Crystallinity- dependent heteroaggregation and co-sedimentation between polystyrene nanoplastics and iron (hydro)oxides

Iron (hydro)oxides (IOs) with different crystallinities are widespread in the aquatic environment. They coexist with nanoplastics (NPs) and tend to interact with each other. The properties of minerals can greatly influence the heteroaggregation of minerals and NPs, being manifested not only in the heteroaggregation capacity but also in the interaction mode. This study investigated the heteroaggregation and co-sedimentation between polystyrene nanoplastics (PSNPs) and IOs with different crystallinity. We found that the zeta potential of IOs increased with rising IOs crystallinity. DLVO calculations indicated stronger electrostatic attraction between negatively-charged NPs and highly-crystalline IOs, resulting in greater heteroaggregation capacity. In addition, the heteroaggregation capacity of PSNPs with IOs declined as the pH value and ion strength increased. The Fourier transform infrared spectra (FTIR) and two-dimensional correlation spectroscopy (2D-COS) analyses indicated that there was strong hydrogen bonding between IOs with high crystallinity and PSNPs, contributing to a greater heteroaggregation capacity of PSNPs on highly-crystalline IOs than low-crystalline IOs. Zeta potential measurements revealed that negatively charged PSNPs heteroaggregated on the surface of positively charged IOs, capable of neutralizing or even reversing the surface charge. This, in turn, affected the sedimentation of IOs-PSNPs agglomerates and their fate. These findings elucidated the key role of crystallinity on the heteroaggregation between IOs and PSNPs, and offered insight into their environmental fate.