Síntese e caracterização da piroaurita e hidroalcita a partir do horizonte laterítico da mina de bauxita de Juruti (PA).

Abstract

Anionic clays, also known as LDH, are nanomaterials with diverse technological applications, as well as versatile nanocomposites with properties such as ion exchangers, adsorbents, catalysers, and the ability to simulate cations and anions in their lamellar structure, which expands their applications as environmental remediators. Mining activities in the Amazon are growing exponentially, so it is necessary to find sustainable and practical ways to minimize environmental charges. Another possibility is the reuse of mining waste with specific applications in the recovery of chemical elements through concentration processes in tailings piles or treatment of acid mining drainage. Bauxite deposits have been explored for 15 years in the Juruti city, Pará state, and are part of the world's largest reserves, with high levels of aluminium oxide (Al2O3). The exploration and processing of aluminium mining leave large lakes where the disposal of tailings occurs. There are several studies that aim to use bauxite dispersion as a starting material for the synthesis of LDH, however this work focuses on the synthesis of LDH from the lateritic horizon, which makes up one of the six horizons of the lateritic-bauxitic profile of Juruti mines. The high levels of Fe3+ and Al3+ observed in the lateritic horizon allow its use as a starting material, together with magnesium chloride and sodium hydroxide to create optimized conditions for the synthesis of LDH by the coprecipitation method at variable pH. X-ray diffraction (XRD), scanning electron microscopy (SEM), spectrometry (FTIR), thermal analysis (TG-DSC) and surface area analysis (BET) techniques were used to characterize the synthesized LDH. The crystalline phases include pyroaurite (Mg2Fe(OH)16CO3.4H2O) and hydrotalcite (Mg6Al2(OH)16CO3.4H2O), in addition to brucite, spinel (mixed oxide) and hematite as secondary phases. Scanning electron microscopy (SEM) showed characteristic LDH morphologies, with small crystals with hexagonal habits, rougher structures and plate-like shapes that resemble honeycombs, when grouped, Infrared spectra (FTIR) indicated the presence of carbonate ions in the interlayer space in both types of LDH, through the characteristic weak absorption bands around 1373.51 –1394.67 cm-1. The conjugate TG/DSC curves indicated three endothermic events for the pyroaurite-type LDH, in addition to an exothermic event at a higher temperature, forming spinel, while the hydrotalcite-type LDH presented four endothermic events. The total mass loss of hydrotalcites is 15%, while that of pyroaurites reaches approximately 50%. Regarding the surface and porosity analysis (BET), a specific surface area (ASEBET) of 74.397 m2/g and a pore volume (VPBJH) of 0.173 cm3g-1 were obtained, and the average pore diameter was 18.64 Å. The ASEBET values of the hydrotalcite-type LDH are considerably higher, with similar VPBJH and pore radius values. The physisorption isotherm is type IV with hysteresis curve H3 that is associated with mesoporous structures as some clays and particle aggregates, such as plates with narrow slit-type pores. Thus, from the defined synthetic routes and having the lateritic horizon as a low-cost starting material, it is possible to continue adsorption tests using pyroaurite and hydrotalcite, synthesized and characterized in this work, with efficient potential to mitigate metal contamination in water resources.