Síntese e caracterização de hidróxidos duplos lamelares a partir da lama vermelha

Abstract

The adsorption is a process in which solids of natural or synthetic origin are used in metal retention, it is an alternative method for wastewater treatment, because combines low cost with high capacity to retain metals. The use of lamellar double hydroxides obtained from the red mud as adsorbent has the advantage of not produce a new generation of residue after the adsorption stage. This study was divided into four parts, with the following goals: synthesize and characterize HDLs formed from the red mud; investigate the anions that are part of the interlayer structure; investigate the ions incorporated into the structure and thermal stability; evaluate the adsorption capacity of HDLs through the removal of ion Cr(VI) in aqueous solution. For the synthesis and characterization of the LDHs, six experiments were performed. The following variables were studied: dripping time (3, 5 and 7h); hydrothermal bath time (5, 10, 20, 30 and 60 days), hydrothermal bath temperature; molar ratio and pH. This study showed that the LDHs formed have good crystallinity, the unit cell parameters a and c and the basal spacing values are similar to those found in the literature. For the investigation of the intercalated anions, molar ratios (M3+ / M2++M3+) close to 1:1were used. No precautions have been taken to avoid contact with atmospheric CO2, therefore some carbonate ions were introduced into the interlayer space. The LDH synthesized at room temperature (298 K), favored the intercalation of carbonate anion in the interlayer space. It occurs probably due the capture of CO2 from the atmosphere. The LDH synthesized at a temperature of 330 K, favored the intercalation of the anion chloride in the interlayer space. The intercalated ions in the structure were investigated using different cation ratios (MII / MII + MIII). The LDH with molar ratio Mg2+ / (Fe3+ + Al3+) higher were more crystalline and shown higher basal spacing values. The decomposition of LDHs occurred in three steps. The first and second step (25-100 °C and 100-235 °C) involve the loss of adsorbed and intercalated water, respectively. In the third step (235-410 °C) occurs dehydroxylation and decomposition of anions in the lamellae. A fourth stage of decomposition can be suggested here. It corresponds to the loss of volatile anionic species (Cl-, CO3 2-) or non-volatile species in which the anions are included by the formation of mixed metal oxides. For the samples synthesized from red mud, the following order of increasing thermal stability can be proposed: LV1> LV3 and LV4> LV2. LDHs were synthesized in the laboratory and its systematic Cr(VI) adsorption behaviour was studied by means of batch experiments. The effect of the parameters pH, concentration of Cr(VI) and the molar ratio MII/MII are here reported. The optimum pH and molar ratio required for maximum adsorption were found to between pH 4.0 – 6.0 and ratio between 3:1 – 4:1, respectively.