A N-doped red mud-biochar magnetic composite for highly efficient Cd2+ removal: Adsorption performance and mechanism

To address cadmium pollution in water bodies, a novel N-doped red mud-biochar magnetic composite (N-RCB) was prepared through one-step co-pyrolysis of corn stover and red mud at 800 °C, using ammonium chloride as a pore-forming agent and urea as a nitrogen source. The obtained N-RCB was applied for the efficient removal of Cd 2+ from aqueous solutions. The equilibrium adsorption capacity of N-RCB for Cd 2+ was 5.0 times and 4.2 times that of the pristine biochar (CB) and non-N-doped magnetic red mud-biochar (RCB), respectively. The maximum theoretical adsorption capacity of N-RCB for Cd 2+ could reach 473.25 mg·g –1 . Adsorption kinetics and isotherm studies indicated that the adsorption process of Cd 2+ onto N-RCB followed the pseudo-second-order kinetic model and the Freundlich isotherm model, suggesting that the process is dominated by chemisorption and heterogeneous multilayer adsorption. The thermodynamic parameters demonstrated that the adsorption process was spontaneous, endothermic, and driven by an increase in entropy. The adsorption mechanisms of Cd 2+ by N-RCB included pore filling, cation-π interactions, surface complexation, and electrostatic attraction. Coexisting ion experiments further demonstrated that N‑RCB exhibited good anti‑interference ability toward common ions such as K + , Na + , Mg 2+ , NO₃⁻, and Cl⁻. In addition, after five consecutive adsorption–desorption cycles, the removal efficiency of Cd 2+ by N-RCB remained above 88 %. These results indicate that N-RCB is a promising green and efficient material for the removal of Cd 2+ from water bodies, demonstrating significant application potential.