Identifying the mechanisms of cation inhibition of phenol oxidation by acid birnessite

Abstract

Many phenolic compounds found as contaminants in natural waters are susceptible to oxidation by manganese oxides. However, there is often variability between oxidation rates reported in pristine matrices and studies using more environmentally relevant conditions. For example, the presence of cations generally results in slower phenolic oxidation rates. However, the underlying mechanism of cation interference is not well understood. In this study, cation co-solutes inhibit the transformation of four target phenols (bisphenol A, estrone, p-cresol, and triclosan) by acid birnessite. Oxidation rates for these compounds by acid birnessite follow the same trend (Na+>K+>Mg2+>Ca2+) when cations are present as co-solutes. We further demonstrate that the same trend applies to these cations when they are absent from solution but pre-exchanged with the mineral. We analyze valence state, surface area, crystallinity, and zeta potential to characterize changes in oxide structure. The findings of this study show that pre-exchanged cations have a large effect on birnessite reactivity even in the absence of cation co-solutes, indicating that the inhibition of phenolic compound oxidation is not due to competition for surface sites, as previously suggested. Instead, the reaction inhibition is attributed to changes in aggregation and the mineral microstructure.