| As an effective technology for soil improvement,carbon emission reduction and pollution remediation,biochar has been a hot area of research.Due to its low nutrient content in biochar,the nutrient that it can directly supply to crop is very limited.Therefore,biochar-based fertilizers that using biochar as carrier of fertilizers have been proposed as an effective way to improving the crop yieldin recent years.However,the pyrolysis production of biochar from farming and forestry wasteinevitably produces polycyclic aromatic hydrocarbons(PAHs)pollution,and the strong absorption of biochar increases the risk of persistent pollution of PAHs in soil-plant systems.In this thesis,biochar-based fertilizers produced from common farming and forestry waste were selected as research objects,and GC-MS detection methods for PAHs in biochar were developedwith the response surface optimization for extraction methods.And on this basis,the formation of PAHs during biochar production were studied.Influencing factors to the formation of PAHs were investigated,especially the impact of different pyrolysis temperatures and raw material on PAHs.Meanwhile,in order to provide scientific basis for practical application of biochar-based fertilizers without PAHs pollution,the impact of biochar in the composting and the degradation of PAHs was investigated,besides,further research was conducted about the migration of PAHs in soil-plant system with biochar-based fertilizer.The mainresults are as follows:1.With the ultrasonic method,the extraction efficiency of 12 different solvents to extract PAHs from biochar-based fertilizerswas investigated.The results showed thatacetone:cyclohexane(1:1,v/v)is the best solvent for 16 kinds of PAHs in biochar-based fertilizers.By comparing different extraction methods for 16 kinds of PAHs in biochar-based fertilizers,we found that the accelerated solvent method is with the best overall average recovery rate of 71.71%,while the ultrasonic method is of 67.47%and the mechanical vibration method is of 58.24%.2.Based on the steepest climbing experiment,the response surface optimizations were conducted for the accelerated solvent method and the ultrasonic extraction method by the designs of the Box-Behnken experiment.Results showed thatwith the accelerated solvent method,the optimal recovery rate for the 16 kinds of PAHs in biochar-based fertilizers is 87.9%when extracting temperature is 78℃for two times of extractionwith 17 min of each duration.It was demonstrated that the LOD of the 16kinds of PAHs is 0.001~0.003 mg/kg,while LOQ is 0.004~0.1 mg/kg,R~2>0.996,the recovery rate is 81.3%~88.2%,and RSDis 4.6%~6.3%.With the ultrasonic extraction method,the optimal recovery rate is 81.69%when extracting temperature was 57℃with 81 min of each duration for two times of extraction.It was demonstrated thatthe LOD of the 16kinds of PAHsis 2.25~23.55μg/kg,while LOQis 7.51~78.49μg/kg,R~2>0.996,the recovery rate is 75.7%~81.2%,and RSDis 3.4%~5.5%.The optimized ultrasonic extractionmethod was used to detect actual samples in market,and it was found that the recovery rate for the 16 kinds of PAHs in three biochar-based fertilizers in market is in the range of 81.1%to 109.4%.3.It was found that the concentration of PAHs in biocharraw materials are187.85,154.51,115.43,103.19 and 72.749μg/kgrespectively in lawn grass,wheat straw,bamboo,fir tree and cow dung.Biochar was produced from the five kinds of farming and forestry wasteunder pyrolysis temperatures between300℃and700℃,and the average results indicated as following:lawn grass>wheat straw>bamboo>fir tree>cow dung,which is the same order as the concentration of PAHs in the fiveraw materials.This indicated that pyrolysis is the major way to form PAHs.With temperature of 500℃,biochar produced from lawn grass,bamboo,fir tree and cow dung have the highest concentration of PAHs,that are2100.02,1617.74,1141.72 and483.49μg/kg.With temperature of 400℃,biochar produced from wheat straw have the highest concentration of PAHs,that is1737.87μg/kg.4.The experiments of composting strengthened by biochardemonstrated thatadding biochar can quickly increase the composting temperature,and can help keep and extend the time of high temperature.The temperature increasing rate and the duration of high temperature both increase with the additive concentration of biochar.Adding biochar can also speed up the increasing rate of pH and EC of composting.The extracting solution from the composting is innocent to lettuce seed,meanwhile it effectively promotes the seed germination and root growth.Also,it can help the degradation of PAHs in the composting.The degradation rate of PAHs risesfirst and then decreases.The degradation degree of PAHs hasa positive correlation with the addition amount of biochar.The degradation of LMW PAHs is mainly during the early-middle stages of composting,while the degradation of HMW PAHs is mainly during the middle and late stages of composting.5.For the polluted soil,it was found that the use of biochar-based organic fertilizer or biochar compost doesnot significantly increase the concentration of PAHs in soil.After using biochar compost,biochar-based organic fertilizer,biochar,and CK respectively in the soil for 40 days,the degradation of PAHs is 35.94%,29.82%,36.82%and 18.56%respectively.It was found that the use of biochar and biochar-based fertilizer can promote the degradation of PAHs in soil.As compared to CK,the concentration of PAHs in vegetables grown with biochar compost and biochar-based organic fertilizer arereduced by 86.98%and 83.03%6.Based on the first-order kinetics equation,a PAHs degradation kinetics model was developed with a good fitting coefficient.The use of biochar and biochar-based fertilizer can both significantly speed up the degradation of PAHs in soil.After using biochar compost,biochar-based organic fertilizer and biochar,the half-life period of PAHs is respectively 59.3,75.4 and 58.1d,with a reduction of 54.49%,42.13%and55.41%than the CK which is of 130.3 d of half-life period.For the polluted soil,after using biochar compost,biochar-based organic fertilizer and biochar,the half-life period of PAHs is respectively 67.5,75.8 and 61.3d,with a reduction of 51.79%,45.86%and 56.21%than the CK which was of 140.0d of half-life period.After using the biochar compost and biochar-based organic fertilizer in polluted soil,the PAHs absorption ability of lettuce is in line with the partition-limited model,witha small difference between the predicted value and the measured value. |
PDF Full Text Request