Efficient generation of ¹O₂ by activating peroxymonosulfate on graphitic carbon nanoribbons for water remediation

The widespread use of pharmaceuticals and industrial chemicals introduces numerous organic pollutants into the environment, posing significant threats to both human health and ecosystems. Acetaminophen (APAP), a common analgesic, serves as a prime example, persisting in aquatic environments despite conventional wastewater treatment. Its persistence stems from its resilience to degradation by typical methods. APAP's potential to cause endocrine disruption and chronic diseases through lipid oxidation and protein denaturation underscores the need for effective remediation strategies. Advanced oxidation processes (AOPs), employing reactive oxygen species (ROS) to degrade recalcitrant pollutants, represent a powerful solution. Among AOPs, the activation of persulfates, such as PMS, offers advantages due to their stability and ease of handling compared to hydrogen peroxide. PMS activation can be achieved through transition metal-based catalysts or carbocatalysts. However, metal-based catalysts risk secondary pollution from metal leaching. Carbocatalysts such as carbon nanotubes, graphene oxide, and nanodiamonds offer a non-toxic and stable alternative, but often exhibit lower activity compared to their metal counterparts. This study focuses on enhancing the catalytic activity of carbocatalysts through the incorporation of heteroatoms and defect engineering to improve PMS activation and subsequent pollutant degradation.

Significant research has focused on improving the performance of carbocatalysts in PMS activation. Heteroatom doping, particularly with nitrogen, has emerged as a promising approach. Nitrogen doping introduces additional functional groups and defective sites, enhancing electron mobility and altering the electron density of the carbon matrix, thus promoting PMS activation and electron transfer. Metal-nitrogen-carbon (M-N-C) materials, particularly those containing iron, have attracted considerable interest due to iron's abundance, low cost, and low toxicity. However, M-N-C materials typically require complex synthesis methods and post-synthesis acid leaching to remove inactive iron species, leading to lower yields and increased complexity. Polyaniline (PANi) has shown promise as a precursor for N-doped carbons due to its ease of synthesis, low cost, and ability to chelate with metal ions. Previous methods often relied on interfacial polymerization, resulting in inhomogeneous iron distribution and lower catalytic activity. This study aims to address these challenges by introducing a novel, simplified synthesis route.

This study employed a novel homogeneous Fenton-like solution polymerization technique to synthesize Fe-chelated polyaniline complexes (FePANi). This method uses H₂O₂ as a green oxidant, with Fe³⁺ ions catalyzing H₂O₂ decomposition to generate hydroxyl radicals that facilitate aniline polymerization and serve as an iron source. The strongly acidic conditions ensure that only a minimal amount of iron is incorporated into FePANi. Pyrolysis of FePANi at 800 °C under N₂ yields few-layer graphitic carbon nanoribbons (GCN). The resulting GCN was characterized using various techniques, including XRD, Raman spectroscopy, N₂ adsorption-desorption, SEM, TEM, HRTEM, and XPS to analyze its phase structure, textural properties, morphology, and elemental composition. The effects of processing parameters, including iron content, pyrolysis temperature, and acid leaching, on the catalytic performance of GCN were investigated using the decolorization of Orange G (OG) as a model reaction. The catalytic degradation of various organic pollutants (OG, RhB, CR, MB, APAP, and SMM) was evaluated to assess the GCN's performance in PMS activation. Reusability tests and anti-interference studies were also conducted. Scavenging tests, spin-trapping electron paramagnetic resonance (EPR) spectroscopy, electrochemical analyses (chronoamperometry, linear sweep voltammetry, and electrochemical impedance spectroscopy), and density functional theory (DFT) calculations were used to elucidate the mechanism of PMS activation and pollutant degradation. HPLC was utilized to analyze reaction intermediates and products.

The researchers successfully synthesized few-layer GCN with rich defective sites using a facile one-step method. The XRD pattern showed two broad peaks indicating low crystallinity, and the Raman spectrum showed a high I_D/I_G ratio indicating abundant defect sites. GCN exhibited a type I N₂ adsorption isotherm with micropores and mesopores. TEM images confirmed the ribbon-like nanostructure. XPS analysis revealed the presence of graphitic-N and Fe-N_x species, which were identified as active sites for PMS activation. The optimization of iron content showed that GCN-0.25 (Fe/aniline molar ratio of 0.25) exhibited the highest catalytic activity and stability. Pyrolysis at 800 °C yielded the best results. Acid leaching did not significantly improve the catalyst's performance. GCN effectively activated PMS to degrade various aromatic pollutants, including dyes and pharmaceuticals. Reusability tests demonstrated good stability, with only a slight decrease in activity after five cycles. The presence of common inorganic anions had a negligible effect on the degradation efficiency. However, HPO₄²⁻ and humic acid showed some inhibitory effects. The scavenging tests revealed that singlet oxygen (¹O₂) was the primary reactive oxygen species (ROS), with a minor contribution from GCN-mediated electron transfer. Electrochemical analyses showed efficient electron transfer between PMS and GCN. EPR spectra confirmed the generation of ¹O₂. DFT calculations indicated synergistic effects between neighboring active sites (C=O and graphitic-N) in promoting PMS activation and electron transfer. XPS analysis of used GCN showed an increase in surface oxygen content, attributed to the adsorption of degradation intermediates.

The findings demonstrate that the novel synthesis method produces a highly active and stable carbocatalyst for PMS activation. The identification of ¹O₂ as the main ROS responsible for pollutant degradation provides valuable insights into the reaction mechanism. The synergistic effect of neighboring active sites enhances PMS adsorption and electron transfer, resulting in efficient ¹O₂ generation. The minor contribution of direct electron transfer further clarifies the reaction pathway. The excellent performance in various water matrices, despite the presence of inorganic anions and humic acid, suggests the practical applicability of the GCN/PMS system. The study's results contribute significantly to the understanding of PMS activation and non-radical oxidation pathways, offering a promising approach for water remediation.

This study presents a facile and efficient method for synthesizing few-layer GCN with rich defective sites, suitable for PMS activation and ¹O₂ generation. The GCN/PMS system demonstrates high catalytic activity and stability for the degradation of organic pollutants in various water matrices. The mechanistic studies revealed that ¹O₂ is the predominant ROS, with a minor contribution from GCN-mediated electron transfer. The synergistic effects of active sites, supported by DFT calculations, significantly enhance catalytic performance. Future research could focus on further optimizing the synthesis method, investigating the long-term stability and scalability of the system, and exploring its application in treating real wastewater samples.

The study primarily focused on the degradation of model pollutants. Further research is needed to evaluate the GCN/PMS system's effectiveness against a broader range of pollutants found in real-world wastewater. The long-term stability of the catalyst under continuous operation requires further investigation. While the study explored the effects of some interfering ions, a more comprehensive analysis of various water matrices and their impact on catalytic performance is warranted. Finally, detailed life cycle analysis and cost-effectiveness studies would be beneficial to assess the feasibility of implementing this technology.