Ionizable pesticides and pharmaceuticals often contain cationic amine groups which facilitate their sorption to soils via cation exchange. This exchange process is well understood and established to be a primary mechanism of organic cation sorption to aluminosilicate clays and natural organic matter. Nevertheless, cooperative or secondary interactions can occur and enhance sorption beyond the extent expected by cation exchange processes alone. However, potential cooperative interaction mechanisms for cationic aromatic amine sorption, as well as corresponding sorbate and sorbent characteristics that influence these interactions are not well established. This study focuses on cooperative interactions pertaining to the sorption of substituted cationic anilines and benzylamines, substructures relevant to ionizable pesticides and pharmaceuticals commonly detected in surface and ground water systems.  We demonstrated that cooperative cation-p, p-p, and van der Waals interactions can increase aromatic cationic amine sorption to Na/Ca-montmorillonite well-beyond the extent expected by cation exchange alone. Cationic amines exhibiting cooperative interactions displayed non-linear S-shaped isotherms and increased affinity for the sorbent at low surface coverage. Our experiments revealed the predominance of cation-p interactions, which occurred between the p system of a compound retained on the surface via cation exchange and the cationic amine group of an adjacent molecule. Compounds with greater amine charge/area and electron-donating substituents that allowed for greater electron density at the center of the aromatic ring showed a greater potential for cation-p interactions. Notably, cooperative interactions were not as important for cationic amine sorption to soils. Benzylamine sorption to nine soils at charge loadings comparable to the experiments with montmorillonite, revealed that cooperative interactions were only likely for soils with exceptionally high cation exchange capacities (> 0.7 mol_c/kg) and low organic matter contents. As such, cation- p interactions are not expected to be significant in all soil systems.

Nonlinearity of cationic aromatic amine sorption to aluminosilicates and soils: Role of intermolecular cation-? interactions

247th National Meeting (2014)

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