Study On Removal Efficiency Of PAHs By Biochar Filler In Bioretention

Polycyclic aromatic hydrocarbons（PAHs）refer to organic compounds with two or more benzene rings in their molecular structure.They are an important component of urban surface runoff pollutants.Some PAHs have carcinogenic,teratogenic and mutagenic effects.PAHs are a class of persistent organic compounds with stable structure,low water solubility and difficult to degrade.Once PAHs enter the water body,they will cause harm to aquatic organisms.The US Environmental Protection Agency（US EPA）has identified 16 PAHs as priority pollutants for control.Bioretention refers to a facility for storing and purifying runoff and rainwater through plant,soil and microbial systems in low-lying areas.The soil filler layer is the main place for purifying pollutants,and other materials besides conventional fillers are usually used to improve the pollutant removal effect.Biochar has good removal efficiency for various pollutants because of its rich pore structure,large specific surface area and various oxygen-containing active groups.In this paper,the adsorption effect of sorghum straw biochar and rice husk biochar on naphthalene in aqueous solution was firstly studied.Through the study of the structure,physical and chemical properties of biochar,as well as the adsorption kinetics and adsorption isotherm of naphthalene,sorghum straw biochar was selected as the filler of bioretention for the improvement of the device.Then,the improved bioretention device was used to remove ten kinds of polycyclic aromatic hydrocarbons in artificial simulated rainfall runoff,and the degradation effect of polycyclic aromatic hydrocarbons adsorbed in the filler and the dominant degradation microorganisms in the degradation process were studied.The above study provides a new idea for the utilization of biochar,and provides some references for the engineering practice of the improvement of the packing layer of the bioretention.The results show that:1.The two kinds of biochar burned at 500℃have the characteristics of large specific surface area and rich pore structure,and the characteristics of sorghum straw biochar are better than rice husk biochar.The naphthalene adsorption capacity of rice husk biochar and sorghum straw biochar increased rapidly within 4h,which were 3.78mg·g^-1 and 3.82mg·g^-1respectively.The adsorption capacity increased slowly from 4 to 8h.After 8h,the adsorption capacity of the two biochar to naphthalene gradually reached a stable state,and the final adsorption capacity was 4.50 mg·g^-1 and 4.61 mg·g^-1,respectively.The kinetic model fitting results showed that the adsorption of naphthalene on the two biochar conformed to the pseudo second-order model,indicating that it was mainly chemical adsorption.2.The fitting results of adsorption isotherm model showed that the adsorption of sorghum straw biochar was more in line with Langmuir model,and the adsorption of rice husk biochar was more in line with Freundlich model.Combining with the pseudo second-order kinetic model,sorghum straw biochar is single molecular layer chemical adsorption and rice husk biochar is multi molecular layer chemical adsorption.According to the comparison of n value in Freundlich model,sorghum straw biochar is more conducive to adsorb naphthalene in aqueous solution.3.At different rainfall intervals,the adsorption and removal effect of three devices improved by sorghum straw biochar filler for naphthalene is the worst,and the average removal rate is 57.85%～65.93%.The adsorption and removal of three-ring PAHs have achieved good results,and the average removal rate ranges from 94.41%to 99.03%.The average removal rate of four-ring PAHs ranged from 82.90%to 98.75%.The removal effect of PAHs in the packing in the device 2 with 5%biochar addition after 45 days of degradation is as follows:the removal rate of naphthalene is 85.33%,the average removal rate of three-ring PAHs was 74.11%,the average removal rate of four-ring PAHs was 36.25%.It can be seen that the microorganisms in the filler have the best degradation effect on two-ring polycyclic aromatic hydrocarbons,followed by three-ring and four-ring is worst.4.With the extension of sampling time,both devices 1 and 2 have the trend of increasing species in varying degrees.The sampling detection results at the final time point show that the growth rates of OTUs of devices 1 and 2 are 19.30%and 39.89%,respectively.Increasing the amount of biochar can help to increase the number of species during microbial degradation of pollutants.The change trend of species number reflected by ACE and Chao indexes in alpha diversity is basically the same as that of OTUs.With the extension of sampling time,the number of species has a significant increase trend.In the beta diversity analysis,at the sampling time points of 0d,22d and 45d,the microbial community composition similarity between the samples of device 1 and 2 is very high.5.In the process of microbial degradation of polycyclic aromatic hydrocarbons,the dominant phylum are Proteobacteria,Bacteroidetes,Acidobacteria,Firmicutes,Actinobacteria,Gemmatimonadetes,Armatimonadetes,Planctomycetes,Verrucomicrobia and Chloroflexi.At the genus level,they are Sphingomonas,Bacillus,Aridibacter,Hydrogenophaga,Flavobacterium,Gemmatimonas,Bdellovibrio,Rhodococcus,Acidovoraxand Janthinobacterium.