Effect Of Soil Aggregate Structure On Aging And Bioavailability Of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons?PAHs?is a class of carcinogenic,teratogenic and mutagenic hydrophobic organic pollutants?HOCs?,of which is 16 US EPA priority controlled.Soil is the main land storage of PAHs.And PAHs can endanger human health through the food chain with the fat-soluble properties.How to governance PAHs-contaminated soil has became a hot issue in the field of science.From the respect of soil aggregates,the aging behavior of pyrene in different size aggregates was simulated in soils that content different amount of organic matter.Meanwhile,PyB-10?Agromyces sp.?,a highly efficient pyrene degrading bacteria screened in our laboratory,was used to study the bioavailability of PAHs in different size aggregates?coarse sand,fine sand,coarse silt,fine silt and clay?of spiked and yield-contaminated soils.The physical and chemical properties of soil aggregates,such as organic carbon content,structural characteristics of organic matter,specific surface area and pore distribution,were analyzed by means of Fourier transform infrared spectroscopy,multi-station extended automatic rapid specific surface and porosity analyzer and elemental analyzer.The aging phenomena and bioavailability of PAHs in different aggregates were explored.The difference between them and the main affecting factors were researched.The results are as follows:?1?Pyrene was added to red soil?1.54%OC?and tidal soil?4.35%OC?with different organic carbon content and their aggregates for 240 days.The changes of pyrene extractability with aging time were studied.The results showed that pyrene was obviously aged in the original soil and its aggregates of red soil and tidal soil with pyrene entering into the soil.With the aging time prolonging,the extractable amount of pyrene decreased gradually.For red soils with low organic carbon content,the amount of pyrene extracted from the bulk soil decreased fastest,followed by fine sand.After 240 days of aging,the extractable amount of pyrene in clay and fine silt treated with NaN₃-added was36.4%and 67.6%of the original amount,respectively.The reduction of extractable pyrene was significantly lower than that of other aggregates?>97%?.The extractable amount of pyrene in bulk soil and aggregates was positively correlated with their average pore size,total pore volume,micropore volume,mesoporous volume and PF_{6 nm}?pore accumulation volume<6 nm in soil?as well as normalized PAHs content(PAH_oc),PAH_ssa content with normalized specific surface area(PAH_mv)and PAH_mv content with normalized micropore volume?p<0.01?.The dynamic changes of pyrene extractability in the bulk soil and aggregates can be well fitted by the first-order kinetic model for the tidal soil with high organic carbon content.The decreasing rate of pyrene extractability?k value?from large to small is in the order of bulk soil>fine sand>coarse silt>coarse sand>fine silt>clay.K values were negatively correlated with average pore size,total pore volume,micropore volume,mesoporous volume,PF_{6 nm} and PAH_oc?p<0.05?.After240 days of aging,the extractable amount of pyrene in soil and aggregates was positively correlated with average pore size,total pore volume and PF_{6 nm}?p<0.05?,and positively correlated with PAH_ssa?p<0.01?.?2?Pyrene was added to red soils?1.54%OC?and tidal soils?4.35%OC?with different organic carbon content and their aggregates.The degradation of pyrene by PyB-10 with different aging time was studied.The results showed that there was no significant difference in the microbial degradation of pyrene between the original soil and the aggregates within 30 days of pyrene aging in red soil,and the degradation rate of pyrene was more than 95%.With the aging time prolonging,the degradation rate of pyrene in each component showed significant difference.The degradation rate of pyrene in clay and fine powder did not change significantly with aging time,while the degradation rate of pyrene in original soil and other aggregates decreased range from98.4%⁹9.0%to 11.1%³7.3%.The degradation rate of pyrene in the aggregates of the original soil decreased with the aging time,and the degradation rate decreased range from85.8%⁹6.5%to 19.4%³5.5%.The microbial degradation rates of pyrene in the two soils and their different aggregates have different trends with aging time,which may be related to the comprehensive effects of organic matter content and structural characteristics,aggregate structure and pore size composition.?3?The degradation experiments of PAHs contaminated soils?cinnamon soil 2.39%OC and paddy soil 1.55%OC?and aggregates with PyB-10 were carried out for 30 days.The results showed that the PAHs content in the original soils and aggregates of cinnamon soil and paddy soil decreased significantly.For cinnamon soil,the degradation rate of 2-3 ring PAHs in original soil and aggregates was high,ranging from 32.3%to61.43%.The total degradation rate of PAHs was clay?72.2%?>coarse sand?52.7%?>fine silt?39.4%?>bulk soil?38.6%?>coarse silt?25.6%?>fine sand?20.5%?.The total degradation rate of PAHs in original soil and aggregates was positively correlated with PAH_mv and PAH_ssa?p<0.05?.For paddy soil,pyrene degradation rate in original soil was the highest,34.4%.Except fine sand and coarse silt,the total amount of PAHs in other components decreased significantly?p<0.05?.The degradation rate of PAHs was coarse sand?47.7%?>clay?38.3%?>bulk soil?21.8%?>fine silt?19.5%?>coarse silt?14.46%?>fine sand?6.4%?.There was no significant correlation between the degradation rate of PAHs and soil physical and chemical properties.