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 ZnFe2O4 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 ZnFe2O4 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 ZnFe2O4 (similar to 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(center dot-) and (OH)-O-center dot 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 ZnFe2O4/biochar composite showed better results when compared to those exposed to raw effluent.