Experimental study on microplastic transport in heterogeneous porous media

As an emerging class of contaminants, microplastics have been widely detected in soil and groundwater. Based on their origin, they can be classified into primary or secondary microplastics, and their loads in terrestrial systems may be 2–23 times those in the oceans. Groundwater serves as the drinking water source for approximately 50% of the global population and supplies 43% of agricultural irrigation water. Therefore, groundwater contamination by microplastics has become an increasing concern. Microplastics can enter groundwater systems through wastewater irrigation, landfill leachate infiltration, atmospheric wet and dry deposition, and surface water–groundwater exchange. However, their transport mechanisms in heterogeneous porous media remain unclear, which limits the prediction of their environmental behaviors and risk assessment.

In this study, laboratory sand-column experiments were conducted to systematically investigate the transport characteristics of fluorescent polystyrene microplastics (200 and 800 nm) in three typical porous media: homogeneous coarse sand (particle size range: 1.18–1.40 mm), homogeneous fine sand (particle size range: 0.12–0.13 mm), and a concentric heterogeneous structure (coarse sand in the center and fine sand at the periphery). Different conditions were set, including pH values (5, 7, 9), flow rates (0.5, 1.0 mL/min), and microplastic particle sizes (200, 800 nm), to observe their breakthrough behaviors. After injecting 3 pore volumes (PVs) of the microplastic suspension into the columns pre-equilibrated with a 0.01 mol/L NaCl background solution, the columns were flushed with 5 PVs of the background solution. The effluent was collected and its fluorescence intensity was measured to construct breakthrough curves, and the columns were subsequently divided into 10 segments to quantify the retained microplastics and establish retention profiles. Batch adsorption experiments and zeta-potential measurements were also performed to characterize the surface properties of the sands and microplastics.

Heterogeneous media significantly altered the transport paths and retention distribution of microplastics. The breakthrough curves exhibited a typical bimodal pattern, reflecting the coexistence of preferential flow (in coarse sand regions) and retention zones (in fine-sand regions). The mobility of microplastics increased with increasing pH and flow rate. For example, in fine sand, the breakthrough rates were 103.17%, 87.06%, and 19.98% at pH 9, 7, and 5, respectively. In coarse sand, they increased from 69.30% to 95.87% as the flow rate increased from 0.5 to 1.0 mL/min. Meanwhile, the transport capacity of 200 nm particles was significantly greater than that of 800 nm particles, with breakthrough rates of 96.20% versus 69.30% in coarse sand and 75.61% versus 67.22% in the heterogeneous medium. The underlying mechanisms are as follows. Increased pH and decreased particle size promote transport by enhancing the electrostatic repulsion between microplastics and the medium surface and by weakening physical clogging, respectively, whereas higher flow rates increase the hydrodynamic force on microplastics, thereby reducing their adsorption capacity onto the medium surface.

This study reveals that the bimodal transport pattern of microplastics in heterogeneous media is dominated by the "fast–slow" dual-domain flow structure. It also clarifies the key regulatory roles of environmental factors such as pH, hydrodynamic conditions, and particle size. These findings provide experimental evidence for the behavior simulation and risk control of microplastic pollution in subsurface environments.