Adsorption–Catalysis Dual-Function Nitrogen-Doped Carbon/CoFe₂O₄ Composite for Efficient Tetracycline Removal

Abstract

Efficient removal of tetracycline (TC) from antibiotic-contaminated wastewater remains a significant challenge. In this study, a nitrogen-doped carbon modified cobalt ferrite (C@CoFe₂O₄, CF) composite was synthesized via a one-step hydrothermal method, and its adsorption–catalysis dual-function performance was systematically evaluated. Structural characterization revealed that CoFe₂O₄ nanoparticles were locally encapsulated by an N-doped carbonaceous layer, providing a high specific surface area and abundant nitrogen-containing active sites. Under optimized conditions, the CF-3/PMS system achieved 93.44% TC removal within 45 min, while CF-3 exhibited a maximum adsorption capacity of 486.5 mg·g⁻¹. Radical quenching experiments suggested that singlet oxygen (¹O₂) and superoxide radicals (O₂•⁻) played major roles in TC oxidation, while sulfate radicals (SO₄•⁻) and hydroxyl radicals (HO•) also contributed, indicating the coexistence of radical and non-radical oxidation pathways. TC adsorption was driven by surface complexation, π–π electron donor–acceptor interactions, and hydrogen bonding. The enhanced TC removal performance may arise from the cooperative contributions of N-containing carbon sites, accessible Co/Fe-containing regions, and interfacial electronic interactions, which promote TC enrichment and PMS-mediated oxidation. This work provides a promising strategy for designing bifunctional materials for antibiotic wastewater treatment.

1. Introduction

The extensive and long-term use of antibiotics has led to their widespread occurrence in aquatic environments, posing significant risks to ecosystems, public health, and drinking water safety through the dissemination of antibiotic resistance and ecological toxicity [1]. Tetracycline (TC), one of the most widely used broad-spectrum antibiotics, is characterized by high water solubility, poor biodegradability, and large consumption. It can enter aquatic environments through livestock effluents, medical wastewater, and surface runoff, resulting in persistent environmental contamination [2]. Therefore, the development of efficient, economical, and environmentally friendly technologies for TC removal is of great importance.

Adsorption and advanced oxidation processes (AOPs) are among the most widely employed technologies for antibiotic wastewater treatment. Adsorption is attractive because of its operational simplicity and low cost; however, its practical application is often limited by insufficient adsorption capacity, reduced efficiency toward low-concentration pollutants, and difficulties in adsorbent regeneration [3,4]. In contrast, peroxymonosulfate (PMS)-based AOPs have received considerable attention due to their ability to generate multiple reactive oxygen species in situ and achieve efficient degradation of refractory organic contaminants. Nevertheless, the efficiency of PMS activation strongly depends on the catalyst structure and surface electronic properties. Insufficient catalytic activity not only lowers oxidant utilization efficiency but also increases treatment costs [5].

Although significant progress has been achieved in PMS-based catalytic systems, the rational integration of adsorption and catalytic functions remains a major challenge. In many conventional catalysts, strong adsorption capacity and high catalytic activity are difficult to simultaneously achieve. Highly crystalline metal oxides generally possess favorable electron-transfer pathways and redox activity but often suffer from limited surface area and insufficient pollutant enrichment capability [6,7]. In contrast, amorphous carbon materials provide abundant surface functional groups and adsorption sites but usually lack efficient catalytic centers for pollutant degradation [8,9]. Consequently, a trade-off frequently exists between pollutant enrichment and catalytic oxidation. Constructing core–shell architectures that integrate a catalytic core with an adsorption shell offers a promising strategy to spatially separate adsorption and catalytic sites, thereby enhancing their synergistic performance.

Spinel cobalt ferrite (CoFe₂O₄) has been widely investigated for PMS activation owing to its excellent magnetic recoverability, chemical stability, and redox activity [10,11]. However, the aggregation tendency of CoFe₂O₄ nanoparticles and their limited adsorption affinity toward TC restrict their practical application. Carbon-based materials have attracted extensive interest because of their high specific surface area and abundant surface functional groups, which facilitate pollutant enrichment and interfacial electron transfer. Moreover, nitrogen doping can effectively tailor the electronic structure of carbon materials, improve pollutant adsorption, and promote PMS activation through enhanced electron transport [12,13,14,15]. Nitrogen-containing functionalities, such as C=N and C–N groups, have been reported to strengthen TC adsorption through π–π interactions, hydrogen bonding, and electron donor–acceptor (EDA) interactions.

In this study, a nitrogen-doped carbon modified CoFe₂O₄ (CF) composite was synthesized via a one-step hydrothermal method. The structural characteristics, adsorption performance, and catalytic activity of the composite toward TC removal were systematically investigated. Furthermore, the adsorption–catalysis synergistic mechanism was elucidated through XRD, FT-IR, XPS analyses, adsorption controls, and quenching tests. This work provides insight into the design of bifunctional materials that integrate pollutant enrichment and catalytic degradation for efficient antibiotic wastewater treatment.

2. Materials and Methods

2.1. Chemicals

All chemicals used were of analytical-reagent (AR) grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). and Aladdin Reagent Co., Ltd. (Shanghai, China), including the following: Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, NH₃·H₂O (25%), ethanol (C₂H₅OH), and NaOH. Tetracycline (TC, purity ≥ 95%) was used as the model pollutant for adsorption and degradation experiments, and peroxymonosulfate (PMS, KHSO₅·0.5KHSO₄·0.5K₂SO₄) served as the oxidant. All chemicals were of analytical grade and used without further purification.

2.2. Synthesis of CF Composites

CF composites were prepared via a one-step hydrothermal method. Fe(NO₃)₃·9H₂O (3.232 g, 0.008 mol) and Co(NO₃)₂·6H₂O (1.164 g, 0.004 mol) were dissolved in 55 mL deionized water under magnetic stirring until fully dissolved. Glucose (0.5–2.0 g), ethanol (0 or 5 mL), and 20 mL ammonia solution were sequentially added, and the pH was adjusted to above 10 using NaOH solution (2 g NaOH in 20 mL water). The mixture was stirred for an additional 30 min before transferring into a Teflon-lined stainless steel autoclave and heated at 180 °C for 10 h. After the reaction, the resulting precipitate was washed several times with deionized water and ethanol, and dried at 60 °C for 12 h. By adjusting the glucose dosage and solvent conditions, four CF samples were obtained and denoted as CF-1, CF-2, CF-3, and CF-4. Detailed precursor amounts are listed in

Table 1. Raw materials used in the fabrication process.

Number	Fe(NO3)3·9H2O (g)	Co(NO3)2·6H2O (g)	Glucose (g)	Ammonia (mL)	Ethanol (mL)	NaOH (mL)
CF-1	3.232	1.164	0.5	20	5	20
CF-2	3.232	1.164	1	20	5	20
CF-3	3.232	1.164	2	20	5	20
CF-4	3.232	1.164	1	20	0	20
Pure hydrothermal carbon	0	0	0.5	20	5	0

.

2.3. Characterization

The crystal structure of the samples was analyzed using X-ray diffraction (XRD, SmartLab SE, Rigaku Corporation, Akishima, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å). The morphology and elemental distribution were characterized by scanning electron microscopy (SEM, GeminiSEM 500, Carl Zeiss Microscopy GmbH, Oberkochen, Germany) equipped with energy-dispersive X-ray spectroscopy (EDS). Surface functional groups were identified using Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA) in the range of 400–4000 cm⁻¹. The surface elemental composition and chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+, Thermo Fisher Scientific, Waltham, MA, USA). Specific surface area and pore size distribution were determined by N₂ adsorption–desorption measurements (V-Sorb 2800P, Gold APP Instruments Corporation, Beijing, China). Thermogravimetric analysis (TG–DSC; instrument model, manufacturer, city, and country to be verified against the original laboratory record) was conducted under a N₂ atmosphere from room temperature to 1100 °C at a heating rate of 10 °C min⁻¹. Solid-state ¹³C magic-angle spinning nuclear magnetic resonance (¹³C MAS NMR) spectra were acquired on a AVANCE 400 MHz WB spectrometer, Bruker BioSpin GmbH, Rheinstetten, Germany using 4 mm zirconia rotors at a spinning rate of 15 kHz. The concentration of solution was measured by ICP-AES techniques using an ICP-AES_OPTIMA7000DV equipment.

2.4. Catalytic Degradation Experiments

TC degradation experiments were conducted in 200 mL of TC solution (50 mg L⁻¹) under continuous stirring at 400 rpm. The initial solution pH was adjusted using 0.1 mol L⁻¹ HCl or NaOH solution. Subsequently, a predetermined amount of catalyst (0.01–0.06 g) was dispersed in the TC solution, and the catalytic degradation reaction was initiated by adding PMS (0.0307–0.123 g). During the reaction, 3 mL aliquots were withdrawn at predetermined time intervals (0–45 min), filtered through a 0.22 μm membrane, and analyzed using a UV–Vis spectrophotometer (UV-5100, Shanghai Metash Instruments Co., Ltd., Shanghai, China) at 357 nm. The raw absorbance data at 357 nm for a representative degradation experiment are provided in Table S2. For the reusability test, five consecutive cycles were conducted under the optimized conditions; after each cycle, CF-3 was recovered by filtration, washed three times with deionized water, dried at 60 °C, and directly reused in the subsequent cycle. The Co concentration in the filtrate collected after the first cycle was quantified by ICP-OES. All degradation experiments were performed in triplicate, and the reported values represent the averages of three independent measurements. The TC removal efficiency (R_e) was calculated according to Equation (1):

$${ R}_e={\displaystyle \frac{C_0-C_t}{C_0}}\times 100\%$$

(1)

2.5. Radical Quenching Procedures and Reactive-Species Assessment

Radical quenching experiments were conducted under the same conditions as the catalytic degradation experiments at pH = 7.0. MeOH, TBA, p-BQ, and FFA were added before PMS addition to quench SO₄•⁻/HO•, HO•, O₂•⁻, and ¹O₂, respectively, with corresponding concentrations of 100, 100, 10, and 10 mmol L⁻¹. Samples were collected at predetermined intervals, filtered through a 0.22 μm membrane, and analyzed at 357 nm by UV–Vis spectrophotometry. A control experiment without scavenger was performed for comparison. The manufacturer, city, and country for MeOH, TBA, p-BQ, and FFA should be verified against the original purchase records before final approval.

2.6. Adsorption Experiments

Adsorption experiments were conducted by adding 0.02 g of CF into TC solutions (70 mL) with different initial concentrations (50–500 mg L⁻¹) at pH =7. The suspensions were continuously stirred, and aliquots were collected at predetermined time intervals to investigate the adsorption behavior of TC. All adsorption experiments were performed in triplicate, and the reported values represent the averages of three independent measurements. The equilibrium adsorption capacity (Q_e) was calculated using Equation (2):

$$Q_e={\displaystyle \frac{(C_0-C_e)V}{m}}$$

(2)

where C₀ and C_e are the initial and equilibrium concentrations of TC (mg L⁻¹), respectively; V is the solution volume (L); and m is the mass of adsorbent (g).

2.7. Adsorption and Thermodynamic Experiments

Adsorption experiments were conducted by adding 0.02 g of CF-3 to 70 mL of TC solutions with initial concentrations ranging from 50 to 500 mg L⁻¹ at pH 7.0. The suspensions were continuously stirred, and aliquots were collected at predetermined time intervals. All adsorption experiments were performed in triplicate, and the results are presented as the mean values of three independent measurements. For the temperature-dependent adsorption experiments, the initial TC concentration was fixed at 200 mg L⁻¹, and the experiments were conducted at 298, 303, and 308 K. Based on the adsorption kinetic results, an equilibration time of 30 min was employed to ensure that adsorption equilibrium was reached. The standard thermodynamic parameters, including the standard Gibbs free-energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°), were calculated according to the equations and procedures provided in Texts S1–S3 of the Supplementary Materials.

3. Results and Discussion

3.1. Crystal and Chemical Structure

Phase Structure Analysis

The phase structures of the as-prepared samples synthesized under different conditions were analyzed by X-ray diffraction (XRD), as shown in Figure 1A. All samples exhibit sharp and well-defined diffraction peaks, indicating good crystallinity of the formed CoFe₂O₄ phase. The diffraction peaks located at 2θ values of 30.0°, 35.4°, 43.0°, 53.5°, 56.9°, and 62.5° are assigned to the (220), (311), (400), (422), (511), and (440) crystal planes, respectively, which are consistent with the standard spinel CoFe₂O₄ structure (JCPDS No. 77-0426). The similar diffraction patterns observed for all samples suggest that variations in glucose dosage and solvent conditions do not alter the intrinsic crystal structure of CoFe₂O₄. The broad diffraction hump at around 20–25° can be attributed to the amorphous carbonaceous component derived from hydrothermal carbonization of glucose, indicating the low graphitization degree of the carbon layer [16]. Notably, CF-3 exhibits slightly lower diffraction peak intensities than the other samples, which can be associated with the presence of a relatively higher amount of amorphous carbon and the resulting decrease in the relative proportion of the crystalline CoFe₂O₄ phase [17].

Fourier transform infrared (FT-IR) spectra of all samples are shown in Figure 1B. Taking CF-3 as a representative example, the absorption bands observed at 402 cm⁻¹ and 579 cm⁻¹ correspond to Co–O/Fe–O stretching vibrations at tetrahedral and octahedral sites of the spinel CoFe₂O₄ structure, confirming the successful formation of the spinel phase. The bands at 1448 cm⁻¹ and 1602 cm⁻¹ are attributed to vibrations of the N-doped carbon framework and the C=C/C=N stretching modes, respectively, indicating successful incorporation of nitrogen into the carbon matrix [13,18]. The presence of the peak at 1602 cm⁻¹ further suggests the existence of nitrogen configurations such as pyridinic N and graphitic N within the carbon structure. The broad absorption band centered at 3388 cm⁻¹ is assigned to O–H stretching vibrations of surface hydroxyl groups, indicating the presence of abundant hydrophilic functional groups on the catalyst surface.

A hydrothermal carbon sphere was synthesized according to Table 1, and solid-state ¹³C NMR spectroscopy was used to analyze both pure hydrothermal carbon and CFx to determine the carbon species. As demonstrated in our previous study [16], ethanol mainly regulates the morphology and dispersion of hydrothermal carbon without altering its principal chemical composition; therefore, CF-3 was selected as a representative sample for ¹³C NMR analysis. The ¹³C NMR spectra show highly similar features, indicating that the carbon in CF retains a structure comparable to that of hydrothermal carbon spheres. The carbon species are mainly assigned to aliphatic carbon, ether carbon, C=C, and carboxyl groups, confirming the presence of abundant oxygen-containing functional groups introduced by the one-step hydrothermal process. The lower signal intensity observed for CF is attributed to its reduced carbon content, which weakens the overall NMR response (Figure 1C).

3.2. Morphological and Physicochemical Properties

3.2.1. Morphology and Elemental Distribution

The morphology and elemental distribution of the CF-3 samples were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), as shown in Figure 2. The SEM images reveal that amorphous hydrothermal carbon is uniformly anchored on the surface of CoFe₂O₄ nanoparticles, forming a loosely structured porous composite. The introduction of ethanol effectively suppresses carbon aggregation, facilitating the formation of smaller and more uniformly dispersed amorphous carbon derived from glucose decomposition [19].

Elemental mapping further confirms the homogeneous distribution of Fe, Co, C, and N throughout the catalyst. The colocalization of C and N indicates that nitrogen atoms introduced via ammonia during the hydrothermal process are successfully incorporated into the carbon framework. Because CF-1–CF-4 were prepared using the same types of precursors and a similar synthesis procedure (Table 1), with differences mainly in the glucose and ethanol dosages, CF-3 was selected as a representative sample for elemental mapping analysis. This N-doped carbon layer not only enhances the adsorption capability toward TC, but also improves the dispersion and stability of CoFe₂O₄ active nanoparticles. Meanwhile, the intimate interfacial contact between the carbon matrix and CoFe₂O₄ is expected to promote interfacial electron transfer, thereby facilitating catalytic performance [16,20].

3.2.2. Thermal Analysis and Carbon Content

Thermogravimetric–differential scanning calorimetry (TG–DSC) curves of CF-1 to CF-4 are shown in Figure 3. All samples exhibit similar thermal behaviors and can be divided into four distinct stages. The first stage (30–50 °C) is associated with the removal of physically adsorbed water accompanied by a weak endothermic signal. The second stage (50–150 °C) corresponds to the elimination of bound water within the structure.

The third stage (150–300 °C) is characterized by a pronounced endothermic peak and significant mass loss (4.2–10.5%), which is attributed to dehydration–condensation reactions, C–O bond cleavage, and structural rearrangement of the hydrothermal carbon component. The fourth stage (>300 °C) reflects the progressive pyrolysis, carbonization, and decomposition of carbonaceous species under nitrogen atmosphere, accompanied by substantial mass loss (35–50%) due to the release of volatile decomposition products. Since the TG–DSC measurements were conducted under nitrogen rather than air, the carbonaceous species were not expected to be completely oxidized before 500 °C; therefore, the absence of a clear plateau is reasonable. Based on the mass loss in the third and fourth stages, the apparent carbon-related mass-loss fractions of CF-1, CF-2, CF-3, and CF-4 were estimated to be 8.5%, 13.4%, 15.5%, and 11.7%, respectively. Among all samples, CF-3 exhibits the highest apparent carbon-related mass-loss fraction, indicating more effective carbon formation during hydrothermal synthesis [21]. Overall, the results suggest that the addition of ammonia and ethanol facilitate the conversion of glucose into hydrothermal carbon, and the carbon source dosage has an important influence on the apparent carbon-related mass-loss fraction of the composites.

3.2.3. Specific Surface Area and Pore Structure

Nitrogen adsorption–desorption isotherms of the CF samples are shown in Figure 4. The specific surface areas of CF-1, CF-2, CF-3, and CF-4 are 81, 98, 94, and 72 m² g⁻¹, respectively. All samples exhibit typical type IV isotherms with H3-type hysteresis loops, indicating the presence of mesoporous structures with pore sizes mainly distributed in the range of 0–25 nm. The variation in specific surface area is closely related to the carbon content in the composites. At relatively low carbon loading, the hydrothermal carbon contributes to pore formation and enhances the surface area [11,22]. However, excessive carbon leads to aggregation and surface coverage on CoFe₂O₄ particles, which partially blocks pore development and results in a decrease in surface area. In addition, the comparison between CF-2 and CF-4 indicates that ethanol addition facilitates pore structure optimization by reducing solvent surface tension and suppressing carbon aggregation, thereby promoting the formation of smaller carbon domains and a more accessible porous network. In contrast, the slight decrease in surface area from CF-2 to CF-3 is mainly associated with the increased glucose-derived carbonaceous component, which may partially cover CoFe₂O₄ particles and block pore development.

The developed mesoporous structure and relatively high surface area provide abundant accessible active sites for tetracycline adsorption and facilitate mass transfer of PMS and reactive species within the pore channels, thereby enhancing both adsorption and catalytic oxidation performance.

3.3. Catalytic Performance and Influencing Factors

3.3.1. Effect of PMS and Catalyst Dosage

As shown in Figure 5, when the catalyst dosage was fixed at 0.03 g, the overall aqueous-phase TC removal increased with increasing PMS dosage from 0.0307 g to 0.123 g, indicating that a higher oxidant concentration increased the availability of PMS-derived oxidizing species within an appropriate concentration range [23]. Because the scavenger experiments are confounded by substantial adsorption and possible probe–surface interactions, these dosage data are not used to rank individual reactive species. Accordingly, 0.123 g was selected as the working PMS dosage, at which the CF-3/PMS system achieved the highest overall TC removal within 45 min. However, for CF-1 and CF-2, excessive PMS led to a slight decrease in overall removal, which may be associated with oxidant self-consumption or scavenging reactions at elevated PMS concentrations.

However, for CF-1 and CF-2, excessive PMS led to a slight decrease in degradation efficiency, which can be attributed to the self-scavenging effect of excess PMS, resulting in reduced concentrations of reactive oxygen species in the system [24,25].

The effect of catalyst dosage on TC removal was further investigated, as shown in Figure 6. With increasing CF dosage, the number of available active sites increased, thereby enhancing PMS decomposition and interfacial electron transfer, which collectively promoted TC degradation. For CF-1, the removal efficiency increased from 66.09% to 81.94% when the dosage was raised from 0.01 to 0.04 g. Similarly, CF-2 showed an increase from 80.25% to 89.51% as the dosage increased from 0.01 to 0.05 g. For CF-3, the efficiency improved from 88.75% to 93.44% with an increase in dosage from 0.02 to 0.05 g. CF-4 exhibited an increase from 85.81% to 92.78% when the dosage was raised from 0.02 to 0.03 g. However, further increase in catalyst dosage led to a slight decline in degradation efficiency, which may be attributed to radical scavenging and recombination effects induced by excessive active sites [26]. Overall, the CF-3/PMS system achieved the highest TC removal efficiency of 93.44% at a catalyst dosage of 0.05 g and a PMS dosage of 0.123 g, indicating its superior catalytic performance.

To evaluate the overall TC-removal performance of the CF-x/PMS systems, matched reaction systems were compared (Figure S1). After 45 min, the CF-1/PMS, CF-2/PMS, CF-3/PMS, and CF-4/PMS systems achieved overall aqueous-phase TC removals of 82.11%, 89.74%, 93.44%, and 92.78%, respectively. Under the same conditions, PMS alone and CF-3 without PMS achieved removals of 38.28% and 67.61%, respectively. The substantial adsorption-only value demonstrates that the CF-3/PMS endpoint combines adsorption with PMS-mediated oxidation. Because the adsorption-only and PMS-only percentages are referenced to the same initial TC inventory and are not independent additive fractions, they were not summed and no quantitative synergy factor was calculated. The CF-3/PMS value was approximately 2.4 times the PMS-only value, supporting catalyst-assisted PMS conversion, while CF-3 was selected for subsequent experiments on the basis of its slightly higher mean overall removal and balanced adsorption/PMS performance.

3.3.2. Effect of Initial pH and Coexisting Anions

The effect of initial pH on TC degradation in the CF-3/PMS system was investigated over a pH range of 3–11, as shown in Figure 7A. The degradation efficiency exhibited a volcano-type trend, increasing first and then decreasing with rising pH. Under acidic conditions (pH = 3 and 5), the TC removal efficiencies were 72.73% and 82.43%, respectively, indicating suppressed catalytic activity. The highest removal efficiency of 93.44% was achieved at neutral pH (pH = 7). When the solution became alkaline (pH = 9 and 11), the efficiencies slightly decreased to 91.75% and 90.66%, respectively. The reduced performance under acidic conditions can be attributed to the excessive consumption of reactive oxygen species (ROS) by H⁺, which inhibits the effective participation of SO₄•⁻ and HO• in TC degradation. In alkaline media, SO₄•⁻ can be partially converted into •OH through reaction with OH⁻, accompanied by the formation of SO₄²⁻. The generated •OH is less selective and more easily consumed under alkaline conditions, thereby decreasing the effective oxidation efficiency toward TC [27]. Overall, the CF-3/PMS system exhibits optimal performance under near-neutral conditions.

Common inorganic anions in natural waters and antibiotic-containing wastewater may influence PMS-based oxidation by interacting with reactive oxygen species or altering the reaction environment [28,29]. Therefore, CO₃²⁻, SO₄²⁻, Cl⁻, and PO₄³⁻ were selected as representative anions to evaluate the tolerance of the CF-3/PMS system to complex water matrices. The influence of coexisting anions was further evaluated by introducing CO₃²⁻, SO₄²⁻, Cl⁻, and PO₄³⁻ into the reaction system (Figure 7B). The effect on TC removal followed the order: PO₄³⁻ (95.67%) > SO₄²⁻ (92.62%) > CO₃²⁻ (85.48%) > Cl⁻ (82.49%). Among them, PO₄³⁻ slightly enhanced TC degradation, while CO₃²⁻ and Cl⁻ significantly inhibited the process. The inhibitory effect of CO₃²⁻ and Cl⁻ can be attributed to their strong scavenging of reactive radicals, resulting in the formation of less reactive secondary species and reduced oxidation capacity toward TC. In contrast, the negligible or slightly positive effect of SO₄²⁻ and PO₄³⁻ may be associated with weaker radical quenching and possible surface interaction effects. These results demonstrate that competitive adsorption and radical scavenging by coexisting anions play a crucial role in regulating PMS-based oxidation performance in complex water matrices [28,30].

3.3.3. Reusability and Co Leaching of CF-3

The reusability of CF-3 was evaluated over five consecutive cycles. As shown in Figure S2, the overall TC removal efficiency decreased from 93.44% in the first cycle to 83.31% in the fifth cycle, indicating that CF-3 retained satisfactory removal performance after repeated use. The gradual decrease may be attributed to catalyst loss during recovery and partial occupation of active sites by adsorbed species or reaction intermediates. The Co concentration in the filtrate after the first cycle was 0.74 mg L⁻¹, indicating limited but measurable Co leaching under the investigated conditions. These results demonstrate the reasonable short-term stability of CF-3, while further investigations are still required to minimize metal release during long-term application.

3.3.4. Radical Quenching Experiments and Reactive Species Identification

To elucidate the activation mechanism of PMS in the CF system for TC degradation, radical quenching experiments were conducted using methanol (MeOH), tert-butanol (TBA), p-benzoquinone (p-BQ), and furfuryl alcohol (FFA) as scavengers for SO₄•⁻/HO•, HO•, O₂•⁻, and ¹O₂, respectively. As shown in Figure 8, the addition of MeOH decreased the TC removal efficiency to 80.42%, while the presence of TBA only slightly affected the degradation efficiency (92.40%), indicating that HO• plays a secondary role in the reaction system. In contrast, the addition of p-BQ and FFA led to significant decreases in TC removal efficiency to 74.37% and 44.29%, respectively, suggesting that O₂•⁻ and especially ¹O₂ are the dominant reactive species responsible for TC degradation [23,29]. Overall, the CF/PMS system does not follow a single radical-dominated pathway. Instead, TC degradation is governed by a synergistic oxidation process involving SO₄•⁻, O₂•⁻, and ¹O₂, with non-radical species (¹O₂) playing a particularly crucial role in the reaction system.

3.4. Adsorption Behavior and Performance of CF-3 Toward TC

3.4.1. Adsorption Capacity of TC on CF-3

To better simulate real-water conditions, adsorption experiments were conducted at pH = 7, consistent with the optimal pH identified in catalytic tests. As shown in Figure 9A, the equilibrium adsorption capacity of CF-3 for TC increases with increasing initial concentration and gradually reaches saturation at approximately 500 mg L⁻¹. CF-3 exhibits the highest maximum adsorption capacity of 486.5 mg g⁻¹. At lower initial concentrations (50–300 mg L⁻¹), the adsorption capacity increases rapidly with concentration due to the abundant availability of active surface sites. As the concentration further increases (300–500 mg L⁻¹), the adsorption rate gradually slows down and approaches equilibrium, which can be attributed to the progressive occupation of active sites and surface saturation. Compared with reported iron-based adsorbents (Table S1), the superior adsorption performance of CF-3 can be ascribed to its locally encapsulated N-doped carbon/CoFe₂O₄ structure and enriched surface functional groups, which provide abundant interaction sites for TC adsorption.

3.4.2. Adsorption Isotherms, Kinetics and Thermodynamics

Adsorption isotherm, kinetic, and thermodynamic analyses were conducted to clarify the adsorption behavior of TC on CF-3. Detailed equations and parameter definitions for the corresponding models are provided in Texts S1–S3 of the Supplementary Materials. The adsorption isotherm results (Figure 9B,C) indicate that the Freundlich model provides a better fit than the Langmuir model, suggesting that TC adsorption on CF is a heterogeneous multilayer process involving energetically different adsorption sites. The kinetic fitting results (Figure 9D,E) show that the pseudo-second-order model (R² = 0.991) fits the experimental data better than the pseudo-first-order model (R² = 0.975), suggesting that the adsorption process is strongly influenced by the availability of surface adsorptive sites and the progressive occupation of these sites during TC removal.

Thermodynamic analysis (Figure 9F, Table S3) reveals that the adsorption of TC on CF-3 is spontaneous, as indicated by negative ΔG values. The positive ΔH value (11.42 kJ mol⁻¹) suggests that the adsorption process is endothermic, implying that higher temperatures favor TC uptake. The positive ΔS value (0.049 kJ mol⁻¹ K⁻¹) indicates increased disorder at the solid–liquid interface during adsorption. Considering the multiple interactions between TC and the surface functional groups of hydrothermal carbon phase in CF-3, the positive entropy change may be associated with adsorption-induced reorganization of interfacial species [16,31]. Overall, the favorable entropy contribution (TΔS = 14.6 kJ mol⁻¹, T = 298K) outweighs the positive enthalpy term (ΔH = 11.42 kJ mol⁻¹), resulting in a negative ΔG. Therefore, the apparent spontaneous adsorption of TC on CF-3 is primarily driven by the entropy contribution.

3.5. Removal Mechanism of TC over CF-3

3.5.1. FT-IR Analysis

FT-IR results (Figure 10) reveal the surface interactions and reaction pathways of CF-3 during TC adsorption and degradation. The bands at 402 and 579 cm⁻¹ correspond to Co–O/Fe–O vibrations of CoFe₂O₄, whose decreased intensity after reaction indicates the involvement of Co/Fe sites in TC complexation and redox cycling during PMS activation. The appearance of the 1080 cm⁻¹ band after adsorption (Figure 10 (C)), assigned to the C–O stretching vibration of TC, supports TC adsorption on the CF-3 surface, while its disappearance after reaction (Figure 10 (B)) suggests the degradation or transformation of adsorbed TC [19,31]. A new band at 1122 cm⁻¹ appears only after reaction, confirming PMS activation on metal sites. The weakened 1448 cm⁻¹ band suggests changes in the local environment of N–C structures during the PMS activation reaction. The C=N band shifts from 1602 to 1630 cm⁻¹ after adsorption and remains shifted after reaction, indicating strong π–π EDA interactions and dynamic electronic modulation induced by Co(II)/Co(III) and Fe(II)/Fe(III) cycling. The N-doped carbon framework may serve as an electron transport pathway and facilitate interfacial charge transfer, as suggested by previous studies [16,20]. The O–H band at 3388 cm⁻¹ strengthens after adsorption, indicating hydrogen bonding, and remains stable after reaction, consistent with ROS analysis.

3.5.2. XPS Analysis

Both CF-3 and CF-3 + TC exhibit characteristic peaks of C, O, N, Co, and Fe in the survey spectra (Figure 11A), indicating that no obvious change occurred in the overall surface elemental composition after the PMS reaction. As shown in Figure 11B–F, the high-resolution C 1s spectra, peaks at 284.8 eV (C=C), 285.1–285.8 eV (C–C/C–N), 286.4–287.5 eV (C–O/C=N), and 288.4–288.6 eV (O–C=O) are observed. After reaction, the increased intensity of oxygenated carbon species suggests enrichment of polar functional groups on the carbonaceous layer, which may enhance surface polarity and favor TC/PMS adsorption. Combined with the preceding analyses, this observation is consistent with the proposed role of the N-doped carbon framework in interfacial electron transfer [16,20]. The O 1s spectra show lattice oxygen (529.7–530.0 eV), surface –OH/defective oxygen (531.5 eV), and adsorbed water/carboxyl oxygen (533.2–531.9 eV). The increased proportion of high-binding-energy oxygen species in CF-3 + PMS indicates an increased abundance of surface hydroxyl groups and defect-associated oxygen species, which may provide additional sites for PMS adsorption and activation. In the N 1s spectra, pyridinic N (~398.3 eV), pyrrolic N (~399.8 eV), and graphitic N (~401.0 eV) are identified. After reaction, the pyridinic N peak shifts to 397.8 eV, corresponding to a negative shift of 0.5 eV, whereas the pyrrolic N peak remains at 399.8 eV and the graphitic N peak shifts slightly from 401.0 to 401.1 eV. In addition, an M–N_x component is fitted at 398.8 eV. These changes suggest variations in the local electronic and coordination environments of the N species. Together with the above FT-IR analysis, these changes are associated with the involvement of M–N_x sites in electron transfer during PMS activation [12,13,14,15]. For Co 2p, four fitted components are observed at 780.0, 784.6, 795.5, and 802.2 eV for CF-3, and at 779.5, 784.6, 795.3, and 802.1 eV for CF-3 + PMS. After reaction, the main Co 2p peaks shift slightly toward lower binding energies. In contrast, Fe 2p spectra (709.8 and 723.7 eV) exhibit a slight positive shift. These opposite shifts suggest changes in the local electronic environments of Co and Fe and reflect electron redistribution between the two metal centers during PMS activation. Together with the catalytic results, these spectral changes are consistent with the involvement of Co/Fe redox transformations in PMS activation [10,11].

3.5.3. Adsorption–Catalysis Synergy

CF-3 integrates adsorption enrichment and PMS-driven catalytic oxidation for efficient TC removal (Figure 12). The N-doped carbon framework and Co/Fe sites contribute to TC enrichment via π–π interactions, hydrogen bonding, and surface complexation. The CoFe₂O₄/N-doped carbon interface is proposed to serve as an important region for PMS activation. Changes in the Co 2p and Fe 2p spectra, together with the catalytic results, are consistent with the involvement of Co/Fe redox transformations in PMS activation. Together with the preceding FTIR and XPS analyses, the observed changes may be associated with electron transfer involving N-containing/M–N_x sites during PMS activation. Quenching results indicate that ¹O₂ and O₂•⁻ dominate TC degradation, with SO₄•⁻ and •OH as auxiliaries. The high adsorption capacity (486.5 mg g⁻¹) enriches TC at the interface and facilitates subsequent PMS-driven catalytic oxidation. The spatial distribution of CoFe₂O₄ and N-doped carbon domains partially differentiate adsorption-dominant regions from PMS-activation sites and thereby alleviate their competition. Accordingly, a proposed “pre-concentration–activation–degradation” pathway is illustrated in Figure 12. This integrated adsorption-assisted catalytic process achieved a TC removal efficiency of 93.44%.

4. Conclusions

A series of N-doped carbon-modified CoFe₂O₄ composites (CF-1–CF-4) were successfully prepared via a one-step hydrothermal method. Comparison among CF-1–CF-4 indicated that the glucose and ethanol dosages influenced the carbon content, pore structure, and TC removal performance in the presence of PMS, with CF-3 exhibiting the highest mean removal efficiency. CF-3 exhibited efficient dual-function performance for tetracycline (TC) removal through synergistic adsorption and catalytic oxidation. CF-3 possesses a high surface area (93.94 m² g⁻¹), mesoporous structure, and well-integrated spinel CoFe₂O₄/N-doped carbon architecture with an appropriate carbon content (~15.5%). Under optimal conditions, CF-3/PMS system achieved 93.44% TC removal within 45 min, while CF-3 exhibited a maximum adsorption capacity of 486.5 mg g⁻¹. The adsorption process was spontaneous and endothermic, while quenching experiments suggested that TC oxidation involved both radical and non-radical pathways, with ¹O₂ and O₂•⁻ playing major roles and SO₄•⁻ and •OH also contributing. The catalytic activity may arise from the cooperative contributions of N-containing carbon sites, accessible Co/Fe-containing regions, and interfacial electronic interactions, which facilitate PMS activation and electron transfer. The locally encapsulated carbonaceous structure may favor the coupling of TC adsorption with PMS-mediated oxidation, rather than indicating a strict spatial separation of adsorption and catalytic sites. Overall, CF-3 provides an effective strategy for antibiotic wastewater treatment with strong adsorption–catalysis synergy and application potential.