Degradation of novel mineral flotation reagent 8-hydroxyquinoline by superparamagnetic immobilized laccase: Effect, mechanism and toxicity evaluation

The environmental impact of the mining industry requires efficient and eco-friendly technologies to mitigate the presence of mineral flotation reagents (MFRs) in mineral processing wastewater (MPW) prior to their discharge into the environment. In this work, for the first time, a robust, easily separable and reusable biocatalyst, Fe3O4@SiO2-NH2-Lac, was used for the degradation of a novel mineral flotation reagent 8-hydroxyquinoline (8-HQ). Under optimized conditions, Fe3O4@SiO2-NH2-Lac achieved 89.2% 8-HQ degradation efficiency within 6 h. The effect of the main constituents of MPW on 8-HQ degradation, including metal ions, organic solvents, surfactant, metal chelator and flotation frother was evaluated. The Fe3O4@SiO2-NH2-Lac also displayed favorable degradation efficiency of 8-HQ in real lead-zinc mine water. The biocatalyst could be easily recovered and had a satisfactory reusability, retaining 64.5% of 8-HQ degradation efficiency in the sixth reaction cycle. Identification of intermediate products revealed that Fe3O4@SiO2-NH2-Lac mediated reaction predominantly generated various structural 8-HQ oligomers/polymers. A potential degradation pathway for 8-HQ was speculated as follows: Fe3O4@SiO2-NH2-Lac initially catalyzed the oxidation of 8-HQ to yield the corresponding reactive radical intermediates, which subsequently undergone self-coupling reaction via C-C and C-O-C covalent coupling at their ortho and/or para positions, finally forming oligomers and polymers. The inhibition assays of marine bacterium (Vibrio fischeri) demonstrated that the toxicity of 8-HQ and its intermediate products was effectively reduced after Fe3O4@SiO2-NH2-Lac treatment. The results of this study might present an alternative immobilized laccase-based clean biotechnology for the clean-up and detoxification of 8-HQ contaminated MPW.