Nanoplastic stimulates metalloid leaching from historically contaminated soil via indirect displacement

It is generally believed that the ability of nanoplastics (NPs) to mobilize other contaminants is due to direct adsorption; however, this intuitive belief is questioned in this study when it comes to a historically contaminated soil where mining activities since 1958 have resulted in arsenic (As) and cadmium (Cd) enrichment. Negatively charged polystyrene (PS) NPs were used in this study, which should theoretically stimulate Cd (metal cation) instead of As (negatively charged oxyanion) leaching if direct adsorption accounted for co-transport. Surprisingly, PS NPs enhanced the leaching of As by up to over 5 times (p value < 0.05), but had almost no effect on Cd leaching (p value > 0.05). A novel indirect displacement model was therefore developed to describe the phenomenon of enhanced As leaching. It has been found that negatively charged NPs interacted with As via competition for soil binding sites. Underlying mechanistic insights were further explored via both theoretical calculations with the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) approach, and direct characterization using Scanning Electron Microscopy (SEM) and Computed X-ray Microtomography (μCT) showing binding sites and flow channels, respectively. The overall results provide new and valuable insights into NP-metal(loid) interactions in the natural soil environment, which can be integrated in future studies regarding the transport and risk assessment of NPs, and toxic metal(loid)s.