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The Fe2+/Fe3+ redox cycle is the rate-determining step of hydroxyl radical formation in the photo-Fenton system. In this research, methyl orange (MO) was used which was degraded using Fe3O4/g-C3N4 in photocatalysis and photo-Fenton systems. The influence of wavelength irradiation on the redox cycle of Fe2+/Fe3+ ions was investigated. The results showed that the MO degradation rate constant reached 0.152 min−1 in photo-Fenton and 0.090 min−1 in photocatalysis using UV–vis light, which is 50 and 30 times higher than Fenton system. Light irradiation affects iron-leaching process and iron photoreduction. The iron leaching occurs more frequently at high energies and does not occur at λ ≥ 420 nm. In photo-Fenton, interaction between H2O2 and Fe3O4 surface makes the leaching process easier to occur. The photoreduction results show that the effective conversion of Fe3+ to Fe2+ occurs at λ ≥ 380 nm with the percentage of Fe2+ reaching ∼90 % in both systems.
Permana et al. (Tue,) studied this question.