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Efficient Removal of Rhodamine B in Wastewater via Activation of Persulfate by MnO2 with Different Morphologies

Key Laboratory of Pollutant Chemistry and Environmental Treatment, College of Chemistry and Environmental Science, Yili Normal University, Yining 835000, China
Key Laboratory for Environmental Pollution Prediction and Control, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China
Author to whom correspondence should be addressed.
Water 2023, 15(4), 735;
Submission received: 25 December 2022 / Revised: 2 February 2023 / Accepted: 8 February 2023 / Published: 13 February 2023
(This article belongs to the Topic Advanced Oxidation Process: Applications and Prospects)


In recent years, typical organic pollutants were frequently found in aquatic environments. Among them, synthetic dyes were widely used in many industries, which resulting in a large amount of wastewater contained dyes. Because of the characteristic of complex components, poor biodegradability and high toxicity, this kind of wastewater brought lots of harm to the ecological environment and organism. In this study, three different types of manganese dioxide (MnO2) with the rod-like, needle-like and mixed morphologies respectively were successfully fabricated by hydrothermal method with changing the preparation conditions and addition of the metal ions, and utilized as activator of persulfate (PS) to remove the dyes aqueous. Subsequently, these MnO2 nanocomposite was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) measurements. In addition, Rhodamine B (Rh B), as a representative substance of xanthene dyes was chosen as the target degradants to researched and compared the efficiency of removal via PS activated by different MnO2. By exploring the influences of different reaction parameters on the result of removal, results indicated that PS activated by the acicular MnO2 (α-MnO2) can remove 97.41% of Rh B over 60 min, with the optimal catalyst/PS ratio of 2:1 (the concentration of the α-MnO2 and PS were 1.2 g/L and 0.6 g/L, respectively), pH value of 3, at the temperature of 20 °C. Meanwhile, the probable degradation mechanism was also proposed. At last, as the catalyst was reused for four times, the degradation rate of target degradants decreased less than 10%.

1. Introduction

In recent decades, with the rapid development of industrialization and urbanization, the environmental problems caused by refractory organic wastewater are becoming increasingly serious and need to be effectively treated [1]. Due to the widespread usage of synthetic dyes in printing, dyeing, papermaking, textile, cosmetics, leather and other industries [2], a large number of highly toxic, complex ingredients, and difficult to biodegradable dye wastewater have been produced, and many organic dyes are difficult to metabolize in the body, gradual accumulation causes great harm to human health and ecological environment [3]. Therefore, it is crucial to find a more efficient and green treatment method to degrade organic dye wastewater. The rhodamine series of dyes are 3’,6’ diaminated xanthene-like fluorescents with excellent pH stability and a variety of structures, and different structures offer fluorescence with different wavelengths and colors. Rhodamine B (Rh B), as a representative substance of xanthene dyes, is most common and most frequently used, similar to diphenylmethane and triphenylmethane derivatives, generally used in industries such as colored glass, paper, textiles, fireworks, and etc. It is difficult to degrade in the environment, highly toxic, persistent and carcinogenic [4]. Herein, Rh B was selected as target pollutants in this study. It is hoped to provide some methods and references for the degradation of such organic dyes.
The usual methods for treating dyeing and printing wastewater include physical, biological and chemical methods. The commonly used physical methods generally include: adsorption, coagulation, membrane treatment, reverse osmosis, ion exchange and etc. However, physical methods face many application limitations: (1) adsorption materials are not suitable for hydrophobic dyes, and the regeneration cost is high and the loss is large; (2) coagulation method is not effective for hydrophilic dyes, and it is easy to produce sludge that needs further treatment; (3) membrane treatment and reverse osmosis membrane can treat printing and dyeing wastewater well, but the cost is too high, the membrane is easy to clog, and the treatment and disposal of waste membrane is also a problem; (4) ion exchange method is only suitable for treating ionic dye wastewater, and it needs to be regenerated continuously, which results in high loss. In addition, the biological treatment method is not ideal for dye wastewater because most of these dyes are poorly biochemical and difficult to biodegrade, among which it has been confirmed that reactive dyes are the most difficult to be degraded in dyeing wastewater [5,6]. Chemical methods refer to the oxidation of organic pollutants using oxidants, and the commonly used oxidants are H2O2, NaClO, O3 and etc. The chemical method has the characteristics of good treatment effect, fast reaction rate, complete degradation of pollutants, high treatment efficiency and wide application range. Commonly used chemical methods are through the oxidant degradation of pollutants, and will use photocatalysis [7], electrocatalysis [8], high temperature and other means to catalyze the oxidant to produce a large number of hydroxyl radicals to improve the degradation effect. However, for some hard-to-degrade organic substances, traditional chemical methods still cannot effectively degrade them.
Advanced oxidation processes (AOPs), which have been developed in recent years, are currently attracting much attention and can make most organic pollutants to be effectively degraded by producing free radicals as the main active substance to eliminate pollutants completely [9]. It is mainly represented by AOPs based on sulfate radical (SO4•−) and hydroxyl radical (OH). By contrast, SO4•− possesses a higher redox potential than OH (redox potential of OH is 2.8 eV, while the SO4•− is 2.5–3.1 eV) [10]; SO4•− has better selectivity through electron transfer, dehydrogenation, addition reaction to degrade organic matter [11]; SO4•− is more stable than OH due to its shorter half-life [12]; the way to produce SO4•− is also easier and the origin of which shows a higher stability [13]. Currently, AOPs based on SO4•− radicals generally generate SO4•− radicals by activating persulfates, including peroxydisulfate (PS)and peroxymonosulfate (PMS). Among them, PS is a solid that exists stably at room temperature, is easily preserved for transport and transfer, dissolves easily in water, and is relatively inexpensive. The PS itself, as a strong oxidizing agent, can also produce SO4•− radicals in aqueous solution without a catalyst. In general, PS is more stable and the above reaction rates are extremely low. After activation by applied energy or catalyst, the peroxygen bond (O-O bond) in the PS breaks and a large number of SO4•− radicals can be efficiently produced with strong oxidizing properties. Herein, SO4•− radicals can degrade difficult-to-treat organic pollutants in wastewater into biodegradable pollutants or even directly mineralize to carbon dioxide and water. In this process, SO4•− radicals will be reduced to sulfate (SO42−), and no toxic and harmful substances will be produced, which is an environmentally friendly oxidant.
Therefore, the activation technology of PS has become the focus of current research. The most commonly used PS activation methods are thermal activation, ultrasound, photoactivation, and transition metal activation [14]. The reactions involved are as follows:
S2O82− + heat → 2 SO4•−
adjust the pH in the reaction parameter influence tests. All the experiments
were carried out for three times to minimize the errors
Mn+ + S2O82− → M(n+1)+ + SO4•− + SO42−
Mn+ + HSO5 → M(n+1)+ + SO4•− + OH
However, some of these methods require a large number of high-cost oxidants, high reaction conditions, or consume large amounts of energy [15]; likewise, some metal oxides or metal ions as catalysts are prone to secondary pollution, such as Co2+, Co3O4, CuO, etc. [16,17,18]. Thus, the exploration and development of green and efficient persulfate activator has become a hot spot and focus now.
Manganese oxides are considered as common mineral oxides found in sediments and soils [19]. Among the various catalyst materials, MnO2 has become a very promising candidate because of its natural abundance, wide source, low cost, high activation, and low toxicity [20]. Because of its superior oxygen mobility, manganese oxides can induce a wider range of redox reactions [21]. More importantly, the reactivity of manganese-based oxides is relatively high. Meanwhile, as a promising alternative to iron minerals, manganese oxides are often applied in Fenton or Fenton-like reactions [22]. In recent reports, MnO2 have attracted considerable attention as activator for PS [23], especially, multi-component magnetic material compounded with it [19,24]. In this paper, three different morphologies of MnO2 activated PS were prepared to degrade organic pollutants, which can provide a certain reference for the technology of activation of PS with MnO2 and research of its multielement composite.
Herein, the main works of this study are: (1) Preparation of catalyst materials: three different morphologies of MnO2 were fabricated by hydrothermal method by changing the reaction conditions and adding metal ions; (2) Characterization of catalysts’ crystallographic structure and micromorphology: X-ray diffraction (XRD) was employed to analyze the materials’ composition and structure of atoms or molecules inside. Scanning electron microscope (SEM) was used to observe the microscopic surface morphology of the MnO2 with different morphologies; (3) Study on the catalytic performance: Rhodamine B was chosen as target pollutants to evaluate the catalytic activity of MnO2 on PS activation. The tests of exploring the influences of different reaction parameters on the result of removal were also conducted; (4) Stability evaluation: the repeated degradation experiments were carried out for 4 times. The main objective of this study was to compare and select the most dominant species among three different morphologies of MnO2 catalysts and to reveal the relationship between the microscopic morphology of the materials and their performance in activating PS for Rh B degradation. Ultimately, this research was expected to provide an important reference for the study of the activation of PS by MnO2 and their multicomponent complexes and the development of new nanomaterials for the efficient removal of organic dyes from wastewater.

2. Materials and Methods

2.1. Chemicals

Potassium permanganate (KMnO4), manganese sulfate monohydrate (MnSO4•H2O), manganese chloride tetrahydrate (MnCl2•4H2O), ferric chloride hexahydrate (FeCl3•6H2O), potassium persulfate (K2S2O8), hydrochloric acid (HCl), absolute ethanol (C2H5OH), sulfuric acid (H2SO4), potassium hydroxide (KOH), Rhodamine B (C28H31ClN2O3, Rh B) were purchased from Kermel Chemical Reagent Co. Ltd. (Tianjin, China). All the chemicals above were directly used when received without further purification. Noted, deionized (DI) water was used throughout this experiment.

2.2. Fabrication of MnO2 with Different Morphologies

In this experiment, MnO2 was prepared by hydrothermal method. By changing the preparation conditions, the reactant ratio and trying to add metal ions as auxiliary agents, three different morphologies of MnO2 were obtained.

2.2.1. Fabrication of Rod-Shaped MnO2

8 mmol (1.3520 g) of MnSO4•H2O and 8 mmol (1.2640 g) of KMnO4 dissolved in 50 mL of deionized water, and performed magnetic stirring for 30 min to dissolve fully. The mixed solution was then transferred to a 100 mL Teflon-lined stainless-steel autoclave, sealed and kept at 140 °C for 12 h. After cooling to room temperature, the precipitate was centrifuged and washed with deionized water and absolute ethanol alternately three times, finally dried in a vacuum oven at 120 °C for 12 h. The sample was ground and stored for following use [25].

2.2.2. Fabrication of Acicular MnO2

The fabrication conditions of the above rod-shaped MnO2 were changed to some extent, instead of changing the dosing ratio of MnSO4•H2O and KMnO4 (1:1), an appropriate amount of iron ions (Fe3+) was added to the reaction system. In this test, 1 mmol (0.2702 g) of FeCl3•6H2O was added to the mixed solution, followed by magnetic stirring for 30 min to dissolve thoroughly. The mixed solution was then transferred to a 100 mL Teflon-lined stainless-steel autoclave, sealed and placed in a vacuum oven at 140 °C for 12 h. After cooling to room temperature, subsequent processing was consistent with the rod-shaped MnO2 above. After grinding, sample was stored for use [26].

2.2.3. Fabrication of Mixed MnO2

To obtain the mixed MnO2, 1.5 mmol (0.2970 g) of MnCl2•4H2O and 2.5 mmol (0.3950 g) of KMnO4 were dissolved in 50 mL of deionized water, and 0.45 mmol (0.1216 g) of FeCl3•6H2O were added to the mixture, which was sufficiently dissolved by vigorously stirring. The mixed solution was poured into a Teflon-lined stainless-steel autoclave, sealed and kept at 150 °C for 24 h. After cooling to room temperature, the treatment of the precipitate simulated the preparation process of the first two types of MnO2 [27].

2.3. Characterizations

Powder X-ray diffraction (XRD) spectrograms of the samples were obtained on Panalytical multifunctional powder X-ray diffractometer (XRD D8 Advance, Rigaku Corporation, Matsumoto, Japan) with a graphite monochromatic Cu Kα radiation (λ = 0.15418 nm). The morphology and microstructure of three types of MnO2 were determined by field-emission scanning electron microscopy (SEM, Japan Electron Optics Laboratory Co. Ltd., Mitaka, Tokyo, JSM-6701F) at 20 kV.

2.4. Catalytic Activity Tests

Prepared a 10 mg/L Rh B solution of the target substance for use and took 50 mL each time into 100 mL Erlenmeyer flasks, added a certain amount of catalyst and PS, then marked according to different reaction conditions. At the same time, set a blank group and pure adsorption group to contrast. Degradation experiments were conducted at the temperature of 20 °C, on the shaking table with the rocking speed kept 180 rpm. After the given time intervals, 5 mL of mixed solution was aspirated with a syringe and filtered through a 0.22 μm to remove the catalyst, and immediately measured at maximum absorption wavelengths of the Rh B (549 nm) in a T6 UV–vis spectrophotometer (Evolution 300, Thermo Fisher Scientific Inc., Shanghai, China) [28]. Besides, the ratio of the measured absorbance of the sample to the absorbance of the initial target solution (At/A0) was regarded as the analysis index. Where At was the absorbance measured for each sampled sample and A0 was the absorbance of the initial 10 mg/L solution of the target pollutant. Meanwhile, in the following equation, C0 was the initial concentration of the target degradant and Ct was the concentration of the target degradant in the solution at time t. The degradation rate of this experiment was calculated using the following Equation (5) [29]. Additionally, H2SO4 (pH = 2) and KOH (pH = 11) were employed to adjust the pH in the reaction parameter influence tests. All the experiments were carried out for three times to minimize the errors.
η% = [(C0 − Ct)/C0] × 100% = [(C0 − Ct)/C0] × 100% = (1 − At/A0) × 100%

3. Results and Discussion

3.1. XRD and SEM Analysis

In order to obtain the crystallographic structure of different types of the as-prepared MnO2, X-ray diffraction (XRD) was carried out and results were displayed in Figure 1. As depicted in Figure 1a, lines appearing at 28.7°, 37.3°, 40.9°, 42.8°, 56.7°, 59,3°, 64.9°, 67.3° and 72.3° can be appreciably consistent with characteristic lines of β-MnO2 (JCPDS: 24-0735), corresponding to the (110), (101), (200), (111), (211), (220), (002), (310) and (301) planes of tetragonal β-MnO2 crystals [30]. Meanwhile, several diffraction lines at 12.6°, 18.0°, 28.7°, 37.4°, 41.8°, 49.7°, 56.4°, 59.9°, 65.0° and 69.2°displayed in Figure 1b can be indexed to the hexagonal phase of K1.33Mn8O16 (JCPDS: 77-1796) with the exposure of (110), (200), (130), (330), (240), (140), (251), (620), (002) and (541) planes. In addition, lines at 12.7°, 18.0°, 25.7°, 28.7°, 37.5°, 42.0°, 49.7°, 56.3°, 60.0°, 65.3°, 69.2° and 72.8° shown in Figure 1c, were attributed to (110), (200), (220), (310), (211), (301), (411), (600), (260), (002), (541) and (321) crystal facets of tetragonal α-MnO2 (JCPDS: 44-0141) [31]. These phenomena could rationally verify that three types of MnO2 were supposed to be pure β-MnO2, mixed K1.33Mn8O16 and pure α-MnO2, respectively. In addition, all three MnO2 catalysts prepared in this study exhibited an excellent crystallinity and no obvious spurious lines appeared in characteristic XRD patterns, indicating that pure MnO2 materials were successfully synthesized.
Besides, to further analyzed the surface morphology of β-MnO2, K1.33Mn8O16 and α-MnO2, SEM consequences of different samples were shown in Figure 2. As exhibited in Figure 2a, as-prepared smooth and evenly distributed rod-like β-MnO2 with an average diameter of about 80 nm showed a number of aggregated states. Moreover, the combination between MnO2 nanoparticles and MnO2 nanorods observed in Figure 2b, were matched with K1.33Mn8O16, a kind of mixed MnO2. While, α-MnO2 displayed acicular-like appearance in Figure 2c were agglomerated with an average diameter of about 60 nm. In addition, rod-like β-MnO2 and acicular-like α-MnO2 are essentially MnO2 nanorods, and the mixed type is a combination one. However, the shorter length and finer diameter of α-MnO2 leaded to larger specific surface area in comparing with β-MnO2 and K1.33Mn8O16, which could theoretically provide more reaction and activation sites to better activate PS [32]. This inference was proved by the subsequent experiments.

3.2. Organic Pollutants Degradation Performance

Herein, the degradation experiments of Rh B were carried out in various systems under the unadjusted pH (4.86) to evaluate and compare the organic pollutants degradation performance of three types of MnO2. As presented in Figure 3, only about 4.01%, 7.13% and 9.46% of Rh B could be removed with absorption of β-MnO2, K1.33Mn8O16 and α-MnO2, respectively, which revealed the decolorization of Rh B had little to do with adsorption. Besides, the degradation efficiency of Rh B was less than 45% over 60 min with the single PS addition, which was ascribed to the weak direct oxidation of PS (E0 = 2.01 V) [33], This result suggested that chemical oxidation alone could not efficiently oxidized organic pollutants. Noted, for as-synthesized β-MnO2, K1.33Mn8O16 and α-MnO2, the Rh B removal efficiency was markedly improved in presence of each type of MnO2 and PS simultaneously, indicated that the activation effect on PS of catalysts in aqueous solution play a vital role in the degradation of organic pollutants [34]. Among them, the efficiency of PS activation on Rh B degradation with different catalysts was α-MnO2 > K1.33Mn8O16 > β-MnO2, which was 78.43%, 67.48% an 47.62%, respectively. The result was consistent with those inference obtained by SEM analysis, which was mainly caused by larger specific surface area α-MnO2 possessed. In detail, the specific surface area of β-MnO2 with a rod-like structure was approximately 7.9~13.2 m2/g, which was much lower than that of α-MnO2 with a needle-like structure (44.4~76.5 m2/g) [32,35], while the specific surface area of hybrid MnO2 should be somewhere in between. Thus, α-MnO2 enhanced the activation of PS with its ability to expose more active sites, facilitating the elimination of Rh B in water.

3.3. Optimization of Reaction Parameters

Generally speaking, as we all know that the addition of catalyst dosage, PS dosage and catalyst/PS dosing ratio play important roles in degradation experiments by PS activation. Moreover, detecting the influence of initial pH is great significance to prove the adaptability of α-MnO2 in complicated practical water with various pH in the different places [36]. Therefore, a serious of tests were conducted to optimize reaction conditions. The effect of α-MnO2 dosage (0.2 g/L, 0.4 g/L, 0.6 g/L, 0.8 g/L and 1.2 g/L) on the Rh B degradation was displayed in Figure 4a. Obviously, the removal rate of Rh B promoted (43.38%, 62.71%, 68.89%, 78.61%, 84.49% and 93.80%) with the increased concentration of the α-MnO2. Likewise, as illustrated in Figure 4b, the decoloration efficiency of Rh B promoted with the increased dosage of PS, loading from 0.2 to 1.2 g/L corresponding to degradation efficiency of 38.31%, 59.85%, 75.25%, 81.39%, 85.61% and 86.71%. These phenomena might imply that with the increase of the amount of catalyst and PS, the available active metal ions increased, which accelerated the activation of PS and generated more active substance to degrade Rh B more efficiently [28]. In consideration of economics and inhibition caused by interaction between the catalyst and PS, reasonable catalyst/PS dosing ratio needed to be determined experimentally. It can be seen from Figure 4c that when the ratio of α-MnO2/PS was 2:1 (the concentration of the α-MnO2 and PS were 1.2 g/L and 0.6 g/L, respectively), the effect of activating PS to degrade Rh B was best. The effects of initial pH were studied at different values of 3, 5, 7, 9 and 11, as shown from Figure 4d, the degradation rate decreases significantly with the pH value increasing. 97.41% of Rh B could be removed for 60 min at the pH of 3, while the pH was increased to 5, 7, or 9, the degradation efficiency decreased by 18%, 25, or 27% over 60 min. In addition, when the pH was increased to 11, only 28.53% of Rh B was removed. The reasons of this result were as follow: (1) The zero point charge of MnO2 and its complexes was reported to generally occur in the pH range of 3 to 5 [35,37,38]. Therefore, when the pH was sufficiently low, the crystalline surface became positively charged, which was more favorable for electrostatic adsorption between the electron-rich contaminants, PS and α-MnO2; (2) When the pH was too high, the large amount of SO42− produced by the reactivity of SO4•− and OH (Equation (6)), caused the loss of SO4•− [39].
SO4•− + OHOH + SO42−

3.4. Mechanism Analysis

Based on the previous study [19,24,28], we proposed a possible mechanism in α-MnO2/PS reaction system. As shown in Figure 5, continuous conversion of Mn (IV) and Mn (III), which occurred on the surface of α-MnO2 were depicted in Equations (7) and (8), a great number of SO4•− produced via the activation of PS. Moreover, moiety SO4•− could further react with H2O to induce ·OH (Equation (9)). Therefore, the mineralization of Rh B was mainly ascribed to strong active species, SRs (SO4•−), hydroxyl radicals (OH) and even persulfate (PS). SO4•− radicals and OH radicals would preferentially attack the central carbon ring position of Rh B to decolorize the dye and further degrade it through a ring opening process. Rh B would be converted into complex xanthene benzene radicals and produce three different forms of phthalic acid (phthalic acid, isophthalic acid and p-phthalic acid) [40]. In addition, an alternative degradation pathway for Rh B under SO4•− radicals and OH radicals attacking has been proposed. The main degradation products were malonic acid, oxalic acid, glycine, m-aminophenol, adipic acid, 2-hydroxybenzoic acid and trans-crotonic acid [41], as long as they were converted to simpler organic acids and inorganic small molecule compounds by stepwise oxidation. Most of the above low molecular weight acids were environmentally friendly and may also be eventually converted to CO2, H2O and inorganic compounds.
Mn(IV) + S2O82− → Mn(III) + S2O8•−
Mn(III) + S2O82− → Mn(IV) + SO4•− + SO42−
SO4•− + H2O → OH + H+ + SO42−

3.5. Reusability of α-MnO2

Generally speaking, stability of catalysts is one of the most important characteristics in all evaluation indicators. Considering practical application potential of α-MnO2, the repeated degradation experiment was performed for four runs. As exhibited in Figure 6, the degradation efficiency got slightly decreased after each cycle, from 97.41% to 89.76%, indicating that α-MnO2 had expectation of putting into practical application in the organic pollutants’ degradation due to its good repeatability. In addition, there were several reasons could explain this reduction of removal rate along the Rh B recycle degradation: (1) the slight leaching of metal ions on the catalyst [42,43]; (2) the absorption of degradation intermediate or remaining Rh B occupied the active sites on the surface of the catalyst in the previous cycle [43]; (3) the by-production produced during the several cycles would compete for active species with the Rh B molecules [44].

4. Conclusions

All in all, rod-shaped, acicular and mixed MnO2 were successfully fabricated by hydrothermal method with controlling the conditions of reaction and addition of the metal ions. From the results of surface morphology and crystallographic structure analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD), three different morphologies of MnO2 were corresponding to β-MnO2, mixed K1.33Mn8O16 and α-MnO2, respectively. In the microscopic state, the α-MnO2 nanorods were more dispersed, and their nanorods were shorter in length and smaller in diameter. The mixed type, K1.33Mn8O16 was a combination of manganese dioxide nanorods and manganese dioxide particles. In contrast, the β-MnO2 mostly showed a polymerized state. Thus, the α-MnO2 with needle-shaped microscopic morphology had a larger specific surface area and could provide more activation sites. As-prepared α-MnO2 had a superior activity for Rh B degradation with the addition of PS, and 97.41% of Rh B could be removed within 60 min. More importantly, the reaction parameters are also optimized in this study, when the catalyst/PS ratio was 2:1 (the concentration of the α-MnO2 and PS were 1.2 g/L and 0.6 g/L, respectively), pH value was 3, and the best catalytic efficiency for PS activation was obtained at the temperature of 20 °C. Meanwhile, the probable degradation mechanism was also proposed. What’s more, the catalyst was reused 4 times, and the degradation rate decreased by less than 10%. In a word, this study provided an important reference for study on activation of PS by manganese oxides and their multicomponent complexes and exploitation of new nano materials for efficient removal of organic dyes in wastewater. In future research, the preparation of composite materials from α-MnO2 with needle-shaped microscopic morphology together with other substances as raw materials could be explored in depth. For example, magnetic composites can be prepared to facilitate the recovery of the catalyst. Or other reactants can be added to obtain composites with better activation and regeneration performance.

Author Contributions

X.Z. contributed to the Data curation, writing—original draft preparation, X.G. and S.C. contributed to the writing—review and editing; J.S. and X.C. contributed to the supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript.


This research received no external funding.

Data Availability Statement

Data available in a publicly accessible repository.


This work was supported by Research and Innovation Team Cultivation Program of Yili Normal University (CXZK2021004) and Natural Science Foundation of Xinjiang Uygur Autonomous Region (2021D01C461).

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. XRD patterns of β-MnO2 (a), K1.33Mn8O16 (b) and α-MnO2 (c).
Figure 1. XRD patterns of β-MnO2 (a), K1.33Mn8O16 (b) and α-MnO2 (c).
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Figure 2. SEM images of β-MnO2 (a), K1.33Mn8O16 (b) and α-MnO2 (c).
Figure 2. SEM images of β-MnO2 (a), K1.33Mn8O16 (b) and α-MnO2 (c).
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Figure 3. Time course of Rh B absorption and degradation via PS activation by different types of catalysts. Experiment conditions: PS concentration = 0.6 g/L, catalyst dosage = 0.8 g/L, Rh B concentration = 10 mg/L.
Figure 3. Time course of Rh B absorption and degradation via PS activation by different types of catalysts. Experiment conditions: PS concentration = 0.6 g/L, catalyst dosage = 0.8 g/L, Rh B concentration = 10 mg/L.
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Figure 4. Effect of reaction parameters on the degradation of Rh B: catalyst dosage (a), persulfate concentration (b), catalyst/PS dosing ratio (c) and initial pH value (d). Experiment conditions: PS concentration = 0.6 g/L, catalyst dosage = 0.6 g/L, catalyst/PS dosing ratio = 2:1, Rh B concentration = 10 mg/L and initial pH value = 4.86 (unadjusted).
Figure 4. Effect of reaction parameters on the degradation of Rh B: catalyst dosage (a), persulfate concentration (b), catalyst/PS dosing ratio (c) and initial pH value (d). Experiment conditions: PS concentration = 0.6 g/L, catalyst dosage = 0.6 g/L, catalyst/PS dosing ratio = 2:1, Rh B concentration = 10 mg/L and initial pH value = 4.86 (unadjusted).
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Figure 5. Proposed mechanism of Rh B degradation under α-MnO2/PS system.
Figure 5. Proposed mechanism of Rh B degradation under α-MnO2/PS system.
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Figure 6. Performance of α-MnO2 upon 4 successive uses for Rh B degradation. Experimental conditions: PS concentration = 0.6 g/L, catalyst dosage = 1.2 g/L, Rh B concentration = 10 mg/L.
Figure 6. Performance of α-MnO2 upon 4 successive uses for Rh B degradation. Experimental conditions: PS concentration = 0.6 g/L, catalyst dosage = 1.2 g/L, Rh B concentration = 10 mg/L.
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MDPI and ACS Style

Zhang, X.; Gan, X.; Cao, S.; Shang, J.; Cheng, X. Efficient Removal of Rhodamine B in Wastewater via Activation of Persulfate by MnO2 with Different Morphologies. Water 2023, 15, 735.

AMA Style

Zhang X, Gan X, Cao S, Shang J, Cheng X. Efficient Removal of Rhodamine B in Wastewater via Activation of Persulfate by MnO2 with Different Morphologies. Water. 2023; 15(4):735.

Chicago/Turabian Style

Zhang, Xinyi, Xinrui Gan, Shihu Cao, Jiangwei Shang, and Xiuwen Cheng. 2023. "Efficient Removal of Rhodamine B in Wastewater via Activation of Persulfate by MnO2 with Different Morphologies" Water 15, no. 4: 735.

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