Competitive Sorption and Carrier-Facilitated Transport of Organic Polymers by Clay Minerals in Limestone Media: Experimental Evidence and Numerical Analysis.

Clay minerals acting as carriers in permeable media can facilitate the transport of less mobile pollutants, including radionuclides, heavy metals, pesticides, polycyclic aromatic hydrocarbons, and organic polymers. In a colloidal dispersed state, clays can increase the mean transport velocity of adsorbed pollutants by several orders of magnitude. Hence, delineated groundwater protection zones and riverbank filtration systems based on pollutant mobility may fail if carrier-facilitated transport is neglected. Yet, leveraging carrier-assisted transport for the controlled release of pollutants from soil may open new remediation opportunities. However, the determination and parametrization of competitive adsorption on mobile and immobile sorbents, crucial for carrier-facilitated transport, are often obscured by the interplay among numerous interaction processes in natural porous media. We present experiments in which montmorillonite minerals facilitated the transport of polyethylene glycol (PEG), resulting in a 10-fold increase in mean transport velocity in limestone media. PEG's high affinity for montmorillonite enabled the mobilization of PEG already adsorbed at immobile interfaces. Additionally, model simulations suggested that high flow rates, for example, due to ponding infiltration, were favorable for carrier-facilitated transport, even for weak adsorption to mobile sorbents. We developed a sequential experimental protocol that permits identification of crucial factors controlling competitive sorption during carrier-facilitated transport.