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Article

Efficiency and Mechanism of a Hollow Carbon-Based Single-Atom Iron Catalyst in Activating Periodate for Bisphenol a Degradation

by
Chen Ling
1,
Mengyue Yuan
1,
Shang Gao
2,*,
Yuzhu Xue
1 and
Yuwei Pan
1,*
1
College of Ecology and Environment, Nanjing Forestry University, Nanjing 210037, China
2
Nanjing Institute of Environmental Sciences, Ministry of Ecology and Environment, Nanjing 210042, China
*
Authors to whom correspondence should be addressed.
Water 2025, 17(18), 2705; https://doi.org/10.3390/w17182705 (registering DOI)
Submission received: 7 August 2025 / Revised: 4 September 2025 / Accepted: 11 September 2025 / Published: 13 September 2025
(This article belongs to the Special Issue The Occurrence, Fate and Removal of Emerging Contaminants in Wastewater, 2nd Edition)
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Abstract

Developing efficient and recyclable periodate (PI)-based advanced oxidation processes (AOPs) for the removal of emerging organic pollutants (EOPs) has attracted considerable attention. However, the structure–activity relationship of single-atom catalyst in PI-AOP systems remains poorly understood. In this study, a hollow carbon-supported single-Fe atom catalyst (HCFe800) was synthesized and applied for PI activation toward bisphenol A (BPA) degradation. Under neutral pH and ambient temperature, HCFe800 enabled complete removal of BPA within 1 min, achieving a degradation rate constant (k) of 5.094 min−1—approximately 3 and 10 times higher than that of Fe-free and solid control catalysts, respectively. After normalization, the apparent degradation rate constant was 1–3 orders of magnitude greater than those of previously reported catalysts. The optimized Fe doping amount and pyrolysis temperature facilitated the formation of atomically dispersed FeN4 sites, which outperformed Fe clusters and iron oxides in catalytic activity. The hollow porous structure further enhanced the exposure of active sites, contributing to the exceptional performance. The HCFe800/PI system remained highly effective across broad pH (3–7) and temperature (5–35 °C) ranges and in the presence of 100-fold concentrations of common inorganic ions. Mechanistic studies revealed that the main reactive species were 1O2, O2•−, and IO3•, with negligible involvement of high-valent Fe species. Eight less-toxic BPA degradation products were identified. Moreover, the system was extendable to various other EOPs and exhibited excellent recyclability via thermal regeneration. This work provided fundamental insights into designing and applying single-atom catalysts for PI-based advanced treatment of EOPs.

1. Introduction

Bisphenol A (BPA), a typical emerging contaminant, has garnered widespread attention due to its environmental persistence, bioaccumulation, and adverse impacts on human health, including reproductive toxicity, neurodevelopmental disorders, and metabolic dysfunction [1]. Its ubiquitous presence in aquatic environments raises serious public health concerns and calls for the urgent development of efficient, co-friendly treatment strategies. Among various methods, advanced oxidation processes (AOPs) have emerged as promising approaches owing to their high efficiency and rapid degradation of recalcitrant organic pollutants [2,3,4,5].
Periodate (PI), as a novel oxidant in AOPs, is gaining increasing interest because of its strong oxidative potential, high chemical stability, and environmental compatibility [6]. Compared to traditional oxidants such as peroxymonosulfate and peroxydisulfate, PI offers unique advantages including a broader pH range of activity and reduced interference from background constituents, positioning it as an attractive alternative oxidant for water treatment applications.
Metal-based catalysts, including metal clusters, metal oxides/sulfides, have demonstrated remarkable potential for PI activation [7], primarily because the metal sites can coordinate with IO4− and engage in the reversible redox cycles, facilitating the electron transfer and promoting the generation of reactive oxygen species (ROS). In our previous work, zero-valent iron exhibited significantly enhanced PI activation after sulfidation treatment, enabling complete degradation of sulfonamide antibiotics within 1 min [8]. However, this system suffered from substantial Fe ion leaching into the aqueous phase during operation, raising concerns of secondary pollution. To overcome the limited efficiency of conventional metal sites, as well as the problems of leaching and instability, recent studies have focused on the development of single-atom catalysts supported on pyrolytic carbon frameworks (PyC-SACs) [9,10]. In these catalysts, individual metal atoms (e.g., Fe, Mn, Cu, Ni, Rh, or dual configurations) are atomically dispersed and stabilized in pyrolytic carbon matrices [11,12]. PyC-SACs exhibit high adsorption capacity, excellent electrical conductivity, and superior thermal and acid–base stability [13]. The first two features, in particular, enhance the collision frequency between reactants and minimize the non-productive consumption of reactive species, ultimately enhancing the catalytic efficiency [14]. Consequently, PyC-SACs have emerged as promising platforms for the efficient catalytic degradation of organic pollutants, especially aromatic compounds such as bisphenol A (BPA).
Nevertheless, compared with other oxidant-based AOP systems, the performance of PyC-SACs for PI activation and their structure–activity relationships remain largely unexplored. For example, the effects of catalyst morphology, metal loading amount, and the water matrices are still poorly understood, yet they are of critical importance in guiding the scientific design and application of PyC-SACs in PI-AOPs. Given the natural abundance, reactivity, and environmental friendliness of iron, it has long been considered one of the most favorable metal centers for SACs. Therefore, in this work, a hollow PyC–supported Fe single-atom catalyst (HCFe800) was synthesized via a template-assisted strategy. We hypothesized that HCFe800 could enable multiple synergistic effects in PI-mediated AOPs. Specifically, atomically dispersed Fe centers serve as highly efficient catalytic sites for PI activation, while the porous graphitic carbon matrix facilitates the adsorption and enrichment of BPA molecules near the active sites. The hollow porous carbon architecture is expected to further increase the exposure of active sites and shorten the mass transfer path, thereby enhancing both adsorption and catalytic efficiency. Overall, this work systematically investigated the performance of the HCFe/PI system in BPA degradation, aiming to deepen the mechanistic understanding of PI-AOPs and to support the development of effective strategies for emerging pollutant removal.

2. Materials and Methods

2.1. Materials

All chemical reagents used in this study were of analytical grade and used without further purification. Bisphenol A, Rhodamine B, and humic acid were purchased from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China. Sulfadiazine, periodate, tert-butyl alcohol (TBA), methanol (MeOH), tetraethyl orthosilicate (TEOS), hydrochloric acid (HCl), and ammonium bifluoride were obtained from China National Pharmaceutical Group Chemical Reagent Co., Ltd., Shanghai, China. L-histidine (L-His), p-benzoquinone (p-BQ), phenol, sodium hydroxide (NaOH), and dimethyl sulfoxide (DMSO) were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Dopamine hydrochloride was supplied by Shanghai Meirui Chemical Technology Co., Ltd., Shanghai, China. Ultrapure water used in all experiments was produced by a Milli-Q water purification system (Millipore, Merck, Darmstadt, Germany) with a resistivity of 18.2 MΩ·cm. All reagents and solutions were stored and used at room temperature.

2.2. Synthesis of Catalysts

The synthesis procedure of HCFe800 is illustrated in Figure 1. Initially, silica nanospheres were synthesized via the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) under alkaline conditions. Subsequently, dopamine (DA) was added, which self-polymerized on the surface of the SiO2 nanospheres. During the self-polymerization process, Fe ions were introduced by the strong affinity between the amino functional groups of DA and Fe ions. After centrifugation and separation, a Fe/SiO2@PDA precursor was obtained.
This precursor was then subjected to high-temperature pyrolysis in a tubular furnace, resulting in the formation of porous FeC@SiO2 material. Finally, the silica template was etched using ammonium bifluoride (NH4HF2) solution, yielding the target hollow-structured PyC-Fe SAC, HCFe800.

2.3. Catalytic Degradation Experiments

Degradation experiments were carried out at room temperature in a 250 mL beaker. A 100 mL aqueous solution of BPA with an initial concentration ranging from 5 to 20 mg/L was added to the beaker, followed by the addition of a specific amount of catalyst under continuous stirring. PI was then introduced to initiate the reaction. At predetermined time intervals, 0.5 mL of the reaction solution was withdrawn and immediately quenched using an excess amount of methanol. It is noteworthy that preliminary experiments showed that over 95% of the BPA adsorbed onto the pyrolytic carbon-based catalysts could be desorbed in the presence of methanol [15]. To investigate the effect of the initial pH of wastewater on the catalytic system, the pH of BPA solutions was adjusted to different values. Various common coexisting solutes, such as inorganic salts and natural organic matter (represented by humic acid), were added to form binary-solute solutions. Catalytic degradation was then performed under otherwise identical conditions to evaluate their impact on BPA removal. To identify the ROS involved, quenching agents with different scavenging capabilities were added to the solution prior to catalyst addition, while maintaining all other experimental conditions the same as in the degradation tests. TBA was used to scavenge hydroxyl radicals (•OH), while MeOH was employed to quench •OH. p-BQ was used to detect superoxide radicals (O2•−), and L-His was added to capture singlet oxygen (1O2) [5]. At the end of each degradation cycle, the catalyst was separated by filtration and immersed in 50% (v/v) methanol solution for desorption under shaking for 1 h. It was then thoroughly washed with deionized water and dried at 70 °C overnight before being reused in the subsequent cycle. Specific experimental parameters such as concentrations and dosages are provided in detail in the corresponding figures.

2.4. Analytical Methods

The concentration of BPA before and after the reaction was determined using a high-performance liquid chromatography system (HPLC, Dionex Ultimate 3000, Thermo Fisher Scientific, Waltham, MA, USA) equipped with a C18 column (4.6 mm × 250 mm, 5 μm). The detection wavelength was set at 276 nm. The mobile phase consisted of methanol and 0.1% phosphoric acid aqueous solution in a volume ratio of 30:70, with a flow rate of 1.2 mL/min. To detect ROS, 0.5 mL of the reaction mixture was sampled during the stirring reaction between the catalyst and PI at specific time points. The sample was immediately mixed with an appropriate amount of 50 mM DMPO or TEMP solution to trap free radicals (e.g., O2•− and •OH) and 1O2. After 2 min of reaction, the mixture was transferred into a quartz capillary tube and analyzed using an electron paramagnetic resonance spectrometer (EPR, Bruker Magnettech ESR5000, Bruker Co., Berlin, Germany) to detect characteristic signal peaks. The intermediate products formed during the reaction were identified using liquid chromatography–mass spectrometry (LC-MS, Q Exactive, Thermo Fisher Scientific, MA, USA). The samples were filtered through a 0.22 μm membrane before injection. A C18 column (2.1 mm × 100 mm, 1.7 μm) was used for separation. The mobile phase was a mixture of 0.1% formic acid aqueous solution and acetonitrile (volume ratio 70:30). The mass spectrometry scanning range was set from 50 to 3000 m/z.

3. Results and Discussion

3.1. Physicochemical and Structural Properties of the HCFe800 Catalyst

Based on our previous characterization using scanning electron microscopy (SEM, Hitachi S-4800, HITACHI, Tokyo, Japan), transmission electron microscopy (TEM, Hitachi-7700, HITACHI, Tokyo, Japan), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, JEM-ARM200F STEM, JEOL Ltd., Tokyo, Japan) [15], the morphology of HCFe800 was confirmed to be a hollow spherical structure with an average diameter of approximately 200 nm and a shell thickness of around 10 nm. The shell was densely packed with abundant nanopores and uniformly dispersed Fe single atoms, with no observable nanoparticles or clusters. HCFe800 exhibited a high specific surface area of ~760 cm2/g, with nitrogen and iron contents of 2.5% and 1.07%, respectively. X-ray diffraction (XRD, Rigaku RINT 2000, Rigaku Co., Tokyo, Japan), X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM, ULVAC PHI Instruments Co., Ltd, Nanjing, China), and extended X-ray absorption fine structure (EXAFS, BSRF, Beijing Synchrotron Radiation Facility, Beijing, China) analyses revealed that the carbon shell possessed a highly graphitized structure, with an ID/IG ratio of 1.42. The nitrogen species were composed of pyridinic N (37.48%), pyrrolic N (29.25%), and graphitic N/Fe–N coordination (33.19%), while no crystalline iron signals were detected [15]. Pyridinic N was identified as the anchoring site for isolated Fe atoms [16], and the surrounding coordination environment indicated a FeN4 configuration, where each Fe atom was coordinated by four nitrogen atoms [15].

3.2. Excellent BPA Degradation by the Combination of HCFe800 and PI

The removal efficiency of BPA in different reaction systems is shown in Figure 2. When only PI was added, the degradation rate of BPA was merely 6.0% within 8 min, indicating that PI alone was nearly ineffective in degrading BPA. In contrast, in the system containing only HCFe800 without PI, the BPA removal rate reached 25% within 8 min, suggesting that the catalyst alone has a rather limited adsorption capacity for BPA. However, when HCFe800 and PI coexisted, the degradation efficiency of BPA increased dramatically, reaching as high as 98% within just 15 s. This significant synergistic effect confirms that HCFe800 can effectively activate PI, thus enabling the efficient degradation of BPA. The HC800 catalyst, which does not contain iron, was compared with HCFe800. It achieved only a 47% degradation of BPA within 8 min, with a rate constant (k) of 0.558 min−1, indicating that the carbon and nitrogen atoms in HC800 contribute limited catalytic activity toward PI activation. In contrast, after the introduction of single-atom Fe, the HCFe800 catalyst completely removed BPA (100%) within 1 min, and the corresponding k value increased significantly to 5.094 min−1, which is 9.1 times higher than that of the HC800 catalyst. This enhanced activity is attributed to the incorporation of iron atoms into the HC800 catalyst, forming FeN4 coordination structures. As identified in previous studies [17,18,19], the FeN4 structure possesses a relatively high d-band center, which endows it with strong affinity toward the oxoanion PI, and thus makes it an efficient activation site, significantly accelerating BPA degradation. The pronounced difference in activity directly confirms the critical role of single-atom Fe centers in PI activation. Moreover, compared with iron-based and other heterogeneous catalysts (Table 1), the HCFe800/PI system exhibited up to 103-fold higher BPA degradation efficiency, as evaluated by two normalized kinetic constants that account for differences in catalyst dosage and pollutant concentration.

3.3. Study of Factors Influencing BPA Degradation

3.3.1. Effect of the Hollow Structure

The catalytic performance of solid SCFe800, in which the SiO2 core was retained, was evaluated for comparison. Under the same Fe–C shell mass, SCFe800 exhibited significantly lower catalytic activity than HCFe800, with the BPA removal efficiency at 1 min decreasing from 99.44% to 35.31% (in Figure 2). This result indicates that the hollow and porous architecture of HCFe800 enables more efficient utilization of both the inner and outer active sites, enhances the collision frequency between BPA molecules and PI/PI-derived ROS, and thereby substantially accelerates the catalytic degradation process.

3.3.2. Effect of Iron Content in the Catalyst

To investigate the effect of Fe content on the activity of HCFe800, a series of catalysts with varying Fe salt dosages were prepared. The FeCl3 dosage in HCFe800 (Fe-1.0, X = 1) was used as the baseline. As shown in Figure 3a,b, when X increased from 0.5 to 4.0, both the rate constant (k) and BPA removal efficiency improved significantly. The k value rose from 1.304 min−1 (X = 1) to 6.249 min−1 (X = 4), while the BPA removal rate within 1 min increased from 70% to 99%. This enhancement is attributed to the increased density of FeN4 active sites formed by atomically dispersed Fe atoms. However, further increasing the Fe content led to a marked decline in catalytic performance. At X = 16, BPA removal within 1 min dropped to 59%, and the k value decreased to 0.880 min−1. This decline is likely due to the aggregation of Fe atoms into Fe3O4 or Fe3C nanoparticles at high loadings, which reduces the number of effective single-atom active sites [25]. These results also indicate that single Fe atom site is the predominant catalytic site.

3.3.3. Effect of Calcination Temperature

The calcination temperature of PyC-based catalysts influences their catalytic degradation performance by affecting their pore structure and the activity of chemical sites. The catalytic performance of HCFe (600–900) for BPA degradation by PI was compared under calcination temperatures ranging from 600 °C to 900 °C. As shown in Figure 3c,d, the degradation rate constants of BPA for the four catalyst-PI systems followed the trend: HCFe800 > HCFe900 > HCFe700 > HCFe600, with HCFe800 exhibiting a rate constant 2 to 8 times higher than those at other temperatures. Previous characterization results showed that the pore volume and specific surface area of HCFe increased with temperature, which facilitates the exposure of active sites and enhances catalytic activity [15]. However, when the calcination temperature reached 900 °C, the iron species in the HCFe catalyst was no longer dominated by single-atom iron sites but also included noticeable iron clusters. The decline in catalytic performance of HCFe900 further confirms that the single-atom iron sites exhibit higher catalytic activity toward PI activation than other forms of iron sites.

3.3.4. Effects of Dosages and Water Quality Conditions

As shown in Figure 4a, when the initial BPA concentration increased from 10 mg·L−1 to 40 mg·L−1, the removal efficiency within 8 min decreased from 100% to 78%. This indicates that, at constant active site availability, higher pollutant concentrations impose a heavier oxidative load on the HCFe800/PI system, requiring more ROS to sustain catalytic activity [26]. Figure 4b demonstrates that increasing the catalyst dosage significantly enhances BPA removal. When the catalyst dosage increased from 0.02 g·L−1 to 0.10 g·L−1, the 8 min removal efficiency improved from 28% to 100%. At 0.08 g·L−1, 99% removal was achieved within 4 min, while at 0.10 g·L−1, the same efficiency was reached in just 30 s. This is attributed to the increased number of active sites, promoting more efficient PI activation and accelerating the degradation rate. Similarly, increasing the PI dosage had a positive effect on BPA degradation (Figure 4c). Raising the PI concentration from 0.2 mM to 0.8 mM improved the 30s BPA removal from 61% to 99%. A higher PI concentration allowed more interaction with the catalyst, generating greater yield of ROS and enhancing the degradation process.
The catalytic performance was also influenced by aqueous pH. As shown in Figure 4d, the HCFe800/PI system maintained excellent degradation performance across pH 3.14 to 6.15, achieving complete BPA removal within 1 min. At an initial pH of 8.72, the time for complete degradation extended to 4 min, suggesting that acidic conditions favor higher catalytic activity. When the pH further increased to 10.84, the 8 min removal rate dropped to 78%. This trend may relate to the pKa values of PI (pKa1 = 1.64, pKa2 = 8.36, pKa3 = 12.20), as its speciation changes with pH. At pH < 8, PI primarily exists as highly reactive IO4−, while at pH > 8, it forms the less reactive dimer H2I2O104−. Since IO4− possesses stronger oxidative potential, this shift in speciation reduces ROS generation, leading to diminished degradation efficiency under alkaline conditions [27]. Temperature had a relatively minor impact on the HCFe800/PI system (Figure 4e). Even when the temperature dropped from 35 °C to 5 °C, the 1 min removal efficiency decreased by only 5%, demonstrating the strong low-temperature adaptability of HCFe800/PI system.
A large number of inorganic anions and natural organic matter are present in natural water bodies, and their presence may interfere with the degradation performance of the HCFe800/PI system. In this study, common anions (Cl−, SO42−, NO3−, and HCO3−) and humic acid (HA) were selected to investigate the anti-interference ability of the system. As shown in Figure 4f, with the exception of SO42−, the other anions exhibited no significant effect on the degradation efficiency of BPA. The slight inhibitory effect observed for SO42− may be attributed to its competitive reaction with •OH. Notably, HA also did not show any marked inhibitory effect. These results strongly demonstrate that the HCFe800/PI system possesses excellent tolerance to common interfering substances found in real water bodies.

3.4. Catalytic Degradation Mechanism Analysis

3.4.1. Role of ROS and High-Valent Iron

According to previous studies [28], various ROS, including •OH, O2•−,•IO3, and 1O2, were typically involved in pollutant degradation processes within PI-based AOPs. In this study, scavenging experiments (Figure 5a,b) combined with EPR spectroscopy were employed to identify the predominant ROS in the catalytic system. Specifically, ethanol and TBA were used as •OH scavengers; L-His served as a scavenger for 1O2; p-BQ was utilized to quench O2•−; and phenol was applied to scavenge both •IO3 and •OH [29,30]. For the EPR experiments, DMPO (5,5-dimethyl-1-pyrroline N-oxide) was used to trap •OH and O2•−, while TEMP (2,2,6,6-tetramethylpiperidine) was employed for the detection of 1O2.
The EPR spectra for the HCFe800/PI system revealed a prominent quartet signal (1:2:2:1), characteristic of the DMPO–•OH adduct (Figure 5c), indicating the presence of •OH in the reaction system. However, the degradation efficiency of BPA was not significantly affected upon the addition of •OH scavengers (TBA and EtOH) in the quenching experiments. This apparent contradiction may arise from the transient interfacial generation of •OH [31], which was formed via PI activation at FeN4 sites and confined to the catalyst surface or pore environment, limiting their diffusion into the bulk solution and thus their direct involvement in pollutant degradation. As shown in Figure 5c, no distinct EPR signal corresponding to O2•− was observed. Nevertheless, the addition of p-BQ led to a notable decrease in BPA removal efficiency compared to the control group. Specifically, within 8 min, the BPA removal efficiency in the HCFe800/PI system decreased from nearly 100% to approximately 60%, with the k value dropping from 6.295 min−1 to 0.764 min−1. These results suggest that O2•− plays a significant role in the degradation process of BPA. The absence of O2•− signals in the EPR spectra could be ascribed to its extremely short half-life under neutral or acidic conditions, during which it undergoes rapid dismutation (2 O2•− + 2H+ → H2O2 + O2) [32], making its transient signal difficult to capture via EPR.
After the addition of L-His, the catalytic activity of the HCFe800/PI system was significantly inhibited, with a 53% decrease in BPA removal efficiency within 8 min. The corresponding rate constant (k) dropped to 0.4721 min−1. EPR analysis detected the characteristic triplet signal of the TEMP-1O2 adduct in the HCFe800/PI system, indicating the involvement of 1O2 in BPA degradation. 1O2 is frequently identified as a prevailing ROS in Fe-site-activated catalytic systems [33,34], supporting the significant role of non-radical pathways in BPA degradation by the HCFe800–PI system. However, since BPA degradation still occurred even after quenching 1O2, it suggests that 1O2 is not the sole reactive species but plays a crucial role, likely acting synergistically with other ROS to promote BPA degradation.
In the PI-AOPs, the contribution of •IO3 cannot be overlooked. Although there are no direct methods for detecting •IO3, phenol is known to effectively inhibit •IO3-driven oxidation reactions [8]. Upon the addition of phenol as a scavenger, BPA removal was markedly suppressed, with only 52% removal efficiency in 8 min and a reduced k value of 0.3208 min−1. Previous scavenging experiments with TBA and EtOH ruled out the involvement of •OH, further supporting •IO3 as the predominant active species in BPA degradation.
To evaluate the role of high-valent iron-oxo species (FeIV = O) in the HCFe800/PI system, DMSO was employed as a probe, as it can be oxidized by FeIV = O to form DMSO2 through an oxygen atom transfer mechanism [35]. As shown in Figure 5a, the addition of DMSO did not significantly affect the degradation efficiency, suggesting that high-valent iron species were not involved in the catalytic process.
In summary, O2•−, 1O2, and •IO3 were identified as the key reactive species responsible for BPA degradation in the HCFe800/PI system. These species collectively enabled both radical and non-radical oxidation pathways.

3.4.2. Electrochemical Analysis

Figure 5e illustrates the current response of the HCFe800 system upon sequential addition of PI and BPA. A sharp increase in current (by 0.65 mA) was observed upon the introduction of PI, indicating a strong interaction between HCFe800 and PI. This provides compelling evidence for electron transfer from HCFe800 to PI adsorbed on its surface, leading to the rapid formation of a metastable HCFe800/PI* complex capable of participating in redox processes. Subsequent addition of BPA also triggered a significant increase in current, suggesting that electrons were transferred from BPA to the HCFe800/PI* complex [15,34]. This implies that BPA underwent oxidative degradation via electron loss. In essence, electron transfer occurred within a ternary system where BPA acted as the electron donor, PI as the electron acceptor, and HCFe800 served as the electron transfer mediator. Further insight into the interaction between the catalyst and PI was obtained through linear sweep voltammetry (LSV). As shown in Figure 5f, when both PI and BPA were present, the current signal was markedly stronger compared to the PI-only system, reinforcing the conclusion that HCFe800 facilitated electron transfer between PI and BPA. Herein, the electron transfer pathway could be illustrated in the inset figure of Figure 5e, in which (i) PI was adsorbed to FeN4 site to form a complex (FeN4-PI*); (ii) The electron of BPA was extracted by FeN4-PI*; (iii) BPA was degraded gradually.

3.4.3. Degradation Pathway and Toxicity Evolution of BPA

LC-MS was employed to identify intermediate products of BPA in HCFe800/PI system. Eight intermediates were detected: phenol (P1), 4-(2-hydroxyprop-2-yl)phenol (P2), 4-hydroxyacetophenone (P3), 5-hydroxybisphenol A (P4), bisphenol-O-quinone (P5), 1,5-hexadiene-3,4-diol (P6), glycolic acid (P7), and glucaric acid (P8). Based on previous literature [36] and experimental evidence, two main degradation pathways of BPA were proposed in Figure 6.
Given that hydroxyl groups are electron-donating, they increase the electron cloud density on the aromatic rings, weakening the bond between the two rings in BPA and making it more susceptible to oxidative attack. In the first pathway, ROS preferentially cleave the C–C bond between the tertiary carbon and the phenyl ring, generating P1 and P2, which were subsequently converted into P3. In the second pathway, hydroxylation of BPA yielded P4, which was further oxidized to P5. Intermediates from both pathways then underwent ring-opening and oxidation reactions, producing low-molecular-weight products such as P7 and P8. Complete BPA degradation was achieved within 8 min, with a TOC removal efficiency of 61%, suggesting that most intermediates were further mineralized to CO2 and H2O.
Toxicity of the intermediates was assessed using the T.E.S.T. (Toxicity Estimation Software Tool, version 5.1.2) model [37,38], with four endpoints: fathead minnow acute toxicity (LC50, 96 h), Tetrahymena pyriformis growth inhibition (IGC50, 48 h), developmental toxicity, and mutagenicity. As shown in Figure 7, the LC50 of BPA was 3.24 mg/L; most intermediates, except for P4 and P5, exhibited lower acute toxicity. For IGC50, BPA had a value of 5.30 mg/L, and nearly all intermediates were less toxic. In terms of developmental toxicity, most intermediates exhibited lower toxicity than BPA, and some showed no developmental toxicity at all. Although the predicted mutagenicity values of several intermediates were higher than that of BPA, all were classified as non-mutagenic. These results indicate that the HCFe800/PI system effectively reduced the overall toxicity of BPA-contaminated water.

3.5. Application Feasibility Evaluation

The degradation performance of the HCFe800/PI system was further evaluated against various types of refractory organic pollutants (Figure 8a). Selected target pollutants included sulfadiazine (SDZ), sulfamethazine (SMT), and rhodamine B (RhB). Remarkably, within just 1 min, RhB was removed with an efficiency of 98%, while SMT and SDZ were degraded by 91% and 84%, respectively. These results demonstrate that the HCFe-800/PI system exhibited excellent degradation performance across different classes of organic contaminants, highlighting its broad applicability.
The stability of the catalyst before and after use is a key factor in determining its practical lifetime. After two catalytic cycles, a slight decrease in performance was observed (Figure 8b), with the 1 min BPA removal efficiency dropping from 99% to 92%. This reduction may be attributed to catalytic site deactivation, caused by the gradual accumulation of product intermediates in the pores and the surface coverage of unreacted peroxides [39]. Nevertheless, complete degradation of BPA was still achieved within 8 min, indicating that the overall catalytic activity remained high. Notably, after thermal regeneration of the catalyst (via recalcination) before the third cycle, its catalytic activity was significantly restored, demonstrating the excellent structural stability and regeneration capability of HCFe800. These findings collectively indicate that HCFe800 possesses a long operational lifespan and strong potential for repeated use.

4. Conclusions

The hollow porous carbon-supported single-atom Fe catalyst, HCFe-800, could activate PI to achieve complete degradation of BPA within 1 min, with a degradation rate constant of 5.094 min−1, which was not only 10 times higher than that of the Fe-free controls; it also exhibited a catalyst-dose-normalized rate constant up to 103 times higher than previously reported catalysts. The optimized Fe doping amount and pyrolysis temperature facilitated the formation of atomically dispersed FeN4 sites, which outperformed Fe clusters and iron oxides in PI catalytic activity. The hollow porous structure further enhanced the exposure of active sites, contributing to the exceptional performance. The HCFe-800/PI system exhibited excellent behavior across pH 3–7, even at low temperature (~5 °C) and under high ionic strength conditions. BPA was degraded via multiple pathways, including radical (O2−•, IO3•), nonradical (1O2), and electron transfer routes, leading to less toxic degradation products. In addition, the HCFe-800/PI system could be extended to antibiotics/dyes and was stably recycled through recalcination. Compared with conventional heterogeneous catalysts and other AOP systems such as chlorination, ozonation, and electrochemical oxidation, PyC-supported SACs like HCFe800 offer distinctive advantages, including maximized metal utilization and reactant contact efficiency, high structural stability with low metal leaching, and the ability to harness both radical and selective nonradical pathways. These merits position HCFe-800/PI as a sustainable and versatile catalytic platform, with strong potential to be extended beyond BPA to other, even more recalcitrant, emerging pollutants. In addition, the high efficiency and reusability of SACs imply the possibility of reducing catalyst dosage and overall operational costs in the long term, although further validation in modularized and pilot-scale systems is still required. Overall, this work contributes to both the practical application and mechanistic advancement of SAC–PI-based advanced oxidation processes.

Author Contributions

Conceptualization, C.L. and Y.P.; methodology, C.L. and M.Y.; software, M.Y.; validation, Y.X. and M.Y.; formal analysis, M.Y. and Y.X.; investigation, M.Y. and Y.X.; resources, C.L. and Y.P.; data curation, M.Y. and Y.X.; writing—original draft preparation, C.L. and M.Y.; writing—review and editing, S.G. and Y.P.; visualization, M.Y. and Y.X.; supervision, S.G. and Y.P.; project administration, S.G. and Y.P.; funding acquisition, C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 22476094 and also by Qing Lan Project of Jiangsu Province for C.L.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors gratefully acknowledge the Modern Analysis Center of Nanjing University for providing support with the EPR measurements in this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 17 02705 g001
Figure 1. Schematic illustration of the synthesis for HCFe800.
Figure 1. Schematic illustration of the synthesis for HCFe800.
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Figure 2. Removal kinetics of BPA (a) in different systems Reaction conditions: [Catalyst] = 0.1 g L−1, [PI] = 0.8 mM, [BPA] = 20 mg L−1, pH0 = 6.25, T = 25 °C.
Figure 2. Removal kinetics of BPA (a) in different systems Reaction conditions: [Catalyst] = 0.1 g L−1, [PI] = 0.8 mM, [BPA] = 20 mg L−1, pH0 = 6.25, T = 25 °C.
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Figure 3. BPA removal efficiency kinetic (a,c) and corresponding reaction rates (b,d) by catalysts with different iron contents or calcination temperature. Reaction conditions: [Catalyst] = 0.1 g L−1, [PI] = 0.8 mM, [BPA] = 20 mg L−1, pH0 = 6.25, T = 25 °C.
Figure 3. BPA removal efficiency kinetic (a,c) and corresponding reaction rates (b,d) by catalysts with different iron contents or calcination temperature. Reaction conditions: [Catalyst] = 0.1 g L−1, [PI] = 0.8 mM, [BPA] = 20 mg L−1, pH0 = 6.25, T = 25 °C.
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Figure 4. Effects of different parameters on BPA removal: (a) Initial BPA concentration; (b) Catalyst dosage; (c) PI dosage; (d) Initial pH; (e) Reaction temperature; (f) Various inorganic anions and HA. Unless otherwise specified, the reaction conditions were as follows: [Catalyst] = 0.1 g L−1, [PI] = 0.8 mM, [BPA] = 20 mg L−1, [salt ions] = 10 mmol L−1, [HA] = 5 mg L−1, pH0 = 6.25, T = 25 °C.
Figure 4. Effects of different parameters on BPA removal: (a) Initial BPA concentration; (b) Catalyst dosage; (c) PI dosage; (d) Initial pH; (e) Reaction temperature; (f) Various inorganic anions and HA. Unless otherwise specified, the reaction conditions were as follows: [Catalyst] = 0.1 g L−1, [PI] = 0.8 mM, [BPA] = 20 mg L−1, [salt ions] = 10 mmol L−1, [HA] = 5 mg L−1, pH0 = 6.25, T = 25 °C.
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Figure 5. Quenching tests to detect ROS for BPA degradation in HCFe800/PI system (a) and the corresponding reaction rate constants (b); ESR spectra of PI activation by HCFe800 under different conditions using DMPO or TEMP as spin-trappers HCFe800 (c,d); Chronoamperometry curves (e) and Chronopotentiometry curves (f) of HCFe800-coated electrodes with adding PI solution (PI: 0.8 mM, 0.5 mL) or BPA solution (20 mg L−1, 0.5 mL). Reaction conditions: [catalysts] = 0.1 g L−1, [PI] =0.8 mM, [BPA] = 20 mg L−1, [EtOH] = 0.2 mM, [L-His] = 0.2 mM, [TBA] = 0.2 mM, [p-BQ] = 0.2 mM, [Phenol] = 1 mM, [DMSO]= 0.2 mM, pH0 = 6.25, T = 25 °C.
Figure 5. Quenching tests to detect ROS for BPA degradation in HCFe800/PI system (a) and the corresponding reaction rate constants (b); ESR spectra of PI activation by HCFe800 under different conditions using DMPO or TEMP as spin-trappers HCFe800 (c,d); Chronoamperometry curves (e) and Chronopotentiometry curves (f) of HCFe800-coated electrodes with adding PI solution (PI: 0.8 mM, 0.5 mL) or BPA solution (20 mg L−1, 0.5 mL). Reaction conditions: [catalysts] = 0.1 g L−1, [PI] =0.8 mM, [BPA] = 20 mg L−1, [EtOH] = 0.2 mM, [L-His] = 0.2 mM, [TBA] = 0.2 mM, [p-BQ] = 0.2 mM, [Phenol] = 1 mM, [DMSO]= 0.2 mM, pH0 = 6.25, T = 25 °C.
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Figure 6. The possible degradation paths of BPA by the activation of PI over HCFe800.
Figure 6. The possible degradation paths of BPA by the activation of PI over HCFe800.
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Figure 7. Theoretically calculated (a) Fathead minnow LC50 (96 h), (b) T. Pyriformis IGC50 (48 h), (c) Developmental toxicity, and (d) Mutagenicity of BPA and its degradation intermediates.
Figure 7. Theoretically calculated (a) Fathead minnow LC50 (96 h), (b) T. Pyriformis IGC50 (48 h), (c) Developmental toxicity, and (d) Mutagenicity of BPA and its degradation intermediates.
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Figure 8. (a) Degradation kinetic curves of various organic pollutants by HCFe800/PI system and (b) reusability of HCFe800 for BPA removal. Reaction conditions: [catalysts] = 0.1 g L−1, [PI] = 0.8 mM, [pollutants] = 20 mg L−1, pH0 = 6.25, T = 25 °C.
Figure 8. (a) Degradation kinetic curves of various organic pollutants by HCFe800/PI system and (b) reusability of HCFe800 for BPA removal. Reaction conditions: [catalysts] = 0.1 g L−1, [PI] = 0.8 mM, [pollutants] = 20 mg L−1, pH0 = 6.25, T = 25 °C.
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Table 1. Comparison of BPA degradation efficiency by HCFe800 with those by previously reported catalysts via PI activation.
Table 1. Comparison of BPA degradation efficiency by HCFe800 with those by previously reported catalysts via PI activation.
CatalystsMass
(g L−1) 		pH0T
(°C)		C0 BPA
(mmol·L−1)		PI Dosage
(mmol·L−1)		k
(min−1)		knorm-1
(L·g−1·min−1)		knorm-2
(L·g−1·min−1)		Refs
FeS1.05.5/0.021.00.03560.03560.001 [20]
S-(nFe0-Ni)/BC0.24.0650.021.00.01670.08350.002 [21]
Fe@PrPOP0.14.0200.0050.50.38383.8380.038 [22]
MgMn2O40.17200.011.00.054610.54610.005 [23]
BC8000.37/0.0220.50.04190.13970.006 [24]
HC8000.16.25250.0880.80.5585.580.614This work
HCFe8000.16.25250.0880.85.09450.945.603This work
knorm-1 = k m c a t a l y s t ; knorm-2 = ( k m c a t a l y s t ) / ( C P I C B P A )
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Ling, C.; Yuan, M.; Gao, S.; Xue, Y.; Pan, Y. Efficiency and Mechanism of a Hollow Carbon-Based Single-Atom Iron Catalyst in Activating Periodate for Bisphenol a Degradation. Water 2025, 17, 2705. https://doi.org/10.3390/w17182705

AMA Style

Ling C, Yuan M, Gao S, Xue Y, Pan Y. Efficiency and Mechanism of a Hollow Carbon-Based Single-Atom Iron Catalyst in Activating Periodate for Bisphenol a Degradation. Water. 2025; 17(18):2705. https://doi.org/10.3390/w17182705

Chicago/Turabian Style

Ling, Chen, Mengyue Yuan, Shang Gao, Yuzhu Xue, and Yuwei Pan. 2025. "Efficiency and Mechanism of a Hollow Carbon-Based Single-Atom Iron Catalyst in Activating Periodate for Bisphenol a Degradation" Water 17, no. 18: 2705. https://doi.org/10.3390/w17182705

APA Style

Ling, C., Yuan, M., Gao, S., Xue, Y., & Pan, Y. (2025). Efficiency and Mechanism of a Hollow Carbon-Based Single-Atom Iron Catalyst in Activating Periodate for Bisphenol a Degradation. Water, 17(18), 2705. https://doi.org/10.3390/w17182705

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