Sílica mesoporosa esférica para aplicação em processo de sorção de corantes catiônicos em meio ácido

Abstract

The demand for novel materials aiming at environmental applications to mitigate the impact caused by the increase in the generation of industrial waste has grown and suggested the approximation of university and productive sector. The textile industry is among the main polluters in the world and this is directly associated with the high production of the sector, and water pollution from dyeing processes is one of the main concerns. The adsorption processes for removing dyes from effluents are among the most investigated and used technologies by the productive sector, however, given the multiplicity of factors and the diversity of characteristics of the textile effluents, there is a demand for novel materials or even for adaptations of materials used for improve the effectiveness of available treatments. In this sense, spherical mesoporous silica (SMPS) was prepared with adaptations in the methodology used for the synthesis of silica MCM-41 in order to apply the method on an industrial scale. Its effectiveness in removing cationic dyes in acidic medium was evaluated using the dye model methylene blue and, for comparison, the anionic dyes Congo Red and Acid Black 172. SMPS was characterized by textural analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrational infrared spectroscopy (FTIR) and pH of zero charge point (pHPZC). SEM images showed the formation of well-defined spherical particles with sizes ranging from 0.5 to 2.0 m. The textural analysis showed a narrow distribution of the pore size in two sets, 1.9 nm and the other between 3 and 6 nm, indicating that the solvothermal process employed was successful. The heat treatment of SMPS was monitored by FTIR and no vibrational band was observed between 2800-2950 cm-1, indicating the removal of the structuring agent of cetyltrimethylammonium bromide (CTAB). SMPS presented a specific area of 348 m² g-1 and pore volume of 0.847 cm³ g-1 . Accuracy in obtaining the pHPZC was not possible, but the result suggested a value of approximately 0.5, being a result of the applied thermal treatment and conversion of silanol to siloxane groups, more hydrophobic and susceptible to protonation only in extremely acidic conditions. The adsorption results showed that SMPS was about four times more effective in removing the methylene blue cationic dye than for anionic dyes. The adsorption kinetics of methylene blue followed the pseudo-second order model with a high adsorption rate in the first 20 minutes of contact. The removal of the cationic dye was more effective at a lower pH, contrary to most studies in which adsorption occurs on surfaces covered with hydroxyl groups, whose pH range that favors adsorption is basic due to the formation of negative charge from the deprotonated hydroxyl groups. The adsorption of methylene blue in SMPS was ruled by physical forces (physisorption) and fitted to the Freundlich isotherm (multilayer) and Temkin isotherm (adsorbate-adsorbate interaction) models. SMPS showed the potential to be applied as an alternative/complementary adsorbent in processes of removal of cationic dyes in acidic medium and, in addition, total regeneration capacity by thermal treatment.