Pyridine sorption to mineral surfaces: a fourier transform infrared spectroscopy study

Abstract

Sorption of organic contaminants affects their bioavailability and therefore the success of biodegradation in the subsurface. The bioavailability of complex mixtures in low organic matter environments is affected by the sorption of polar compounds to mineral surfaces. Nonpolar compounds then adhere to the organic coatings on the minerals formed by the sorbed polar compounds. Understanding how the geology and solution chemistry affect the mechanisms responsible for the sorption of polar compounds to mineral surfaces will therefore lead to more accurate predictions of the success of biodegradation in those conditions. The objective of this study was to determine the mechanisms responsible for the sorption of polar organic molecules, represented by pyridine, to various mineral surfaces. Four minerals were used in this study: montmorillinite, kaolinite, iron oxide and silica gel. Conditions were varied by sorting pyridine to the minerals in both the gaseous and aqueous phase and by altering the exchangeable cations of the minerals and pH of the mineral solutions. FTIR can provide direct measurements of sorption interactions. Shifts of the 19b, 19a, and 8a vibrational modes of pyridine provide evidence for the molecular interactions responsible for the sorption of pyridine to the various mineral surfaces. Results indicated that hydrogen bonding and bonding to Lewis acid sites are responsible for the sorption of gaseous phase pyridine which sorbed to all mineral surfaces. FTIR spectra provided evidence for the sorption of aqueous phase pyridine through hydrogen bonding to the various montmorillinite surfaces at pH=10. Bioavailability is therefore expected to be reduced in subsurface environments dominated by montmorillinite when compared to a kaolinite, iron oxide, or silica environment. Altering the exchangeable cations did not affect sorption.