Mechanochemical degradation of pentachlorophenol onto birnessite

The efficiency of mechanochemical treatments in degrading pentachlorophenol (PCP) molecules in presence of the highly reactive phyllomanganate birnessite, and without using organic solvents, has been demonstrated in this study. By combining the information from solid-phase analyses with that from solvent extract analyses, and comparing the resulted data with those from previous batch-type experiments, it has been possible to get a comprehensive picture of the most reliable reaction mechanism of PCP molecules onto KBi surfaces. This allowed the individuation of specific sites in the synthesized oxides on which pentachlorophenol was preferentially adsorbed, and thus degraded. The mechanochemically induced PCP degradation mainly occurs through the formation of a surface monodentate inner-sphere complex between the phenolic group of the organic molecules and the structural Mn(IV), as indicated by changes induced in the MnO6 layers of birnessite as a consequence of the prolonged milling with the pollutant. The mechanism implies reduction of Mn(IV), consequent formation of both Mn(III) and new vacancies, and release of free Mn2+ ions. This is quite low if compared to the formation of Mn(III). Therefore the cannibalization of the birnessite structure due to the redox reaction induced by mechanochemical treatments with PCP can be considered negligible. PCP mechanochemical degradation mainly occurs by dehalogenation.

The existence of a lot of worldwide pentachlorophenol-contaminated sites has induced scientists to concentrate their effort in finding ways to degrade it. Therefore, an effective tool to decompose it from soil mixtures is needed. In this work the efficiency of the phyllomanganate birnessite (KBi) in degrading pentachlorophenol (PCP) through mechanochemical treatments was investigated. To this purpose, a synthesized birnessite and the pollutant were ground together in a high energy mill. The ground KBi-PCP mixtures and the liquid extracts were analyzed to demonstrate that mechanochemical treatments are more efficient in removing PCP than a simple contact between the synthesized birnessite and the pollutant, both in terms of time and extent. The mechanochemically induced PCP degradation mainly occurs through the formation of a surface monodentate inner-sphere complex between the phenolic group of the organic molecules and the structural Mn(IV). This is indicated by the changes induced in birnessite MnO6 layers as a consequence of the prolonged milling with the pollutant. This mechanism includes the Mn(IV) reduction, the consequent formation of Mn(III) and new vacancies, and free Mn2+ ions release. The PCP degradation extent is limited by the presence of chloro-substituents on the aromatic ring. © 2012 Elsevier B.V.