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

## TL;DR

Clay minerals can speed up the movement of pollutants like PEG in limestone by acting as carriers, which has implications for groundwater protection and soil remediation.

## Contribution

A new experimental protocol was developed to identify factors controlling competitive sorption during carrier-facilitated transport.

## Key findings

- Montmorillonite minerals increased PEG transport velocity by 10-fold in limestone media.
- High flow rates enhance carrier-facilitated transport even with weak adsorption.
- PEG's strong affinity for montmorillonite mobilized PEG already adsorbed at immobile interfaces.

## Abstract

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.