Date of Completion
Field of Study
Doctor of Philosophy
With the increasing number of emerging contaminants that are cationic at environmentally relevant pH, there is a need for robust predictive models for organic cation sorption coefficients (Kd). Column chromatography was validated as a method to obtain organic cation sorption isotherms for environmental solids; thereby enabling a less labor intensive means to generate the extensive datasets of linear Kd values. Current predictive models fail to account for the differences in the abundance and affinity of inorganic exchange ions naturally present on environmental solids. To better understand how organic cation sorption is influenced by inorganic exchange ions, sorption coefficients of ten cationic pharmaceuticals were determined for six homoionic forms of the aluminosilicate clay mineral, montmorillonite. Regular trends in compound sorption coefficients across the clays were consistent with competition according to the inorganic cation selectivity sequence. Such regular changes in competition suggested that a probe compound, such as phenyltrimethylammonium, could capture soil-to-soil variations in inorganic cations for the prediction of organic cation sorption to soils and soil minerals. Structure-affinity relationships from a literature model allowed for extrapolation of phenyltrimethylammonium Kd values for soils to those of different compound structures. The use of a probe compounds is only valid if organic cations interact with all sites on clay minerals in the manner. However, charge sites on clay minerals vary in hydration and the area over which the charge is spread. To understand the how differences in clay site structure influence sorption, a number of specific structural moieties were investigated from the base structure of benzylamine. The ordered hydration and increased charge focus of sites within a clay interlayer caused for large differences in the sorption coefficients of organic cations with polar moieties. These interactions where exemplified by deviations in quantitative trends in sorption coefficients of complex cationic pharmaceuticals, making organic cation sorption to clay minerals a function of hydration and charge focus.
Jolin, William C., "Sorption of Organic Cations to Aluminosilicate Clay Minerals and Soils" (2016). Doctoral Dissertations. 1194.