Adsorption And Desorption Of Di-n-butyl Phthalate On Different Size-particles Of Paddy Soil

In recent years,phthalic acid esters(PAEs)have been widely detected in agricultural soil owing to the widespread use of plastic products,especially the agricultural plastic film(including plastic greenhouses).Di-n-butyl phthalate(DBP),as one of the typical PAEs,has been widely detected in the environment,and identified as priority pollutants by US Environmental Protection Agency(USEPA).Adsorption and desorption are essential processed to affect the migration,degradation,bioavailability of pollutants in the soil.It is of great significance to investigate the DBP adsorption and desorption in the soil,especially on different size-particle fractions of soil.In this paper,adsorption and desorption characters and mechanisms of DBP in paddy soil and its six different particle-size fractions(>180μm,61~180μm,30~61μm,2~30μm,<2μm and HA)were investigated using batch adsorption and desorption experiments with the help of high performance liquid chromatography,Surface Area Analyzer,X-Ray Diffraction,Fourier Transform Infrared Spectroscopy and Scanning electron microscope.Besides,the biochar derived from rice straw was used to explore the influence for DBP sorption on six different particle-size fractions of paddy soil.The results will provide essential data for the environmental pollution control and assessment of ecological risk of DBP.The main results are as follows:(1)The adsorption of DBP in the paddy soil reached equilibrium within 48 h.The adsorption kinetic of DBP followed the pseudo-second-order kinetic model with R~2>0.999,and it was controlled by the boundary layer diffusion and intra-particle diffusion.The DBP adsorption isotherms followed the Freundlich model well with R~2>0.975 under different temperatures(15,25,35℃).The sorption was a spontaneous,endothermic,and physical sorption process,and the adsorption increased when temperature rises.Soil organic matters acted as predominant role of DBP adsorption process.Besides,the DBP desorption isotherm also fitted well the Freundlich model with R~2>0.870 at 25℃.Obvious desorption hysteresis was observed for the paddy soil.(2)The adsorption of DBP on six different particle-size fractions of paddy soil reached equilibrium within 1~7 d.Similarly,the sorption kinetics followed the pseudo-second-order kinetic models,and they were also controlled by the boundary layer diffusion and intra-particle diffusion.The DBP sorption isotherms of different particle-size fractions followed the Freundlich models well,and it was a spontaneous,endothermic,and physical sorption process.All particle-size fractions of paddy soil were mainly controlled by physical adsorption,but the other size paddy soils may include chemical mechanisms(except>180μm and humic acid).The DBP adsorption capacity increases with the decrease of the particle size,showing that DBP is easier to be adsorbed on clay and small particle size composition.The sorption capacity of different particle-size fractions have positive correlation with soil organic matter,cation exchange capacity,specific surface area and pore volume,indicating that soil physicochemical properties could significantly affect the adsorption of DBP.(3)The biochar derived from rice straw contained high carbon content and great specific surface area and abundant functional structures.After adding the biochar(1.0%,5.0%),the adsorption capacity of DBP for all particle size compositions of paddy soil increased significantly compared to the one without addition of biochar.Langmuir and Freundlich equations fitted well the data of DBP sorption with the latter fitting better.