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 (MnO 4 − ) and manganese dioxide (MnO 2 ) under acidic conditions, this study was conducted to systematically investigate the oxidation kinetics and mechanisms of contaminants by MnO 4 − and MnO 2 under acidic conditions. The process of phenol oxidation by MnO 4 − showed an auto-accelerating trend at pH 2.0-6.0, which was mainly associated with the generation of MnO 2 , an oxidant more active than MnO 4 − under acidic conditions. However, an auto-inhibiting trend was observed during phenol oxidation by MnO 2 , which could be ascribed to the generation of Mn( ii ) and its subsequent adsorption on the surface of MnO 2 . The presence of excessive pyrophosphate (PP) greatly accelerated the oxidation of phenol by MnO 4 − and MnO 2 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 MnO 2 and Mn( ii ) to Mn( iii )-PP, respectively, in the processes of organic contaminant oxidation by MnO 4 − and MnO 2 . In short, the reduction products of MnO 4 − and MnO 2 determined the auto-accelerating or auto-inhibiting phenomenon observed in the process of phenol oxidation by MnO 4 − and MnO 2 . The accelerating and inhibiting behavior of organic contaminant oxidation by MnO 4 − and MnO 2 in the presence and absence of pyrophosphate (PP) under acidic conditions.