Development And Performance Of Sustaiinable Solidifying Agents For Multi-Heavy Metal-Contaminated Soil

At present,a large number of heavy metal contaminated sites need to be repaired at home and abroad.Solidification and stabilization of heavy metal contaminated sites by solidifying agents is the most widely used in practice.In this study,bentonite,zeolite,fly ash,blast furnace slag and cement were selected to compose different solidifying agents and applied to the remediation of Pb,Zn,Cu and Ni contaminated soils.The strength properties,freeze-thaw properties,leaching characteristics and morphological distribution of the specimens after solidification were used to explore the effect of the solidifying agents on the remediation of heavy metal contaminated soils.The results of this study are expected to provide theoretical support for the engineering practice of soil remediation.The main conclusions are as follows:(1)Preliminary screening of solidifying agents showed that there were four groups with good results.Two groups were traditional control group composed of cement and fly ash.The other two groups were sustainable group with minerals,namely cement-fly ash-bentonite and cement-fly ash-zeolite.The total amount of four groups of solidifying agents was 5%.(2)The unconfined compressive strength of the specimens corresponding to the four groups of solidifying agents was positively correlated with solidifying age,and the unconfined compressive strength was greater than 1 MPa after 28 days,which met the requirements of 0.35 MPa of EPA.(3)The strength loss of the four specimens after 10 freeze-thaw cycles is less than 33%,and the lowest loss is the sustainable group with zeolite,the strength loss of the corresponding specimens after 10 freeze-thaw cycles is 5.5%.In addition,the mass loss of each specimens is less than 3% after 10 freeze-thaw cycles,which meets the relevant requirements of "Test Method for Freeze-thaw Test of Inorganic Binder Stabilized Materials"(T0805-2009).The mass loss of the specimens corresponding to the sustainable group cement-fly ash-bentonite is the smallest after 10 freeze-thaw cycles,which is 2.21%.(4)The leaching concentration of zinc,copper and nickel in contaminated soil could be reduced by four groups of solidifying agents,and the percentage of reduction was between 14.47%-47.66%,50.94%-76.28% and 41.54%-79.50% respectively.In addition,freeze-thaw has little effect on the leaching concentration of Pb in the specimens corresponding to the four groups of solidifying agents,but has great influence on the leaching concentration of Zn,Cu and Ni.The leaching concentration of Zn,Cu and Ni in the specimens corresponding to the sustainable group cement-fly ash-bentonite decreases after freeze-thaw.Sustainable group with zeolite will decrease the leaching concentration of Ni in the specimens after freeze-thaw,while traditional group will increase the leaching concentration of Zn and Cu in the specimens after freeze-thaw.(5)(1)The four groups of solidifying agents can convert Zn and Ni from exchangeable state to carbonate-bound state and iron-manganese-bound state.(2)The sustainable combination of cement-fly ash-bentonite can make the copper in the contaminated soil from carbonate bonded state to iron-manganese bonded state.(3)Freeze-thaw has little effect on the morphological distribution of Pb,Zn,Cu and Ni in each specimen.