Importance of cation exchange for colloid-associated transport of organic cations on clays

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The potential for colloid-associated transport (co-transport) of pollutants in groundwater caused by natural and artificial colloids has been subject of research since the 1980s up to the present day. In particular, the potential for co-transport of nonpolar, hydrophobic substances to natural and artificial organic but also inorganic colloids is experimentally well documented. However, organic pollutants often occur in the aquatic environment as ionic species. Especially with small contents of organic carbon in the aquifer sorption and thus also environmental and transport behavior of cations is not characterized by hydrophobic sorption but by ion exchange. Predictions on the transport of pollutants can then lead to both underestimation and overestimation in mobility. Despite the high adsorption potential of organic cations on clays by ion exchange and a sorption hysteresis similar to hydrophobic sorption, cation exchange has not yet been systematically investigated as a sorption mechanism during co-transport. There are no fundamental studies on the importance of cation exchange for co-transport of organic ionic species to clay colloids. The aim of this proposal is to systematically quantify and evaluate the importance of co-transport of organic cations on clay colloids by experimental laboratory methods. For this purpose, batch and column experiments with monovalent and divalent organic cations as sorbent and clay minerals (illites, kaolinites and montmorillonite) of the size fractions 200, 500 and 1000 nm under different defined conditions (pH, ionic strength, valence of organic cations and residence times) are carried out in/with a silicate matrix. Understanding the complexities of co-transport provides a foundation for estimating the environmental potential of organic pollutants in the presence of clay minerals. In addition, the project provides the methodological basis for subsequent experimental laboratory experiments on complex materials and in field studies.