Using colloidal AFM probe technique and XDLVO theory to predict the transport of nanoplastics in porous media

The plastic concentration in terrestrial systems is orders of magnitude higher than that found in marine eco-systems, which has raised global concerns about their potential risk to agricultural sustainability. Previous research on the transport of nanoplastics in soil relied heavily on the qualitative prediction of the mean-field extended Derjaguin-Landau-Verwey-Overbeek theory (XDLVO), but direct and quantitative measurements of the interfacial forces between single nanoplastics and porous media are lacking. In this study, we conducted multiscale investigations ranging from column transport experiments to single particle measurements. The maximum effluent concentration (C/C0) of amino-modified nanoplastics (PS-NH2) was 0.94, whereas that of the carboxyl-modified nanoplastics (PS-COOH) was only 0.33, indicating PS-NH2 were more mobile than PS-COOH at different ionic strengths (1-50 mM) and pH values (5-9). This phenomenon was mainly attributed to the homogeneous aggregation of PS-COOH. In addition, the transport of PS-NH2 in the quartz sand column was inhibited with the increase of ionic strength and pH, and pH was the major factor governing their mobility. The transport of PS-COOH was inhibited with increasing ionic strength and decreasing pH. Hydrophilicity/ hydrophobicity-mediated interactions and particle heterogeneity strongly interfered with interfacial forces, leading to the qualitative prediction of XDLVO, contrary to experimental observations. Through the combination of XDLVO and colloidal atomic force microscopy, accurate and quantitative interfacial forces can provide compelling insight into the fate of nanoparticles in the soil environment.

Automated summary

The plastic concentration in terrestrial systems is much higher than that in marine ecosystems, leading to concerns about their impact on agricultural sustainability. Previous research on nanoplastic transport in soil relied on qualitative predictions, but lacked direct measurements of interfacial forces. This study conducted multiscale investigations and found that amino-modified nanoplastics were more mobile than carboxyl-modified nanoplastics. The transport of both types of nanoplastics was influenced by ionic strength and pH. Hydrophilicity/hydrophobicity-mediated interactions and particle heterogeneity interfered with interfacial forces, contradicting the qualitative predictions of existing theory. The combination of theory and atomic force microscopy can provide valuable insights into the fate of nanoparticles in soil.