Desenvolvimento de um adsorvente à base de engaço de uva e poli(vinil álcool) para remoção de safranina de matrizes aquosas

Abstract

The contamination of water resources with dyes alters light penetration and poses risks to the environment, due to toxicity and potential carcinogenic effects towards animals and humans. Thus, it is imperative to develop effective processes for removing these contaminants. Hence, this work proposes the reuse of grape stalks, a wine industry byproduct, in producing a composite adsorbent for removing safranin cationic dye (SA) from aqueous media. The stalks were blended with poly(vinyl alcohol) (PVA), producing a material with a 5:1 grape stalk-to-PVA ratio, 6% m/mPVA 10% of glycerol, and 2% m/mPVA of citric acid, as a 0.183 cm thickness bar, which was cut into 1 cm² squares. Fourier transform infrared spectroscopy demonstrated grape stalk contribution to functional group diversity on adsorbent surface, such as aromatic rings, methoxy groups, hydroxyls and carbonyls. In turn, the presence of PVA was associated with a slight improvement in the thermal stability of the composite relative to the raw biomass, as indicated by thermogravimetric analysis, as well as with a smoother surface and possible pore obstruction compared with isolated grape stalk particles, as observed in field-emission scanning electron microscopy micrographs. The point of zero charge (pHPCZ) of the composite was determined to be 4.02. Accordingly, electrostatic attraction between SA molecules and negatively charged functional groups on the adsorbent surface is favored at pH values above 4.02. The adsorption process was optimized at a contact time of 240 min, the natural initial pH of the dye solution (~5.9), an adsorbent mass of 600 mg, an agitation speed of 180 rpm, and an initial SA concentration of 195 mg L-¹ at 25 °C. Under these conditions, an adsorption capacity of 10.56 mg g-¹ and a removal efficiency of 65.03% were achieved. Kinetic analysis showed that the adsorption process was best described by the Elovich model and suggested the combined contribution of intraparticle and external diffusion to the overall adsorption kinetics. The equilibrium isotherm was favorable and best represented by the Sips equation. Thermodynamic analysis revealed that the adsorption process was endothermic, spontaneous, and predominantly governed by physisorption. The adsorbent also demonstrated reusability potential for at least five cycles, regenerated with acetic acid 0.1 mol L-¹. The adsorption mechanism was attributed to the combined action of electrostatic attraction and multiple intermolecular interactions, including hydrogen bonding, van der Waals forces, and π-π interactions between the aromatic structures of the adsorbent and the SA molecules. Overall, the proposed composite exhibited adsorption capacity for SA and reusability, while offering easier recovery and reuse than the raw biomass, requiring substantially less PVA than adsorptive films, and promoting the valorization of an abundant agro-industrial byproduct. [resumo fornecido pelo autor]