PAHs And Environmentally Persistent Free Radicals In Biochars And Biochar-amended Soils

Polycyclic aromatic hydrocarbons（PAHs）and Environmentally persistent free radicals（EPFRs）are typical pollutants with mutagenic,carcinogenic,and teratogenic properties.They have frequently been found in contaminated water,air,and soil,posing serious threats to human health and ecological safety.Use of biochar in agricultural production has attracted attention of the public due to numerous merits.However,during biochar production PAHs are generated,and crops have the ability to absorb the PAHs and accumulate them in their leaves and tissues.Also,EPFRs in biochars induce ROS which have been associated with inhibition of germination in seeds and human health risks.Therefore,the use of PAHs and EPFRs rich biochar in agricultural production can pose negative impact on plant growth and human health risks?there is a lack of understanding at home and abroad.This may pose human health deficit to those consuming them.In this dissertation the underlying factors of EPFRs formation,their fate,risks and regulatory drawbacks were reviewed.A literature data study was carried out to assess the risks in food crops grown in biochar amended soils.Additionally,a comparative study was carried out to determine the influence of corn straw,straw-based biochar,and straw-based compost on PAH accumulation in soils and three vegetable species Chinese cabbage（Brassica chinensis L.）,Pakchoi（Brassica campestris L.）and Malabar spinach（Spinacia oleracea L.）.The main results were as follows:（1）Clarified the formation,fate and environmental risks of EPFRs in biochar.EPFRs are produced by the homogeneous cleavage of C-C and C-O bonds in α-and β-alkyl aryl ethers during the carbonization of biomass.Pyrolysis temperature and raw material characteristics are the key factors influencing the formation of EPFRs in biochar.Raw materials rich in lignin fibers form more EPFRs at low temperature pyrolysis.Biochar and the gas produced during the pyrolysis of biochar also contain a certain amount of EPFRs,which have a certain negative impact on the external environment.Therefore,in order to protect the ecological environment and improve the quality of biochar,the International Biochar Association has formulated relevant laws,regulations and policies to reduce the content of EPFRs in biochar.In addition,in the preparation process of biochar,the development of advanced preparation technology to control the production of EPFRs is expected to improve the quality of biochar and reduce the production of EPFRs in biochar and its environmental risks.（2）Based on the literature data,the crop contamination risk of PAHs in biochar after returning to the field was revealed.Based on the previously published literature on biochar PAHs pollution（26）,after data collection and analysis,it was found that the PAHs content in the collected biochar ranged from 12-355295 μg/kg,mainly 2-3 ring PAHs（40%-71%）.Furthermore,after the biochar rich in PAHs was returned to the field,the migration coefficient of PAHs from biochar to soil and the enrichment coefficient of crops to PAHs were studied.It was found that PAHs in biochar after returning to the field can lead to an increase in PAHs content in soil and crops;the average lifetime cancer risk index（ILCR）of adult males in soil and crop PAHs pollution caused by biochar application is 2.0 × 10-6-1.85×10-5.（3）The effects of straw biochar,straw and straw-based composting on soil and crop PAHs pollution were compared.It was found that the PAHs content in Shanghai Qing（Brassica chinensis L.）,Pakchoi（Brassica rapa L.）and spinach（Spinacia oleracea L.）growing in the soil with straw and straw piles was very low（1071-6316 μg/kg）,May be mainly derived from the soil background.However,the PAHs content of vegetables grown in the soil with straw biochar applied was higher（1315-6581 μg/kg）,and as the amount of biochar application increased,the accumulation of PAHs in soil increased and the PAHs content of vegetables increased.Straw biochar,straw,and straw-based compost can all increase vegetable biomass,but the effect of different application rates（2%,5%,10%）on vegetable yield is not significant.These results show that,similar to straw or straw composting,straw biochar can also increase vegetable yield,but PAHs in biochar have potential pollution risks to soil and crops.