Adsorption interactions of aromatics and heteroaromatics with hydrated and dehydrated silica surfaces by Raman and FTIR spectroscopies

Raman and FTIR spectroscopies are used to investigate the sorption mechanisms of benzene, toluene, and 2- and 4-picoline onto silica as models for volatile aromatic pollutant interactions with a soil constituent. Benzene and toluene vapor adsorption on silica occurs via weak pi-system-hydrogen bonding with silanols on the silica surface. This weak interaction would likely result in low vadose zone retention, especially in wet conditions where water adsorption would successfully compete for surface sites. The vapor adsorption of 2- and 4-picoline (2- and 4-methylpyridine) is studied to model aza-arene environmental contaminants. These species adsorb to surface silanols by a more specific and stronger hydrogen-bonding mechanism involving the lone pair electrons on the N atom. The strength of these interactions is probably sufficient to result in their retention by dry or damp vadose zone soil, slowing their transport. This work illustrates the utility of these vibrational spectroscopic techniques in elucidating specific surface interactions of pollutants with mineral oxides and in helping to predict the fate of pollutants in the environment.