Free radical behaviours during methylene blue degradation in the Fe2+/H2O2 system

Behaviours of the free radicals during the methylene blue (MB) oxidation process in the Fe2+/H2O2 system were studied to reveal the reason for the low utilization efficiency of H2O2. The roles of O2−∙, ∙OH and HO2∙ radicals were proven to be different in the MB oxidation process. The results showed that O2−∙ radicals had a strong ability to oxidize MB; however, they were not the main active substances for MB degradation due to the low concentration in the traditional Fe2+/H2O2 system. HO2∙ radicals could not oxidize MB. ∙OH radicals were the main active substances for MB oxidation. In the short initial stage, the utilization efficiency of H2O2 was high, because the generation rate of ∙OH was much higher than that of HO2∙. More ∙OH radicals were involved in the MB oxidation reaction. In the long deceleration stage (after the short initial stage), a large amount of H2O2 was consumed, but the amount of oxidized MB was very small. Most of the ∙OH radicals were consumed via the rapid useless reaction between ∙OH and HO2∙ in this stage, resulting in the serious useless consumption of H2O2. It is a feasible method to improve the utilization efficiency of H2O2 by adding suitable additives into the Fe2+/H2O2 system to weaken the useless reaction between ∙OH and HO2∙.