Influence of solution chemical properties and porous medium surface coatings on the transport and retention behavior...

The transport and retention behavior of microplastics (MPs) in saturated porous media is a hot issue in the fields of ecological security and public health in contemporary society. In this study, the transport and retention behaviors of polystyrene microplastics (PS-MPs) in saturated porous media were systematically investigated under various environmental conditions, including humic acid (HA) concentration, ionic strength (IS), mass fraction of iron oxyhydroxide coated quartz sand (ω), and pH in the presence of HA. A series of column experiments were conducted, and all breakthrough curves and retention profiles were successfully simulated using a two kinetic sites attachment-detachment model (TKSADM) that distinguishes between chemical adsorption (Site 1) and physical retention (Site 2). Results demonstrated that increasing HA concentration from 1 to 10 mg L^-1 enhanced PS-MP transport, which was attributed to HA adsorption creating steric hindrance and enhancing electrostatic repulsion. In contrast, HA conditions, increasing IS from 0.1 to 100 mM progressively suppressed MP transport, with model parameters revealing a dramatic 30-fold increase in physical retention rate (k _att2) and a 2.8-fold increase in chemical adsorption capacity (S _max1/C ₀), reflecting the dual role of IS in compressing the electrical double layer and inducing HA conformational collapse. Increasing the mass fraction of iron oxyhydroxide coated quartz sand (ω from 0.15 to 0.75) substantially inhibited PS-MP transferability, with both chemical adsorption rate (k _att1) and physical retention rate (k _att2) increasing by 57% and 83%, respectively, accompanied by a 79% increase in S _max1/C ₀. This balanced enhancement reflects the dual role of iron oxyhydroxide coatings: increased surface roughness creating physical straining sites and [triple bond, length as m-dash]Fe-OH groups providing abundant reactive sites for specific adsorption. Increasing pH from 6.0 to 10.0 substantially enhanced MP mobility, with all retention - related parameters decreasing monotonically: k _att1 decreased by 34%, k _att2 decreased by 52%, and S _max1/C ₀ exhibited the most dramatic decline (71%), revealing a "pH-switching effect" on physical retention sites - activated at low pH and deactivated at high pH due to enhanced electrostatic repulsion and HA conformational extension. This study provides quantitative mechanistic insights into MP fate in subsurface environments and underscores the need to consider coupled effects of multiple environmental factors when predicting MP transport behavior.