Auto-accelerating and auto-inhibiting phenomena in the oxidation process of organic contaminants by permanganate and manganese dioxide under acidic conditions: Effects of manganese intermediates/products

Considering the confused/controversial kinetics law of organic contaminant oxidation by permanganate (MnO₄^-) and manganese dioxide (MnO₂) under acidic conditions, this study was conducted to systematically investigate the oxidation kinetics and mechanisms of contaminants by MnO₄^- and MnO₂ under acidic conditions. The process of phenol oxidation by MnO₄^- showed an auto-accelerating trend at pH 2.0-6.0, which was mainly associated with the generation of MnO₂, an oxidant more active than MnO₄^- under acidic conditions. However, an auto-inhibiting trend was observed during phenol oxidation by MnO₂, which could be ascribed to the generation of Mn(ii) and its subsequent adsorption on the surface of MnO₂. The presence of excessive pyrophosphate (PP) greatly accelerated the oxidation of phenol by MnO₄^- and MnO₂ under acidic conditions due to the generation and accumulation of highly reactive Mn(iii)-PP. The presence of PP also changed the manganese species at equilibrium from MnO₂ and Mn(ii) to Mn(iii)-PP, respectively, in the processes of organic contaminant oxidation by MnO₄^- and MnO₂. In short, the reduction products of MnO₄^- and MnO₂ determined the auto-accelerating or auto-inhibiting phenomenon observed in the process of phenol oxidation by MnO₄^- and MnO₂.