Formation Of Polycyclic Aromatic Hydrocarbons In The Pyrolysis Process Of Iron Impregnated Biomass And Environmental Risks

Biochar,the carbonaceous material produced during the pyrolysis process of biomass can contribute positively to carbon sequenstration,soil improvement and contamination remediation.But in some cases,the actual effect of biochar application is not satisfactory due to the limitation of its physicochemical property.Recently,much attention has been reseived in the context of engineered biochar,which is defined as a biochar-base composite with improved physicochemical and sorptive properties.Iron impregnated biochar could greatly increase As³⁺and As⁵⁺sorption capacities.However,biochars are prodeced from the pyrolysis of biomass and can contain considerable amounts of coproduced polycyclic aromatic hydrocarbons（PAHs）.The modification process by iron impregnation would affect the formation of PAHs during the pyrolysis.In this study,barley grass impregnated with Fe（NO₃）₃ and FeCl₃ was pyrolyzed in the temperature range of 350-650^oC to produce Fe-biochar composite.TG-MS was employed to explore the influence of Fe impregnation on PAHs formation through the dynamic changes of noncondensable gases in the pyrolysis process.The potential environmental risks of PAHs was evaluated by toxic equivalent quantity（TEQ）.The main results are as follows:（1）The total amounts of PAHs produced from the pyrolysis of barley grass were increased with temperature increase,from 19.6 mg·kg^-1（350^oC）to 88.3 mg·kg^-1（650^oC）.And the PAHs were mainly concentrated in the liquid tars,accounting for more than 85%of the total PAHs.Naphthalene,fluorine,and phenanthrene were the main compounds of PAHs,resulting in the dominance of 2-3 rings PAHs（>75%）.（2）The total amounts of PAHs produced from the pyrolysis of Fe（NO₃）₃ impregnated barley grass were increased with temperature increase,from 31.6 mg·kg^-1（350^oC）to 64.3 mg·kg^-1（650^oC）.And the produced PAHs were mainly concentrated in the liquid tars,accounting for more than 90%of the total PAHs.Naphthalene,fluorine,and phenanthrene were the main compounds of PAHs,resulting in the dominance of 2-3rings PAHs（>75%）.At 350^oC,the formation of PAHs,especially naphthalene was enhanced with Fe（NO₃）₃ impregnation.Total amount of PAHs was increased from 22.1 mg·kg^-1 to 31.6 mg·kg^-1.The main reason might be iron catalyzed the cracking of polycyclic aromatic clusters that are linked via-CH2-,-C-C-,and-C-O-C-groups.The cleavage of such molecular bridges in biochar structure would lead to the release of polycyclic aromatic compounds,especially smaller PAHs at low temperature from the solid biochar into the gas phase.At 500 and 650^oC,total amounts of PAHs were decreased with Fe（NO₃）₃ impregnation,from60.6 and 97.9 mg·kg^-1 to 54.2 and 64.3 mg·kg^-1.And more obvious reduction（about 34%）in the concentration of PAHs was observed at650^oC,which was due to the existence of Fe⁰ within biochar since Fe⁰ had higher catalytic activity on PAHs decomposition.（3）The total amounts of PAHs produced from the pyrolysis of FeCl₃impregnated barley grass were increased with temperature increase,from8.50 mg·kg^-1（350^oC）to 64.2 mg·kg^-1（650^oC）.And the produced PAHs were mainly concentrated in the liquid tars,accounting for more than 90%of the total PAHs.Naphthalene,fluorine,and phenanthrene were the main compounds of PAHs,resulting in the dominance of 2-3 rings PAHs（>65%）.At 350^oC,the concentration of PAHs was reduced by about 70%with FeCl₃ impregnation,from 27.1 mg·kg^-1 to 8.50 mg·kg^-1.Iron and chlorine could catalyze the cracking and reforming of naphthalene on the surface of biochar,which was supported by the significant reduction of naphthalene.At 500 and 650^oC,total amounts of PAHs were both decreased with FeCl₃impregnation,from 88.0 and 106 mg·kg^-1 to 54.4 and 64.2 mg·kg^-1.Besides,more obvious reduction in the concentration of 5-6 rings PAHs was observed at 650^oC,which was due to the existence of Fe⁰ within biochar since Fe⁰ had higher catalytic activity on the decomposition of high molecular weight PAHs.（4）The concentrations of PAHs in Fe-biochar composite produce at350-650^oC were all below the threshold values set by the International Biochar Initiative（IBI）of 6-20 mg·kg^-1.Only biochar produced at 650^oC exceeded the threshold values set by the European Biochar certificate（EBC）of 4 mg·kg^-1 for premium grade biochar.In general,with Fe（NO₃）₃and FeCl₃ impregnation,PAHs concentration and TEQ were both decreased.Furthermore,low pyrolysis temperature condition could be an economical and feasible choice to reduce the potential environmental risks caused by PAHs produced from the pyrolysis of Fe-biochar composite.