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 $$ , $$ and $$ radicals were proven to be different in the MB oxidation process. The results showed that $$ 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. $$ radicals could not oxidize MB. $$ 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 $$ was much higher than that of $$ . More $$ radicals were involved in the MB oxidation reaction. In the long deceleration stage (after the short initial stage), a large amount of H₂O₂ was consumed, but the amount of oxidized MB was very small. Most of the $$ radicals were consumed via the rapid useless reaction between $$ and $$ 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 $$ and $$ .