Experimental evidence and numerical analysis of competitive sorption and carrier-facilitated transport: How mobile clays shape solute mobility in limestone media

Authigenic clay minerals may serve as effective solute carriers, enabling the movement of less mobile pollutants, including pesticides, heavy metals, and polycyclic aromatic hydrocarbons through subsurface environments. When present as colloidal suspensions, clays are highly mobile and can dramatically accelerate the transport of pollutants adsorbed to their surfaces, sometimes increasing mean transport velocities by several orders of magnitude. As a result, delineated groundwater protectionzones and riverbank filtration systems designed solely based on pollutant mobility may be inadequate if the impact of carrier-facilitated transport is ignored. Clay’s ability to mobilize pollutants may be also exploited by using carrier-facilitated (carrier-assisted) transport to release harmful substances from soil or groundwater in in-situ remediation techniques. However, quantitatively evaluating carrier-facilitated transport—especially the parameters governing co-sorption and competitive adsorption between mobile and immobile sorbents—is challenging due to the complex interplay of transport and interaction processes in natural porous media. In this case study, we conducted column experiments demonstrating an enhanced mobilization and transport of poly(ethylene glycol) polymers by montmorillonite in limestone media by a factor of ten. The polymer’s strong affinity for montmorillonite promotes competitive adsorption and enables clays as carriers to mobilize polymers that were previously adsorbed at the immobile phase. Our numerical analysis revealed that high flow rates, e.g. during events like ponding or flooding, further promote carrier-facilitated transport, even when mobile sorbent adsorption is weak. By combining experimental observations with a comprehensive numerical sensitivity analysis, we advanced an experimental protocol to identify and infer the multitude of parameters present in models describing carrier-facilitated transport in an uncorrelated manner, thereby overcoming ambiguity in parameter estimation.