Avaliação do uso de adsorventes a base de óxido de grafeno e celulose para a remoção de íons cromo hexavalente de soluções aquosas

Abstract

The water contamination is characterized as one of the major environmental problems caused by the increase of manufacturing industries, among these contaminants, the chromium ions can be highlighted. High concentrations of this substance in water can cause serious harms to human health, especially in its hexavalent form. In this way, the objective of this work consists in evaluate the adsorption capacity of hexavalent chromium ions, in aqueous solution, using graphene oxide, cellulose aerogels, and cellulose aerogels containing graphene oxide. To this end, initially, the synthesis of graphene oxide was performed by Hummers method and the manufacture of cellulose aerogels, from nanocellulose, and cellulose aerogels containing graphene oxide, in concentrations of 0,5 and 1% (w/w), being these produced by freeze-drying process. The characterization of these materials was carried out in order to verify their physical, morphological and chemical properties. The adsorption tests were, then, carried out in a batch system, evaluating the influence of experimental factors such as the system pH, initial solute concentration and temperature. Parameters such as adsorption isotherms models, through reaction models, as well thermodynamic parameters of the process were also evaluated. Finally, an attempt was made to recover the adsorbent materials, being realized two regeneration cycles. From this, it was found that the Langmuir isotherm model was the one that best fit the experimental data, for all adsorbent materials investigated, demonstrating that adsorption occurs through the formation of a monolayer, with energetically homogeneous sites. In relation to the adsorption kinetics, it was observed that the pseudo-second order reaction model presented the best fit to the experimental data, suggesting, thus, that the diffusion through the bulk film and the intraparticle diffusion are the controlling steps of this process. The thermodynamic parameters verified indicate that the chromium adsorption process occurs spontaneously and that the same happens through physical adsorption, being the same of exothermic nature. In this way, from this study, it can be concluded that the evaluated materials have a good ability to remove hexavalent chromium from aqueous solutions, obtaining adsorbent amounts of 142,1 mg.g-1, 108,52 mg.g-1, 129,22 mg.g-1 and 121,78 mg.g-1 for graphene oxide, pure cellulose aerogel, and aerogels containing 0.5 and 1% (w/w) of graphene oxide, respectively. Furthermore, it was possible to verify that they can be reused in more than one removal cycle, without losing properties in up two regeneration cycles. In this way, it is observed that materials based on cellulose and graphene oxide can be used in adsorption process for the removal of hexavalent chromium from aqueous solutions. This, in this way, demonstrates the potential of using materials from renewable sources in adsorption processes. [resumo fornecido pelo autor]