Study On The Remediation Effect Of Sludge-palygorskite Co-pyrolysis Biochar On Soil Heavy Metal Pollution

The rapid development of industrialization and agriculture not only brings convenience to people,but also causes serious pollution to the soil.Because heavy metals will enter the human body along the food chain and harm human health,the remediation of heavy metal contaminated soil has become an urgent environmental safety issue.With the continuous improvement of sewage treatment standards,the surplus sludge produced after sewage treatment also increases,and the safe treatment and disposal of sludge has become an urgent problem to be solved.Palygorskite is a kind of magnesia-aluminosilicate mineral with layered chain structure,which has the characteristics of large specific surface area,strong adsorption,alkalinity,etc.There are abundant palygorskite mineral resources in Gansu,but the exploitation and utilization degree of palygorskite is low.The development of diversified palygorskite products will play a positive role in the development and utilization of palygorskite in Gansu.In this study,starting from the resource treatment of sludge,the sludge and palygorskite were mixed and pyrolysis for 2.0 h at 200℃,250℃,300℃,350℃,400℃and 450℃to prepare six kinds of sludge-palygorskite co-pyrolysis biochar,and the six kinds of biochar were characterized by SEM,FT-IR and XRD.Six kinds of sludge-palygorskite co-pyrolysis biochar were added to the heavy metal contaminated soil to conduct the stabilization experiment of contaminated soil.PH,EC and CEC of stabilized soil were determined.Bioavailability content of heavy metals was extracted by H₂O,Ca Cl₂,DTPA and TCLP.Chemical speciation of heavy metals was determined by BCR sequence extraction.After that,pot experiments were carried out to determine the growth status of plants and the enrichment of heavy metals in different parts of plants,so as to evaluate the remediation effect of sludge-palygorskite co-pyrolysis biochar on heavy metal contaminated soil,so as to achieve the purpose of turning waste sludge into treasure.The results showed that:（1）After passivation for 30 days,the pH and CEC of the stabilized soil increased firstly and then decreased.When the co-pyrolysis biochar prepared at 400℃was added,the pH and CEC of the stabilized soil increased by 16.7%and 17.95%,respectively,compared with the control treatment group.The EC of stabilized soil showed a trend of constant increase,and the EC reached the maximum value when the co-pyrolysis biochar prepared at 450℃was added,which increased by 8.94%compared with the control group.（2）After stabilization for 30 days,the contents of four extractive states of heavy metals in the contaminated soil showed a decreasing trend on the whole,indicating that the risk of heavy metals being absorbed by plants was reduced.The extracted contents of H₂O,Ca Cl₂,DTPA and TCLP of heavy metals Cu,Zn,Cr and Ni reached the lowest level after adding co-pyrolysis biochar prepared at 400℃to stabilize heavy metal contaminated soil.The extracted contents of H₂O,Ca Cl₂,DTPA and TCLP of heavy metals Cr and Pb reached the lowest after adding co-pyrolysis biochar prepared at 450℃to stabilize heavy metal contaminated soil.The addition of co-pyrolysis biochar passivates contaminated soil,which promotes the conversion of heavy metals from unstable acid-soluble and reduced states to stable oxidation and residue states,reduces the migration capacity of heavy metals to the food chain.Compared with the control treatment,the residual states of Cu,Cd,Zn,Pb,Cr and Ni increased by 18%,22%,9%,19%,17%,20%,and the acid soluble states decreased by 12%,26%,8%,17%,17%,18%,respectively.（3）The addition of co-pyrolysis biochar to contaminated soil promoted the growth of plants and increased the biomass of plants.The plant height,root length,fresh weight and dry weight of the plants were significantly higher than those of the control treatment.Compared with the control treatment,plant height,root length,fresh weight and dry weight were increased by 21.70%,49.61%,25.49%and 32.56%,respectively.At the same time,the addition of co-pyrolysis biochar to contaminated soil reduced the accumulation and transport of heavy metals in plants.In order to limit the accumulation of heavy metals from the soil to the plant body,from the underground part to the aboveground part of transport.Therefore,sludge-palygorskite co-pyrolysis biochar can be used as an environmental protection material for the stabilization and remediation of heavy metal contaminated soil,and it is also a kind of high-quality plant growth fertilizer,which has a very broad application prospect.