Efficient Nitrate Adsorption and Selective Reduction to Nitrogen gas Using Zero-Valent Fe/Mn Bimetallic Biochar Derived from Water Bamboo Husk

Nitrate contamination of water poses significant environmental and public health concerns, necessitating efficient, selective reduction strategies. In this study, a zero-valent Fe-Mn bimetallic biochar was synthesized from water bamboo husk via a one-step co-pyrolysis process, providing a sustainable approach that avoids the use of additional chemical reductants. The influence of Fe:Mn molar ratios (1:2, 1:1, and 2:1) on nitrate removal and nitrogen transformation was systematically evaluated under acidic conditions (pH 2.0) at an initial nitrate concentration of 100 mg/L and a dosage of 2.5 g/L. Among the tested materials, Fe–Mn/BC21 achieved the highest nitrate removal efficiency (81.56 ± 1.31%) and an estimated N 2 selectivity of up to 80.08%, as determined by nitrogen mass balance analysis. Compared with monometallic counterparts, the Fe–Mn composite exhibited suppressed NH 4 + formation, suggesting a synergistic redox interaction between Fe and Mn species that may regulate electron flux and influence nitrogen product distribution. Kinetic modeling indicated that nitrate removal follows surface-controlled behavior described by pseudo-second-order adsorption and pseudo-first-order reduction kinetics. Equilibrium data were better represented by the Langmuir isotherm, with a maximum adsorption capacity of 43.10 mg/g for Fe–Mn/BC21. Structural and compositional characterizations revealed physicochemical transformations associated with redox-driven nitrate reduction.