JBCS

INTRODUCTION

Organic dye is one of the main pollutants discharged into water environment by textile and printing industry, and its discharge is increasing at an alarming rate. At present, the research on dye wastewater treatment mainly focuses on adsorption enrichment¹ and catalytic degradation.² Among them, the removal of dye molecules by the adsorption method requires combination with other methods for better removal, such as photocatalysis.³ In contrast, the catalytic degradation method is much simpler and more efficient because it breaks down the dye molecules directly in water. However, this method also has many problems, for example, the applicable pH range is often very narrow, and the catalyst is not easy to separate and recover, etc.^4,5

In recent years, advanced oxidation processes have shown great potential in removing organic pollutants from water, especially H₂O₂ oxidation catalyzed by Fe₃O₄ magnetic nanoparticles (MNPs).^6-8 Firstly, Fe²⁺ in Fe₃O₄ has good catalytic performance and can form a Fenton system with H₂O₂ greatly accelerating the degradation of organic pollutants by H₂O₂.⁹ Secondly, Fe₃O₄ nanoparticles have a high specific surface area, which can effectively adsorb and enrich organic pollutants, so as to further promote the complete oxidation and decomposition of dye molecules by H₂O₂.¹⁰ Thirdly, the strong magnetic properties of Fe₃O₄ MNPs make it easy to be separated and regenerated in external magnetic field, and this excellent recyclability reduces the economic cost of H₂O₂ degradation of dye wastewater.¹¹ Therefore, Fe₃O₄ MNPs catalyzed H₂O₂ treatment of organic polluted wastewater has become a research hotspot.^12,13 However, Fe₃O₄ MNPs tend to agglomerate, which usually requires various surface modification when used.¹⁴ By coating SiO₂ to modify the Fe₃O₄ magnetic core, it can overcome the disadvantage of easy agglomeration, improve its dispersibility in water, and effectively prevent the oxidation of Fe²⁺ into Fe³⁺, thus ensuring its good catalytic performance and recyclability.^15,16

Malachite green (MG) is a triphenylmethane cationic dye, which is widely used in the dye industry.^17-19 MG can also treat fungal or bacterial infections in fish, so it used to be a common drug in aquaculture.²⁰ Once MG enters the food chain with fish, it will cause carcinogenesis, teratogenesis, mutagenesis and other toxic side effects on human body.^21-23 In addition, its metabolite leucomalachite green is more toxic in living organisms.²⁴ Although MG has been listed as a prohibited drug in aquaculture by many countries, it is still illegally abused by illegal businessmen due to its low cost and good therapeutic effect on fish diseases.²⁵ In recent years, MG has also been found to be used in large quantities to treat ornamental fish diseases and to enter aquatic environments with municipal sewerage systems.²⁶ Under this background, it is of great significance to study how to quickly and effectively remove the residual MG from all kinds of complex wastewater.

In this work, Fe₃O₄@SiO₂ MNPs with good dispersibility, strong magnetism and good catalytic performance were prepared by co-precipitation and sol-gel method, and were used to catalyze the degradation of MG by H₂O₂ in wastewater at different pH. The catalytic effect of Fe₃O₄@SiO₂ on the oxidative decomposition of MG by H₂O₂ were systematically investigated with the UV-Vis spectrophotometer. The conditions of H₂O₂ concentration, catalyst dosage, initial concentration of MG, temperature and pH range were optimized to perform the best treatment effect of MG wastewater. Besides, the recycling performance of Fe₃O₄@SiO₂ catalyst was also investigated. By using Fe₃O₄@SiO₂ nanocatalysts, we expect the degradation rate of malachite green to be greatly enhanced, much higher than that without Fe₃O₄@SiO₂; and the stability, efficiency and regeneration of such kind of catalysts are expected to be excellent. At the same time, this new MG treatment method may be applied within a much wider pH range. Therefore, this new technology of Fe₃O₄@SiO₂ MNPs catalyzing H₂O₂ to rapidly and completely degrade MG wastewater under acid-base conditions provides a new economic and eco-friendly idea for treating complex industrial wastewater with different pH values.

 

EXPERIMENTAL

Chemicals and reagents

All chemicals used in this work were analytically pure. Ferric chloride hexahydrate (FeCl₃·6H₂O), ethyl orthosilicate (TEOS), anhydrous ethanol (CH₃CH₂OH), tetramethylammonium hydroxide, polyethylene glycol (PEG-4000), hydrochloric acid (HCl), hydrogen peroxide (H₂O₂, 30%), and isopropanol (IPA, C₃H₈OH) were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China); ferrous chloride tetrahydrate (FeCl₂·4H₂O) and sodium hydroxide (NaOH) were purchased from Xilong Chemical Company Limited (Guangdong, China); ammonia (25%) was purchased from Chongqing Chuandong Chemical Reagent Co. (Sichuan, China); and malachite green (MG) was purchased from Tianjin Chemical Reagent Research Institute (Tianjin, China). Deionized water (0.55 μS cm^-1) was used throughout the experiments. Deionized water prepared with HHitech Eco-S15G from Shanghai HHitech Instruments Co., Ltd. (Shanghai, China) was used to prepare the sample solutions and to wash the samples.

Preparation of Fe₃O₄@SiO₂

In a 4 mL of hydrochloric acid solution dissolved with 0.886 g (3.28 mmol) FeCl₃·6H₂O and 0.326 g (1.64 mmol) FeCl₂·4H₂O, it was rapidly added a certain volume of 1 mol L^-1 ammonia under the protection of nitrogen gas, followed by rapid stirring (120 rpm, 60 W) and sonication (40 kHz, 50 W). After mechanical stirring for 30 min, the solution was kept standing and stratifying under the action of an applied magnetic field, and then the upper clear layer was removed. The residual solids were dispersed in deionized water and separated again using an applied magnetic field. This process was repeated until the upper clear layer turned clear. Finally, a bright black Fe₃O₄ MNPs was obtained.

A mass of 30 mg of Fe₃O₄ was added to 1.5 mL of water and mixed thoroughly. Then 100 mL of IPA was added, and the solution was sonicated for 30 min. Afterwards, 3.0 g PEG-4000 was added and mechanically stirred for 10 min, and 9 mL of water was added and stirred for another 10 min. Then, TEOS was added slowly and stirred for 12 h. In the end, the final mixture was placed in an external magnetic field, removing the upper clear layer. The solid was washed with deionized water, and then removed the upper clear layer again. This process was repeated until the upper clear layer turned clear. Then the residue was sonicated for 1 h, a black solution of Fe₃O₄@SiO₂ MNPs was obtained, and stored at room temperature.

Characterization

The crystal structures of the prepared materials were characterized by the X-ray powder diffractometer (XRD, MAX-2500, Rigaku, Tokyo Japan). The morphology of the materials was observed by the scanning electron microscopy (SEM, JEM-3010, Shimadzu, Kyoto, Japan). The characteristic functional groups were investigated by the Fourier transform infrared spectrometer (FTIR, Avatar 360, Nicolet, Wisconsin, USA). The absorption spectra were obtained by ultraviolet visible spectrophotometer (UV-Vis, TU-1901, PERSEE, Beijing, China). A nitrogen adsorption/desorption apparatus (ASAP 2460, Micromeritics, Norcross, GA, USA) was employed to evaluate the Brunauer-Emmett-Teller (BET) surface areas distributions of the samples. The magnetic properties of MNPs were investigated using a vibrating sample magnetometer (VSM, VSM 7307, Lake Shore, Ohio, USA). pH was detected by a pH meter (PHS-3E, YOKE, Shanghai, China). Heating and temperature monitoring of solutions were done using thermostatic baths (thermostatic baths, DF-101Z, LICHEN, Shanghai, China).

Determination of MG standard working curve

The MG standard solutions were prepared at the concentrations of 1, 2, 3, 4, 5, 6 and 7 mg L^-1, and the maximum absorption wavelength of 617 nm was selected, and the MG standard working curve was plotted. The MG concentration in the upper clear layer will be calculated by the standard working curve afterwards.

Degradation of MG by H₂O₂ catalyzed under Fe₃O₄@SiO₂

In order to study whether the prepared Fe₃O₄@SiO₂ MNPs would show catalytic properties or not, the degradation of MG by H₂O₂ in the presence/absence of Fe₃O₄@SiO₂ MNPs was investigated by using UV-Vis spectrophotometer. Two portions of 25 mL MG solution (10 mg L^-1) were prepared: one was added with 0.06 g L^-1 of Fe₃O₄@SiO₂, while the other contained no Fe₃O₄@SiO₂. Both solutions were then treated with 0.2 mol L^-1 H₂O₂ and subjected to oscillation at 130 rpm for 25 ºC. After a specified reaction time, Fe₃O₄@SiO₂ was separated using a magnetic field, and the upper clear layer was then placed in a spectrophotometer for analysis. At the same time, the concentration of MG in the solution without Fe₃O₄@SiO₂ was measured, which was used to compare with the result obtained from the solution that contained Fe₃O₄@SiO₂. In the experiment, the degradation rate (η) was calculated according to Equation 1:

where C₀ (mol L^-1) is the initial concentration of MG; C_t (mol L^-1) is the concentration of MG at any moment.

Condition optimization

In order to improve the efficiency of degradation of MG by H₂O₂ catalyzed under Fe₃O₄@SiO₂, some corresponding parameters were optimized, including the concentration of H₂O₂, the concentration of Fe₃O₄@SiO₂, the initial concentration of MG, the degradation temperature and the initial pH of MG solution. All of these experiments were carried out according to the same procedure as the above.

Reusability experiments

The Fe₃O₄@SiO₂ was washed with deionized water to neutral and the magnetic field was applied to separate the Fe₃O₄@SiO₂ and dried under vacuum at 60 ºC to obtain the regenerated catalyst. The regenerated Fe₃O₄@SiO₂ was added to 25 mL of 10 mg L^-1 MG solution, the pH was adjusted to 6 with hydrochloric acid solution, and then 1.0 mol L^-1 H₂O₂ was added, and the Fe₃O₄@SiO₂ was separated by an external magnetic field after 5 min. The concentration of MG in the upper clear layer was measured by UV-Vis, and the degradation rate of MG was calculated accordingly and repeated five times. The regeneration performance of Fe₃O₄@SiO₂ catalyst was judged according to the degradation rate.

 

RESULTS AND DISCUSSION

Characterization of Fe₃O₄@SiO₂

The structure, morphology and magnetic properties of the prepared Fe₃O₄ and Fe₃O₄@SiO₂ are investigated by FTIR, XRD, SEM and VSM (Figures 1 to 3).

 

 

 

 

 

 

Figure 1A shows the FTIR spectra of Fe₃O₄ and Fe₃O₄@SiO₂. In Figure 1Aa, the characteristic absorption peak at 582 cm^-1 can be attributed to the vibration of Fe−O bond, which proves the successful synthesis of Fe₃O₄. In Figure 1Ab, 1090 and 944 cm^-1 are attributed to the stretching vibration of Si−OH bond; 800 cm^-1 is attributed to the bending vibration of Si−O bond; and 464 cm^-1 is attributed to the bending vibration of Si−O−Si bond. At the same time, the characteristic absorption peak of Fe−O at 582 cm^-1 is blueshifted to 570 cm^-1; and the characteristic absorption peak of −OH can be observed at 3450 cm^-1. All of these results indicate the formation of Fe−O−Si chemical bond and the successful synthesis of Fe₃O₄@SiO₂.

Figure 1B shows the XRD spectrum of the prepared Fe₃O₄ and Fe₃O₄@SiO₂. In Figure 1Ba, the characteristic peaks located at 30º, 35º, 43º, 53º, 57º and 62º correspond to the (220), (311), (400), (422), (511) and (440) crystal planes of Fe₃O₄, respectively, which is in accordance with the JCPDS card (No. 88-315). These results imply that the prepared Fe₃O₄ has a good crystal structure. In Figure 1Bb, the broad and strong diffraction peaks appeared between 20-30º are caused by the amorphous SiO₂, while the characteristic peaks of Fe₃O₄ can still be found, indicating that Fe₃O₄ has been successfully encapsulated by SiO₂ and the crystalline structure has not been changed.²⁷

Nitrogen adsorption-desorption experiments were carried out to evaluate the specific surface area and pore size distribution of Fe₃O₄@SiO₂. The results are shown in Figure 2. The overall shape of the nitrogen adsorption-desorption indicates the presence of micropores and macropores in the catalysts, with an average pore size of 7.4994 nm. Based on the BET theory, the specific surface area of Fe₃O₄@SiO₂ was calculated to be 102.1323 m² g^-1. The results suggest that the high specific surface area of Fe₃O₄@SiO₂ can provide lots of binding sites for the dye molecule MG, which facilitates its adsorption and enrichment and the final quick removal by H₂O₂.²⁸

Figure 3 shows the SEM images of Fe₃O₄ (Figure 3A) and Fe₃O₄@SiO₂ (Figure 3B). It can be seen from Figure 3A that Fe₃O₄ magnetic nanonuclei are well dispersed and uniform in size, about 10 nm. In Figure 3B, the modified Fe₃O₄@SiO₂ MNPs are also uniform spherical particles in size, about 80 nm, bigger with better dispersibility due to the coating of SiO₂.

Figure 4 shows the VSM diagrams of Fe₃O₄ and Fe₃O₄@SiO₂. Both of the magnetic hysteresis lines are symmetric to the origin, and the saturation magnetization strengths of the prepared Fe₃O₄ and Fe₃O₄@SiO₂ are 68.07 and 50.45 emu g^-1, respectively. It can be seen compared to Fe₃O₄, the magnetization value of Fe₃O₄@SiO₂ decreases due to the coating of non-magnetic SiO₂. However, it still exhibits strong magnetic properties, which ensures rapid separation under the action of an external magnetic field and provides great feasibility for easy recovery and regeneration required by various applications.

 

 

Standard curve for malachite green

The working wavelength of malachite green solution was chosen to be 617 nm, and the standard curve was obtained as in Figure 5, with a linear relationship of y = 0.1482x + 0.0032, R² (coefficient of determination) = 0.9995.

 

 

Performance of Fe₃O₄@SiO₂ catalyzed H₂O₂ degradation of MG

As mentioned above, after verifying the good magnetic properties of Fe₃O₄@SiO₂, its catalytic performances for wastewater degradation are studied, and results are shown in Figure 6. Figures 6A and 6B are the UV-Vis spectra of the upper clear layers (see Experimental section), obtained from the reaction systems of the degradation of MG by H₂O₂ in the presence/absence of Fe₃O₄@SiO₂, respectively. When Fe₃O₄@SiO₂ was added, the degradation rate (η) reached 97% in just 5 min, while it took 720 min to achieve the same rate without the addition of Fe₃O₄@SiO₂. Obviously, the degradation time can be greatly shortened by adding Fe₃O₄@SiO₂, which proves its excellent catalytic properties for MG degradation.

 

 

The pseudo-first-order kinetic model is used to better investigate the catalytic effect of Fe₃O₄@SiO₂ on the degradation of MG by H₂O₂, and the degradation rate is calculated according to the following Equation 2.

where C₀ (mol L^-1) is the initial concentration of MG; C_t (mol L^-1) is the MG concentration at any moment; k (min^-1) is the degradation rate constant, and t (min) is the degradation time. Figures 6C and 6D show the diagrams according to Equation 2 for the pseudo-first-order kinetic models in the presence/absence of Fe₃O₄@SiO₂, respectively. It can be seen that the degradation rate constant is only 0.005 min^-1 without Fe₃O₄@SiO₂ but 0.893 min^-1 with the addition of Fe₃O₄@SiO₂, a great increase of 179 times. This indicates that the prepared Fe₃O₄@SiO₂ has good catalytic performance, and it can effectively catalyze H₂O₂ to degrade MG at a very rapid speed. After determining the catalytic performance of Fe₃O₄@SiO₂, the degradation conditions are optimized, including the concentration of H₂O₂, Fe₃O₄@SiO₂ or MG, the degradation temperature and the pH.

Condition optimization

Firstly, the influence of the concentration of H₂O₂ in the degradation effect is investigated. It was added 0.12 g L^-1 of Fe₃O₄@SiO₂ to 10 mg L^-1 of MG solution and H₂O₂ concentrations were altered from 0.04 to 1.2 mol L^-1, at the room temperature and pH 7. The results are shown in Figure 7. Obviously, from Figure 7A, it can be seen that all the investigated concentration of H₂O₂ (even as low as 0.04 mol L^-1) can achieve an MG degradation rate of higher than 98% within 10 min. This is precisely because of the outstanding catalytic effect of the synthetic MNPs Fe₃O₄@SiO₂. For further investigation, data reprocessing and reanalysis on Figure 7A were carried out and the results are shown in Figures 7B-7D. Figure 7B shows the relationship between H₂O₂ concentration and degradation rate at 10 min of degradation, from which it can be seen that the degradation rate gradually increases as the H₂O₂ concentration rises to 1.0 mol L^-1, and the degradation rate reaches up to 99.6% at 1.0 mol L^-1; while the degradation rate gradually decreases after exceeding 1.0 mol L^-1, but still remains above 98%. Figure 7C shows the relationship between H₂O₂ concentration and degradation time when the degradation rate reaches 95%, from which it can be seen that the degradation time shortens rapidly as the H₂O₂ concentration increases to 1.0 mol L^-1, and the degradation time is the shortest at 1.0 mol L^-1, even less than 5 min; while the degradation time has no significant change after exceeding 1.0 mol L^-1. Figure 7D shows the relationship between H₂O₂ concentration and degradation rate constant. It can be seen that as the H₂O₂ concentration increases to 1.0 mol L^-1, the degradation rate also goes up and the maximum degradation rate reaches 1.45 min^-1; while the degradation rate constant is almost unchanged after exceeding 1.0 mol L^-1. These phenomena may be due to the fact that the degradation of MG is achieved through the decomposing of H₂O₂ into ·OH. When the concentration of H₂O₂ is low, the amount of ·OH is less, and the degradation rate of MG is low. With the increase of H₂O₂ concentration and the amount of ·OH, the degradation rate of MG is accelerated, and the degradation time is shortened. Finally, when the concentration of H₂O₂ increases to a certain extent, excess H₂O₂ can become a scavenger of ·OH for they can react to form hydroxyl peroxide radical (HOO·), leading to a decrease in the degradation rate of MG.^29,30

 

 

Secondly, the influence of the concentration of Fe₃O₄@SiO₂ in the degradation effect is investigated. It was added 1.0 mol L^-1 of H₂O₂ to 10 mg L^-1 of MG solution and Fe₃O₄@SiO₂ concentrations were altered from 0.03 to 0.24 g L^-1, at room temperature and pH 7. The results are shown in Figure 8. As can be seen from Figure 8A, the Fe₃O₄@SiO₂ concentration in all studies (even as low as 0.03 g L^-1) shows good catalytic effect, which is reflected in the MG degradation rate higher than 95% within 10 min. For further investigation, data reprocessing and reanalysis on Figure 8A were carried out and the results are shown in Figures 8B-8D. Figure 8B shows the relationship between the Fe₃O₄@SiO₂ concentration and the degradation rate at 10 min. Obviously, the degradation rate rises as the concentration of Fe₃O₄@SiO₂ increases and all of them are above 95%. When the Fe₃O₄@SiO₂ concentration is greater than 0.12 g L^-1, the degradation rate can even reach higher than 99%. Figure 8C shows the relationship between the Fe₃O₄@SiO₂ concentration and the degradation time when the degradation rate is 95%. The required time of reaching 95% degradation rate keeps shortening with the increase of the Fe₃O₄@SiO₂ concentration, and all of them are less than 10 min. When the concentration is greater than 0.12 g L^-1, the required time is even less than 3 min. Figure 8D shows the relationship between the Fe₃O₄@SiO₂ concentration and the degradation rate constant. The degradation rate constant rises quickly before the Fe₃O₄@SiO₂ concentration increases up to 0.12 g L^-1, and it rises slowly after exceeding it. These phenomena indicate the excellent catalytic performance of the prepared Fe₃O₄@SiO₂. This catalyst can provide more active centers before its concentration is increased to 0.12 g L^-1, leading to faster degradation reactions and greater improvement in degradation rates. Meanwhile, more catalysts can adsorb more MG so as to accelerate its decomposition. When the Fe₃O₄@SiO₂ concentration exceeds 0.12 g L^-1, due to the fixed amount of H₂O₂, no matter how much the catalyst is increased, it can only catalyze a limited amount of H₂O₂. Therefore, when the catalyst is increased to a certain number, the acceleration in the degradation reaction will slow down.

 

 

Thirdly, the influence of the initial MG concentrations in the degradation effect is investigated. It was added 1.0 mol L^-1 of H₂O₂ and 0.12 g L^-1 of Fe₃O₄@SiO₂ to different initial MG concentrations solutions (10-30 mg L^-1), at room temperature and pH 7. The results are shown in Figure 9. As can be seen from Figure 9A, the degradation rate of all the initial MG concentrations studied (even up to 30 mg L^-1) can reach more than 99% within 120 min. For further investigation, data reprocessing and reanalysis on Figure 9A were carried out and the results are shown in Figures 9B-9D. Figure 9B shows the relationship between the initial concentration of MG and the degradation rate at the time of 120 min. The degradation rate rises a little bit before the initial MG concentration increases to 20 mg L^-1 and the degradation rate reaches 99.7% at 20 mg L^-1. After exceeding 20 mg L^-1, the degradation rate decreases a little but still remains above 99%. Figure 9C shows the relationship between the initial MG concentration and the degradation time when the degradation rate reaches 95%. As the initial concentration of MG increases to 30 mg L^-1, the degradation time keeps prolonging but it is still within 80 min. Figure 9D shows the relationship between the initial MG concentration and the degradation rate constant. Before the initial MG concentration is increased to 30 mg L^-1, the degradation rate constant decreases significantly at first, and then remains basically unchanged (0.034 min^-1), but is obviously higher than the rate without the use of catalyst (0.005 min^-1). With the increase of the initial concentration of MG, the amount of ·OH required for degradation increases, but the amount of ·OH produced by H₂O₂ decomposition is limited. Therefore, when the initial concentration of MG exceeds a certain value, the amount of hydrogen peroxide that it can consume is still limited, so the change of the initial MG concentration no longer significantly affects the degradation rate.

 

 

Then, the influence of the degradation temperature in the degradation effect is investigated. It was added 1.0 mol L^-1 of H₂O₂ and 0.12 g L^-1 of Fe₃O₄@SiO₂ to 10 mg L^-1 of MG solution, at pH 7 and different temperatures (15-70 ºC). The results are shown in Figure 10. As can be seen from Figure 10A, the degradation rate of MG can reach more than 95% within 15 min when the degradation temperature is above 25 ºC. For further investigation, data reprocessing and reanalysis on Figure 10A were carried out and the results are shown in Figures 10B-10D. Figure 10B shows the relationship between the degradation temperature and the degradation rate at 15 min. Before the degradation temperature is increased to 70 ºC, the degradation rate rises significantly at first and then remains basically unchanged, and all the degradation rates can reach more than 99% when the temperature is above 25 ºC. Figure 10C shows the relationship between the degradation temperature and the degradation time when the degradation rate is 95%. With the temperature rising to 70 ºC, the degradation time is continuously shortened, all within 10 min. Figure 10D shows the relationship between the degradation temperature and the degradation rate constant. The degradation rate constant increases with the temperature rising to 70 ºC, and are all greater than 0.2 min^-1. It can be seen that the catalyst still has high activity when the temperature is 70 ºC, indicating that the catalyst has good stability. These phenomena may be due to the fact that with the increase of temperature, the molecular motion speed is accelerated, which increases the chance of MG molecules colliding with Fe₃O₄@SiO₂ surface and H₂O₂, thus improving the degradation rate of MG. As is well known to us, higher reaction temperatures can provide more energy to overcome the activation energy barrier. The rate of decomposition of H₂O₂ into •OH increases exponentially with the increase of reaction temperature. Therefore, as the temperature increases, more ·OH will be produced in the system, so the degradation effect of MG will naturally be better. Therefore, it seems that the degradation temperature of 70 ºC is the best, but considering that the degradation rate at 25 ºC has reached 99%, increasing the temperature will also increase the cost and operation difficulty, so we believe that room temperature can be selected as the degradation temperature of MG.

 

 

Lastly, the influence of the pH value in the degradation effect is investigated. It was added 1.0 mol L^-1 of H₂O₂ and 0.12 g L^-1 of Fe₃O₄@SiO₂ to 10 mg L^-1 of MG solution, at the room temperature and at different pH (1-10). The results are shown in Figure 11. It can be seen from Figure 11A that under all pH conditions tested, MG degradation can reach equilibrium within 10 min. Figure 11B shows the relationship between the degradation pH and the degradation rate at 10 min. When the pH value increases from 1 to 4, the degradation rate of MG gradually decreases within 10 min, especially when pH is 4, the degradation rate is only 22.7%. When the pH value increases from 4 to 10, the degradation rate first rises remarkably and then remains basically unchanged, and when the pH is 10, the degradation rate is the highest, reaching 99.8%. Figure 11C shows the relationship between the degradation pH and the degradation time when the degradation rate reaches 95%. When the pH value increases from 1 to 4, the degradation time exceeds 20 min, especially when pH is 4, the degradation time is as long as 120 min. When the pH value is increased from 5 to 10, the degradation time is less than 5 min, and when the pH value is 10, the degradation time is the shortest, even less than 3 min. Figure 11D shows the relationship between the pH value and the degradation rate constant. When the pH value increases from 1 to 4, the degradation rate constant is lower than 0.08 min^-1, especially when pH is 4, the degradation rate constant is as low as 0.01 min^-1. When the pH value increases from 5 to 10, the degradation rate constant is greater than 1 min^-1, especially when pH is 10, the degradation rate constant is as high as 1.5 min^-1. This indicates that the catalyst Fe₃O₄@SiO₂ can maintain good activity in a wide range of pH values. The phenomena from pH 1 to 4 can be explained by the existence of the surface zero point of charge (pHzpc) of Fe₃O₄@SiO₂ (about 2.8-3.4). When pH < pHzpc, the surface of Fe₃O₄@SiO₂ is positively charged, which will repel the positively charged MG, and the closer the pH is to pHzpc, the stronger the repulsive force will be. Therefore, in the range of pH 1 to 4, the degradation rate of MG gradually decreases. When pH > pHzpc, the surface of Fe₃O₄@SiO₂ is negatively charged, which can attract positively charged MG, leading to the increase in the degradation rate of MG in the pH range of 5-10.

 

 

Reusability experiments

After optimizing the degradation conditions of MG oxidized by H₂O₂ under the catalyzation of Fe₃O₄@SiO₂, the reusability of the catalyst is investigated, and the results are shown in Figure 12. It can be seen that the degradation rate can still reach more than 80% after 5 cycles, indicating the outstanding reusability of the prepared Fe₃O₄@SiO₂ MNPs. Combined with its good catalytic performance and unique characteristics of easy separation in magnetic field, its application in the treatment of industrial dye wastewater will have excellent economic value and important environmental significance.

 

 

Comparative study and limitations

In this study, the removal performance of MG in the wastewater by the Fenton-like reaction has been compared. The experimental conditions such as removal time, amount of catalysts, pH, and maximum removal capacity are listed in Table 1. It is found that Fe₃O₄@SiO₂ has an outstanding catalytical effect to enhance the degradation efficiency of MG by H₂O₂. In this paper, a low dose of remover is used, which can save materials and reduce costs. Moreover, the time required for the 95% removal of MG by H₂O₂ under the catalyzation of Fe₃O₄@SiO₂ is only 3 min, greatly reducing the time cost and enhance the removal efficiency. All of these results indicate that Fe₃O₄@SiO₂ can be used as a promising catalyst to catalyze H₂O₂ for the treatment of MG in real wastewater.

 

 

CONCLUSIONS

In this study, Fe₃O₄@SiO₂ is used to catalyze the rapid and efficient degradation of MG by H₂O₂. Firstly, the structure, morphology and magnetic properties of the catalyst Fe₃O₄@SiO₂ are characterized by FTIR, XRD, VSM, SEM. Then, systematical investigations about this degradation reaction are carried out by UV-Vis, including the mechanism, the kinetic model and the optimization of degradation conditions. Results show that under the optimal conditions of C (H₂O₂) = 1.0 mol L^-1, C (Fe₃O₄@SiO₂) = 0.12 g L^-1, C₀ (MG) = 10 mg L^-1, at room temperature and at pH 5-10, Fe₃O₄@SiO₂ can efficiently catalyze MG degradation by H₂O₂ and the degradation time to reach 95% degradation rate is less than 3 min. The catalyst can be separated quickly under the applied magnetic field, and the degradation rate is 82% after 5 times of recycling. Compared with no catalyst, the degradation time is greatly shortened and the degradation efficiency is significantly improved. Under acidic or alkaline conditions, the catalyst exhibits good performance at both room temperature and higher temperature. In conclusion, the new process of Fe₃O₄@SiO₂ catalyzed H₂O₂ degradation of MG wastewater developed in this paper is convenient, rapid, economical and environmentally friendly, and is expected to be widely used in the treatment of industrial dye wastewater under various complex conditions.

 

ACKNOWLEDGMENTS

This research was funded by the National Nature Science Foundation of China (NSFC No. 21505005), Hunan Provincial Natural Science Foundation (No. 2018JJ2424), International Cooperative Project for “Double First-Class”, CSUST (2019IC21). No potential conflict of interest was reported by the authors. All data generated or analyzed during this study are included in this published article.

 

REFERENCES

1. Abdelhamid, H. N.; Dalton Trans. 2023, 52, 2506. [Crossref]

2. Xiong, H.; Shi, K.; Han, J.; Cui, C.; Liu, Y.; Zhang, B.; Environ. Sci. Pollut. Res. 2023, 30, 59366. [Crossref]

3. Renda, C. G.; Goulart, L. A.; Fernandes, C. H. M.; Mascaro, L. H.; de Aquino, J. M.; Bertholdo, R.; J. Environ. Chem. Eng. 2021, 9, 104934. [Crossref]

4. Mahmoud, S. M.; Ammar, S. H.; Ali, F. D.; Ali, N. D.; J. Iran. Chem. Soc. 2023, 20, 1135. [Crossref]

5. Takdastan, A.; Pourfadakari, S.; Yousefi, N.; Orooji, N.; Desalin. Water Treat. 2020, 197, 402. [Crossref]

6. Nas, M. S.; Kaya, H.; Inorg. Nano-Met. Chem. 2021, 51, 614. [Crossref]

7. Zhang, F.; Xue, X.; Huang, X.; Yang, H.; J. Mater. Sci. 2020, 55, 15695. [Crossref]

8. Barakat, M. A.; Kumar, R.; Lima, E. C.; Seliem, M. K.; J. Taiwan Inst. Chem. Eng. 2021, 119, 146. [Crossref]

9. Cen, H.; Nan, Z.; J. Phys. Chem. Solids 2018, 121, 1. [Crossref]

10. Mansur, A. A. P.; Leonel, A. G.; Krambrock K.; Mansur, H. S.; Catal. Today 2022, 397, 129. [Crossref]

11. Angamuthu, M.; Satishkumar, G.; Landau, M. V.; Microporous Mesoporous Mater. 2017, 251, 58. [Crossref]

12. Hu, X.; Liu, B.; Deng, Y.; Chen, H.; Luo, S.; Sun, C.; Yang, P.; Yang, S.; Appl. Catal., B 2011, 107, 274. [Crossref]

13. Zhang, M.; Niu, Y.; Xu, Y.; J. Colloid Interface Sci. 2020, 579, 269. [Crossref]

14. Bai, J.; Wang, L.; Wu, J.; Zhang, X.; Liu, B.; Mao, Y.; Gao, H.; Li, S.; Zhu, X.; Wang, X.; Water, Air, Soil Pollut. 2023, 234, 1871. [Crossref]

15. Ye, L.; Zhu, P.; Wang, T.; Li, X.; Zhuang, L.; Nanoscale Adv. 2023, 5, 4852. [Crossref]

16. Ahmadian, M.; Derakhshankhah, H.; Jaymand, M.; J. Ind. Eng. Chem. 2023, 124, 102. [Crossref]

17. Goswami, R.; Mishra, A.; Prasad, B.; Bhatt, N.; Water, Air, Soil Pollut. 2023, 234, 1972. [Crossref]

18. Mehdinia, A.; Shilsar, S. M. S.; Mozaffari, S.; Abedi, S.; J. Iran. Chem. Soc. 2023, 20, 1073. [Crossref]

19. Hassan, A. F.; El-Naggar, G. A.; Braish, A. G.; El-Latif, M. M. A.; Shaltout, W. A.; Elsayed, M. S.; Int. J. Biol. Macromol. 2023, 249, 126075. [Crossref]

20. Zhao, S.-S.; Ma, C.-J.; Xu, Y.; Tan, X.-C.; Wang, Q.; Yan, J.; Microchim. Acta 2023, 190, 200. [Crossref]

21. Shetty, B.; Yashodha, S. R.; Johns, J.; Fibers Polym. 2023, 24, 1297. [Crossref]

22. Çiftçi, G.; Türkeli, U. D.; Özen, E. Y.; Özdemir, M.; Sanin, F. D.; İmamoğlu, İ.; Water Sci. Technol. 2023, 87, 1072. [Crossref]

23. Aridi, A.; Basma, H.; Chehade, W.; Hassan, R. S.; Yaacoub, N.; Naoufal, D.; Awad, R.; Environ. Sci. Pollut. Res. 2023, 30, 58399. [Crossref]

24. Zheng, C.; Chen, J.; Ling, Y.; Zhang, Z.; Microchem. J. 2024, 205, 111299. [Crossref]

25. Du, H. Y.; He, L.; Zhang, M.; Manyande, A.; Chen, H.; Journal of Food Measurement and Characterization 2023, 17, 3368. [Crossref]

26. Wang, J.; Hong, C.; Lin, Z.; Huang, Z.-Y.; J. Food Drug Anal. 2022, 30, 369. [Crossref]

27. Ji, X.; Qiao, R.; Xu, Z.; Liu, J.; Ma, Q.; Yuan, H.; Xing, H.; J. Mater. Sci.: Mater. Electron. 2024, 35, 1912. [Crossref]

28. Cai, X.; Zhang, Y.; Hu, H.; Huang, Z.; Yin, Y.; Liang, X.; Qin, Y.; Liang, J.; J. Cleaner Prod. 2020, 258, 120741. [Crossref]

29. Hou, K.; Wang, G.; Zhu, Y.; Ezzatahmadi, N.; Fu, L.; Tang, A.; Yang, H.; Xi, Y.; Appl. Clay Sci. 2019, 181, 105243. [Crossref]

30. Ansari, M. S.; Rases, K.; Khan, M. A.; Rafiquee, M. Z. A.; Otero, M.; Polymers 2020, 12, 2246. [Crossref]

31. Wu, J.; Yang, J.; Feng, P.; Wen, L.; Huang, G.; Xu, C.; Lin, B.; J. Ind. Eng. Chem. 2022, 113, 206. [Crossref]

32. Van Tran, T.; Nguyen, D. T. C.; Kumar, P. S.; Din, A. T. M.; Qazaq, A. S.; Vo, D.-V. N.; Environ. Res. 2022, 214, 113925. [Crossref]

33. Yulizar, Y.; Abdullah, I.; Surya, R. M.; Alifa, N. L.; J. Environ. Manage. 2023, 342, 118139. [Crossref]

34. Jabeen, S.; Ganie, A. S.; Ahmad, N.; Hijazi, S.; Bala, S.; Bano, D.; Khan, T.; Inorg. Chem. Commun. 2023, 152, 110729. [Crossref]

35. Puthukkara, P. A. R.; Jose, T. S.; Lal, S. D.; Mater. Today Commun. 2022, 33, 104759. [Crossref]

36. Jiang, J.; Song, W.; Zhang, M.; Feng, Y.; Bai, H.; Zhang, H.; Yu, K.; Kan, G.; Liu, B.; Jiang, Y.; ACS Appl. Nano Mater. 2023, 6, 12882. [Crossref]

37. Khattak, R.; Begum, B.; Qazi, R. A.; Gul, H.; Khan, M. S.; Khan, S.; Bibi, N.; Han, C.; Rahman, N. U.; Catalysts 2023, 13, 227. [Crossref]

38. Jing, H.; Ji, L.; Li, Z.; Wang, Z.; Li, R.; Ju, K.; Biochar 2023, 5, 1. [Crossref]

39. Kolya, H.; Kang, C.-W.; Sustainability 2022, 14, 15800. [Crossref]

 

Associate Editor handled this article: Lucia Mascaro