Degradation of Tetracycline Hydrochloride by Cobalt-Doped Biochar-Activated Peroxymonosulfate

Abstract

The presence of tetracycline hydrochloride (TC) in the environment poses significant risks to human health and ecological stability, necessitating the development of effective and rapid removal strategies. In this research, we investigate the efficacy of degrading tetracycline hydrochloride using cobalt-doped-biochar (Co-BC)-activated peroxymonosulfate (PMS) and the underlying mechanisms of this process. The research objectives and conclusions were as follows: (1) Co-BC materials were synthesized from balsa wood powder through a process of impregnation followed by high-temperature calcination. Characterization techniques such as SEM, XRD, FTIR, and XPS were used to confirm the material’s structure and composition. (2) In a TC solution of 20 mg L⁻¹, the use of 100.0 mg L⁻¹ of Co-BC and 1.0 mM PMS led to a TC degradation efficiency of 96.2% within 30 min. (3) The Co-BC+PMS system exhibited wide pH adaptability (4.34–9.02) and strong resistance to environmental matrix interference (Cl⁻, ${\mathrm{NO}}_3^{-}$, and ${\mathrm{SO}}_4^{2-}$). (4) Free-radical quenching experiments indicated that sulfate radicals ($\mathrm{S}{\mathrm{O}}_4^{•-}$) were the primary reactive species in TC degradation. The 11 intermediates of TC were analyzed using LC-MS, and two possible degradation pathways were deduced. In summary, this study offers significant, valuable insights into and technical support for the green, efficient, and environmentally friendly removal of antibiotics from sewage.

1. Introduction

Antibiotics, categorized as emerging pollutants, often accumulate as residuals in aquatic environments, posing potential risks to human health and aquatic ecosystems [1,2,3]. Tetracycline hydrochloride (TC), a broad-spectrum antibiotic characterized by its biotoxicity and resistance to degradation, is frequently detected in environmental water bodies [4,5]. Prolonged exposure to TC can induce antibiotic resistance in pathogenic bacteria, further threatening human health [6]. However, conventional wastewater treatment methods, including biodegradation, membrane separation, and adsorption, do not facilitate efficient TC removal [7,8]. Consequently, there is an urgent need to develop efficient and eco-friendly technologies to eliminate residual TC from water.

In recent decades, advanced oxidation processes (AOPs) have become increasingly prevalent in the degradation of various recalcitrant and toxic pollutants. This trend is attributed to their rapid degradation kinetics, environmental friendliness, ease of induction, lack of harmful byproducts, and cost-effectiveness [9,10]. Sulfate radicals ($\mathrm{S}{\mathrm{O}}_4^{•-}$) have drawn significant attention in water and wastewater treatment for their strong oxidizing power and high reactivity towards recalcitrant organic pollutants [11]. $\mathrm{S}{\mathrm{O}}_4^{•-}$ exhibits a high redox potential (2.5–3.1 V) [12], a relatively long half-life (3–4 × 10⁻⁵ s), and a broad pH reaction window [13]. Typically, permonosulfate (PMS, ${\mathrm{HSO}}_5^{-}$) and peroxydisulfate (PDS, ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$) can be activated to generate $\mathrm{S}{\mathrm{O}}_4^{•-}$ through the cleavage of the peroxygen bond [14,15]. The literature suggests that PMS is more readily activated than PDS and demonstrates superior performance in the degradation of organic pollutants; this performance can be attributed to its asymmetric structure and longer O-O bonds [16]. However, the practical application of inactivated PMS is limited by its slow autoxidation kinetics [17]. Consequently, the development of catalysts with high catalytic activity and environmental adaptability for PMS activation is of critical importance [18,19].

Biochar (BC), an environmentally friendly material characterized by its abundant raw materials, straightforward preparation process, and low cost, is widely used for the catalytic activation of PMS [20,21]. However, pure biochar has few active sites, seriously affecting its effectiveness in activating PMS for pollutant removal. Consequently, biochar must be modified and optimized to improve its activation capacity [22]. Doping biochar with transition metals (Co, Fe, Ni, etc.) is an effective way of enhancing its catalytic performance [23]. Among these metals, Co and its composites show superior performance to other metals because of reversible Co²⁺/Co³⁺ redox reactions when activating PMS to produce reactive oxygen species [24]. The incorporation of Co onto a biochar substrate with a high specific surface area and stability not only mitigates the agglomeration of Co metal particles but also enhances the rate of PMS activation by the catalytic materials, thereby improving the efficiency of pollutant degradation [25,26]. Therefore, the introduction of metal ions into biochar is a feasible and eco-friendly method of improving the performance of biochar-activated PMS.

In this study, cobalt-doped biochar composites (Co-BCs) were synthesized through an impregnation method, followed by high-temperature calcination under an inert atmosphere, and used to activate PMS for TC degradation. The synthesized Co-BC was characterized to analyze its surface morphology, crystal structure, and elemental composition. The effects of the catalyst dosage, PMS concentration, initial solution pH, inorganic anions, and different aqueous matrices on TC removal in the Co-BC+PMS system were investigated. Finally, free-radical quenching experiments were performed to determine the primary active species involved in TC degradation, and the TC degradation intermediates and degradation pathways were detected and analyzed using LC-MS.

2. Results and Discussion

2.1. Characterization of the Catalyst

2.1.1. SEM Characterization

The morphology and structural characteristics of the catalyst were examined through scanning electron microscopy (SEM) analysis characterization, and the results are shown in Figure 1. Figure 1a shows that the BC has an abundant open structure with a smooth surface, and this structure is favorable for making contact with PMS during the catalytic process. The Co-BC obtained after Co doping still retained an intact open structure, which did not have much of an effect on the surface morphology of the BC (Figure 1b). Analyzing the surface elements of Co-BC using EDS revealed that C, N, O, and Co were uniformly distributed on the surface of the material (Figure 1c,d), indicating that Co was successfully doped into the biochar. Figure 1e indicates that the content of Co in the Co-BC composite is 5.95 wt %. This characterization underscores the efficacy of biochar as a support for cobalt, facilitating the dispersion of Co particles on its surface, thereby mitigating particle agglomeration and enhancing the catalytic performance of Co-BC [27].

2.1.2. XRD Characterization

The crystalline phase and chemical structure of the material were examined using X-ray diffraction (XRD). As illustrated in Figure 2, the broad diffraction peak at 22.6° in the XRD pattern of BC represents the (002) crystalline plane of graphitic carbon [28,29]. The diffraction peaks of graphitic carbon can also be observed in the XRD pattern of Co-BC, indicating that the process of Co doping also helped enhance the graphitization of the biochar. Furthermore, the two characteristic peaks appearing at 44.2° and 51.5° correspond to the (111) and (200) crystallographic planes of the Co standard card (PDF#15-0806) [30]. This analysis further confirms the successful incorporation of elemental cobalt into the biochar matrix.

2.1.3. FTIR Characterization

The FTIR spectra of BC and Co-BC are shown in Figure 3. The absorption peaks of BC and Co-BC at 1045 cm⁻¹ and 1381 cm⁻¹ are mainly related to the stretching vibration of C-O [31] and the deformation vibration of C-O-H [32], while the absorption band at 1633 cm⁻¹ was caused by the vibration of C=O [33,34]. In addition, the peak at 2918 cm⁻¹ corresponds to the stretching vibrations of C-H, while the peak at 3444 cm⁻¹ was mainly caused by the stretching vibration of O-H present in the biochar [35,36].

2.1.4. XPS Characterization

The chemical composition and valence states of the Co-BC composites were analyzed using XPS tests. As shown in Figure 4, the XPS spectrum of Co 2p (Figure 4a) indicates the presence of Co⁰ (780.2 eV), Co³⁺ (781.8 eV), and Co²⁺ (783.1 eV) [37,38] as well as a satellite peak located at 787.0 eV [39]. The weaker signal for Co 2p is due to the smaller Co doping. As shown in Figure 4b, the C 1s peak (284.8 eV) was used to correct all the binding energies obtained from the XPS spectra, and the C 1s peak spectrogram of Co-BC was categorized into four peaks at 284.8, 285.4, 288.5, and 291.1 eV, which correspond to C-C, C-N, C-O, and O-C=O, respectively [38,40]. Figure 4c shows three peaks of the N 1s spectrum corresponding to pyridine N (397.9 eV), pyrrole N (400.9 eV), and graphite N (404.0 eV) [30,38]. Previous studies have shown that the presence of N facilitates the activation of PMS, which may add additional active sites or accelerate electron transfer, wherein graphite N is able to enhance the activity towards PMS by accelerating the electron transfer between neighboring carbon atoms, thus improving the catalytic performance of the materials [41,42]. Additionally, the fine spectrum of O 1s (Figure 4d), with peaks at 532.1 and 533.4 eV, can be attributed to O-H and C-O [43,44]. The above results indicate the successful synthesis of cobalt-doped biochar Co-BC composites.

2.2. Catalytic Performance

The catalytic performance of Co-BC composites was evaluated by analyzing the TC degradation efficiency of different systems, and the results are illustrated in Figure 5a. The Co-BC adsorption experiments showed that the catalyst alone had a poor adsorption effect on TC, adsorbing only about 10% of the TC, while the degradation rate of the PMS system alone was 44.5% in 50 min, attributed to the fact that PMS has a strong oxidizing property by itself, but its oxidation rate was slow and the degradation efficiency of TC was not high. Additionally, the degradation rate of the BC+PMS system was comparable to that of PMS alone, indicating that the activation effect of BC on PMS was weak. In addition, the Co-BC+PMS system exhibited a significant TC degradation effect. Its TC degradation rate reached 97% after the addition of PMS for 30 min, and the degradation rate constant reached 0.1118 min⁻¹ (Figure 5b). The enhanced activation performance of PMS induced by Co-BC is likely due to the doping of Co, and the homogeneous distribution of Co on the biochar provided effective catalytic active sites for the activation of PMS.

The influence of catalyst dosage on TC degradation was investigated (Figure 5c). The effect of an increasing catalyst dosage on the degradation process is shown in Figure 5. With the gradual increase in catalyst dosage from 25.0 mg L⁻¹ to 200.0 mg L⁻¹, the adsorption effect of the material increased from 2% to 24.1%. Concurrently, the TC removal rate improved from 55.5% to 99.7%, and its degradation rate constant increased from 0.0259 min⁻¹ to 0.1935 min⁻¹ (Figure 5d). When the catalyst dosage was increased from 25.0 mg L⁻¹ to 100.0 mg L⁻¹, the degradation performance was significantly improved, a result attributed to the introduction of more active centers in the reaction system, an action that facilitated the activation of PMS and thus improved the TC degradation efficiency. However, further increases in catalyst dosing served to decrease the additional increase in the overall removal rate of the system. This result is consistent with a previous report, which showed that more catalyst does not necessarily equate to better performance [45] and is contrary to the concept of green chemistry. Therefore, a catalyst dosage of 100.0 mg L⁻¹ was selected for the subsequent experiments.

The influence of the PMS concentration is illustrated in Figure 5e. The findings indicate that an increase in the PMS concentration from 0.2 mM to 1.0 mM significantly improved the catalytic performance. Additionally, the TC degradation rate increased from 73.8% to 96.2%, and the degradation rate constant increased from 0.0405 min⁻¹ to 0.1017 min⁻¹ (Figure 5f). These results are due to the rapid consumption of PMS by TC, resulting from the insufficient production of active species when the amount of PMS was low (0.2 mM). When 1.0 mM PMS was added, almost complete degradation of TC was achieved, while a further increase in the PMS dosage decreased the rate of increase. This observation suggests that while elevated PMS concentrations generally facilitate TC removal, the incremental benefit is attenuated beyond a certain concentration. This attenuating effect may be attributed to the quenching of free radicals by excess PMS [15]. From both economic and environmental perspectives, a PMS concentration of 1.0 mM was selected for all of the subsequent experiments.

The initial pH of a solution is a critical determinant of the catalytic efficacy of the Co-BC+PMS system. As illustrated in Figure 5g, the ultimate degradation rate of TC was typically over 95% when the pH ranged from 4.34 to 9.02. This indicates that the system is effective in a wide pH range. However, when the initial pH was lowered to 3.04, the final TC degradation rate decreased to 82.3%, and its degradation rate constant decreased to 0.0548 min⁻¹ (Figure 5h). This diminished performance at a strongly acidic pH can be attributed to the formation of CoOH⁺ (Equation (1)), in addition to the acidic conditions at pH 3.04, at which point H⁺ binds to free radicals and quenches some of them (Equations (2) and (3)) [46], further contributing to the reduction in the degradation rate.

Co²⁺ + H₂O → CoOH⁺ + H⁺

(1)

$$\mathrm{S}{\mathrm{O}}_4^{•-}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} \to {\mathrm{HSO}}_4^{•-}$$

(2)

^•OH + H⁺ + e⁻ → H₂O

(3)

Environmental matrices, including inorganic substances, are commonly found in natural water bodies. They can affect the performance of catalysts through various pathways, such as by adjusting pH, quenching free radicals, or influencing the decomposition of PMS [47]. As shown in Figure 6, the effects of different anions on the TC degradation rate at 1 mM, 10 mM, and 100 mM concentrations were investigated. Based on previous studies, it is generally believed that the presence of Cl⁻ has an inhibitory effect on the degradation of organic pollutants [48,49]. As shown in Figure 6a, it can be seen that 1 mM, 10 mM, and 100 mM of Cl⁻ reduced the TC degradation rate to 89.6%, 85.4%, and 77.1%, respectively. This phenomenon may be attributed to the fact that Cl⁻ quenches $\mathrm{S}{\mathrm{O}}_4^{•-}$ and ^•OH radicals, producing less reactive chloride anions (Cl^•, ${\mathrm{Cl}}_2^{•-}$, and ${\mathrm{ClOH}}^{•-}$) (Equations (4)–(6)) [50]. Figure 6b shows that ${\mathrm{NO}}_3^{-}$ had a small effect on the system, and the TC degradation rate was reduced to 86.5% when the ${\mathrm{NO}}_3^{-}$ concentration of 1 mM was increased to 100 mM. Figure 6c shows that ${\mathrm{SO}}_4^{2-}$ had no effect on TC degradation. The effects of different ions on the Co-BC+PMS system are different. Overall, the Co-BC+PMS system has some resistance to ionic interference.

$${\mathrm{Cl}}^{-}+\mathrm{S}{\mathrm{O}}_4^{•-}\to {\mathrm{Cl}}^{•}+{\mathrm{SO}}_4^2$$

(4)

$${\mathrm{Cl}}^{-}+{\mathrm{Cl}}^{•} \to {\mathrm{Cl}}_2^{•-}$$

(5)

$${\mathrm{Cl}}^{-}{+}^{ •}\mathrm{OH} \to {\mathrm{ClOH}}^{•-}$$

(6)

Furthermore, we examined the degradation of TC in various aqueous matrices. As illustrated in Figure 7, compared with deionized water, tap water and river water slightly decreased the TC degradation rate to 87.6% and 86.1%, respectively, which might have been a result of the different types and contents of ions and natural organic matter in the different types of water with different levels of purity, but the overall difference was not significant. In conclusion, the Co-BC+PMS system demonstrates resilience to variations in water quality, indicating its potential for widespread application across diverse aquatic environments.

The reusability of the synthesized Co-BC catalyst was assessed through cyclic degradation experiments. A series of four consecutive tetracycline (TC) degradation trials were conducted under identical reaction conditions, as illustrated in Figure 8a. The findings indicated a decline in the TC degradation rate from 96.5% to 67.2% over the course of the four cycles. This reduction can be attributed to the oxidation of the active sites on the surface of Co-BC, as well as the accumulation of TC degradation intermediates, which may have impeded the efficiency of TC degradation [51]. The stability of the catalyst was further examined through X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) analyses of Co-BC before and after the reaction, as depicted in Figure 8b,c. The results revealed no significant alterations in the XRD peaks or the functional groups of the catalyst, indicating that the Co-BC catalyst maintained its stability throughout the experiments.

2.3. Free Radical Identification

In order to compare the contributions of various free radicals to TC degradation, we employed methanol (MeOH) and tert-butanol (TBA) as free-radical quenchers. MeOH and TBA are commonly used to quench $\mathrm{S}{\mathrm{O}}_4^{•-}$ and ^•OH, respectively [52]. As shown in Figure 9, the degradation rate of TC without adding any quencher was up to 96.2% at 30 min after casting PMS. The final degradation rate of TC was 87.4% after 500 mM of TBA was added to the system, and this rate decreased to 61.9% after 500 mM of MeOH was added to the system. This finding suggests that both ^•OH and $\mathrm{S}{\mathrm{O}}_4^{•-}$ were involved in the degradation of TC by the Co-BC+PMS system, with $\mathrm{S}{\mathrm{O}}_4^{•-}$ being the major reactive oxygen contributor. Our findings align with the research conducted by Luo et al. [14,15], who examined the degradation of hygromycin hydrochloride and ciprofloxacin by a cobalt-based MOF catalyst (ZIF-67) activated by PMS. The results revealed that ZIF-67 has significant activation potential for PMS, with $\mathrm{S}{\mathrm{O}}_4^{•-}$ being the main active species.

3. Materials and Methods

3.1. Materials

Balsa wood powder (100 mesh) was sourced from China Changsha Jirui Chemical Glass Instrument and Equipment Co. (Changsha, China). Tetracycline hydrochloride (TC, C₂₂H₂₄N₂O₈·HCl), potassium persulfate (PMS, KHSO₅·0.5KHSO₄·0.5K₂SO₄), and cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O) were obtained from McLean Biochemistry Co. Ltd. (Shanghai, China). Methanol (MeOH, CH₃OH), concentrated sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), sodium nitrate (NaNO₃), anhydrous sodium sulfate (Na₂SO₄), sodium chloride (NaCl), and tert-butanol (TBA, C₄H₁₀O) were supplied by Sinopharm Chemical Reagent Co. (Shanghai, China). All chemicals were of analytical grade and used without further purification. Deionized water (18.24 MΩ·cm) was utilized for all experiments. River water samples were collected from the Jinjiang River in Changsha, Hunan Province, China, and tap water samples were taken from the municipal water supply in Changsha, Hunan Province, China.

3.2. Synthesis of Catalysts

The 100-mesh balsa wood powder was cleaned with deionized water to remove surface impurities and then dried at 80 °C. Subsequently, 2 g of the treated powder was dispersed in 200 mL of deionized water. Then, 1.75 g of Co(NO₃)₂·6H₂O was added to the solution, which was then stirred for 12 h. The resultant material was then separated, washed with deionized water, and dried overnight at 60 °C. The material was subsequently heated to 800 °C in a tube furnace under a nitrogen atmosphere at a rate of 5 °C min⁻¹ and held at this temperature for 2 h to yield Co-BC. For comparison, pristine biochar (BC) was synthesized by directly calcining balsa wood powder using a similar method.

3.3. Experimental Procedure

A TC stock solution was prepared using deionized water. All catalytic experiments were conducted in a 200 mL beaker containing 100 mL of TC solution (20 mg L⁻¹) at 25 °C with constant stirring. Typically, a measured amount of catalyst was added to the TC solution and stirred for 20 min to establish an adsorption equilibrium. Subsequently, a predetermined amount of PMS was introduced to initiate the degradation process. The initial pH of the TC solution was adjusted using 0.1 mol L⁻¹ H₂SO₄ or NaOH solutions. At designated time intervals, samples were collected, and the absorbance at 355 nm was measured using a UV–visible spectrophotometer. The TC concentration in the degradation system was determined using a pre-established standard curve. The degradation efficiency of TC was calculated using Equation (7):

R = (1 − C/C₀) × 100%

(7)

where R represents the degradation efficiency of TC, C₀ is the initial concentration of TC, and C is the final concentration (mg L⁻¹).

3.4. Material Characterization

The samples’ morphologies were examined using scanning electron microscopy (SEM, Sigma 300, ZEISS, Oberkochen, Germany), and elemental distribution was analyzed via EDS. X-ray diffractograms were acquired with an X-ray diffractometer (XRD, MiniFlex600, Rigaku, Tokyo, Japan), operating at a scan rate of 5° min⁻¹ over a 5°~90° range with 40 mA and 40 kV settings. Fourier-transform infrared spectra were recorded using an infrared spectrometer (FT-IR, NICOLET iS20, Thermo Scientific, Waltham, MA, USA) across a 4000–400 cm⁻¹ range with 32 scans. X-ray photoelectron spectroscopy (XPS, PHI VersaProbe 4, ULVAC-PHI, Kanagawa, Japan) was employed to analyze the chemical states of the samples’ main elements.

4. Conclusions

In this study, cobalt-doped biochar catalysts were successfully prepared via a simple impregnation method and high-temperature calcination using balsa wood powder as biomass. These catalysts were employed to activate PMS for TC degradation. The successful preparation of the catalyst was proved through various characterization techniques, such as SEM, XRD, FTIR, and XPS. Using 100.0 mg L⁻¹ of Co-BC and 1.0 mM of PMS, an initial concentration of 20 mg L⁻¹ of TC was effectively removed in 30 min, with a degradation rate constant of 0.1017 min⁻¹. In addition, the Co-BC+PMS system exhibited a high TC degradation efficiency over a wide pH range (4.34–9.02). Different ions had different effects on the Co-BC+PMS system. Overall, the Co-BC+PMS system has some resistance to ionic interference. Only when 100 mM Cl⁻ was added to the system did the TC degradation rate decrease to 77.1%. At the same time, the effects of water from different water bodies on the TC degradation rate were weak. The free-radical quenching experiments revealed that $\mathrm{S}{\mathrm{O}}_4^{•-}$ was the main reactive oxygen species in the TC degradation process. Therefore, the Co-BC+PMS system shows great potential for practical application in wastewater treatment and enables the efficient removal of antibiotics from polluted water sources.