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Article

Synthesis of Fe(III)-g-C3N4 and Applications of Synergistic Catalyzed PMS with Mn(VII) for Methylene Blue Degradation

by
Lin Li
1,
Huangling Gu
1,
Qiong Wang
1,*,
Meiyin Chen
1,
Wenjing Ma
1 and
Hongwei Zhang
2
1
School of Urban and Environment, Hunan University of Technology, Zhuzhou 412000, China
2
School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(6), 2364; https://doi.org/10.3390/su16062364
Submission received: 4 February 2024 / Revised: 1 March 2024 / Accepted: 11 March 2024 / Published: 13 March 2024
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Abstract

:
Refractory organic pollutants pose a great threat to public health in water bodies due to their toxicity and low biodegradability. Developing a method of activating persulfate efficiently and in an environmentally friendly way has become a popular topic of research in current advanced oxidation water treatment technologies. Fe(III)-g-C3N4 was prepared by the calcination method. Fe(III) was anchored on the framework of g-C3N4. The characterization analysis indicated that Fe(III) was successfully loaded on g-C3N4. The best effect for MB degradation was Fe(III)-g-C3N4 (0.1 g/L) dosed with 30 µmol/L KMnO4 for synergistic catalyzed PMS (0.1 g/L), where the degradation rate could reach 95.4%. The optimum temperature for MB degradation was determined to be 10 °C. The optimum pH range of Fe(III)-g-C3N4/Mn(VII) synergistic catalyzed PMS for MB degradation was pH 4.4–6.6 under acidic conditions, and the optimum pH range for MB degradation was pH 8–10 under alkaline conditions. The Fe(III)-g-C3N4/Mn(VII) synergistic catalyzed PMS system was also tested for the degradation of methyl orange and rhodamine b, and good degradation results were obtained with the degradation rates of 87.37% and 84%, respectively. It facilitates the reduction in pollutant emissions, improves water quality and will have a positive impact on the sustainability of the environment.

1. Introduction

The construction of an ecological civilization in the new era of China is an important strategic initiative for promoting sustainable development, and it is also an important component of global ecological civilization construction [1,2]. During the period of the 14th Five-Year Plan, the government will improve the prevention and control of environmental pollution with higher standards and continue to improve the pollution control of key industries in the classification and treatment of water pollution. In recent years, the pollution of water bodies by recalcitrant organic pollutants (ROPs) has become a serious problem [3]. ROPs are widely present in the wastewater of industries such as chemical engineering, printing and dyeing, pharmaceuticals, and papermaking, generally including polycyclic aromatic hydrocarbons, halogenated hydrocarbons, organic phosphorus pesticides, surfactants, organic dyes, and other organic compounds [4,5]. These substances have characteristics such as persistence, high toxicity, bioaccumulation and semi-volatility. Many toxic and recalcitrant organic pollutants cannot be removed by conventional treatment methods [6,7]. Therefore, the development of economically effective ROPs control and treatment technologies is of great significance.
An effective method of degrading ROPs through redox reactions is considered to be the Advanced Oxidation Process (AOP) [8,9]. Among them, the advanced oxidation processes based on the sulfate radicals (SR-AOP) can generate highly oxidative sulfate radicals (SO4−·) and hydroxyl radicals (HO·), which can degrade ROPs rapidly and efficiently, and thus has attracted much attention [10,11].
However, persulfate can generate highly oxidative sulfate radicals (SO4−·) from various activating factors such as oxidants, heat, strong alkalis, transition metal ions, etc. If persulfate is not activated, the persulfate anion will react with some organic chemicals [12,13]. However, the degradation effect is much less than that in the form of free radicals after activation [14]. Therefore, the activator of persulfate and its properties will have a crucial role in pollutant degradation. Among the various activators, the transition metal activation method is uncomplicated and economical in the formulation of the reaction system.
Among the many available activating metal ions, iron is widely used for catalyzing the degradation of various pollutants by persulfate due to its non-toxicity and its abundance, cheapness and ease of production [15,16,17,18,19,20]. Currently, there is a growing interest in loading transition metal ions onto nanomaterials [21,22]. On the one hand, these transition metal ions can activate persulfate through single electron transfer, and on the other hand, the surface of nanomaterials provides numerous active sites for the activation reaction, which are often centered on the transition metal ions as the center of the active reaction [23,24]. As a new type of non-metallic polymer semiconductor with a two-dimensional graphene-like structure and superior electronic properties, g-C3N4 is an excellent carrier with the advantages of structural stability, abundant binding sites, non-toxicity, acid and alkali resistance, and cost-effectiveness. In addition, g-C3N4 may have some unique properties that make it more advantageous for specific applications. For example, it may have better electrical conductivity, larger specific surface area, higher chemical stability or better interaction with the target substance. In addition, g-C3N4 may be easier to chemically modify or functionalize. g-C3N4 has a high density of nitrogen atoms as well as special nano-cavities that provide a large number of lone-pair electrons for the formation of Fe-N-C, which leads to iron stabilization [25,26].
In addition, permanganate (Mn(VII)) is one of the common oxidants with as selective oxidizing power for organic pollutants [27,28]. Mn(VII) has a high reduction potential compared to other chemical oxidants and is attractive because of its relative stability, ease of handling, low cost, and lack of formation of chlorination or bromination by-products [29]. Moreover, the reduction products of Mn(VII) are environmentally benign and can be used as coagulants to improve the performance of the coagulation/filtration process or as adsorbents or oxidizers to facilitate the removal of pollutants [30,31,32]. There are few studies on activating persulfate using Mn(VII) combined with Fe(III) [33,34]. Related studies found synergistic effects of iron and manganese bimetallic systems in hydrogen peroxide activation [35,36,37]. The role of Mn(VII) may vary depending on factors such as catalysts, reaction conditions and pollutants [38,39,40]. Therefore, experiments about the optimum dosage and reaction conditions are needed to discover the best degradation effect and catalyst stability [41,42].
In the preliminary study, our project team initially constructed a non-homogeneous system Fe(III)-g-C3N4/Mn(VII) to catalyze peroxymonosulfate (PMS) and peroxodisulfate (PDS), respectively, and compared the removal effect on methylene blue (MB); the results showed that the degradation of MB by PMS which catalyzed by Fe(III)-g-C3N4/Mn(VII) was more significant. Therefore, in this paper, composite catalysts of Fe(III)-g-C3N4 were prepared by the calcination method and experiments were designed to verify the synergistic catalytic performance of Fe(III)-g-C3N4/KMnO4 synergistic catalytic PMS in the degradation of MB. At the same time, the influencing factors such as the Fe(III)/Mn(VII) dosing ratios, temperature and pH were investigated. It was tested for the degradation of other difficult-to-degrade organic compounds. This work was expected to provide an environmentally friendly strategy in the field of catalytic degradation of ROPs based on the above studies. It also facilitates the reduction in pollutant emissions, improves water quality and will have a positive impact on the sustainability of the environment.

2. Materials and Methods

2.1. Materials of Test

2.1.1. Reagents

The main experimental reagents that will be used are shown in Table 1.

2.1.2. Instruments and Equipment

In Table 2, the main instruments and equipment that will be used in the experiment are listed.

2.2. Synthesis of Fe(III)-g-C3N4

The Fe(III)-g-C3N4 (denoted as FeCN) was prepared as follows. A total of 1 g of anhydrous ferric chloride and 25 g of melamine were accurately weighed, placed in a mortar, and grinded for 40 min so that the anhydrous ferric chloride and melamine were completely mixed. Then, the mixture was put into a crucible and heated at 575 °C for 4 h. After the muffle furnace was turned off, the temperature was allowed to return naturally to room temperature and then the mixture sample was ground to powder [43]. The powder samples were washed three times with deionized water in a high-speed centrifuge at 10,000 rpm, then dried and evaporated at 80 °C. The powder samples of FeCN were obtained and then stored for use. The overall process for the preparation of FeCN is illustrated in Scheme 1.

2.3. Characterization Methods

All characterizations were carried out on the FeCN catalyst. The crystalline phase and the surface morphology of the FeCN were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM); the valence state of the Fe element on the surface of FeCN was analyzed by X-ray photoelectron spectroscopy (XPS). During the reaction, the mixture samples were collected and characterized with a UV-Vis spectrophotometer for degradation characterization.

2.4. Redox Catalytic Activity and Degradation Experiment of Methylene Blue

In order to find the best combination of FeCN synergistic KmnO4-activated peroxymonosulfate degradation of methylene blue with the best efficiency, 30 mg/L methylene blue was configured as a simulated wastewater using character water, and the absorbance were measured and analyzed by designing different sets of experiments. The experimental steps were as follows: (1) Configure 30 mg/L solution of methylene blue in a 500 mL beaker using character water. (2) Add 0.1 g/L of FeCN powder to the solution, and characterize on a magnetic stirrer for 1 h. (3) Determine the zero point specimen and measure its absorbance. (4) Weigh different combinations of oxidants into the beaker and start timing while adding. (5) Samples were taken at 30 s intervals during the reaction and diluted three times with character water. (6) The samples were filtered and the absorbance was measured. The different combinations of oxidants were grouped as follows: ① 0.1 g/L potassium persulfate; ② 20 µmol/L KmnO4; ③ 0.1 g/L potassium persulfate and 20 µmol/L KmnO4.

2.5. Effect of Different Factors on Catalysis

The influencing factors such as the reaction temperature and pH were investigated. Six sets of different reaction temperatures (10 °C, 15 °C, 20 °C, 30 °C, 40 °C and 50 °C) were set. Based on the results of methylene blue residual rate, degradation rate and kinetic fitting analysis of methylene blue degradation effect under different temperature, the optimum temperature for FeCN/Mn(VII) co-catalyzed PMS was explored. The pH of the reaction solution was adjusted using HCl and NaOH and based on the results of methylene blue residual rate, degradation rate and the kinetic fitting analyses at different pH, the optimum pH or range for the FeCN/Mn(VII) co-catalyzed PMS was explored.

2.6. Degradation Effection for Other Pollutants

To test the degradation effect of the FeCN/Mn(VII) synergistic catalyzed PMS system on other persistent organic pollutants, methyl orange and rhodamine B were selected as the target substrates with reference to the methylene blue degradation experiment. The optimal absorption wavelengths of methyl orange solution (0.05 g/L) and rhodamine b solution (0.03 g/L) were finally determined to be 464 nm and 554 nm, respectively.

2.6.1. The Degradation of Methyl Orange

(1) Prepare 20mg/L methyl orange solution; (2) Add catalyst and stir: weigh 0.1 g/L FCN catalyst, add into the conical flask and stir for 1 h, take 4 mL of the sample as the zero point sample; (3) Add oxidant: weigh an appropriate amount of potassium permanganate and potassium persulfate, add to the conical flask and start timing; (4) Timed sampling: take 4 mL of sample every 1 min, filter and measure the absorbance at 464 nm by spectrophotometer.

2.6.2. The Degradation of Rhodamine B

(1) Prepare 10 mg/L rhodamine b solution; (2) Add catalyst and stir: weigh 0.1 g/L FCN catalyst, add into the conical flask and stir for 1 h, take 4 mL of sample as the zero point sample; (3) Add oxidant: weigh an appropriate amount of potassium permanganate and potassium persulfate, add into the conical flask and start timing; (4) Timed sampling: take 4 mL of sample every 3min, filter and measure the absorbance at 554 nm by spectrophotometer.
The concentrations of potassium permanganate and potassium persulfate were obtained from the results of the previous steps. The concentrations, residual ratios and degradation rates of the samples were calculated and the corresponding kinetic fit analyses were performed.

3. Results

3.1. Characteristics of Structure and Morphology

The morphologies of the catalysts as prepared were characterized by means of the SEM technique (Figure 1). Figure 1a,b showed the morphological structure of the composite samples. The results show that the structure of the samples has a typical g-C3N4 lamellar stacking structure, and the porous structure of the surface increases the surface area of the load (Figure 1a), which is favorable for Fe loading. The FeCN samples’ structure has a greater disorder and is looser than that of g-C3N4. Fe particles are embedded between the g-C3N4 layers (Figure 1b). The as-prepared catalyst was also named FeCN. Figure 1c showed that Fe(III) modification has no effect on the overall structure of the sample. The EDS elemental mapping results in Figure 1d showed that C, N, O and Fe elements are distributed in FeCN samples, with 1.82 wt% Fe distributed in FeCN. Figure 1e showed the results of element mapping; it was indicated that the element Fe is uniformly distributed in the sample and the nanocomposites have been successfully prepared.
In order to understand the phase composition and the crystal structure of g-C3N4 and FeCN, the prepared samples were analyzed by XRD, and the results were as shown in Figure 2. There are two crystalline facet characteristic peaks of g-C3N4 that can be seen from the XRD patterns of the g-C3N4, corresponding to the (100) and (002) crystalline planes, respectively. The strong diffraction peak at 27.8° corresponds to the (002) crystallographic plane and it is attributed to the interlayer stacking of conjugated aromatic rings; the weaker diffraction peak at 12.9° corresponds to the (100) crystallographic plane and corresponds to in-plane reflection. The two peaks are in agreement with the classical crystallographic phase of g-C3N4 [44]. The I(100)/I(002) value of g-C3N4 sample is 0.163, while the I(100)/I(002) value of FeCN sample is 0.059. Based on these data, it can be assumed that the FeCN sample is more disordered than g-C3N4 [45].
However, no diffraction peaks of Fe oxide are observed on FeCN (blue line); this suggests that Fe may be chemically coordinated to the main body of g-C3N4 in the form of Fe-N bonds. If this is the case, the intensity of the XRD peak of FeCN will be smaller than that of g-C3N4, because the doping of iron destroys the structure of g-C3N4. In addition, the local magnification of the (002) crystal plane shows that the corresponding diffraction peaks are slightly shifted to a higher diffraction angle and the intensity of the diffraction peaks decreases, which suggests that Fe has been successfully introduced into the structure of g-C3N4 and agrees with the theory. No other stray peaks appear in the figure; it showed that the purity of the FeCN composites we obtained was high.

3.2. X-ray Photoelectron Spectroscopy Analysis

In order to observe the chemical states of g-C3N4 and FeCN, the X-ray photoelectron spectroscopy was carried out and the results are shown in Figure 3. As shown in Figure 3a, the signals of C, N, and O of FeCN samples before and after the reaction (freshed and used) were observed in the spectroscopic investigation. However, due to the low iron loading, only weak iron signals were observed in the samples. As shown in Figure 3b, no chemical shift occurred in the C1s spectrum after iron doping. Following charge calibration, the characteristic peak of C1s at 284.8 eV is attributed to adsorbed amorphous carbon or sp3 graphite carbon formed during polymerization processes on the surface. The binding energy at 285.3 eV belongs to the sp2 hybridized carbon in N−C=N. The peak at 395.7 eV in Figure 3c is assigned to the hybridized aromatic nitrogen atom (C-N=C). The peak at 398.1 eV represents the tertiary nitrogen group (N-(C)3 or C-NH-C). In addition, the broad peak at 401.7 eV is due to π-π* excitations between stacked interlayers [46].
Furthermore, all post-reaction samples were collected for characterization during the experimental process. Figure 3a shows the detected Mn2p and S2p peaks on the surface, suggesting the involvement of Mn ions in redox reactions, with some S elements also detected. Figure 3d showed the presence form of Fe element, located at the 711.48/724.07 eV binding energy position which is the characteristic signal peak of Fe3+, corresponding to the content of 76.42 at% which indicates that the Fe element in this sample is dominated by Fe3+. The 718.16/732.99 eV binding energy position at the satellite peaks of Fe3+ 2p3/2 and Fe3+ 2p1/2 further proves the presence of Fe3+ in the sample. These results indicated that Fe(III) successfully coordinates with N atoms to form Fe(III)-N, which is in agreement with the XRD analysis.

3.3. Redox Catalytic Activity and Degradation Experiment of MB

The redox catalyst degradation of MB by FeCN was investigated. The catalytic efficiency of three different combinations with the FeCN catalyst were investigated for the degradation of MB (30 mg/L). The results were as seen in Figure 4a; the concentration of MB was reduced to different degrees after the reaction of FeCN with different combinations. From Figure 4b it indicated that the lowest degradation rate was the combination of FecN + KMnO4, but due to the oxidative degradation of organic pollutants by KMnO4 itself, the concentration of MB still decreased and the degradation rate was 22.9%. The concentration of MB was greatly reduced in 30 s in the other two combinations; the combination of FeCN + PMS was 81.1%, and the degradation rate of FecN + KMnO4 + PMS combination was 91.9%, which showed that the non-homogeneous Fe(III)-g-C3N4/Mn(VII) catalysts have a better activation effect on PMS and a higher efficiency in degrading MB. Mn(IV) helps to improve the co-catalytic performance of Fe(III)-g-C3N4 on PMS. In Figure 4c, the pseudo-first-order constants (Kobs) for MB removal in the three combinations were 0.4056 min−1 (FeCN + PMS), 0.0589 min−1 (FecN + KMnO4), and 0.5950 min−1 (FecN + KMnO4 + PMS), respectively, which indicated that KMnO4 could be an excellent co-catalyst for Fe(III)/PMS system.

3.4. Degradation of MB by Different Fe(III)/Mn(VII) Ratios

In order to obtain the optimal Fe(III)/Mn(VII) ratios, the experiment was designed with different concentrations of KMnO4, and the results are shown in Figure 5a. The degradation of MB was mainly rapid within 30 s, after which the reaction gradually leveled off. Figure 5b shows that with different concentrations of KMnO4, the concentrations of MB were all significantly decreased. Among them the highest degradation rate reached 95.4% when using 30 µmol/L KMnO4. When the concentration of KMnO4 was further increased, the MB degradation rate decreased instead. The promotion effect of Mn(VII) was no longer increased when a certain concentration was reached. In Figure 5c it also showed the pseudo-first-order constants (Kobs) for MB removal. An amount of 30 µmol/L KMnO4 is most favorable for the degradation of MB which enhanced the co-catalytic capacity of FeCN synergistically, and the Kobs reached 0.7304 min−1.

3.5. Effect of Different Factors on Catalysis

3.5.1. Different Reaction Temperature

Different temperature affects the degradation efficiency. An appropriate range of reaction temperatures must be determined based on the economics of practical applications. The results of the effect of different temperatures (10 °C, 15 °C, 20 °C, 30 °C, 40 °C, 50 °C) on the catalytic reaction efficiency were shown in Figure 6a. The degradations of MB were different at different temperatures and the degradation rate gradually decreased with the increase in temperature, which showed that high temperature was not conducive to the degradation of MB by FeCN/Mn(VII) co-catalyzed PMS. The reaction was carried out rapidly within 30 s at 10 °C, 15 °C and 20 °C, and the concentration of MB was greatly reduced, with the fastest reaction and the highest degradation rate reaching 95% at 10 °C. According to the linear relationship between ln(C/C0), the kinetics were fitted; the pseudo−first−order constants (Kobs) for MB removal are shown in Figure 6b. Kobs of the six different temperatures were 0.06578 min−1, 0.6327 min−1, 0.6233 min−1, 0.5437 min−1, 0.5064 min−1, 0.5159 min−1, respectively. Therefore, it also proved that 10 °C was the optimum temperature in theory. Since the difference in degradation rates between the three temperatures (10 °C, 15 °C and 20 °C) was relatively small, a higher MB degradation rate could be achieved by carrying out the degradation process in the range of temperatures (10−20 °C) for saving energy.

3.5.2. pH

Different surface charge properties would be exerted in different pH values during the catalytic degradation process which would then affect the catalytic performance for the degradation. Therefore, different pH values (pH = 0.7, 1.5, 3, 4.4, 5.8, 6.6; pH = 8, 9.3, 10, 11, 12, 13.3) were considered to study the effect on the catalytic degradation of MB (30 mg/L), and the results are shown in Figure 7. With increasing pH under acidic conditions (pH = 0.7, 1.5, 3, 4.4, 5.8, 6.6), the acidity of the solution gradually weakened, and the degradation rate of MB gradually increased. According to the residual ratio in Figure 7a, it can be seen that the MB degradation rate was very low at pH 0.7, at only 37.9%, and the reaction basically ended in 30 s, with weak subsequent degradation. The reaction proceeded slowly at pH 1.5, where the MB concentration reached a significant decrease in 30 s for all the cases of pH 3–6.6, and the degradation rate reached a maximum of 91% when the acidity approached neutrality. As the pH of the original MB solution was alkaline, acidity would affect the adsorption force of MB, thus affecting the final decolorization efficiency, so the degradation effect of MB decreased as acidity was increased. It can be seen that the strong acid is unfavorable for the degradation of MB, and the weak acid environment of partial neutrality is more conducive to the co−catalysis of PMS by the FeCN/Mn(VII) for the MB degradation. The pseudo-first-order constants (Kobs) for MB removal are shown in Figure 7b. It also indicated that the optimum pH range for MB degradation was pH 4.4–6.6 under acidic conditions. And the Kobs were 0.5942 min−1 (pH 5.7), 0.5638 min−1 (pH 6.6).
With increasing pH under alkaline conditions (pH = 8, 9.3, 10, 11, 12, 13.3), the degradation rate of MB gradually decreased. According to the residual ratio in Figure 7c, it can be seen that the MB degradation rate was only 26.8% at pH 13.3, and the reaction basically ended in 30s, with weak subsequent degradation. The reactions proceeded slowly at pH 11–12; the MB concentration all reached a significant decrease in 30 s at pH 8–10, and the biggest degradation rate increased to 95.4% when the alkalinity approached neutrality. When the alkalinity of the solution was increased, Fe3+ and Mn7+ would precipitate, which in turn led to a decrease in the degradation of MB. It can be seen that the strong alkali is unfavorable for the reaction of MB degradation to proceed, and the weak alkaline environment of partial neutrality is more conducive to the co-catalysis of PMS by the FeCN/Mn(VII) for the MB degradation. The Kobs for MB removal was shown in Figure 7d. It also indicated that the optimum pH range for MB degradation was pH 8–10 under alkaline conditions. And the Kobs were 0.7104 min−1 (pH 8), 0.6276 min−1 (pH 9.3), 0.6404min−1 (pH 10).

3.6. Degradation Effection for Other Pollutants

As can be seen from Figure 8a, the degradation curve of the FeCN/Mn(VII) co-activated PMS system for the degradation of methyl orange decreases rapidly in the first 4 min when the reaction is proceeding rapidly. After 4 min, the curve tends to flatten but still decreases. At this time, the reaction rate is very slow, but the reaction is still occurring. When the reaction reached equilibrium after 7 min, the final degradation rate of methyl orange was 87.37%.
As can be seen from Figure 8b, the slope of the degradation curve of the FeCN/Mn(VII) co-activated PMS system for the degradation of rhodamine b is greater in the first 12 min when the degradation reaction rate is faster. The slope of the curve decreased 12 min after the start of the reaction, at which time the reaction rate was very slow, although the reaction was still ongoing. After 30 min since the start of the reaction, the degradation rate of rhodamine b was 84%.
In summary, it can be concluded that the FeCN/Mn(VII) co-activated PMS system has a good degradation effect on a variety of difficult-to-degrade organic pollutants; the reaction rate is fast at first, and then slows until the final reaction to reach equilibrium. Most of the degradation reaction in 10 min that has been close to the reaction is complete, the final degradation rate is more than 80%, the treatment effect is good, and it can be used to deal with dyestuff wastewater.

4. Conclusions

In summary, a Fe(III)-g-C3N4 composite catalyst was constructed for the degradation of methylene blue. The characterization analysis indicated that Fe(III) was successfully loaded on g-C3N4. The best combination for MB degradation was Fe(III)-g-C3N4 + KMnO4 synergistic catalyzed PMS. By adjusting the dosage of KMnO4, the best reaction condition was 0.1 g/L Fe(III)-g-C3N4 dosed with 30 µmol/L KMnO4 to synergistically catalyzed 0.1 g/L PMS for the degradation of 30 mg/L methylene blue, and the removal rate could reach 95.4%. The optimum temperature for methylene blue degradation was determined to be 10 °C. The optimum pH range of Fe(III)-g-C3N4/Mn(VII) synergistic catalyzed of PMS for methylene blue degradation was pH 4.4–6.6 under acidic conditions and the optimum pH range for methylene blue degradation was pH 8–10 under alkaline conditions. Mn(VII) could be conducive to enhance the synergistic catalytic performance with Fe(III)-g-C3N4 on PMS and increase the degradation of methylene blue. Compared with the degradation rate of methylene blue that catalyzed PMS by g-C3N4, loaded iron and cobalt nanobimetallic was 88.4% after 30 min of the reaction (Wang et al., 2018) [47], and the degradation rate of methylene blue was 90% within 20 min, which catalyzed PMS by Fe-g-C3N4 (Peng et al., 2022) [48]. The composite catalysts in our study have the characteristics of a short reaction time and high efficiency. Meanwhile, the Fe(III)-g-C3N4/Mn(VII) synergistic catalyzed PMS system was tested for the degradation of methyl orange and rhodamine b, and good degradation results were obtained, with the degradation rates of 87.37% and 84%, respectively. This research illustrates that the Fe(III)-g-C3N4/Mn(VII) synergistic catalyzed of PMS is prospective in recalcitrant organic pollutants. And it is also provided the theoretical foundation for the development of efficient and environmentally friendly persulfate activation technology and the deep treatment of recalcitrant organic pollutants. With the advantages of reducing pollutant emissions, improving water quality, renewability and recyclability, it will have a positive impact on the sustainability of the environment.

Author Contributions

Conceptualization, L.L.; formal analysis, L.L., M.C., W.M. and H.Z.; resources, L.L.; writing—original draft preparation, L.L.; writing—review and editing, L.L. and H.G.; visualization, L.L.; supervision, L.L. and Q.W.; project administration, L.L.; funding acquisition, L.L., H.G. and Q.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Teaching Reform Research Project of Hunan University of Technology (2022YB18); Hunan Provincial Natural Science Foundation of China (Grant No. 2022JJ50063); The Key Scientific Research Project of Hunan Provincial Department of Education (No. 22A0413); The Natural Science Foundation of Hunan Province (No. 2021JJ50046).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Scheme 1. Schematic characterization of the Fe(III)-g-C3N4 synthesis.
Scheme 1. Schematic characterization of the Fe(III)-g-C3N4 synthesis.
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Figure 1. SEM images of FeCN samples (a–c); EDS images of FeCN samples (d); EDX element mapping (e).
Figure 1. SEM images of FeCN samples (a–c); EDS images of FeCN samples (d); EDX element mapping (e).
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Figure 2. The XRD patterns of the g-C3N4 and the FeCN samples.
Figure 2. The XRD patterns of the g-C3N4 and the FeCN samples.
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Figure 3. The XPS survey spectra of FeCN samples (freshed and used) (a); high resolution C1s (b); the N1s XPS spectra of FeCN (c); Fe2p XPS spectra of FeCN (d).
Figure 3. The XPS survey spectra of FeCN samples (freshed and used) (a); high resolution C1s (b); the N1s XPS spectra of FeCN (c); Fe2p XPS spectra of FeCN (d).
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Figure 4. Degradation effect of MB in different combinations during the reaction times (a); the final degradation efficiency (b); and the corresponding Kobs of three combinations (c). Conditions: [FeCN] = 0.1 g/L, [KMnO4] = 20 µmol/L, [PMS] = 0.1 g/L, [MB] = 30 mg/L, [temperature] = 15–25 °C, pH = 4.0–5.0.
Figure 4. Degradation effect of MB in different combinations during the reaction times (a); the final degradation efficiency (b); and the corresponding Kobs of three combinations (c). Conditions: [FeCN] = 0.1 g/L, [KMnO4] = 20 µmol/L, [PMS] = 0.1 g/L, [MB] = 30 mg/L, [temperature] = 15–25 °C, pH = 4.0–5.0.
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Figure 5. Degradation effect of MB with the different ratios of Fe(III)/Mn(VII) during the reaction times (a); the final degradation efficiency (b); the corresponding Kobs of five concentrations of KmnO4 (c). Conditions: [FeCN] = 0.1 g/L, [PMS] = 0.1 g/L, [MB] = 30 mg/L, [temperature] = 15–25 °C, [Initial pH] = 4.0–5.0.
Figure 5. Degradation effect of MB with the different ratios of Fe(III)/Mn(VII) during the reaction times (a); the final degradation efficiency (b); the corresponding Kobs of five concentrations of KmnO4 (c). Conditions: [FeCN] = 0.1 g/L, [PMS] = 0.1 g/L, [MB] = 30 mg/L, [temperature] = 15–25 °C, [Initial pH] = 4.0–5.0.
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Figure 6. Degradation effect of MB with the different temperatures (a); the corresponding Kobs of six different temperatures (b). Conditions: [FeCN] = 0.1 g/L, [KmnO4] = 30 µmol/L, [PMS] = 0.1 g/L, [MB] = 30 mg/L, [Initial pH] = 4.0−5.0.
Figure 6. Degradation effect of MB with the different temperatures (a); the corresponding Kobs of six different temperatures (b). Conditions: [FeCN] = 0.1 g/L, [KmnO4] = 30 µmol/L, [PMS] = 0.1 g/L, [MB] = 30 mg/L, [Initial pH] = 4.0−5.0.
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Figure 7. Degradation effect of MB with the different pH (pH = 0.7, 1.5, 3, 4.4, 5.8, 6.6) (a) and the corresponding Kobs of MB degradation by different pH (b). Degradation effect of MB with the different pH (pH = 8, 9.3, 10, 11, 12, 13.3) (c) and the corresponding Kobs of MB degradation by different pH (d). Conditions: [FeCN] = 0.1 g/L, [KmnO4] = 30 µmol/L, [PMS] = 0.1 g/L, [MB] = 30 mg/L, [temperature] = 10–20 °C.
Figure 7. Degradation effect of MB with the different pH (pH = 0.7, 1.5, 3, 4.4, 5.8, 6.6) (a) and the corresponding Kobs of MB degradation by different pH (b). Degradation effect of MB with the different pH (pH = 8, 9.3, 10, 11, 12, 13.3) (c) and the corresponding Kobs of MB degradation by different pH (d). Conditions: [FeCN] = 0.1 g/L, [KmnO4] = 30 µmol/L, [PMS] = 0.1 g/L, [MB] = 30 mg/L, [temperature] = 10–20 °C.
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Figure 8. Methyl orange degradation curve (a); conditions: [FeCN] = 0.1 g/L, [KMnO4] = 30 µmol/L, [PMS] = 0.1 g/L, [methyl orange] = 20 mg/L, [temperature] = 0–20 °C, [pH] = 5.0–6.0. Rhodamine b degradation curve (b); conditions: [FeCN] = 0.1 g/L, [KMnO4] = 30 µmol/L, [PMS] = 0.1 g/L, [rhodamine b] = 10 mg/L, [temperature] = 10–20 °C, [Initial pH] = 5.0–6.0.
Figure 8. Methyl orange degradation curve (a); conditions: [FeCN] = 0.1 g/L, [KMnO4] = 30 µmol/L, [PMS] = 0.1 g/L, [methyl orange] = 20 mg/L, [temperature] = 0–20 °C, [pH] = 5.0–6.0. Rhodamine b degradation curve (b); conditions: [FeCN] = 0.1 g/L, [KMnO4] = 30 µmol/L, [PMS] = 0.1 g/L, [rhodamine b] = 10 mg/L, [temperature] = 10–20 °C, [Initial pH] = 5.0–6.0.
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Table 1. Experimental reagents.
Table 1. Experimental reagents.
ReagentSpecificationManufacturer
melamineAnalytical pure ARShandong Yusuo Chemical Technology Co., Ltd, Heze, China
anhydrous ferric chlorideAnalytical pure ARShanghai Macklin Biochemical Co., Ltd., Shanghai, China
Fe(III)-g-C3N44%self-made
potassium permanganateAnalytical pure ARShanghai Macklin Biochemical Co., Ltd., Shanghai, China
potassium persulfateAnalytical pure ARXilong Science Co., Ltd., Shantou, China
methylene blue(MB)Analytical pure ARShanghai Macklin Biochemical Co., Ltd., Shanghai, China
Methyl OrangeAnalytical pure ARShanghai Macklin Biochemical Co., Ltd., Shanghai, China
rhodamine bAnalytical pure ARShanghai Macklin Biochemical Co., Ltd., Shanghai, China
Table 2. Instruments and equipment.
Table 2. Instruments and equipment.
InstrumentSpecification/ModelManufacturer
muffle furnaceSX2-5-12YuanDong Electrical Furnace Factory, Changsha, China
analytical balanceME104E/02Mettler-Toledo Instruments (Shanghai) Co., Ltd., Shanghai, China
high speed centrifugeTG16-WSHunan Xiangyi Instrument Development Co., Ltd., Changsha, China
electric blast drying ovenDHG-9070ATianjin Kenuoyi Electronic Technology Co., Ltd., Tianjin, China
multi-head magnetic stirring apparatusHJ-4ABonsee Technology (Shanghai) Co., Ltd., Shanghai, China
X-ray energy spectrometerThermo Scientific ESCALAB Xi+Thermo Fisher Scientific, Waltham, MA, USA
X-ray diffractometerBruker D8 advanceBruker (Beijing) Technology Co., Ltd., Beijing, China
scanning electron microscopZEISS Sigma 300Carl Zeiss AG, Oberkochen, German
ultraviolet-visible spectrophotometerUV-1800Shanghai Meipuda Instrument Co., Ltd., Shanghai, China
electron paramagnetic resonanceBruker EMXplusBruker (Beijing) Technology Co., Ltd., Beijing, China
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Li, L.; Gu, H.; Wang, Q.; Chen, M.; Ma, W.; Zhang, H. Synthesis of Fe(III)-g-C3N4 and Applications of Synergistic Catalyzed PMS with Mn(VII) for Methylene Blue Degradation. Sustainability 2024, 16, 2364. https://doi.org/10.3390/su16062364

AMA Style

Li L, Gu H, Wang Q, Chen M, Ma W, Zhang H. Synthesis of Fe(III)-g-C3N4 and Applications of Synergistic Catalyzed PMS with Mn(VII) for Methylene Blue Degradation. Sustainability. 2024; 16(6):2364. https://doi.org/10.3390/su16062364

Chicago/Turabian Style

Li, Lin, Huangling Gu, Qiong Wang, Meiyin Chen, Wenjing Ma, and Hongwei Zhang. 2024. "Synthesis of Fe(III)-g-C3N4 and Applications of Synergistic Catalyzed PMS with Mn(VII) for Methylene Blue Degradation" Sustainability 16, no. 6: 2364. https://doi.org/10.3390/su16062364

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