Valorization of red mud and biomass waste via pre-pyrolysis activation for high-performance magnetic biochar in heavy metal remediation

Heavy metal contamination of water remains a critical environmental challenge, demanding efficient, low-cost, and sustainable treatment technologies. This study presents an innovative strategy to address both wastewater pollution and industrial waste disposal by converting red mud (RM), a hazardous byproduct of aluminum production, and maple wood (MW) biomass into magnetic biochar (MBC) adsorbents. Unlike traditional post-pyrolysis biochar (BC) activation methods, a novel pre-pyrolysis biomass chemical activation approach was employed using acid (HNO3) and base (KOH) to tailor the surface properties of the biomass-RM mixture prior to co-pyrolysis. The resulting materials, HNO3-MBC and KOH-MBC, displayed distinct physicochemical characteristics and adsorption behaviors. Despite having a lower surface area, KOH-MBC exhibited superior removal efficiencies (∼100 %) for Cu2+ and Pb2+ due to its abundant oxygen-containing functional groups (-OH, -COOH). HNO3-MBC achieved slightly lower removal (∼95 %) but offered higher mesoporosity. Adsorption was governed by chemisorption mechanisms, including electrostatic attraction, ion exchange, complexation, precipitation, and redox reactions, with both materials fitting pseudo-second-order kinetics and Langmuir isotherm models. Economic analysis highlighted the cost advantage of KOH-MBC (CAD 15.47/kg) over HNO3-MBC (CAD 41.29/kg), reinforcing its potential for scalable environmental applications. Overall, this work offers a sustainable and cost-effective pathway to transform industrial wastes into high-performance adsorbents for heavy metal remediation in water.