Cr(III) exchange on zeolites obtained from kaolin and natural mordenite

Abstract

Zeolites with high Cr(III) exchange capacity were synthesized from kaolin and natural mordenite. The intermediate phases and final products were characterized by X-ray diffraction, FTIR spectroscopy, scanning electron microscopy, thermogravimetric analysis, N-2-adsorption and chromium exchange capacity (CrEC). In addition, precise zeolitic phases were identified using the TOPAS program based on Rietveld refinement. Hydrothermal synthesis from kaolin leads to the formation of a mixture of zeolites-X and A. At higher hydrothermal treatment period, zeolite-X (space group Fd-3) appears as the dominant phase. In the synthesis from natural mordenite a mixture of zeolite-Y (Fd-3m) and orthorhombic zeolite-P2 (Pnma 62) is formed, obtaining a more pure zeolite-P with the increase in reaction time. The differences in the course of the crystallization/transformation process in both systems are explained in terms of the differences in the dissolution rate of the starting materials in alkaline medium. The CrEC of synthesis products was determined by the type of zeolite and the fraction of amorphous phase in the solid product. It was found that the highest CrEC is obtained for synthesis products containing FAU-type zeolites. The chromium exchange on FAU zeolites is favored due to the larger pore opening, which facilitates the diffusion of large hydrated chromium ions into the internal cation exchange sites. Synthesized zeolite products presented higher Cr(III) exchange capacity than commercial zeolites. These results suggest that the use of these synthesized materials in Cr(III) removal from industrial wastewater could be promising.