Highly selective generation of singlet oxygen via peroxymonosulfate activation by Mn–N supported biochar

In recent years, water pollution has increasingly garnered public attention, with the emergence of antibiotics further exacerbating the issue.^1,2 With the growing population, antibiotic levels in soil have been increasing annually, especially for sulfa drugs like sulfamethoxazole (SMX), sulfafurazole (SIZ), and sulfasalazine (SSZ).^3–5 Studies have shown that 50%–90% of sulfonamides and their metabolites enter the environment and persist through urine, feces, and medical wastewater.^6,7 In addition, trace amounts of sulfa drugs remaining in water bodies will produce significant toxicity and cause irreversible pollution to organisms.⁸ Consequently, it is of great significance to explore efficient and sustainable approaches for the degradation of sulfa drugs.

Advanced oxidation technologies (AOTs) are widely used in the degradation of persistent or hard-to-degrade pollutants in water because they can generate a large number of reactive oxidizing species (ROS).⁹ Peroxymonosulfate (PMS), as a common oxidizing agent, has been widely used in AOTs due to its high efficiency and mild reaction conditions.¹⁰ It was reported that PMS activation mainly consists of thermal activation, electrical activation, transition metal activation, etc.^11–13 Among these approaches, transition metals are extensively utilized due to their low cost and high safety.¹⁴ It is worth noting that doping transition metals with non-metals could greatly improve catalytic activity and accelerate the degradation of pollutants.¹⁵ However, transition metals frequently demonstrate a limited binding affinity with substrate materials and exhibit suboptimal stability.¹⁶ Therefore, exploring new loading materials is crucial to addressing this limitation effectively.

Loading active sites onto a carbon-based material is an effective solution to overcome this limitation.¹⁷ Biochar (BC) is prevalently applied in pollutant degradation because of its considerable specific surface area, excellent adsorption performance, and reduced metal ion spillover.¹⁸ Research indicated that incorporating active sites onto BC improved PMS activation and facilitated the production of ROS.¹⁹ At the same time, loading transition metals will significantly increase the activation degree of PMS and further promote the degradation of pollutants.²⁰ Among various transition metals, the Mn atom exhibits high stability, excellent electron transfer capabilities, and enhanced reactivity with PMS.²¹ The simultaneous introduction of non-metals and transition metals enhanced the adsorption capacity of BC, promoted the activation of PMS, and accelerated the rapid degradation of pollutants.²² Specifically, anchoring N atom onto BC activates the free-flowing π electrons, causing the O–O bond in PMS to break and generating a significant amount of radicals.²³ The creation of Mn–N bonds alters the electronic polarization of BC, enhances the stability of the activated carbon particle structure, and significantly facilitates the degradation of contaminants.²⁴ Nevertheless, research on the activation of PMS by Mn–N sites loaded on BC for pollutant degradation was limited, and the underlying mechanism was unclear. Therefore, further investigation into its pollutant degradation mechanism was necessary.

In this study, a novel material with Mn–N reactive sites loaded on BC (MnN@BC) was developed to activate PMS for the degradation of SMX. The factors influencing the degradation process and the efficiency of synergistic effects were systematically examined. This study aims to utilize X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) to describe MnN@BC and examine the structural alterations in BC resulting from the incorporation of active sites. Density functional theory (DFT) analyzed the changes in the degradation mechanism after anchoring the Mn–N active sites to BC. In addition, cyclic experiments explored its application in actual water environments. In summary, this study clarified the strategic design and precise manufacturing of transition metal–nonmetal dual active sites to promote pollutant degradation, laying a theoretical foundation for the development of water treatment innovation.

All chemicals used in the experiment were of analytical grade and obtained from McLean Reagent Company.

The straw underwent calcination at 1073.15 K for 3 h, and the resultant carbonization product was labeled as BC. A total of 0.4 g of BC, 0.7 g of urea, and 0.5 g of manganese chloride tetrahydrate (MnCl₂ · 4H₂O) were meticulously disseminated in a solution of 35 ml of N,N-dimethylformamide (DMF) and 15 ml of deionized water (DI). The resultant solution is subjected to heating at 453.15 K for a duration of 12 h. MnN@BC was acquired using centrifugation and subsequent drying following washing. The specific preparation method is illustrated in Fig. 1.

The morphological characteristics of the MnN@BC catalyst were inspected by SEM (ZEISS Sigma 360). The existence and proportion of elements in the material were determined by energy dispersive spectroscopy (EDS). The specific surface area and pore size distribution of the catalyst material were measured and analyzed through BET (Micromeritics ASAP 2460). The crystal structure and phase composition of MnN@BC were characterized with a Rigaku Ultima IV X-ray diffractometer (XRD). The chemical state of the elements in the MnN@BC catalyst was analyzed via X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) using Kα radiation (hv = 1486.6 eV). The surface functional groups of MnN@BC were characterized using attenuated total reflection Fourier transform infrared spectroscopy (Thermo Fisher Scientific Nicolet iS5).

All experiments were conducted in a 250 ml beaker, which served as the reaction vessel. A magnetic stirrer was employed to maintain a stirring speed of 800 rpm at a temperature of 298.15 K. During the experiment, the catalyst was added to 200 ml of a solution containing PMS and pollutants to initiate the degradation reaction. The pH and temperature were not adjusted unless otherwise specified in the experiment.

The impact of multiple parameters on the degrading efficiency of organic pollutants was examined by altering the catalyst quantity, PMS concentration, experimental temperature, and beginning pH value within the MnN@BC/PMS system. NaOH and H₂SO₄ were used to adjust the pH value of the reaction.

The involvement of SMX-degrading free radicals in the MnN@BC/PMS system was thoroughly examined. A specific quantity of quenching agents, including ethanol, tert-butyl alcohol, p-benzoquinone, and L-histidine, was introduced into the reaction mixture to investigate the effects of various free radicals on the reaction process. The rate constants between the quenchers and ROS are detailed in Table S1. All other experimental parameters were maintained constant.

The utilized MnN@BC was recycled, subjected to three washings with deionized water and ethanol, and subsequently placed in a 373.15 K oven to dry prior to the subsequent experiment.

The detailed calculation method can be found in reference Text S1.

The specific calculation details can be found in Text S2.

SEM and EDS were utilized to investigate the structural characteristics and elemental composition of MnN@BC. As shown in Fig. 2(a), the SEM image of BC reveals a fibrous structure characterized by numerous cracks and exfoliations, indicating a relatively rough surface that provides a favorable substrate for the loading of Mn–N sites. After the Mn–N active site was anchored to BC, distinct point-like sediments were observed, exhibiting a highly porous structure that may have significantly enhanced the specific surface area of BC [Fig. 2(b)]. Figure 2(c) illustrated that MnN@BC had an intricate flower-like architecture, characterized by a substantial specific surface area that facilitated the uniform distribution of Mn–N sites throughout its surface and defect regions. The EDS spectra further validated the uniform distribution of C, O, Mn, and N in MnN@BC [Figs. 2(d)–2(g)]. Additionally, the elemental compositions are detailed in Fig. S1. It was affirmed that the Mn–N site was successfully loaded onto BC.

The crystal structures of MnN@BC were analyzed using XRD. Figure 3(a) illustrated a refraction peak at 26° corresponding to the characteristic plane (002) of graphitic carbon (JCPDS No. 01-0640), signifying the formation of graphitic carbon.²⁵ The diffraction peaks at 28.62°, 36.059°, 37.685°, 38.994°, and 41.634° corresponded to the (002), (012), (020), (210), and (102) crystal planes of N₂O₄ (JCPDS No. 74-2264), respectively. The diffraction peaks at 32.382°, 44.409°, 53.864°, 56.023°, and 58.498° were indicated for Mn₃O₄ (JCPDS No. 80-0382) of (103), (220), (312), (303), and (321) crystal faces. In addition, the characteristic peaks of MnN@BC at 24.341° (012), 31.475° (104), 41.593° (113), 45.359° (202), 49.877° (024), 51.914° (116), 59.417° (211), and 60.382° (122) were consistent with MnCO₃ (JCPDS No. 86-0173). This suggested that MnN@BC exhibits high crystallinity, which can further enhance the activation of PMS and facilitate the degradation of pollutants.^25,26

FTIR was employed to further investigate the surface functional groups of MnN@BC [Fig. 3(b)]. Distinct peaks at 3423, 2923, 1404, and 1067 cm⁻¹ were seen in the spectra of BC and MnN@BC, corresponding to –OH, C–H, C=O, and C–O–C functional groups, respectively.²⁷ Compared with BC, MnN@BC presented a new characteristic peak at 1580 cm⁻¹, corresponding to the C–N functional group.²⁸ It was stated that the distinctive peaks of transition metals are mainly centered at 400–800 cm⁻¹.²⁹ Therefore, the new characteristic peak of MnN@BC at 602 cm⁻¹ corresponded to the functional groups of Mn–N. The results show that the Mn–N active site is successfully anchored into BC.

The BET analysis was further conducted on the pore structure of MnN@BC. Figure 4 illustrated that the adsorption–desorption curve of MnN@BC for nitrogen corresponds to a type IV isotherm with an H3 hysteresis loop, signifying a mesoporous structure.³⁰ Furthermore, following the incorporation of Mn–N active sites, both the specific surface area and pore volume of BC increased (Table I), likely attributable to structural alterations induced by the doping of Mn and N particles. This suggested that Mn–N doping of BC significantly enhanced its specific surface area, likely promoting PMS activation and facilitating pollutant degradation.³¹ 

The XPS analysis was conducted to further investigate the elemental composition and functional groups present in MnN@BC. The XPS full spectrum [Fig. 5(a)] indicated that the prepared catalyst contains C, O, Mn, and N elements with contents of 56.82%, 28.54%, 11.62%, and 3.02%, respectively. The C 1s spectra exhibited three diffraction peaks at 284.6, 286.33, and 289.31 eV, which correspond to C=O, C=N, and MCO₃ bonds, respectively [Fig. 5(b)]. Figure 5(c) presented the Mn 2p spectrum, with peaks at 642.1, 645.32, and 653.91 eV corresponding to Mn(III), Mn(IV), and Mn2p_1/2, respectively. Additionally, the N 1s spectrum shown in Fig. 5(d) has diffraction peaks at 398.09, 400.06, and 401.97 eV, corresponding to oxidized N, pyrrolic N, and pyridine N, respectively. These findings further validated the effective anchoring of Mn–N active sites onto BC.

The MnN@BC/PMS system was developed to facilitate the degradation of SMX in order to evaluate its effectiveness in pollutant removal. Figure 6(a) illustrated that the removal of SMX predominantly took place within the initial 15 min following the introduction of PMS, leading to a reduction of 20.2%. After 15 min, the ability of PMS to degrade SMX significantly weakened, with only a 7% reduction, indicating that non-activated PMS has limited oxidative degradation capacity for SMX.³² The adsorption and degradation efficiencies of SMX in the BC and MnN@BC systems were 10.47% and 19.14%, respectively, within 40 min. This indicated that the introduction of Mn–N active sites enhanced the adsorption capacity of BC for pollutants. In the BC/PMS system, the SMX degradation efficiency after 40 min was only 70.8%, whereas the MnN@BC/PMS system significantly enhanced the efficiency to 96.95% within the same timeframe. This indicated that the incorporation of Mn–N sites into BC significantly improved the degrading efficiency of SMX, likely due to the augmented number of active sites and the expanded specific surface area. Figure 6(b) showed the reaction rate constants for SMX degradation under different systems. The rate constants for PMS alone, BC, MnN@BC, BC/PMS, and MnN@BC/PMS systems were 0.0109, 0.0043, 0.0079, 0.0462, and 0.1301 min⁻¹, respectively. It was worth noting that, as shown in Table II, the synergy index (SI) for SMX removal in the MnN@BC/PMS system was 6.92, indicating a significant synergistic effect between the PMS and MnN@BC adsorption processes. It was further confirmed that the incorporation of Mn–N sites onto BC significantly enhanced the activation efficiency of PMS, thereby improving the removal of SMX.

To find out the optimal efficacy of MnN@BC/PMS on the degradation of SMX, the influences of various amounts of MnN@BC, PMS dose, initial pH, and initial temperature were examined (Fig. 7). As shown in Fig. 7(a), as the MnN@BC concentration increased from 0.1 to 0.15 g/l, the degradation efficiency of SMX increased by 10.45% within 40 min (the reaction rate constant increased by 0.0313 min⁻¹). After the MnN@BC concentration was subsequently increased to 0.2 g/l, the degradation efficiency of SMX was further improved, with 95.32% degraded in only 30 min (the degradation efficiency was 96.95% within 40 min). This was due to the increased MnN@BC concentration, which provided more active sites and promoted the degradation reaction. However, after the MnN@BC concentration increased to 0.3 g/l, the degradation efficiency of SMX only increased slightly to 97.81% (the reaction rate constant only increased by 0.006 min⁻¹). This phenomenon may be attributed to the excessive presence of MnN@BC in the system, which results in a depletion of $SO4•−$⁠.³³ Therefore, the MnN@BC concentration of 0.2 g/l was selected for subsequent experiments.

The initial pH is a significant factor influencing the breakdown of contaminants in a PMS-activated system. In the MnN@BC/PMS system, the influence of pH on the degradation of SMX is relatively minimal [Fig. 7(c)]. As the solution pH was within the range of 3.00–9.00, the degradation efficiency of SMX exceeds 95%, with a reaction rate constant greater than 0.683 min⁻¹ (Fig. S2). It is noteworthy that, relative to neutral conditions, the degradation efficiency of SMX under acidic conditions exhibits a modest increase of 2.7%. This behavior may be ascribed to the enhanced activation of PMS in an acidic environment. However, after the pH further increased to 11.0, the degradation efficiency of SMX decreased to 92.87%. This is due to $SO4•−$ and •OH being converted into $SO5•−$ and $HSO5−$ with poor oxidizing properties, thereby inhibiting the degradation of SMX.³⁵ Therefore, a neutral environment with pH = 7.00 was selected for further experiments.

The initial temperature is another critical factor influencing pollutant degradation. As illustrated in Fig. 7(d), the elevation of the initial temperature from 298 to 318 K led to a notable reduction in SMX degradation efficiency, decreasing from 96.95% to 81.03%. This is different from most experiments. In other studies, as the initial temperature increases, the reaction accelerates and the pollutant degradation efficiency increases. The reason for this result may be that the material itself is more suitable for activating PMS under 298 K conditions, while higher temperatures may lead to side reactions, thereby inhibiting the catalytic activity of MnN@BC.³⁶ The degrading efficiency of SMX at a starting temperature of 288 K was examined to corroborate this. Under the initial temperature of 288 K, the degradation efficiency and reaction efficiency constant of SMX were 89.2% and 0.0704 min⁻¹ (Fig. S3), respectively, which were lower than the initial temperature of 298 K. The results presented earlier demonstrate the existence of an optimal initial temperature, specifically at 298 K. Therefore, the MnN@BC concentration of 0.2 g/l, PMS dosage of 5 mM, neutral conditions, and room temperature were selected for subsequent research.

The evaluation of ROS generated from the degradation of SMX in the MnN@BC/PMS system was conducted through quenching studies. Figure 8(a) illustrated that in the MnN@BC/PMS system, the degradation efficiencies of SMX diminished by 24.94% and 41.12% within 40 min, respectively, upon the introduction of 50 mM of L-histidine and p-BQ. This indicated that $O2•−$ and ¹O₂ were significant in the degradation of SMX. It is inferred that part of the ¹O₂ is produced by $O2•−$ [Eqs. (2) and (3)].³⁷ In addition, the degradation efficiency of SMX in the MnN@BC/PMS system decreased by 36.07% and 35.18% upon the addition of 2000 mM TBA and EtOH, respectively. This result shows that •OH and $SO4•−$ in the MnN@BC/PMS system are produced to a certain extent to degrade SMX. After EtOH and TBA, the K_obs decreased from 0.1301 to 0.0258 and 0.0243 min⁻¹, respectively [Fig. 8(b)]. It is important to highlight that the addition of L-histidine to the MnN@BC/PMS system resulted in a significant decrease in the reaction rate constant, which was reduced by 92.46%. This further underscores the significant role that ¹O₂ plays within the reaction system. Furthermore, at the addition of 50 mM p-BQ, the K_obs diminished to 0.0187 min⁻¹. The aforementioned results reaffirmed the existence of •OH, $SO4•−$⁠, $O2•−$⁠, and ¹O₂ within the MnN@BC/PMS system.

The potential mechanisms by which MnN@BC activates PMS to produce ROS were further investigated. XPS analysis was conducted on the MnN@BC after four reactions. As shown in Fig. 5(b), the intensities of the C=C bond, C=N bond, and MCO₃ diffraction peaks in the C 1s spectrum significantly decreased, indicating that the chemical structure of MnN@BC was altered during the activation of PMS to remove SMX. The Mn 2p spectrum [Fig. 5(c)] revealed a decrease in the proportions of Mn(IV) and Mn(III), indicating that valence transformation of Mn occurred during the activation of PMS for SMX degradation, highlighting its involvement in the reaction process. The N 1s spectrum [Fig. 5(d)] showed that after four reactions, the pyridine N and oxidized N peaks in MnN@BC disappeared, further confirming that the Mn–N sites were likely the key active sites for activating PMS to remove SMX.

Furthermore, to gain deeper insights into the role of Mn–N active sites in facilitating PMS activation, a comprehensive study was conducted utilizing DFT calculations. As shown in Fig. 9, loading Mn–N sites onto BC increased the charge transfer during PMS adsorption from 0.054e⁻ to 0.086e⁻ compared to BC alone. This indicated that the loading of Mn–N sites significantly enhanced electron transfer, thereby further promoting the generation of ROS. In addition, the introduction of Mn–N active sites significantly altered the electronic structure of the BC surface, redistributed local charges, and disrupted the symmetry of the electron distribution. This asymmetric electron distribution further enhanced the adsorption of PMS and pollutants, thereby promoting their degradation. The O–O bond length of PMS increased from 1.480 to 1.487 Å following the introduction of active sites, which enhances ROS formation and consequently facilitates more effective pollutant degradation.

The durability and recyclability of catalyst materials are crucial for their practical applications. As shown in Fig. 10(a), the SMX degradation efficiencies within 40 min across four consecutive cycles were 96.95%, 94.96%, 90.99%, and 87.16%, respectively. Correspondingly, the reaction rate constant decreased from 0.1301 to 0.0884 min⁻¹ [Fig. 10(b)]. This indicated that MnN@BC still maintains a high degradation rate after four consecutive cycles. In addition, the reduction of MnN@BC after four times of recycling may be due to the loss of active sites and pore blockage on the BC surface, which hinders PMS activation and results in lower degradation efficiency.⁴² This was consistent with the Mn 2p and N 1s XPS spectra obtained after four reactions. Additionally, the leaching amount of Mn ions in all four experiments remained below 0.1 mg/l.

This study effectively synthesized a new Mn–N loaded BC material for the activation of PMS to enhance the breakdown of SMX. Under optimal conditions, the degradation efficiency of MnN@BC/PMS for SMX achieved 96.95% within a duration of 40 min. The findings indicated that MnN@BC possesses a graphitized structure, an abundance of active sites, and a high specific surface area. DFT calculations indicated that the incorporation of the Mn–N site alters the electron polarization of BC, enhances electron transfer between PMS and BC, accelerates the cleavage of O–O bonds in PMS, and consequently facilitates the generation of ROS. The free radical quenching experiments confirmed that •OH, $SO4•−$⁠, $O2•−$⁠, and ¹O₂ can be generated in the MnN@BC/PMS system, thereby facilitating the degradation of SMX. In conclusion, this study presented a novel strategy for the development of BC materials that leverages dual active sites, thereby offering valuable insights for the efficient degradation of pollutants.

See the supplementary material for detailed information about the degradation of pollutants and density functional calculations.

This work was supported by the 2023 Science and Technology project of Chongqing City Vocational College (Grant No. XJKJ202300002) and the Research and Innovation Program for Graduate Students in Chongqing Jiaotong University (Grant No. 2024S0048).

The authors have no conflicts to disclose.

Huan Wu: Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Fangfang Ji: Writing – review & editing (equal). Bin Zhou: Data curation (supporting); Investigation (supporting). Shikun Gao: Conceptualization (equal); Project administration (equal). Zhe Zhang: Methodology (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.