Influence of pH and exchangeable cation on oxidation of methylphenols by a montmorillonite clay

Research efforts on the use of clay minerals as environmental reactants to “transform” mobile organic contaminants to stationary compounds are needed for eventual field solutions to reduce organic contaminant migration. This study addresses the oxidation of 2,6-dimethylphenol (DMP) and o-methylphenol (MP) by Na+-, Ca2+-, Al3+- and Fe3+-saturated montmorillonite with pH values varying between 2 and 12. Phenolic dimers, trimers and tetramers resulting from oxidative coupling have been identified by mass spectrometry. Because of alumina sheet hydrolysis at low pH conditions, more Al is exposed at the edges of the mineral particles, thus resulting in the highest oligomer yields. The dissociated phenolic -OH group (pH > 9) is a greater benzene ring activator than the non dissociated -OH group, and shows a greater tendency for oxidation by montmorillonite. Exchangeable Fe3+ accounts for a significant portion of the oxidizing power of clay. Simple factors such as water, exchangeable cation and pH can drastically affect the ability of clays to oxidize phenols to oligomers.