Síntese e caracterização de carbonos mesoporosos visando a aplicação na adsorção de corantes têxteis catiônicos e aniônicos

Abstract

Synthetic dye effluents are generated on a large scale annually. Adsorption with biomass-derived activated carbons is widely employed to treat these effluents, but there is difficulty in controlling pore size and textural characteristics that undermine the efficiency of the process. The "hard templating" method can be applied using a predefined template to enhance this control. Therefore, the objective of this work was to produce mesoporous carbons by the hard templating method using mesoporous silicas and apply them in textile dye adsorption. Mesoporous carbons were synthesized using sucrose as a carbon source and Aerosil A-380 and A-200 silicas as templates under a controlled N2 atmosphere at 800°C. Characterizations indicated that CM-A200 has a particle size of 0.61 mm and CM-A380 of 1.22 mm. SEM identifies both solids heterogeneous, rough, and porous surfaces. EDS analysis revealed a predominance of carbon and oxygen, with smaller amounts of silicon (Si). The HRTEM technique identified a network of mesopore ordering in a disorganized pattern. The pHpcz obtained was 6.2 and 7.1 for CM-A200 and CM-A380, respectively, with a large range of pH (5 < pH < 9) with neutral residual charge. N2 physisorption identified the formation of mesoporous solids with associated micropores, where CM-A200 and CM-A380 showed respectively surface areas of 1081 m² g-1 and 766 m² g-1, pore volumes of 0.62 cm³ g-1 and 0.48 cm³ g-1, and pore diameters of 8.30 nm and 5.93 nm. Adsorption experiments showed better performance of the anionic reactive Blue 5G dye at pH 2 and cationic methylene blue dye at pH 10. During adsorption equilibrium, both solids showed sigmoidal isotherm behavior (Type S), with the BET model being the best fit. FTIR identified the presence of hydroxyls, C-H groups, C-OH, C-O, C=O and C=C groups, observing possible mechanisms related to van der Waals interactions, hydrogen bonds, and π-π stacking. For the AM adsorption, the best capacities were 177 mg L-1 to CM-A380 and 193 mg L-1 to CM-A200. In A5G adsorption, CM-A200 showed a capacity of 191 mg g-1 and CM-A380 of 153 mg g-1. The RITM model was the best for CM-A380 and to CM-A200 in a temperature of 30°C, while the MMA model was the best fit to the CM-A200 in 40°C and 50°C. The difference in textural properties impacted on the molecule diffusion and, consequently, adsorption capacity and mass transfer mechanisms (related to the pore filling effect). In general, it was possible to obtain relevant information about the mechanisms and get fundamental data for the design, optimization, and possible scale-up of the adsorption process of these materials.