The Efficient Degradation of Oxytetracycline in Wastewater Using Fe/Mn-Modified Magnetic Oak Biochar: Pathways and Mechanistic Investigation

Abstract

Antibiotic resistance has been recognized as a global threat to human health. Therefore, it is urgent to develop effective strategies to address the contamination of water environments caused by antibiotics. In this study, Fe/Mn bimetallic-modified biochar (FMBC) was synthesized through a one-pot oxidation/reduction-hydrothermal co-precipitation method, demonstrating an exceptional photocatalytic-Fenton degradation performance for oxytetracycline (OTC). Characterization techniques including FTIR, SEM, XRD, VSM, and N₂ adsorption–desorption analysis confirmed that the Fe/Mn bimetals were successfully loaded onto the surface of biochar in the form of Fe₃O₄ and MnFe₂O₄ mixed crystals and exhibited favorable paramagnetic properties that facilitate magnetic recovery. A key innovation is the utilization of biochar’s inherent phenol/quinone structures as reactive sites and electron transfer mediators, which synergistically interact with the loaded bimetallic oxides to significantly enhance the generation of highly reactive ·OH radicals, thereby boosting catalytic activity. Even after five recycling cycles, the material exhibited minimal changes in degradation efficiency and bimetallic crystal structure, indicating its notable stability and reusability. The photocatalytic degradation experiment conducted in a Fenton-like reaction system demonstrates that, under the conditions of pH 4.0, a H₂O₂ concentration of 5.16 mmol/L, a catalyst dosage of 0.20 g/L, and an OTC concentration of 100 mg/L, the optimal degradation efficiency of 98.3% can be achieved. Additionally, the pseudo-first-order kinetic rate constant was determined to be 4.88 min⁻¹. Furthermore, this study elucidated the detailed degradation mechanisms, pathways, and the influence of various ions, providing valuable theoretical insights and technical support for the degradation of antibiotics in real wastewater.

1. Introduction

Tetracyclines (TCs) are a class of natural or semi-synthetic broad-spectrum antibiotics, characterized by their tetracyclic structure comprising four carbon rings (Figure 1), which have been widely used globally for the treatment of diseases [1,2]. This antibiotic category demonstrates potent antibacterial activity against diverse pathogenic bacteria, including both Gram-positive and Gram-negative strains [3]. Notably, tetracycline (TC), oxytetracycline (OTC), chlortetracycline (CTC), and doxycycline (DOX) represent the four commonly employed variants within this group [4], which are widely utilized in veterinary medicine or as feed additives [5].

Some studies indicated that more than 75% of TCs could not undergo complete metabolism or absorption by organisms, leading to their inevitable release into aquatic environments [6]. Moreover, the rapid global expansion of livestock farming and the misuse of TC antibiotics have resulted in a significant increase in antibiotic concentrations in river and lake water [7], surpassing safe levels [8]. This has led to the prolonged exposure of aquatic organisms to these compounds, thereby exacerbating the development of human drug resistance [9]. Currently, antibiotic resistance has been recognized as a global threat to human health by the World Health Organization (WHO) [10]. Therefore, it is urgent to develop an effective method to address the new pollution problem caused by TC antibiotics in water environments.

At present, the photo-Fenton degradation technology is widely recognized as an advanced oxidation process (AOP) capable of effectively removing organic pollutants, such as tetracycline antibiotics [11,12]. This technology demonstrates exceptional reactivity and broad applicability for refractory organic pollutants. It is characterized by simple operation, mild reaction conditions, a rapid degradation rate, and high cost-effectiveness. These advantages have facilitated its extensive application in degrading tetracycline antibiotics in water bodies, drawing significant attention from researchers [13,14,15]. The fundamental principle of this technology involves the generation of highly oxidizing hydroxyl radicals (·OH) through the decomposition of hydrogen peroxide (H₂O₂) under the action of light and a metal catalyst, enabling the efficient degradation of organic pollutants like tetracyclines. However, the traditional Fenton oxidation method faces limitations due to its narrow applicable pH range and relatively low cycling efficiency of iron (III)/iron (II) [16]. To address these challenges, recent studies have demonstrated that the catalytic performance can be enhanced by incorporating diverse metal complexes into various carrier materials (e.g., g-C₃N₄, MOFs, carbon quantum dots (CQDs), etc.) [17,18,19]. For instance, composite materials, such as g-C₃N₄@ZIF-8/Ag₃PO₄, MoS₂@ZIF-67, MIL-101(Fe)/g-C₃N₄/FeOCl, CQDs/TiO₂, and CQDs/N-BiOCl, have been successfully synthesized and shown remarkable degradation capabilities [18,19,20,21,22]. Nevertheless, certain materials continue to encounter significant obstacles, such as intricate preparation procedures, insufficient degradation efficiency, elevated industrial costs due to the use of precious metals, and potential secondary pollution resulting from heavy metal ion leaching [23,24]. Consequently, the design and development of novel multi-level composite materials to further enhance catalytic activity while overcoming current limitations remains a critical focus for future research.

In recent years, numerous studies have demonstrated that bimetallic nanoparticles can efficiently activate oxidants or dissolved oxygen (DO) to generate reactive oxygen species (ROS) while also exhibiting significant reduction capabilities. This dual functionality enables them to effectively degrade refractory organic pollutants (ROPs) in high-salinity organic wastewater (HSOW) [25]. Therefore, bimetallic nanoparticles (NPs) have gained considerable attention and been extensively synthesized for application in groundwater remediation. However, their small particle size and high surface activity render them susceptible to aggregation and corrosion in aqueous environments, thereby restricting their practical applicability. To overcome these limitations, researchers have employed porous carbon materials, which possess unique pore structures, abundant reducing functional groups, and superior adsorption capacities, to immobilize the nanoparticles [26,27,28]. Porous carbon not only facilitates the adsorption of reactants onto the catalyst surface but also enhances electron transfer between reactants and metal nanoparticles, thereby significantly improving the degradation efficiency of pollutants [25]. However, most porous carbon materials either involve complex preparation processes or incur high costs, which hinder their large-scale application. In contrast, biochar derived from biomass is considered an ideal carrier material for metal nanoparticles due to its unique physicochemical properties, wide availability, and low cost [29,30,31].

Therefore, this study developed a novel biochar-supported Mn-Fe bimetallic hierarchical composite material, FMBC. The phenolic/quinone structures within the biochar of this material function as active reaction sites and electron transfer mediators. These structures synergistically interact with the supported bimetallic oxides, thereby significantly enhancing the generation of ·OH radicals. This material exhibits several notable advantages: (i) Its strong paramagnetic properties allow it to be readily recovered using a magnet following the degradation reaction, effectively addressing the challenge of recovering traditional catalysts and demonstrating excellent potential for reuse. (ii) This material is synthesized via a straightforward one-pot oxidation/reduction-hydrothermal co-deposition method, which is operationally convenient and highly feasible. (iii) Most importantly, FMBC exhibits a remarkable degradation performance during the photocatalytic-Fenton synergistic degradation of oxytetracycline (OTC). Furthermore, this study thoroughly investigates and elucidates the degradation mechanism and pathway of oxytetracycline using this material. In summary, these research findings suggest that this material holds great promise for practical applications.

2. Materials and Methods

2.1. Materials and Reagents

Oak sawdust was bought from Ruyi Timber Factory, Shijiazhuang, China. All the chemicals used in the experiments, including MnSO₄⋅H₂O, FeCl₃⋅6H₂O, HCl, NaHCO₃, EDTA-2Na, isopropanol (IPA), H₂O₂ (30 wt%), and vitamin C, were of analytical grade and purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Oxytetracycline (OTC) was purchased from Shengxue Dacheng Pharmaceutical Co., Ltd. (Shijiazhuang, China).

2.2. Characterization

The materials were analyzed for their specific surface area, pore volume, pore width, and pore size distribution using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) models. The N₂ adsorption/desorption isotherms of the materials were determined at 77 K utilizing a Kubo X1000 automatic surface area and pore analyzer (Beijing Builder Co., Ltd., Beijing, China). The morphology and elementary composition of the materials were measured by S-4800 cold-field emission scanning electron microscopy and INCA 350 energy-dispersive X-ray spectroscopy (SEM-EDS; Hitachi, Tokyo, Japan). The X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Fisher, Dartford, UK) was employed for the analysis of the coordination structures and the chemical valence states of Fe and Mn atoms in materials. The elemental composition and crystal structure of the material were determined using an X-ray diffraction (XRD) analyzer (Bruker D8 Advance, Karlsruhe, Germany). The functional group information of the material was analyzed using Fourier transform infrared spectroscopy (FTIR) with KBr pellets (Bruker AXS, Karlsruhe, Germany). The concentration of OTC was detected at 357 nm by the UV–Vis spectrometry (PERSEE TU-1901, Beijing, China). The intermediate product of OTC degradation at the end of the experiment was measured by LC-MS (CORUI Apus Plus, Beijing, China; SCIEX Triple TOF 5600+, Framingham, MA, USA).

2.3. Synthesis of Fe/Mn Bimetal-Modified Biochar

The oak sawdust was initially subjected to carbonization treatment in a tubular furnace at 500 °C for 3 h under a N₂ atmosphere. After being cooled to room temperature, it was ground into powder using a grinder and sieved through a 100-mesh screen to ensure a consistent particle size distribution, resulting in the production of BC.

The Fe/Mn bimetal-modified biochar (FMBC) was synthesized via a one-pot oxidation/reduction-hydrothermal co-precipitation method, as follows: Firstly, the BC prepared above (3.0 g) was dispersed in 80 mL of ultra-pure water and stirred for 30 min. Subsequently, 10 mL mixture containing 0.2 mol/L FeCl₃·6H₂O and 0.2 mol/L MnSO₄·H₂O (molar ratio of 1:1) was added to the stirring system. Following this, 10 mL of vitamin C (0.2 mol/L) was introduced into the mixture, which was then stirred for an additional 10 min before adjusting the pH to 8.0 using saturated NaHCO₃ solution while continuing stirring at 60 °C for 30 min. The resulting mixture was subsequently transferred into a reaction vessel and continuously heated at an oven temperature of 120 °C for 6 h. After cooling down, the sample was subjected to washing with ultra-pure water and then dried to obtain FMBC. Furthermore, materials with varying ratios of Fe/Mn were also prepared and designated as FM_1/3BC, FM_1/2BC, FM_2/1BC, and FM_3/1BC, respectively.

2.4. Experimental Procedure

The non-homogeneous photo-degradation capability was assessed by the photo-Fenton assisted degradation efficiency of oxytetracycline at room temperature, utilizing a 500 W xenon (Xe) lamp. All experiments were conducted using a 250 mL glass photo-reactor system. FMBC was introduced into the OTC aqueous solution within the photo-reactor. The different pH of OTC solution was adjusted using 0.1 mol/L H₂SO₄ or NaOH solution. The suspension was brought to adsorption–desorption equilibrium after stirring in the dark for 30 min. Then, the photo-Fenton reaction was initiated under continuous stirring and the appropriate H₂O₂ was added at the same time. In the process of photo-degradation, 2 mL analytical suspension was periodically extracted from the reactor and subjected to filtration through a membrane (0.45 μm) in order to effectively eliminate all remaining photocatalysts. The UV-visible spectrophotometer was employed to record the absorbance values of the degradation samples. The concentration of OTC antibiotics was determined by the absorption peak within the wavelength range of 200~800 nm (λ max = 357 nm). Finally, the relationship between C_t/C₀ and Time (t) was plotted to elucidate the adsorption performance and photocatalytic degradation efficiency of OTC in the purification system. Afterwards, the photo-degradation rate (η) was quantified and assessed by employing Equation (1):

$$\eta ={\displaystyle \frac{C_0-C_{\mathrm{t}}}{C_0}}\times 100\%$$

(1)

The kinetic rate of photo-Fenton degradation of OTC was investigated by analyzing the experimental data using kinetic models like pseudo-first-order and pseudo-second-order (Equations (2) and (3)) [32].

$$\ln {\displaystyle \frac{C_0}{C_{\mathrm{t}}}}=k_{1}t$$

(2)

$${\displaystyle \frac{1}{C_{\mathrm{t}}}}={\displaystyle \frac{1}{C_0}}+k_{2}t$$

(3)

where C₀ is the initial degradation concentration of OTC and C_t is the degradation concentration of OTC after a certain exposure time t (min), and k₁ and k₂ are the first- and second-order kinetic apparent rate constants, respectively. Typically, the slope of the graph is used to calculate the apparent rate constant.

In addition, the HPLC-MS verified the intermediate products formed during the OTC’s degradation using the C18 column (4.6 × 200 mm, 5 μm). For 30 min, the flow rate was adjusted to 0.1 mL/min in the isocratic mode. The HPLC gradient elution method was employed using a mobile phase of methanol–water (0.1% formic acid, Table S2). Employing identical procedures for the photocatalytic reaction as above described, the recycling experiments of photocatalytic degradation were conducted to assess the stability and reusability of FMBC. Each iteration of the antibiotic degradation process involved magnetic separation catalysts, rinsing with deionized water and drying in preparation for the subsequent operations.

3. Results and Discussion

3.1. Characterization of Materials

The specific surface area and pore structure of BC and FMBC samples were evaluated by the N₂ adsorption–desorption method. Firstly, the N₂ adsorption isotherms and pore size distributions in Figure 2 indicate that both samples possess microporous and mesoporous structures, and the N₂ adsorption isotherms exhibit a typical Type I isotherm pattern. This finding is consistent with the d_BJH data (Table S1). Additionally, as depicted in Table S1, it is evident that a significant proportion of the total specific surface area (S_BET) and total pore volume (V_total) can be attributed to the specific surface area and pore volume of micropores (S_micro and V_micro), thereby indicating the microporous nature of the prepared BC and FMBC materials. On the other hand, the BET specific surface area of FMBC (322.1 m²/g) exhibits a significant increase compared to that of BC (215.5 m²/g) (Table S1). This observation suggests that the modification with Fe and Mn can effectively enhance the specific surface area of biochar, potentially attributed to the incorporation of iron oxides and manganese salts facilitating the decomposition of oxygen complexes in biochar [33,34].

The XRD patterns of FMBC and BC are shown in Figure S1. The FMBC has diffraction peaks at 2θ = 18.07°, 30.62°, 36.05°, 43.72°, 54.12°, 57.72°, and 63.32°, relating to the (111), (220), (311), (400), (422), (511), and (440) crystal planes of FMBC, respectively, which could be well matched to spinel MnFe₂O₄ (JCPDS 10-0319) and Fe₃O₄ (JCPDS 19-0629) [35,36]. Therefore, based on the material preparation conditions, it can be inferred that the crystal structure of Fe and Mn in the FMBC material is a mixed structure consisting of MnFe₂O₄ and Fe₃O₄. The spinel-structured materials mentioned above typically exhibit robust paramagnetic properties [36,37]. Therefore, to investigate the magnetic performance of FMBC, VSM was employed to measure the hysteresis loop and its parameters. As depicted in Figure 3, the magnetization curve of FMBC displays a symmetrical ‘S’ shape with two overlapping loops. It can be observed that the curve passes through the origin and exhibits zero coercivity with no remanence. The saturation magnetization intensity is approximately 21.5 emu/g, indicating that FMBC demonstrated pronounced paramagnetic properties at room temperature and can be readily recovered and reused by applying a magnetic field (Figure 3). In addition, during the experiment, we recycled and reused the material using a magnet and conducted XRD characterization on the samples recovered after five cycles (Figure S1c). The results indicate that the diffraction patterns of the recovered FMBC are highly consistent with those of the original FMBC, suggesting that the crystal structure of the material remained stable and did not undergo significant alteration during the experimental process.

The functional groups present on the surfaces of BC and FMBC can be qualitatively identified in the FTIR spectrum shown in Figure S2. The spectrum reveals characteristic infrared absorption at 3438, 3135, 2920, 1619, 1417, 1260, and 1127 cm⁻¹, corresponding to –OH, =C–H, C–H, C=O, C–O–C, and C–O groups, respectively [38]. After comparing the spectra of FMBC and BC, a significant absorption peak at 3135 cm⁻¹ was observed in FMBC, indicating C–H stretching vibrations of unsaturated bonds. Moreover, the increase in the intensity of the absorption peak between 1576 cm⁻¹ and 1642 cm⁻¹ can be attributed to the stretching of the carbon ring structure or C=C/C–O bonds in biochar. The pronounced enhancement at 1260 cm⁻¹ further confirms the presence of abundant C–O–C cyclic structures within the material [39]. Therefore, considering all these factors collectively, it is plausible that complex reactions such as intramolecular or intermolecular dehydration involving active hydroxyl groups, carboxyl esterification, and cyclization occur during Fe/Mn-modified biochar production process. Additionally, the Fe–O and Mn–O bonds were remarkably enhanced due to bimetallic doping, corresponding to wavenumbers of 620 and 570 cm⁻¹, respectively [34,40]. Similarly, FTIR characterization was conducted on the samples recovered after five cycles (Figure S2c). The results indicate that the FTIR spectra of the recovered FMBC are in high agreement with those of the original FMBC. This observation aligns with the XRD analysis results, providing further evidence that no significant alteration occurs in the functional group structure of the material during the experimental process.

The XPS spectra of the FMBC, as depicted in Figure 4a, exhibit the presence of C (284.5 eV), O (532.6 eV), Fe (711.9 eV), and Mn (643.8 eV) elements. As shown in Figure 4b, four distinct C 1s binding energy peaks are observed at 284.2, 284.9, 285.9, and 288.4 eV, corresponding to the C–C, C=O, C–O, and O=C–O groups, respectively [41], which is consistent with the FTIR spectra. The Fe 2p spectrum exhibited two distinct peaks, namely Fe 2p_3/2 (711.9 eV) and Fe 2p_1/2 (725.4 eV) [42], as shown in Figure 4c. Specifically, the binding energy of Fe (II) was observed at 711.2 eV (50.8%) for the Fe 2p_3/2 peak, while that of Fe (III) appeared at 715.2 eV (49.2%). The high-resolution XPS spectra of Mn 2p in FMBC also exhibited the characteristic double peaks of Mn 2p_3/2 (642.9 eV) and Mn 2p_1/2 (654.7 eV) (Figure 4d). Notably, the three distinct peaks observed at binding energies of 642.4 (52.1%), 645.2 (38.5%), and 647.3 eV (9.4%) corresponded to different oxidation states of manganese: Mn (II), Mn (III), and Mn (IV), respectively [38]. We characterized the samples recovered after five cycles of operation using XPS (Figure S3). The results revealed that the crystal structures of both metals remained largely unchanged, whereas the valence state of the Mn element exhibited subtle variations. Specifically, the proportion of Mn (II) decreased slightly, while those of Mn (III) and Mn (IV) increased marginally. This finding suggests that the Mn element played a critical role in the efficient degradation of OTC. Notably, the total amount of oxidized carbon species in the material, such as C=O and C–O, increased significantly, which could be attributed to the partial oxidation of surface-active groups on biochar during the OTC degradation process. Additionally, a slight decrease in the fitting area ratio of –COOH was observed. It is speculated that this may result from the decarboxylation reaction of –COOH under the influence of hydroxyl radicals (OH).

The morphology and surface elemental composition of BC and FMBC were characterized using SEM–EDS analysis (Figure 5). In the SEM images, it was apparent that some solid particles adhered to the surface of FMBC (Figure 5b), resulting in its relatively rough texture, in contrast to the original biochar (BC, Figure 5a) with its smooth surface and uniform pore structure. Meanwhile, upon magnifying the FMBC crystal for observation, it became evident that each nanoflower consisted of multiple nanorod crystals of varying sizes. The average dimensions of these nanorods were approximately 300 nm in length and 90 nm in width (Figure S4). In addition, the element mapping images (Figure 5c,d) demonstrated the successful modification of Fe/Mn on the surface of BC with a uniform distribution. Upon comparing Figure 5c,d, it can be inferred that the distribution of Fe and Mn elements was relatively consistent, indicating their presence in a compound state, which aligns with the conclusion drawn from XRD analysis. Additionally, the EDS spectrum (Figure 5e) and elemental ratio distribution table (Figure 5f) of FMBC reveal successful surface modification of BC by Fe/Mn, exhibiting a weight ratio and molar ratio close to 1:1, which is consistent with the amounts of Fe and Mn added during material preparation.

3.2. Influence of Experimental Conditions

3.2.1. Influence of FMBC Dosage and Fe/Mn Ratios

Initially, comparisons between pure BC and bimetallic materials with varying Fe/Mn ratios (Figure S3) revealed that the FMBC composite material with a Fe/Mn ratio of 1:1 exhibited a superior performance, which may be related to the multivalent state changes of manganese and the mixed crystal structure of iron and manganese. Subsequently, under the conditions of pH = 6 and the OTC concentration of 100 mg/L, the influence of various catalyst dosages was investigated to screen and optimize the most suitable material (Figure 6a). It was observed that as the catalyst dosage increased from 0.01 g to 0.05 g in 250 mL reactor, both the degradation efficiency and reaction rate gradually improved. This is attributed to adequate catalyst providing sufficient reactive sites for H₂O₂ reaction and enhanced ·OH generation and promoting OTC pollutant degradation. However, a further increasing FMBC dosage decreased efficiency. This decline is likely due to excessive catalyst hindering light transmittance, impeding photo-generated charge excitation and thus reducing ·OH production. Moreover, excess Fe(II) or Mn(II) from the catalyst can scavenge generated ·OH [43]. The optimal FMBC dosage was determined to be 0.05 g.

3.2.2. Influence of pH on the System Performance

Solution pH significantly influences the efficiency of Fenton-like processes. Experiments varying pH (Figure 6b) demonstrated the optimal degradation efficiency (>94.1%) within the pH range of 4.0 to 6.0, with a corresponding decline outside this range. Kinetic analysis (

Table 1. The kinetic rate parameters of photo-Fenton degradation of OTC.

Parameters		Pseudo-First-Order Kinetic Model		Pseudo-Second-Order Kinetic Model
		k1 (×102, min−1)	R2	k2 (×10, min−1)	R2
FMBC dosages (g)	0.01	2.38 ± 0.03	0.9985	0.65 ± 0.03	0.9889
	0.03	3.63 ± 0.05	0.9982	1.73 ± 0.08	0.9887
	0.05	4.93 ± 0.11	0.9961	3.09 ± 0.11	0.9918
	0.07	3.07 ± 0.06	0.9971	1.06 ± 0.04	0.9904
	0.10	2.48 ± 0.05	0.9964	0.78 ± 0.04	0.9876
H2O2 concentrations (mmol/L)	1.29	1.57 ± 0.03	0.9979	0.32 ± 0.02	0.9778
	2.58	2.07 ± 0.06	0.9932	0.53 ± 0.02	0.9878
	5.16	4.52 ± 0.05	0.9990	3.21 ± 0.13	0.9883
	10.32	5.26 ± 0.06	0.9989	4.69 ± 0.23	0.9827
pH	2.0	2.61 ± 0.04	0.9981	0.63 ± 0.05	0.9629
	4.0	4.66 ± 0.07	0.9980	3.19 ± 0.12	0.9886
	6.0	3.47 ± 0.03	0.9994	1.28 ± 0.10	0.9621
	7.0	1.91 ± 0.03	0.9977	0.36 ± 0.03	0.9616
	9.0	1.15 ± 0.02	0.9979	0.16 ± 0.02	0.9987
C0 of OTC (mg/L)	25	3.59 ± 0.06	0.9979	1.84 ± 0.11	0.9798
	75	3.87 ± 0.09	0.9963	2.23 ± 0.12	0.9823
	100	4.88 ± 0.07	0.9983	3.54 ± 0.20	0.9817
	125	3.25 ± 0.08	0.9949	1.39 ± 0.08	0.9764
	150	2.68 ± 0.05	0.9978	0.82 ± 0.04	0.9887

) confirmed the optimal reaction rates (3.47 to 4.66 min⁻¹) in this range. A possible explanation is that a medium-strength acidic solution (pH 4.0–6.0) facilitates the efficient decomposition of H₂O₂ by the catalyst, thereby enhancing the generation rate of ·OH. At highly acidic pH values, H⁺ may compete with other species for active sites, or the reaction rate of Fe²⁺ with H₂O₂ may slow down. At a neutral or alkaline pH, H₂O₂ will decompose into O₂, resulting in decreased free radical production. Considering the typical OTC wastewater pH (4.5–7.1 [44]), the optimal range of FMBC suggests a good potential for practical application with potential minor pH adjustment.

3.2.3. Effect of H₂O₂ on the System Performance

The H₂O₂ concentration is a crucial factor as it is the primary precursor for oxidizing radicals. Increasing the H₂O₂ concentration enhanced OTC removal (Figure 6c). Efficiency exceeded 98% at 5.16 mmol/L, with no significant improvement beyond this point. This initial enhancement is due to increased H₂O₂ providing more reactant for ·OH formation. However, excess H₂O₂ can scavenge ·OH radicals, forming less reactive HO₂· (H₂O₂ + ·OH → HO₂· + H₂O), or undergo self-decomposition. Thus, 5.16 mmol/L was identified as the optimal concentration, balancing radical generation and scavenging/resource consumption. Kinetic analysis (pseudo-first-order) also showed a substantial increase in rate with H₂O₂ concentration, peaking at >4.52 min⁻¹ at the optimum (Table 1).

3.2.4. Effects of Different Concentrations of OTC

Variations in wastewater lead to different pollutant concentrations, making it crucial to investigate their impact. As shown in Figure 6d, the initial OTC concentration had a notable influence on the degradation rate. Higher OTC concentrations resulted in slower degradation rates. This is because, under constant conditions (light, catalyst, H₂O₂), the rate of ·OH production is relatively constant. At higher pollutant concentrations, these available radicals are distributed among more OTC molecules, effectively lowering the reaction rate. Conversely, lower OTC concentrations exhibit faster relative degradation rates due to a more favorable ratio of radicals to pollutant molecules and less limitation by molecular collision frequency between ·OH and OTC. The degradation efficiency remained high (>96%) at lower concentrations, reaching an optimal efficiency (98.3%) at 100 mg/L, then slightly declining at higher concentrations.

In conclusion, this study on the photocatalytic degradation of OTC using a Fenton-like reaction system revealed that the optimal degradation efficiency was achieved at pH 4.0, H₂O₂ concentration of 5.16 mmol/L, catalyst dosage of 0.05 g, and OTC concentration of 100 mg/L.

3.2.5. Effects of Various Ions on the Degradation of OTC

The presence of common ions, such as Cl⁻, NH₄⁺, CO₃²⁻, and H₂PO₄⁻, in the matrix of agricultural or pharmaceutical wastewater can potentially impact the photocatalytic reaction. Firstly, as depicted in Figure 7, the effects of Cl⁻ ions on the photocatalytic degradation of OTC were investigated. The results indicated that Cl⁻ has no significant impact on the purification reaction of OTC. The impact of Cl⁻ on advanced oxidation processes is relatively complex [45]. On the one hand, Cl⁻ can react with ⋅OH, potentially acting as a scavenger (e.g., Cl⁻ + ⋅OH → ClOH·). On the other hand, these reactions can lead to the formation of secondary reactive chlorine species, such as Cl⋅ and Cl₂⋅⁻, which also possess oxidizing capabilities and can promote the degradation of pollutants. The observation of no significant effect of Cl⁻ in experiments indicates that the inhibitory effects of Cl⁻ (such as initial ⋅OH scavenging) are largely offset by the formation and reactivity of secondary reactive chlorine species. Therefore, the degradation rate decreases slightly with the addition of a small amount of chloride ions. However, as the concentration of chloride ions continues to increase, the degradation rate exhibits a slight upward trend. CO₃²⁻ ions also impact the degradation process through competing mechanisms. These ions can act as buffers, reacting with enough H⁺ in water to produce CO₂ (Equation (4)), thereby reducing the ion concentration and mitigating their impact on the degradation efficiency of OTC. Meanwhile, HCO₃⁻ transformed from CO₃²⁻ is an efficient scavenger of ·OH (Equations (5) and (6)) [46], leading to a significant reduction in the concentration of this key oxidizing species and thus inhibiting the degradation. Therefore, the observed marginal effect of CO₃²⁻ and HCO₃⁻ on the degradation efficiency of OTC is the net outcome of these competing effects: potential buffering/pH benefits versus significant radical scavenging inhibition. Like the behavior of CO₃²⁻ or HCO₃⁻, the reaction of H₂PO₄⁻ with ·OH could also generate fewer active free radicals, as demonstrated in Equation (7), resulting in reduced efficiency for OTC removal. Lastly, the presence of NH₄⁺ ions during the photo-Fenton process may result in their oxidation and consumption of ·OH in solution (Equation (8)). Thus, this reduction of ⋅OH could impede the degradation rate of OTC.

CO₃²⁻ + 2H⁺(sufficient) → H₂O + HCO₃⁻

(4)

CO₃²⁻ + H⁺ → HCO₃⁻

(5)

HCO₃⁻ + ·OH → H₂O + CO₃⋅⁻

(6)

H₂PO₄⁻ + ·OH → H₂PO₄⋅/HPO₄⋅⁻ + OH⁻/H₂O

(7)

NH₄⁺ + 4⋅OH → ·NO₂ + 2H₂O

(8)

In general, as illustrated in Figure 7, the chloride ion concentration exhibits a minimal influence on the degradation efficiency of OTC. However, with increasing concentrations of NH₄⁺, CO₃²⁻, and H₂PO₄⁻ ions, the degradation efficiency of OTC decreases markedly. This phenomenon is attributed to the interference of these ions with the photo-Fenton reaction process via distinct reaction mechanisms. Furthermore, an OTC solution was prepared using tap water (Figure S16). The experimental findings indicate that despite the presence of various complex ions in tap water, it exerts no significant effect on the degradation efficiency of the OTC simulated wastewater. Consequently, the FMBC catalyst demonstrates promising potential for application in the treatment of real water pollution.

3.3. The KINETICS Study of the OTC Decomposition

The kinetic rate of the photo-Fenton degradation of OTC was investigated by analyzing the experimental data using first-order and second-order kinetic models (Equations (2) and (3)). The study results are presented in Table 1. When comparing the correlation coefficients (R²) of the pseudo-first-order and pseudo-second-order kinetic models, it is evident that the pseudo-first-order kinetic model more accurately describes the behavioral characteristics of the catalyst during the OTC degradation process, thereby demonstrating its superior applicability. However, under varying conditions, the rate constant exhibits significant variation, as follows:

(i) Kinetic analysis of different catalyst dosages reveals that both pseudo-first-order (k₁) and pseudo-second-order (k₂) rate constants increase with a catalyst dosage up to 0.05 g. Beyond this point, further increases in catalyst lead to a decline in the rate constant, indicating that an excessive catalyst concentration inhibits OTC degradation. This indicates that the optimal dosage of the catalyst is 0.05 g. (ii) As the concentration of H₂O₂ increases, both the first-order and second-order kinetic constants increase. This indicates that an increase in the H₂O₂ content generates more ·OH, thereby enhancing the degradation rate of OTC. However, an excessive amount of H₂O₂ will scavenge ·OH, generating HO₂· with lower reactivity (H₂O₂ + ·OH → HO₂· + H₂O), or undergo self-decomposition. Therefore, a concentration of H₂O₂ of 5.16 mmol/L is the optimal concentration for the degradation of OTC. (iii) The pH of the solution significantly affects the degradation rate in Fenton-like reactions. As pH increases, both pseudo-first-order and pseudo-second-order kinetic constants rise. However, when pH exceeds 7.0, the reaction rate drops sharply. The optimal pH range for the maximum degradation efficiency is 4.0 to 6.0. Beyond this range, efficiency decreases gradually. (iv) Differences in wastewater characteristics cause significant variations in pollutant concentrations, making it crucial to study their impact on degradation processes. Kinetic data analysis reveals faster pollutant degradation under low concentrations and slower rates under high concentrations. At high concentrations, hydroxyl radicals are distributed among more oxytetracycline molecules, reducing effective reactions per unit time and slowing degradation. Conversely, lower concentrations allow for less restricted collisions between hydroxyl radicals and oxytetracycline, enhancing the reaction efficiency and speeding up degradation.

In conclusion, the results of pseudo-first-order and pseudo-second-order kinetic analyses match the experimental findings on factor influences. The optimal OTC degradation conditions are 0.05 g FMBC catalyst, 5.16 mmol/L H₂O₂, pH 4.0, and a 100 mg/L initial OTC concentration. Comparing the R² values shows that the pseudo-first-order model better describes the catalyst’s behavior during OTC degradation, confirming its superior applicability.

3.4. The Degradation Mechanism of Oxytetracycline (OTC)

In this study, the activation ability of Fe/Mn bimetallic-modified biochar (FMBC) for H₂O₂ during the photocatalytic-Fenton synergistic degradation of OTC was investigated through quenching experiments, as can be seen from Figure 8. Upon introducing various quenchers, the degradation efficiency of OTC exhibited significant changes, indicating that multiple free radicals played important roles in the degradation process. Specifically, without any quencher, the removal rate of OTC by 0.05 g FMBC reached 98.3% within 80 min. However, upon adding isopropanol (IPA), the degradation efficiency of OTC dropped sharply to 22.8%, indicating that ·OH were the predominant active species in the synergistic degradation of OTC (Equation (9)). Importantly, the quinone structures in biochar can accept and transfer photogenerated electrons, thereby reducing H₂O₂ to generate a large amount of ·OH and ·O₂⁻ radicals, thus maintaining the redox capacity of the photocatalytic-Fenton system. In addition, when N₂ was introduced into the reaction system or EDTA-2Na was added, the degradation efficiencies decreased to 75.3% and 41.2%, respectively, indicating that superoxide radicals (·O₂⁻) and holes (h⁺) also had a significant impact on the degradation efficiency (Equation (10)) [47].

According to relevant literature reports [48,49] and experimental results, the redox catalytic cycles of Fe(II)/Fe(III) and Mn(II)/Mn(III) (Equations (11) and (12)) can effectively generate a large amount of ·OH. In this cyclic reaction, based on the redox potentials of E₀ (Mn(III)/Mn(II)) = 1.51 V, E₀ (Mn(IV)/Mn(II)) = 1.22 V, and E₀ (Fe(III)/Fe(II)) = 0.771 V, Fe(II) can thermodynamically reduce Mn(III)/Mn(IV) to Mn(II) (Equation (13)), thereby facilitating electron transfer in the reaction system and breaking the dependence of the conversion of Mn(III)/Mn(IV) to Mn(II) on H₂O₂ (Equation (14)). Simultaneously, Fe(II) can be regenerated via the photolysis of Fe(III) (Equation (15)), which can significantly promote the degradation activity of OTC. Moreover, during the degradation process, HO₂· can rapidly transform into ·O₂⁻, which becomes one of the main sources of ·O₂⁻ (Equation (16)). This transformation promotes the reduction of Fe(III) and Mn(III)/Mn(IV) to Fe(II) and Mn(II), as shown in Equations (17) and (18) [50]. In summary, the mechanism of photocatalytic-Fenton synergistic degradation of OTC in wastewater in this study is primarily attributed to the combined action of active free radicals such as ·O₂⁻, h⁺, and ·OH; however, the contribution levels of each reactive species differ significantly.

⋅OH + OTC →degraded products

(9)

h⁺ + H₂O → ⋅OH + H⁺

(10)

Fe(II) + H₂O₂ → Fe(III) + ⋅OH + OH⁻

(11)

Mn(II) + H₂O₂ → Mn(III) + ⋅OH + OH⁻

(12)

Fe(II) + Mn(III)/Mn (IV) → Mn(II) + Fe(III)

(13)

Mn(III)/Mn (IV)+ H₂O₂ → Mn(II) + HO₂⋅ + H⁺

(14)

Fe(III) + OH⁻ + hυ → Fe(II) + ⋅OH

(15)

HO₂⋅ ↔ ⋅O₂⁻ + H⁺

(16)

Fe(III) + ⋅O₂⁻ → Fe(II) + O₂

(17)

Mn(III)/Mn (IV) + ⋅O₂⁻ → Mn(II) + O₂

(18)

3.5. The OTC Degradation Pathways

To explore the intermediate products in the photocatalytic-Fenton degradation of OTC, the composition of the reaction solution was analyzed using LC-MS technology. The mass spectrum peaks of the corresponding intermediates in the reaction solution are shown in Figures S9–S12. Based on the analysis of the MS spectrum and with reference to previous reports [21], the analysis revealed that the degradation pathways of OTC were analogous to those of TC. Three main possible degradation pathways have been proposed: Pathway I involves attacks on acylamino and tertiary amine groups; Pathway II targets the aromatic ring; and Pathway III focuses on the hydroxyl groups at the C6 and C8 sites.

As shown in Pathway I of Figure 9, the acylamino and tertiary amine groups of OTC are initially attacked and oxidized [51], followed by the protonation and removal of the hydroxyl groups at the C6 and C8 positions, leading to the formation of the intermediate P1 (m/z = 338) [52]. Subsequently, P1 is further oxidized to a quinone structure, which then undergoes ring-opening to yield intermediate P2 (m/z = 318). P2 subsequently undergoes a series of transformations including esterification, dehydrogenation, oxygenation, and ring-opening events, ultimately converting to CO₂ and water via P21 (m/z = 102).

As illustrated in Pathway II of Figure 9, the aromatic ring of OTC readily undergoes hydroxylation upon attack by ·OH, leading to the formation of a diphenol structure comprising o-, m-, and p-isomers with m/z = 476 [53]. Diphenol structures are readily oxidized to quinone structures, which can subsequently undergo ring-opening to form P10 (m/z = 437). As previously mentioned, following the decarboxylation of P10, it undergoes further oxidation to form the quinone structures P11 (m/z = 374) and P16 (m/z = 360). Notably, P16 exhibits greater stability compared to P11, which is consistent with the MS spectrum. Then, both P11 and P16 are subjected to additional oxidation, leading to the formation of P17 (m/z = 338). After undergoing a series of oxidation and ring-opening reactions, they are ultimately converted into CO₂ and water via P21 (m/z = 102).

Pathway III involves the loss of hydroxyl and methyl groups, which initially occurs at the C6 and C8 sites, resulting in the intermediate compound P12 (m/z = 413). However, the ongoing photo-Fenton degradation of P12 proceeds via two distinct pathways [51,53]: Pathway III-1, which involves the attack on the aromatic ring; and Pathway III-2, where the acylamino and tertiary amine groups are targeted. This results in the formation of P13 (m/z = 376) and P14 (m/z = 318), respectively, as illustrated in Figure 9. Subsequently, P13 can be oxidized to transform into P16 (m/z = 360), and ultimately converted into CO₂ and water via the P17 (m/z = 338) to P21 pathways. Likewise, P14 undergoes oxidation and degradation to yield P15 (m/z = 284), which is subsequently transformed into CO₂ and water through the P18 (m/z = 174) and P21 pathways.

In summary, each degradation pathway is interconnected rather than isolated, facilitating the mutual transformation of various compounds through fundamental chemical reaction mechanisms such as hydroxylation, dealkylation, dehydration, oxidation, deamination, and deamidation, ultimately leading to the efficient degradation of OTC.

3.6. Stability and Reusability of FMBC

The recovery and reusability of photocatalysts play pivotal roles in the practical implementation of wastewater treatment systems. Therefore, the FMBC composite material, achieved by surface modification of BC with Mn/Fe oxides, exhibited remarkable magnetism, thereby addressing the challenge of photocatalyst separation through the use of a permanent magnet. In the recycling experiment, the spent catalyst was initially isolated using an external magnetic field and then washed three times with distilled water and dried for subsequent operations. The entire recovery process does not result in any significant loss of catalyst mass (recovery > 98%). The FMBC composite material exhibited exceptional stability in the continuous five cycles of photo-degradation experiments, with a removal rate of OTC consistently exceeding 96%, as illustrated in Figure 10. Additionally, XRD and FTIR measurements were conducted to verify the structure of the regenerated FMBC composite after the fifth cycle. The results revealed no significant alterations in either crystal structure or functional group composition (Figures S1 and S2), indicating its exceptional chemical stability during the OTC photocatalytic reaction. Consequently, FMBC demonstrates promising industrial value in terms of stability and recyclability.

4. Conclusions

In this study, magnetic Fe/Mn bimetallic-modified biochar (FMBC) was successfully synthesized via a one-pot hydrothermal co-precipitation method with oxidation/reduction steps. Leveraging the electron-mediating properties of biochar’s quinone structures, the loaded Fe₃O₄ and MnFe₂O₄ mixed crystals synergistically enhanced photocatalytic-Fenton degradation of OTC. The material exhibited facile magnetic recovery due to its paramagnetism. FMBC demonstrated an excellent performance, achieving an optimal degradation efficiency of 98.3% for OTC at pH 4.0, 5.16 mmol/L H₂O₂, 0.05 g catalyst, and 100 mg/L OTC, with a pseudo-first-order rate constant of 4.88 min⁻¹. Crucially, it showed notable stability and reusability, maintaining over 96% efficiency after five cycles with minimal structural changes. Detailed degradation pathways, mechanisms, and the impact of various ions were elucidated, providing theoretical and technical insights. In summary, the simple synthesis, high efficiency, excellent stability, and good recyclability of FMBC highlight its significant potential for pharmaceutical wastewater treatment.