Manganese (Mn) oxides are redox active minerals that are formed naturally, generated as byproducts of engineered systems (e.g., acid mine drainage remediation), or synthesized for water treatment applications. These minerals react selectively with common functional groups in organic chemicals, including phenols. Mn oxides can therefore be applied to oxidize phenolic compounds in environmentally relevant systems (e.g., in soils or at the sediment-water interface) or be applied in passive <i>in situ</i> treatment systems. In addition to being present in many contaminants of concern, phenolic functional groups are common in naturally occurring dissolved organic matter (DOM). Therefore, Mn oxides can also react with DOM, potentially complicating the application of these minerals for water treatment. In this talk, we will take a mechanistic look at the reactivity of manganese oxides with phenols as this system becomes increasingly complex. The reactivity of individual phenols with Mn oxides, including the rate limiting step of the reaction, depends on the structure of the phenolic compound. However, common quantitative structure activity relationships are ineffective at predicting the behavior of complex contaminants or reactions with non-synthetic manganese oxides . In phenol mixtures, the impact of competition among phenols also varies with phenol structure. Oxidation rates of electron transfer-limited phenols (i.e., triclosan, resorcinol, bisphenol A) are inhibited in mixtures, whereas the oxidation rate and extent of sorption for sorption-limited 4,4’-biphenol is enhanced in mixtures.  In more environmentally relevant systems, DOM reacts with Mn oxides to form organic matter that is generally more aliphatic and lower in apparent molecular weight. However, the extent of reaction depends on water type (e.g., wastewater, rivers) and highly aromatic DOM undergoes greater changes.  These results have important implications for predicting the reactivity of phenols with Mn oxides in natural and engineered systems.

Reactivity of manganese oxides with phenolic moieties in organic contaminants and dissolved organic matter

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