The rapid increase of global solid waste poses significant environmental challenges. In this work, two abundant wastes— red mud and apple peel—were used as precursors to prepare zero-valent iron biochar for efficient pollutant removal. This study innovatively developed a green, low-temperature in-situ hydrogen reduction strategy via one-step coppercatalyzed ethanol decomposition, which generated in-situ hydrogen and uniformly dispersed high-load Fe0 without the need for external hydrogen or hazardous reagents. Compared with N2 pyrolysis, in-situ H2 treatment enlarged the pore size by 17.2%, increased surface oxygen functionalities, and enhanced active site exposure and electron transfer, markedly improving reactivity. The composite exhibited high saturation magnetization (33.13 emu g–1) for rapid magnetic separation, low iron leaching (≤ 0.13 mg L–1), and retained over 63% removal efficiency after four cycles. Removal efficiencies reached 87.77 − 98.50% for MB, RhB, and TC in single-dye systems, and remained high at 70.09 − 84.32% in multi-dye wastewater. Synergistic mechanisms involving porous adsorption, Fe–O coordination, π–π interaction, and NZVI-mediated reduction contributed to superior performance. This sustainable strategy enhances the waste value and provides effective and environmentally safe solutions for complex wastewater treatment, promoting resource recovery and pollution control.

In-situ hydrogen-reduced red mud/apple peel biochar with highly loaded nZVI for efficient removal of organic pollutants
    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Materials
        2.2 Single-factor optimization of preparation parameters
        2.3 Synthesis of RMAPBC under optimized conditions
        2.4 Characterization
        2.5 Evaluation of RMAPBC adsorption performance and mechanistic insights for multi-dye wastewater treatment
        2.6 Assessment of RMAPBC reusability, iron leaching and structural stability
        2.7 Statistical analysis
    3 Results and discussion
        3.1 Comparative adsorption evaluation under N2 and in-situ H2 atmospheres
        3.2 Sequential single-factor optimization under in-situ H2 atmosphere
            3.2.1 Effect of acid modification
            3.2.2 Effect of biomass parameters
            3.2.3 Effect of pyrolysis heating rate and temperature
    3.3 Structural characterization and performance evaluation of RMAPBC under different pyrolysis atmospheres
        3.3.1 Crystal structure and magnetic properties
        3.3.2 Surface morphology and porosity
        3.3.3 Chemical composition and surface functionality
        3.3.4 Performance of optimized RMAPBC under different pyrolysis atmospheres
    3.4  Adsorption kinetics and thermodynamics
        3.4.1 Adsorption kinetics
        3.4.2 Adsorption isotherm and thermodynamic analysis
    3.5 Effect of operational parameters on removal efficiency
        3.5.1 Effect of adsorbent dosage
        3.5.2 Effect of pH and surface charge (pHpzc) on adsorption mechanism
    3.6 Reusability and structural stability
    3.7 Adsorption mechanism
    4 Conclusions
    References

• Xinxin Li(School of Environment, Beijing Jiaotong University, 100044 Beijing, China)
• Fumin Ren(School of Environment, Beijing Jiaotong University, 100044 Beijing, China) Corresponding author
• Junshi Liu(School of Environment, Beijing Jiaotong University, 100044 Beijing, China)
• Sibo Jia(School of Environment, Beijing Jiaotong University, 100044 Beijing, China)
• Xiaoyu Ge(School of Environment, Beijing Jiaotong University, 100044 Beijing, China)
• Houliang Guo(School of Environment, Beijing Jiaotong University, 100044 Beijing, China)