High-Resolution Characterization of the Size Exclusion Effect on the Transport of Low-Concentration Mixed-Size Colloids in Porous Media Using a Synthetic DNA-Labeling Method

The transport of colloids in groundwater plays a critical role in pollutant dispersion, with the size exclusion effect (SEE) being a key mechanism influencing colloid fate. However, accurately measuring low-concentration colloids and differentiating between colloids of different particle sizes remains challenging. In this study, we encapsulated specific DNA fragments within colloids of sizes 0.1, 0.2, 0.5, 1.0, 3.0, and 5.0 μm to conduct column-scale experiments with either single-size or multi-size colloids. These experiments were designed to characterize the colloid breakthrough curves and quantify their SEE. This approach significantly enhanced detection signals, enabling accurate colloid detection at very low concentrations (≤10 mg L^-1) and precise colloid size differentiation. Furthermore, the colloid breakthrough curves were fitted to quantify a key SEE parameter, <i>i.e.</i>, the water saturation inaccessible to mobile colloids (<i>γ</i>). Regression analysis revealed a power function correlation between the <i>γ</i> parameter and the colloid diameter, indicating that the SEE intensifies with increasing colloid diameter. These findings highlight the potential of DNA labeling for high-resolution characterization of low-concentration and mixed-size colloidal particle transport in porous media. This report enhances our understanding of colloid transport behavior and its environmental implications.