Effect And Mechanism Of Straw-based Biochar On Soil Improvement And Remediation Of Heavy Metal Pollution

As a kind of common agricultural waste,the preparation of biochar from maize straw and its application on soil improvement and pollution remediation are common ways of resource utilization.However,due to the lack of nutrients and poor adsorption performance,the efficiency of maize straw biochar on soil application needs to be improved.In this paper,maize straw was used as the raw material to prepare biochar and enhanced by adding high concentration fermentation wastewater and sewage sludge,which were named as fermentation wastewater-maize straw biochar?FMB?and sewage sludge-maize straw biochar?SMB?,respectively.By comparing the physical and chemical properties of the two biochars,we try to determine their respective application scopes.On this basis,pot experiments were carried out to investigate the sandy soil improvement after FMB application and remediation efficiency and mechanism of heavy metals(Pb²⁺,Cd²⁺)polluted soil after SMB application.The results were shown as following:The yield,fixed carbon,N content,N/C ratio and CEC of FMB increased with the increase addition of fermentation wastewater,which was related to the high concentrations of organic compounds and nitrogen compounds in the fermentation wastewater,and these were beneficial to the increase of nutrient contents in sandy soil.Meanwhile,the specific surface area and pore volume of FMB,aromatic C-H and C-N groups increased significantly with fermentation wastewater dosage increased.The results of the pot experiment after FMB application in sandy soil showed that FMB could improve the physical and chemical properties of soil and promote the growth of plants.With the increase dosage of fermentation wastewater and the proportion of FMB application,the physical and chemical properties of soil and the activity of catalase?CAT?also increased.When 5%FMB2 biochar was applied,the soil organic carbon content and total nitrogen content reached the highest values?34.50g/kg and 1.32 g/kg?,which increased by 57.46%and 78.38%respectively.Through co-pyrolysis of fermentation wastewater and maize straw,C and N can be fixed in biochar,and the contents of CEC,organics and nutrients in sandy soil can also be increased.Therefore,FMB can improve the properties of sandy soil and promote the growth of plants.For SMB,when the preparation temperature increased from 350°C to 600°C,the contents of C and N,specific surface area and micropore volume of SMB increased,and the surface functional groups such as-COOH,-OH and-PH also increased,which were benificial for its adsorption of heavy metals.In the pot experiments of heavy metal-polluted soil remediation by SMB,it was found that SMB can effectively passivate the Pb²⁺and Cd²⁺in the soil,making their speciations transformed from exchangeable and reducible to stable residue fraction.SMB could also increase the abundance of bacteria with lead fixation ability,and then reduce the lead content in the plant.With the application ratio of SMB increased from 1%to 5%?w/w?,the passivation ability of SMB for lead and cadmium improved.The results showed that SMB had a passivation function on heavy metal ions in the Pb-Cd polluted soil.The presence of cadmium in the Pb-Cd polluted soil was conducive to the fixation of lead and the transformation into residual fractions,while the presence of lead could improve the activity of cadmium in soil.The adsorption and passivation of heavy metals was mainly due to the complexation of functional groups?such as-COOH,-OH?,precipitation and ion exchange of on the surface of SMB.It was also related to the increase of soil p H and CEC.In conclusion,this study could provide a useful exploration for the application of modified maize straw biochar in soil improvement and heavy metal pollution remediation,and also provide a promising way to realize the resource utilization of fermentation wastewater and agricultural wastes at the same time.