Amino-Functionalized Hydrothermal Biochar for Synergistic Removal of Cr(VI) and Mn(II): Insights into Competitive Adsorption and Redox Mechanisms

Utilizing waste resources enables sustainable circular economy strategies. The application of hydrochar in industrial wastewater treatment is usually limited due to poor porosity and surface area. This study addressed the critical need for heavy metal removal from industrial wastewater by developing an amino-functionalized hydrochar while offering sustainable resource recovery potential. The novel amino-functionalized hydrochar (NMHC) was derived from garden waste via a one-step hydrothermal process using nitric acid, tannic acid, and nicotinamide as modifiers. The adsorption batch experiment found that NMHC exhibited different adsorption behaviors for Cr(VI) and Mn(II) in sole and mixed systems. NMHC exhibited exceptional adsorption capacities of 245.0 mg/g for Cr(VI) and 183.9 mg/g for Mn(II), which were 3–4 times higher than those of unmodified hydrochar. The adsorption mechanisms analysis indicated that Cr(VI) removal involved synergistic adsorption and reduction to less toxic Cr(III) via oxygen-containing functional groups, while Mn(II) uptake relied on electrostatic interactions with nitrogen functionalities. Competitive adsorption in mixed systems revealed concentration-dependent inhibition, with Mn(II) displaying higher selectivity at elevated concentrations. NMHC demonstrated robust anti-interference capability against coexisting ions, maintaining >78% efficiency after five adsorption–desorption cycles. Practical validation using smelting wastewater confirmed NMHC performance under real conditions. This work shows that NMHC is a promising and environmentally friendly material for removing Cr(VI) and Mn(II) from wastewater.