Passivation Remediation Of Cadmium,Lead And Zinc In Lead-Zinc Mining Soils By Biochar-Hydroxyapatite Composites

Soil is the foundation of human survival and development.It is an urgent environmental problem to repair and improve heavy metal contaminated soil.China is rich in mineral resources.In the process of exploitation and utilization of mineral resources,ecological and environmental problems such as vegetation destruction are not only caused,but also serious heavy metal pollution is caused by the discharge of"three wastes"from industrial and mining enterprises.The remediation of soil heavy metal pollution is very difficult due to the characteristics of varied forms of heavy metal elements in soil,easy enrichment in organisms and difficult to be degraded and eliminated.Biochar has the characteristics of large specific surface area,developed pore structure,rich functional groups and alkalinity,which is commonly used to absorb and solidified heavy metals in soil.Studies have shown that the remediation effect of a single type of biochar on soil heavy metal pollution is limited,while the composite of biochar and other passivating agents can improve the soil remediation effect.In this study,Cd,Pb and Zn-contaminated soil in lead-zinc mining area of northwest Guizhou was taken as remediation object,and hydroxyapatite（H）,corn straw charcoal（M）and walnut shell charcoal（W）were taken as raw materials.Hydroxyapatite was mixed with corn straw carbon and walnut shell carbon according to the mass ratio of 1:4（denoted as HMA,HWA）,1:2（denoted as HMB,HWB）and 1:1（denoted as HMC,HWC）,and then activated at high temperature to prepare 6 kinds of composite materials.Each composite material was added to the contaminated soil at the ratio of 1%,2%and 4%respectively for passivation remediation test.The main research conclusions are as follows:（1）Hydroxyapatite mainly contains two functional groups,PO₄^3-and-OH,and the functional groups in biochar are mainly-OH,C-H,C-O,C=O and C=C.Under scanning electron microscope,hydroxyapatite is a small particle spherical structure with many regular pores on the surface,while biochar is mainly massive structure with obvious stratification and pores.After high temperature activation,the pores of HWA（HWB,HWC）composite increase and the surface becomes coarser.X-ray diffraction（XRD）analysis shows that hydroxyapatite is mainly composed of Ca₅（PO₄）₃OH,the mineral components of corn straw carbon are mainly SiO₂ and KCl,and the content of SiO₂ and Ca CO₃ in walnut shell carbon is more.（2）Soil pH increased with the extension of passivating agent adding time.Soil pH after composite treatment was higher than that of single material,and reached the maximum at 4%.The soil electrical conductivity（EC）increased most significantly(158.3μs·cm^-1)after 40 days of 4%corn straw carbon treatment.The cation exchange capacity（CEC）of soil increased gradually with the extension of time when single passivator was applied,and the water and fertilizer retention capacity of soil was obviously improved.The contents of soil organic matter（SOM）in the single application of passivating agent increased to different degrees.The contents of soil organic matter（SOM）increased better when the proportion of HMA was 1%,and the contents of soil organic matter（SOM）increased significantly when the proportion of HWB was 1%and 4%.（3）The contents of available DTPA-Cd and DTPA-Zn in the soil showed a general trend of decrease after passivation agent was added.The content of partially treated DTPA-Pb increased after 70 d passivation,and the content of available state increased with the extension of time.The mass ratio of 1:4（HMA,HWA）and 1:2（HMB,HWB）composite has better passivation effect on DTPA effective states,which is obviously superior to single passivation agent treatment.The contents of Cd,Pb and Zn in soil toxic leaching state（TCLP）can be effectively reduced by passivating agents.With the extension of passivation time,the contents of TCLP-Cd and TCLP-Pb on the whole show a continuous decreasing trend,and the contents of TCLP-Zn do not change significantly after 40 and 70 d of passivation treatment.Among all treatments,the best passivation effect of Cd,Pb and Zn in toxic leaching state was added 2%HMB and 1%HMC and HMA,and the passivation effect of composite was better than that of single application.After single material and composite material treatment,the contents of residual Cd,Pb and Zn in most treatments are increased,which can significantly reduce the migration and toxicity of heavy metals.The content of residual Cd after 40 d passivation is basically higher than that after 70 d passivation,and the passivation effect is greatly affected by the time scale.After 70 days of soil passivation agent cultivation,residue content in most treatments increased,and the passivation effect of Pb was better than that of Cd and Zn.The best passivation effect of Cd,Pb and Zn were added 4%HMA,HMB and H,respectively.Compared with the control group（CK）,The percentage of residual state content increased by 34.94%,45.05%and 36.78%.