Facile synthesis of alpha-FeOOH/gamma-Fe2O3 by a pH gradient method and the role of gamma-Fe2O3 in H2O2 activation under visible light irradiation

The development of an efficient and recyclable catalyst at a wide working pH range is crucial for the practical application of heterogeneous Fenton process. In this work, magnetic alpha-FeOOH/gamma-Fe2O3 nanocomposite was one-pot synthesized by a simple pH gradient method. The obtained sample was found to be an efficient Fenton catalyst for the degradation and mineralization of tetracycline hydrochloride (TC) over a wide pH range under visible light irradiation. It exhibited higher adsorption capacity towards TC at neutral pH, which facilitates the surface reactions of TC with active species. The contribution of homogeneous reactions can be ignored at neutral and alkaline pH, but increased significantly at acidic pH. The mechanism for H2O2 activation by alpha-FeOOH/gamma-Fe2O3 at neutral pH is quite different from that by FeOOH. In the alpha-FeOOH/gamma-Fe2O3 photo-Fenton process,gamma-Fe2O3 could act as an electron trap and inhibit the reaction between photogenerated electrons and H2O2. Consequently, the production of HO center dot was reduced, whereas more holes were separated and the production of center dot O-2(-) from the reaction of holes with H2O2 was significantly enhanced. TC should be degraded by HO center dot, holes and center dot O-2(-). Finally, the intermediates were determined by liquid chromatography-mass spectrometry and a possible pathway for TC degradation was proposed.