| Freeze-thaw cycles(FTCs)are a typical climate phenomenon in North China,which could significantly change the physical and chemical properties of soils,thus resulting in the changes of environmental behavior of organic pollutants in soils.Polycyclic Aromatic Hydrocarbons(PAHs)are a kind of semi-volatile organic pollutants that connected by two or more benzene rings and widely existing in the environments.Due to the toxicity,bioaccumulation,carcinogenicity,teratogenicity and mutation,they have been attracted widely attention.The pollution of PAHs pollution in Chinese soils is serious.It is important to understand the behavior of PAHs in soil,which has a great significance to evaluate the potential risks of PAHs to ecosystems.So far,the migration and bioavailability of PAHs under FTCs have not been studied.In this study,effects of FTCs on the migration and bioavailability of PAHs(phenanthrene,pyrene and benzopyrene)in three kinds of typical soils(black soil,brown soil and loess)were studied.The influencing factors of FTCs on soil environmental behavior of PAHs were clarified and preliminarily revealed the migration and bioavailability mechanism of PAHs in soil under FTCs.Analitical method of PAHs in soil has been established and optimized in this study.The method is accurate and reliable,which lays a foundation for the sample analysis.The effects of freeze-thaw frequency(4,8,16 and 32),freezing temperature(-5 ℃,-15 ℃ and-25 ℃)and moisture content(30%,50% and 100%)on the migration and bioavailability of PAHs in three kinds of soils were studied under different FTCs conditions.The results showed that FTCs impact the migration and bioavailability of PAHs by affecting the physical and chemical properties of soils.During FTCs,changes of soil physical and chemical properties in vertical direction vary with soil types.After FTCs,contents of clay colloid increased from the top of columns to the bottom of columns.The content of clay colloids in black soil changed significantly.Similarly,the porosity of other soils increases after FTCs.At 0 ℃,the migration of PAHs in soils have no changes.FTCs enhanced the migration of PAHs in soil columns.The migration depth of PAHs in different soils were significantly different.The migration rate of PAHs was the highest in black soil,because of the clay colloid content is higher in black soil.Among the three PAHs,the mobility of phenanthrene is the highest,while benzopyrene is less affected by FTCs.Higher water contents cause deeper migration of PAHs in soil.The lower freezing temperature leads to the fragmentation of large particles in soils,and more clay colloids are produced.FTCs promote the movement of water,then the water permeates into soil,which increases the content and movement of clay colloid,thus promoting the movement of PAHs.The mobility of PAHs was significantly affected by freezing temperature and moisture content.The migration of PAHs can be promoted by low freezing temperature and higher moisture content.The migration distances of three PAHs in Loess and Brown soil column are less affected by freezing temperature,which may be related to their low clay content.Generally,the clay colloid content of the soils in the middle and bottom of the column increased under the FTCs,and the concentrations of PAHs also showed similar trends,indicating that PAHs would migrate with soil colloids.The effect of FTCs on the bioavailability of PAHs in soils was a complex process and depend on various factors.The biological effectiveness of PAHs in three typical soils was evaluated by Tenax(60-80 mesh)under different freezing and thawing conditions(freezing temperature,moisture content and frequency).The results showed that the FTCs significantly affected the bioavailability of PAHs in soils.The bioavailability of PAHs is affected by the frequency of freezing and thawing,water content,freezing temperature,soil organic matter,soil physical and chemical properties and PAHs properties.Compared with the constant temperature control,FTCs significantly improved the bioavailability of phenanthrene in three soils.The moisture content has different effects on the bioavailability of PAHs in different soils.In black soil and loess,higher soil moisture content is beneficial to improve the biological efficiency of phenanthrene;on the contrary,higher moisture content will reduce the bioavailability of phenanthrene in brown soil.In this study,benzopyrene was not detected,which was related to the higher octanol water distribution coefficient(Log Kow)of benzopyrene,and the bioavailability of phenanthrene and pyrene in three soils was different.The bioavailability of phenanthrene and pyrene in black soil was the lowest,and the bioavailability of phenanthrene was significantly higher than that of pyrene.With increasing freeze-thaw times,the bioavailability of phenanthrene and pyrene increased at first and then decreased.After 8 FTCs,the bioavailability of phenanthrene and pyrene in black soil,loess and brown soil was higher.Compared with the CK,FTCs improved the bioavailability of pyrene in black soil and reduced the bioavailability of pyrene in loess.The bioavailability of phenanthrene and pyrene increased first and then decreased with the increase of freeze-thaw times in three soils.The higher moisture content and lower freezing temperature can improve the bioavailability of phenanthrene and pyrene in three soils.FTCs significantly affected the contents of humus,including HA,FA and HM in three kinds of soils(p < 0.05).The HA/FA(1.28)of black soil was significantly higher than loess(0.85)and brown soil(0.47),indicating that the humification degree of black soil was the highest.In the process of FTCs,changes of humus were different in different soils.In general,with the increase of freezing temperature and moisture content,and HA and FA were transformed to HM,which significantly affected the degree of HA/FA(p< 0.05).The lower freezing temperature,the higher the soil humification degree.The specific surface area of black soil is largest,and the loess is the lowest among the three soils.The average pore volume of loess is higher than that of black soil and brown soil.Generally,the pore volume is decreased after FTCs.The specific surface area of black soil decreases with the increase of freeze thaw frequency,and the decreases of adsorption points may lead to the increase of PAHs bioavailability;for brown soil and loess,the specific surface area increases with the increase of freeze-thaw frequency,which provides more adsorption points,leading to the decrease of biological effectiveness.The frequency of FTCs,moisture content and freezing temperature have significant effects on the specific surface area,pore size and pore volume of soils(p < 0.05),but the changes of specific surface area on three kinds of soil was different.FTCs had a significant effect on soil particle size,and changes with the change of freeze-thaw conditions.The proportion of silt in black soil and loess was the largest in initial conditions,which was significantly higher than that of other components(p < 0.05).The distribution of soil particle size varies in the process of FTCs.In the process of FTCs,the moisture content has a significant effect on the distribution of soil particle size.The soil moisture content,frequency of FTCs and freezing temperature affect the bioavailability of PAHs in the process of FTCs by changing soil pore structure,specific surface area and soil organic matter components(HA,FA,HM,HA/ FA,DOM).During the FTCs,the bioavailability of PAHs can be improved by the decrease of temperature,the change of organic matter components,the change of water phase into ice and the change of soil pore structure.During the high amplitude temperature change of day or season,the thermal stress will drive the soil particles to break.The stress depends on the temperature change,and the stress increases with the increase of temperature.Under the contact stress,the coarse mineral particles are broken into smaller particles,forming more adsorption points,which affect the bioavailability of pollutants.FTCs may reduce the diffusion rate of pollutants on soil organic matters,thus improving the bioavailability.In addition,FTCs may increase the adsorption of chemical molecules on glass organic matter,leading to the decrease of bioavailability and the transition of rubber organic matter(HA,FA)to glass organic matter(HM)caused by FTCs may also lead to the decrease of bioavailability.When the initial values of soil specific surface area and porosity are very large,the bioavailability of PAHs will be significantly affected. |
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