Effects of Iron-Organic Matter Colloids on Transport of Rare Earth Elements in Saturated Porous Medium

Colloids, such as iron-humic acid (Fe-HA) colloids, are key vectors for the transport of potentially hazardous elements such as rare earth elements (REEs) due to their strong adsorption capacity and high mobility. These heterogeneous particles vary in size and composition and are highly sensitive to changing biogeochemical conditions (e.g., pH, redox potential). However, their transformation during transport and the resulting impact on contaminant behavior remain underexplored. In this study, batch and column experiments were combined to investigate how Fe-HA colloid heterogeneity influences REE mobility in a quartz sand column under water-saturated conditions. Elemental fractionation, evident from discrepancies in the breakthrough curves of Fe and organic matter, became more pronounced at lower Fe-HA concentrations, suggesting shifts in colloid composition with contaminant loading. Dynamic light scattering (DLS) revealed two major colloid populations (∼100 and ∼500 nm). The smaller particles eluted earlier, whereas the larger, aggregation-derived colloids appeared later and were enriched in Fe. Distinct REE patterns, reflecting subtle differences within the 14-element series, were used to probe REE surface speciation, competitive binding, and deviations from equilibrium during transport. This work provides new insights into colloid-facilitated REE migration in water-saturated porous media, with implications for contaminant mobility in various soils and sediments.