Investigation of catalytic activation of peroxydisulfate on cu-doped mesoporous silica-based particles (Cu-BMS) for efficient degradation of methylene blue

Catalysis Communications 186 (2024) 106833 Available online 28 December 2023 1566-7367/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Investigation of catalytic activation of peroxydisulfate on cu-doped mesoporous silica-based particles (Cu-BMS) for efficient degradation of methylene blue Saeed Sajjadi a , * , Akrity Anand a , Ana M. Beltr´ an b , Dana Dvoranov´ a c , Aldo R. Boccaccini d , Dagmar Galuskov´ a a , David Jaˇ ska e , R´ obert Klement a , * a Centre for Functional and Surface Functionalized Glass, Alexander Dubˇ cek University of Trenˇ cín, Trenˇ cín 911 50, Slovakia b Departamento de Ingeniería y Ciencia de Los Materiales y Del Transporte, Universidad de Sevilla, Escuela Polit´ ecnica Superior, Virgen de ´ Africa 7, Seville 41011, Spain c Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinsk´ eho 9, Bratislava SK-812 37, Slovakia d Institute of Biomaterials, University of Erlangen-Nuremberg, Erlangen 91058, Germany e Department of Polymer Engineering, Faculty of Technology, Tomas Bata University in Zlín, Vavreˇ ckova 5669, 760 01 Zlín, Czech Republic ARTICLE INFO Keywords: Mesoporous catalyst Persulfate activation Advanced oxidation process (AOP) Organic pollutant ABSTRACT The Cu-doped mesoporous silica-based particles (Cu-BMS) were prepared using an evaporation-induced selfassembly sol-gel procedure as a heterogeneous catalyst for the activation of peroxydisulfate (PDS). The formation of well-organized mesoporous structures with amorphous nature and high surface area of 286 m 2 /g was demonstrated. The catalytic activity of Cu-BMS in the degradation of Methylene Blue (MB) and the effects of operating parameters, including Cu-BMS dosage, initial PDS amount, initial MB concentration, temperature and initial pH, were investigated in details. The Cu-BMS demonstrated a remarkable catalytic activity (93.5% degradation efficiency within 60 min) and good stability. 1. Introduction Large volumes of organic pollutants have been released by the rapidly developing chemical sector, posing major risks to the environment and public health. The presence of dyes in industrial wastewater from the textile industry is a major concern in developing regions, mainly due to the presence of various persistent pollutants. The textile industry’s daily consumption of freshwater is a significant concern, given the vast range of over 100,000 possible colors worldwide. Methylene blue (MB) is a common color found in textile effluent, along with other dye compounds. This particular basic dye, based on thiazine, has the ability to cause long-term environmental damage by blocking sunlight in water bodies, thereby affecting the aquatic ecosystem and its inhabitants. From a public health perspective, it should be noted that MB can cause ocular burns, which may lead to lasting injury. When ingested, this substance causes a feeling of intense heat, resulting in the expulsion of stomach contents through vomiting [1,2]. Advanced oxidation processes (AOPs) have been widely demonstrated as promising technologies for environmental remediation. AOPs can generate reactive oxygen species (ROS) to degrade organic contaminants into smaller molecules or water and carbon dioxide [3–6]. Traditional oxidants, such as hydrogen peroxide (H 2 O 2 ), can be converted into hydroxyl radicals ( • OH, E 0 =1.89–2.72 V vs. NHE) by iron ions (Fe 2+ ) and are used in AOPs. However, this system cannot be applied on a large scale because it produces a large amount of sludge and works only within a narrow pH range (2–3). In recent decades, extensive research has been conducted on the sulfate radical (SO•− 4, E 0 =2.5–3.1 V vs. NHE)-based AOPs, which are viable alternatives [7,8]. In a wide pH range (3–9), persulfates, such as peroxymonosulfate (PMS, E 0 =1.4 V vs. NHE) and peroxydisulfate (PDS, E 0 =2.01 V vs. NHE), are stable solid oxidants with excellent AOP efficiency. PDS has longer O–O bonds than PMS, which makes it easier to activate and shows better oxidation capacity in the commercial oxidation process. Moreover, AOP based on PDS provide the benefits of low sludge output and gentle working conditions. It has been previously stated that sulfate radicals can be generated via ultraviolet (UV) light, heat, ultrasound (US), and transition metals, according to Eqs. (1–3) [9–11]. * Corresponding authors. E-mail addresses: [email protected] (S. Sajjadi), [email protected] (R. Klement). Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom https://doi.org/10.1016/j.catcom.2023.106833 Received 9 October 2023; Received in revised form 11 December 2023; Accepted 25 December 2023 Catalysis Communications 186 (2024) 106833 2 S2O2− 8→ heat 2SO•− 4(1) S2O2− 8→ US or UV 2SO•− 4(2) S2O2− 8+Mn+(Metal)→SO2− 4+SO•− 4+M(n+1)+ (3) Among these, homogeneous systems with transition metal ions (Fe 2+ , Cu 2+ , Mn 2+ , Co 2+ , etc.) are regarded as the most effective. Recent studies have demonstrated that copper is better than other transition metals for catalyzing chemical reactions, particularly in the efficient activation of PDS [12] and PMS [13], using a wider range of methods [14]. Cu-based catalysts can activate both PMS and PDS to produce free radicals (like SO4 • ‾, • OH, and O 2 • ‾), singlet oxygen ( 1 O 2 ), and Cu(III) [15–17]. Nevertheless, homogeneous catalysts have the drawback of being difficult to recover and prone to causing secondary pollution throughout the wastewater treatment process [18]. Compared to homogeneous systems, heterogeneous catalysts such as metals, metal oxides, and supported metal oxides have less environmental impact and are more recyclable [8]. In recent years, numerous research efforts have been made to increase the generation rate of activated species and accelerate reaction kinetics to maximize the efficiency of organic compound degradation. In particular, catalysts based on Cu, which have the benefits of variable valence, abundant active sites, and high surface reactivity, have been the subject of intense research because of their potential to efficiently degrade organic pollutants [14,19]. Mesoporous silicas, such as SBA-15 (Santa Barbara Amorphous-15, a mesoporous silica sieve), are frequently used as support matrices for heterogeneous Cu-based catalysts owing to their large surface area, thermal stability, flexibility for chemical modification, and environmental friendliness [20–23]. Only a few studies have been conducted on the catalytic effectiveness of welldispersed Cu species supported on mesoporous silica for AOP [24–27]. Mesoporous silica SBA-15 catalysts containing Cu nanoparticles have shown excellent catalytic activity for the reduction of different dyes [28]. The main aim of this study was to investigate the catalytic activation of PDS using Cu-doped mesoporous silica-based particles (Cu-BMS, SiO 2 - CaO-CuO-P 2 O 5 ) for the degradation of MB, a typical organic dye in wastewater. Cu-BMS was prepared through the sol-gel method. The effects of the operating parameters, including Cu-BMS dosage, initial PDS amount, initial MB concentration, and initial pH, were investigated. Quenching tests were also conducted to determine the participation of the reactive radical species in the degradation process. Finally, the stability of the Cu-BMS was evaluated using 12 repetitive runs. To the best of our knowledge, this is the first investigation on the catalytic activity of Cu-doped mesoporous silica particles as a heterogeneous catalyst for PDS activation. 2. Materials and methods 2.1. Preparation of Cu-BMS Cu-BMS, with a composition of 80% SiO 2 , 11% CaO, 4% CuO, and 5% P 2 O 5 (mol%), was prepared using an evaporation-induced self-as- sembly (EISA) sol-gel procedure [29]. In a typical procedure, 0.875 mL of hydrochloric acid (HCl, 0.5 mol/L, Merck, Germany) was mixed with 79.84 mL of ethanol (Mikrochem, Slovakia), and the resulting solution was mixed with 4.2 g of Pluronic® P-123 (Poly(ethylene glycol)-poly (propylene glycol)-poly(ethylene glycol), M n =5800, Merck, Germany). In addition, 8.25 mL of tetraethyl orthosilicate (TEOS, ~98.0%, Merck, Germany), 0.78 mL of triethyl phosphate (TEP, ~99.8%, Merck, Germany), 1.2 g of calcium nitrate tetrahydrate (Ca(NO 3 ) 2 .4H 2 O, ~99%, VWR International GmbH, United States), and 0.248 g of copper (II) chloride (CuCl 2 , ~99%, Merck, Germany) were dissolved in the above solution. At 32 ◦C and 75% humidity, the prepared sol was cast into Petri dishes (100 mm ×20 mm) for the EISA process. The EISA process took place over approximately nine days. To obtain Cu-BMS particles, the final gel was calcined at 700 ◦C (2 ◦C/min) for 5 h. 2.2. Characterization Powder X-ray diffraction (XRD) patterns of the as-prepared Cu-BMS particles were recorded on an X-ray diffractometer (Empyrean, Malvern Panalytical, Almelo, Netherlands) using Cu-K α (1.54184 Å) radiation at 45 kV and 40 mA. The presence of a mesoporous structure was verified using the small-angle X-ray scattering (SAXS) technique. The measurement was conducted using an Anton Paar SAXSpace instrument, employing Cu-K α (1.54184 Å) radiation at 40 kV and 50 mA, in line collimation mode. The sample and detector were separated by a distance of 307 mm, and the exposure time was 3 min. The sample surface was inspected by X-ray Photoelectron Spectroscopy (XPS) using Nexsa G2 XPS Surface Analysis System (Thermo Scientific). The microstructure of the Cu-BMS particles was examined using a JSM-7600F scanning electron microscope (SEM, Jeol, Japan). To prove the mesoporous structure of prepared particles and the presence of the mentioned elements in the structure, transmission electron microscopy (TEM) and energydispersive X-ray spectroscopy (EDX) were carried out using a Talos F200X system (Thermo Fisher Scientific, USA); an accelerating voltage of 200 kV was applied for imaging. Optical emission spectrometry with inductively coupled plasma (ICP-OES, Agilent 5100 SVDV, USA) was used to analyze the chemical composition of the Cu-BMS particles. The sample for analysis was prepared by dissolving 25 mg of Cu-BMS in a solution containing 6 mL of 30% hydrochloric acid (30%, Merck), 2 mL of nitric acid (67–69%, Analytika Ltd.), and 0.5 mL of hydrofluoric acid (47%, VWRchemicals). After the complete dissolution of the sample, the resulting solution was analyzed using ICP-OES. To determine the possible leaching of Cu ions from the Cu-BMS particles into the aqueous solution, a leaching test was performed using the following procedure. The amount of Cu ions released into the solution during the reaction was analyzed using ICP-OES. After completing the degradation process, a 10 mL sample of the solution was collected, and the catalyst was separated by centrifugation. Subsequently, two drops of concentrated nitric acid (68%) were introduced into the sample to reach pH <2 and keep analyte in dissolved form prior the analysis. In the ICP-OES measurement, the carrier gas flow, wavelength, and radiofrequency power were 0.55 L/ min, 327.395 nm, and 1.2 kW, respectively. Infrared (IR) spectra were recorded using Fourier-transform infrared (FTIR) spectrometer IRAffinity-1S (scan speed:40 scans/min, resolution:4 cm −1 , SHIMADZU, Japan). The pore volume and Brunauer–Emmett–Teller (BET) surface area were determined using a Belsorp-mini II analyzer (Microtrac, Japan). A simultaneous thermal analyzer (STA 449F1, Netzsch, Germany) was used for thermal analyses (DTA and TGA) of the Cu-BMS particles. These analyses were performed at a standard heating rate of 10 K/min up to 1000 ◦C in air atmosphere. The EPR experiments at room temperature (295 K) with dispersed systems in deionized water under air atmosphere were carried out employing EPR spectrometer (Bruker, Germany) operating in X-band at 100 kHz field modulation in the TM resonator (Bruker) with the small quartz flat cell (WG 808-Q, Wilmad-LabGlass, optical cell length 0.045 cm). The spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO, Sigma-Aldrich) was distilled before application and stored at −18 ◦C. The 4-oxo-2,2,6,6-tetramethylpiperidone (TMPO, Sigma-Aldrich) was used as supplied. The 50 μ L DMPO (0.212 M stock solution in deionized water) was added to the reaction suspension containing 100 μ L of Cu- BMS suspension (1.0 g/L) and 100 μ L of PDS solution (4 g/L), i.e. similar concentrations as in experiments of Cu-BMS catalytic activity assessment. The reaction mixture was transferred into quartz flat cell and the EPR spectra were recorded in situ. The EPR spectra acquisition started 3 or 11 min after mixing. For comparison, the experiment at the same conditions was conducted without the Cu-BMS catalyst; the 50 μ L DMPO (0.212 M stock solution in deionized water) was added into the S. Sajjadi et al. Catalysis Communications 186 (2024) 106833 3 mixture of 100 μ L of PDS solution (4 g/L) diluted with 100 μ L of deionized water. The relative concentration of paramagnetic species was evaluated from the double-integrated EPR spectra. The experimental EPR spectra were analyzed using WinEPR software (Bruker) and the calculations of spin Hamiltonian parameters and relative concentrations of individual spin adducts were performed with the EasySpin toolbox working on MatLab® platform [30]. 2.3. Catalytic activity assessment The catalytic activity of Cu-BMS for PDS activation was evaluated based on the degradation of MB. The degradation process was carried out in a 250 mL Pyrex beaker under magnetic stirring at 25 ◦C. According to commonly used methodology [31,32], a specific quantity of sodium persulfate (Na 2 S 2 O 8 , 99%, Merck, Germany) was added to 100 mL of MB solution (100 mg/L). The catalytic degradation process was initiated by adding a defined amount (0.05, 0.1, 0.2, 0.3, and 0.4 g) of Cu-BMS. To follow the MB degradation kinetics, 3 mL of the reaction solution was collected at regular intervals (every 10 min) during the degradation process, and the catalyst was separated by centrifugation. The concentration of MB in the solution was determined spectrophotometrically using a UV–Vis-NIR spectrophotometer (Cary 5000, Agilent, United States). The effects of the operating parameters, including the Cu-BMS dosage, initial amount of PDS, initial MB concentration, initial pH, and radical species scavengers, on the degradation efficiency (DE%) were studied. DE% of the MB was determined using the Eq. (4) [33]: DE%=(1−At A0)×100 (4) where A 0 and A t are the initial absorbance and absorbance at a particular time in the process (t, min), respectively. The stability of the Cu- BMS catalyst was investigated by collecting, washing (with isopropanol and deionized water), and drying (overnight at 100 ◦C) the catalyst after each degradation step. 3. Results and discussion 3.1. Cu-BMS characterization The transformation temperatures and mass losses of the Cu-BMS sample prior to calcination at 700 ◦C for 5 h were determined by DTATGA analysis. Fig. 1a shows the DTA-TGA thermograms of the Cu-BMS prepared with the Pluronic® P-123 surfactant. Because the sol-gel EISA process was carried out under high-humidity conditions and a surfactant was present, additional weight loss was expected. During heating to 1000 ◦C, three exothermic peaks were observed in the DTA curve at approximately 66–104, 127–201, and 250–470 ◦C. The first two peaks, which appeared below 201 ◦C, were linked to the release of adsorbed water molecules from the surface, resulting in a weight loss of 11%. A weight loss of up to 55% was measured between 250 and 470 ◦C, which can be attributed to the decomposition of the remaining nitrate and burning of the residual surfactant. Burning of the surfactant may release a significant amount of CO 2 and H 2 O, resulting in a broad Fig. 1. (a) DTA-TGA thermograms, (b, c) SEM images, and (d) particle size distribution histogram of Cu-BMS particles prior to calcination at 700 ◦C for 5 h. S. Sajjadi et al. Catalysis Communications 186 (2024) 106833 4 exothermic peak [34,35]. Based on the DTA-TGA analysis, 700 ◦C was selected for calcination to obtain mesoporous Cu-BMS particles. According to the SEM images (Fig. 1b, c) of the as-prepared Cu-BMS particles, the final particles exhibited a polygonal shape, with smaller particles adhering to larger particles. From the SEM images using ImageJ (National Institutes of Health, MD, USA) and applying the Gaussian distribution function, the average particle size was estimated to be 58 ±19 μ m (Fig. 1d). The XRD pattern obtained from the as-prepared Cu-BMS particles (Fig. 2a) reveals the amorphous nature of the Cu-BMS particles, which is documented by the amorphous halo that appears from 15◦to 36◦2θ, which is typical for amorphous systems, such as silicate glasses [36]. Furthermore, the presence of hints of peaks at specific 2θ values, namely 32.5◦, 35.5◦, 38.8◦, and 48.8◦, which correspond to the crystalline planes of (110), (002), (111), and (202) respectively, provides evidence for the creation of CuO (JCPDS-01–089-5899) [37] within the structure of BMS. The low intensity of the CuO peaks in XRD trace could be attributed to the low loading and highly dispersed nature of the copper species within the mesoporous silica structure, which might result in low crystallinity or small crystal sizes. It is also reasonable to expect, that copper ions can be bounded to non-bonding oxygens within the amorphous structure/framework of the BMS. Additionally, the small-angle X- ray diffraction analysis was performed, and the result is illustrated in Fig. 2a (inset). A distinct diffraction peak at 1.2◦is evident, corresponding to the reflections of the 100 plane. Two minor peaks at 1.75◦ and 2.03◦were also observed, and identified as the reflection of the 110 and 200 planes [38]. The reflections confirmed the presence of a mesoporous structure, which closely match the findings of the TEM examination. To gain a better understanding of the characteristics of the copper species in Cu-MBS, XPS analysis was conducted. According to the XPS survey results (Fig. S1, Supplementary Material), O, Si, P, Ca, and Cu elements were detected. The presence of the C peak in the XPS survey is due to impurities that were absorbed during storage and analysis. The peak fit of Cu 2p core level XPS spectrum is shown in Fig. 2b. The peaks located at 936.8 and 955.9 eV can be attributed to the Cu(II) state, whereas the peaks positioned at the binding energy values of 934.4 and 953.3 eV are assigned to the Cu(I) state. The satellite peaks, as usually observed for copper, at around 944 and 963 eV can be attributed to the shake-up phenomenon, which arises as a consequence of the unoccupied 3d orbital of Cu(II) [39]. It is worth noting that the presence of Cu(I) in the Cu-BMS benefits the PDS activation process [40]. The FTIR spectrum of Cu-BMS (Fig. 2c) revealed that all organic compounds used in the preparation were removed after heat treatment at 700 ◦C for 5 h. The bands around 450 and 800 cm −1 can be assigned to Si-O-Si symmetric vibrations, and the band located at 1060 cm −1 can be Fig. 2. (a) XRD pattern (the inset represents the Small-angle XRD pattern), (b) Cu 2p high-resolution XPS spectra, and (c) FT-IR spectrum of Cu-BMS particles. S. Sajjadi et al. Catalysis Communications 186 (2024) 106833 5 ascribed to the Si-O-Si anti-symmetric stretching mode. Moreover, this spectrum exhibited a very low-intensity peak at approximately 540 cm −1 , which was assigned to the vibrations of P–O in calcium phosphates [41,42]. The chemical composition of the Cu-BMS particles was determined using ICP-OES (after sample decomposition) and EDX analyses. Based on ICP-OES analysis of the Cu-BMS sample, Table 1 lists its composition and standard deviation. Silica-based compounds dissolve in hydrofluoric acid (HF) by breaking strong Si–O bonds and forming hexafluorosilicic acid (H 2 [SiF 6 ]), which exists in its dissociated form (SiF 6 2− ) in water. Owing to the high SiO 2 content (80 mol%) of Cu-BMS, the partial formation of volatile silicon tetrafluoride (SiF 4 ) cannot be excluded, which could result in Si loss during decomposition. Therefore, this method cannot provide complete recovery for silicon analysis, and the values for SiO 2 present in Cu-BMS were estimated by normalizing it to 100%. Although the presence of Cu was confirmed by ICP-OES analysis, the actual composition of the prepared sample differed from the nominal composition, especially in terms of the amount of phosphorus, which was significantly lower than expected. This discrepancy can be attributed to the different hydrolysis rates of the two precursors (TEP and TEOS) under acidic pH conditions during preparation. Specifically, the rapid hydrolysis of TEOS and condensation of SiO 2 particles, compared to TEP, prevents the incorporation of PO 4 structural units into the amorphous SiO 2 structural network, thus leading to a reduced level of P 2 O 5 in the Cu-BMS system [43]. The remaining P 2 O 5 may act as a network former and form bonds with silica and calcium [44]. Additionally, EDX analysis confirmed the high purity of the synthesized Cu- BMS particles by detecting Si, Ca, P, O, and Cu in their structures without the presence of other unwanted elements (Table 1). TEM images of the specimen confirmed the existence of a wellordered, cylindrical-like mesoporous structure (Fig. 3a). Mesoporous structures are formed as a result of the regular packing (self-assembly) of micelles and homogenous distribution of organic–inorganic species [45]. The study employed an optimized amount of surfactant to achieve homogenization of inorganic-organic pairs. This approach ensures that the formation of ordered mesopores is not interrupted by any additional surfactant species. Moreover, the EISA method was utilized to condense inorganic species, and control of temperature and humidity led to an organized mesoporous structure within the Cu-BMS particles [46]. N 2 adsorption–desorption analysis was performed to investigate the pore structure and BET surface area. The Cu-BMS exhibited a type-IV curve with an H1 hysteresis loop according to the IUPAC classification [47], as shown in Fig. 3b. The H1 hysteresis loop confirms the formation of narrow distribution of uniform cylindrical mesoporous structures which is open at both ends [48]. Based on the multipoint BET method, the Cu-BMS particles have a large specific surface area (a s,BET ) of 286.94 m 2 /g. This mesoporous structure was further confirmed using the Barrett-Joyner-Halenda method [49], which determined an average pore size of 5.54 nm. The total pore volume and monolayer adsorption amount (V m ) of the Cu-BMS were 0.3976 cm 3 /g and 65.926 cm 3 (STP)/g, respectively. 3.2. Catalytic activity of Cu-BMS catalyst A comparison of the degradation of MB by different systems, including Cu-BMS/PDS, Cu 2+ /PDS, Cu-BMS (adsorption), and PDS, is shown in Fig. 4a. The insignificant DE% (<13%) within 60 min indicates that inactivated PDS alone cannot result in high degradation of MB, which is in accordance with previous reports [50–52]. Additionally, the DE% of the Cu 2+ /PDS system showed that homogeneous Cu 2+ ions were not powerful enough to activate the PDS. As a source of Cu 2+ ions, we used 20 mg/L of copper(II) sulfate (CuSO 4 , Merck, Germany), because in this comparison our catalyst dosage was 0.5 g/L and contains 4 mol% of Cu 2+ in the Cu-BMS catalyst (according to the ICP-OES analysis). The data in Fig. 4a show that 25.5% (within 60 min) of the MB was adsorbed onto the surface of the Cu-BMS particles. The high surface adsorption indicates that the Cu-BMS catalysts have a high specific surface area, which in turn indicates a high number of reaction sites. Therefore, BMS is an excellent support for loading Cu 2+ ions. Regarding the degradation of MB, the adsorption of Cu-BMS did not reach the required DE% level. In contrast, the Cu-BMS/PDS system increased the DE% of MB to 93.5% within 60 min. The Fig. S2 displays the UV–Vis spectra of MB degradation at various times during the degradation process. The absorbance peak of MB was observed in the range of 500–700 nm, representing the heteropoly aromatic linkage (involving chromophore and auxochrome (-CH 3 )) [53], with its maximum at λ =664nm, that absorbance was monitored during the MB degradation process. In addition, the catalytic efficacy of Cu-BMS with varying amount of copper (1%, 2%, and 4%) was examined, and the findings are illustrated in Fig. S3. The increase in the copper content in the BMS structure resulted in an increase in the DE %. To determine the reaction rate constant (k) of MB degradation, pseudo-first order kinetics was applied (Eq. (5)), and a linear fit was performed between ln(A 0 /A t ) and time, where A 0 and A t are the initial and time-related MB absorption, respectively [54]. lnA0/At=kt(5) The data pertaining to the pre-equilibrium stage (up to 40 min) are described by Eq. (5). Linear plots representing these data are shown in Fig. 4b. From the figure, it can be observed that the rate constant k Cu- BMS/PDS (0.0641 min −1 ) is approximately 8 times greater than k Cu-BMS (0.0077 min −1 ), almost 17 times greater than K2+ Cu /PDS (0.0036 min −1 ), and almost 20 times greater than k PDS (0.0031 min −1 ). The higher rate constant of Cu-BMS/PDS compared to that of the other systems indicates its potential as a more effective catalyst for the degradation of MB. The results indicate that the Cu-BMS catalyst can activate PDS and accelerate the degradation of MB. Furthermore, it was observed that the Cu-BMS catalyst exhibited a higher level of reactivity in terms of the degradation rate constant during the activation of PDS compared to some of the catalysts reported in the literature (Table 2). 3.3. Effect of the operating parameters on DE% 3.3.1. Cu-BMS catalyst dosage Owing to economic considerations, it is important to use an appropriate dosage of the catalyst in the degradation reactions [55]. An investigation of the effect of Cu-BMS dosage on the DE% of MB is shown in Fig. 5a. The DE% of MB increased steadily as the Cu-BMS catalyst dosage was increased until it reached 93.5% at 0.5 g/L catalysts. A higher catalyst dosage enhances PDS activation sites, resulting in increased SO•− 4 generation and higher DE% [56]. However, increasing the Cu-BMS dosage from 0.5 to 1.0 g/L did not have a significant effect on DE% (97.3% within 60 min). This could possibly be explained by the fact that the adsorption of persulfate (2 g/L) for activation reached equilibrium when the Cu-BMS dosage was at 0.5 g/L [57]. Table 1 ICP-OES and EDX-TEM data of Cu-BMS particles. ICP-OES SiO 2 CaO P 2 O 5 CuO Nominal composition (Wt%) 74.50 9.56 11 4.93 Actual composition (ICP-OES) (Wt%) 77.6 ± 0.6 11.2 ± 0.5 1.5 ± 0.6 5.3 ± 0.2 EDX-TEM Element Wt% At.% Cu 5.80 1.93 O 49.66 65.79 Si 38.11 28.76 P 0.72 0.49 Ca 5.68 3.00 S. Sajjadi et al. Catalysis Communications 186 (2024) 106833 6 3.3.2. PDS dosage In Fig. 5b, the catalytic activity of the Cu-BMS catalyst was investigated at different concentrations of PDS (0.5, 1.0, 2.0, 3.0, and 4.0 g/L) in the presence of a fixed quantity of catalyst (0.5 g/L) and MB (100 mg/ L). By increasing the PDS dosage from 0.5 to 3.0 g/L, the DE% increased from 63.5% to 95.6% within 60 min. At high PDS dosages, high concentrations of active radicals facilitated the oxidation of contaminant molecules (Fig. S4). Nonetheless, DE% decreased significantly with a further increase in the PDS dosage (4.0 g/L). In the presence of high dosages of PDS, SO•− 4 can react with PDS to form less active radicals, as shown in Eq. (6) and the chain-termination reaction between SO 4 •− radicals (Eq. (7)) may hinder the oxidation of MB [58,59]. SO•− 4+S2O2− 8→S2O•− 8+SO2− 4(6) SO•− 4+SO•− 4→S2O2− 8(7) 3.3.3. Initial pH It is well known that the DE% of organic contaminants can be significantly affected by adjusting the solution’s initial pH. We investigated the DE% of MB at five different pH levels (2.5, 4.5, 6.5, 8.5, and 10.5), and the results are summarized in Fig. 5c. Solutions of NaOH (0.1 mol/L) and H 2 SO 4 (0.1 mol/L) were used to adjust the initial pH of the pollutant solution. As shown in Fig. 5c, the acidic environment improved the DE% of MB, whereas the alkaline environment had a negative impact. The higher DE% under acidic conditions was due to the generation of more SO•− 4 radicals, according to the reactions given in Eqs. (8–11) [56,60]. Furthermore, at high pH levels, high concentrations of OH‾ can scavenge SO•− 4 (Eq. (12)) [61]. S2O2− 8+H+→HS2O− 8(8) HS2O− 8→SO•− 4+SO2− 4+H+(9) SO•− 4+H2O→SO2− 4+OH•+H+(10) SO2− 4+OH•+H+→SO•− 4+H2O(11) SO•− 4+OH−→SO2− 4+OH•(12) 3.3.4. Initial MB concentration The degradation of MB at different initial concentrations (50, 100, 200, and 400 mg/L) was also investigated in the presence of a fixed quantity of catalyst (0.5 g/L) and PDS (2.0 g/L) at pH 6.5. By increasing Fig. 3. (a) TEM images at different magnification and (b) Nitrogen adsorption isotherm of the Cu-BMS particles. S. Sajjadi et al. Catalysis Communications 186 (2024) 106833 7 Fig. 4. (a) DE% of MB; and (b) the pseudo-first-order kinetic rate plot for different systems (Parameters: [Cu-BMS] =0.5 g/L, [PDS] =2.0 g/L, [MB] =100 mg/L and pH =6.5 (original pH of MB)). S. Sajjadi et al. Catalysis Communications 186 (2024) 106833 8 the initial concentration of MB from 50 to 400 mg/L within 60 min, the DE% decreased from almost 100% to 44.6% (Fig. 5d). A constant number of active radicals were generated under specific operating conditions, such as the catalyst dosage, PDS dosage, and reaction time. Hence, insufficient active radicals would be present in the presence of a high concentration of MB molecules, resulting in a low DE%. Furthermore, when MB is present at high concentrations, the active sites of the catalyst can be clogged with MB molecules, which can reduce the effectiveness of the catalyst in degrading MB molecules [62]. 3.3.5. Reaction temperature The influence of different temperatures (25, 45, and 65 ◦C) on the Cu-BMS/PDS system is shown in Fig. 6a. The degradation of MB exhibited a consistent upward trend as the temperature increased. For example, the decomposition of MB increased from 61% to 97% after 10 min of reaction time as the temperature rose from 25 ◦C to 65 ◦C. This demonstrates that PDS was also activated by heat producing reactive sulfate radical species active in MB degradation process [63]. 3.3.6. Stability Catalyst stability and reusability play major roles in the costeffectiveness of heterogeneous water treatment. Therefore, twelve consecutive MB degradation tests were performed on the Cu-BMS catalyst, and the results are shown in Fig. 6b. In the first cycle, 93.5% of MB was degraded within 60 min. During the fourth, eighth, and twelfth cycles, the MB DE% decreased to 84.3%, 75.7%, and 70.4%, Table 2 Degradation efficiency of organic molecules during activation of persulfate with different catalysts. Catalyst Conditions DE (%) - Time (min) Ref. Mesoporous silica spheres [MB] =50 mg/L, [PDS] =3.7 mM, [Catalyst] =1 g/L, pH =5 90% - 60 [59] Fe–Co-SBA [Orange II] =70 mg/L, [PDS] =20 mM, [Catalyst] =1 g/L, pH =3–7 >80% - 120 [73] α -MnO 2 / Palygorskite [RhB] =20 mg/L, [PMS] =0.9 mM, [Catalyst] =0.1 g/L, pH <5.5 ~100% - 300 [74] CuO/Fe 2 O 3 / CuFe 2 O 4 [Levofloxacin] =10 mg/L, [PDS] = 7 mM, [Catalyst] =0.5 g/L, pH = 6.4 75.5% - 120 [75] Cu/ZSM5 [RhB] =50 mg/L, [PMS] =5 mM, [Catalyst] =1 g/L, pH =7 99% - 60 [76] Cu-BMS [MB] =100 mg/L, [PDS] =10 mM, [Catalyst] =0.5 g/L, pH =6.5 93% - 60 This study Fig. 5. Effect of (a) Cu-BMS catalyst dosage, (b) PDS dosage, (c) initial pH, and (d) initial MB concentration on DE% in the Cu-BMS/PDS system (Parameters: [Cu- BMS] =0.5 g/L, [PDS] =2.0 g/L, [MB] =100 mg/L, and pH =6.5). S. Sajjadi et al. Catalysis Communications 186 (2024) 106833 9 respectively. The Cu-BMS catalyst maintained an acceptable DE% (70%) even after the twelfth cycle of degradation tests, indicating its durability and reusability. Additionally, structural stability of Cu-BMS catalyst was examined by XRD (Fig. S5) and FESEM (Fig. S6) analysis after twelve times use. The comparable morphology and XRD pattern observed in the utilized catalyst and the pristine Cu-BMS provides evidence for the structural integrity and stability of the Cu-BMS catalyst after several time usage. This indicates that the Cu-BMS synthesized in this study exhibits a notable level of structural stability and satisfactory reusability when employed in the degradation of organic contaminants within the Cu-BMS/PDS system. The concentration of Cu leaching (0.5 g/L Cu-BMS in 100 mL solution with a pH of 6.5) was measured by ICP-OES. The results showed that the Cu concentrations in the above-mentioned tests were 0.105 mg/L (first run), 0.088 mg/L (fourth run), 0.026 mg/L (eighth run), and 0.017 mg/L (twelfth run). The World Health Organization (WHO) and the European Water Quality Directive recommend a Cu concentration below 2.0 mg/L in drinking water. Therefore, the concentration of Cu leached from the catalyst was lower than the standard Cu concentration [64]. 3.4. Investigation of the reactive radical species The EPR spin trapping technique was applied to confirm the formation of reactive radical species as their paramagnetic intermediates with spin trapping agents. DMPO (5,5-dimethyl-1-pyrroline N-oxide), as in many studies often applied, was used as a spin trapping agent of the SO•− 4 and HO • radicals, and TMPO was used to detect the singlet oxygen species 1 O 2 . In the system Cu-BMS/PDS/DMPO formation of two spinadducts was identified (Fig. S7), the • DMPO-OH (spin Hamiltonian parameters: a N =1.504 mT, a H β =1.459 mT; g =2.0057, c rel =52%) and the signal of the • DMPO-SO 4 − adduct (spin Hamiltonian parameters: a N =1.370 mT, a H β =1.007 mT, a H γ1 =0.142 mT, a H γ2 =0.076 mT; g = 2.0057, c rel =48%). The EPR signal of the • DMPO-SO 4 − adduct was weaker than that of • DMPO-OH, and the intensity of • DMPO-OH signal increased with time. This is because the capture efficiency of DMPO is higher for HO • radicals than for SO•− 4 [65], and additionally, • DMPOSO 4 − adduct can undergo rapid nucleophilic substitution to form hydroxyl radical adduct • DMPO-OH which is monitored [66]. Also, sulfate radicals can transfer into hydroxyl radicals at neutral pH, according to Eq. (10). The same species ( • DMPO-SO 4 − c rel =44%, • DMPO-OH c rel = 56%), however in lower concentration, were observed also in reaction Fig. 6. (a) Effect of temperature on DE% in the Cu-BMS/PDS system; (b) MB degradation of Cu-BMS/PDS system in the first, fourth, eighth, and twelfth cycles; and (c) effect of reactive radical species scavengers on the DE% (Parameters: [Cu-BMS] =0.5 g/L, [PDS] =2.0 g/L, [MB] =100 mg/L and pH =6.5). S. Sajjadi et al.