Effects Of Biochars On The Biodegradation Of Polycyclic Aromatic Hydrocarbons In Coking-site

Coking industry is an indispensable pillar industry for regional economic development in China.However,the coking industry is also an important source of pollution.Long-term coking process discharged various pollutants that adhere to particulate matter and eventually fall to the ground,making the soil in coking site and its surrounding area subject to long-term serious pollution.Polycyclic aromatic hydrocarbons(PAHs)are the most serious organic pollution in coking sites.PAHs are persistent organic pollutants with teratogenic,carcinogenic and mutagenic effects.Therefore,PAHs in the soil from the coking site pose a potential risk to human health and ecological environment.Therefore,it is in urgent need of effective remediation and treatment.However,the existing remediation methods have limitations,such as the high cost of physical methods,the risk of secondary pollution caused by chemical methods,and the slow effectiveness of biological methods,which restrict their application.In this study,biochars were produced from agricultural waste(walnut shell,corn straw,corn cob and rice straw)at 250 ?,400 ? and 600 ? under anoxic condition,which were added to the contaminated soil at the application rate of2.5% to improve soil properties and stimulate microbial growth.Laboratory simulation experiment was carried out to examine the effects of biochars on the biodegradation of PAHs in the soil from heap coking(low concentrations of PAHs)and in the soil from coke factory(high concentration of PAHs)to explore the mechanism on the effects of biochars on the biodegradation of PAHs in soil contaminated by coking process.The major aim of this study was to find economic,effective and environmentally friendly technical methods for the remediation PAHs in the soil from coking site.The results show that:(1)The characterization results of biochars showed that the physical and chemical properties of biochars were related to the feedstocks and pyrolysis temperatures.In terms of feedstocks,the ash content of biochar from rice straw was significantly higher than that from other feedstocks.With the increase inpyrolysis temperatures,the aliphatic carbon content in biochars decreased,whereas the aromatic cabon content increased.Meanwhile,the ash contents and surface areas of the biochars increased with the increase in pyrolysis temperature.(2)For the relative low concentration of PAHs in indigenous coking site(less than 0.5 mg/kg),the microbial quantities(bacteria and fungi)were strongly depend upon the pyrolysis temperatures.Microbial quantities had significantly positive correlations with the aliphatic carbon in the biochars(p < 0.01),but negative correlations with the aromatic carbon in the biochars(p < 0.01).These results indicate that the increase in recalcitrant aromatic carbon in biochars that occurs with the increase in pyrolysis temperature is unfavorable for the microbial growth in the soil studied.The biodegradation rates of PAHs were obviously associated with specific bacteria and fungi rather than the microbial quantities.Specifically,biodegradation of low-molecular-weight PAHs such as naphthalene and phenanthrene was related to Firmicutes(bacteria),while biodegradation of high-molecular-weight PAHs such as pyrene and chrysene was primarily dominated by norank?k?Fungi(fungi)in the soil studied.Generally,the addition of biochars to the soils increased the biodegradation percentages of Chr by 2.50%?45.00% compared to those of the control.Additionally,both the benzene ring number and angular pattern of the ring linkage also affect the biodegradation rates of PAHs in biochar-treated soil.(3)For the relative high concentration of PAHs in coking plant(up to 200mg/kg),the biochars primarily influenced the bacterial community at the genus level,while they strongly affected the fungal community not only at the genus level but also at the phylum level.Principal component analysis(PCA)indicated that the inorganic minerals in biochars played a key role in stimulating microbial abundance(bacteria)and diversity(bacteria and fungi)in soil from the coking plant.Significantly negative correlations between the residual concentrations of PAHs in biochar-treated soils and the ash contents in biochars(p < 0.05)were observed.These findings further suggested a positive effect of the inorganicminerals of biochars on stimulating changes in microorganisms,which could contribute to the biodegradation of PAHs in soil from the coking plant.Generally,the addition of biochars to the soils increased the biodegradation percentages of individual PAHs by 3.0%?60.3% compared to those of the control.Among these biochars,rice straw-derived biochar pyrolyzed at 600 ?(RS6)effectively degraded PAHs from soil,with increasing biodegradation percentages of individual PAHs from 40.00%?58.84%.This result was likely attributed to the substantial appearance of some specific Acidobacteria-related genera in RS6-treated soil,which contributed to the effective biodegradation of PAHs.Accordingly,biochars pyrolyzed at a relatively high heating temperature with rice straw feedstock containing relatively high mineral nutrients are recommended for application to heavily PAH-contaminated soils.