Enhanced mobility and dynamic retention of nanoplastics in mineral coated porous media.

This study examines the transport and retention of carboxylated and amine-modified polystyrene nanoparticles (NP) in quartz-, kaolinite-, and goethite-coated sands under saturated flow. Column experiments at varying flow velocities (1- 50 m d⁻¹), supported by adsorption kinetics, (X)-DLVO modeling, hydrodynamic torque analysis, and advection-dispersion modeling (ADE), were used to identify controlling factors of NP mobility.

Increasing flow velocity enhanced NP breakthrough and shifted deposition zones further along the column length, indicating a transition from diffusion- to reaction-limited attachment. Deposition followed the order goethite > kaolinite > quartz. Despite similar surface charge, carboxylated polystyrene NP showed unexpectedly strong retention on kaolinite, attributed to hydrogen bonding between carboxyl and kaolinite hydroxyl groups, as indicated by IR spectroscopy, an interaction not captured by DLVO theory. Force tensiometry showed variations in contact angles between various mineral coatings, yet no evidence for earlier proposed “long range” hydrophobic forces between NP and macroscopic surfaces could be found from Peak Force QNM measurements. ADE simulations incorporating reversible attachment-detachment and site blocking best reproduced observations, highlighting the combined roles of hydrodynamics and mineral surface chemistry. The modelling exercise further suggests that the low density of polymeric nanoparticles limits gravitational settling, challenging the transferability of trends established for denser mineral engineered nanoparticles (ENPs) such as silica.

Overall, the results demonstrate greater NP mobility and more dynamic retention in natural, heterogeneous flow systems than inferred from DLVO interactions, as supported by experiments and kinetic ADE modeling.