Influence of natural organic matters on fate of polystyrene nanoplastics in porous media

Natural organic matters (NOMs) are widely present in aqueous environments. The effect of NOMs on the fate of nanoplastics that are gradually receiving widespread attention in porous media needs to be noticed, but relevant research is lacking. To fill this gap, the present study focused on elucidating the influence of NOMs and metal cations with varying concentrations upon the transport, long-term release, and particle fracture of polystyrene nanoplastics (PS-NPs) in saturated porous media. The adsorption, transport, long-term release, and particle fracture tests were conducted. A mathematical model and the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were used in this research. NOMs could adsorb onto PS-NPs leading to a reduction in the PS-NPs' zeta potential and an increase in the energy barrier and steric hindrance between PS-NPs and quartz sand, ultimately facilitating the transport of PS-NPs through porous media. On the other hand, an increase in concentration and valence of metal ions enhanced the PS-NPs' zeta potential, resulting in PS-NPs' aggregation and increased size when NOMs were present. This reduced the energy barrier between porous media and PS-NPs, resulting in increased blocking and straining, allowing decreased PS-NPs' transport. Long-term release tests demonstrated release ability and mobilities of PS-NPs decreased as the enhanced NOM concentration, addition of metal cations, and decreased valence of metal ions, in agreement with the transport test findings. In the research about particle fracture, NOMs were found to inhibit the fracture of PS-NPs by adsorbing on their surface to protect them from fracture. Metal cations and increased metal cation valence promoted the fracture of released PS-NPs when NOMs were present by promoting NOM aggregation and thus hindering the protection of NOMs for the nanoplastics.