Combined 1H NMR and LSER study for the compound-specific interactions between organic contaminants and organobentonites

The compound-specific mechanisms for the sorption of organic contaminants onto cetyltrimethylammonium-saturated bentonite (i.e., CTMA-Bentonite) in water were evaluated by 1H NMR study and Linear Solvation Energy Relationship (LSER) approach. In 1H NMR study, comparing with pure CTMAB, the up-field shifts of hydrogen peaks for CH2N+ and CH3N+ of CTMA+ in CTMAB-aromatics (1-naphtylamine, aniline and phenol) mixtures are much greater than that in CTMAB-aliphatics (cyclohexanone and cyclohexanol) mixtures. Meanwhile, the peak position of hydrogen on amino- and hydroxyl-groups of aromatic compounds also changes greatly. 1H NMR data demonstrated the strong molecular interaction between the positive ammonium group of CTMA+ and the delocalized π-systems of aromatic solutes, whereas the interactions of CTMA+ with aliphatic compounds having electron-donating groups (such as cyclohexanol and cyclohexanone) or aromatic ring substituted by electron-withdrawing groups (i.e., nitrobenzene) or nonpolar aromatic compounds with single phenyl ring (i.e., toluene) are weak. The derived LSER equation was obtained by a multiple regression of the solid–water sorption coefficients (Kd) of 16 probe solutes upon their solvation parameters, and demonstrates aromatics sorption onto CTMA-Bentonite is concurrently governed by the π-/n-electron pair donor–accepter interaction and the cavity/dispersion interaction, while the predominant mechanism for aliphatic compounds is the cavity/dispersion interaction, consisting with the 1H NMR results.

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