Catalytic activity of iron species in layered clays for photodegradation of organic dyes under visible irradiation

The iron species in layered clays are active for catalytically oxidizing synthetic dyes with H2O2 under visible irradiation (λ > 420 nm). Three layered clays, Montmorillonite, Laponite, and Nontronite, in which the iron species exist in different chemical environment, were used to investigate the role of the iron species in the clays. It was found that the reactivity of the iron species greatly depends on their chemical environments. The exchangeable interlayer iron ions in the clay exhibit much better ability to catalyze the mineralization of malachite green (MG) than the structural iron at the centers of octahedrons that are sandwiched by two layers of SiO4 tetrahedrons. The interactions among the foreign ligands, such as phosphate and EDTA, and the two types of iron species are dissimilar, causing the different effects of the ligands on the degradation kinetics. The EPR results show explicitly that the reaction over the two iron species proceeds through different pathways. This explains the differences in the degradation kinetics for the photodegradation of MG by H2O2 over different clays. These iron-containing clay minerals are chemically and mechanically stable, we do not observe any noticeable loss of activity and damage of the clay structure after 14 recycles.