the decrease of soil particle size basically,and the content of residual form of Cr and Pb were found in sand grade particles,especially fine sand.(4)The background concentration was one of the key factors that influenced the removal amount and rate of heavy metals of the different particle-size fractions of soil.The order of removal rate of heavy metals by leaching with CA and FeCl3 according to particle size was roughly as follows:silt&clay>coarse sand>fine sand,and the much residual is the major reason for the worse removal effect of Cr and Pb in the fine sand.After leaching,a layer of relatively-dense crystalline mineral compound was formed on the surface of soil particles.The leaching rate of heavy metals of the different particle-size fractions of soil was significantly different from that of the original soil,but the original soil had a high leaching rate too,thus the sieving pretreatment of soil has slight significance in the leaching experienment.(5)The removal of heavy metals from contaminated soil in stirring reactor with leaching with CA and FeCl3 was similar to the results in centrifugal pipe system.After 24 h,the removal rates of Cd,Cr,Pb and Zn were 98.9%,79.6%,92.1%and 100.3%respectively.The leaching with CA and FeCl3 is an ideal method to remediate the highly contaminated soil with multi-heavy metals,with great reduction of the content of Cd,Cr,Pb,Zn and Cr(VI)and the ecological environment and human health risk after leaching.