Extração, caracterização e utilização de lignina de Eucalyptus grandis na remoção do corante azul de metileno

Abstract

In the last decades, environmental problems have become increasingly critical and frequent mainly due to excessive population growth and increased industrial activity. The textile sector stands out in this scenario for generating large volumes of wastewater containing dyes. These wastewaters, when discarded in an inadequate way in the environment, are responsible for the contamination of several ecosystems, mostly of the aquatic systems, which are indispensable to the life and maintenance of the balance in the planet. Due to the greater rigor of legislation and increasing environmental awareness, the improvement and development of new technologies for this purpose has been the subject of great interest by the scientific community. In this context, adsorption has shown to be a very promising technique for the removal of color, since it has a relatively low cost, besides having considerable efficiency. Considering these aspects, the aim of this work was to evaluate the efficiency of the Eucalyptus grandis lignin in the removal of the methylene blue dye from synthetic aqueous solutions. Initially, the eucalyptus sawdust was collected, processed and subjected to the extraction of the lignin from this wood residue. Then, isolated lignin was characterized by several techniques, such as elemental analysis, Fourier transform infrared spectroscopy, solid-state carbon-13 nuclear magnetic resonance spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy and pyrolysis coupled to gas chromatography/mass spectrometry. The specific surface area, the average pore size and the pH at the zero charge were also determined. The adsorption capacity of eucalyptus lignin was further investigated in a batch system, after the influence of experimental parameters, such as initial dye concentration, lignin mass, medium pH, stirring rate and temperature were previously evaluated. In general, the characterization analyzes showed the presence of several oxygenated functional groups, among them phenolic hydroxyls and methoxyls, and aromatic rings in the chemical structure of lignin, accompanied by a very heterogeneous morphology, as well as a relatively low surface area (20 m2 g-1) compared to other lignocellulosic materials. It has further been found that the pHPCZ required to neutralize the surface of the material is equal to 3.15, and that this characteristic directly influences the adsorption capacity of the material. Regarding the adsorption equilibrium, the data were well fitted with the Langmuir isotherm (R² = 0.9540) presenting, respectively, an isothermal constant of Langmuir (kL) of 0.0228 L mg-1 and a maximum adsorption capacity (qmax) of 31.97 mg g-1. From the thermodynamic point of view, the results evidenced a spontaneous (with values of ΔG°ads < 0), physical and endothermic (ΔH°ads = + 38.64 kJ mol-1) process, that occurs with an increase in the randomness of the system (ΔS°ads = + 141.26 J mol-1 K-1). Concerning the adsorption kinetics, the pseudo-second-order model was the one that best described the process. Lastely, the lignin of Eucalyptus grandis showed to be a good adsorbent for the removal of the methylene blue cationic dye, which can be reused at least four times without significant loss of its adsorbent capacity.(sic)