Intrinsic fast kinetics on the degradation of azo dye by iron in alkaline condition

It is well-known that the performance of reducing azo dyes using iron is significantly weakened with increasing pH, leading to the weak degradation performance in alkaline condition. Herein, an unexpected promoting effect of high alkalinity on the degradation of azo dye has been found, and the crucial impact of oxygen and the underlying mechanism of high alkalinity advantage have been revealed. The degradation performance can be largely restrained when oxygen participates in the reaction, as the adsorption of oxygen on iron hinders the electron transfer between iron and azo dye and covers up the real effect of hydroxyl ions on the reaction, leading to the conventional misunderstanding. The effect of oxygen on the reaction thermodynamics is minutely investigated by electrochemical tests, and degradation experiments indicate that large improvement in kinetic rate constant can be achieved in alkaline condition by eliminating the dissolved oxygen. It is experimentally found that, at pH 10.0, up to 89.9% improvement in kinetic degradation rate can be achieved by removing dissolved oxygen in the degradation of Orange II (25 mg L − 1) using iron. Moreover, less than 10 min is needed to decompose half of the dye in the solution (12.5 mg L − 1) of pH 10.0.