Quantifying Ion‐Specific Effects on the Size‐Exclusion Transport of Multi‐Sized Colloids in Porous Media Using a Novel Synthetic DNA Labeling Approach

Abstract

Colloids strongly adsorb pollutants and promote their co‐transport in aquatic environments, posing substantial ecological contamination risks. However, quantifying the size exclusion effect (SEE), a critical mechanism that enhances colloid transport, remains challenging in multi‐sized colloid systems, owing to limited high‐resolution labeling methods and ion‐induced dynamic aggregation. Here, we developed a high‐resolution DNA‐based tracing framework that employs synthetic DNA fragments to individually label multi‐sized colloids (0.1, 0.2, 0.5, and 1.0 μm), and utilizes quantitative real‐time PCR (qPCR) to quantify their transport behavior in saturated quartz sand under the typical environmental ions (NaCl/CaCl ₂ ). The results demonstrated that Ca ²⁺ promoted colloid aggregation and retention more strongly than Na ⁺ , thereby modifying size‐exclusion behavior, although the magnitude of SEE depended on colloid size and transport conditions. While the SEE generally exhibited a positive size dependence in the single‐sized system, the multi‐sized system exhibited higher SEE for 0.1 and 0.2 μm colloids relative to their single‐sized counterparts under certain solution chemistry conditions. This discrepancy may be attributed to pore blockage caused by larger colloids within the multi‐sized system. Furthermore, two transport patterns, blocking and ripening, were identified from the breakthrough curves of DNA‐labeled colloids. Correlation analysis revealed that the SEE parameter ( γ ) was positively correlated with the normalized retention parameter ( S _max / C ₀ ) under blocking conditions ( r  = 0.655). This study highlights the effectiveness of synthetic DNA labeling approach in tracking the transport of multi‐sized colloids, providing new insights into the fate of colloids in natural and engineered porous systems.