Interaction between microplastics and humic acid and its effect on their properties as revealed by molecular dynamics simulations

The characteristics and fates of microplastics (MPs) and humic acid (HA) in the environment are significantly influenced by their interactions. Thus, the influence of the MP-HA interaction on their dynamic characteristics was explored. Upon MP-HA interaction, the number of hydrogen bonds established in the HA domains decreased significantly, and the water molecules bridging the hydrogen bonds shifted to the exterior regions of the MP-HA aggregates. The distribution intensity of Ca2+ located at ∼0.21 nm around HA deceased, indicating that the coordination of Ca2+ with the carboxyl on HA was impaired in the presence of MPs. Additionally, the Ca2+-HA electrostatic interaction was suppressed because of the steric hindrance of the MPs. However, the MP-HA interaction improved the distribution of water molecules and metal cations around the MPs. The diffusion coefficient of HA decreased from 0.34 × 10-5 cm2/s to 0.20-0.28 × 10-5 cm2/s in the presence of MPs, implying that the diffusion of HA was retarded. The diffusion coefficients of polyethylene and polystyrene increased from 0.29 × 10-5 cm2/s and 0.18 × 10-5 cm2/s to 0.32 × 10-5 cm2/s and 0.22 × 10-5 cm2/s, respectively, indicating that the interaction with HA accelerated the migration of polyethylene and polystyrene. These findings highlight the potential environmental hazards posed by MPs in aquatic environments.