Immobilizing chromate reductase NfoR on magnetic biochar reduced Cr(VI) in copper-containing wastewater

Heavy metals (e.g. copper) in smelting wastewater inhibited microbial Cr(VI) reduction, previous report found that a novel chromate reductase NfoR exhibited Cu(II)-enhanced activity and broad-spectrum metal tolerance. In this study, magnetic biochar (MB) from pine sawdust was fabricated to immobilize NfoR via covalent binding. MB-NfoR showed a 2.6-and 2.1-folds higher activity in reducing Cr(VI) to Cr(III) compared to free NfoR and MB, respectively. XPS and EPR analysis showed that NfoR catalysis and persistent free radicals (PFRs) are responsible for Cr(VI) reduction mechanism of MB-NfoR. The addition of Cu(II) increased the MB-NfoR activity 2.3-fold compared to that of without Cu(II). The maximum Cr(VI) removal by MB-NfoR in the pure Cr(VI) solution was 98% at 45 C and pH 6.0, and the MB-NfoR retained 68.3% of its initial activity even after five consecutive cycles. Application of MB-NfoR in smelting wastewater can simultaneously remove 94% Cr(VI) and 52.1% Cu(II). Furthermore, treated smelting wastewater by MB-NfoR showed no phytotoxicity and enriched numerous oligotrophic bacteria. This study provides a novel 'one-stop' strategy for decontamination of effluents combined with heavy metals, utilizing synergies offered by constituents in such waste streams.

Automated summary

This study developed a magnetic biochar (MB) to immobilize a novel chromate reductase NfoR via covalent binding, which showed enhanced activity in reducing Cr(VI) to Cr(III). The mechanism of Cr(VI) reduction by MB-NfoR was found to involve NfoR catalysis and persistent free radicals (PFRs). The addition of Cu(II) further increased the activity of MB-NfoR. Application-wise, MB-NfoR was effective in simultaneous removal of Cr(VI) and Cu(II) in smelting wastewater, without phytotoxicity and with enrichment of oligotrophic bacteria.