InterPore2026

Pore-Scale Hydrodynamically Driven Trapping of Microplastics in Soils

19 May 2026, 11:35

15m

Oral Presentation (MS02) Environmental Porous Media: Water, Agriculture, and Remediation MS02

Speaker

Marjan Ashrafizadeh (Institute for Geosciences, Applied Geology, Friedrich-Schiller-University Jena, 07749 Jena, Germany)

Description

Microplastic (MP, < 5 mm) pollution is widespread in soils, where particles can persist, migrate with groundwater, and carry harmful chemicals and microorganisms 1-2. Understanding how MPs are transported and retained in porous media is therefore essential for assessing their environmental impacts 3. However, standardized methods to investigate MP transport in soils are still limited 4-5. Most existing studies rely on indirect column experiments, where total retention is estimated from outlet concentrations 6-7. The MP localization is often determined using destructive sampling and chemical extraction, which disturb soil structure and remove important pore-scale information 8-9. Recent studies have shown that X-ray computed tomography (µCT) can be used to visualize MPs in porous media 8-11. However, these studies mainly focused on static systems, where millimeter-sized MP fragments were manually placed in the soil. As a result, the dynamic transport behavior of fine MPs, with sizes of only a few micrometers, remains largely unexplored.
In this study, we introduce a non-destructive pore-scale workflow that combines micro-scale flow-through column experiments with high-resolution X-ray micro-computed tomography (µCT; 0.7–2 µm voxel size) and digital rock physics (DRP) analysis. This integrated approach allows direct three-dimensional visualization and quantitative analysis of the transport and retention of fine MPs (2 µm polystyrene spheres) in soil-relevant porous media. MP suspensions were injected at different flow rates (0.5 to 2 mL min⁻¹) and injection volumes (15 and 30 mL). After injection, the columns were flushed to remove mobile MP particles, dried to stabilize the pore structure, and scanned using µCT. Machine-learning-based segmentation was used to create digital models of the pore space for image-based DRP analysis and interpretation of retention mechanisms.
The results reveal a non-monotonic relationship between flow rate and MP retention, which contradicts predictions from classical colloid filtration theory (Figure 1). Retention decreases as flow rate increases from low to intermediate values. At the highest flow rate, however, retention increases strongly, leading to permeability reductions of up to about 5%. The analyses indicate that at low flow rates, retention is mainly controlled by surface deposition. At intermediate flow rates, advective transport dominates, resulting in lower but more evenly distributed retention. This occurs because increasing the flow rate reduces the time available for particles to approach grain surfaces by Brownian motion, facilitating their transport. At high flow rates, elevated pore-scale velocities force more particles through constricted throats per unit time, increasing particle-particle collisions and interactions with surface roughness. These interactions promote particle clustering and hydrodynamic bridging, as well as trapping of detached MPs from upstream. This interpretation is supported by strong permeability reductions and larger MP clusters observed at high flow rates. This indicates that high-flow events, such as heavy rainfall or irrigation, can cause MPs to accumulate locally in soils rather than continue downward transport. Overall, this study provides clear pore-scale evidence that MP retention depends on flow conditions and improves predictions of their environmental fate.

Figure 1. Effect of flow rate and injected volume on microplastic retention: a) patterns and b) total retention along sand-packed columns.