Heteroaggregation and sedimentation of natural goethite and artificial Fe3O4 nanoparticles with polystyrene nanoplastics in water

Abstract Iron oxide nanomaterials play important roles in biogeochemical processes. This study investigates the effects of representative natural carbonaceous materials (humic acid [HA ] and extracellular polymeric substances [EPS ] ) and cations on the heteroaggregation and sedimentation of engineered and natural iron oxide nanomaterials with montmorillonite and sulfate- and amine-modified polystyrene (PS) nanoparticles (NPs) (S- and N-PS NPs, respectively) in water, assessing their environmental behavior and differences in colloidal stability parameters. In addition, a novel extended Derjaguin–Landau–Verwey–Overbeek theory (XDLVO) was developed to describe the mechanism of colloidal behavior that concurrently considers gravitational and magnetic attraction forces. In CaCl 2 solution and most natural water samples, negatively charged S-PS NPs promoted heteroaggregation with goethite and iron oxide (Fe 3 O 4 ) NPs more than positively charged N-PS NPs with increased nanoplastic particle concentration. In seawater, the introduction of S- and N-PS NPs increased the maximum net energy (barrier) (Φ MAX ) of heteroaggregation and sedimentation with goethite and Fe 3 O 4 NPs, facilitating dispersal and suspension of the system. The X-ray photoelectron spectroscopy (XPS) and molecular dynamics simulation results suggested that Ca 2+ forms bridging interactions between Fe 3 O 4 and S-PS NPs to promote aggregation, while competitive adsorption occurs between the N atoms of N-PS NPs and Ca 2+ on the surface of Fe 3 O 4 NPs. The study findings will help to improve the understanding of interfacial processes affecting ions at nanomaterial/water interfaces and assessments of the geochemical behavior and ecological risks of nanoplastics.