Synthesis of different crystallographic FeOOH catalysts for peroxymonosulfate activation towards organic matter degradation

In this study, different crystalline structures of FeOOH have been prepared. alpha-FeOOH was synthesized through a hydrothermal method, whereas beta-FeOOH was synthesized via a direct hydrolysis method. Moreover, gamma- and delta-FeOOH were prepared by precipitation methods through slow and quick oxidation, respectively. On this basis, their crystal structure, morphology, and surface area were measured. Then, all the synthesized materials were applied to activate peroxymonosulfate (PMS) to generate sulfate radicals (SO4-center dot) for acid orange 7(AO7) degradation. Compared with alpha-FeOOH, beta-FeOOH, and gamma-FeOOH, delta-FeOOH showed more efficient decolorization of AO7 in the catalytic system because of its abundant surface area and crystalline structure. The effects of several parameters in the delta-FeOOH/PMS/AO7 system were investigated. The results show that the initial pH, which is related to the features of surface hydroxyl groups, is the decisive factor, and excellent catalytic activity is maintained in the pH range 5-8. The increase of catalyst dosage and appropriate increase of PMS concentration contributed to promote the degradation effect. However, self-quenching was observed in a high PMS concentration system. Moreover, delta-FeOOH was stable after six consecutive cycles, and the leaching of iron ions was negligible. According to the quenching test and electron spin resonance analysis, both SO4-center dot and (OH)-O-center dot were the dominant radicals for AO7 degradation.