Study On The Migration And Transformation Of Heavy Metals In Polluted Soil And Sediment Under The Remediation Of Biochar

Long-term of heavy metal pollution in soil and sediment can cause serious damage to the ecological environment and human health,which is an urgent problem to be solved.Biochar as a low-cost carbonaceous material has been widely used in soil amendment and water pollution treatment,which can serve as a potential adsorbent for in situ remediation of soil and sediment.In this paper,the preparation methods,basic physical and chemical properties of biochar materials,and their application in the sequestration and transformation of heavy metals in soil and sediments were studied.Biochars produced from different feedstocks and at different pyrolysis temperatures were applied into heavy metal polluted soil and sediment,to investigate the migration and transformation of heavy metal in soil and sediment under the remediation of different biochars.The effect of chemical activation treatment on biochar properties and sediment remediation were also studied.The biochar/attapulgite composite material was synthesized to further improve the sequestration and transformation efficiency of heavy metals in sediments,The results can provide theoretical basis and practical guidance for application of biochar based materials for soil and sediment remediation.In the first part,the biochars prepared from four kinds of feedstocks including bamboo,coconut shell,wood chips and bagasse were applied in red soil to study their effects on Cd???adsorption and immobilization.After a long-term of incubation,biochars significantly improved the physical/chemical properties of the soil.The addition of biochar to the red soil significantly increased soil pH,cation exchange capacity CEC,functional group content,and improved soil pore structure.The improvement of soil properties was helpful to improve the adsorption capacity of Cd???.The results showed that the maximum adsorption capacity of Cd???increased from 8.02 to 9.07-11.51 mmol/kg,and bamboo biochar exhibited the highest Cd???immobilization ability.The Langmuir model?R²>0.983?fitted the adsorption data better than the Freundlich model?R² is 0.902-0.937?.Soil column leaching experiments showed that biochar can also increase the immobilization of Cd?II?under leaching conditions.Biochar mainly increased the weak/unstable binding of Cd?II?to soil,such as ion exchange,electrostatic attraction,physical adsorption and carbonate precipitation.In addition,significant improvement of surface complexation were also observed.The application of biochar in soil can provide an effective method for soil Cd remediation and carbon sequestration.The amendment effects of biochar on total microbial activity was measured by fluorescein diacetate?FDA?hydrolytic activity,and phytotoxicity in Pb?II?-contaminated soils was examined by the application of 4 different biochars to soil,with rice as a test plant.The physical/chemical properties of biochar are closely related to the raw materials.Elemental analysis showed that four biochars were rich in carbon?75.27%-81.34%carbon?.The oxygen and hydrogen contents in biochars were 11.30%-18.21%and 4.19%-4.88%,respectively.The pH of all biochars is alkaline?9.77-10.92?,indicating that they could be used to mitigate soil acidity.Community Bureau of Reference?BCR?heavy metal speciation analysis showed that biochar could reduce the bioavailability of Pb in soil.Biochar could significantly reduce the toxic effects of Pb on soil microbes and the phytotoxicity of Pb on rice.FDA hydrolytic activities of biochar-amended soils were much higher than that of the control.The survival rate and growth of rice in lead contaminated soil were significantly improved under the treatment of different biochars,especially the effect of bamboo biochar was the best.The immobilization of Pb,improvement on soil properties,and positive effects on soil microbes are the main mechanisms of biochar on the remediation of Pb contaminated soil.These results suggested that biochar can have an excellent ability to reduce the toxic effects of Pb.Therefore,biochar can be used as an effective solution for Pb-contaminated soil remediation,which can achieve the win-win results of soil remediation and carbon sequestration.Different biochars were produced at different pyrolysis temperatures?400,500,600,700 ??using rice husk and rice straw as feedstocks.In addition,nine kinds of biomass,including rice straw,rice husk,peanut shell,saw dust,sugarcane bagasse,camellia shell,water hyacinth,bamboo waste,and coconut husk were selected as feedstocks for biochar preparation.Pyrolysis temperature and feedstock have great influence on the element composition of biochar.With the increase of pyrolysis temperature,the carbon content of biochar increased gradually,while the contents of nitrogen,hydrogen and oxygen decreased gradually.H/C,O/C and?O + N?/C moles ratio also decreased with the increase of pyrolysis temperature.Pyrolysis temperatures and raw materials also had significant effect on the specific surface area of biochar.The specific surface area of biochar increased with the increase of pyrolysis temperature.As the pyrolysis temperature is low,the specific surface area of the biochars produced from different feedstocks were small.With the increase of pyrolysis temperature,the surface functional groups of biochar had different changes.The concentration of overlying water and pore water decreased with the increase of pyrolysis temperature,which indicated that the pyrolysis temperature of biochar had a significant effect on the adsorption of heavy metals by sediment.The effect of biochar produced from different raw materials on the overlying water and pore water concentration were different,and water hyacinth biochar?WHB?exhibited the best performance.The content of oxygen-containing functional groups and the specific surface area of biochar were two important properties that affect the adsorption capacity of biochar,both of which determined the performance of biochar.Biochar could significantly reduce the acid-extractable heavy metals and increase the residue heavy metal content in the sediment.These effects increased with the increase of pyrolysis temperature,and varied with the feedstocks.The heavy metal concentration of TCLP leachate decreased with the increase of pyrolysis temperature,and the feedstocks of biochar had significant effect on the heavy metal concentration of Toxicity Characteristic Leaching Procedure?TCLP?leachate.It can be seen that the pyrolysis temperature and feedstock of biochar were the important factors that affected the properties of biochar,which determined the remediation efficiency of sediment.In order to improve the immobilization ability of biochar to heavy metals in the sediment,the microwave heating method was applied to modify the rice husk biochar using H₃PO₄,HNO₃,H₂SO₄,H₂O₂ and ZnCl₂ as the activating agent?PAB,NAB,SAB,OAB,and ZAB?.The carbon content of the activated biochar decreased,and the content of oxygen increased obviously.The specific surface area of rice husk biochar significantly increased under the effect of modifier and microwave heating.The specific surface area of different biochar followed the order of NAB>PAB>ZAB>SAB>OAB.At the same time,the modification can also introduce or add many functional groups?e.g.,hydroxyl functional groups,carboxyl functional groups and other oxygen-containing functional groups?on the surface of the biochar.Compared with the raw biochar,the contents of heavy metals in the overlying water and pore water of sediment under the treatment of different activated biochars decreased at different extent.Activated biochar could significantly reduce the acid extractable heavy metals in the sediment,and increase the heavy metal content of the residue.Compared with the raw biochar,heavy metals concentrations in TCLP leachates after treatment of different activated biochars decreased at different extent.The above results suggested that modification could effectively increase the adsorption capacity of biochar to heavy metals.Of which,H₂O₂ and ZnCl₂ showed the most obvious effect.The performanrce of H₂O₂ and ZnCl₂ activated biochar were equivalent to commercial activated carbon.It can be inferred that activated biochar can be used as a cheap and efficient in situ remediation material to improve the heavy metals retention capacity of sediment,and the selection of active agent is important for the performance of activated biochar.The slow pyrolysis of attapulgite pretreated rice straw biomass under the activation of ZnCl₂ was applied to prepare various biochar-based attapulgite composites?ATP/BCs?.Compared with the original biochar?BC?and attapulgite?ATP?,the specific surface area and pore volume of ATP/BCs were greatly increased,and average pore size significantly reduced.The specific surface area of ATP/BCs increased significantly with the addition of activator.SEM,EDS and XRD analysis showed that attapulgite was successfully loaded onto biochar.The concentration of heavy metals in the overlying water and pore water of sediment under the treatment of biochar/attapulgite composites decreased significantly compared with the raw biochar.The content of acid extractable heavy metals in the sediment treated with different biochar/attapulgite composites decreased significantly,and the contents of residue As and Cd in the sediment increased at different extent.Heavy metals concentrations in TCLP leachates after treatment of different biochar/attapulgite composites followed the order of BC>ATP/BC1>ATP/BC2.The results showed that the biochar/attapulgite composite material could effectively increase the adsorption and immobilization ability of heavy metal in sediment,which had a great improvement compared with the raw biochar.At the same time,the function of ATP/BC was related to the amount of zinc chloride activator in the preparation process,and increased with the increase of the amount of activator.In addition,the addition amount of ATP/BCs had a great effect on their performance,and the treatment efficiency was significantly improved with the increase of the addition amount of ATP/BCs.It can be seen that biochar/attapulgite composites can be used as an efficient in-situ remediation material to improve the adsorption and immobilization ability of heavy metals.The optimization of the preparation conditions and increase of addition amount of composite materials can improve the remediation ability.