Bonding mechanisms of salicylic acid adsorbed onto illite clay: An ATR-FTIR and molecular orbital study

Understanding the nature of chemical bonding between natural organic matter (NOM) and clays is an important step toward understanding the chemistry controlling the fate and bioavailability of organic contaminants in soils and sediments. Many organic contaminants preferentially associate with NOM, and adsorption of NOM to mineral surfaces affects this association. To examine possible bonding mechanisms of NOM to clays, adsorption mechanisms of salicylic acid onto illite clay were investigated through the use of attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy and molecular orbital (MO) calculations. Batch adsorption experiments under acidic and neutral conditions were performed to sorb salicylic acid onto the illite surface. ATR-FTIR spectra were collected to determine the vibrational frequencies of organic surface complexes. MO calculations on salicylate-aluminate and salicylate-silicate clusters were used to model possible surface complexes. Strong correlations were found between theoretical and observed infrared frequencies. Based on these correlations, we suggest that Al-O-C linkages form between salicylate anions and Al3+ octahedra at the illite clay edges. A monodentate complex involving one carboxylate oxygen is most probable under neutral conditions; a bidentate complex involving both carboxylate oxygen atoms and two vicinal Al3+ octahedra is likely to dominate at lower pH. The predicted monodentate surface complex is similar to that suggested by Murphy et al. (Sci. Total Environ. 1992, 117/118, 413) for humic acid-clay bonding. This similarity between salicylic and humic acids suggests that simple organic ligands are good analogues for more complex humic acid-clay interactions.