Spectroscopic Study of Reaction of Pentachlorophenol on Fe(III)-Montmorillonite Clay Surface.

Polychlorinated dibenzo-p-dioxins (PCDDs) and especially octachlorodibenzo-p-dioxin (OCDD), are ubiquitous in even “pristine” surface soils and prehistoric clay deposits. Characteristic features of soil or clay-associated PCDDs are the dominance of octachlorodibenzo-p-dioxin (OCDD) as the most abundant congener and very low levels of polychlorinated dibenzofurans (PCDFs). These propensities led to the hypothesis of in situ PCDD formation in soils and geologic clay deposits. Our previous results showed that OCDD can be formed from precursor pentachlorophenol (PCP) under environmental relevant conditions, and a clay-catalyzed reaction mechanism was proposed. In this study, in situ FTIR spectroscopy is used to monitor the reaction process at different water contents. Based on our preliminary results, when PCP is sorbed on the Fe-exchanged clay, the vibrational modes of PCP are clearly perturbed. As the decrease of humidity, the ring vibrations at 1418 and 1382 cm^-1 are perturbed, the –OH bending modes at 1279 cm^-1 shift to 1286 cm^-1, two new vibrational bands at 1359 and 1333 cm^-1 are observed. These changes may be due to the charge transfer between the metal center on the clay and sorbed PCP. Ab initio IR spectra from quantum calculations also match the peaks observed from FTIR measurements and support our proposed reaction pathway.