To effectively treat refractory azo dye wastewater, microwave advanced catalytic oxidation technology was adopted to degrade the model pollutant methyl orange using activated carbon fiber (ACF)/CuO as the catalyst and potassium persulfate (K2S2O8) as the oxidant. The optimized experimental parameters and the degradation pathway of methyl orange were determined. The results showed that when the microwave power was 500 W, the irradiation time was 2 min, the dosage of potassium persulfate was 0.6 g/L, and the dosage of ACF/CuO was 10 g/L, the removal rate of methyl orange solution was close to 100%, the COD removal rate was 89.65%, and the TOC removal rate was 72.36%. Mechanism analysis indicated that the double bond was broken to generate acid and p-nitrophenol, which were gradually degraded to benzene and phenol under the oxidation of sulfate radical. Subsequently, the benzene and phenol underwent chain cleavage to form maleic anhydride, and part of the benzene, phenol, and the generated maleic anhydride were ultimately degraded to water and carbon dioxide.
Research Article
Study on the Treatment of Methyl Orange Contaminated Water by Activated Carbon Fiber/Copper Oxide as Persulfate Activator under Microwave Irradiation
https://doi.org/10.21203/rs.3.rs-7737009/v1
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advanced catalytic oxidation technology
ACF/CuO nanocomposites
microwave
methyl orange
A combinative strategy involving microwave and heterogeneous catalysis based on ACF/CuO/K2S2O8 was innovatively applied to remove methyl orange contaminated water.
Methyl orange, COD and TOC removal efficiency in the combinative process were achieved to 100%, 89.65% and 93% , respectively.
Double bonds in methyl orange were broken to generate acid and p-nitrophenol, and they gradually degraded into benzene and phenol by oxidation of sulfate radicals. Water and carbon dioxide are the final degradation products.
The treatment of dye wastewater faces two primary challenges: high chromaticity and high organic concentration. To address the environmental pollution caused by organic dyes, various methods and technologies have been employed to treat wastewater contaminated with organic dyes. The primary goal is to separate and remove the color-forming substances and to break down these substances to achieve decolorization and degradation of the organic matter. (Lan 2021; Al-Tohamy 2022; Solayman 2023).As the most commonly used microwave-induced catalysts, activated carbon fiber and copper oxide enhance the treatment effect of microwave advanced oxidation technology on methyl orange dye. They also reduce reaction energy consumption and costs. However, both have their respective drawbacks. A high dosage of activated carbon fibers can lead to a minor amount of fibers or ash floating on the water surface after the fibers, treated by multiple wastewater soakings, are immersed. This can cause a certain level of contamination to the water body. Copper oxide nanowires are difficult to collect and are not practical for future applications. In practical use, some of the copper oxide dissolves into the solution, resulting in secondary water pollution. (Wang 2019; Peng 2019).Furthermore, when comparing energy consumption and treatment efficacy, both activated carbon fiber and copper oxide exhibit certain levels of consumption. This renders them unsuitable for future practical applications. However, employing a composite material of copper oxide nanowires on activated carbon fiber (ACF/CuO) could reduce catalyst consumption and prevent secondary water pollution, all while maintaining catalytic effectiveness. Additionally, exploring the combined effect of this composite material with microwave advanced oxidation technology could lead to the production of sulfate radicals, which are effective in treating methyl orange dye wastewater.
In this paper, methyl orange dye wastewater was used as the target pollutant. Potassium persulfate served as the oxidant, andACF/CuO was employed as the catalyst. The study investigated the effects of the microwave-potassium persulfate-ACF/CuO system, optimized its process parameters, and analyzed the catalytic mechanism of ACF/CuO nanocomposites. Additionally, the degradation process of methyl orange was examined, providing a foundation for the practical application of the microwave-potassium persulfate-ACF/CuO system in treating dye wastewater.
1.1 Materials.
Methyl orange, potassium persulfate (K2S2O8), Oxone, activated carbon fiber, copper chloride, polyethylene glycol 20000, polyvinylpyrrolidone (PVP), polyvinyl alcohol, ammonia, sodium hydroxide, anhydrous ethanol, and all other reagents used in the experiment were of analytical reagent (AR) grade, (sourced from Sinopharm Chemical Reagent Co., Ltd.
The ultraviolet-visible photometer T6 (manufactured by Beijing Pulse Analytical General Instrument Co., Ltd.), the ultrasonic reactor SK7210LHC (produced by Shanghai Kedao Ultrasonic Instrument Co., Ltd.), the X-ray polycrystal diffractometer (from Rigaku Denki, Japan), the scanning electron microscope SEM515 (from Philips Co., Ltd.), the electronic balance (from Shanghai Jingke Co., Ltd.), and the microwave oven (a modified Midea microwave oven).
The diagram of the modified microwave equipment is shown in Figure 1. The PTFE reactor is positioned on the support table within the resonance cavity of the microwave oven, ensuring it avoids the metal guide tube. A hole is punctured at the top of the microwave oven to accommodate the insertion of a glass tube, which is sealed with a sealant at the interface to prevent microwave leakage. Connected to the reactor within the microwave resonance cavity are buffer bottles and wash bottles. The dimensions of the resonance cavity are 340 mm by 320 mm by 220 mm, and the PTFE container has a diameter of 145 mm at the base and a height of 195 mm. The rotating fan at the top of the microwave oven alters the propagation direction of the microwaves, ensuring that the water samples in the reactor are uniformly irradiated.
1.2 Experimental
1.2.1 Preparation of activated carbon fiber loaded with copper oxide
Pre-treatment of activated carbon fiber: The activated carbon fiber was cut into small squares measuring 1 cm by 1 cm and soaked in alcohol for 24 hours to remove oil stains from its surface. It was then subjected to ultrasonic cleaning with ultra-pure water several times to wash away any residual alcohol. The activated carbon fiber was subsequently placed in ultra-pure water, heated to boiling, and this process was repeated five times to eliminate ash and weak fibers from within. The treated fibers were then placed in an oven at 60℃ for 12 hours.
Sensitizing solution preparation: The ratio of stannous chloride to hydrochloric acid was set at 30 g/L to 150 ml/L. The treated activated carbon fibers were immersed in the sensitizing solution for 12 hours and ultrasonically cleaned with ultra-pure water several times to remove any remaining sensitizing solution. The fibers were then placed back in the oven and heated at 60℃ for another 12 hours.
Activation solution preparation: The ratio of silver nitrate to ammonia was set at 5 g/L to 150 ml/L. The treated activated carbon fibers were immersed in the activation solution for 12 hours and ultrasonically cleaned with ultra-pure water several times to wash away any residual sensitizing solution. Finally, the treated fibers were placed in an oven and heated at 60℃ for 12 hours.
Activated carbon fiber loaded with copper source: Begin with one liter of deionized water, to which add 10 grams of anhydrous copper sulfate, 10 grams of anhydrous sodium carbonate, 15 mL of formaldehyde, 40 mL of a sodium potassium tartrate solution, and 10 mg of 2,2'-bipyridine. Introduce the treated activated carbon fiber into the mixture, then adjust the pH to 12 using hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions. Ultrasonicate for 10 minutes, followed by electrical stirring for 24 hours at room temperature. After removing the fibers, wash the solution several times with ultra-pure water and dry in an oven at 60°C for 12 hours. Subsequently, dry the solution in an oven at 60°C for an additional 12 hours.
Preparation of activated carbon fiber loaded with copper oxide (ACF/CuO): Remove the dried activated carbon fiber with the loaded copper source, place it in a drying dish, and allow the activated carbon fiber to cool to room temperature. Then, lay the activated carbon fiber flat to ensure full contact with the air's oxygen. Subsequently, place it in an atmosphere furnace at 300°C, calcinate for 2 hours, cool, and store it in the drying dish.
1.2.2 Catalytic experimental methods
The experiment was conducted in a 250 mL special conical flask, adjusting the microwave power from 0 W to 1000 W, the concentration of the activated carbon fiber loaded copper oxide material from 1 g/L to 25 g/L, adding 100 mL of a 100 mg/L methyl orange solution, and potassium persulfate from 0 g/L to 1 g/L. The sample was then irradiated for 0 to 3 minutes to calculate the concentration change and reaction rate of the methyl orange solution.
2.1 Analysis of micro-structural properties of ACF/CuO
2.1.1 Scanning electron microscope(SEM)
To observe the morphology and dimensions of the synthetic products, various morphologies of CuO and ACF/CuO were examined and photographed using scanning electron microscopy. Figure 2 presents the scanning electron microscope images of activated carbon fibers before (Fig. 2a) and after (Fig. 2b) preparation. The surface of the treated activated carbon fiber appears smooth, with small pointed copper oxide particles visible on its surface. This is attributed to the copper source deposited on the activated carbon fiber's surface, which, when heated to 300 ℃, reacts with the oxygen in the air. The growth of the nuclei is favored at lower temperatures, and once the nuclei are formed, they grow along the surfactant chains, resulting in the formation of copper oxide nanowires. (Zheng 2022; Yang 2023).
2.1.2 Fourier infrared spectra
Figure 3 presents the IR spectra of activated carbon fiber and ACF/CuO. Figure 3a depicts the activated carbon fiber, which exhibits a prominent absorption peak at approximately 3444.59 cm-1, primarily attributed to the -OH stretching vibration within the activated carbon fiber. The absorption peak at 1637.62 cm-1 is caused by the vibrations of C=C and C=O bonds in the activated carbon fiber, with the C=C bonds also interacting with those in the benzene ring, resulting in a peak shift to around 1600 cm-1. The absorption peak at 1398.50 cm-1 is due to the stretching vibration of -OH in the water of crystallization, along with the symmetric stretching vibration of -CO2- and the vibrational peaks of C-O in the carboxyl group and phosphate ester. At 1117.01 cm-1, there is a less pronounced absorption peak, which corresponds to the vibrations of C-OH- and C-O-CO.
Figure 3b illustrates the infrared spectra of ACF/CuO. When compared to Figure 3a, the infrared peaks at 527.17 cm-1, 1637.62 cm-1, and 3444.59 cm-1 are notably enhanced. This increase is due to the vibrational characteristic absorption peaks of Cu-O at 527.17 cm-1 and the vertical surface dangling bonds in the copper oxide crystals (on the surface of the lattice) at 1637.62 cm-1. Additionally, the telescopic vibrational peak at 3444.59 cm-1 is associated with the end-terminated bond, and the chemical bond is unsaturated due to the presence of unpaired electrons. (Ihsan 2024; Cuarán-Rosero 2024).This indicates that copper oxide nanowires have been successfully loaded onto activated carbon fibers.
2.1.3 XRD
Figure 4 presents the XRD patterns for copper oxide nanowires and ACF/CuO. Figure 4a depicts the XRD pattern of copper oxide nanowires, which was identified as pure CuO by comparison with the PDF card (05-0661). Figure 4b illustrates the XRD pattern of ACF/CuO. When compared to Figure 4a, characteristic diffraction peaks of graphitization are observed at 2θ = 18.26° and 43.63°, indicating that copper oxide crystals have been deposited onto the surface of the activated carbon fibers.(Kolahalam 2022; Jabli 2023; Prabu 2024).
2.2 Removal of methyl orange in different reaction systems
The microwave power was set at 500 W, the irradiation time was 180 seconds, the catalyst dosage was 10 g/L, the oxidant dosage was 0.6 g/L, and the pollutant concentration was 100 mg/L. The changes in methyl orange concentration within various reaction systems were examined, and the reaction rates were calculated. The experimental results are depicted in Figure 5. As can be observed from the figure, when only microwave irradiation or potassium persulfate was present in the reaction system, the concentration of methyl orange remained essentially unchanged, and the reaction rates were also low, at 0.0008 and 0.0013 s-1, respectively. In the combined microwave and potassium persulfate reaction system, the concentration of methyl orange decreased, and the reaction rate increased to 0.0064 s-1, indicating that the microwave irradiation promoted the decomposition of potassium persulfate and the generation of free radicals for the degradation of organic matter. Upon the addition of activated carbon fiber to the microwave and potassium persulfate reaction system, the efficiency of the reaction system was further enhanced, and the reaction rate reached 0.0138 s-1. When ACF/CuO was used as the catalyst, the reaction rate of the system peaked at 0.021 s-1, demonstrating that activated carbon fiber loaded with CuO exhibited a higher activation activity for potassium persulfate than pure activated carbon fiber.
2.3 Study on microwave-assisted K2S2O8-ACF/CuO synergistic catalytic oxidation system and influencing factors
2.3.1 Effect of ACF/CuO dosage
Adjust the microwave power to 500 W. Take fiber-activated carbon fiber loaded with copper oxide material at concentrations of 1 g/L, 5 g/L, 10 g/L, 15 g/L, and 20 g/L. Add 100 mL of a 100 mg/L methyl orange solution and potassium persulfate at a concentration of 0.6 g/L. Irradiate for 2 minutes, and the changes in methyl orange solution concentration are shown in Figure 6.
As illustrated in Figure 6, the absorbance of the methyl orange solution gradually decreased with the progressive increase in the dosage of ACF/CuO. When the ACF/CuO dosage reached 10 g/L, the removal rate of methyl orange increased from 55% at 1 g/L to 93%. Upon further increasing the ACF/CuO dosage, the concentration of the methyl orange solution continued to decrease slowly, but the removal rate of methyl orange remained essentially constant. Consequently, the optimal dosage of ACF/CuO was determined to be 10 g/L.
2.3.2 Effect of potassium persulfate dosage
Adjust the microwave power to 500 W, add 10 g/L of fiber-activated carbon fiber loaded with copper oxide material, and 100 ml of a methyl orange solution with a concentration of 100 mg/L. Then, introduce potassium persulfate at concentrations of 0.2 g/L, 0.4 g/L, 0.6 g/L, 0.8 g/L, and 1 g/L. After irradiating for 3 minutes, observe the change in the reaction rate of the system, as depicted in Figure 7.
As illustrated in Figure 7, the removal rate of methyl orange in the microwave-potassium persulfate-ACF/CuO system progressively increased with the incremental addition of potassium persulfate. Upon reaching a dosage of 0.6 g/L, the removal rate peaked at 93.2%. At a dosage of 0.8 g/L, the removal rate of methyl orange further increased to 95.4%. However, when the potassium persulfate dosage was elevated to 1 g/L, the removal rate unexpectedly dropped to 90%. This decline was attributed to the self-quenching reaction of excessive potassium persulfate, which decreased the concentration of free radicals in the system, thereby reducing the pollutant removal rate (Peng, 2024). Consequently, the optimal dosage of potassium persulfate was determined to be 0.6 g/L.
2.3.3 Effect of microwave power
The microwave power was adjusted to 100 W, 300 W, 500 W, 800 W, and 1000 W. A sample of 10 g/L of fiber activated carbon fiber loaded with copper oxide was taken and added to 100 ml of a solution containing 100 mg/L of methyl orange. Potassium persulfate was dosed at 0.6 g/L, and the solution was then irradiated for 120 seconds. The absorbance change of the methyl orange solution was observed and is depicted in Fig. 8.
As illustrated in Figure 8, the reaction rate of the microwave-ACF/CuO-potassium persulfate system progressively increased with the incremental rise in microwave power. When the microwave power reached 500 W, the reaction rate climbed from 0.005 at 100 W to 0.021 s-1. However, upon increasing the microwave power from 500 W to 1000 W, the reaction rate only increased to 0.024 s-1. Consequently, the optimal microwave power for this reaction system was determined to be 500 W.
2.3.4 Reutilization experiment of ACF/CuO
Set the microwave power to 500 W, the radiation time to 2.5 minutes, the potassium persulfate dosage to 0.6 g/L, and the ACF/CuO dosage to 10 g/L. The experiment was repeated 10 times under the same conditions. According to the changes in the decolorization rate of methyl orange, as shown in Figure 9, it can be observed that with an increase in the number of tests, the removal rate of the methyl orange solution consistently reached 100% after ten reuses. This is because, during the reaction, ACF/CuO acts solely as a catalyst, utilizing its own hotspots to promote the production of SO4- from persulfate without any consumption. Consequently, as a continuous catalyst in microwave advanced oxidation reactions, ACF/CuO not only reduces the energy consumption and cost of the reaction but is also reusable. The experiment confirms that ACF/CuO is an effective and environmentally friendly catalyst.
2.4 Analysis of the mechanism in the microwave-potassium persulfate-ACF/CuO system
In the catalytic system, two types of free radicals are generated: sulfate radicals and hydroxyl radicals. These free radicals can efficiently degrade organic matter in water. Therefore, it is crucial to ascertain the role of each free radical within the catalytic system to explore the catalytic mechanism and subsequently achieve micro-control of the catalytic reaction. During the experiments, tert-butanol and ethanol were introduced into the reaction system under optimal conditions. The reaction rates of ethanol with sulfate and hydroxyl radicals were similar, whereas the reaction rate of tert-butanol with hydroxyl radicals was significantly higher than with sulfate radicals (Xie, 2023; Liao, 2024). Consequently, by separately adding ethanol and tert-butanol to the reaction system, it was possible to determine which free radical played a more significant role in methyl orange degradation based on their impact on the degradation rate of methyl orange. The experimental results, depicted in Fig. 10, indicate that the addition of ethanol resulted in a dramatic decrease in methyl orange degradation efficiency. In contrast, the addition of tert-butanol did not lead to a significant change in the degradation of methyl orange compared to when no trapping agent was introduced.
The results above indicate that in the microwave-potassium persulfate-ACF/CuO system, sulfate radicals primarily drive the degradation process, while hydroxyl radicals contribute only minimally. Furthermore, it is evident that the free radicals produced within the system are predominantly sulfate radicals.
2.5 Analysis of methyl orange degradation and transformation rules in microwave-potassium persulfate-ACF/CuO system
2.5.1 Analysis of methyl orange degradation process by high performance liquid chromatography-mass spectrometry
Under optimal process conditions, raw and treated water samples were taken, filtered, and diluted 50 times. The treatment results of methyl orange solution were analyzed using high-performance liquid chromatography, with an injection volume of 5 μL. The mobile phase consisted of a V(methanol):V(water) ratio of 50:50, and the separation was performed using a Shimadzu C18 column (4.6×150 mm) at a flow rate of 1.0 ml/min and a column temperature of 30 ℃. The test results are presented in Figure 11.
Figure 11 presents the high-performance liquid chromatograms of methyl orange aqueous samples at wavelengths of 461 nm, 365 nm, and 248 nm at various times. Figure 11(a) illustrates that the retention time for methyl orange was 2.93 minutes at a wavelength of 461 nm, and the absorption peak of methyl orange diminished rapidly as time progressed. This suggests that the chromophore group was compromised and methyl orange underwent degradation. Figure 11(b) indicates that the organic compounds at 283 nm, with a retention time of 2.8 minutes, initially increased and then gradually decreased. This implies that organic compounds with a co-chromophoric structure were formed during the degradation of methyl orange, and these intermediates were progressively destroyed as treatment time increased. Figure 11(c) reveals a new absorption peak at 248 nm, signifying that organic intermediates with a benzene ring structure emerged during the degradation process. As time continued, these intermediates were also degraded, leading to the complete breakdown of methyl orange.
2.5.2 Analysis of methyl orange degradation process
During the degradation of methyl orange, a series of intermediate products are produced. The generation of these intermediate products can effectively reveal the degradation process of methyl orange and can also reflect the reaction mechanism of microwave advanced catalytic oxidation technology in degrading methyl orange. Further analysis of the methyl orange solution treated by microwave advanced catalytic oxidation technology using GC-MS and comparison with the standard substances in the NIST spectral library revealed that the intermediate products were determined as shown in Table 1.
As indicated in Table 1, under the advanced catalytic oxidation of methyl orange by microwave, initially, the double bond is cleaved to produce benzenesulfonic acid and p-nitrophenol (Hojjati-Najafabadi 2024; Maravilla Jr 2024). Subsequently, benzenesulfonic acid and p-nitrophenol undergo gradual degradation, transforming into a benzene ring and phenol. Through oxidation by sulfate radicals, portions of the benzene ring and phenol experience chain cleavage, resulting in the formation of maleic anhydride. Ultimately, both the benzene ring and phenol, along with the produced maleic anhydride, are degraded into water and carbon dioxide. Based on this analysis, the potential degradation pathways of methyl orange solution are depicted in Figure 12.
2.5.3 Water quality analysis of methyl orange water samples before and after treatment
COD, TOC, and copper concentrations dissolved in the methyl orange solution were examined both before and after treatment, with the results presented in Table 2.
As indicated in Table 2, the removal rates of COD and TOC before and after treatment were 89.65% and 72.36%, respectively. These results suggest that microwave-potassium persulfate-ACF/CuO effectively degrades methyl orange solutions. Additionally, the concentration of copper ions in the treated solution was 0.005 g/L, demonstrating that activated carbon fiber loaded with copper oxide can effectively address the issue of copper oxide dissolution.
(1) Scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction (XRD) analyses confirmed the successful preparation of activated carbon fiber loaded with copper oxide. Scanning electron microscopy revealed that the smooth surface of the activated carbon fiber was uniformly coated with copper oxide fibers after pretreatment, immersion, and heat treatment. Infrared spectroscopy and XRD results indicated the presence of both activated carbon fiber and copper oxide components.
(2) The dosage of ACF/CuO, potassium persulfate, microwave power, and radiation time all significantly affect the decolorization rate of methyl orange solutions. The optimal experimental conditions were determined to be a microwave power of 500 W, a radiation time of 2 minutes, a potassium persulfate dosage of 0.6 g/L, and an ACF/CuO dosage of 10 g/L, achieving a decolorization rate of 93% for methyl orange solutions. Furthermore, ACF/CuO maintained a high decolorization rate even after being reused 10 times.
(3) Experiments involving the injection of free radical trapping agents revealed that in the microwave-potassium persulfate-ACF/CuO system, sulfate radicals are primarily responsible for the degradation process, with minimal generation of hydroxyl radicals during the catalytic reaction.
(4) The potential degradation mechanism of methyl orange under microwave advanced catalytic oxidation was elucidated using liquid chromatography and gas chromatography-mass spectrometry (GC-MS) analyses. Testing of actual water samples showed that the removal rates for chemical oxygen demand (COD) and total organic carbon (TOC) were 89.65% and 72.36%, respectively. Additionally, the concentration of copper ions in the treated solution was 0.005 g/L, suggesting that activated carbon fiber loaded with copper oxide effectively addresses the issue of copper oxide dissolution.
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Funding
This work was supported by the the Natural Science Foundation of Chongqing, China (Grant No. CSTB2023NSCQ-MSX0277) and the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJQN202212901).
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Competing interests
The authors declare no competing interests.
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