Use of a new ZnFe2O4/biochar composite for degradation and ecotoxicity assessment of effluent containing methylene blue dye

In this study, biochar from coffee grounds was used for the first time as a support for ZnFe 2 O 4 and its application and evaluation as a photocatalyst in methylene blue dye degradation reactions. The new composites were verified by different characterization techniques: XRD, FT-IR, SEM, EDS and band gap . The effect of different proportions of ZnFe 2 O 4 and biochar was investigated on the dye degradation efficiency. The composites ZFO1HC3, ZFO1HC1 and ZFO3HC1 showed 93, 100 and 100% degradation in 60 min of reaction, respectively. The composites were more efficient than pure ZnFe 2 O 4 (∼37% degradation), and this may be related to the fact that the biochar has a high concentration of phenolic, hydroxyl and carboxylic functional groups on its surface. Furthermore, this may be associated with a decrease in band gap energy and in the recombination rate of e¯/h + pairs. Radical identification tests indicated that h + , O 2 − and OH were the reactive species involved in the degradation. In addition, to assess the potential toxicity of the effluent containing methylene blue, Danio rerio fish were exposed for 96 h to the crude and treated dye solution. Results showed changes in biomarkers in the gills, such as an increase in catalase activity in the group exposed to untreated methylene blue, and a reduction in glutathione S-transferase activity in this same group. Thus, the set of biomarkers evaluated in fish exposed to effluent containing methylene blue treated with a new ZnFe 2 O 4 /biochar composite showed better results when compared to those exposed to raw effluent.