| Polycyclic aromatic hydrocarbons(PAHs)are a kind of organic pollutants with strong hydrophobicity in the environment.Because PAHs have carcinogenic,teratogenic and mutagenic effects,and can threaten human health through the food chain,16 PAHs,such as phenanthrene and naphthalene,were listed as priority monitoring pollutants by the U.S.Environmental Protection Agency in 1976.When PAHs enter into the environment,it will accumulate in the soil and groundwater for a long time,in which the soil carries more than 90%of the PAHs load in the environment.Therefore,the characteristics of adsorption and desorption of PAHs by soil directly affect its environmental fate and ecological toxicity.In this paper,three typical soils in Guanzhong Basin were selected to study the adsorption and desorption dynamics and thermodynamics characteristics of PAH phenanthrene in loess,silts and silty sand.The influence of different factors on the adsorption and desorption process was discussed.The optimal adsorption conditions and theoretical maximum adsorption rate were determined by response surface method,so as to provide environmental chemical behavior of PAHs in soil Theoretical basis.The main research results are as follows:(1)The adsorption process of loess,silts and silty sand to phenanthrene is characterized by "rapid adsorption at the initial stage and gradual balance at the later stage".The adsorption capacity of the three kinds of soils is loess>silts>silty sand.The order of the fitting of the dynamic equation is quasi second order dynamic equation>quasi first order dynamic equation>Elovich equation.Under the same temperature,the order of adsorption capacity of three soils to phenanthrene is the highest in loess,the lowest in silty sand,and the adsorption process is exothermic reaction.The increase of temperature is not conducive to the adsorption reaction.At the same temperature,the blocking ability of loess to phenanthrene is the highest.(2)The desorption and adsorption process of phenanthrene in three soils are the same,which are divided into two stages:fast desorption and slow desorption.The desorption capacity is loess>silts>silty sand.The fitting order of the three dynamic equations is quasi second order dynamic equation>Elovich equation>quasi first order dynamic equation.In the study of desorption thermodynamics,the desorption capacity and desorption capacity of the three soils are the highest in Loess and the lowest in silty sand,and the temperature rise is conducive to the desorption reaction.Langmuir equation fitting is more significant.The calculation of desorption lag coefficient shows that the desorption lag coefficient of loess is the highest,followed by silt and silt.The desorption lag coefficient of the same soil increased with the increase of temperature.(3)Combined with the single factor experiment in the process of adsorption and desorption,the initial phenanthrene concentration,solution pH,temperature and soil organic matter are selected as the influencing factors,and the adsorption rate is the response value.The BBD method of response surface is used to design the four factor and three level experiment.Through the analysis of the model results,the best influencing conditions of phenanthrene adsorption on loess are pH=10,the initial phenanthrene concentration is 30mg/L,the temperature is 20.14℃,and the soil temperature is 30.14℃ The code value of organic matter content is 1,the best theoretical adsorption rate is 98.89%;the best influence condition of adsorption of phenanthrene by silts is pH=10,the initial phenanthrene concentration is 30mg/L,the temperature is 21.15℃,the code value of soil organic matter content is 1,the best theoretical adsorption rate is 96.59%;the best influence condition of adsorption of phenanthrene by silty sand is pH=2,the initial phenanthrene concentration is 29.96mg/L,the temperature is 40℃,the coding value of soil organic matter content is 1,the best theoretical adsorption rate is 93.37%. |
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