Attenuated total reflectance Fourier-transform infrared spectroscopy of carboxylic acids adsorbed onto mineral surfaces

A suite of naturally-occurring carboxylic acids (acetic, oxalic, citric, be nzoic, salicylic and phthalic) and their corresponding sodium salts were ad sorbed onto a set of common mineral substrates (quartz, albite, illite, kao linite and montmorillonite) in batch slurry experiments. Solution pH's of a pproximately 3 and 6 were used to examine the effects of pH on sorption mec hanisms. Attenuated total reflectance Fourier-transform infrared (ATR FTIR) spectroscopy was employed to obtain vibrational frequencies of the organic ligands on the mineral surfaces and in solution. UV/visible spectroscopy o n supernatant solutions was also employed to confirm that adsorption from s olution had taken place for benzoic, salicylic and phthalic acids. Molecula r orbital calculations were used to model possible surface complexes and in terpret the experimental spectra. In general, the tectosilicates, quartz and albite feldspar, did not chemiso rb (i.e., strong, inner-sphere adsorption) the carboxylate anions in suffic ient amounts to produce infrared spectra of the organics after rinsing in d istilled water. The clays (illite, kaolinite and montmorillonite) each exhi bited similar ATR FTIR spectra. However, the illite sample used in this stu dy reacted to form strong surface and aqueous complexes with salicylic acid before being treated to remove free Fe-hydroxides. Chemisorption of carbox ylic acids onto clays is shown to be limited without the presence of Fe-hyd roxides within the clay matrix. Copyright (C) 1999 Elsevier Science Ltd.