Kinetics of microwave-enhanced oxidation of phenol by hydrogen peroxide

Abstract

Aqueous solutions of phenol were oxidized by hydrogen peroxide assisted by microwave (MW) irradiation. A simple kinetic model for the overall degradation of phenol in the presence of excess H_2O_2 is proposed in which the degradation rate of phenol is expressed as a linear function of the concentrations of phenol and H_2O_2. A detailed parametric study showed that the degradation rate of phenol increased with increasing [H_2O_2] until saturation was observed. Phenol degradation followed apparent zero-order kinetics under MW radiation or H_2O_2 oxidation. However, after 90 min of irradiation, the observed kinetics shifted to pseudo first order. The overall reaction rates were significantly enhanced in the combined MW/H_2O_2 system, mainly because microwave could accelerate H_2O_2 to generate hydroxyl radical (•OH) and other reactive oxygen intermediates. The observed synergetic effects of the MW/H_2O_2 process resulted in an increased in the net reaction rate by a factor of 5.75. When hydrogen peroxide is present in a large stoichiometric excess, the time required to achieve complete mineralization is reduced significantly.