Study On Combined Remediation And Mechanical Properties Of Chromium Contaminated Soil

Heavy metal chromium and its compounds are widely used as raw materials for industrial production in various fields,resulting in a large number of Cr(?)and Cr(?)contaminated soil in their long-term production process.Chromium contaminated soil is harmful to the environment and human beings.The research on remediation technology of chromium contaminated soil has strong theoretical value and practical significance.It is a major issue to remedy chromium contaminated soil by solidification/stabilization technology in the field of environmental geotechnical engineering at present.According to remediation of chromium contaminated soil by combined effect of solidification,reduction and adsorption and its mechanical properties,taking the typical chromium contaminated site in Shenyang city,Liaoning province,as a research object,through indoor experiments,theory analysis and numerical simulation,studying from deep level,multi-discipline,multi-angle,the main research results are as follows:(1)By tests of adding single curing agent,reducing agent,or adsorbent and orthogonal tests of adding the above three kinds of additives,the effect of different kinds of additives and each additive dosage,curing time,initial pH value,chromium content and valence on remediation chromium contaminated soil was determined.The best formula of the complex preparation containing curing agent,reducing agent and adsorbent was determined.By tests of toxicity characteristics leaching,the effect of different types of curing agent,water content,curing age,chromium content and pH value of extract on combined remediation chromium contaminated soil was studied.From the toxicity leaching characteristics point of view,the effect of remediation was evaluated.The results show that complex preparation of cement,CaS5 and fly ash can solidify and stable the contaminated soil containing Cr(?)and Cr(?)effectively by combined effect of solidification,reduction and adsorption.The best ratio of complex preparation is as follows:cement content of 20%,3 times amount of CaSs and synthetic zeolite content of 15%.The complex preparation can reduce the hindering effect of Cr(?)and Cr(?)on hydration and pozzolanic reaction and strengthen the solidification/stabilization effect by the combined effect of solidification,reduction and adsorption.So the leaching concentration of solidified soil significantly reduces and all the toxic leaching concentration meets the requirements of relevant standards.(2)By tests of unconfined compression strength,the effect of different types of curing agent,water content,curing age and chromium content on combined remediation chromium contaminated soil was studied,and the microstructure of the samples was analyzed by SEM.From the unconfined compression strength characteristics point of view,the effect of remediation was evaluated.The results show that the complex preparation has obvious effect on increasing strength of solidified chromium contaminated soil.After proper curing age,the maximum unconfined compressive strength of the solidified chromium contaminated soil is relative to the lower water content.The unconfined compressive strength of the solidified soil shows a continuous growth trend with the increase of curing age.Compared with adding single cement,the use of complex preparation can effectively avoid the phenomenon of the solidified soil strength rebound.With the increase of chromium content,the weakening effect of chromium on the strength of the solidified soil is intensified.The analysis of microstructure with SEM is consistent with the results of unconfined compressive strength test.(3)Based on the stress and strain relation in the unconfined compressive strength,damage variable D was introduced,Popovics model was improved and the elastoplastic nonlinear constitutive relative model of the combined remediation chromium contaminated soil was established with complex preparation.The model of the solidified soil was verified by origin software.The results show that the calculated values of the model fit well with the experimental data.(4)By tests of dry-wet cycle and freeze-thaw cycle,the effect of different chromium content and cycle times on combined remediation chromium contaminated soil was studied.Unconfined compression strength,toxicity leaching concentration and mass loss rates of the solidified soil were determined,and the microstructure of the samples was analyzed by SEM.From the durability characteristics point of view,the effect of remediation was evaluated.The results show that with the increase of dry-wet cycle times the unconfined compression strength of the solidified soil increases first and then decreases,and the concentration of Cr(?)and total chromium in leaching solution reduces first decreases and then increases.The higher the chromium content of the contaminated soil is,the lower the unconfined compressive sthength is,and the greater the concentration of total chromium and Cr(?)is.With the increase of freeze-thaw cycle times the unconfined compression strength of the solidified soil decreases gradually,and the concentration of total chromium and Cr(?)in leaching solution increases continuously.The higher the chromium content of the contaminated soil is,the lower the unconfined compressive sthength is,and the greater the concentration of total chromium and Cr(VI)is.Both in the dry-wet cycle experiment and freeze-thaw cycle test,with the increase of cycle times,the cumulative mass loss rate of solidified soil is gradually increased,and the higher the chromium content is,the greater the cumulative mass loss rate is.The both mass loss rate is less than 2%,which represents that the combined remediation chromium contaminated soil with complex preparation has better durability.(5)In order to study the feasibility of using solidified contaminated soil for subgrade filling,taking Shen Kang expressway as the engineering background from the Yalu River street to Xinchengzi district,the numerical simulation analysis of displacement,plastic zone,shear stress and shear strain increment was carried out with FLAC software.Contrasting with the original soil,the advantages of the solidified chromium contaminated soil with complex preparation were obvious.The results provide theoretical basis for engineering application of the combined remediation chromium contaminated soil.