Avaliação da produção de fotocatalisador à base de zinco reciclado e emprego no tratamento de resíduos líquidos orgânico

Abstract

The improper disposal of batteries represents an environmental problem due to the presence of metals such as zinc. This work aimed to produce a photocatalyst with a high percentage of zinc oxide (ZnO) in its composition, from the recovery of zinc present in the anode of spent alkaline batteries, and to evaluate its application in the degradation of organic pollutants via heterogeneous photocatalysis. Specifically, it sought to optimize the synthesis process by acid leaching (using sulfuric acid [H2SO4] and oxalic acid [H2C2O4]) and calcination, characterize the obtained material, and test its efficiency in the degradation of the reactive blue 5G dye. The potential of the produced photocatalyst in the degradation of the herbicide 2,4-D and the drug metformin hydrochloride was also evaluated. A 32 fractional factorial design was used to optimize the acid concentrations and calcination temperature, aiming to maximize photocatalytic degradation and mass yield. The photocatalyst obtained under optimized conditions (FPOD) was characterized by XRD, FTIR, SEM, TGA, and N₂ physisorption. The photocatalytic tests were carried out in a batch system with 200 mL of reactive blue 5G dye solution (C₀ = 100 mg L-1 and initial pH = 7) and 1 g L-1 of photocatalyst, with the mixture kept under stirring for 30 min in the dark and subsequently for 3 h with exposure to UV light. Aliquots were taken from the reaction medium over time, subjected to centrifugation (3000 rpm, 5 min) and analyzed in a UV-Vis spectrophotometer (614 nm). From the XRD analysis, the formation of ZnO was confirmed, with the presence of Zn-O bonds observed by FTIR. The TGA analysis verified the purity of the material (approximately 90.6% ZnO) and the adequacy of the calcination temperature, with the last mass loss events at 374.67 °C and 409 °C, followed by stabilization. From the results of the experimental design, it was possible to observe that the variables [H2C2O4] and calcination temperature were statistically significant at the 5% significance level for photocatalytic degradation. The adjusted quadratic model was validated (Fcalculated = 86.2 > Ftabulated = 3.11) and the optimal point (FPOD) defined as 0.4 mol L-1 of H2SO4, 0.5 mol L-1 of H2C2O4, and calcination at 381 °C. Photocatalysis tests, under UV irradiation and sunlight, demonstrated high efficiency in the degradation of the reactive blue 5G dye (96.5% and 98.8%, respectively), following pseudo-first-order kinetics (kUV = 0.019 min-1 and ksol = 0.016 min-1). The degradation of 2,4-D was shown to be pH-dependent, with degradation being higher at pH 7 (97.2%), with no significant photocatalytic degradation observed for metformin hydrochloride. The ecotoxicity test with Eisenia fetida indicated no toxicity to this bioindicator from the reactive blue 5G dye solution before and after treatment by photocatalysis with FPOD. It is concluded that the route of zinc recovery from alkaline batteries for the production of ZnO photocatalysts shows indications of viability, generating an efficient material for the remediation of liquid effluents containing different organic pollutants, contributing to waste valorization and environmental sustainability.