ABSTRACT

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 ${\text{O}}_2^{-\bullet }$, ${}^{\bullet }\text{OH}$ and ${\text{HO}}_2^{\bullet }$ radicals were proven to be different in the MB oxidation process. The results showed that ${\text{O}}_2^{-\bullet }$ 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. ${\text{HO}}_2^{\bullet }$ radicals could not oxidize MB. ${}^{\bullet }\text{OH}$ 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 ${}^{\bullet }\text{OH}$ was much higher than that of ${\text{HO}}_2^{\bullet }$. More ${}^{\bullet }\text{OH}$ 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 ${}^{\bullet }\text{OH}$ radicals were consumed via the rapid useless reaction between ${}^{\bullet }\text{OH}$ and ${\text{HO}}_2^{\bullet }$ 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 ${}^{\bullet }\text{OH}$ and ${\text{HO}}_2^{\bullet }$.