| The raw materials for the preparation of biochar are mostly waste biomass such as agricultural and forestry residues,livestock and poultry manure,and household waste,which may contain pollutants such as heavy metals,polycyclic aromatic hydrocarbons,pesticides and so on.The preparation process of biochar will concentrate pollutants in raw materials and produce organic pollutants such as organic acids,tar and polycyclic aromatic hydrocarbons.Therefore,the potential ecological and environmental risks of biochar should be considered in the practical application of biochar,or the surface pollutants should be removed by appropriate pretreatment and environmental risk assessment should be carried out on biochar before application.In this study,three kinds of biochar were prepared from three typical waste biomass(rice straw,biogas residue and kitchen waste).By using organic and inorganic eluents,the probability of removing surface pollutants of biochar was explored and its environmental risks were evaluated.Based on this,the toxic and side effects of biochar extract on soil plants and microorganisms during its practical application were explored through seed germination,seedling cultivation test and biochar soil culture test,and its potential environmental risks were comprehensively assessed.The main conclusions are as follows:(1)Shock elution with deionized water has a significant effect on the removal of soluble inorganic salts on the surface of biochar.After elution,the removal of heavy metals on the surface is 11.2%～70.8%,and the removal of mineral elements is1.3%～51.7%.The ratio of solid to liquid during elution is positively correlated with the content of inorganic salts in the eluent.Anhydrous ethanol can remove polycyclic aromatic hydrocarbons(PAHs)from biochar surface,and the total amount of PAHs on the surface of biochar after elution is reduced by 41.0%～73.4%.The total amount of PAHs before elution exceeds the limit of PAHs content in biochar of the International Biochar Association,and the total amount of PAHs after elution reaches the standard.The ultrasonic elution method can effectively save time,energy and water resources.Under the same solid-liquid ratio and temperature,the removal rate of heavy metals by ultrasonic(100 k Hz,1 h)is 1.03～2.69 times that by shock(200r·min-1,24 h).Under the condition of 1:20 solid-liquid ratio,the removal effect of ultrasonic on some heavy metals is even better than that under the condition of 1:50solid-liquid ratio.In addition,the removal rate of total soluble ions on the surface of biochar by ultrasonic method is 1.14～2.03 times of that under the condition of vibration,and the damage degree of pore structure of biochar with fragile carbon skeleton is smaller.(2)The biomass charcoal extract had significant inhibitory effect on radish seed germination.Compared with the control group,the radish germination rate was reduced by 10.26%～56.41%.The inhibitory effect was kitchen waste charcoal(KB)>rice straw charcoal(RB)> pig manure biogas residue charcoal(PB),1:20 solid-liquid ratio >1:50 solid-liquid ratio.Under the condition of 1:20 solid-to-liquid ratio,the survival rate of radish seedlings was less than 12%.Under the condition of 1:50solid-liquid ratio,the seedling apparent indexes(fresh weight,root length,seedling length,etc.)were decreased by 29.55%-51.89%,20.77%-76.04%,37.73%-56.01%compared with the control group,respectively,showing obvious growth inhibition with inhibition effect of KB>RB>PB.Compared with the control group,chlorophyll content of radish seedlings was decreased by 12.00%～24.32%,root activity was decreased by 39.95%～83.57%,and malondialdehyde(MDA)content was increased by 17.49%～59.46%,indicating that the integrity of cell membrane of seedling roots was damaged and lipid peroxidation occurred under the biochar extraction.It showed obvious toxic effect.(3)Compared with raw materials,the carbonization of kitchen waste reduced the soil microbial diversity,the carbonization of biogas residue increased the soil microbial diversity,and the carbonization of rice straw increased the soil microbial diversity.The effects of kitchen carbon on soil microbial community structure mainly came from the high EC values of water-soluble Na,As and kitchen carbon in biochar.Na,As and EC values affected the composition of soil microbial species,and Na changed the relative abundance of dominant bacteria.The effects of biogas residue biochar on soil microbial community structure were related to the water-soluble Cu,Ca,Mg and Zn in biochar.Cu,Ca and Mg affected the soil microbial species composition and the relative abundance of dominant bacteria,while Zn caused the relative abundance changes of some sensitive bacteria and resistant bacteria.The effects of rice straw biochar on soil microbial community structure were related to the water-soluble K and As in biochar,among which As was the main factor,and the relative abundance of other phyla was mainly affected by the species composition.In conclusion,shock elution with deionized water has a significant effect on removing soluble inorganic salts on the surface of biochar,anhydrous ethanol can remove PAHs on the surface of biochar,and ultrasonic elution can effectively save time,energy and water resources.Rice straw biomass carbon,biogas residue biomass carbon and kitchen material carbon had different potential hazards to radish seed germination and seedling growth.The application of biochar to soil after elution can reduce the effect of biochar on soil microbial diversity,and the selection of appropriate elution agent can make biochar play a role in improving soil microbial richness.This study comprehensively verified the feasibility of elution in reducing potential environmental risks of biochar,and provided theoretical basis for screening and safe utilization of typical waste biomass carbonization for agricultural application potential,technical support for safe application of biochar,and theoretical guidance for the wide application of pyrolytic biochar technology. |
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