Template-Based Fabrication of Copper Oxide for Persulfate Activation: Investigating Non-Radical Mechanisms in Efficient Bisphenol A Degradation

doi:10.21203/rs.3.rs-4853615/v1

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Template-Based Fabrication of Copper Oxide for Persulfate Activation: Investigating Non-Radical Mechanisms in Efficient Bisphenol A Degradation

https://doi.org/10.21203/rs.3.rs-4853615/v1

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Presently, Advanced Oxidation Processes (AOPs) emerge as highly effective methods for breaking down organic environmental pollutants. The current investigation involves the synthesis of copper oxide (CuO) catalysts utilizing a mesoporous silica (SiO₂) template for activation of peroxymonosulfate (PMS), thereby targeting the elimination of bisphenol A (BPA). Characterized by their ideal pore volume and nanopore dimensions, mesoporous silica nanoparticles, serve as excellent templates for catalyst preparation. The CuO catalysts were meticulously synthesized through blending and calcination, guided by the use of mesoporous silica as a structure-directing template. This method aimed to induce significant changes in the conventional morphological characteristics of CuO. This alteration led to an increase in specific surface area and active sites of CuO, thereby enhancing PMS activation for the breakdown of organic contaminants. The work essentially demonstrates a BPA degradation rate of 89.57% within 5 minutes, incrementing to a remarkable 97.8% at the 60-minute mark, effectively achieving complete degradation. A comprehensive investigation of the CuO/PMS system's reaction mechanism for degradation of BPA involved an electron paramagnetic resonance (EPR) spectroscopy-based analysis along with a series of radical quenching tests. The findings suggest that non-free radicals play a predominant role in the degradation of BPA. Furthermore, the efficiency of BPA degradation remained at 72% following five reuses over 60 minutes, demonstrating consistent and effective degradation. This research emphasizes the pivotal role of non-radicals as the key driving force behind BPA degradation, providing compelling proof for their key role in the breakdown of organic pollutants.

copper oxide

peroxymonosulfate

mesoporous silica

bisphenol A

advanced oxidation process

In recent times, significant scientific and technological advancements have elevated the quality of life but have also aggravated the level of pollution in the environment. Organic contaminants in aquatic environments have significantly affected human health. This surge in pollution, particularly in aquatic environments, has dire consequences for human health due to the presence of organic contaminants[1]. Bisphenol A (BPA), a chemical with extensive utilization in farming and industrial manufacturing for diverse applications like pesticides, adhesives, plasticizers, epoxy surface coatings, and flame retardants, poses substantial risks. Research has identified its teratogenic and embryotoxic effects, and its role in heightening the risk of leukemia, ovarian cancer, and prostate cancer in animals[2]. Being highly stable under standard conditions, BPA has been recognized as a persistently occurring organic pollutant found in water sources. Conventional methods for water treatment and purification face challenges in effectively removing organic contaminants of this kind. As a consequence, many researchers are turning towards advanced oxidation processes (AOPs) since they show promise in addressing these issues. AOPs are gaining recognition for their ability to produce high levels of reactive oxygen species, facilitating the efficient degradation of organic pollutant species[3, 4, 5].

Advanced oxidation processes (AOPs) encompass ozone oxidation, electrochemical oxidation, photocatalytic oxidation, Fenton oxidation, and several coupled technologies, displaying remarkable degradation efficacy, non-selectivity, and accelerated oxidation reaction rates[6, 7]. During the course of oxidation, these methods generate hydroxyl radicals (•OH), superoxide radicals (O₂•−), sulfate radicals (SO₄•−), singlet oxygen species (¹O₂), and other species that classify as reactive oxygen species, effectively breaking down organic contaminants[8]. Physical or chemical activation of persulfates (for instance peroxydisulfate- PDS or peroxymonosulfate - PMS) via physical or chemical methods gives rise to free radicals[9]. PMS specifically demonstrates an asymmetric molecular geometry indicating the facile cleavage of its (-O-O-) peroxide linkage in the presence of catalysts. As a result, PMS is recognized as a robust oxidizing agent for breaking down water-based organic pollutants. PMS (Peroxymonosulphate) exhibits remarkable stability, a high oxidation potential, and convenient storage requirements. These characteristics, coupled with its potential for direct catalytic activation, underscore its significant advantages in the degradation of organic water contaminants.

Transition metals including iron (Fe), cobalt (Co), manganese (Mn), and copper (Cu) are frequently utilized as cost-effective catalysts in PMS-based catalytic systems. Eq. (1) elucidates the catalytic mechanism associated with these transition metals[10]:

Mⁿ⁺ + HSO₅−→ M⁽ⁿ⁺¹⁾⁺ + OH− + SO₄•−

(1)

Cobalt (Co) serves as an effective activator for PMS and displays impressive thermal stability. Several studies have detailed methods for creating diverse cobalt oxide (Co₃O₄₎ based nano-architectures[11, 12, 13, 14]. In spite of these advances, the real-time use of cobalt-based catalysts is hindered by the likelihood of leaching of cobalt ions from the reaction medium, resulting in secondary pollution. PMS activation for the treatment of wastewater has frequently employed iron oxides, in particular the catalyst Fe₃O₄. However, Fe₃O₄ has a tendency to clump together during the course of its synthesis, leading to a decrease in specific surface area and masking of active sites, eventually compromising the overall effectiveness of the catalyst. Furthermore, iron-based catalysts can be affected by anion interference during reactions, limiting their suitability for real-time wastewater treatment applications. Within the realm of transition metal catalysis, some researchers have explored MnO₂ as a catalyst for activating PMS. However, manganese-based catalysts often exhibit weaker catalytic performance and have a more limited range of applications. On the other hand, CuO is particularly noteworthy for its exceptional reactivity for PMS activation, positioning it as a pivotal catalyst in this field. Its widescale recognition is credited to its cost-effectiveness, straightforward methods of preparation, and low toxicity[15, 16]. Functioning under mild reaction conditions, CuO manifests remarkable efficiency in activating PMS to degrade compounds like 2,4-dichlorophenol. Moreover, CuO showcases significant catalytic potential for phenol degradation in the presence of PMS. The present work highlights CuO's effective catalysis in generating both radicals and non-radicals from PMS, as evidenced by quenching experiments. Recent research has also indicated a partial substitution of SO₄•− by•OH. Due to its higher redox potential and longer half-life compared to•OH, SO₄•− has a greater likelihood of effectively reacting with organic pollutants and facilitating their breakdown. Recent data suggests that the activated PMS system may not have only SO₄•− and•OH as the primary active species. Instead, ¹O₂ appears to dominate in systems involving the degradation of sulfamethoxazole by biochar-activated PMS, showcasing the prevalence of non-radicals (¹O₂) in oxidation reactions, which operate effectively across a wide pH range[17, 18]. Still, there is an ongoing discussion about PMS activation by CuO proceeding through a radical or non-radical process. This discrepancy necessitates a thorough exploration of the mechanistic steps involved in the action of CuO-catalyzed PMS, focusing on pinpointing the primary pathways regulating the generation of radical or non-radical species.

The present study is focused on the fabrication of a CuO catalyst via calcination, employing mesoporous silica as a structure-directing template employing an impregnation technique. This CuO/PMS system effectively targets the degradation of the pollutant BPA. The degradation of the organic pollutant BPA (30 mg/L) demonstrated high efficiency, achieving significant degradation within a short timeframe. Additionally, the catalysts developed in this study were not only convenient to synthesize but also exhibited robust catalytic performance and stability, allowing for their reusability. Furthermore, free radical quenching experiments unveiled that the primary products of BPA degradation were ¹O₂, shedding new light on the pathway of ¹O₂ production in the breakdown of organic contaminants.

2.1. Chemicals

Triethanolamine (C₆H₅NO₃), cetyltrimethylammonium chloride (CTAC), tetraethyl silicate (TEOS), commercial copper oxide (CuO, 99%), copper nitrate trihydrate (Cu(NO₃)₂.3H₂O), bisphenol A (BPA), peroxodisulfate (KHSO₅), tertiary butyl alcohol (TBA), furofuryl alcohol (FFA) and benzoquinone (BQ) and methanol ( MeOH) were all purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Anhydrous ethanol (C₂H₅OH), concentrated sulphuric acid (H₂SO₄), hydrochloric acid (HCl) and ammonium nitrate (NH₄NO₃) from Tianjin Fuyu Fine Chemical Co., Ltd. Sodium chloride (NaCl), sodium bicarbonate (NaHCO₃), sodium sulphate (NaSO₄) and sodium hydroxide (NaOH) from Tianjin Beilian Fine Chemical Co., Ltd. 5,5-dimethyl-1-pyrolin-Noxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidinol (TEMP) were purchased from Sigma-Aldrich Co., Ltd. All chemicals and reagents were used as raw samples without further processing. Solvents were taken from deionised water from Eco-Q deionised water purifier.

2.2. Fabrication of mesoporous silica templates

The mesoporous silica templates (SiO₂) were prepared based on our previous studies using the surfactant cetyltrimethylammonium chloride (CTAC) [19]. To begin, 0.3 g of triethanolamine and 10 g of CTAC were solubilized in water (100 mL) and stirred for 1 hour at 95°C. While stirring, tetraethyl silicate (TEOS) (7 g) was slowly added dropwise. The mixture was continuously stirred at 95°C for 2 hours. Upon the conclusion of the reaction, the white precipitate was collected by centrifugation, washed three times each in water and anhydrous ethyl alcohol, and then dried at 60°C under vacuum. Next, 100 mL of anhydrous ethanol was used to dissolve 0.6 g of ammonium nitrate. The previously obtained dried precipitate was dissolved in a solution of 100 mL of alcoholic ammonium nitrate, and the mixture was condensed under reflux at 60 ℃ for 12 hours. Afterward, the resulting material was centrifuged, washed thrice with distilled water, followed by another three washes with anhydrous ethanol, and subsequently dried under vacuum at 60 ℃. This process yielded white mesoporous silica.

2.3 Catalyst preparation

As presented in Scheme 1, the preparation of CuO catalyst involves a straightforward process involving three essential steps; hydrothermal treatment, ethanol impregnation, and subsequently alkaline pyrolysis. First, dissolution of copper nitrate trihydrate Cu(NO₃)₂.3H₂O (1.75 g) was carried out in 25 mL of ethanol. Subsequently, SiO₂ was dispersed in this solution and stirred at 85° C until complete evaporation of the solvent. The resulting mixture was vacuum-dried for 24 hours. Following this, the powder underwent calcination at 500°C for 5 hours. Upon cooling down to ambient temperature, it was washed using NaOH (3 M) at 85°C for 2 hours with continuous stirring. The material was then washed with water via centrifugation until it attained a neutral pH, followed by three washes with anhydrous ethanol. Finally, it was dried at 60°C under reduced pressure.

2.4. Catalytic degradation of contaminants and analytical methods

The catalytic degradation experiments were carried out in a 250 mL catalytic reactor and the initial pH was adjusted with 1.0 M H₂SO₄ and NaOH. To a total volume of 100 mL of organic pollutant solution (30 mg/L), 0.1 g/L of catalyst was added and stirred for 15 min, followed by the addition of 0.5 mM of PMS to start the reaction. At regular intervals, 1 mL of the reaction solution was removed, immediately filtered through a 0.22 µm filter and then quenched with excess pure methanol (0.5 mL). The residual BPA concentration was analyzed by ultra-high performance liquid chromatography (UHPLC) using a C18 column (5µm, 4.6mm×250mm) and a UV absorption detector. The mobile phase consisted of methanol and water (75:25, v/v) with a flow rate of 0.7 mL/min. The pH of the non-homogeneous solution was measured using a pH meter and the fluctuation of the measured values was controlled so that it was within a pH range of ± 0.1. Free radicals were determined by electron paramagnetic resonance (EPR, Bruker EMXplus-6/1, Germany) using DMPO and TEMP as self-selected capture agents.To increase the reliability of the experimental results, each group was measured three times in parallel to control the standard deviation. After completion of each experimental group, solid-liquid separation was carried out by filtration to collect the solid catalyst after use.

2.5 Material characterisation

The crystal structure of the samples was analyzed by X-ray diffraction (XRD, Bruker D8 Advance, Germany) using a Cu-Ka radiation source with a scan speed of 5°/min and 2θ scans at 40 kV and 40 mA, with a 2θ range from 5° to 90°. The morphology and crystal structure were characterized using an ultra-high resolution field emission scanning electron microscope (SEM, ZEISS Sigma 300, Germany), a transmission electron microscope (TEM, JEOL JEM-F200, Japan) and a high resolution transmission electron microscope (HR-TEM, JEOL JEM-F200, Japan). The specific surface area of the samples was analyzed using a fully automated specific surface area and porosity analyzer (BET, Mike ASAP2460, USA), and the chemical state of the different elements of the samples before and after the reaction was determined using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA).A total organic carbon analyser (TOC, TOC-L, Japan) was used to detect the organic carbon content in the system solution. An atomic absorption spectrometer (Agilent 240FS + GTA120, USA) was used to determine the leaching concentration of metal ions. Liquid chromatography-mass spectrometry (LC-MS, Agilent 1290 uplc, USA) was used to identify the oxidation products of BPA during the degradation system.

3.1. Characterization

The X-ray diffraction (XRD) spectra in Fig. 1 depict a distinct diffraction pattern that aligns with (PDF#48-1548), with 2θ values centered at 32.5°, 35.5°, 38.7°, 48.7°, 53.5°, 58.3°, 61.5°, 66.2°, and 68.1° Upon comparison with commercially available CuO, the XRD spectra of the two specimens displayed only minimal disparities, lacking any noticeable impurity peaks. This indicates that the CuO samples prepared maintained a high level of purity and structural integrity throughout the synthetic process. For reference, the XRD spectra of mesoporous silica can be found in Fig. S1.

The SEM images of commercial CuO and the CuO sample obtained in this study have been shown in Figs. 2a-b, showcasing their respective morphological features. The visual comparison highlights that the commercial CuO possesses an overall irregular bulk structure, whereas the CuO sample synthesized in this work comprises notably smaller particles. This reduction in size is primarily attributed to the nanostructure of mesoporous silica (Fig. S2). The TEM image, featured in the inset of Fig. 2b, illustrates that these minute particles are constructed from smaller particles, displaying pores and open spaces within them. Such a structural arrangement offers advantages by increasing the surface area of CuO, facilitating efficient catalysis of PMS. Furthermore, in Fig. S3, it's evident that the CuO sample shows improved dispersion in water, a factor that contributes to its efficacy in catalytic degradation. Figure 2c displays the HR-TEM image and the corresponding SAED map of the specimen, revealing well-defined lattice edges measuring 0.254 nm. Additionally, the N₂ adsorption-desorption technique was employed for assessing the specific surface area of the CuO specimen. As depicted in Fig. 2d, CuO fabricated in this work has a significantly higher specific surface area equivalent to 9.19 m²/g in comparison to the commercial CuO, for which the corresponding value is 0.56 m²/g. This substantial difference suggests that the technique employed for CuO preparation is favorable for expanding the catalyst's specific surface area, allowing the availability of a greater number of surface active sites for PMS molecule adsorption.

3.2. Activation of CuO by PMS for catalytic degradation of BPA

The degradation impact of various reaction systems on BPA was examined to validate the efficacy of CuO catalysts synthesized using a mesoporous SiO₂ template. Applying a set of identical conditions, a series of comparative experiments were conducted to assess the catalytic effectiveness of various reaction systems. In Fig. 3a, it can be observed that the efficiency of the CuO/PMS system for BPA degradation reached 89.57% following 5 minutes of reaction and was recorded to be 97.8% following a duration of 60 minutes under the same conditions. This rate significantly surpassed the degradation levels achieved by other systems, signifying that the optimized CuO demonstrates superior performance and acts as an efficient catalyst for PMS. Moreover, the solid-state EPR spectrum of the CuO specimen, as shown in Fig. S4 does not exhibit a distinct peak for an oxygen vacancy signal, indicating the exclusion of the effect of oxygen vacancy (V_O) on PMS activation by CuO. Additionally, the enhanced specific surface area of the CuO specimen fabricated in this study has been verified to play a crucial role in the degradation efficacy of the CuO/PMS system.

For a comparison of the rate of degradation of BPA by different systems, the corresponding pseudo-first-order kinetic constants ln (C/C₀) = -kt have been presented in Fig S5, where the reaction rate constant is represented by the value of K in min^− 1, C_0, and C_t respectively represent the BPA concentration in mg/L before the reaction and after a time t and s represents the reaction time in seconds. The CuO exhibited a reaction rate constant of 28%, significantly surpassing other systems, and showcasing the exceptional catalytic potential of the CuO/PMS system.

Catalysts based on transition metals release metal ions in the course of the reaction, influencing the process [20]. In acidic conditions, PMS activates through dissolved Cu²⁺, primarily yielding SO₄^•―. To explore Cu²⁺'s impact on BPA degradation, a Cu^2+/PMS degradation system was established. The outcome as presented in Fig. 3b, shows a 32% degradation efficiency of BPA after 60 minutes. This suggests that Cu²⁺ does not significantly contribute to the efficient BPA degradation observed in the CuO/PMS system, thereby allowing us to exclude its influence. As all experiments in this study were conducted indoors under natural lighting, the catalytic activity of the catalyst might generate reactive oxygen species in response to light exposure. Hence, experiments involving the CuO/PMS system were additionally conducted under conditions avoiding exposure to light. Figure S6 demonstrates that indoor natural light does not affect the degradation efficiency of CuO/PMS, indicating that the factor of natural light can be disregarded.

The study investigated multiple factors—BPA concentration, amounts of PMS and catalyst, temperature, initial pH, and the timing of delayed BPA addition—in terms of their influence on the degradation reaction. Figure 4a depicts a decline in BPA degradation efficiency as the pollutant concentration rises. Notably, at 40 mg/L, the degradation effect seemed significantly hindered. In contrast, concentrations of 5 mg/L and 10 mg/L of BPA exhibited almost complete degradation efficiency within 30 minutes, indicating a distinct correlation between BPA concentration and its degradation efficiency. Figure 4b portrays the influence of PMS dosage on BPA degradation. The efficiency of BPA degradation diminishes with higher PMS dosages. However, there's an interesting trend: while increased PMS dosages initially reduce the degradation efficiency, further increments lead to enhanced efficiency. This phenomenon occurs due to the catalyst action on PMS, generating more -O-O- and subsequently producing a larger quantity of ROS. Likewise, adjusting the quantity of the catalyst also influences the breakdown of BPA. As evident from Fig. 4c, an increase in the catalyst amount to 0.2 g/L results in a degradation rate of 99.48%. Simultaneously, the solution's pH significantly affects degradation. Previous research points to the existence of SO₄•− as the primary active species under acidic conditions. However, with the increase in pH to 11,•OH surfaces as the principal active component within the system. Thus, this study investigated how the reaction is influenced by the initial solution pH (Fig. 4d), and therefore pH was managed accordingly. The experimental data indicates that extreme acidity or alkalinity does not favor the degradation of BPA within the CuO/PMS system. Optimal degradation occurs at pH 6.8. Given that water has a typical pH range of 5–9, it suggests that CuO/PMS holds promise for real-time water treatment applications. Moreover, the impact of temperature was investigated as well. As depicted in Fig. S7, the influence of temperature elevation on BPA degradation is minimal. This suggests that BPA degradation can effectively take place at room temperature.

Previous reports propose that the introduction of BPA at various points in time following the mixing of PMS and catalyst can identify if the PMS-catalyst system is primarily driven by ROS or material-mediated electron transfer oxidation[21]. Figure 5 presents data indicating an initial degradation rate of 97.8% (0 min) and 93.9% after a 30-minute delay. This suggests that with the increase in the interval of time, the reaction rate (K) and BPA removal by the CuO/PMS system undergoes a gradual decrease. A substantial amount of ROS is generated post-PMS catalysis by CuO, but these ROS gradually diminish over time. This trend sufficiently demonstrates that ROS predominantly governs the oxidation process in the CuO/PMS system investigated in this study. Conversely, if the reaction rate constant (K) and the degradation rate exhibit minimal variation despite the extended time intervals, it signifies that the reaction operates through material-mediated electron transfer oxidation.

Using DMPO as a self-selected scavenger, electron spin resonance (EPR) was performed for the detection of free radicals. DMPO-SO₄•− and DMPO-•OH, adducts have been depicted with their corresponding peaks in Fig. 6a. This indicates that the generation of reactive oxidants, for instance, SO₄•− and •OH is facilitated by the CuO catalyst. In order to verify the generation of SO₄•− and •OH as primary reactive species in the BPA elimination reaction, TBA with a K(•OH) = 3.8–7.6 × 10⁸ M^− 1 s^− 1 and a K(SO₄•−) = 4–9.5 × 10⁵ M^− 1 s^− 1 and MeOH with a K(SO₄•−) = 2.5 × 10⁷ M^− 1 s^− 1 and a K(•OH ) = 9.7 × 10⁸ M^− 1 s^− 1 were utilized as quenching agents to eliminate SO₄•− and •OH, respectively. Figure 6b illustrates that adding an excess of TBA or MeOH minimally affected the degradation of BPA, confirming the potential involvement of SO₄•− and •OH in the reaction, yet indicating that they might not be the primary ROS responsible for BPA degradation. This further suggests the presence of other kinds of ROS species within the CuO/PMS system, presumably dominating the reaction process.

Equations (2) and (3) show the quenching mechanism of •OH by TBA and MeOH[22] :

(CH₃)₃COH + •OH → CH₂C(CH₃)₂OH• + H₂O

(2)

CH₃OH + •OH → CH₂OH• +H₂O

(3)

The quenching mechanism of SO₄•− has been presented in the equations (4) and (5):

(CH₃)₃COH + SO₄•− → CH₂C(CH₃)₂OH• + H⁺ + SO₄²−

(4)

CH₃OH + SO₄•− → CH₂OH• + H⁺ + SO₄²−

(5)

¹O₂ detection occurred during the transition metal-activated PMS reaction, revealing its primary role in degrading p-hydroxybenzoic acid, overshadowing free radicals. Our current experimental system's indication that free radicals aren't central to BPA degradation strongly suggests the probable involvement of non-radical ¹O₂ in the process. As a typical ¹O₂ quenching agent, FFA has a reaction rate constant of 1.2×10⁸ M^− 1 s^− 1[23, 24].

The BPA degradation rate diminishes progressively with rising FFA concentration as shown in Fig. 7b and Fig. 7c, showcasing a notable hindrance in the degradation process—from the initial value of 0.028 min^− 1 to 0.0022 min^− 1. This strongly implies that ¹O₂ takes precedence as the primary agent in BPA degradation. Simultaneously, EPR analysis utilized TEMP to trap the specific reactive intermediates or radicals during the experiment, which thereby functions as a spin trap agent[25]. The typical peaks associated with the triplet signal of ¹O₂ within the CuO/PMS system have been depicted in Fig. 7a. Hence, it sufficiently signifies that the principal reactive species driving BPA degradation within the CuO/PMS catalytic system is unequivocally ¹O₂.

Figure 7. (a) TEMP spin-trapping EPR spectrum of ¹O₂ ([TEMP]₀ = 10 mM, [CuO]₀ = 0.1 g/L, [PMS]₀ = 0.5 mM, T = 25°C, pH = 6.8); (b) Effects of FFA dose on BPA removal; (c) the rate constant for degradation([BPA]₀ = 30 mg/L, [PMS]₀ = 0.5 mM, [CuO]₀ = 0.1 g/L, T = 25°C, pH = 6.8).

Earlier literature documents the involvement of O₂•− in the reaction that leads to ¹O₂ generation. Eq. (6) describes the corresponding reaction mechanism.

2 HO + O•− → HO + HO− + O

(6)

In an attempt to determine the predominant pathway for the formation of ¹O₂ within the CuO/PMS system, experiments involving O₂•− quenching were conducted. O₂•− is quenched using benzoquinone (BQ) which demonstrates a reaction rate constant of 2.9×10⁹ M^− 1s^− 1. In Fig. 8a, upon the addition of a substantial amount of BQ, only a minimal alteration was observed in the degradation rate of BPA, showing no significant inhibition. Even with an increased concentration of BQ up to 10 mM, the rate of BPA degradation consistently remained above 90%. Concurrently EPR analysis targeted O₂•− within the system by employing DMPO as a self-selected trapping agent, as depicted in Fig. 8b. Although the CuO/PMS system showed characteristic peaks of O₂•− signals, suggesting the generation of O₂•−, it does not appear to be the primary pathway for ¹O₂ formation within the system.

The findings thus propose the direct generation of ¹O₂ via the autolytic breakdown of PMS[26]. Eq. (7) mechanistically represents the corresponding reaction.

HSO₅− + SO₅²− → HSO₄− + SO₄²− + ¹O₂

(7)

Figure 7b demonstrates that low-intensity characteristic peaks corresponding to the ¹O₂ triplet signal were detected even in the single-system PMS. This observation suggests that the self-decomposition of PMS alone has the capability to generate ¹O₂.

Metal catalysts have been reported to activate PMS molecules according to literature reports, leading to the direct generation of singlet oxygen states (¹O₂)[27, 28]. Equations (8)-(9) depict the mechanism for this process.

Catalyst + HSO₅− →¹O₂

(8)

organic pollutants + ¹O₂ → intermediates

(9)

It may be inferred that the secondary pathway for producing ¹O₂ involves the self-decomposition of PMS, while the primary pathway for generating ¹O₂ stems from the catalytic activation of PMS by CuO.

XPS spectroscopy was utilized to analyze the CuO before and after use. Figure 9a illustrates that prior to use CuO exhibits a single peak at 934.4 eV corresponding to Cu 2p3/2, representing the Cu(II) state. This indicates that the surface composition of copper oxide is primarily based on Cu(II) before its application. However, after the reaction, the CuO displays both the Cu(II) and Cu(I) peaks. The relative composition on the surface of the utilized CuO revealed 80.4% Cu(II) and 19.6% Cu(I). This indicates that in the course of the catalytic reaction, a transition between Cu(II) and Cu(I) occurs on the CuO surface, highlighting the involvement of the same transition in the process of PMS activation. After analyzing the experimental results mentioned earlier, a potential mechanism for CuO's activation of PMS was suggested which is detailed in the following equations[22].

Cu(II) + HSO₅−→Cu(I) + SO₅−+H⁺

(10)

Cu(I) + HSO₅−→Cu(II) + SO₄•−+OH−

(11)

As seen, Cu(II) and Cu(I) respectively generate sulphate radical anion and peroxymonosulphate anion respectively. Sulphate radical anion subsequently reacts with water forming hydroxyl radicals.

SO₄•−+H₂O→•OH + H⁺+SO₄²−

(12)

Equations 12 and 13 further represent the interconversion of copper ions (Cu) between Cu(I) and Cu(II) in the presence of oxygen (O₂) and its reactive form, the superoxide anion (O₂•−).

Cu(I) + O₂→Cu(II) + O₂•−

(13)

Cu(II) + O₂•−→Cu(I) + O₂

(14)

3.3. Reusability and stability studies

Cycling experiments were performed to assess the CuO sample's stability and reusability. In 60 minutes, 72% of the BPA in the system had degraded by the time the CuO catalyst was reused (Fig. 10). This suggests that the CuO sample can be used again. As shown in Fig.S9, the TOC removal remained above 76% after 5 cycles, indicating that part of the BPA was mineralised into CO₂ and H₂O.

As Fig. 11 illustrates, the morphological structure of the CuO catalyst used is still loose and porous, and its internal structure is uniformly composed of smaller particles. This indicates that the catalyst has not changed its morphology and structure much after the catalytic degradation reaction, which proves that the catalyst prepared in this experiment has very good structural stability. From the HR-TEM and the corresponding Selected Area Electron Diffractograms of the samples after use, clear lattice stripes with a spacing of 0.258 nm can be distinguished, and the SAED diagrams prove that the catalyst is a polycrystalline structure. Elemental scans show that the Cu and O elements are still present and uniformly distributed in the structure of the CuO used.

Real wastewater has a lot of inorganic anions, which are pollutants that help break down BPA in a passive way. Additionally, the typical inorganic anions affect the reaction system's pH, which lowers the amount of active oxygen species[29, 30]. 10 mM of common anions (Cl−, HCO₃−, SO₄²−) were introduced to the CuO/PMS degradation system in order to investigate the impact of anions. As Fig. 12a illustrates, the anions in the mixture had no discernible effect on the breakdown of BPA. This further confirms that the principal byproduct resulting from the CuO/PMS breakdown of BPA is the non-radical ¹O₂. This conclusion arises from the observation that certain anions adversely impact BPA degradation when they interact with free radicals. Specifically, free radicals have a tendency to combine with Cl − and HCO₃ − to form less active free radicals, thereby hindering the degradation process[31, 32].

Equations (15)-(18) outline the potential mechanisms illustrating how chloride anions could participate in the reaction.

Cl^– + SO₄•− → Cl• + SO₄²−

(15)

Cl− + Cl• → Cl₂•−

(16)

2Cl− + HSO− + H → SO − + Cl + HO

(17)

Cl₂ + H₂O → HOCl

(18)

Cl− + HSO₅− → SO₄ ²− + HOCl

(19)

The reaction equations (20)-(21) represent the possible mechanism for the involvement of SO₄ ²− ions.

•OH + SO₄ ^2– → OH− + SO₄•−

(20)

SO₄•− + SO₄ ²− → S₂O₈ ²− + e−

(21)

The possible involvement of HCO₃ − ions has been shown in the following equations.

•OH + HCO₃− → H₂O + CO₃•−

(22)

SO₄•− + HCO₃− → SO₄ ²− + H⁺ + CO₃•−

(23)

Furthermore, a complete investigation regarding the effect and potential interference from natural organic matter (NOM) on BPA breakdown was conducted by the introduction of Humic acid (HA) into the CuO/PMS degradation system. In Fig. 12b, a slight inhibition in BPA degradation is evident with increasing HA concentration. This inhibition arises from the interaction between ¹O₂ and HA, resulting in a minor decrease in ¹O₂ reactivity. Concurrently, HA serves as a scavenger in free radical oxidation systems due to its constituents—such as carboxyl, phenol, alcohol hydroxyl, ketone, quinone groups, and other functionalities. These components compete with and consume the BPA target during the reaction involving SO₄•− and •OH consequently diminishing free radicals[33, 34]. However, the overall impact on the degradation rate following HA addition remains insubstantial. Consequently, when combined with NOM removal methods like membrane filtration, this degradation system demonstrates strong resilience to anions due to the dominance of ¹O₂-driven non-radical oxidation processes, offering significant merits in the treatment of wastewater.

The reaction intermediates of BPA were analysed by LC-MS (Table S1.). Combining the previous studies and the detected oxidised intermediates, a possible catalytic degradation pathway of BPA in the CuO/PMS system was proposed (Fig. 13). BPA undergoes cleavage under the attack of large amounts of reactive oxygen species to produce 4-isopropylphenol. Then, driven by reaction time, 4-isopropylphenol is gradually oxidised to 4-hydroxyacetic acid and the intermediates formed are further degraded to acetylacetone and acetone[35, 36]. Finally, these compounds were mineralised to CO₂ and H₂O.

This study focused on the preparation of CuO catalysts via impregnation methods using mesoporous silica as a structural control agent, followed by calcination. Comparative analysis against commercial CuO showcased the significantly enhanced catalytic efficacy of the prepared CuO sample, which effectively degraded the targeted pollutant BPA. Under optimal conditions, the CuO catalyst achieved an impressive 89.57% removal of BPA within 5 minutes and the removal rate soared to 97.8% within 60 minutes. Moreover, a comprehensive series of quenching experiments and EPR studies unveiled a crucial discovery: contrary to prevailing assumptions; the primary active agents responsible for eliminating BPA within the CuO/PMS system were neither SO₄•− nor •OH. Instead, the investigation pinpointed non-radical ¹O₂ as the key player in the degradation process. This revelation prompted the delineation of the principal pathways contributing to the formation of ¹O₂, providing crucial insights into its role in pollutant degradation. These findings hold substantial significance in advancing the development of multiphase catalysts. By identifying ¹O₂ as the primary degradation agent and elucidating its formation pathways, this research lays a foundation for the fabrication of straightforward, stable, highly efficient, and cost-effective catalysts essential for removing organic pollutants from water. This study serves as a valuable reference for future endeavors aimed at refining water treatment technologies.

ACKNOWLEDGEMENTS

This work is grateful for the support of the National Nature Science Foundation of China (82060646) and Program of Science and Technology Innovation Team in Xinjiang (2020CB006).

CONFLICT OF INTEREST

The authors declare no conflict of interest.

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Template-Based Fabrication of Copper Oxide for Persulfate Activation: Investigating Non-Radical Mechanisms in Efficient Bisphenol A Degradation

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Version 1

Abstract

Figures

Introduction

Material and methods

2.1. Chemicals

2.2. Fabrication of mesoporous silica templates

2.3 Catalyst preparation

2.4. Catalytic degradation of contaminants and analytical methods

2.5 Material characterisation

Results and discussion

3.1. Characterization

3.2. Activation of CuO by PMS for catalytic degradation of BPA

2 HO + O•− → HO + HO− + O

3.3. Reusability and stability studies

2Cl− + HSO− + H → SO − + Cl + HO

Conclusions

Declarations

References

Supplementary Files

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