Black liquor as biomass feedstock to prepare zero-valent iron embedded biochar with red mud for Cr(VI) removal: Mechanisms insights and engineering practicality.

• BL firstly acted as the biomass feedstock to synthesize engineered ZVI/biochar. • Graphitic structure in red mud/black liquor (RM/BL) enhanced the electron transfer. • ZVI acted as an electron tank for Fe(II) during Cr(VI) reduction. • Chemical reduction and formation of complexations were the dominant mechanisms. • The industrial applicability of magnetic RM/BL was evaluated by fixed-bed column study. Black liquor (BL) is an agro-industrial residue with high number of lignocellulosic components which could be recognized as a biomass feedstock. In this work, BL coupled with red mud (RM), were applied to prepare cost-effective zero-valent iron (ZVI) embedded in biochar. The oligomers in BL acted as reductants for RM to generate ZVI, while the organic components could be converted into biochar during pyrolysis. The RM/BL demonstrated excellent performance in the removal of Cr(VI) (349.5 mg/g), as the mechanisms were reduction and adsorption. The fixed-bed column study was conducted and 1.7 L simulated wastewater could be treated by 1.0 g RM/BL. After reaction, 95.5% ± 0.8% and 82.5%±3.2% Cr-loaded adsorbents could be recovered by an external magnet for batch and fixed-bed experiments, respectively. All these results shed light on valorizing these two widespread agro-industrial byproducts, and bridged the knowledge gap between magnetic bio-adsorbent preparation and its industrial practicality on wastewater purification. [ABSTRACT FROM AUTHOR]