Preparation of Fe₃O₄/α-MnO₂ Magnetic Nanocomposites for Degradation of 2,4-DCP through Persulfate Activation

Abstract

In this study, Fe₃O₄ magnetic nanoparticles (MNPs) were loaded on α-MnO₂ nanowires using an improved hydrothermal synthesis method combined with an ultrasonic coprecipitation method, the loading ratio was optimized, the efficiency of the prepared Fe₃O₄/α-MnO₂-activated persulfate (PS) system for the degradation of 2,4-dichlorophenol (2,4-DCP) was investigated, and the effects of PS concentration, Fe₃O₄/α-MnO₂ magnetic nanocomposites (MNCs) dosage, pH value and initial pollutant concentration on the degradation of 2,4-DCP were investigated. The results showed that when the initial concentrations of 2,4-DCP, PS, and Fe₃O₄/α-MnO₂ MNCs were 100 mg/L, 30 mmol/L, and 0.4 g/L, the degradation rate of 2,4-DCP reached 96.3% after 180 min of reaction at 30 °C under a neutral condition, and the fitting results showed that the degradation of 2,4-DCP by the Fe₃O₄/α-MnO₂-activated PS system conformed to quasi-first-order kinetics. The degradation of 2,4-DCP by different Fe₃O₄/α-MnO₂-activated PS systems was compared, and a possible PS activation mechanism was proposed. The Fe₃O₄/α-MnO₂ MNCs exhibited excellent reusability, and by introducing Fe₃O₄/α-MnO₂ MNCs as the PS activator into the advanced oxidation process (AOP) system, the electron transfer of Mn(III/IV) and Fe(III/II) on the surface of MNCs was realized, thus greatly improving the reaction efficiency.

1. Introduction

Advanced oxidation processes (AOPs) are a new type of remediation technology developed in recent years that uses free radicals to remove organic pollutants in the environment [1,2]. Hydroxyl radicals (•OH), sulfate radicals (SO₄⁻•), superoxide radical (•O₂⁻) and singlet oxygen (¹O₂) have all been shown to be able to effectively degrade organic pollutants in various AOPs systems [3,4,5,6]. Compared with the traditional Fenton system and other •OH-based AOPs, SO₄⁻•-based AOPs have obvious advantages. For example, the stability of the commonly used oxidant persulfate (PS) is better than that of H₂O₂, the degradation of organic pollutants occurs under pH conditions [7], and the lifetime of SO₄⁻• is much longer than that of •OH [8]. Therefore, the application of activated PS-based AOP systems in soil and water remediation has been extensively studied [9,10,11,12].

According to the difference in the morphology of SO₄⁻• produced by activated PS, the AOP systems can be divided into homogeneous activation systems and heterogeneous activation systems. Ultraviolet (UV) activation, thermal activation, and transition metal ion activation are commonly used homogeneous activation systems. However, they have significant limitations, such as high cost, strict pH requirements, and secondary pollution. Therefore, heterogeneous activation systems have gradually attracted attention [12]. Among them, iron and manganese oxides are excellent heterogeneous PS activators [8,13]. They can effectively activate PS to produce SO₄⁻• to degrade organic pollutants, and secondary pollution is less likely to occur.

In last few years, magnetic nanoparticles (MNPs) have driven much consideration for several applications. For instance, Ni@SiO₂-PMo is applied as a recyclable antibacterial agent [14], Fe₃O₄ MNPs are used as heterogeneous Fenton-like catalysts [15]. Among these MNPs, Fe₃O₄ can steadily under mild conditions and be easily reused. Besides, Fe₃O₄ is a mixed-valence iron oxide which Fe(II) species included in its structure can initiate the activation of PS [16]. Some scholars have prepared and modified Fe₃O₄ magnetic nanomaterials and used Fe(II) to activate PS and effectively degrade organic pollutants, such as 4-aminobenzenesulfonic acid and polychlorinated biphenyls (PCBs) [17,18]. However, due to its magnetic properties, Fe₃O₄ extremely readily agglomerates, which reduces the number of active sites on the surface. After the reaction, Fe(II) becomes Fe(III), so the efficiency of the activator decreases over time, making the activator less stable and less reusable [13,19]. In addition, Fe₃O₄ is more effective under acidic conditions, which limits its application.

In recent years, combining the advantages of individual metal compounds to prepare bimetallic core-shell nanomaterials (CSNs) for AOPs has become a popular research topic. Of these compounds, MnO₂ can take different crystal forms, such as α-, β-, γ-, δ-, ε- and η-. Because these MnO₂ crystal forms can exhibit different heterogeneous activation potentials, they have attracted extensive attention. Among them, α-MnO₂ with an open 2 × 2 tunnel structure and can facile oxygen vacancy formation on its surface. This characteristic lead to superior catalytic performance compared to other MnO₂ polymorphs in different application, such as the oxygen reduction reaction (ORR), supercapacitors, and heterogeneous catalytic [20]. Xu et al. evaluated the degradation of steroid estrogen with MnO₂ as an activator and showed that MnO₂ is an ideal activator for removing estrogen from water [21]. Edy et al. synthesized different crystalline phases of MnO₂ and tested their heterogeneous activation of PS to degrade phenol [22]. Tushar Kanti Das, et al. synthesized single-layer δ-phase MnO₂ nanosheet nanocatalyst toward environmental remediation of hazardous nitroaryl compounds [23]. Although MnO₂ has a good catalytic effect on PS, its lack of magnetism leads to difficulties in recovery and reuse. Theoretically, combining the excellent performance of MnO₂-activated PS with the advantages of Fe₃O₄ magnetic recovery, the synthesis of Fe₃O₄-MnO₂ CSNs can simultaneously make up for the shortcomings and deficiencies of the two materials. However, the conditions for the synthesis of a certain CSN are relatively harsh and difficult to control, resulting in an increase in cost. Importantly, wrapped by MnO₂, the Fe₃O₄ nucleus can play only a role in electron transfer, which is not convenient for direct activation of PS to remove organic pollutants. The use of a simple coprecipitation method to dispersedly load Fe₃O₄ particles on MnO₂ is a convenient way, which can not only exploit the catalytic performance of MnO₂ but also achieve magnetic recovery.

In view of this, in this study, Fe₃O₄ MNPs were loaded on α-MnO₂ nanowires by the hydrothermal synthesis method combined with the ultrasonic coprecipitation method to synthesize Fe₃O₄/α-MnO₂ magnetic nanocomposites (MNCs), an activated PS-based AOP system was constructed using the MNCs, and the degradation efficiency of 2,4-dichlorophenol (2,4-DCP) by this system was investigated.

2. Materials and Methods

2.1. Reagents and Instruments

Main reagents: 2,4-DCP was purchased from Sigma-Aldrich (Shanghai, China) Trading Co., Ltd. Sodium PS was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Ferrous sulfate, ferric chloride, potassium permanganate, hydrochloric acid, acetic acid, ammonia, and other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The reagents used in the experiment were all analytically pure, and all solutions were prepared with ultrapure water.

Main instruments: tube furnace (SG-GL1400K, Shanghai Institute of Optics and Fine Mechanics, Shanghai, China), vacuum drying oven (BZF-30, Shanghai Boxun Medical Biological Instrument Corp., Shanghai, China), high-resolution transmission electron microscope (HRTEM, Tecnai G2 F20 S-TWIN, Thermo Fisher Scientific, Shanghai, China), and UV Spectrophotometer (722 N, Shanghai Precision and Scientific Instrument Corp., Shanghai, China) with wavelength ranging from 250 nm to 800 nm.

2.2. Experimental Methods

Preparation of α-MnO₂ nanowires: The α-MnO₂ nanowires were prepared using an improved hydrothermal synthesis method [24]. First, 0.16 g of KMnO₄ was dissolved in 40 mL of ultrapure water, and 0.7 mL of CH₃COOH was slowly added dropwise under ultrasonication. The mixed solution was transferred to an autoclave lined with polytetrafluoroethylene, which was sealed and heated to 140 °C. After 12 h, the cooled product was washed several times with ultrapure water and ethanol, placed in a vacuum drying oven, and dried at 60 °C for 8 h. Using this synthesis method, 0.05 g of α-MnO₂ could be prepared each time. The α-MnO₂ black powder obtained from several preparations was ground, mixed, and set aside for use.

Preparation of Fe₃O₄/α-MnO₂ MNC: The Fe₃O₄ MNPs were loaded on the α-MnO₂ nanowires by the ultrasonic coprecipitation method, and the preparation process was carried out under the nitrogen protection. First, for bottle A, 2.705 g of FeCl₃·6H₂O and 20 mL of oxygen-free water were added, two drops of (1 + 1) HCl were added, and 2.780 g of FeSO₄·7H₂O was added. The mixture was shaken until the solids dissolved. Bottle B was placed in a 70 °C water bath/ultrasonic generator at 40 Hz. For bottle B, the prepared α-MnO₂ and 30 mL of oxygen-free water were first added, and 10 mL of concentrated ammonia was added after 10 min. The liquid in bottle A was added dropwise to bottle B. After 1 h, bottle B was removed from the ultrasonic generator and shaken in an air shaker for 12 h. Finally, the prepared nanomaterials were washed with oxygen-free water and ethanol until they were neutral. The obtained composites were dried in a tube furnace at 80 °C for 4 h under the nitrogen protection. After drying, the products were ground in a vacuum bag with an agate mortar to a fine powder, i.e., the Fe₃O₄/α-MnO₂ MNCs. The MNCs were sealed in a glass bottle filled with nitrogen and stored in a refrigerator. During the preparation process, MNCs with different iron and manganese ratios were obtained by adjusting the amount of α-MnO₂.

Experimental procedure: A certain amount of Fe₃O₄/α-MnO₂ MNCs and 2,4-DCP stock solution were sequentially added to the reaction vessel and ultrasonically dispersed. After the ultrasonication feature was turned off, an amount of PS stock solution was added to trigger the activation reaction, with shaking at a constant temperature. At different times, 0.2 mL liquid samples were taken from the reaction vessel, and excess methanol was added to quench the free radical reaction. The mixture of water and methanol mixture was then passed through a 0.22 μm filter membrane and used for the 2,4-DCP determination.

2.3. Analysis Methods

The 2,4-DCP was measured by spectrophotometry [25]. A 600 μL liquid sample was placed in a 10 mL colorimetric detector tube, and 100 μL of 20.8 mM 4-aminoantipyrine solution, 100 μL of 83.4 mM potassium ferricyanide solution, and 200 μL of 0.25 M sodium bicarbonate solution (pH = 8.4) were added. After ultrapure water was added until the marked line was reached, the color was developed for 10 min, the solution was transferred to a 10 mm cuvette, and the optical density (OD) was measured at 510 nm using a spectrophotometer. The standard solution was prepared using the same procedure, and the standard curve was drawn after the measurement.

3. Results and Discussion

3.1. Characterization of the Fe₃O₄/α-MnO₂ MNCs

The X-ray diffraction (XRD) results of the Fe₃O₄/α-MnO₂ MNCs are shown in Figure 1a. In the scanning range of 10°–90°, the characteristic peaks match the peaks of Fe₃O₄ and (44-0141) α-MnO₂ in JCPDS card (19-0629) [15,26], proving that the synthesized MNCs are composed of Fe₃O₄ and α-MnO₂.

The elemental composition of the Fe₃O₄/α-MnO₂ MNCs was analyzed using energy dispersive X-ray (EDX) microanalysis. The ratio of Fe₃O₄ to α-MnO₂ in the measured samples is 0.7 to 1, and results are shown in Figure 1b. In the EDX spectrum, the measured ratios of Fe and Mn are similar to those during the preparation; the molar percentages of Fe and Mn are 16.1% and 7.3%, and the mass percentages are 35.6% and 15.9%, respectively.

The Fe₃O₄/α-MnO₂ MNCs were characterized by HRTEM, and the results are shown in Figure 2a–d. As shown in the figure, the diameter of Fe₃O₄ MNPs is approximately 20 nm, and the width of α-MnO₂ nanowires is 10 nm–50 nm. The Fe₃O₄ MNPs are dispersedly loaded on the α-MnO₂ nanowires, so the dispersion of the Fe₃O₄ MNPs is improved and the α-MnO₂ nanowires can be exposed, which can help the two materials simultaneously perform activation.

3.2. Effectiveness of the Fe₃O₄/α-MnO₂ MNCs in the Removal of 2,4-DCP by PS Activation

3.2.1. Comparison of 2,4-DCP Removal by Different Activated PS-Based Systems

The removal of 2,4-DCP in different systems was compared to verify the effectiveness of Fe₃O₄/α-MnO₂ MNCs. Systems with only PS, only Fe₃O₄/α-MnO₂ MNCs, and both PS and Fe₃O₄/α-MnO₂ MNCs were prepared. The concentration of PS added was 30 mM, the dosage of MNCs was 0.4 g/L, the initial concentration of 2,4-DCP was 100 mg/L, the initial pH was 7, and the temperature was 30 °C. The changes in the concentration of 2,4-DCP in each system within 180 min of the reaction are shown in Figure 3. When only the oxidant PS or the activator Fe₃O₄/α-MnO₂ MNC is added, the final removal rate of 2,4-DCP is only 3.1% and 1.5%, respectively. These results indicate that the pollutants adsorbed by the PS direct oxidation and Fe₃O₄/α-MnO₂ MNCs are negligible. In the Fe₃O₄/α-MnO₂-activated PS system, the degradation rate of 2,4-DCP reaches 96.3%, which is much higher than that of the other two systems. This result shows that the prepared Fe₃O₄/α-MnO₂ MNCs can effectively activate PS to remove 2,4-DCP.

3.2.2. Optimization of the Loading Ratio of Fe₃O₄ on α-MnO₂

Fe₃O₄/α-MnO₂ MNCs with molar ratios of Fe₃O₄ MNPs and α-MnO₂ nanowires of 0.5:1, 0.7:1, 1:1, 2:1, and 5:1 were prepared, and the effectiveness of these 5 types of Fe₃O₄/α-MnO₂ MNCs in activating PS to degrade 2,4-DCP was investigated. The results are shown in Figure 4. At 30 °C, the PS concentration was 30 mM, the concentration of Fe₃O₄/α-MnO₂ MNCs is 0.4 g/L, the 2,4-DCP concentration is 100 mg/L, and the pH is 7. After 180 min of reaction, the removal rates of 2,4-DCP by degradation are 92.1%, 96.3%, 83.7%, 66.8%, and 52.7% for the above 5 Fe₃O₄/α-MnO₂ MNCs, respectively. The results show that when the molar ratio of Fe₃O₄ to α-MnO₂ is 0.7:1, the Fe₃O₄/α-MnO₂ MNC show the best effectiveness in terms of the degradation of 2,4-DCP by activating PS. When the proportion of Fe₃O₄ is further increased, the activation ability of the activator worsens. On the one hand, the results in 3.5.1 show that the activation ability of α-MnO₂ nanowires is better than that of Fe₃O₄ MNPs of the same mass, and a high molar ratio of Fe₃O₄ MNPs and α-MnO₂ indicates the low α-MnO₂ proportion in the prepared MNCs, which decreases the activation performance of the MNCs. Moreover, MnO₂ nanowires support the Fe₃O₄ MNPs as the skeleton, which enhance the dispersion and reduce the aggregation effect of Fe₃O₄ MNPs. Herein, more activated sites on the surface of Fe₃O₄/α-MnO₂ MNCs are exposed to enhance the activation performance of the MNCs. Similar phenomenon has been observed in other scholars’ studies [27,28]. Wu et al. improved the problem of agglomeration by synthesizing composite material of D-ATP-nFe/Ni, which also utilizes the support function of the attapulgite to weaken the aggregation effect of Fe/Ni bimetallic nanoparticles [28]. On the contrary, due to the agglomeration of Fe₃O₄, a high molar ratio for Fe₃O₄ MNPs in the prepared MNCs can easily lead to uneven dispersion of MNPs on α-MnO₂ nanowires, or even the complete coverage of α-MnO₂ nanowires by excessive MNPs. These two effects reduce the number of activated sites on the MNCs and lead to a reduction in 2,4-DCP removal. On the other hand, a low molar ratio for Fe₃O₄ MNPs and α-MnO₂ may affect the synergistic catalytic effect between Fe₃O₄ and α-MnO₂, thus reducing the removal rate of 2,4-DCP. Based on the experimental results, Fe₃O₄/α-MnO₂ MNCs prepared with a molar ratio of 0.7:1 between Fe₃O₄ and α-MnO₂ were used as activators in subsequent experiments.

3.3. Influencing Factors of 2,4-DCP Removal through the Activated PS by Fe₃O₄/α-MnO₂ MNCs

3.3.1. Effect of Activator Dosage

The effect of Fe₃O₄/α-MnO₂ MNCs on the removal efficiency of 2,4-DCP was investigated at 30 °C, PS concentration of 30 mM, 2,4-DCP concentration of 100 mg/L, and pH of 7. The results are shown in Figure 5a. The activator concentrations were 0.1–0.8 g/L. After 180 min of reaction, the removal rates of 2,4-DCP are 50.9%, 61.6%, 96.3%, 90.3%, and 79.6%, respectively. As the dosage of activator increases from 0.1 g/L to 0.4 g/L, the degradation rate of 2,4-DCP increases accordingly; however, as the dosage of activator continues to increase from 0.4 g/L to 0.8 g/L, the removal rate of 2,4-DCP decreases instead.

In the Fe₃O₄/α-MnO₂-activated PS system, both Fe₃O₄ and α-MnO₂ can activate PS to generate free radicals to degrade and remove 2,4-DCP. Previous studies have shown that when α-MnO₂ alone is used to activate PS, an increase in activator dosage does not lead to a decrease in pollutant removal and reaction rate [29]. While Fe₃O₄ alone is used to activate PS, the results are similar to those of this study [15,30]. Therefore, with a further increase in the activator dosage, the decrease in the 2,4-DCP removal rate is caused by the excess of Fe₃O₄. On the one hand, Fe₃O₄ provides Fe(II) to activate PS, but with further increase in Fe₃O₄ concentration, Fe(II) also increases, and consequently, more SO₄⁻• is produced. The excess Fe(II), PS and SO₄⁻• all have a quenching effect on SO₄⁻• [31], resulting in less SO₄⁻• in the system to participate in the oxidative degradation of 2,4-DCP, which causes a decrease in the removal rate of 2,4-DCP. On the other hand, due to the magnetic properties of the Fe₃O₄/α-MnO₂ MNCs, excessive dosage would lead to agglomeration of the activator, which would affect the contact area between the PS and the activator, resulting in a decrease in the removal rate of 2,4-DCP.

The results showed that when the dosage of the activator Fe₃O₄/α-MnO₂ is 0.4 g/L, the degradation rate of 2,4-DCP is the highest. After 180 min of reaction, the removal rate of 2,4-DCP reaches the highest rate, of 96.3%. Therefore, an activator dosage of 0.4 g/L was used in subsequent experiments.

3.3.2. Effect of Oxidant Dosage

Systems with 10–40 mM PS were prepared, and other experimental conditions were as follows: the initial 2,4-DCP concentration was 100 mg/L, the dosage of Fe₃O₄/α-MnO₂ MNCs was 0.4 g/L, the reaction temperature was 30 °C, and the initial pH was 7. Figure 5b shows that, as the PS concentration increases from 10 mM to 40 mM, after 180 min of reaction, the removal rates of 2,4-DCP are 50.7%, 79.1%, 96.3%, 91.3%, and 89.1%, respectively. With increasing PS dosage, the removal rate shows a trend of first increasing and then decreasing. The highest removal rate of 2,4-DCP by the Fe₃O₄/α-MnO₂-activated PS system is reached when the PS concentration is 30 mM.

When the PS concentration is too high, the reaction system can generate a large amount of SO₄⁻• in a short time, and the high concentration of SO₄⁻• can cause a self-quenching reaction (K = 8.9 × 10⁸ M⁻¹·s⁻¹) [32]. At the same time, excessive PS can also become the quencher of SO₄⁻• [27]. Therefore, the concentration and the effective utilization rate of SO₄⁻• are reduced, resulting in decreases in the removal rates of 2,4-DCP.

The above two experimental results show that for the 2,4-DCP solution with a concentration of 100 mg/L, when the dosage of Fe₃O₄/α-MnO₂ MNCs is 0.4 g/L and the initial concentration of PS is 30 mM, the degradation rate of 2,4-DCP reaches 96.3% after 180 min of reaction. Therefore, Fe₃O₄/α-MnO₂ MNCs (activator) of 0.4 g/L and the initial PS (oxidant) concentration of 30 mM are used in subsequent experiments.

3.3.3. The Effect of the Initial pH of the System

Systems with initial pH values of 3, 5, 7, 9 and 11 were prepared to investigate the effect of the initial pH of the system on the removal of 2,4-DCP, and the other reaction conditions were as follows: the dosage of Fe₃O₄/α-MnO₂ MNCs was 0.4 g/L, the initial concentration of PS was 30 mM, the initial concentration of 2,4-DCP was 100 mg/L, and the temperature was 30 °C. As shown in Figure 5c, after 180 min of reaction, the removal rates of 2,4-DCP are 97.9%, 97.6%, 96.3%, 18.4%, and 5.2%, respectively. The lower the pH is, the better the removal of 2,4-DCP and the faster the degradation rate. When the initial pH is 3, the removal rate of 2,4-DCP reaches 86.9% at 20 min. With increasing pH, the reaction rate observed decreases. When the pH is neutral, the removal rate of 2,4-DCP reaches 87.8% at 120 min, which is closely to that under acid condition (pH = 3) in 20 min. Under alkaline conditions, the oxidative capacity of the Fe₃O₄/α-MnO₂-activated PS system is inhibited, and the removal rate of 2,4-DCP decreased to less than 20% in 180 min. In addition, although the reaction rate observed under pH = 3, 5, and 7 are different at the beginning of the experiment, the difference in the removal rate of 2,4-DCP after 180 min is less than 1%. This phenomenon indicates that under acidic and neutral initial conditions, the Fe₃O₄/α-MnO₂-activated PS system can effectively degrade 2,4-DCP.

Studies have shown that the redox conditions of S₂O₈²⁻ are different in acidic, neutral, and alkaline environments and that the free radicals that degrade pollutants in the system are different. When pH is approximately 2–7, SO₄⁻• is the dominant active free radical; when pH is approximately 9, SO₄⁻• and •OH coexist; when pH > 12, •OH is the dominant active free radical [33]. Studies have shown that when pH is low, the formation of SO₄⁻• by S₂O₈²⁻ can be accelerated under acid catalysis [34], which may increase the degradation rate of pollutants. The acidic environment (low pH) can also lead to the dissolution of Fe²⁺ in the Fe₃O₄/α-MnO₂ MNCs, thereby increasing the removal rate of 2,4-DCP.

When pH = 7, the degradation rate of 2,4-DCP is 96.3% at 180 min, and when the pH continues to increase to 9 and 11, the degradation rates of 2,4-DCP are only 18.4% and 5.2% at 180 min, respectively. As the pH increases, •OH becomes the main active free radical, while the decomposition of H₂O₂ and the quenching of SO₄⁻• and •OH under alkaline conditions reduce the amount of •OH. Meanwhile the redox potential of •OH decreases with increasing pH. These two effects result in a decrease in the removal rate of pollutants [35].

The surface charge of α-MnO₂ may also be one of the influencing factors. Previous studies have shown that the electrical properties of the surface charge of metal oxides are related to the pH of the solution at the point of zero charge (pH_pzc) [36,37]. When pH < pH_pzc, metal oxides have a positive surface charge; when pH > pH_pzc, metal oxides have a negative surface charge. Prélot et al. reported the pH_pzc of several manganese oxides, including the pH_pzc of α-MnO₂ = 4.5/4.6 [38]. When pH = 3, i.e., pH < pH_pzc, the surface of α-MnO₂ is positively charged. Due to the attraction of unlike charges, S₂O₈²⁻ is more likely to move to the surface of the activator to generate free active radicals to oxidatively degrade 2,4-DCP. When pH = 5–11, i.e., pH > pH_pzc, the surface of α-MnO₂ is negatively charged. Due to the repulsion of the like charges, the contact of S₂O₈²⁻ with the surface of the activator is blocked, resulting in the inhibition of the free active radical production [37], so the removal rate of 2,4-DCP decreases.

3.3.4. Effect of Initial Pollutant Concentration in the System

Systems with initial 2,4-DCP concentrations of 100, 500, and 1000 mg/L were prepared to investigate the effect of the initial 2,4-DCP concentration on the activation. The other reaction conditions were as follows: the dosage of Fe₃O₄/α-MnO₂ MNCs was 0.4 g/L, the initial concentration of PS was 30 mM, the initial pH was 7, and the temperature was 30 °C. As shown in Figure 5d, after 180 min of reaction, the removal rates of 2,4-DCP are 96.3%, 86.3%, and 74.1%, respectively, indicating that with an increasing initial concentration of 2,4-DCP, the removal rate gradually decreases. When the pollutant concentration is too high, the system cannot generate enough active free radicals to degrade and remove pollutants when the amount of added oxidizer and activator is a constant.

3.3.5. Effect of Temperature

Systems with temperatures of 10 °C, 15 °C, 25 °C, 30 °C, 40 °C and 80 °C were prepared to investigate the effect of system temperature on the removal of 2,4-DCP, and other reaction conditions were as follows: the dosage of Fe₃O₄/α-MnO₂ MNCs was 0.4 g/L, the initial concentration of PS was 30 mM, the initial pH was 7, and the initial concentration of 2,4-DCP was 100 mg/L. Figure 5e shows that under the experimental conditions of this study, as the temperature decreases from 80 °C to 10 °C, the removal rate of 2,4-DCP decreases from 97.4% to 91.9% at 180 min. The removal rate decreases by only 5.5% when the temperature decreases by 70 °C. This result indicates the Fe₃O₄/α-MnO₂-activated PS system for 2,4-DCP degradation is not sensitive to the reaction temperature. Therefore, this system can be applied in both high and low-temperature environments and can still effectively degrade pollutants. In particular, compared with other activated PS systems for degradation of organic pollutants, the Fe₃O₄/α-MnO₂ MNCs are found to have excellent activation performance for PS in a low-temperature environment [19,39]. Temperature has a weaker effect on the degradation rate than the oxidant dosage, activator dosage, initial pH, and initial pollutant concentration; i.e., the system is the least sensitive to the influencing factor of temperature.

3.4. Reusability

The Fe₃O₄/α-MnO₂ MNCs were recovered and repeatedly added to the reaction system under the same initial conditions to investigate their reusability. The reaction conditions were as follows: the temperature was 30 °C, the PS concentration was 30 mM, the dosage of Fe₃O₄/α-MnO₂ MNCs was 0.4 g/L, the initial 2,4-DCP concentration was 100 mg/L, and the initial pH was 7. Figure 6 shows that after four repeated uses, the Fe₃O₄/α-MnO₂ MNCs still had good activation performance, and the degradation rate of 2,4-DCP reached 94.0%, which is only 2.0% lower than that of the first use, indicating that the Fe₃O₄/α-MnO₂ MNCs have excellent reusability.

3.5. Reaction Mechanism of 2,4-DCP Removal by the Fe₃O₄/α-MnO₂-Activated PS System

3.5.1. Comparison of 2,4-DCP Removal by Different Activated PS Systems

The removal of 2,4-DCP in different systems was compared. The four systems were as follows: ① Fe₃O₄ MNPs + PS, ② α-MnO₂ nanowires + PS, ③ Fe₃O₄ MNPs + α-MnO₂ nanowires + PS, and ④ Fe₃O₄/α-MnO₂ MNCs + PS. The concentration of the solid activator in the system was 0.4 g/L, the concentration of PS was 30 mM, the initial concentration of 2,4-DCP was 100 mg/L, the initial pH was 7, and the temperature was 30 °C. The variation in 2,4-DCP concentration with time for each system within 180 min is shown Figure 7. For systems ① and ②, the removal rates are 47.4% and 66.4%, respectively, indicating that both can effectively activate PS to remove 2,4-DCP in water. At the same dosage, the activation effect of α-MnO₂ nanowires on PS is better than that of Fe₃O₄ MNPs. For system ③, the 2,4-DCP removal rate is 53.3%, which is in between those of systems ① and ②, indicating that physical mixing of the activators and oxidant cannot significantly increase the activation capacity of the system. For system ④, the degradation rate of 2,4-DCP reaches 96.3%, which is much higher than that of other systems, indicating that for the Fe₃O₄/α-MnO₂ MNCs prepared by the hydrothermal synthesis method and the ultrasonic coprecipitation method, the activation capacity is significantly improved, and the activation performance is much better than that of the systems ① (Fe₃O₄ MNPs alone), ② (α-MnO₂ nanowires alone) and ③ (mixture of Fe₃O₄ and α-MnO₂), indicating that there is a synergistic catalytic effect between Fe₃O₄ and α-MnO₂ in the MNCs. Similar synergistic effects due to electron transfer have been found in other studies [40,41]. In addition, in the process of multiple recycling and reuse cycles, the Fe₃O₄/α-MnO₂ MNCs still maintain high activation performance, indicating that in the process of activating PS to degrade pollutants, the leaching effect is limited, and the reaction mainly occurs on the surface of the MNCs.

3.5.2. Speculation of the Reaction Mechanism for the Removal of 2,4-DCP by the Fe₃O₄/α-MnO₂-Activated PS System

The speculated mechanism for the removal of 2,4-DCP by the Fe₃O₄/α-MnO₂-activated PS system based on the experimental results is shown in Figure 8. In the process of 2,4-DCP degradation by the Fe₃O₄/α-MnO₂-activated PS system, Fe(II) and Mn(IV) on the surface can react with PS to generate free radicals, as shown in Equations (1) and (2).

S₂O₈²⁻ + ≡Fe(II)→≡Fe(III) + SO₄⁻• + SO₄²⁻

(1)

S₂O₈²⁻ + ≡Mn(IV)→S₂O₈⁻• + ≡Mn(III)

(2)

The Mn(III) formed by the reaction of Mn(IV) with PS can continue to activate PS to produce sulfate radicals, as shown in Equation (3).

S₂O₈²⁻ + ≡Mn(III)→≡Mn(IV) + SO₄⁻• + SO₄²⁻

(3)

In addition, a small amount of Fe²⁺ leached into the solution can activate PS to produce sulfate radicals, as shown in Equation (4).

S₂O₈²⁻ + Fe²⁺→Fe³⁺ + SO₄²⁻ + SO₄⁻•

(4)

These three ways of generating free active radicals are the same as those in the system with the physical mixture of Fe₃O₄ MNPs, α-MnO₂ nanowires and PS. However, the comparison of the results of different systems shows that the 2,4-DCP removal rate in the Fe₃O₄/α-MnO₂ MNCs + PS system is 43% higher than that in the system with the physical mixture of Fe₃O₄ MNPs, α-MnO₂ nanowires, and PS, indicating that there might be other reactions for radical generation in the Fe₃O₄/α-MnO₂-activated PS system, which enhances the activation performance of the Fe₃O₄/α-MnO₂ MNCs, thus increasing the degradation rate of 2,4-DCP. Therefore, it is speculated that in the Fe₃O₄/α-MnO₂-activated PS system, there may be electron transfer between the transition metals on the surface of MNCs, i.e., the Fe(III) and Mn(III) on the surface of MNCs undergo redox reaction to realize electron transfer, Mn(III) loses an electron to produce Mn(IV), and Fe(III) gains an electron to produce Fe(II), as shown in Equation (5).

≡Fe(III) + ≡Mn(III)→≡Fe(II) + ≡Mn(IV)

(5)

Therefore, the number of Fe(II) sites on the MNC surface that can activate PS is increased, as illustrated in the dashed box in Figure 8. The electron transfer on the surface may be the reason for the synergistic catalytic effect between Fe₃O₄ and α-MnO₂ in Fe₃O₄/α-MnO₂ MNCs, and this synergistic catalytic effect enhances the activation performance of Fe₃O₄/α-MnO₂ MNCs and significantly improves the degradation rate of the pollutant 2,4-DCP.

4. Conclusions

In this study, a novel Fe₃O₄/α-MnO₂ MNCs were prepared by loading Fe₃O₄ MNPs on α-MnO₂ nanowires by an improved hydrothermal synthesis method combined with an ultrasonic coprecipitation method, and the optimal loading ratio was 0.7:1. The MNCs were characterized by XRD and TEM. The results showed that the Fe₃O₄/α-MnO₂ MNCs could effectively activate PS to remove 2,4-DCP from water. Under the present experiment conditions, the degradation rate of 100 mg/L 2,4-DCP reached 96.3% after 180 min of reaction. The possible reaction mechanism of the Fe₃O₄/α-MnO₂-activated PS system for the degradation of 2,4-DCP was described. Electron transfer may occur on the surface of the MNCs to produce a synergistic catalytic effect, which greatly improves the activation performance of the MNCs to PS.