Fabrication and environmental applications of multifunctional mixed metal-biochar composites (MMBC) from red mud and lignin wastes

Highlights: • Novel MMBC fabrication from RM and lignin wastes via one-step pyrolysis in N₂ atmosphere. • MMBC exhibited hierarchical porous structure, various functional groups, and metallic Fe. • Incorporated RM promoted syngas generation and hydrocarbons decomposition of lignin. • Demonstrated multifunctionality (adsorption, reduction, and catalysis) of MMBC. -- Abstract: This study fabricated a new and multifunctional mixed metal-biochar composites (MMBC) using the mixture of two abundant industrial wastes, red mud (RM) and lignin, via pyrolysis under N₂ atmosphere, and its ability to treat wastewater containing various contaminants was comprehensively evaluated. A porous structure (BET surface area = 100.8 m² g⁻¹) was created and metallic Fe was formed in the MMBC owing to reduction of Fe oxides present in RM by lignin decomposition products during pyrolysis at 700 °C, which was closely associated with the transformation of liquid to gaseous pyrogenic products. The potential application of the MMBC was investigated for the removal of heavy metals (Pb(II) and Ni(II)), oxyanions (As(V) and Cr(VI)), dye (methylene blue), and pharmaceutical/personal care products (para-nitrophenol and pCBA). The aluminosilicate mineral, metallic Fe, and porous carbon matrix derived from the incorporation of RM and lignin contributed to the multifunctionality (i.e., adsorption, chemical reduction, and catalytic reaction) of the MMBC. Thus, engineered biochar composites synthesized from selected industrial wastes can be a potential candidate for environmental applications.