Preparation Of Biochars Via Co-pyrolysis Sewage Sludge With Cotton Stalks For For Remediation Of Lead And Cadium Contaminated Soil

In response to the rapid economic development and urbanization in China,many municipal sewage treatment plants have been constructed around the country,which produce an enormous amount of sewage sludge annually,however,most of them is not being disposed of properly.The traditional disposal methods of sewage sludge include incineration,landfill dumping,and land application,which always cause secondary environmental pollution and pose high ecological risk.Therefore,it is a practical need to develop an alternative technology that can reuse sewage sludge in a sustainable way.Meanwhile,combined pollution of heavy metals in soil is widespread and always low in concentration,which is another serious environmental problem facing China.In this case,in-situ passivation remediation maybe the most economical method for remediation of heavy metal contaminated soil in China.Passivation agents is the key to in situ passivation remediation.Among numerous passivation agents,biochar has unique advantages because of its low cost,good passivation effect of heavy metals,and the ability to improve soil quality at the same time.Therefore,conversation of sewage sludge into biochar via pyrolysis,which can be used as passivation agents for remediation of heavy metal contaminated soil,is a promising strategy to safe dispose of and reuse sewage sludge.However,biochars prepared via pyrolysis of sewage sludge alone always have a poorly developed pore structure and small specific surface area because of the low C content and easily thermal decomposed organic components such as protein and starch.At the same time,toxic and harmful heavy metals such as Pb,Cr,Cu,Zn,Ni and Cd in sludge will enrich in the biochar,which pose high ecological risk.These faults limit the application of sludge biochar as a passivation agents.Raw materials,pyrolysis temperature,and pyrolysis residence time are the three main factors affecting the migration and transformation of elements in the pyrolysis process of biomass,and determine the performance and cost of biochar.Therefore,the pyrolysis process should be optimized to improve the performance of sludge biochar and reduce the potential ecological risk of heavy metals.Co-pyrolysis of sludge and agricultural straw with high C content and low heavy metals contents may be the key to achieve this goal.In this study,biochars were prepared via co-pyrolysis of sewage sludge and cotton stalks.The effects of pyrolysis temperature,pyrolysis residence time,and cotton stalk addition ratios of cotton stalks on the migration and transformation of elements(C,H,N,Pb,Cr,Cu,Zn,Ni,and Cd,etc.),physical-chemical properties,pore structure,and surface chemical characteristics of the biochars were studied,and the potential ecological risks of heavy metals in the biochars were quantitatively evaluated.Response surface methodology(RSM)was used to optimize the co-pyrolysis process.The effects and mechanisms of adsoiption of Pb2+ and Cd2+ by mixed-based biochar in solution and soil were studied by adsorption experiments of Pb2+ and Cd2+,and passivation experiments of Pb and Cd contaminated soil.The ryegrass pot experiment was conducted to study the effects of mixed-based biochar on phytoavailability in Pb and Cd contaminated soils.The main results are as follows:Pyrolysis temperature,pyrolysis residence time,and cotton stalk addition ratios had significant effects on the migration and transformation of heavy metals during co-pyrolysis process.Increase in pyrolysis temperature and prolong of pyrolysis residence time can increase the concentration of heavy metals in mixed-based biochar,promote the transformation of heavy metals from weak acid extractable fraction,reducible fraction to oxidizable fraction and residue fraction.And increase in cotton stalk addition ratios reduced the concentration of heavy metals in mixed-based biochar,enhanced the reduction atmosphere in pyrolysis device,and significantly increased the organic binding fractions of heavy metals.Heavy metals were converted from weak acid extractable fraction and reducible fraction to oxidizable fraction,but increase in cotton stalk addition ratios reduced the content of Si in the raw materials,resulting in a decrease in the proportion of heavy metal residues fraction.Potential ecological risk index(RI)evaluation results showed that increasing pyrolysis temperature,prolonging pyrolysis residence time and increasing cotton stalk addition ratios could reduce the potential ecological risks of heavy metals in mixed-biochar.Co-pyrolysis reduced the migration and bioavailability of heavy metals in sludge,and reduced the potential ecological risks of using sludge-based biochar.Pyrolysis temperature,pyrolysis residence time,and cotton stalk addition ratios had significant effects on the properties of the biochar.Increasing pyrolysis temperature and prolonging pyrolysis residence time reduced the yield and C,H,N content of mixed-based biochars,but increased the aromatization degree,pH value,and ash contents of mixed-based biochars.The specific surface area increased first and then decreased with the increase of pyrolysis temperature and the prolong of pyrolysis residence time,reaching the maximum at 650? and 90 min.Increasing the cotton stalk addition ratios decreased the yield,H,N content and ash contents of biochars,but increased the C content and aromatization degree.The specific surface area increased with the increase of the cotton stalk addition ratios.The sludge was a macroporous material,and the mixed-biochars prepared by co-pyrolysis were all mesoporous materials.The addition of cotton stalks during pyrolysis of sewage sludge promoted the development of the pore structure of the mixed-biochars and increased their specific surface area.The results of response surface methodology(RSM)showed that there was an important interaction between pyrolysis residence time and cotton stalk addition ratio,but there was no interaction between pyrolysis temperature and cotton stalk addition ratio,pyrolysis residence time,pyrolysis temperature and cotton stalk addition ratio.The optimum technological conditions for the preparation of mixed-based biochar are as follows:pyrolysis temperature 638?,pyrolysis residence time 80 min,cotton stalk addition ratio 50%.Under these conditions,the mixed-based biochar has the greatest adsorption capacity.The experimental results of Pb2+ and Cd2+ adsorption in solution show that the initial pH of the solution has an important effect on the adsorption of Pb2+,Cd2+ by the mixed-based biochar.For Pb2+and Cd2+,the optimum initial pH of the solution is 5.0 and 6.0,respectively.The kinetic process of the adsorption of Pb2+,Cd2+ by the mixed-based biochar is more in accordance with the pseudo-second-order kinetic equation(R2>0.99).Compared with the Langrnuir model,the Freundlich model can better characterize the adsorption characteristics of the biochar on Pb2+,Cd2+.The adsorption process of mixed-based biochar for Pb2+,Cd2+ is a coexistence process of physical adsorption and chemical adsorption.The main mechanisms include ion exchange,precipitation coprecipitation,complexation,and physical adsorption.The results showed that the application of mixed-based biochar could improve soil water holding capacity,CEC and pH,increase plant nutrient supply of organic carbon,available N,P and K,enhance soil microbial activity,and improve soil quality.And the application of mixed-based biochar can effectively immobilize Pb and Cd in soil.After 90 days of culture with mixed-based biochar,the proportion of Pb and Cd in fraction F4 and F3 increased significantly,while the proportion in fraction F1 and F2 decreased.Although the application of mixed-based biochar increased the contents of Cu,Ni,Zn,and Cr in soil,the leaching contents of Cu,Ni,and Cr decreased.But Zn was activated,the proportion of Zn in F4 decreased while those in F1 increased,and TCLP leaching concentration of Zn increased with the increasing additional ratios of the biochar indicating the increase in mobility and bioavailability of Zn in the soil.The results showed that both the biochars increased the germination rate of ryegrass,increased the biomass of ryegrass,and reduced the accumulation of Pb and Cd in ryegrass plants due to the improvement of soil physical and chemical conditions,the increase of soil available nutrients,and the passivation of' Pb and Cd.Compared with the biochar prepared by pyrolysis of the sludge alone,the biochar prepared by co-pyrolysis of the sludge and cotton stalks has advantages in improving soil water holding capacity,reducing the mobility and bioavailability of Pb and Cd,and reducing the accumulation of Pb and Cd in ryegrass plants,but it has disadvantage in increasing soil available nutrient supply.The improvement of ryegrass growth should be the result of multiple factors.In general,co-pyrolysis of the sludge and cotton stalks can improve the performance of biochar and reduce the potential ecological risks of heavy metals in mixed-based biochar.Compared with the biochar prepared by pyrolysis of the sludge alone,the biochar prepared by co-pyrolysis was more effective in remediation Pb and Cd contaminated soil.Thus,co-pyrolysis of sewage sludge and cotton stalks is a green and sustainable way for the safe disposal and resource utilization of the sludge.