Application of a laccase-catalyzed oxidative coupling for the removal and detoxification of typical flotation reagent salicylhydroxamic acid

[Display omitted] •The laccase-mediated oxidative coupling mechanism of SHA was elucidated.•More attention is paid to both, product identification and toxicity evolution.•SHA transformation predominantly generated various structural oligomers/polymers.•Transformation of SHA is accompanied by detoxification and TOC reduction.•The first report on the use of laccase-mediated coupling in MPW decontamination. Residual organic flotation reagents in mineral processing wastewater (MPW) can pose environmental hazards and hinder the sustainable development of the mining industry. Laccase as a versatile biocatalyst can eliminate various recalcitrant contaminants from the environment, but its potential applications for the removal and detoxification of organic flotation reagents have until now been overlooked. Herein, the influence of superparamagnetic immobilized laccase (SPM-IMLac) on the enzymatic removal of salicylhydroxamic acid (SHA), a typical mineral flotation reagent, as well as the mechanism of this process have been explored for the first time. The results indicated that SPM-IMLac can effectively remove SHA over a broad pH range and this process follows the pseudo-first-order reaction kinetics. A methodology which combined high resolution mass spectrometry (HRMS) and a solid phase extraction (SPE) technique revealed that SPM-IMLac could catalyze a single-electron oxidation of SHA to generate self/cross-oligomers via radical-driven C-C and/or C-O-C covalent coupling pathways. The Vibrio fischeri bioluminescence inhibition assay and a total organic carbon (TOC) analysis corroborated that the SHA transformation was accompanied by detoxification and by a reduction in TOC of the reaction solution, the extent of which correlated with the extent of the oxidative coupling. Moreover, SPM-IMLac could be easily recovered and reused, while keeping its high removal efficiency of SHA in the actual MPW matrices. This study not only provides fascinating insights into the transformation mechanisms and environmental fate of SHA mediated by SPM-IMLac, but it also exploits a viable and sustainable approach for the remediation of organic flotation reagents-contaminated MPW.