The Effect Of Nano-zero-valent Iron On The Exudation Risk Of Heavy Metals In Sludge Biochar

Resource utilization is an inevitable way of municipal sludge treatment and disposal.The preparation of biochar by sludge pyrolysis is one of the effective ways to utilize municipal sludge resources,but the heavy metal seepage seriously restricts the practical application of sludge biochar.Therefore,reducing the risk of heavy metal seepage has important scientific significance and practical value for the utilization of sludge resources.In this study,municipal sludge was used as raw material,nano-zero-valent iron?nZVI?was used as additive,and sludge biochar was prepared by pyrolysis.The effects of pyrolysis temperature,heating rate,nZVI addition and heat preservation time on the morphology of typical heavy metals such as Zn,Cu,Pb,Ni and Cd in sludge biochar were studied.Combined with XRD,XPS,FTIR,SEM/EDS and other characterization results,the mechanism of heavy metal migration and transformation was discussed.Using the comprehensive pollution index method,the risk assessment index method,the geological accumulation index method and the potential ecological risk analysis method,the ecological risk of sludge biochar was evaluated.The following conclusions are drawn:?1?The main order of the influence of each factor on the steady morphology of Zn was determined by orthogonal experiment:heat preservation time>pyrolysis temperature>heating rate>nano-zero-valent iron addition.The optimal preparation conditions were:pyrolysis temperature 700°C,heating rate 8°C/min,heat preservation time 30 min,The amount of added was 1000mg/kg.The main order of the influence of each factor on the steady morphology of Cu was determined by orthogonal experiment:heat preservation time>heating rate>pyrolysis temperature>nano-zero-valent iron addition.The optimal preparation conditions were:pyrolysis temperature 500°C,heating rate 10°C/min,heat preservation time 60 min,The amount of added was 1000mg/kg.The main order of the influence of each factor on the steady morphology of Pb was determined by orthogonal experiment:heating rate>nano-zero-valent iron addition>pyrolysis temperature>heat preservation time.The optimal preparation conditions were:pyrolysis temperature 700°C,heating rate5°C/min,heat preservation time 60 min,The amount of added was 500mg/kg.The main order of the influence of each factor on the steady morphology of Ni was determined by orthogonal experiment:nano-zero-valent iron addition>heating rate>heat preservation time>pyrolysis temperature.The optimal preparation conditions were:pyrolysis temperature 700°C,heating rate 5°C/min,heat preservation time 60min,The amount of added was 500mg/kg.The main order of the influence of each factor on the steady morphology of Cd was determined by orthogonal experiment:pyrolysis temperature>nano-zero-valent iron addition>heating rate>heat preservation time.The optimal preparation conditions were:pyrolysis temperature 500°C,heating rate 5°C/min,heat preservation time 30 min,The amount of added was 200mg/kg.?2?The optimized nZVI addition,pyrolysis temperature,heating rate and constant temperature time by single factor experiment were 800mg/kg,800°C,4°C/min and 60min.The following results can be obtained by combining XRD,SEM,BET,FTIR and XPS analysis.The nano-iron zero-valent iron was successfully loaded on the sludge biochar.The main crystal in the sludge is quartz?SiO₂?,and a series of functional groups such as NH,C=O,C-H,C-O-C and-OH were formed,different surface morphology and pore structure were formed under different preparation conditions.Through a series of characterization nano-zero-valent iron can participate in the reaction of heavy metal content and morphological change.Fe⁰ was oxidized to Fe₂O₃ and then oxidized to Fe₃O₄.The reaction also forms FeOOH.FeOOH and Fe₃O₄ were transformed into each other in the whole reaction system,indicating that the immobilization of heavy metals is a complicated process.The ecological risk of sludge biochar was evaluated.Under RAC evaluation system,the ecological risk of sludge biochar was low or no risk.In the potential ecological risk assessment,the ecological risk of other heavy metals is low except Cd.In addition to Zn and Cd in the geological accumulation index method and comprehensive pollution index method,the ecological risks of other heavy metals are lower.In general,the ecological risk of the prepared biochar is low.In practical applications,the ecological risks of Zn and Cd should be paid attention to.