Enhanced removal of trivalent chromium from leather wastewater using engineered bacteria immobilized on magnetic pellets

Leather wastewater contains various toxic contaminants, with trivalent chromium (Cr(III)) having high concentration and adversely affecting wastewater treatment. In this study, a Cr(III) adsorption protein (MerP) was displayed on the cell surface of Escherichia coli and then coupled with a magnetic pellet system to facilitate Cr(III) adsorption. The results showed the engineered strain M-BL21 achieved an in vitro Cr(III) adsorption capacity of 2.38 mmol/g. Next, the magnetic pellets were prepared as component ratios of sodium alginate (2.5%), polyvinyl alcohol (8%), Fe3O4 nanoparticles (3.5%), and M-BL21 at 3 g/L. The optimized system was capable of Cr(III) adsorption at an efficiency of 91.29%, which was substantially higher than that of the magnetic carrier alone (67%). Results of scanning electron microscopy with energy-dispersive X-ray analysis proved that Cr(III) was absorbed on the magnetic pellet. The recyclable performance of magnetic property (13.34185 emu/g) and high Cr(III) adsorption efficiency (68.75%) remained after five cycles of Cr(III) absorption. In the medium-scale experiment, 25 L of leather wastewater were treated with magnetic pellet and the Cr(III) removal efficiency reached 88.2%. Thus, our results present an advanced, fully operational, and eco-friendly method for in situ removal of Cr(III) from contaminated wastewater.