Sustainable conversion of Brazilian Amazon kaolin mining waste to zinc-based Linde Type A zeolites with antibacterial activity

Kaolin is a clay mineral with several applications, especially as filler in the paper industry. Brazil has the largest kaolin processing plant in the world located in the Amazon region. Kaolin mining activities generate high volumes of waste, resulting in additional costs and environmental impacts, and therefore, new approaches for their reduction are necessary. This work elucidates, for the first time, the technological valorization of abundant Amazon Brazilian kaolin mining waste to synthesize pure-phase zeolitic material with the antibacterial application. For the synthesis optimization, it was evaluated the influence of the type of kaolin (natural or thermally treated at 600 C), the crystallization period (4, 6 and 24 h), the suppression of aluminum source (sodium aluminate) and the zinc ion-exchanging procedure. The materials were characterized by various techniques including powder X-ray diffraction (XRD), aluminum nuclear magnetic resonance with magic-angle spinning (Al-27 MAS NMR) and X-ray photoelectron spectroscopy (XPS), N-2 adsorption/desorption isotherms, chemical analysis, thermogravimetric analysis (TGA), infrared spectroscopy (IR) and scanning electron microscopy (SEM). The results reveal that it is possible to direct the synthesis to the different zeolite structure with simple thermal treatment of the raw material, obtaining crystalline Linde Type A (LTA) or sodalite (SOD) zeolites using thermally treated or natural kaolin, respectively. Under the optimized synthesis conditions, the LTA zeolite was obtained after rapid crystallization in 6 h and without commercial aluminum supplementation. The Znexchanged LTA zeolites containing 4 and 7 wt% of Zn were applied as antibacterial materials. They presented growth inhibition zones between 7 and 8 mm for Gram-positive bacteria (Listeria monocytogenes and Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli and Salmonella enterica) while pure kaolin and LTA zeolite without Zn did not show antibacterial activity. These results offer new opportunities on the way that kaolin-mining industries manage their wastes.