Biochar From Different Feedstock For Remediation Of Heavy Metal Contaminated Tailings Soil

Lead-zinc mining areas in China are widely distributed and rich in reserves.High-intensity mining activities have caused many abandoned tailings land,destroyed the ecological environment,and caused serious heavy metal pollution.Adding exogenous organic amendments can effectively combine with phytoremediation to achieve the dual purposes of soil remediation and vegetation restoration in abandoned tailings land.Biochar is one of the commonly used organic amendments.The basic physical and chemical properties of biochar determine its remediation behavior and application potential,and the types of feedstocks used to prepare biochar play a leading role in its physical and chemical properties.In this study,two soil samples of heavy metal contaminated soil and tailings were collected from abandoned mining areas.Straw biochar(SB),coconut shell biochar(CSB)and sludge biochar(SSDB)were mixed with soil samples at a ratio of 0.5%,2.5%and 5%respectively,and incubation at 25℃ and 70%field water holding capacity for 30 days.It is of great theoretical and practical value to systematically compare the physicochemical properties of biochar prepared from different feedstock and analyze its effects on remediation behavior of heavy metal contaminated soil.The details are as follows:(1)In this study,the properties of heavy metal contaminated soil and tailings slag are different to some extent.The pH conditions are in two extremes,and the soil is acidic and alkaline respectively.The contents of Heavy metals Pb and Zn were up to 2676 mg·kg-1 and 1091 mg·kg-1,and the contents of heavy metals Zn and Pb were up to 2113 mg·kg-1 and 1207 mg·kg-1,respectively,far beyond the risk screening values in soil pollution risk control.The surface structure and physicochemical properties of the three biochar were quite different.Straw biochar has higher aromatization degree and larger specific surface area,but lower pH value.The content of C was 56.29%,and the content of PO43-dissolved by biochar was 10.5mg/L.Coconut shell biochar has the highest aromatization degree and has a large pore size,and its C content is 69.23%.The sludge biochar was not aromatized enough,and its C content was only 5.66%.Sludge biochar is rich in mineral components,among which the content of SO42is four times that of the other two biochar,and the available phosphorus content is up to 174.2 mg/kg.(2)After the soil improvement experiment,compared with the control group,the concentrations of Cd,Cu,Pb and Zn in the TCLP leaching solution of the soil with 5%dosage showed a decreasing trend on the 30th day.In the straw biochar treatment group,Cd,Cu,Pb and Zn decreased by 7.1%,37.6%.10%and 20.2%respectively.In coconut shell biochar treatment group,Cd,Cu,Pb and Zn decreased by 10.8%,4%,10.5%and 8.1%respectively.In the sludge biochar treatment group,Cd,Cu,Pb and Zn decreased by 10.2%,36.9%,12.7%and 8.5%respectively.In the detection results of heavy metal forms,we found that the heavy metal forms in the soil were mainly residual,indicating that the leaching risk of heavy metals in this area was reduced.In general,there was no obvious difference in the passivation effect of the three biochar to heavy metals in acidic soil.As the curing time goes on,the stabilization effect does not change significantly.(3)After the soil improvement experiment,compared with the control group,the three biochar can significantly reduce the leaching concentration of heavy metals in the soil,and the stabilization effect of biochar on heavy metals increases with the increase of the dosage and the extension of curing time.Among them,5%dosage of sludge biochar obtained the optimal treatment effect,the concentration of Cd,Cu,Pb and Zn in TCLP leaching solution decreased by 13.9%,33.4%,49.2%and 23.6%,respectively,and the acid soluble and reducibility of the four heavy metals in BCR sequential extraction decreased significantly.The residue increased by 10.5%,12.69%,23.85%and 11.77%,respectively.In terms of time,coconut shell biochar has a rapid stabilization effect on short-term TCLP leaching results of heavy metals,while straw biochar and sludge biochar have a slow stabilization effect on long-term TCLP leaching results.(4)The micromorphology,surface functional groups,bond energy and crystal structure of biochar were characterized by FTIR,XPS and XRD,and the surface complexation and chemical precipitation of heavy metals in soil and anions such as-COOH,-OH,-S=O-in biochar were analyzed.Redundancy analysis showed that there was a great difference between coconut shell biochar treatment group and sludge and straw biochar treatment group,and sludge biochar treatment group showed a good response to its dosage.The pore size and carbon content of biochar were negatively correlated with the short-term leaching of heavy metals.The conductivity,available phosphorus and anion contents were negatively correlated with the longterm leaching of heavy metals.Straw biochar has a high content of available phosphorus,a microporous structure of vascular bundles,and a large specific surface area,which is suitable for heavy metal remediation by surface complexation,surface adsorption and precipitation of oxygen-containing functional groups.Coconut shell biochar has higher pH value,larger pore size,higher H/C atomic ratio and hydrophobicity,which is suitable for heavy metal remediation by surface complexation,electrostatic attraction and adsorption of oxygen-containing functional groups.Sludge biochar has high conductivity,pH value and anionic content,but low degree of aromatization,which is suitable for heavy metal restoration by precipitation method.