Recuperação de prata metálica de pilhas do tipo botão para a preparação de híbridos de quitosana com propriedades bactericidas

Abstract

The recovery of metals is an extremely important issue that has attracted the attention of several researchers, both for the environmental and economic concerns. Some metals, such as silver or its derivatives, can be incorporated into biopolymers and used to treat industrial wastewaters. In view of these facts, this work aimed to recover metallic silver from discarded Ag-Zn button cells and use it in the preparation of silver nitrate (AgNO3) for later incorporation into a chitosan matrix, aiming the production of hybrid materials with bactericidal properties. At the end of these two processes (recovery and synthesis), metallic silver and AgNO3 were analyzed by several techniques, including energy dispersive X-ray fluorescence (EDX), scanning electron microscopy with field emission gun (SEM/FEG), energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD), which evidenced a high degree of purity for both materials (99.3% for metallic silver and 99.4% for silver nitrate), and a dendritic morphology and a face-centered cubic crystalline system for silver and a plaque-shaped morphology and orthorhombic crystalline system for AgNO3. Subsequently, silver nanoparticles (AgNPs) were synthesized with different concentrations of AgNO3 and pH ranges, using sodium borohydride and sodium citrate as a reducer and stabilizer, respectively. The obtained AgNPs were then evaluated by molecular absorption spectroscopy in the ultraviolet and visible region and in terms of inhibition of bacterial growth, using Escherichia coli and Enterococcus faecalis bacteria. The AgNPs solution that showed the best bacterial growth inhibition was used to produce chitosan pellets, which were then employed in the disinfection of an industrial wastewater. This wastewater was evaluated in relation to the Escherichia coli bacterium and the amount of silver leached. Finally, the bactericidal activity of the chitosan pellets with AgNPs was investigated. In general, the results obtained in the second stage of this work evidenced that AgNPs synthesized at pH 10.0 and with a higher concentration of AgNO3 presented greater inhibition in relation to bacterial growth. Moreover, chitosan pellets containing AgNPs proved to be effective in disinfecting industrial wastewater, destroying the Escherichia coli bacterium after 1 h of treatment. Finally, considering the ease of application, low environmental impact, reduced costs and satisfactory inhibitory action, it can be concluded that the hybrids developed in this work have great potential to be used as auxiliaries in the tertiary treatment of industrial wastewaters.