Key Takeaways & Executive Findings
- •• • Ball-milled Fe/DBC composite (BM-Fe/DBC) achieved 79.9% nitrobenzene removal at Fe:C mass ratio 2:1, dosage 1.0 g·L−1, pH 5, outperforming physically mixed composite (PM-Fe/DBC) and demonstrating the critical role of interfacial Fe–C coupling. • • Aniline yield from BM-Fe/DBC was 1.85 times higher than that from PM-Fe/DBC, indicating enhanced electron selectivity toward nitrobenzene reduction over competing hydrogen evolution. • • Electrochemical analysis revealed that BM-Fe/DBC exhibited a corrosion current density approximately 2.15 times higher and lower charge transfer resistance than PM-Fe/DBC, confirming superior electron transfer kinetics. • • BM-Fe/DBC maintained high reduction efficiency across a wide pH range (3–9), overcoming the strong pH dependency typically observed for bare ZVI, which is critical for practical wastewater treatment under variable pH conditions.
Abstract
Zero-valent iron (ZVI) suffers from surface passivation and low electron utilization in reductive removal of nitrobenzene (NB). To address these issues, a ball-milled iron/digestate biochar composite (BM-Fe/DBC) was prepared and compared with a physically mixed counterpart (PM-Fe/DBC). Characterization revealed that ball milling tightly embedded ZVI particles into the carbon matrix, forming Fe–C chemical bonds and a strong interfacial coupling structure that established efficient electron transfer channels. This structure significantly enhanced the micro-galvanic effect between iron and carbon, yielding superior reduction performance across a wide pH range (3–9). Under optimal conditions (Fe:C mass ratio 2:1, dosage 1.0 g·L−1, pH 5), BM-Fe/DBC achieved 79.9% NB removal, and the generation of aniline (AN) was 1.85 times that of PM-Fe/DBC. Mechanistic studies indicated that the intimate Fe–C interfacial coupling promoted sustained ZVI corrosion and enhanced the production of indirect reducing species, including adsorbed Fe(II) and atomic hydrogen (H*). Electrochemical analyses showed that BM-Fe/DBC exhibited a lower corrosion potential, a higher corrosion current density (approximately 2.15 times higher), and lower charge transfer resistance, kinetically confirming its superior electron transfer capability. These findings reveal that constructing strong interfacial coupling in iron–carbon composites via mechanochemical methods can effectively overcome key limitations of ZVI in reduction reactions, providing a theoretical basis and practical pathway for designing high-performance water treatment materials.
1. Introduction
Zero-valent iron (ZVI) is a widely used reductant for removing persistent organic pollutants such as nitroaromatics from wastewater. However, its practical application is hindered by surface passivation, low electron selectivity (with significant electron loss to hydrogen evolution), and particle agglomeration. Various modification strategies, including sulfidation, noble metal doping, and coupling with carbonaceous supports, have been explored. Carbon materials, such as biochar, can enhance ZVI performance by adsorbing pollutants, dispersing particles, and forming galvanic cells. Yet, inconsistent results across studies suggest that the mode of Fe–C interfacial contact—whether physical mixing or strong chemical bonding—may be a more fundamental factor controlling electron transfer and reactivity than the carbon's intrinsic properties.
This study addresses this bottleneck by employing a mechanochemical ball-milling method to create a composite (BM-Fe/DBC) with intimate Fe–C interfacial coupling, compared to a physically mixed composite (PM-Fe/DBC). The hypothesis is that strong interfacial coupling, characterized by Fe–C chemical bonds, enhances electron transfer and promotes indirect reduction pathways, thereby overcoming ZVI's limitations. The experimental results confirm that BM-Fe/DBC achieves significantly higher nitrobenzene removal and aniline yield, with improved corrosion kinetics and reduced charge transfer resistance, providing a rational design principle for high-performance iron–carbon water treatment materials.
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LIU Daicheng, LI Yongqiang, CHEN Jingkang, XIONG Lei, GUO Dajiang, JU (2026). Dominant Role of Digestate Biochar-Modified Zero-Valent Iron Interfacial Structure in Regulating Nitrobenzene Reduction Efficiency. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511043
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Frequently Asked Questions
What is the specific role of Fe–C interfacial coupling in enhancing electron transfer and nitrobenzene reduction, and how does it compare to physical mixing?
Ball milling creates Fe–C chemical bonds and a tight interface, which significantly enhances the micro-galvanic effect, leading to a corrosion current density approximately 2.15 times higher and lower charge transfer resistance compared to physical mixing. This promotes sustained ZVI corrosion and generates more indirect reducing species (adsorbed Fe(II) and atomic H*), resulting in 79.9% nitrobenzene removal and 1.85 times higher aniline yield under optimal conditions.
How does the BM-Fe/DBC composite perform across different pH conditions, and what is the underlying mechanism?
BM-Fe/DBC maintains high reduction efficiency across a wide pH range (3–9), unlike bare ZVI which is strongly pH-dependent. The buffering effect of biochar and the enhanced interfacial electron transfer mitigate passivation and maintain reactivity, making it suitable for wastewater with fluctuating pH.
What are the potential scalability and cost implications of the ball-milling method compared to physical mixing?
Ball milling is a scalable, solvent-free (or minimal solvent) mechanochemical process that can be easily integrated into industrial production. While it requires additional energy input, the improved performance (higher removal efficiency and aniline yield) may offset costs by reducing material dosage and reaction time. A detailed techno-economic analysis is not provided in the paper, but the method is considered industrially feasible.
What is the long-term stability and reusability of BM-Fe/DBC in continuous or repeated batch operations?
The paper does not explicitly report long-term stability or reusability data. However, the strong Fe–C interfacial coupling and reduced passivation suggest improved resistance to deactivation. Further studies are needed to evaluate performance over multiple cycles and under continuous flow conditions.
How does the presence of biochar influence the selectivity of electrons toward nitrobenzene reduction versus hydrogen evolution?
The biochar matrix, through interfacial coupling, enhances the generation of adsorbed Fe(II) and atomic H*, which are key indirect reducing species. This shifts electron utilization away from hydrogen evolution, as evidenced by the higher aniline yield (1.85 times) and improved nitrobenzene removal, indicating better electron selectivity.
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