The retention mechanism of the adherent iron corrosion pipe scale on polystyrene nanoplastics in drinking water distribution systems

The retention mechanism of iron corrosion pipe scales (ICPS), including the adsorption and the hetero-aggregation/co-sedimentation, on polystyrene nanoplastics (PSNPs) in drinking water distribution systems (DWDSs) was investigated for the first time. Adsorption kinetic results indicated that the equilibrium adsorption capacity was up to 0.244-2.172 mg/cm with the initial PSNPs concentration of 5-50 mg/L. Adsorption kinetics and isotherm results revealed that the adsorption of PSNPs onto adherent ICPS is multilayer involving chemisorption. Adsorption thermodynamic results suggested that this adsorption was spontaneous, exothermic, and entropy-decreasing. Density functional theory calculations revealed that the intermolecular forces between PSNPs and ICPS are the weak C-H···O and O-H···π hydrogen-bonding, strong O-H···O hydrogen-bonding and Fe-O coordination-bonding forces. The residual chlorine in DWDSs enhances the PSNPs adsorption by introducing stronger O-H···Cl and O-H···O hydrogen bonds. The adsorption was driven by both short-range intermolecular forces and long-range Coulomb electrostatic forces. The adsorption will not be significantly inhibited at conventional drinking water pH, ion strength, and organics content ranges. Significant hetero-aggregation/co-sedimentation between the released ICPS nanoparticles and PSNPs was observed. The α-FeOOH and FeO in ICPS dominate this process, while the γ-FeOOH component enhances the dispersion stability of PSNPs. These results improve our understanding of the NPs fate in DWDSs.