The role of quinone cycle in Fe2+-H2O2 system in the regeneration of Fe2.

The reaction between Fe2+ and H2O2 generates highly reactive ·OH. However, the weak conversion from Fe3+ to Fe2+ limits its continuous reaction. Here, the difference between the Fenton system and modified Fenton system for the regeneration of Fe2+ was analyzed. A UV-vis spectrometer and redox potential measurements were used to detect Fe2+ concentration. Results indicated that Fe2+ could be better regenerated in the modified Fenton system. The regeneration of Fe2+ was facilitated by the consumption of NH2OH, while in hydroquinone (HQ)- and 1,4-bezoquinone (1,4-BQ)-modified Fenton systems, the quinone cycle could be built up and Fe3+ could be converted to Fe2+ continuously. However, results showed that HQ and 1,4-BQ reacted with ·OH, which caused a gradual decline in the enhancement effect. In order to keep Fe2+ concentration stable for a longer time, the influence of [HQ/1,4-BQ]0/[Fe2+]0 on Fe2+ concentration was carefully studied. When the mole ratio was 5:1, Fe2+ concentration remained nearly 90% of total iron at 40 min. But when the mole ratios were 0.5:1 and 0.1:1, Fe2+ concentration decreased to a very low level at 20 min. Oxidation-reduction potential (ORP) results further confirmed the role of quinone cycle.