Effects Of Surfactants On The Transport Of Microplastics And Lead In Saturated Porous Media

The microplastic pollution has become an important scientific issue in the field of environment and ecology.Once microplastics enter the groundwater environment,they may alter the transport and fate of pollutants due to the adsorption of microplastics.Therefore,the transport of microplastics in the underground environment has become an environmental issue requiring urgent attention.Pb(Ⅱ)is a potentially toxic element ubiquitous in the environment,which poses serious threats to the ecological environment and public health.The microplastics usually have strong hydrophobicity and are not easy to migrate in aqueous media or react with metal ions.Surfactants are amphoteric compounds containing hydrophobic and hydrophilic groups.As a hydrophobic material,microplastics are easily combined with the hydrophobic chains of surfactant molecules to improve the hydrophilicity of microplastics,which not only affects the affinity of microplastics for heavy metals,but also aggravates the transport and diffusion of microplastics in underground media.However,the mechanism of surfactant molecules on the transport of microplastics and their cotransport with pollutants is still unclear,and relevant research is urgently needed.In this study,the influence of different factors(ionic strength,p H,flow rate,multivalent cations,surfactant concentration,surfactant type and microplastic type)on the migration of microplastics in the presence of surfactants was investigated through the quartz sand packed column test,and the transport behavior of microplastics under different physical and chemical conditions was clarified.The dynamic contact angle of surfactants on the surface of microplastics was measured and the work of adhesion of surfactants on the surface of microplastics was calculated according to the Young-Dupre equation,which clarified the potential mechanism of retention of microplastics in the presence of surfactants.The common dominant factors of anionic,cationic and nonionic surfactants affecting the migration of microplastics in media were further revealed.The adsorption-desorption behavior of microplastics for Pb(Ⅱ)under surfactant and the co-transport behavior of microplastics and Pb(Ⅱ)in porous media were evaluated by batch adsorption and column experiments.The adsorption mechanism of Pb(Ⅱ)on microplastics under the action of surfactant was analyzed,and the interaction between different microplastics and Pb(Ⅱ)in porous media was clarified.It is expected that the results of the study will clarify the migration and diffusion risks of microplastics under the influence of surfactants,which is of great significance for preventing and reducing the environmental hazards of microplastics.The main conclusions were as follows:(1)The experimental results showed that the mobility of PE decreased with increasing ionic strength,cation valence,and decreasing flow rate.The transport abilities of PE increased with increasing surfactant concentration when the surfactant concentration was less than the critical micelle concentration.The mobility of PE under CTAB was much greater than that of PE under SDBS.Because quartz sand could absorb more CTAB molecules through electrostatic attraction to weaken the collision between microplastics and quartz sand.The Extended-Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory formed by adding osmotic,elastic,and hydrophobic force could well described the migration behavior of microplastics in CTAB and SDBS well.(2)The transport ability of microplastics followed the order of PTFE <PMMA <PS<PE <PP,which was mainly attributed to the difference in the adhesion of the surfactant on the surface of the microplastics,which reflected the ability of surfactant molecules to detach from microplastic surfaces.When the microplastics were close to each other,if the reaction force of the electrostatic force was greater than the adhesion force of the surfactant molecules on the surface,the surfactant molecules would be separated from the microplastics and the stability of the microplastics would decrease.The greater the adhesion work of the surfactant on the surface of the microplastic,the higher the stability of the microplastic and the stronger its migration in the porous media.In addition,the migration ability of microplastics in anionic surfactants was weaker than that of cationic surfactants,because the osmotic and elastic repulsion produced by SDBS were weaker than CTAB.(3)The different surfactants promoted the transportation of PE in the column to different degrees.The promotion degree of cationic,anionic and nonionic surfactants on PE transport from large to small were BG > PQ28 > CTAB > BLB > TLB,SDS > SLDED >DSS> SL > SDBS,POES > T40 >T80 > X100 > NP40,respectively.The promoting effect of surfactant on the migration of PE microplastics could also be attributed to the adhesion work of surfactant molecules on the surface of microplastics.The difference in the molecular structure of the surfactant made the adsorption area of the surfactant molecule on the microplastic surface different.The surfactant molecule had a long carbon chain and no heterochain,so the adsorption area on the solid surface was small,and the adhesion work was large,which made the microplastics have strong mobility.However,when the adhesion work was the same,the migration ability of PE microplastics under anionic or nonionic surfactants was weaker than that of cationic surfactants,because the osmotic and elastic energy between microplastics and quartz sand would played a dominant role.(4)The adsorption capacity of different particle sizes of PE and PP for Pb(Ⅱ)in the presence of surfactants was PP-SDBS > PE-SDBS > PP-CTAB > PE-CTAB.The application of surfactants could enhance the adsorption capacity of microplastics by promoting the entry of Pb(Ⅱ)into the fissures,wherein that of PP and PE on Pb(Ⅱ)was increased by up to 8.04 and 7.35 times,respectively.The FTIR and XPS results indicated that that the adsorption of Pb(Ⅱ)by microplastics in the presence or absence of surfactants was a physical adsorption process.In addition,the XPS etching results determined that the Pb(Ⅱ),S,and N contents did not significantly decrease at different depths in the presence of surfactants,and the Pb(Ⅱ)content without surfactants decreased with an increasing depth,indicating that the crack structure in the microplastic was the main site for Pb(Ⅱ)adsorption,and the surfactant accelerated the diffusion of Pb(Ⅱ)in the microplastic fissure structure.(5)When the microplastics were cotransport with Pb(Ⅱ),the transport ability of the microplastics did not change compared with the migration of the microplastics alone.The PTFE,PMMA,PS,PE and PP all enhance the mobility of Pb(Ⅱ)under CTAB compared to the migration of Pb(Ⅱ)alone in porous media.The recovery of Pb(Ⅱ)increased from 86%to 97.55%,95.11%,92.95%,90.12% and 87.55%.In the presence of SDBS,PS,PE and PP inhibited the migration of Pb(Ⅱ),and the recoveries decreased from 79% to 61.75%,57.53%and 70.96%,respectively.However,PTFE and PMMA slightly promoted the migration of Pb(Ⅱ),the recoveries increased to 83.27% and 81.36%,respectively.There were two main reasons for promoting or inhibiting Pb(Ⅱ): one was the adsorption rate of Pb(Ⅱ)by microplastics,and the adsorption rate determined whether the migration trends of Pb(Ⅱ)and microplastics were consistent;the other was the stability of microplastics itself.The adsorption reaction of microplastics and Pb(Ⅱ)in the column was not as violent as that in the shaker,and it was a relatively slow and peaceful process.