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Article

Nitrogen-Enriched Porous Carbon from Chinese Medicine Residue for the Effective Activation of Peroxymonosulfate for Degradation of Organic Pollutants: Mechanisms and Applications

by
Xiaoyun Lei
1,*,
Dong Liu
1,
Weixin Zhou
1,2,
Xiao Liu
1,
Xingrui Gao
1,
Tongtong Wang
3,* and
Xianzhao Shao
1
1
Shaanxi Key Laboratory of Catalysis, School of Chemistry and Environment Science, Shaanxi University of Technology, Hanzhong 723001, China
2
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
3
Institute for Interdisciplinary and Innovation Research, Xi’an University of Architecture and Technology, Xi’an 710055, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(10), 926; https://doi.org/10.3390/catal15100926
Submission received: 14 August 2025 / Revised: 23 September 2025 / Accepted: 27 September 2025 / Published: 1 October 2025
(This article belongs to the Special Issue Catalytic Materials for Hazardous Wastewater Treatment)
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Abstract

Advanced oxidation processes (AOPs) utilizing peroxymonosulfate (PMS) have recently gained attention for effectively removing organic dyes. Biochar, a carbon-based material, can act as a catalyst carrier for PMS activation. This study developed a nitrogen-doped biochar catalyst (NCMR800–2) from waste Chinese medicine residue (CMR) through one-step pyrolysis to efficiently remove Rhodamine B (RhB) from wastewater. Results indicate that NCMR800–2 rapidly achieved complete removal of 20 mg/L Rhodamine B (RhB), the primary focus of this study, within 30 min, while maintaining high degradation efficiencies for other pollutants and significantly outperforming the unmodified material. The material demonstrates strong resistance to ionic interference and operates effectively across a wide pH range. Quenching experiments and in situ testing identified singlet oxygen (1O2) as the primary active species in RhB degradation. Electrochemical analysis showed that nitrogen doping significantly enhanced the electrical conductivity and electron transfer efficiency of the catalyst, facilitating PMS decomposition and RhB degradation. Liquid chromatography–mass spectrometry (LC-MS) identified intermediate products in the RhB degradation process. Seed germination experiments and TEST toxicity software confirmed a significant reduction in the toxicity of degradation products. In conclusion, this study presents a cost-effective, efficient catalyst with promising applications for removing persistent organic dyes.

Graphical Abstract

1. Introduction

The rise in domestic sewage and industrial pollutants has led to the infiltration of micropollutants into water bodies, posing potential threats to human health and ecosystems [1,2,3]. Synthetic dyes, prime examples of these pollutants, are extensively used in industries like textiles, leather, and papermaking [4,5]. With over 10,000 varieties and an annual production surpassing 700,000 tons, synthetic organic dyes represent a major concern [6]. As the leading textile producer, China faces significant challenges relating to organic dye pollution. During dyeing, 10% to 45% of dyes are lost, resulting in nearly 2 × 105 tons of colored wastewater annually [7]. Traditional methods for treating contaminated water, namely, physical–chemical techniques like adsorption and electrochemistry, and biological approaches such as contact oxidation and membrane bioreactors face challenges including high operational costs, secondary pollution, and incomplete pollutant removal [8,9,10]. In contrast, AOPs utilizing PMS have gained attention for their efficiency and environmental compatibility. PMS oxidation in particular has emerged as a key area of research concentration due to its effectiveness in degrading common organic pollutants in water [11]. The development of AOPs leveraging PMS has received notable attention in recent years, attributed to their high efficacy in removing pollutants and alignment with green economy principles [12,13]. Among these, PMS oxidation approaches have gained prominence in research due to their high efficiency in degrading organic pollutants in water bodies [14,15]. While PMS is a potent oxidizing agent, its direct oxidation and degradation of organic pollutants under standard conditions is challenging, necessitating activation through appropriate means to generate free radicals or other reactive species. Current activation strategies include the use of alkaline conditions, organic substrates, and carbon-based materials [16,17,18]. Therefore, there is an urgent need to develop environmentally friendly, cost-effective, and highly efficient catalysts. Activation approaches leveraging multifunctional synergistic effects can effectively enhance pollutant degradation efficiency.
Human activities have led to the annual production of over 1.6 billion tons of biomass waste, often dismissed as useless [19]. However, biomass waste is a renewable resource with considerable potential as an energy carrier [20]. Recent advancements in AOPs have highlighted the role of carbon-based materials in activating PMS processes, making them a research focal point [21,22]. Studies on biochar derived from CMR have predominantly examined its physical adsorption capabilities for pollutant removal from water. For instance, Wang et al. developed a chitosan-modified magnetic biochar from ligusticum chuanxiong residues, achieving removal efficiencies exceeding 95%, 99%, and 99% for Cr(VI), As(III), and Pb(II) in contaminated water, respectively [23]. Yuan et al. found that biochar made from traditional CMR has good potential for adsorbing heavy metal lead (II) [24]. Li et al. employed a one-step carbonization method to produce biochar from atropine residue and revealed its exceptional adsorption efficacy for norfloxacin [25]. Despite these compelling findings, research endeavors exploring the application of biochar derived from CMR within AOPs remain relatively scarce. This study, therefore, explores the use of TCM residues as a precursor for biochar in AOPs. Modifying biochar is crucial for its practical application in environmental remediation. Widely recognized modification methods include acid–base treatment [26], oxidant treatment [27], and heteroatom doping [28]. The literature documents that nitrogen (N) doping confers enhancements in catalytic performance through the augmentation of surface functional group abundance, pore dimensions, and pore volume; these effects collectively contribute to the amelioration of both adsorption and degradation efficiencies toward target pollutants [29,30,31]. Urea, a cost-effective biofertilizer with high nitrogen content, has been extensively used in biochar modification research [32,33]. For instance, Ge et al. successfully synthesized N-doped biochar polymer composites using urea modification, achieving a tetracycline removal rate of 97.83% [34]. Similarly, Lin et al. utilized urea-modified porous carbon materials to attain a bisphenol A removal rate of 97.20% [35]. While significant progress has been made, the underlying removal mechanisms for various pollutants warrant further investigation. Although PMS is an efficient oxidant, it presents certain safety and environmental concerns [36,37]. PMS is corrosive and may generate halogenated by-products or residual toxicity if not properly managed [38,39]. Therefore, strict handling, quenching of residual PMS, and monitoring of by-products are necessary to ensure safe application.
In summary, this study utilized herbal medicine residue as a biomass precursor, which was modified with urea and subjected to a one-step pyrolysis method to prepare an eco-friendly, economical, and high-performance catalyst. The degradation experiment conditions were optimized, and the reaction mechanism for activating PMS to degrade pollutants was explored to address current water pollution issues. Structural characterization analyses were conducted to examine the physical structure of the obtained catalyst material. Further investigations were carried out to determine the optimal conditions for pollutant removal, including evaluating the effects of catalyst and PMS dosage, pollutant type and concentration, initial pH, and water environment. The practicality of the catalyst was assessed through ion interference experiments, cyclic stability tests, and toxicity experiments. The catalytic activation of PMS for organic pollutant removal from water was investigated through radical quenching experiments, electron paramagnetic resonance (EPR) measurements, Raman spectroscopy, electrochemical analyses, and LC-MS. This research offers an efficient and sustainable approach to water treatment, enabling the elimination of contaminants from aquatic environments under mild reaction conditions.

2. Results and Discussion

2.1. Material Characterization

Figure 1a illustrates the preparation procedures of NCMRT-X material, where T represents the activation temperature and X represents the mass ratio of urea to Chinese medicine residue. Since the catalytic and adsorption properties of materials are strongly influenced by their microstructure [40], we used scanning electron microscopy (SEM) to investigate the morphological characteristics of NCMR800–2. Figure 1b illustrates its highly porous nature, while Figure 1c reveals distinctive wrinkles on the catalyst surface, probably resulting from urea decomposition at elevated temperatures. This process generates gases that form numerous pores within the material. The presence of abundant large pores and channels in the catalyst minimizes mass transfer resistance, facilitating pollutant penetration into the catalyst interior and thereby improving removal efficiency. Further microstructural analysis of NCMR800–2 through transmission electron microscopy (TEM), as shown in Figure 1d,e, reveals a strip-like morphology with well-defined pore structures. Figure 1f–j demonstrate uniform distribution of C, N, and O elements on the material surface, indicating success as well as uniform nitrogen doping into the catalyst framework. Moreover, Figure 1i,j reveal the presence of Si and Al elements in the material, likely to be trace elements self-doped from the CMR.
The pore characteristics and specific surface area (SSA) of the catalysts with varying mass ratios were evaluated through nitrogen adsorption–desorption measurements [12]. The results depicted in Figure 2a demonstrate that the introduction of nitrogen elements led to a decrease in SSA from 59 m2/g to 55 m2/g. The nitrogen adsorption–desorption isotherm of the catalyst exhibits a distinct hysteresis loop, signifying the development of a mesoporous structure post-doping. This finding aligns with the pore size distribution curve in Figure S1 and Table S1. The mesopores enhance the availability of reactive sites for Rhodamine B (RhB) degradation. The crystalline architecture of catalysts was determined through X-ray diffraction (XRD) analysis, as depicted in Figure 2b. A diffraction peak at 2θ = 23.5° indicates the presence of an amorphous (002) diffraction plane characteristic of carbonaceous materials [41]. Furthermore, distinct diffraction peaks at 2θ = 20.8° and 2θ = 26.6° correspond to the (100) and (101) diffraction planes of silicon dioxide (SiO2) [42]. The crystallinity of the sample changes, structure, and defects were assessed using Raman spectroscopy [43]. The D peak and G peak are common features in carbon-based materials like graphite and graphene. The D peak, located at approximately 1350 cm−1, indicates structural defects or disorder, whereas the G peak, at around 1580 cm−1, represents the planar vibrational mode of sp2 hybridized carbon atoms [12]. The ratio of the D peak to the G peak (ID/IG) is a pivotal metric for assessing the crystalline integrity and defect density of carbon materials [44]. In Figure 2c, the ID/IG value decreases progressively with increasing modification ratio and temperature, indicating a relatively complete crystal structure and reduced defect level in the modified and activated carbon material. Additionally, a prominent absorption peak at 1080 cm−1 was observed, assigned to the high-temperature SiO2 phase [45].
X-ray photoelectron spectroscopy (XPS) was employed for analyzing the elemental composition and chemical state of materials. The XPS spectrum of NCMR800–2, depicted in Figure 2d, revealed the presence of five elements: C, O, N, Si, and Al. This observation validates the effective doping of N, with incidental detection of Si and Al in minute quantities originating from the herbal residue. This deduction is supported by the XRD and energy-dispersive energy dispersive spectroscopy (EDS) spectra. Subsequent XPS analysis was performed to investigate the molecular structure and chemical surroundings of the doped impurity atoms in NCMR800–2. The C 1s peak exhibited four distinct components at 284.65 eV (graphitic C/C-C/C-H), 286.4 eV (C=N), 288.5 eV (C=O), and 291 eV (COOH) [46]. Similarly, the O 1s spectrum of NCMR800–2 displayed peaks for C=O (530.6 eV), O-Al (531.5 eV), O-H (532.8 eV), and O-Si (533.7 eV). The N 1s spectrum revealed peaks at 398.2 eV, 399.5 eV, 401.2 eV, and 402.8 eV, corresponding to pyridine-N (N−6), pyrrole-N (N−5), graphite-N (N-Q), or oxidized-N (N-O) [47]. Analysis of the Si 2p spectrum revealed peaks at 101.8 eV, 102.65 eV, and 103.5 eV, representing Si3N4, Si-O bonds, and Si-OH bonds, respectively [48]. Furthermore, high-resolution Al 2p spectroscopy identified characteristic peaks at 73.79 eV, 74.65 eV, and 75.02 eV, corresponding to Al(OH)3, AlOOH, and Al2O3, respectively [49].

2.2. Evaluation of Catalyst Activation Performance

Catalyst performance in PMS activation was evaluated by synthesizing catalysts at different temperatures and ratios, followed by an assessment of their catalytic efficiency to optimize the preparation process [50]. Figure 3a illustrates the adsorption–degradation profiles of the unmodified and modified catalysts. Incorporation of the catalyst effectively triggers the activation of PMS, thereby augmenting its adsorption and degradation capabilities towards pollutants. Furthermore, in contrast to the unmodified biochar, the nitrogen-doped catalyst demonstrates a significant improvement in catalytic efficiency, achieving full pollutant degradation within 30 min, highlighting the effectiveness of our modification strategy. The impact of temperature on catalyst performance was also scrutinized, with the degradation efficiency of the catalyst evaluated across different preparation temperatures. As depicted in Figure 3b, the adsorption efficiency of the catalyst remained largely unaffected by escalating calcination temperatures. Additionally, the degradation rate curves reveal that the NCMR800–1 catalyst degraded pollutants more rapidly. Consequently, 800 °C was chosen as the calcination temperature. To identify the optimal modification ratio, catalysts with varying ratios were calcined at this temperature. As depicted in Figure 3c, increasing the urea ratio enhances the adsorptive capacity of the catalyst and pollutant removal efficiency. Notably, NCMR800–2 achieved 100% pollutant removal in 20 min, while NCMR800–3 reached the same removal rate in just 10 min. Both catalysts demonstrated exceptional performance; however, for economic reasons, NCMR800–2 was selected to achieve effective catalytic degradation at a reduced cost.
Catalyst dosage critically influences pollutant removal efficiency. This study evaluated various catalyst dosages on degradation performance. As depicted in Figure 3d, increased catalyst dosage correlates with higher pollutant adsorption during the initial stage, with both 30 mg and 40 mg doses achieving complete pollutant removal within 20 min. For optimal efficiency and cost-effectiveness, a 30 mg catalyst dose was chosen for further research. Similarly, oxidant dosage impacts pollutant removal efficiency. We examined different PMS dosages under a constant catalyst dosage, as shown in Figure 3e. Even at a low PMS dosage of 20 mg, effective pollutant degradation was achieved, leading to its selection for subsequent studies. Further analysis explored the effect of varying pollutant concentrations on degradation performance, illustrated in Figure 3f. Increased pollutant concentration diminishes removal efficiency. To ensure efficient removal within a limited timeframe, a 20 mg/L RhB solution was selected for further investigation. These findings suggest that N co-doping significantly enhances PMS activation and RhB degradation.
In natural aquatic systems, anions such as chloride ions (Cl−), nitrate ions (NO3−), sulphate ions (SO42−), and phosphate ions (PO43−) coexist alongside natural organic compounds like humic acid (HA). This study investigated the system of NCMR800–2/PMS resistance to interference during pollutant degradation by introducing these common anions and organic matter at specific concentrations. As depicted in Figure 4, the presence of these ions did not notably impact pollutant adsorption by the catalyst. However, they variably influenced the catalytic activation of PMS for pollutant degradation. Figure 4a,b illustrate that Cl− and NO3− mildly inhibited PMS activation for RhB degradation. Figure 4c indicates that increasing SO42− concentration progressively heightened this inhibitory effect. Notably, Figure 4d reveals that PO43− strongly suppressds the catalytic degradation efficiency; once pollutants reached adsorption saturation, degradation virtually ceased. The addition of HA (Figure 4e) enhances the catalyst-activated PMS degradation of pollutants at low concentrations (5, 10 mg/L), significantly boosting catalytic efficiency. However, excessive HA can inhibit catalyst performance. The initial pH of the solution is critical in determining the reactive oxygen species (ROS) type and intermediate products formed. As depicted in Figure 4f, acidic conditions, with increased H+ ions, improved the catalytic efficiency of NCMR800–2. In contrast, weakly acidic, neutral, and weakly alkaline conditions showed negligible effects on catalytic degradation. Under strongly alkaline conditions, while the adsorptive capacity of the catalyst is enhanced, its degradation performance declines markedly, probably due to metal ions within the catalyst. Studies indicate that in alkaline environments, OH− ions react with metal ions to form complexes, significantly inhibiting the catalytic efficiency of many metal ions.

2.3. Practical Application of the NCMR800–2/PMS System in Water Treatment

The zeta potential test results (Figure 5a) indicate that the surface of catalyst was negatively charged. In aqueous solution, RhB molecules undergo ionization, generating positively charged species and releasing Cl−. This process significantly enhances the adsorption capacity on the catalyst surface, primarily due to the electrostatic interaction between its cationic form and the negatively charged catalyst surface. However, in strongly alkaline conditions, PMS exists as SO52−, leading to strong electrostatic repulsion with the catalyst. This repulsion hinders PMS adsorption on the catalyst surface, resulting in poor catalytic degradation efficiency under alkaline conditions. The catalytic performance of NCMR800–2 was evaluated in real water bodies to assess its applicability under more complex environmental conditions. As illustrated in Figure S3, the catalytic degradation performance of NCMR800–2 in actual water samples was limited, likely due to the presence of interfering ions such as HCO3− and PO43−, which significantly impacted the catalytic performance of the catalyst. Furthermore, the abundant microorganisms in natural water bodies may have inhibited the activation of PMS by the catalyst to some extent. Despite these challenges, NCMR800–2 exhibited exceptional catalytic degradation efficiency for various pollutants, including RhB, AO7, HTC, OTC, and MB, each at a concentration of 20 mg/L, with removal rates of 100%, 100%, 87.57%, 97.29%, and 72.63%, respectively (Figure 5b). To evaluate the stability of the catalyst, a fixed-bed flow reactor was used to determine its catalytic efficiency (Figure S4). As shown in Figure 5c, the NCMR800–2 catalyst exhibited excellent removal performance for RhB in the flow mode in the presence of PMS. After 40 h of cycling, NCMR800–2 still exhibited good stability. To test the recyclability of the catalyst, used NCMR800–2 was recovered and recycled. After four cycles, the catalyst still exhibited strong catalytic activity, with a pollutant removal rate of 88.54% (Figure 5d). This result indicates that the catalyst has strong practical applicability.

2.4. Investigation of Active Species and Mechanistic Pathways

To explore the activation mechanism, free radical quenching experiments were conducted to identify the reactive species involved (Figure 6a) [51]. Tert-butyl alcohol (TBA) is the preferred •OH scavenger [52]. When TBA was added, it did not significantly affect the reaction, indicating that •OH was not generated during the reaction. Additionally, methanol was used to quench •OH and SO4•−, and the addition of methanol (MeOH) did not significantly inhibit the degradation of pollutants, indicating that SO4•− was not present during the reaction. p-benzoquinone (p-BQ) and L–histidine (L–His) were selected as quenchers for O2•− and 1O2, respectively. The addition of p-BQ did not inhibit the degradation of RhB, indicating that free radicals were not present in the reaction process, while the addition of L–His significantly inhibited the degradation of RhB, indicating that 1O2 participated as a potential active species in the degradation of RhB. FFA is another commonly used 1O2 quencher, but at the same concentration, its inhibitory effect was much smaller than that of L–His; so, the addition of FFA did not significantly affect the degradation of RhB in this system [53,54]. Additional research was carried out to examine the reactive species responsible for the activation of PMS by NCMR800–2 using EPR spectroscopy [55]. TEMP was employed as a scavenger for 1O2, DMPO was utilized to trap •OH and SO4•− in water-based solutions, and DMPO in methanol solution was used as a scavenger for O2•−. As shown in Figure 6b, when detecting 1O2, a weak 1:1:1 ternary signal was observed as early as 3 min into the reaction, and this signal continued to strengthen over time, indicating the presence of 1O2 in the system [56]. As shown in Figure S5, no significant signals for •OH/SO4•− or O2•− were detected in the NCMR800–2/PMS system. This outcome aligns with the results observed in the radical quenching experiment.
Subsequently, Raman spectroscopy was employed to analyze the characteristics of the PMS and the resultant polymer on its surface. As depicted in Figure 6c, vibrational peaks corresponding to HSO5− in PMS were observed at 1060 cm−1 and 889 cm−1, while the signal for SO42− manifested as a broad peak at 963 cm−1. The attenuation of the HSO5− peak at 889 cm−1 and the SO42− peak at 963 cm−1 in the NCMR800–2/PMS system can be attributed to the degradation of PMS induced by the catalyst introduction. The shift in the HSO5− vibrational peak further validates the interaction of the O-O bond of PMS with the active sites on NCMR800–2, resulting in alterations in the stretching vibration amplitude [57]. To delve deeper into the electron transfer mechanism in the NCMR800–2/PMS system, its electrochemical characteristics were evaluated utilizing a three-electrode configuration [58]. The cyclic voltammetry curves of the catalyst material are shown in Figure 6d. NCMR800–2 exhibited a large charge potential difference and good cyclic reversibility, demonstrating that this material possessed strong conductivity and redox reaction capability at the reaction interface. Electrochemical impedance spectroscopy (EIS) indicated the charge transfer resistance characteristic of the material, with a smaller semicircular diameter in the EIS plot indicating lower resistance [59]. This suggests the material exhibited faster electron transfer kinetics and enhanced electrical conductivity. Figure 6e shows that NCMR800–2 had a smaller diameter, demonstrating its superior conductivity and electron transfer capability compared to the original carbon. Hybrid atom-doped carbon-based catalysts have garnered significant attention owing to their distinctive physicochemical characteristics, diverse active sites, and their ability to facilitate electron transfer between PMS and the catalyst [60,61]. The charge transfer between NCMR800–2, PMS, and RhB molecules was further investigated using the chronoamperometry method. As shown in Figure 6f, the addition of PMS had a significant impact on the current output, indicating the formation of a metastable complex (NCMR800–2/PMS). The current experienced a sudden increase following the addition of RhB, suggesting the formation of a current path from RhB to the complex on the NCMR800–2 surface, which may have been related to the oxidative degradation of RhB.

2.5. Possible Degradation Pathways

To further investigate the reaction mechanism of RhB degradation in the NCMR800–2/PMS system, LC-MS was used to confirm the intermediate degradation products of RhB. Based on the LC-MS analysis results for the NCMR800–2/PMS system (Figure S6) and previous research reports [62,63], three possible degradation pathways for RhB under this reaction system were proposed (Figure 7). Among these, pathways 1 and 3 are initiated by the decarbonization of carbon atoms, with 1O2 attack and electron transfer catalyzing the cleavage of RhB [64]. Pathway 2 primarily involves cleavage reactions and ring-opening reactions. Throughout degradation, large compounds were fragmented into smaller molecules, ultimately mineralizing into H2O and CO2. The total organic carbon (TOC) concentration decreased by approximately 33% after 40 min in the NCMR800–2/PMS system (Figure S7); the remaining 67% of TOC was attributed to small molecular byproducts derived from RhB degradation.

2.6. Evaluation of the Toxicity of RhB Degradation Products

Toxicity assessment is a very important aspect of the NCMR800–2/PMS system. Although the RhB solution appeared visually degraded and decolorized after treatment, complete mineralization into CO2 and H2O remains challenging, and the degradation intermediates may still pose environmental risks. Therefore, it is essential to evaluate the toxicity of the byproducts formed during the RhB degradation process. Previous studies have commonly used seed germination experiments to study the toxic effect of pollutant solutions [12,65]. In this study, Shanghai greens, wheat, and pea seeds were employed as model crops. The toxicity of both treated and untreated RhB solutions was assessed by examining the average root and shoot growth of the three model crop seeds. Lower root and stem growth inhibition indicates lower solution toxicity. As shown in Figure 8a–c, for peas, those grown in distilled water exhibited longer roots and stems, with an average length longer than those grown in the degraded culture medium and the pollutant culture medium, while the treated culture medium also performed better than the pollutant solution. Similarly, the same growth trend was observed in wheat and Shanghai greens. Additionally, it was found that the average root length after treatment was closer to that in distilled water but significantly better than that in the untreated RhB solution; this indicates that the ecological toxicity of the RhB solution treated with the NCMR800–2/PMS system was significantly reduced, and NCMR800–2 demonstrated satisfactory catalytic performance.
The evaluation of several toxicity parameters using TEST software (Toxicity Estimation Software Tool, version 5.1) provides a comprehensive understanding of the potential environmental risks of this catalyst system during degradation. As shown in Figure 8d–f, developmental toxicity, bioaccumulation factors and mutagenicity were evaluated. In these tests, the degradation products P11 and P12 showed low or negligible toxicity. Test results showed that the biological toxicity and developmental toxicity of the main intermediate products generated during degradation exhibited a significant decreasing trend, with the toxicity of the main end products significantly reduced or non-toxic; although some intermediate products still exhibited toxicity during degradation, their concentrations were reduced by extending the reaction time, thereby lowering their toxicity to aquatic ecosystems. In summary, the present study introduces an economical and straightforward way to prepare modified carbon-based catalysts for the green and efficient removal of pollutants from water.

3. Experiments

3.1. Chemicals

Urea (CH4N2O), peroxymonosulfate (PMS), L–histidine(L–His), tert-butanol (C4H10O), furfuryl alcohol (FFA), methylene blue (MB), tetracycline hydrochloride (HTC), Acid Orange 7(AO7), Oxytetracycline(OTC), sodium chloride (NaCl), sodium hydrogen carbonate (NaHCO3), sodium nitrate (NaNO3), sodium sulfate (Na2SO4), sodium hydroxide (NaOH), trisodium phosphate (Na3PO4), and methanol (MeOH) were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Naphthol, humic acid (HA), and Rhodamine B (RhB) were obtained from Macklin Biochemical Co., Ltd. (Shanghai, China). 5,5-dimethyl−1-pyrroline-N-oxide (DMPO) and 2,2,6,6-tetramethyl−4-piperidinol (TEMP) were purchased from J&K Scientific Ltd., Shanghai, China. Peas, wheat, shanghai cabbage, and Chinese medicine residue (CMR) were collected from Hanzhong Tianyang Biotechnology Co., Ltd., Hanzhong, Shaanxi, China. All chemicals are highly purified reagents without further purification.

3.2. Preparation and Characterization of Catalysts

The preparation procedure of NCMRT-X material is illustrated in Figure 1a, with comprehensive experimental details available in the Supporting Information.

4. Conclusions

This work reports the development of a highly efficient catalyst based on nitrogen-doped biochar (NCMR800–2) derived from waste CMR using a one-step pyrolysis method, specifically designed for the removal of synthetic dyes (RhB) from water. Experimental results demonstrate that NCMR800–2 achieved 100% removal of RhB—a primary target dye in this study—within 30 min, significantly outperforming untreated traditional Chinese medicine residues and other comparison samples; its high catalytic performance is attributed to the significant increase in specific surface area due to nitrogen doping, which introduces abundant mesoporous structures and active sites, thereby enhancing adsorption capacity and PMS activation capacity for RhB. Further characterization techniques confirmed the successful nitrogen doping and its regulatory effect on material structure and chemical state. In practical applications, the NCMR800–2/PMS system demonstrated excellent RhB removal efficiency under various pH conditions and ionic interference, exhibiting strong interference resistance and broad applicability. Cycling experiments showed that the catalyst maintained an 88.54% removal rate after four cycles, demonstrating good cycling stability. Investigations into the reaction mechanism indicate that 1O2 serves as the predominant reactive species in the NCMR800–2/PMS system for RhB degradation. Electrochemical analysis further revealed that nitrogen doping improved conductivity of the catalyst and enhanced its electron transfer efficiency, thereby facilitating the decomposition of PMS and the degradation of RhB. Additionally, toxicity assessment experiments confirmed the active sites and reaction mechanism of NCMR800–2 and demonstrated that the treated RhB solution exhibited significantly reduced toxicity in simulated crop growth conditions. In summary, NCMR800–2, as a low-cost, high-efficiency catalyst, holds promising application prospects in the field of water treatment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15100926/s1, Figure S1: Pore size distributions of catalysts; Figure S2: The XPS gross mapping of NCMR800–2 (a) C 1s; (b) O 1s; (c) N 1s; (d) Si 2p; (e) Al 2p; Figure S3. RhB removal experiments in various water matrixes; Figure S4: Flow cycle apparatus diagram; Figure S5: EPR spectra of (a) TEMP-O2•−, (b) DMPO-•OH and DMPO-SO4•− adduct; Figure S6: m/z of intermediates obtained by LC-MS analysis; Figure S7: TOC removal efficiency for RhB in the PMS process ([RhB] = 20 mg/L, [PMS] = 20 mg/L, catalyst dosage = 0.15 g/L, T = 298 K); Table S1: Pore characteristics of carbon materials.

Author Contributions

Conceptualization, X.L. (Xiao Liu) and D.L.; methodology, W.Z.; software, X.G.; validation, W.Z. and X.G.; formal analysis, X.S.; investigation, D.L.; resources, X.S.; data curation, D.L. and W.Z.; writing—original draft preparation, X.L. (Xiaoyun Lei); writing—review and editing, X.L. (Xiaoyun Lei); visualization, D.L. and X.G.; supervision, T.W.; project administration, X.S.; funding acquisition, T.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Shaanxi Province Science and Technology Innovation Team Project, grant number 2025RS-CXTD-040 and the Shaanxi University of Technology Doctoral Research Start Foundation, grant number SLGRCQD004.

Data Availability Statement

Data will be made available on request.

Acknowledgments

The original contributions presented in this study are included in the article and Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00926 g001
Figure 1. (a) Preparation scheme for NCMRT-X material, (b,c) SEM, (d,e) TEM, and (f–j) TEM mapping.
Figure 1. (a) Preparation scheme for NCMRT-X material, (b,c) SEM, (d,e) TEM, and (f–j) TEM mapping.
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Figure 2. (a) N2 adsorption–desorption isotherms, (b) XRD patterns of prepared samples, (c) Raman spectra, (d) full-range XPS surveys of NCMR800–2.
Figure 2. (a) N2 adsorption–desorption isotherms, (b) XRD patterns of prepared samples, (c) Raman spectra, (d) full-range XPS surveys of NCMR800–2.
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Figure 3. Removal of RhB using (a) various processes, (b) different preparation temperatures, (c) different doping ratios, influence of (d) NCMR800–2 dose, (e) PMS dose, (f) pollution concentration. Reaction conditions: [RhB] = 20 mg/L, [Catalyst] = 0.15 g/L, [PMS] = 0.15 g/L, T = 25 °C.
Figure 3. Removal of RhB using (a) various processes, (b) different preparation temperatures, (c) different doping ratios, influence of (d) NCMR800–2 dose, (e) PMS dose, (f) pollution concentration. Reaction conditions: [RhB] = 20 mg/L, [Catalyst] = 0.15 g/L, [PMS] = 0.15 g/L, T = 25 °C.
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Figure 4. Influence of inorganic anions on the NCMR800–2/PMS system: (a) Cl−, (b) NO3−, (c) SO42−, (d) PO43−, (e) HA and (f) impact of initial pH on RhB removal. Reaction conditions: [RhB] = 20 mg/L, [Catalyst] = 0.15 g/L, [PMS] = 0.15 g/L, T = 25 °C.
Figure 4. Influence of inorganic anions on the NCMR800–2/PMS system: (a) Cl−, (b) NO3−, (c) SO42−, (d) PO43−, (e) HA and (f) impact of initial pH on RhB removal. Reaction conditions: [RhB] = 20 mg/L, [Catalyst] = 0.15 g/L, [PMS] = 0.15 g/L, T = 25 °C.
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Figure 5. (a) Zeta potential distribution at different pH, (b) experiments on the universality of the NCMR800–2/PMS system, (c) pollutant removal by catalyst over 40 h, and (d) cycling experiments.
Figure 5. (a) Zeta potential distribution at different pH, (b) experiments on the universality of the NCMR800–2/PMS system, (c) pollutant removal by catalyst over 40 h, and (d) cycling experiments.
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Figure 6. (a) Effect of chemical quenchers, (b) EPR spectra of PMS activation in TEMP solutions by the NCMR800–2/PMS syste m, (c) Raman of single PMS and NMCR800–2/PMS systems, electrochemical testing (d) CV scans, (e) EIS analysis, (f) I-t curves.
Figure 6. (a) Effect of chemical quenchers, (b) EPR spectra of PMS activation in TEMP solutions by the NCMR800–2/PMS syste m, (c) Raman of single PMS and NMCR800–2/PMS systems, electrochemical testing (d) CV scans, (e) EIS analysis, (f) I-t curves.
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Figure 7. Proposed degradation pathways of RhB degradation by NCMR800–2/PMS system.
Figure 7. Proposed degradation pathways of RhB degradation by NCMR800–2/PMS system.
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Figure 8. Seed germination experiments: (a) pea, (b) wheat, (c) Shanghai greens (d–f) ecotoxicity analysis of RhB and its intermediates.
Figure 8. Seed germination experiments: (a) pea, (b) wheat, (c) Shanghai greens (d–f) ecotoxicity analysis of RhB and its intermediates.
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Lei, X.; Liu, D.; Zhou, W.; Liu, X.; Gao, X.; Wang, T.; Shao, X. Nitrogen-Enriched Porous Carbon from Chinese Medicine Residue for the Effective Activation of Peroxymonosulfate for Degradation of Organic Pollutants: Mechanisms and Applications. Catalysts 2025, 15, 926. https://doi.org/10.3390/catal15100926

AMA Style

Lei X, Liu D, Zhou W, Liu X, Gao X, Wang T, Shao X. Nitrogen-Enriched Porous Carbon from Chinese Medicine Residue for the Effective Activation of Peroxymonosulfate for Degradation of Organic Pollutants: Mechanisms and Applications. Catalysts. 2025; 15(10):926. https://doi.org/10.3390/catal15100926

Chicago/Turabian Style

Lei, Xiaoyun, Dong Liu, Weixin Zhou, Xiao Liu, Xingrui Gao, Tongtong Wang, and Xianzhao Shao. 2025. "Nitrogen-Enriched Porous Carbon from Chinese Medicine Residue for the Effective Activation of Peroxymonosulfate for Degradation of Organic Pollutants: Mechanisms and Applications" Catalysts 15, no. 10: 926. https://doi.org/10.3390/catal15100926

APA Style

Lei, X., Liu, D., Zhou, W., Liu, X., Gao, X., Wang, T., & Shao, X. (2025). Nitrogen-Enriched Porous Carbon from Chinese Medicine Residue for the Effective Activation of Peroxymonosulfate for Degradation of Organic Pollutants: Mechanisms and Applications. Catalysts, 15(10), 926. https://doi.org/10.3390/catal15100926

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