Effect of cationic, anionic and non-ionic surfactants on transport of microplastics: Role of adhesion of surfactants on the polyethylene surface

In view of the amphiphilicity of surfactants, which can improve the hydrophilicity of polyethylene (PE) microplastics and increase the mobility of PE microplastics, the research on the transport behavior of PE microplastics under the coexistence of surfactants is still lacking and necessary. In this work, column experiments were carried out on the suspensions made of 15 surfactants mixed and dispersed with PE microplastics to explore the promoting effect of various surfactants on the transport of PE in saturated porous media. The dynamic contact angle of surfactant on PE surface and the surface tension and zeta potential of PE / surfactant dispersion system were measured. The interaction energy between PE / surfactant and medium, the adhesion of surfactant on PE surface were calculated respectively according to DLVO theory and Young's equation. The experimental results showed that 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 > TX20, respectively. The recovery of PE in the column ranged from 17% to 99.93% under the surfactants. The calculation results of the adhesion of surfactant on PE surface also showed the same law. The minimum single molecule adsorption area (AsL) of surfactant on PE surface was negatively correlated with the recovery of PE in the column under the same type of surfactant. The high adhesion of surfactant on PE surface was caused by the small adsorption area of single molecule, which finally made PE obtain higher stability and penetration. These findings provide new insights and methods for the fate and transport of microplastic and surfactant coexisting systems in soil and the prediction of their potential risk of groundwater pollution.