Key Takeaways & Executive Findings
- •• • Fe/N–C catalyst achieves >98% phenol degradation within 30 min and 60% TOC removal, demonstrating superior performance for refractory pollutant treatment in coking wastewater. • • The catalyst features a high defect density (sp3-C/sp2-C = 0.66) and atomically dispersed iron, which are critical for enhancing PDS activation and electron transfer. • • Singlet oxygen (1O2) is identified as the primary reactive oxygen species, with both radical and non-radical pathways contributing to degradation, offering a robust mechanism for various organic pollutants. • • The synthesis uses coal-tar pitch, a low-cost by-product, enabling high-value conversion and aligning with circular economy principles, potentially reducing catalyst costs for industrial water treatment.
Abstract
Phenolic compounds are typical refractory organic pollutants in coal chemical coking wastewater, posing significant risks to ecosystems and human health. Conventional treatment methods are inefficient, necessitating advanced oxidation processes (AOPs). Here, we report a low-cost Fe/N–C catalyst synthesized from coal-tar pitch, a common by-product of the coal chemical industry, via a self-assembly and pyrolysis strategy using graphitic carbon nitride (g-C3N4) as a template and nitrogen source, with dicyandiamide as an auxiliary nitrogen source and FeCl3·6H2O as the iron precursor. The resulting nitrogen-doped carbon nanosheets possess abundant defects (sp3-C/sp2-C = 0.66) and atomically dispersed iron species. The Fe/N–C catalyst exhibits outstanding catalytic activity for peroxydisulfate (PDS) activation, achieving over 98% phenol degradation within 30 minutes and a 60% total organic carbon (TOC) removal rate. Mechanistic studies, including radical quenching and electron paramagnetic resonance (EPR) experiments, reveal that both radical and non-radical pathways contribute to phenol degradation, with singlet oxygen (1O2) as the primary reactive oxygen species. Electrochemical analyses demonstrate that atomically dispersed Fe sites significantly enhance interfacial electron transfer. Post-reaction characterization indicates the consumption of pyrrolic-N, C=O, and carbon defects as active sites, while graphitic-N and Fe–N structures remain stable, confirming the catalyst's stability. This work provides an economical route to convert coal-tar pitch into high-performance catalytic materials for efficient water treatment, embodying the circular economy concept of waste-to-resource utilization.
1. Introduction
Phenolic compounds, prevalent in coal chemical coking wastewater, are notoriously recalcitrant and toxic, posing severe threats to ecosystems and human health. Conventional water treatment methods, such as biological processes and physical adsorption, often fall short in completely mineralizing these pollutants, leading to secondary pollution or incomplete removal. Advanced oxidation processes (AOPs), particularly persulfate-based AOPs (PS-AOPs), have emerged as promising alternatives due to their ability to generate highly reactive oxygen species (ROS) capable of degrading refractory organics. However, the practical application of PS-AOPs is often hindered by the high cost and complex synthesis of efficient catalysts, as well as the need for sustainable and scalable production methods.
This study addresses these bottlenecks by introducing a low-cost Fe/N–C catalyst derived from coal-tar pitch, an abundant by-product of the coal chemical industry. The synthesis leverages a self-assembly and pyrolysis strategy using g-C3N4 as a template, resulting in nitrogen-doped carbon nanosheets with abundant defects and atomically dispersed iron. This design not only enhances catalytic activity for peroxydisulfate activation but also ensures stability and reusability. The catalyst achieves over 98% phenol degradation within 30 minutes, with singlet oxygen as the primary ROS, offering a highly efficient and economically viable solution for phenolic wastewater treatment. This work exemplifies the circular economy concept by transforming industrial waste into high-value catalytic materials, potentially revolutionizing water treatment technologies.
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WANG Ruixue, QIU Zihan, ZHANG Runmeng, LENG Changyu, WANG Xuzhen, QIU Jieshan (2026). Single-atom iron catalysts on defect-rich nitrogen-doped carbon nanosheets for efficient phenol degradation via peroxydisulfate activation. New Carbon Materials. https://doi.org/10.1016/S1872-5805(26)61101-8
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Frequently Asked Questions
What is the catalytic mechanism of Fe/N–C for peroxydisulfate activation, and how does the presence of single-atom iron enhance the degradation of phenol?
The Fe/N–C catalyst activates peroxydisulfate (PDS) via both radical and non-radical pathways. Radical quenching and EPR experiments identified singlet oxygen (1O2) as the primary reactive oxygen species, with sulfate radicals (SO4•−) and hydroxyl radicals (•OH) also contributing. The atomically dispersed iron sites significantly enhance interfacial electron transfer, as verified by electrochemical analyses, facilitating the generation of 1O2 and promoting phenol degradation. The abundant defects (sp3-C/sp2-C = 0.66) and nitrogen species (pyrrolic-N, C=O) act as active sites, while graphitic-N and Fe–N structures remain stable, ensuring sustained catalytic activity.
How does the Fe/N–C catalyst compare to traditional iron-carbon composites in terms of cost and performance for wastewater treatment?
The Fe/N–C catalyst is synthesized from coal-tar pitch, a low-cost by-product of the coal chemical industry, using a simple self-assembly and pyrolysis method. This significantly reduces material costs compared to conventional iron-carbon composites that often require expensive precursors or complex synthesis steps. Performance-wise, Fe/N–C achieves over 98% phenol degradation within 30 minutes and 60% TOC removal, which is comparable or superior to many reported catalysts. The high activity is attributed to the synergistic effect of single-atom iron and defect-rich nitrogen-doped carbon, which enhances electron transfer and ROS generation. This cost-performance advantage makes Fe/N–C a promising candidate for large-scale water treatment applications.
What are the long-term stability and reusability of the Fe/N–C catalyst in continuous operation?
The Fe/N–C catalyst exhibits excellent stability, as evidenced by post-reaction characterization showing that graphitic-N and Fe–N structures remain intact, while pyrrolic-N, C=O, and carbon defects are consumed as active sites. This suggests that the catalyst maintains its structural integrity and catalytic activity over multiple cycles. However, the gradual consumption of active sites may eventually lead to a decrease in performance, necessitating regeneration or replacement. The catalyst's stability is also supported by its high TOC removal efficiency, indicating effective mineralization of phenol without significant fouling. For industrial applications, further studies on long-term continuous operation and regeneration strategies are recommended.
Can the Fe/N–C catalyst be scaled up for industrial wastewater treatment, and what are the potential challenges?
The synthesis of Fe/N–C is relatively straightforward and uses low-cost precursors, making it amenable to scale-up. However, challenges include ensuring uniform dispersion of single-atom iron and maintaining defect density at larger scales, as well as optimizing pyrolysis conditions for batch-to-batch consistency. Additionally, the catalyst's performance in real wastewater matrices, which contain various coexisting ions and organic matter, needs to be evaluated. The use of coal-tar pitch as a precursor also requires consistent quality and supply. Despite these challenges, the economic and environmental benefits of converting waste to valuable catalysts make Fe/N–C a viable option for industrial adoption, provided that process engineering and quality control are addressed.
What is the environmental impact of using Fe/N–C for phenol degradation, particularly regarding the formation of toxic by-products?
The Fe/N–C/PDS system achieves 60% TOC removal, indicating significant mineralization of phenol to CO2 and H2O, thereby reducing the formation of toxic intermediates. The primary ROS, singlet oxygen (1O2), is a selective oxidant that reacts with phenol to form less harmful products. However, complete mineralization is not achieved, and some intermediates may remain. The study does not report detailed toxicity analysis of the treated water, but the high TOC removal suggests a substantial reduction in organic load. Further studies should assess the ecotoxicity of the effluent to ensure the treated water meets discharge standards. The catalyst itself is composed of iron and carbon, which are environmentally benign, and its synthesis from coal-tar pitch contributes to waste valorization, aligning with green chemistry principles.
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