Influência do método de secagem nas características físico-químicas de hidróxidos duplos lamelares e sua utilização como material adsorvente

Abstract

Layered Double Hydroxides (LDH) are natural or synthetic materials nanostructured in two dimensions that feature positively charged layers with interspersed anions to balance charge and structure. Being recognized as promising materials for various applications, a complete exploration of their possible attractive properties and their synthesis process is essential. However, drying, a necessary step in the process, is still little studied. This work aimed to evaluate the influence of the drying method on the physicochemical characteristics in the synthesis of MgAl–CO3/LDH and its use as an adsorbent material. LDH synthesis was carried out using the coprecipitation method, maintaining a Mg/Al ratio of 2:1, and dried in an oven, freeze dryer and atomizer. The oven drying experiments were carried out at temperatures of 60 °C, 70 °C, 80 °C and 90 °C. For freeze-drying, the samples were frozen at a temperature of -50 °C and dried at temperatures of 30 °C, 40 °C, and 50 °C. The experiments for spray drying evaluated the drying temperature and concentration of the feed paste, in addition to discussing a statistical approach to obtain the volumetric heat transfer coefficient through an experimental design. The results obtained for drying kinetics and rate, effective diffusivity and Biot number indicate that external conditions control LDH drying. MgAl-CO3/LDH microspheres were obtained by atomization in all conditions studied, presenting a smooth surface and no holes. XRD results showed that changes in the drying process produced LDHs with variation in basal spacing, lattice parameter c and average crystallite size. The FTIR analysis presented the three characteristic types of chemical bonds in LDH, identifying the functional groups and the intercalated anion. The SEM showed different morphologies for the HDL obtained, with the oven-dried HDLs presenting distinct, defined and angled shapes of varying sizes, the freeze-dried ones presenting a rough and inhomogeneous surface, and the atomized ones in a spherical format, showing a difference in the size of the microspheres with the drying parameter. Preliminary adsorption tests were carried out, and LDH efficiently removed some pollutants. Given the results obtained, the materials produced showed physical-chemical and structural differences with the variation in the drying stage, directly influencing the adsorption process.