Transformação de rejeitos de lavagem de bauxita em hidróxidos duplos lamelares e óxidos mistos para aplicações e remediação ambiental.

Abstract

In the present thesis, bauxite washing wast were explored as a low-cost precursor for the preparation of a layered double hydroxide (LDH) nanoadsorbent similar to pyroaurite (Mg-Fe-Al-NO3). Subsequently, this LDH precursor was calcined to obtain the so-called mixed metal oxides (MMO) and encoded as CLDH-X, where X represents the calcination temperature. The LDH was synthesized by the classical co-precipitation method from a solution rich in trivalent cations Fe3+ and Al3+ obtained after acid digestion of the bauxite wash tailings. The synthetic materials HDL and CLDH-X were characterized structurally and morphologically by X-ray diffraction (XRD), infrared spectroscopy (FTIR) and Raman, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and specific surface area measurements (BET method). The first material was tested for the removal of erythrosine B (EB) from aqueous solutions, while the second was applied for the adsorption of ponceau 4R (P4R). Effect of temperature, adsorbent dosage, pH, contact time, and initial concentration were investigated. LDH has proven to be suitable for rapid and efficient removal of EB. In fact, almost complete adsorption of the dye onto the nanostructured pyroaurite occurred within the first 20 min leading to an adsorption capacity (qe) equal to 94.25 mg/g for a specific surface area of 81 m2/g. Its dye removal rate (percentage) increased from 89% to 93% as the temperature increased from 308 K to 328 K. Parallel to the adsorption experiments, using CLDH-X, the so-called memory effect suffered by these mixed oxides was investigated and how this structural regeneration directly affects the removal of P4R. Both CLDH-X showed excellent performance in P4R adsorption, CLDH-400 achieved a removal rate of 99% in just 30 min of contact time, while CLDH-600 achieved a removal rate of 98% in 240 min. This contact time was drastically reduced when the dye solution was heated to 313 K, in just 60 min the removal rate reached 99.5%. It could be inferred that the excellent adsorption capacity of CLDH-400 was mainly influenced by the memory effect. On the other hand, this same phenomenon negatively interfered in the adsorption experiments, causing the removal efficiency to drop by approximately 40%. The results suggest that both HDL and mixed metal oxides are promising adsorbents and useful materials in the treatment of water contaminated by synthetic dyes.