Migration Behavior Of Heavy Metals And Characteristics Of Phosphorus-enriched Biochar

Municipal sludge is a kind of typical biomass waste and its output is increasing year by year.Municipal sludge is rich in phosphorus.It could be converted into phosphorus-enriched biochar by microwave pyrolysis,which could not only realize the efficient collaborative treatment of municipal sludge,but also realize the recovery and utilization of phosphorus,which is in line with the sustainable development concept of waste management.In this paper,microwave was used as a heat source to produce the phosphorus-enriched biochar.On the basis of exploring the effects of microwave pyrolysis conditions and additives on the characteristics of phosphorus-enriched biochar,the influence of individual and interaction effects of different additives on the migration and transformation of phosphorus were deeply studied.In addition,the leaching characteristics and occurrence forms of heavy metals in sludge and phosphorus-enriched biochar were investigated,finally,the safety of phosphorus-enriched biochar was evaluated using OPTI and RAC risk assessment methods.The results showed that the pH,BET and TP contents of phosphorus-enriched biochar increased with the increase of temperature or power.The aromaticity and hydrophobicity of phosphorus-enriched biochar were gradually enhanced,and their properties were more stable.It is found that temperature is an important factor affecting the conversion of amorphous carbon to graphene like structure.After microwave pyrolysis,the structure of phosphorus-enriched biochar is more rough and porous,which is more conducive to the immobilization of heavy metal ions.Extending residence time could promote the transformation of NAIP to AP to a certain extent.In addition,the addition of CaO can effectively promote the transformation of NAIP to AP（mainly Ca9Al（PO4）7,Ca3（PO4）2,Ca3Mg3（PO4）4,Ca10（PO4）6（OH）2）.The addition of MgCl2 promoted the conversion of AP to NAIP（mainly AlPO4）.When CaO and MgCl2 were added at the same time,they showed antagonistic effect,and CaO played a leading role.When the temperature was 500-600℃ and the residence time was 150 min,the phosphorus recovery effect was relatively better.The ratio of 9%CaO or 6%CaO+6%MgCl2 is enough to convert the inherent phosphorus of sludge into AP.After microwave pyrolysis,the stability of As,Co,Cr,Cd,Cu,Ni,Mn,Pb and Zn was significantly enhanced,and the potential environmental risk was small.In conclusion,microwave pyrolysis is an application technology with advantages and development prospects to convert municipal sludge into phosphorus-enriched biochar.