In situ synthesis of Co-MOF@BC hybrid catalyst for enhanced BPA degradation via sulfite activation

Zhanmei Zhang; Yunxuan Huang; Xilin Chen; Huaili Zheng; Xinyue Li

doi:10.4491/eer.2024.572

Environ Eng Res > Volume 30(5); 2025 > Article

Zhang, Huang, Chen, Zheng, and Li: In situ synthesis of Co-MOF@BC hybrid catalyst for enhanced BPA degradation via sulfite activation

Research

Environmental Engineering Research 2025; 30(5): 240572.

Published online: February 7, 2025

DOI: https://doi.org/10.4491/eer.2024.572

In situ synthesis of Co-MOF@BC hybrid catalyst for enhanced BPA degradation via sulfite activation

Zhanmei Zhang^1,^2,^†

, Yunxuan Huang¹, Xilin Chen¹, Huaili Zheng², Xinyue Li¹

¹College of River and Ocean Engineering, Chongqing Jiaotong University, Chongqing, 400074, China

²College of Environment and Ecology, Chongqing University, Chongqing, 400044, China

^†Corresponding author: E-mail: zhanmei2003@126.com, Tel: +86-18680837981. ORCID: 0000-0001-7259-8011

Received September 6, 2024 Revised February 5, 2025 Accepted February 6, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A novel synthesis approach was employed to prepare the Co-MOF@BC composite catalyst, utilizing CoCl₂·6H₂O, 2,5-dihydroxyterephthalic acid (DHTA), and N,N-dimethylformamide (DMF) for the in situ growth of metal-organic frameworks (MOFs) on a two-dimensional biochar (BC) substrate. The physicochemical properties of Co-MOF@BC were characterized using SEM, BET, XRD, FT-IR, and XPS. A heterogeneous catalytic system, Co-MOF@BC/PMS, was established for bisphenol A (BPA) removal. The optimal BPA removal efficiency reached 93.66% under the following conditions: BPA concentration of 10 mg•L⁻¹, Co-MOF@BC dosage of 100 mg•L⁻¹, PMS dosage of 0.20 mM, initial pH of 7.0, and reaction temperature of 20°C. The proposed degradation mechanism involves PMS-catalyzed radical pathways (SO^•−₄, •OH and O^•−₂) and non-radical pathways (¹O₂ oxidation and direct electron transfer), as supported by material characterization and degradation efficiency assessment. This study provides insights into MOFs modification and lays the foundation for developing efficient heterogeneous catalysts in sequential redox-acidic oxidation processes (SR-AOPs)

Keywords: Biochar, Bisphenol A, Metal-organic Framework, Persulfate

Graphical Abstract

Keywords: Biochar, Bisphenol A, Metal-organic Framework, Persulfate

1 Introduction

Nowadays, the pollution problem caused by endocrine disruptors (EDCs) is becoming increasingly severe. Concerns have been raised regarding the impact of EDCs present in the environment on human health and ecological systems. As a typical endocrine disruptor, bisphenol A (BPA) is widely used in the production of epoxy resin, polycarbonate and other materials. It has been observed that during the production and application of products containing BPA, this compound is directly or indirectly released into the natural environment and surface water systems. Consequently, BPA can often be detected in sediments, groundwater, and drinking water sources [1], posing significant risks to the reproductive health of humans and animals [2]. However, conventional water treatment methodologies are constrained in practical utility owing to their elevated costs or diminished treatment efficacy [3–4]. Advanced oxidation processes utilizing peroxymonosulfate (PMS) have garnered significant interest in the scientific community due to their robust oxidative capabilities and broad pH applicability, among which heterogeneous catalysis is an economical and environmentally friendly method, which can activate PMS under mild conditions [5–7].

Metal-organic frameworks (MOFs), a novel category of porous materials, are assembled from metal ions and organic ligands via ligand bonding. Owing to their attributes, including uniform and abundant catalytic sites, high specific surface area, and favorable mass transfer capacity [9], MOFs have attracted extensive attention in various fields. In recent years, the preparation of persulfate-activating catalysts by combining MOFs materials with transition metals has become a research focus. Among numerous transition metals, Co has emerged as an ideal choice for this research direction due to its excellent activation ability. The asymmetric structural unit and linkage pattern of Co-MOF can be observed from Fig. 1 (a) [10]. The asymmetric Co-MOF structure is composed of Co²⁺, three deprotonated BPDCH₂ ligands, one coordinated water molecule, and one free water molecule. In the Co-MOF structure, each metal ion exhibits a six-coordinate geometry, binding to four O atoms (O(1), O(3), O(4), O(5)) and two chelating N atoms (N(1A) and N(2A)). These four O atoms originate from two different ligands and the coordinated water, respectively. Fig. 1 (b) and Fig. 1 (c) show the Π - Π interaction between Co-MOF molecules and the 1D chain structure, respectively.

However, in homogeneous Co-PMS system, the dosage of cobalt ions in application was extremely high, far more than the emission standard [10–18]. The superfluous cobalt ions may pose great threats to the aquatic environment and human health, restricting its wide application. To address this issue, the fabrication of multiphase composite catalysts can be undertaken. Biochar (BC), a type of carbon–based material, is generated through the pyrolysis of carbon-rich biomass. It features a relatively large specific surface area, favorable electrical conductivity, and certain catalytic properties. BC has been regarded as an ideal support for compositing with MOFs materials [19]. Recent studies have confirmed that MOFs can be included and immobilised within biochar to form affordable MOF composites with strong adsorption and versatile capacity. The incorporation of MOF compounds can elevate the resultant composite’s porosity, surface chemistry and selectivity, while the embedded BC can contribute to increased stability and durability, making the composite a sustainable and eco-friendly material for large-scale applications in separation, bioremediation, catalysis, energy conversion, and storage [20–21]. The research found that cow dung contains a relatively high content of organic matter, such as cellulose, hemicellulose, and lignin. During the pyrolysis process, these organic matters decompose and transform, forming the rich pore structure and large specific surface area of BC [22]. Moreover, cow dung is produced in large quantities in agricultural production. It is an easily accessible raw material that requires almost no additional cost and is an ideal choice for preparing BC.

Consequently, in this study, cow dung biochar was selected to be combined with MOFs for the preparation of the composite material Co-MOF@BC. Single-factor experiments were conducted to investigate the effects of various parameters, such as catalyst dosage, PMS dosage, initial pH, and reaction temperature, on the removal rate of BPA in the Co-MOF@BC/PMS degradation system, and the interactions between these factors were evaluated. Moreover, the reaction mechanism of Co-MOF@BC catalyzing PMS to degrade BPA was further elucidated through quenching experiments, electron spin resonance (ESR) analysis, and X-ray photoelectron spectroscopy (XPS) characterization. This research aims to provide technical insights for the effective treatment of wastewater contaminated with BPA and other EDCs pollutants.

2 Materials and Methods

2.1. Materials

Peroxide sulfate was purchased from Shaoguan Su Noi Chemical Reagent Factory(Shaoguan, China), BPA, CoCl_2·6H₂O, 2,5-dihydroxyterephthalic acid (DHTA) and N,N-Dimethylformamide (DMF) were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Deionized water was used throughout the experiment.

2.2. Preparation

The catalyst Co-MOF@BC was synthesized by in-situ growth of Co-MOF on the surface of BC using a solvothermal method. Initially, cow dung was dried and pulverized, then placed in a carbonization furnace, where it was heated to 400°C at a rate of 10°C/min and maintained at this temperature for 2 h for carbonization. After naturally cooling to room temperature, it was ground and sieved through a 60-mesh sieve to obtain the BC required for the experiment.

To synthesize Co-MOF@BC, 0.714 g of CoCl2·6H2O and 0.594 g of DHTA were dissolved in 30 mL of DMF and stirred magnetically for 30 min. BC was then added, and stirring was continued for another 30 min. The mixture was transferred to a 100 mL polytetra-fluoroethylene-lined reactor, sealed, and heated at 120°C for 20 h. The product was washed with DMF, ethanol, and water, then vacuum-dried at 70°C. The Co-MOF material without BC was synthesized using a similar method. The experimental method is shown in the supplementary material, Fig. S1.

2.3. Characterization

The micro-morphology of BC, Co-MOF and Co-MOF@BC was characterized and analyzed by scanning electron microscope (Model: Zeiss Sigma 300 scanner). The crystallinity and crystal phase structure of the material were characterized by XRD analyzer (Model: Panaco X'Pert PRO). The functional group types and corresponding chemical structures were characterized by Fourier infrared spectrometer (Model: Nicolet 670). The parameters such as specific surface area, pore size and pore distribution of catalyst were detected by automatic specific surface area tester (Model: ASAP-2460). X-ray photoelectron spectroscopy (Model: Thermo Scientific K-Alpha X) was used to analyze the elemental composition and chemical valence state of the catalyst.

2.4. Experimental Procedures

The experiment investigated the ability of the Co-MOF@BC/PMS system to degrade BPA. In the experiment, 10 mg of Co-MOF@BC and 0.2 mM PMS were weighed and added to 100 mL of 10 mg•L⁻¹ BPA solution. The mixture was placed in a thermostatic oscillation chamber, with the speed of the magnetic stirrer set to 250 rpm. During stirring, samples of the solution were taken at specific time intervals. The obtained solution was filtered through a 0.22 μm filter tip and then analyzed for BPA concentration using high-performance liquid chromatography (HPLC: LC-2010, Shimadzu, Japan). After each experiment, the remaining catalyst was collected, washed three times with deionized water and ethanol, and then dried at 70°C for 8 h. While keeping the reaction conditions consistent, the experiment was repeated five times. The reusability of the material was determined based on its BPA degradation rate.

3 Result and Discussion

3.1. Characterization

3.1.1. SEM

The micromorphology of the obtained samples was observed by SEM (Fig. 2). The particles on the surface of Co-MOF@BC are observed to be uniformly dispersed on the surface of BC. These particles display a rod-like hexagonal prism structure and exhibit a morphology that is similar to that of Co-MOF. The synthesis of Co-MOF@BC materials and the doping of BC will not change the structure of Co-MOF. Notably, the uniform dispersion of Co-MOF particles may be attributed to the interaction between the biochar structure and Co-MOF particles, which leads to the weakening of Co-MOF magnetism and alleviates its self-agglomeration, making Co-MOF@BC provide more active sites.

3.1.2. BET

Nitrogen adsorption-desorption tests (Fig. 3 and Table 1) revealed that the adsorption curve of Co-MOF@BC exhibited a distinct Type IV isotherm characteristic. Moreover, an H3-type hysteresis loop appeared at a relative pressure P/P₀ > 0.4, indicating the presence of a large number of mesoporous structures. These mesoporous structures facilitate the mass transfer between oxidants and substrates [23]. Additionally, they provide more adsorption sites for gas molecules, thus significantly enhancing the specific surface area, which is far greater than that of the individual biochar and Co-MOF.

3.1.3. XRD

The crystallinity information of the sample material was obtained by X-ray diffractometer (Fig. 4). The two prominent peaks located at 23.1° and 43.1° correspond to the characteristic peaks of amorphous carbon (002) and graphite carbon (100) [24], respectively. The most distinct peak observed in the synthesized Co-MOF@BC material appeared at 2θ = 11.75°, which is related with the (300) crystal plane of the Co-MOF material. When studying the crystal structure of Co-MOF after incorporating BC, it was found that the characteristic diffraction peaks of the Co-MOF@BC composite material did not change significantly compared to the individual Co-MOF. This observation indicates that the incorporation of BC as a carrier did not affect the crystalline structure of Co-MOF, thus confirming the findings from SEM.

3.1.4. FT-IR

FT-IR spectroscopy was used to analyze the molecular structure and functional group characteristics of the sample materials (Fig. 5). Within the low-frequency region, the prominent absorption peak observed near 583 cm⁻¹ is likely the vibration mode of Co–O [25–26]. The characteristic peaks at 815 cm⁻¹ and 881 cm⁻¹ correspond to the stretching [27] and bending vibrations [28] of the C–H bond associated with the benzene ring in DHTA. Additionally, the peaks at 1192 cm⁻¹ and 1242 cm⁻¹ are due to the C–O bonds in DHTA, indicating the successful coordination of organic ligands in the Co-MOF@BC structure [29]. The distinct sharp peaks at 1552 cm⁻¹ and 1410 cm⁻¹ are attributed to the symmetric and asymmetric stretching of the central −COO group in DHTA.

3.1.5. XPS

The elemental composition and valence distribution of the sample materials were obtained by XPS spectroscopy (Fig. 6). The entire XPS spectra of the composites (Co-MOF@BC) revealed the presence of Co, C, and O. In the Co-MOF@BC sample, the Co 2p3/2 and Co 2p1/2 peaks are observed at binding energies of 781.3 eV and 797.3 eV, respectively, indicating the presence of both Co(II) and Co(III) simultaneously in the main peaks of Co 2p3/2 and Co 2p1/2, along with associated satellite peaks [30]. Further investigation revealed that the binding energy peaks at 781.2 eV and 797.2 eV correspond to Co(II), while the peak at 782.6 eV was related to Co(III) [31]. In the O 1s spectrum, the two peaks at 531.6 eV and 532.7 eV were attributed to the Co–OH bond and O=C–O group of the organic ligand, respectively. In the C 1s spectrum, the four peaks of 284.8 eV, 284.4 eV, 288.4 eV and 290.9 eV correspond to C=C/C–C, C–O, C=O and π-π*, respectively.

3.2. Catalytic Performance and Stability of Catalysts

3.2.1. The influence of biochar dosage on the catalyst

Investigations were carried out on the heterogeneous Co-MOF@BC/PMS systems with different biochar contents. The kinetic fitting of the concentration of BPA at different reaction time points in the Co-MOF@BC/PMS system was performed using the quasi-first-order kinetic model, and the results are shown in the supplementary material, Fig. S2. With the increase in biochar content, the degradation rate of BPA by Co-MOF@BC accelerates. Specifically, the degradation rate of BPA by Co-MOF@BC(5%) reaches up to 6.37×10⁻² min⁻¹, and the removal rate reaches 93.48%. This can be attributed to two factors. The hierarchical pore structure of biochar provides a large specific surface area and a large number of active sites, which can adsorb and activate PMS more effectively, thereby improving the degradation rate of BPA. In addition, the introduction of Co-MOF can enhance the electron transfer efficiency between biochar and PMS, further promoting the degradation of BPA. However, when the BC doping amount increases to 10%, the degradation rate, the BPA removal rate, and the reaction rate constant of the Co-MOF@BC/PMS system all decrease correspondingly with the increase in BC content. This is because the excessive BC content may lead to excessive accumulation in the composite material, blocking the active sites in Co-MOF or hindering the contact between the active sites, BPA molecules, and PMS, thereby reducing the catalytic activity. Therefore, Co-MOF@BC (5%) material is selected for subsequent experiments in this study.

3.2.2. BPA degradation in Co-MOF@BC/PMS system

Comparative experiments were conducted to verify the removal performance of the Co-MOF@BC/PMS system for BPA in water, in order to assess the removal efficiency of BPA by Co-MOF@BC (Fig. S3). The study found that PMS alone was insufficient to effectively degrade BPA. Additionally, in systems without PMS, BC, Co-MOF, and Co-MOF@BC could partially degrade BPA due to their inherent adsorptive capabilities. After 50 minutes of reaction, the Co-MOF@BC/PMS system achieved a removal rate of 93.48% for BPA, which was higher than that of the Co-MOF/PMS and BC/PMS systems.

3.2.2. Stability and repeatability of Co-MOF@BC

To understand the stability of Co-MOF@BC materials before and after PMS catalysis, FT-IR and XRD were used to characterize the surface functional groups and crystal structure of Co-MOF@BC (Fig. S4). It was found that the characteristic peaks of O–C=O and C=C did not change significantly, while the intensity of Co–O characteristic peak decreased slightly, indicating that the structure of Co-MOF@BC remained stable during the reaction process, with only minor degradation of Co–O bonds. After the catalytic degradation experiment, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to analyze the Co concentration in the solution. The measured Co concentration was 66.09 μ g•L⁻¹, and the calculated loss rate of Co was 0.02%. The minimal dissolution of Co indicates that the Co-MOF@BC composite material has high stability.

Referring to the reusability experiments for evaluating catalyst materials in most of the literature, in this study, four cycle tests were performed on the Co-MOF@BC material (Fig. S5), and the removal rates of BPA in each cycle were recorded as 94.37%, 90.11%, 86.86%, and 81.82%, respectively. Although the removal rate shows a downward trend, this decline is relatively gentle, and it can still maintain a relatively high level above 80% after multiple cycles of use. By calculating the rate of change of the removal rate between adjacent cycles to evaluate the stability more accurately, it is found that these rates of change are relatively small, indicating that the attenuation of the catalyst performance is relatively uniform and slow during different cycling processes, without a sudden significant drop in performance, thereby demonstrating the stability of the performance.

3.3. Influence of Different Parameters

To determine the optimal process parameters for the degradation of BPA using Co-MOF@BC catalyzed persulfate, the effects of catalyst dosage (50 mg•L⁻¹, 75 mg•L⁻¹, 100 mg•L⁻¹, 125 mg•L⁻¹), persulfate dosage (0.10 mM, 0.15 mM, 0.20 mM, 0.25 mM), pH(3.0, 5.0, 7.0, 9.0, 11.0), and temperature(10°C, 20°C, 30°C, 40°C) on BPA removal was investigated using a single-factor experiment approach. In addition, the effect of inorganic ions on the removal of BPA was also investigated, which provided a theoretical basis for the application in actual water.

3.3.1. Catalyst dosage

The degradation efficiency is significantly enhanced as the concentration of Co-MOF@BC rises from 50 mg•L⁻¹ to 100 mg•L⁻¹, For specifics, please refer to the supplementary material, Fig. S6. This can be attributed to the fact that with the increase in catalyst dosage, the active sites, such as the metal ion Co and its surrounding coordination environment, are capable of interacting with PMS, thereby prompting its decomposition and generating highly active radicals. Additionally, in the reaction system, BPA molecules and PMS need to come into contact with the active sites of the catalyst for the reaction to occur. When the catalyst concentration is low, the opportunities for them to interact with the active sites are relatively limited. However, as the catalyst concentration increases, the collision probability between the reactant molecules and the active sites rises, enabling more BPA molecules to come into contact with the active sites on the catalyst surface and subsequently react with the generated active radicals, thus accelerating the degradation process of BPA and improving the degradation efficiency.

Nevertheless, when the concentration of the Co-MOF@BC catalyst is further increased, several factors come into play. On the one hand, according to the stoichiometry of the reaction, even though the number of active sites increases, the amount of PMS available for activation becomes relatively insufficient. During the reaction process, PMS is consumed rapidly, and once it exceeds a certain level, the limited amount of PMS cannot fulfill the demand of more active sites, thereby restricting the subsequent radical generation process. On the other hand, when the catalyst dosage is further augmented, excessive radicals will be generated, and these radicals will react with each other, resulting in self-quenching phenomena. Specifically, the highly active radicals originally intended for the degradation of BPA will combine with each other and transform into relatively stable products, thereby reducing the number of effective radicals capable of reacting with BPA molecules and leading to a decrease in the degradation rate of BPA.

3.3.2. PMS concentration

Fig. S7 in the supplementary material demonstrates that within the range of PMS dosage from 0.1 mM to 0.2 mM, the removal rate of BPA in the Co-MOF@BC/PMS heterogeneous catalytic system is positively correlated with the PMS dosage, with a significant impact. However, when the PMS dosage is increased to 0.25 mM, there is a slight increase in the removal rate of BPA, but the rate of increase in degradation efficiency significantly decreases. This trend may be attributed to the fact that when the PMS dosage is increased to 0.25 mM, a large number of reactive species (SO^•−₄ and •OH) will undergo the aforementioned self-quenching reactions, resulting in a decrease in the number of effective reactive species and thus reducing the degradation efficiency of BPA. Additionally, the active sites of the catalyst are limited. As the active sites have already been fully utilized, the excess PMS cannot interact effectively with the catalyst, which causes the generation of reactive species not to increase proportionally. Instead, due to the influence of self-quenching and side reactions, the degradation rate of BPA decreases slightly.

3.3.3. pH

Fig. S8 in the supplementary material shows that under strong acidic conditions (pH=3.0), the degradation efficiency of BPA in the reaction system over 50 min is only 66.74%. The slower degradation rate may be due to the fact that under strong acidic conditions, SO^•−₄, •OH and PMS react with the excess H⁺ in the system, as shown in Eq. (1)–(3), resulting in a reduction in effective free radicals and a decrease in PMS content, thereby leading to the decrease in degradation efficiency. When the pH value is in the range of 5.0–7.0, the degradation efficiency increased with the increase of pH. Under the condition of pH=7.0, the degradation rate is more than 2 times that at pH=3.0, mainly because under neutral conditions, the above-mentioned weakening reactions are reduced, greatly improving the degradation rate of BPA. When the pH value rises to 11.0, the degradation effect of BPA decreased to 49.73%, because excessive OH⁻ would react with SO^•−₄ to generate •OH, as shown in Eq. (4)

(1)

S O_{4}^{\cdot -} + H^{+} + e^{-} \to H S O_{4}^{\cdot -}

(2)

\cdot O H + H^{+} + e^{-} \to H_{2} O

(3)

H S O_{5}^{-} + H^{+} \to S O_{4}^{2 -} + H_{2} O

(4)

S O_{4}^{\cdot -} + O H^{-} \to S O_{4}^{2 -} + \cdot O H

3.3.4. Temperature

As shown in the supplementary material, Fig. S9 although the reaction temperature increases from 10°C to 40°C, it has little impact on the final removal effect of BPA, but the initial rate of the reaction is significantly accelerated. This is because when the temperature rises from 10°C to 40°C, the electrons on the surface of the catalyst acquire more energy, and the rate of electron transfer reaction accelerates, making it easier to overcome the energy barrier required for electron transfer, so that electrons can be transferred from the active sites of the catalyst to PMS more easily. Moreover, the decomposition of PMS to generate free radicals is an important step in the degradation of BPA, and its decomposition reaction is generally an endothermic reaction. Increasing the temperature provides the required thermal energy for this reaction, enabling PMS molecules to decompose faster and accelerating the degradation process of BPA.

3.3.5. Effect of humic acid and inorganic anions

Considering that inorganic anions and NOM prevalent in natural water and wastewater have significant effects on advanced oxidation processes (AOPs) [32]. The effects of several common inorganic anions and typical natural organic matter (HA) on BPA degradation in the Co-MOF@BC/PMS system were studied.

As indicated in the supplementary material, Fig. S10, the degradation efficiency of BPA is negatively correlated with the concentration of HA. As the concentration of NOM represented by HA increases, the removal rate of the target pollutant in water decreases gradually and significantly. When the added concentration is 2 mg·L⁻¹, the removal rate of BPA after a final reaction of 50 min is approximately 90.14%, and the degradation rate constant k is 4.70×10⁻² min⁻¹, and the decrease in the pollutant removal rate is relatively small compared to the case without adding HA. However, when the concentration of HA is increased to 5 mg·L⁻¹ and 10 mg·L⁻¹, the removal rate of the pollutant BPA within 50 min will decrease to 84.53% and 77.72%, respectively. It can thus be seen that NOM in natural water bodies has a certain influence on the pollutant removal effect of the Co-MOF@BC/PMS system. NOM contains abundant functional groups such as carboxyl (−COOH), hydroxyl (−OH), and phenolic (−R−OH) groups. On the one hand, −COOH and −OH, etc., will react with SO^•−₄, •OH, and ¹O₂, resulting in the quenching of free radicals, thereby consuming more free radicals and further reducing the source of effective free radicals used for pollutant degradation. On the other hand, functional groups such as carboxyl (−COOH) and phenolic (−R−OH) are prone to interact with metal ions in the reaction system and adsorb on the surface of the catalyst in the form of binding with the heterogeneous catalyst, thereby further hindering the contact between active sites and PMS and affecting the efficiency of the material in activating PMS to degrade pollutants.

As shown in the supplementary material, Fig. S11, Cl⁻, HCO⁻₃ and NO⁻₃ have different degrees of adverse effects on the performance of the catalyst. Cl− has a dual effect on the degradation of BPA in the Co-MOF@BC/PMS system, which is manifested by inhibiting the reaction at low concentrations and promoting the reaction at high concentrations. When the Cl⁻ concentration increases from 0 mM to 2 mM, the removal rate of BPA after a final reaction of 50 min decreases from 93.32% to 76.24%. In the persulfate catalytic oxidation system, Cl⁻ can react with SO^•−₄ and •OH, generating radicals with lower oxidation potentials and lower reactivity, namely Cl^•, HOCl^•−, and Cl^•−₂, as shown in Eq. (5)–(9), with reaction rate constants of 3.2×108 M⁻¹s⁻¹ and 4.3×109 M⁻¹s⁻¹, indicating that there is a competitive relationship between Cl⁻ and BPA. When the Cl⁻ concentration is further increased to 5 mM, Cl⁻ has a slight promoting effect on the degradation of BPA. There is a competitive relationship between Cl⁻ and organic substances, which consumes part of SO^•−₄ and •OH, but the generated oxidized active chlorine species such as Cl^•, HOCl^•−, and Cl^•−₂ can also react with the target pollutant. The oxidation/reduction potentials of Cl^•, HOCl^•−, and Cl^•−₂ are 2.4 eV, 2.0 eV, and 1.5 – 1.8 eV, all of which are lower than those of SO^•−₄ ((SO^•−₄/SO²⁻₄/SO²⁻₄)=2.5–3.1Ev) and can only partially offset the inhibitory effect.

(5)

S O_{4}^{\cdot -} + C l^{-} \to S O_{4}^{2 -} + C l^{\cdot}

(6)

{}^{\cdot}O H + C l^{-} \to H O C l^{\cdot -}

(7)

C l^{-} + C l^{\cdot} \to C l_{2}^{\cdot -}

(8)

2 C l^{\cdot} \to C l_{2} + 2 C l^{-}

(9)

C l^{\cdot} + H_{2} O \to H O C l^{\cdot -} + H^{+}

With the increase in the concentration of HCO⁻₃, the degradation rate of BPA slightly decreases. When the concentration of HCO⁻₃ increases to 10 mM, the removal rate of the pollutant BPA within 50 min drops to 87.36%. This is mainly because HCO⁻₃ can act as a scavenger to react with SO^•−₄ and •OH to generate HCO^•₃ and CO^•−₃, as shown in Eq. (10)–(11). The redox potential of CO^•−₃, E0(CO^•−₃) = 1.78 eV, is lower than that of SO^•−₄ ((E(SO^•−₄/SO²⁻₄) = 2.5–3.1eV), and thus the degradation efficiency of BPA will be inhibited. Meanwhile, some studies have shown that HCO⁻₃ may form a complex on the material surface, which will also hinder the reaction between the active sites of the material and PMS [33]. However, it can be known from this study that the change in inorganic carbon concentration in natural water has little effect on the pollutant degradation effect of the Co-MOF@BC/PMS system.

(10)

S O_{4}^{\cdot -} + H C O_{3}^{-} \to S O_{4}^{2 -} + H C O_{3}^{\cdot}

(11)

\cdot O H + H C O_{3}^{-} \to H_{2} O + C O_{3}^{\cdot -}

The addition of NO⁻₃ leads to a decrease in the removal effect of BPA, but not significantly. This is because NO⁻₃ can react with the radicals SO^•−₄ and •OH generated in the Co-MOF@BC/PMS heterogeneous catalytic system, and the oxidizing property of the generated NO^•₃ is not as strong as that of SO^•−₄ and •OH, thus only having a slight influence on the removal of BPA. The specific reaction is shown in Eq. (12)–(13).

(12)

S O_{4}^{\cdot -} + N O_{3}^{-} \to S O_{4}^{2 -} + N O_{3}^{\cdot}

(13)

\cdot O H + N O_{3}^{-} \to O H^{-} + N O_{3}^{\cdot}

3.4. Identification of Catalytic Substances

3.4.1. EPR

EPR spectroscopy was used to assess the presence of •OH, SO^•−₄, ¹O₂, and O^•−₂ in the reaction. The spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to capture •OH, SO^•−₄, and O^•−₂, while 2,2,6,6-tetramethylpiperidine (TEMP) was used to detect singlet oxygen.

From the supplementary material, Fig. S12(a), it is evident that a series of characteristic peaks belonging to DMPO-OH and DMPO-SO^•−₄, i.e., the typical seven-line (1:2:1:2:1:2:1:2:1) spectra corresponding to the oxidation product of DMPO, DMPO-X (5,5-dimethylpyrrolidinium-(2)-oxo-(1)), appeared in the spectra 5 and 10 min after the addition of DMPO [33–34], which indicates that Co-MOF@BC can indeed catalyze PMS to generate SO^•−₄ and •OH. Fig. S12(b) displays the EPR spectrum with an equal intensity triplet signal characteristic of ¹O₂ oxidizing TEMP, with a peak ratio of 1:1:1. The appearance of TMPE-¹O₂ signal confirms the involvement of ¹O₂ in the trapping experiment. From Fig. S12(c), the special spectrum of the quadruple signal with the intensity ratio of 1:1:1 is observed, which represents the characteristic peak of DMPO-O^•−₂ adduct, indicating that O^•−₂ is generated in the Co-MOF@BC/PMS system.

3.4.2. Free radical quenching

p-Benzoquinone (p-BQ) was used to quench superoxide radicals (O^•−₂), and furfural (FA) was employed as the ¹O₂ scavenger. The experiment was conducted under standardized conditions of 10 mg·L⁻¹ BPA, 0.22 mM PMS, and 122 mg·L⁻¹ Co-MOF@BC. The reaction temperature was maintained at 20°C, and the molar ratio of quencher to oxidant was set at 500:1 to facilitate radical scavenging.

The supplementary material, Fig. S13 illustrates that the addition of MeOH significantly inhibits the removal of BP A in the degradation system, with a degradation rate of only 30.37% at 50 min compared to the blank control. This decline indicates that methanol has a substantial adverse effect on the degradation of BPA in the Co-MOF@BC/PMS system. After adding TBA, the removal rate at 50 min was 86.78%, which is 6.54% lower than the control group. Under the same conditions, the inhibitory effect of MeOH on free radicals exceeds that of TBA, suggesting that SO^•−₄ and •OH radicals are produced during the process of Co-MOF@BC catalyzing PMS. When FA and p-BQ were added to the system, the degradation rates of BPA were 82.17% and 43.95% respectively after 50 min, which are 11.15% and 49.37% lower than the control group, indicating the generation of O^•−₂ and ¹O₂ radicals. Additionally, while O^•−₂ and •OH are also produced, the effects are not significant. Therefore, the predominant free radicals in the Co-MOF@BC/PMS degradation system are SO^•−₄, •OH, ¹O₂, and O^•−₂, with SO^•−₄ and ¹O₂ being the most predominant radicals.

3.4.3. Valence transition of Co

To investigate the interactions between elements, XPS analysis was conducted before and after the reaction (Fig. S14). Fig. S14 reveals a shift of the Co 2p energy spectrum towards higher binding energy after the reaction, indicating electron loss and transfer in the Co-containing catalyst. Co(II) serves acts as an electron donor and reacts with PMS to generate SO^•−₄, while being oxidized to Co(III). The characteristic O1s peaks of Co-MOF@BC appear at 531.7 eV and 532.7 eV, corresponding to Co–OH and O–C=O [35], respectively. After the reaction, the amount of O–C=O decreases, while the amount of Co–OH increases, indicating that hydroxylated reactions occur in Co-MOF@BC during the reaction process, further verifying effectiveness of the Co-MOF@BC material [36]. After the reaction, the content of C=C decreases, while the contents of C–O and C=O increase. The presence of C=O can promote the formation of non-radical ¹O₂, while the increased electron density of C=C/C–C can facilitate electron transfer during the catalytic process, thereby promoting the generation of catalytic species and the degradation of BPA [37].

3.5. Mechanism

Under the catalysis of Co-MOF@BC, PMS degrades BPA through a combined pathway of radicals and non-radicals (Fig. 7). In the radical pathway, Co(II) donates electrons to PMS, forming SO^•−₄ and oxidizing Co(II) to Co(III). Subsequently, SO^•−₄ can react with water and OH⁻ to generate •OH. The oxygen-containing functional groups on the surface of BC facilitate the formation of SO^•−₄ through the catalytic action of PMS. The non-radical pathway includes the generation of ¹O₂ from O^•−₂ and OH⁻ as well as the self-decomposition of PMS. The redox cycle of Co (Co(III)→Co(II)) generates SO^•−₅, which can further react with water to generate ¹O₂. These reactive oxygen species (ROS), primarily SO^•−₄, •OH, ¹O₂, and O^•−₂, contribute to the mineralization of BPA into CO₂ and H₂O.

3.6. Pathways

The degradation pathway of BPA was investigated using ultra-performance liquid chromatography-mass spectrometry (UPLC/MS) to identify intermediate oxidation products. The m/z ratios of the products were confirmed by comparing molecular ion spectra and analyzing fragment ion peaks. By correlating with literature data, the chemical structures of the intermediates were inferred.

Based on the analysis of intermediate degradation products detected by LC-MS and the comparison with literature [38–39], a possible degradation pathway for BPA was proposed (Fig. S15). The SO^•−₄ and •OH produced by Co-MOF@BC-catalysed PMS attacked the C–C bond connecting the benzene ring of BPA to the isopropyl group, and the between the hydroxyl group and the benzene ring to break it [40–41], generating phenol radical intermediates, and consequent phenylmethyl bond breaking between the two benzene rings of BPA to generate 4-isopropylphenol radical and phenol radical [42]. 4-Isopropylphenol radical generates 4-isopropylphenol via dehydrogenation reaction. The phenol radical undergoes hydroxylation of the benzene ring by electrophilic addition of •OH to produce hydroquinone, which is converted to maleic acid by breaking the ring of hydroquinone to form under the attack of SO^•−₄ and •OH. Under the catalysis of ¹O₂, BPA generates monohydroxylated BPA through hydroxylation reaction, while hydroxylated BPA is unstable in nature, and under the attack of reactive substances it is easy to undergo bond breakage and form the products 4-isopropenylphenol and phenol [43]. Under the combined catalytic action of SO^•−₄, •OH and ¹O₂, these substances are further oxidized and transformed into CO₂ and H₂O.

3.7. Assessment of the Biotoxic Potential of BPA and Its Metabolic Byproducts

Long-term exposure to low concentrations of pollutants in aquatic environments also leads to serious ecological risks. Therefore, in addition to focusing on the efficiency of pollutant removal, quantitatively assessing the toxicity of degradation products is crucial for reducing the risk of water pollution and ensuring the health of aquatic ecosystems [44]. In this study, the ECOSAR model was used to evaluate the acute toxicity of BPA and its six degradation products to three representative aquatic organisms (fish, daphnia, and green algae). By determining the half-lethal concentration (LC50) and half-effective concentration (EC50) for different species, the varying levels of toxicity of the intermediate products were analyzed. The results are presented in the supplementary material, Table S2.

According to the Global Harmonized System of Classification and Labelling of Chemicals, acute toxicity is classifed into four classifications, include acute toxicity (LC50/EC50 ≤ 1 mg•L⁻¹), toxic toxicity (1 mg•L⁻¹ < LC50/EC50 ≤ 10 mg•L⁻¹), harmful toxicity (10 mg•L⁻¹ < LC50/EC50 ≤ 100 mg•L⁻¹), and non-toxic (EC50 > 100 mg•L⁻¹) [44]. As shown in the supplementary material, Fig. S16, the degradation of BPA significantly reduces the toxicity of the intermediate products. Moreover, as the degradation process continues, the toxicity of these intermediates gradually decreases until the benzene ring is cleaved, leading to a sharp decline in toxicity and ultimately forming non-toxic and harmless end products. Therefore, the toxicity analysis of the intermediates indicates that in the Co-MOF@BC/PMS system, the toxicity of BPA continuously decreases throughout the degradation process, eventually resulting in non-toxic and harmless substances.

4 Economic Feasibility Analysis

4.1. Raw Material Cost

The synthesis of Co-MOF@BC primarily involves cow dung biochar, CoCl₂·6H₂O, DHTA, and DMF. Cow dung is a widely available and inexpensive biomass source, significantly reducing the cost of the biochar precursor. CoCl₂·6H₂O and DHTA are commercially available chemicals, and their costs can be managed through bulk purchasing. DMF, although a solvent with a certain cost, can be recycled and reused in the synthesis process to some extent, minimizing its overall consumption and cost impact.

4.2. Synthesis Process Cost

The synthesis of Co-MOF@BC employs a solvothermal method, which requires heating equipment and reaction vessels. While there is an initial investment in equipment, the process can be scaled up relatively easily in industrial settings. The reaction conditions (120°C for 20 h) are not overly energy-intensive compared to some high-temperature and high-pressure synthesis processes, resulting in manageable energy costs. Additionally, the synthesis steps are relatively straightforward, reducing labor costs associated with complex manufacturing procedures.

4.3. Catalytic Performance and Longevity

The Co-MOF@BC catalyst demonstrates high catalytic activity in the degradation of BPA. A relatively low dosage of 100 mg•L⁻¹ can achieve a high removal rate of 93.66% under optimal conditions. This efficient performance implies that less catalyst is required to treat a given volume of wastewater, reducing the overall catalyst cost per unit of wastewater treatment. Moreover, the catalyst exhibits good stability and reusability. After four cycles of experiments, the degradation rate of BPA can still reach 81.82%. This reusability further extends the lifespan of the catalyst, lowering the long-term cost of catalyst replacement and making it a cost-effective option for continuous wastewater treatment operations.

5 Conclusion

In this study, novel Co-MOF@BC were successfully synthesized and applied as PMS activators for BPA degradation in aqueous solution. The BC-loaded MOFs heterogeneous material effectively avoids the self-agglomeration of MOFs and greatly enhance the PMS activation and BPA degradation, which can be summarized as follows:(i) The introduction of BC formed a hierarchical pore structure, which effectively avoided the agglomeration of Co-MOF and provided more active sites. (ii) Co-MOF has a large number of unsaturated metal sites, which can react with PMS to generate reactive oxygen radicals and enhance the oxidation ability of PMS. (iii) The unsaturated metal sites of Co-MOF can act as electron acceptors to react with PMS, promote electron transfer, accelerate the redox cycle of Co(II)/Co(III), and improve the activation efficiency of PMS. Benefiting from these above synergistic effects between BC and Co-MOF, the resulting Co-MOF@BC presented excellent catalytic activity for PMS activation, and the degradation rate of BPA reached 93.66%. Furthermore, Co-MOF@BC showed good recycling performance, and the degradation rate of BPA could reach 81.82% after four cycles of experiments. This research offers a novel methodology for the in-situ formation of Co-MOF on the surface of BC, resulting in the preparation of Co-MOF@BC activated PMS for the degradation of endocrine-disrupting pollutants, while also confirming the synergistic impact of Co-MOF and BC. The degradation mechanisms of BPA were systematically explored and a new mechanism of electron transfer was proposed. The acute toxicity of BPA and its six degradation byproducts was assessed utilizing the ECOSAR model. The analysis revealed that the toxicity of the intermediary products progressively diminished throughout the degradation sequence, culminating in the formation of non-toxic and innocuous end products. This demonstrates that the Co-MOF@BC/PMS system is capable of not only effectively decomposing BPA but also mitigating its toxicity to a harmless threshold. Consequently, this approach offers a novel perspective for the development of environmentally benign water treatment technologies.

Supplementary Information

eer-2024-572-supplementary.pdf

Notes

Acknowledgements

This work completed with the support from the Joint Graduate Training Base for Resources and Environment between Chongqing Jiaotong University and Chongqing Gangli Environmental Protection Co., Ltd., Chongqing Jiaotong University, Chongqing 400074 ; Chongqing Postgraduate Joint Training Base Project (JDLHPYJD2022005).

Conflict-of-Interest Statement

The authors declare no conflict of interest.

Author Contributions

Zhanmei Zhang(Associate Professor) conceptualized the research framework, designed the methodology, and critically reviewed and edited the manuscript. Yunxuan Huang(Graguate student) conducted formal analysis, performed experimental investigations, curated data, and wrote the original draft. Xilin Chen(Graguate student) validated experimental protocols and contributed to data curation. Huaili Zheng(Professor) participated in manuscript review and editing. Xinyue Li(Graguate student) assisted in manuscript review and editing.

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