Effect Of Low-molecular-weight Organic Acids On The Transport Of Polystyrene Nanoplastics And Pb In Saturated Porous Media

Nanoplastics（NPs）have attracted widespread attention as a contaminant with high migration capacity.So far,more studies have focused on the deposition and transportation behavior of NPs,while the large specific surface area of NPs can produce strong adsorption properties for heavy metals,which in turn influence the transportation and transformation of Pb in natural environments,resulting in greater environmental risks.Low-molecular weight organic acids（LMWOAs）as the soluble organic substances are common in natural environment and it is important in the transport and transformation of soil nutrients and other life activities,which can influence the physical and chemical properties of soils.However,there is relatively a little research on the effects of LMWOAs on the transportation behavior of nanoparticles.Therefore,in this study,Polystyrene Nanoplastics（PSNPs）which has a diameter of 400 nm and the heavy metal Pb were used as the objects of study.To effects of three LMWOAs,acetic acid,malic acid and citric acid,different p H and IS on the adsorption characteristics of PSNPs on Pb were investigated.The transportation of PSNPs and Pb in saturated porous media with different physical and chemical conditions was studied.Lastly,the effects of different LMWOAs on the co-transport of PSNPs and Pb in porous media were studied.This study is important for understanding the migration pattern and mechanism of small organic acids to nanoparticles and heavy metals in soil-water environment,and evaluating the environmental risk of compound pollutants in soil.The main results of this study are as follows:（1）LMWOAs have significant effect on the transportation of PSNPs in saturated porous media.Low concentration(0.0001 mol L^-1)of LMWOAs boosted the transportation of PSNPs,while the LMWOAs(0.001 and 0.01 mol L^-1)obstructed the transportation of PSNPs in quartz sand column at p H=7 and IS=0.001 mol L^-1.The deposition of PSNPs in the medium was enhanced.In the range of 0.001-0.05 mol L^-1IS and 5-9 p H,with the increase of IS or the decrease of p H,the mobility of PSNPs also increased,and the effect of organic acids was more significant.In addition,the effects of LMWOAs on the transportation of PSNPs under different physicochemical conditions followed:acetic acid<malic acid<citric acid.This result is due to the LMWOAs have different relative molecular mass and different amounts of functional groups.（2）The adsorption types of Pb by PSNPs are mainly monolayer physical adsorption.Isothermal kinetic adsorption experiment was used to analyze the adsorption characteristics of PSNPs on Pb,and the result of kinetic adsorption experiment fitted the pseudo-second-order kinetic model.We can come to a conclusion that the maximum adsorption capacity of PSNPs on Pb was 148.35 mg kg-1 by the isothermal adsorption experiment.Langmuir model was better than Freundlich model to fit the adsorption process.Thermodynamic adsorption experiments further prove that the adsorption process of Pb by PSNPs is non-spontaneous（ΔG>0）and exothermic（ΔH<0）,which belongs to physical adsorption.The addition of LMWOAs inhibited the adsorption of Pb by PSNPs,and the inhibition effect became stronger with higher LMWOAs concentration.In addition,the adsorption of Pb by PSNPs decreased with increasing IS.（3）LMWOAs produce different effects on both when PSNPs and Pb co-transport in saturated porous media.The presence of Pb reduces the mobility of PSNPs in quartz sand columns but free PSNPs carry more Pb out of the sand columns,and LMWOAs can promote the transportation of both,and the promotion effect follows:citric acid>malic acid>acetic acid.LMWOAs inhibit the transportation of PSNPs in quartz sand columns in the presence of Pb,but can release more of the Pb deposited in the media.Conversely,the presence of LMWOAs enhanced the transportation of Pb in the sand column in the presence of PSNPs and inhibited the re-release of PSNPs which deposited in the quartz sand.