Sorption Behavior And Mechanisms Of Perfluorooctanoic Acid And Perfluorooctane Sulfonate On Different Size-particles Of Paddy Soil

Perfluorinated compounds（PFCs）,a highly toxic and non-degradable organic contaminant,have been widely detected in the environment.Perfluorooctanoic acid（PFOA）and Perfluorooctane sulphonate（PFOS）are representative PFCs.However,there are few reports about their sorption behaviors and mechanisms in soils,especially soil different fractions.In this study,different particle size fractions of paddy soil were prepared by wet sieve-centrifugal-precipitation method.The sorption behaviors of PFOA and PFOS in the prepared soil fractions were studied using batch experiments;the sorption mechanisms were examined by analyses of BET specific surface area,fourier transform infrared spectroscopy（FTIR）,etc.The findings of this study would be helpful to ecological risk assessment and soil pollution control of PFASs.Main results obtained in this study are as follows.（1）Six particle size fractions of paddy soil were prepared,i.e.,coarse sand（>180μm,F1）,fine sand（61¹80μm,F2）,coarse powder（30⁶1μm,F3）,fine powder（2³0μm,F4）,clay（<2μm,F5）,and humic acid（HA,F6）.The quality percentage of each fraction decreased with the decreasing particle size,with F1 and F6 fractions being the highest（31.9%）and the lowest（1.8%）proportions,respectively.The organic matter content and cation exchange capacity of the soil fractions ranged from 9.1 to 40.5 g/kg and 59.0 to 391.4 cmol/kg,respectively,with the highest content of F6 fraction.（2）Sorption of PFOA in all the soil fractions reached equilibrium within 12 h,and the sorption kinetics conformed to the pseudo-second order kinetic equation（R²>0.998）which involved both intraparticle diffusion and liquid film diffusion,with the former being the rate-limiting step.PFOA isotherms of the soil fractions conformed to both linear equation（R²>0.964）and Freundlich equation（R²>0.935）,with the former better.The sorption process is a spontaneous endothermic physical sorption process with a relatively lowΔG⁰ andΔH⁰ value,and the elevated temperature is favorable for the sorption.Organic matter content,cation exchange capacity,pore volume,and specific surface area were positively correlated with the PFOA soil-water partition coefficients（K_d values）,and correspondingly the sorption capacity decreased as F6>smaller fractions>bigger fractions.However,the fraction contributions to the overall PFOA sorption were generally opposite to the sorption capacity trend due to the impacts of their weight distributions in the original soil.According to the partition coefficient of organic carbon(logK_oc)of PFOA（2.77³.21）,PFOA was difficult to be adsorbed in the soil fractions.The main mechanism underlying the PFOA sorptionare hydrophobic force,divalent cation bridging effect,and hydrogen bonding.（3）Sorption of PFOS in all the soil fractions reached equilibrium within 8 h,and the sorption kinetics also conforms to the pseudo-second order kinetic equation（R²>0.999）,which involved intraparticle diffusion and liquid film diffusion processes,with the former being the rate-limiting step.PFOS sorption isotherms are in accordance with both Freundlich equation（R²>0.954）and Langmuir equation（R²>0.916）at different temperatures（15,25,35°C）,with the former better.The sorption process is a spontaneous endothermic physical sorption process,and the increasing temperature is favorable to the sorption.There are significantly positive correlations between PFOS sorption capacity and contents of organic matter content and cation exchange capacity,as well as pore volume and specific surface area.Thus,the sorption capacity showed the pattern F6>smaller fractions>bigger fractions.However,the fraction contributions to the overall PFOS sorption were opposite pattern,owing to the impacts of their weight distributions in the bulk soil.The main mechanisms causing the PFOS sorption are hydrophobic interactions,divalent cation bridging effect,and hydrogen bonding.These results were similar with those of PFOA.However,there were much higher partition coefficient of organic carbon(logK_oc)of PFOS（3.76⁴.34）than that of PFOA（2.77³.21）,indicating that PFOS was adsorbed much easier than PFOA in the soil fractions.This was associated with higher hydrophobicity of PFOS due to its longer C-F chains than PFOA.