Effect of red mud added to zeolite LTA synthesis: Where is Fe in the newly-formed material?

In this work, silica and alumina pure sources were used to perform the synthesis of zeolite A (LTA) by both conventional and pre-fused hydrothermal processes. Increasing amount of red mud (RM, a waste formed during the caustic leaching of bauxite to produce alumina and rich in Fe) was added during the synthesis processes. We attempted to investigate if the use of pure sources with the addition of different amounts of red mud promotes, or hinder, the crystallization of the zeolite A and if Fe is structurally-incorporated into the newly-formed zeolite structure (i.e., as framework or extra-framework cation) or if it is concentrated in a proper Fe-rich phase. The results carried out using a multi-methodological approach (laboratory X-ray powder diffraction; synchrotron radiation powder X-ray diffraction; scanning electron microscopy and electron transmission microscopy coupled with EDX elemental mapping) show that LTA zeolite crystals (with cubic morphology) can be efficiently formed using pure source combined with variable amount of red mud. The use of complementary analytical techniques indicate that Fe coming from the red mud is not structurally-incorporated into the newly-formed zeolite crystals, but it is mainly concentrated in nano-clusters of Fe-oxides (with spinel-type structure) located on the zeolite crystal surface. The utilization of non-conventional Fe source, here represented by red mud, can be considered as the first step toward a new solidification/stabilization process for this waste, as dictated by the regenerative economy route. These data make an important contribution to understanding the role played by surface phenomena on zeolite crystals when transition elements are used in the synthesis procedure.