Mn(III) Center Availability as a Rate Controlling Factor in the Oxidation of Phenol and Sulfide on δ-MnO2

Abstract

Manganese oxides are involved in many environmentally important redox reactions. This work focuses primarily on the reaction of phenol and sulfide with δ-MnO₂ (birnessite) and the inhibitory effect of pyrophosphate on these reactions. The reactions were modeled in terms of Mn(III) center surface availability. The model partitioned the observed rate constants between two different hypothetical reaction pathways. One of these pathways was deemed to be dependent on Mn(III) center concentration, while the other was Mn(III) center independent. The relative contribution of each pathway was then calculated based on the equilibrium concentration of free Mn(III) centers at a given pyrophosphate concentration. Using this approach it was possible to model the observed pyrophosphate effects and to predict inhibition with respect to reactant concentration. Finally, the effects of pyrophosphate and orthophosphate on the reaction of sulfide and hydroquinone with δ-MnO₂ were observed and compared to previously published observations. The observed orthophosphate and pyrophosphate effects were consistent with the two reaction pathway model in terms of Mn(III) center complexation. These findings have important implications for modeling and understanding the fate and transport of redox reactive material.