The Properties Of Naphthalene Adsorption On Soil And Its Site-Energy Distribution Analysis

Polycyclic aromatic hydrocarbons(PAHs),which many countries view as the priority pollutants to deal with,are important organic contaminants with the nature of carcinogenic,harmonic and mutagenic.More than 90%of PAHs load in the environment was performed by soil medium and adsorption and desorption of polycyclic aromatic hydrocarbons(PAHs)by soils and sediments plays an important role in determining their transport and fate in the soil environment.The study will have a great significance for realizing the retention,distribution,fate and transportation,bioavailability of PAHs and exploring the rules of transportation and transformation of PAHs in soil environment.Adsorption of naphthalene by six different soils,including surface layer of lou soil,claying layer of lou soil,old soil layer of lou soil,yellow earth,purple soil,limestone soil,was examined using batch adsorption equilibrium methods.The adsorption kinetics and thermodynamics of naphthalene on different soils was studied,and the relationship between soil physicochemical properties and adsorption fitting parameters were analyzed.Further more,the site-energy distribution function for naphthalene adsorption in soils was obtained.The main findings are as follows:(1)All sets of experimental adsorption data were fitted using five different sorption kinetic models:the Elovich equation,the dual constant model,the parabolic diffusion model,the Pseudo-first order model,and the Pseudo-first order model.The kinetics of adsorption prcesses were best described by Pseudo-first order model for all the soils studied other than claying layer of lou soil(25℃).The activation energy of adsorption was calculated using the rate constant k₂ of Pseudo-second order model, and it was found to be 27.50,27.21,132.74 KJ·mol^-1for the old soil layer oflou soil, yellow earth and purple soil,respectively.(2)Henry model,Freundlich model and deBoer-Zwikker model can better fit the adsorption isotherm of naphthalene by soils compared to Langmuir model and BET model.The adsorption isotherm by adsorption and relative balance concentration at 45℃were obviously higher than that at 25℃,these showed that adsorption capacity increased with the temperature increasing.At 25℃condition,naphthalene distribution between aqueous solution and soil organic carbon was the main adsorption mechanism,but the adsorption mechanism became more complicated as the temperature increasing.The adsorption capacity K_F of Freundlich model and the initial adsorption potential of deBoer-Zwikker model had obvious difference at two temperature degree,but the parameter n in Freundlich model was not sensitive to temperature change because it only characterizes the surface property of the soil particles.(3)The evaluated Gibbs free energy change and enthalpy change indicated the spontaneous and endothermic nature of naphthalene adsorption in soils,and the entropy gain in whole adsorption system is the main adsorption driving force.(4)The adsorption energy E^* decreased significantly in the initial stage with increasing the adsorption qe,and then decreased slowly.The site-energy distribution for naphthalene sorption on soils decreased exponentially with adsorption energy E^* increasing over the concentration range.(5)At the experimental concentration the adsorption energy E^* ranged from 4.15～12.25 KJ·mol^-1,8.19～17.95 KJ·mol^-1at 25℃and 45℃respectively.At 25℃condition,the area under the site energy distribution curve of naphthalene sorption on soils other than claying layer of lou soil and old soil layer of lou soil was larger than that at 45℃.The value of claying layer of lou soil was from 32.98μg·g^-1to 35.92μg·g^-1.The adsorption capacity of naphthalene in soils had no difference at two different temperatures(ρ=0.727).The adsorption capacity of naphthalene in soils appeared to positively correlate with soil organic content at 25℃(ρ=0.067),but as the temperature increasing this relationship was not obvious(ρ=0.306).