Fe₃O₄–MWCNT magnetic nanocomposites as efficient peroxidase mimic catalysts in a Fenton-like reaction for water purification without pH limitation

Abstract

The Fenton-based reaction is powerful enough to decompose refractory organic pollutants, but it is limited by having a low pH range and it is necessary to have a secondary disposal of the iron sludge. This study demonstrates that Fe₃O₄–multi-walled carbon nanotube (Fe₃O₄–MWCNT) magnetic hybrids can be used as an efficient peroxidase mimic catalyst that could overcome such pH limitations in a Fenton-like reaction and could be reused after a simple magnetic separation. The Fe₃O₄–MWCNT hybrid was prepared using a simple one-pot strategy via in situ growth of Fe₃O₄ magnetic nanoparticles onto the surface of the MWCNTs. In this process, MWCNTs act as an excellent dispersant, which ensures that the Fe₃O₄ is well dispersed. The Fe₃O₄–MWCNT hybrid was characterized by X-ray diffractometry, Fourier transform infrared spectrometer, thermogravimetric analysis and vibrating sample magnetometry, which indicated that the Fe₃O₄ nanoparticles were successfully deposited on to the surface of MWCNTs. Furthermore, it was revealed that the Fe₃O₄–MWCNTs could catalyze H₂O₂ decomposition by acting as a peroxidase mimic catalyst. Then heterogenous Fenton-like reactions were performed using the Fe₃O₄–MWCNT nanocomposites as a catalyst to degrade methylene blue (10.0 mg L⁻¹; MB) in aqueous solution. The results showed that MB could be efficiently removed in a broad pH range of 1.0–10.0, with a degradation efficiency of 88.13% to 98.68% in two hours, and a highest total organic carbon removal efficiency of 35.6% in 12 hours. Furthermore, the magnetic nanocomposites exhibited an enhanced removal efficiency for MB compared with the Fe₃O₄ magnetic nanocomparticles and MWCNTs used individually. In addition, Fe₃O₄–MWCNT nanocomposites exhibited strong magnetism, and thereby could be easily separated from aqueous solution using an external magnetic field. Therefore, the as-prepared Fe₃O₄–MWCNT nanocomposites could be used as a promising and effective catalyst in Fenton-like reactions for the purification of MB polluted water in a wide pH range.

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1. Introduction

Concern over the organic pollutants released from the dyestuff industry into water has long existed and attracted much attention. To purify such polluted water, various methods, including physical,^1–3 chemical^4–6 and biological methods^7,8 have been developed. Among them, a homogeneous solution mixture of iron ions and H₂O₂ as the classic Fenton reagent is very effective. However, it has some disadvantages such as a limited pH range and it requires secondary disposal of the iron sludge. In order to overcome these shortcomings, Fenton's catalyst can be incorporated onto a solid support. For example, Hu et al. have employed two clay-supporting Fe nanocomposites (Fe-B (Fe supported on bentonite clay) and Fe-Lap-RD (Fe supported on laponite clay)) for degradation of Orange II dye.⁹ Zhou et al. have reported the use of porous carbon microspheres as a heterogeneous Fenton's catalyst for decomposing methylene blue (MB).¹⁰ Following the report of the intrinsic enzyme mimetic activity possessed by Fe₃O₄ magnetite nanoparticles (MNPs),¹¹ the use of Fe₃O₄ has also been explored as alternative heterogeneous Fenton-like catalyst for the degradation of organic pollutants such as phenolic and aniline compounds,^12,13 rhodamine B¹⁴ and MB.^15a More recently, use of magnetic Fe₃O₄@C nanoparticles as a Fenton-like catalyst was reported for the decoloration of MB.^15b Unfortunately, Fe₃O₄ still has some shortcomings such as spontaneous aggregation because of high surface energy, which usually leads to a significant loss of their activity and low hydrogen peroxide (H₂O₂) activating ability. In addition, this type of magnetic catalyst is limited by a low and narrow pH working range for the removal of the target in the aqueous phase. Thus, improving the activity of Fe₃O₄ magnetic nanocomposites is highly desirable and valuable.

In recent years, multi-walled carbon nanotubes (MWCNTs) have received great attention because of their remarkable features of a large specific surface area, and hollow and layered structures. Also, the distinctive sp² hybridized carbon bonds in MWCNTs enables them to be a good electronic transfer support. Thus, to improve the H₂O₂ activating ability of Fe₃O₄ MNPs, in this paper, a simple one-pot approach was adopted for the preparation of Fe₃O₄–MWCNT nanocomposites via in situ growth of Fe₃O₄ MNPs onto the surface of MWCNTs. In this process, the MWCNTs act as an excellent dispersant, which ensures that the Fe₃O₄ is well dispersed. The Fe₃O₄–MWCNT hybrid obtained presents an improved catalytic effect toward the reduction of H₂O₂. The as-prepared nanocomposites were then used to remove MB by a Fenton-like reaction. MB is a typical organic dye and has been widely used for coloring paper, in temporary hair colorants, dyeing cottons, wools, and as a coating for paper stock.¹⁶ MB is also resistant to biodegradation. A long-term exposure to MB will cause harmful effects in humans.¹⁷

Our results demonstrated that Fe₃O₄–MWCNT magnetic nanocomposites could effectively adsorb and degrade MB in the presence of H₂O₂ at a wide pH range of 1.0 to 10.0. Furthermore, the magnetic nanocomposites exhibited an enhanced removal efficiency for MB compared with the Fe₃O₄ MNPs and MWCNTs alone. In addition, the resulting Fe₃O₄–MWCNTs presented a high magnetic sensitivity under an external magnetic field, providing an easy and efficient way for the separation of catalysts from aqueous solution. The higher H₂O₂-activating ability, high magnetism and good reusability make Fe₃O₄–MWCNT nanocomposites a promising and an effective catalyst free of pH limitation, for the removal of MB by Fenton-like reaction from wastewater.

2. Experimental section

2.1. Chemicals and reagents

Methylene blue, ferrous ammonium sulfate ((NH₄)₂Fe(SO₄)₂·6H₂O), ammonium ferric sulfate (NH₄Fe(SO₄)₂·12H₂O), ammonia, H₂O₂, sodium hydroxide (NaOH), nitric acid (HNO₃) and hydrochloric acid (HCl) were purchased from Chongqing Chemical Reagents Co., Ltd. (Chongqing, China). All chemicals used in this work were of analytical grade and used as received without further purification. 3,3′,5,5′-Tetramethylbenzidine (TMB) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were obtained from Sigma-Aldrich (Shanghai, China). Ultrapure water was prepared in the laboratory using a water treatment device. The solution pH was adjusted using diluted HCl and NaOH solutions. All glassware was soaked in the diluted HNO₃ and thoroughly cleaned before use.

2.2. Preparation of Fe₃O₄ MNPs and Fe₃O₄–MWCNT magnetic nanocomposites

The Fe₃O₄–MWCNT magnetic nanocomposites were synthesized based on the method reported by Wang et al.¹⁸ with a minor modification. Briefly, 2.0 g MWCNTs were added to a three-necked bottle. After adding 10 mL of the concentrated HNO₃, the suspension was transferred to a 60 °C water bath with constant stirring for 12 h. Then, the reaction mixture was naturally cooled down to room temperature and washed four times with ultrapure water. After drying for 10 h in a 100 °C oven, 1.5 g of the MWCNTs obtained were added to 200 mL of an aqueous solution containing 1.7 g (NH₄)₂Fe(SO₄)₂·6H₂O and 2.5 g NH₄Fe(SO₄)₂·12H₂O at 50 °C in a water bath under a N₂ atmosphere and mixed ultrasonically for 10 min. During this time, 10 mL of 8 M ammonia was added to precipitate the iron oxide. After the suspension was kept stirring for 30 min, the resulting Fe₃O₄–MWCNT magnetic nanocomposites were separated using a magnet and then washed three times with ultrapure water. The solids obtained were dried and used for characterization by scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). The synthesis of Fe₃O₄ MNPs was similar to the previous procedure except for the addition of MWCNTs.

2.3. Characterization and detection techniques

The XRD patterns of the as-prepared products were measured using an XD-3 X-ray diffractometer (PuXi, Beijing, China) with nickel filtered Cu Kα radiation (λ = 0.15406 nm) at a current of 20 mA and a voltage of 36 kV. The scanning rate was 4° min⁻¹ in the angular range of 5–80° (2θ). FT-IR spectra were recorded on a Nicolet 170-SX instrument (Madison, WI, USA) in transmission mode using KBr pellets of the sample. The TGA was performed on a TA-STD Q600 (TA Instruments Inc., New Castle, DE, USA) instrument. The magnetic properties of Fe₃O₄ MNPs and Fe₃O₄–MWCNT magnetic nanocomposites were analyzed using a HH-15 VSM. Total organic carbon (TOC) measurement was made using a Hach IL TOC 550 Total Organic Carbon Analyzer (Hach, USA) by subtracting the inorganic carbon value from total carbon value which were determined using a standard curve. TEM images were captured on a Tecnai G2 T20 electron microscope (FEI) at 200 kV. The concentration of MB was obtained by measuring the absorbance of the solution at 662 nm on a model UV-2450 spectrophotometer (Shimadzu, Suzhou, China).

2.4. Removal of MB by Fenton-like reaction

The Fenton-like experiment for the catalytic removal of MB was carried out at room temperature by adding Fe₃O₄–MWCNT magnetic nanocomposites into 25 mL of a 10 mg L⁻¹ MB solution in the presence of H₂O₂. After the suspension was shaken with a thermostatted shaker at 180 rpm for 30 min, H₂O₂ was added to initiate the catalytic reaction. After 12 h, an Nd-Fe-B strong magnet (50 × 50 × 25 mm) was deposited at the bottom of the beaker for the magnetic isolation of the Fe₃O₄–MWCNTs from the solution. The supernatant solution was analysed for MB and TOC. The isolated Fe₃O₄–MWCNT magnetic nanocomposites were washed three times with ultrapure water and used for the next process cycle.

3. Results and discussion

3.1. Characterization and enhanced peroxidase-like activity of Fe₃O₄–MWCNT magnetic nanocomposites

The TEM images in Fig. 1A show that the MWCNTs had sizes from 10 to 20 nm. The Fe₃O₄ particles with diameter range of 10–20 nm were in a sphere-like shape in the prepared Fe₃O₄–MWCNTs. TEM also revealed that the iron oxide MNPs were successfully decorated on the surface of the MWCNTs (Fig. 1B). The XRD patterns of MWCNTs, Fe₃O₄ and Fe₃O₄–MWCNT nanocomposites are shown in Fig. 1C. For the MWCNTs, the strong diffraction peak at 2θ = 25.8° was indexed as the (002) reflection of the hexagonal graphite structure. For Fe₃O₄, the diffraction peak at 2θ values of 18.2° (111), 30.0° (220), 35.3° (311), 42.9° (400), 53.4° (422), 56.9° (511), and 62.5° (440) were associated with the characteristic peaks of the cubic phase Fe₃O₄ MNPs. For Fe₃O₄–MWCNT magnetic nanocomposites, both the diffraction peaks of the MWCNTs and the Fe₃O₄ MNPs were observed, suggesting that Fe₃O₄–MWCNT magnetic nanocomposites had been formed.¹⁹ Fig. S1† shows the FT-IR spectra of Fe₃O₄, MWCNTs and Fe₃O₄–MWCNT nanocomposites. The peaks at 582 cm⁻¹ and 442 cm⁻¹ in the Fe₃O₄ MNPs were assigned to the Fe–O modes.¹⁹ The characteristic peaks of Fe₃O₄ MNPs could still be observed in Fe₃O₄–MWCNT nanocomposites, although a slight red shift or blue shift (574 cm⁻¹ or 449 cm⁻¹) had occurred. This suggested the formation of the Fe₃O₄–MWCNT nanocomposites and the interaction between Fe₃O₄ MNPs and MWCNTs. The TGA curves of Fe₃O₄, MWCNTs, and Fe₃O₄–MWCNT nanocomposites are shown in Fig. 1D. The weight loss at temperatures lower than 100 °C was because of water vapour. It was noted that for Fe₃O₄–MWCNT magnetic nanocomposites, the weight loss at temperatures higher than 500 °C was because of the oxidation loss of MWCNTs and the weight loss of the Fe₃O₄–MWCNT nanocomposites was lower than that of the MWCNTs. This further proved that Fe₃O₄–MWCNT nanocomposites had been formed. From Fig. S2,† the maximum saturation magnetization value of the Fe₃O₄–MWCNT nanocomposites was 44.04 emu g⁻¹. Thus, its magnetic response was sufficiently high.

Fig. 1 (A) TEM image of MWCNTs, scale bar = 100 nm; (B) TEM image of the Fe₃O₄–MWCNTs, scale bar = 50 nm; (C) XRD patterns of Fe₃O₄, MWCNTs and Fe₃O₄–MWCNTs; (D) TGA curves of Fe₃O₄, MWCNTs and Fe₃O₄–MWCNTs.

Fe₃O₄ MNPs exhibit an intrinsic enzyme mimetic activity¹¹ and have been used as alternative heterogeneous Fenton-like catalyst for the degradation of organic pollutants.^12–15 Recently, some novel amino particles based on Fe³⁺, Mn²⁺, and Cu²⁺ ions in active metal centers were reported to exhibit a special ability to activate hydrogen peroxide²⁰ and were used instead of peroxidase in an immunoassay for lung cancer detection.²¹ Thus, the peroxidase-like activity of the Fe₃O₄–MWCNT nanocomposites compared to Fe₃O₄ or MWCNTs alone was evaluated by the catalytical oxidation of a typical peroxidase substrate (TMB) by H₂O₂. From Fig. 2, it is seen that the Fe₃O₄–MWCNT nanocomposites could catalyze the oxidation of TMB by H₂O₂ in sodium acetate buffer, and produced an intensified color reaction when compared to that of Fe₃O₄ or MWCNTs alone (Fig. 2). This suggested that the Fe₃O₄–MWCNT nanocomposites had a high H₂O₂-activating ability and confirmed the peroxidase-like activity²² of Fe₃O₄–MWCNTs. Fig. 1B shows that depositing Fe₃O₄ MNPs onto the surface of MWCNTs could prevent Fe₃O₄ NPs from aggregating, and thus greatly improve their dispersibility^23,24 and lead to an enhanced H₂O₂-activating ability and catalytic activity. Thus, the Fe₃O₄–MWCNTs could be potentially used as an enzyme mimicking catalyst for the removal of organic pollutants in the presence of H₂O₂.

Fig. 2 UV-vis spectroscopy of 0.5 mM TMB after a 10 min reaction with 1 mM H₂O₂ in 0.2 M acetate buffer (pH 3.5) in the presence of different catalysts.

3.2. Reduction efficiency of MB dye

Fig. 3 presents the kinetics of MB removal in the presence of and absence of H₂O₂ and different catalysts. Clearly, the removal of MB was insignificant when Fe₃O₄ or H₂O₂ was individually added into MB solution. This result indicated that the adsorption of MB by Fe₃O₄ or direct oxidation of MB by H₂O₂ was very limited. By contrast, when only MWCNTs were added into the MB solution, the removal efficiency was 27.21%, which was mainly caused by the adsorption of MB onto the MWCNTs. When Fe₃O₄ MNPs and H₂O₂ were both added into the MB solution, 17.89% MB removal was observed. This was mainly because the degradation of MB by Fe₃O₄ MNPs in the presence of H₂O₂ originates mainly from ferrous ions at the surface of Fe₃O₄ MNPs.¹¹ Notably, when Fe₃O₄–MWCNT nanocomposites and H₂O₂ were added to the solution of MB, the removal efficiency of MB sharply increased to approximately 97%, in which the MB adsorption by the Fe₃O₄–MWCNT nanocomposites accounted for about 52%. The MB removal by Fe₃O₄–MWCNT nanocomposites in the presence of H₂O₂ was higher than the sum of MB removals by H₂O₂ in the absence and presence of Fe₃O₄–MWCNT MNPs, implying a possible synergistic effect. To further investigate this, we measured the specific surface areas of MWCNTs, Fe₃O₄–MWCNTs and Fe₃O₄ MNPs using a N₂ adsorption-desorption method. The values were 115.6 m² g⁻¹, 91.9 m² g⁻¹ and 75.3 m² g⁻¹, respectively. Clearly, depositing Fe₃O₄ onto MWCNTs served to reduce the specific surface area of MWCNTs. This was probably attributed to coverage of the iron oxide nanoparticles (about 10–20 nm) on the surface of the MWCNTs, causing them to block some of the pore structures of the MWCNTs. However, the MB adsorption by the Fe₃O₄–MWCNTs was higher than the sum of MWCNTs and Fe₃O₄, suggesting that both specific surface areas and active sites had played key roles in MB adsorption onto the Fe₃O₄–MWCNT nanocomposites. It was highly possible that depositing of Fe₃O₄ MNPs onto MWCNTs provided additional adsorption sites for MB. Thus, in the composites, there were two types of adsorption sites, namely MWCNTs and Fe₃O₄ MNPs.²⁵ So, it could be concluded that the synergistic effect between the MWCNTs and Fe₃O₄ MNPs increased the MB removal by the Fe₃O₄–MWCNTs. As can be seen from Fig. 1B, the deposit of Fe₃O₄ MNPs onto the surface of MWCNTs could prevent the Fe₃O₄ MNPs from aggregation, thus greatly improving their dispersibility^23,24 and leading to enhanced H₂O₂ activating ability and catalytic activity. It should be noted that upon the addition of H₂O₂, ˙OH radicals were generated, which was confirmed by electron spin resonance (ESR) data (Fig. S3 ESI†). As can be seen, similar to ESR spin-trapping spectra of the Fe₃O₄ MNPs, the ESR spectra in the presence of Fe₃O₄–MWCNTs displayed a 4-fold characteristic peak of the DMPO–˙OH adduct with an intensity ratio of 1 : 2 : 2 : 1. The radicals produced attack MB molecules, resulting in decolorization.

Fig. 3 MB removal versus time in different systems. Reaction conditions: pH 5.5, temperature 298 K, 0.3 g L⁻¹ of Fe₃O₄–MWCNTs (or 0.2 g L⁻¹ Fe₃O₄ or 0.1 g L⁻¹ MWCNTs), 0.4 mol L⁻¹ of H₂O₂, 10 mg L⁻¹ of MB. Error bars represent 1 standard deviation (n = 3).

3.3. The effect of Fe₃O₄–MWCNT mass, H₂O₂ concentration and pH

Fig. 4A shows the effect of Fe₃O₄–MWCNT nanocomposite loading on MB removal efficiency in the dosage range from 0 to 0.5 g L⁻¹. It could be seen that the removal of MB was neglected when no Fe₃O₄–MWCNT magnetic nanocomposites were added, so the direct oxidation of MB by H₂O₂ was very limited. When Fe₃O₄–MWCNT nanocomposites were added, the removal efficiency of MB increased with increasing Fe₃O₄–MWCNT nanocomposites loading in the range of 0 to 0.3 g L⁻¹, probably because of the fact that more ˙OH radicals were formed when more catalysts were present (Fig. S4 ESI†). However, when the loading of Fe₃O₄–MWCNT nanocomposites was above 0.3 g L⁻¹, only a slight change of MB removal efficiency was observed. This may be related to the concentration of MB. In this work, the concentration of MB in the aqueous solution is fixed at 10 mg L⁻¹, thus a further increase in catalyst dosage above 0.3 g L⁻¹ caused no obvious decolorization of MB, although the H₂O₂ decomposition was accelerated as the dose of Fe₃O₄–MWCNTs increased and more hydroxyl radicals were generated (Fig. S4 ESI†). In order to confirm this, the decolorization of MB was conducted at a relatively high concentration of MB at 15 mg L⁻¹. Clearly, the degree of decolorization of MB at 15 mg L⁻¹ was lower than that at 10 mg L⁻¹ (Fig. S5 ESI†). Based on previous discussion, 0.3 g L⁻¹ of Fe₃O₄–MWCNT nanocomposites was selected in the subsequent experiments.

Fig. 4 (A) Effect of Fe₃O₄–MWCNTs loading on the removal of MB. Reaction conditions: pH 5.5, 0.4 M H₂O₂, 10 mg L⁻¹ MB. (B) Effect of H₂O₂ concentration on the removal of MB. Reaction conditions: pH 5.5, 0.3 g L⁻¹ Fe₃O₄–MWCNTs, 10 mg L⁻¹ MB. Error bars represent 1 standard deviation (n = 3).

The effect of H₂O₂ concentrations on MB removal efficiency is shown in Fig. 4B. It can be observed that the removal of MB was caused by the adsorption of Fe₃O₄–MWCNT nanocomposites when no H₂O₂ was added. When H₂O₂ was added, the removal efficiency of MB increased rapidly with increasing concentration of H₂O₂ in the range of 0 to 0.4 mol L⁻¹. However, only a slight change in MB removal was observed when the concentration of H₂O₂ was above 0.4 mol L⁻¹. Furthermore, it was observed that the Fe₃O₄–MWCNT nanocomposites required a higher H₂O₂ concentration of 0.5 mol L⁻¹ to reach the maximal level of catalytic activity, which was in agreement with the results reported elsewhere.¹¹ Finally, 0.4 mol L⁻¹ of H₂O₂ was selected for use in subsequent experiments.

The dependence of removal efficiency of MB on solution pH was investigated. As can be seen from Fig. 5, the MB removal efficiency was slightly changed in the range from 88.13% to 98.68%, when the solution pH varied in the range from 1 to 10. This was very different from the Fe₃O₄-based degradation system, in which the removal efficiency sharply decreased when the solution pH was higher or lower than the optimal value.^26,27 This phenomenon might be attributed to the fact that Fe₃O₄ MNPs loaded on the MWCNTs were more stable compared to the Fe₃O₄ MNPs alone, and thus, they did not require harsh conditions to reach the maximal level of catalytic activity. The wide range of the working pH is very attractive for the practical treatment of wastewater. Furthermore, it could also be found that the removal efficiency of MB under acidic conditions was higher than that under alkaline conditions, which may be because of two reasons. Firstly, H₂O₂ is not stable in alkaline solution and can be decomposed immediately to produce H₂O and O₂.¹² Secondly, an acidic condition was beneficial to producing ˙OH or O₂⁻ free radical species. In addition, the higher degradation activity at acidic conditions may be related to the released Fe³⁺ ions in aqueous solution. In order to confirm this, the leaching of Fe³⁺ after suspending Fe₃O₄–MWCNTs in water at different pHs for 12 h was investigated. The experimental results (Table S1 ESI†) suggested that the leaching of Fe³⁺ had occurred and the leaching rate of Fe³⁺ was above 2% at pH ≤ 3.0. Thus, the higher degradation activity at acidic conditions is surely related to the released Fe³⁺ ions in aqueous solution. However, it should be noted that only a small amount of iron ions were leached out (<1%) at pH > 3.0 in the aqueous solution of the Fe₃O₄–MWCNT catalyst. For example, at pH 5.0 to 6.0, only 0.31% Fe³⁺ was leached out. This suggested that the Fe₃O₄–MWCNT catalyst played a major role in the removal of MB at the conditions used (pH 5.5).

Fig. 5 Effect of pH on the removal of MB. Reaction conditions: 0.3 g L⁻¹ Fe₃O₄–MWCNTs, 0.4 mol L⁻¹ H₂O₂, 10 mg L⁻¹ MB. Error bars represent 1 standard deviation (n = 3).

3.4. The comparison of MB mineralization and decolorization

The complete decolorization of MB does not mean that the MB was mineralized completely. So, the MB and TOC removal efficiency versus time in the reaction process was monitored as shown in Fig. 6. It was shown that the TOC removal efficiency was significantly lower than the MB removal efficiency. After 12 h, the removal efficiency of MB reached 99.78%, while the TOC removal efficiency reached 35.6%. A further increase in reaction time from 15 to 24 h caused no obvious variation of MB decolorization and TOC removal (Fig. 6). The result implied that there were still significant amounts of intermediates in the solution. In addition, compared with the results in Fig. 2 (Fe₃O₄–MWCNTs–MB), the removal efficiency versus time for adsorption of MB on Fe₃O₄–MWCNT magnetic nanocomposites was higher than that for degradation of MB on Fe₃O₄–MWCNT magnetic nanocomposites in the presence of H₂O₂. This indicated the existence of a process which degraded parent molecules into small molecules.

Fig. 6 Comparison of MB decolorization and TOC removal. Reaction conditions: pH 5.5, 0.3 g L⁻¹ of Fe₃O₄–MWCNTs, 0.4 M H₂O₂, 10 mg L⁻¹ of MB. Error bars represent 1 standard deviation (n = 3).

3.5. Stability of Fe₃O₄–MWCNT nanocomposites

In order to evaluate the durability of the Fe₃O₄–MWCNT magnetic nanocomposites, the recycling experiment of Fe₃O₄–MWCNT magnetic nanocomposites was studied as shown in Fig. 7. The results demonstrated that the removal efficiency of MB was still 100% after the first two runs and the removal efficiency decreased from 95.55% to 79.96% as the recycling number increased to five. This indicated that the Fe₃O₄–MWCNT magnetic nanocomposites were stable and could be easily used for the repeated treatment of MB. The decrease in MB degradation efficiency might be because of the loss and aggregation of Fe₃O₄–MWCNT magnetic nanocomposites during the recycling process, and the adsorption of MB or accumulation of intermediates on the surface of Fe₃O₄–MWCNT magnetic nanocomposites.

Fig. 7 Removal efficiency of MB for different cycles of Fe₃O₄–MWCNTs using identical reaction conditions. Reaction conditions: pH 5.5, 0.3 g L⁻¹ of Fe₃O₄–MWCNTs, 0.4 M H₂O₂, 10 mg L⁻¹ of MB. Error bars represent 1 standard deviation (n = 3).

It should be mentioned that the operational cost of the Fenton system is crucial once it is applied in real water purification. Fortunately, it could be remarkably reduced by the fact that the Fe₃O₄–MWCNT magnetic nanocomposites could be recycled five times. Also, the easy regeneration of Fe₃O₄–MWCNT nanocomposites reduced the operating cost, making the Fe₃O₄–MWCNT nanocomposites more applicable from the economic perspective. In addition, the cost is increased by the pH adjustment through chemical addition that is needed during the conventional homogeneous Fenton system and this is not needed in this heterogeneous system. Obviously, the reduction of cost, brought by the recycling of Fe₃O₄–MWCNT magnetic nanocomposites, added economic competition of the Fenton-like system from the standpoint of engineering.

4. Conclusion

In summary, we have successfully demonstrated that the Fe₃O₄–MWCNT nanocomposites possess a high peroxidase-like activity and can be used as an efficient peroxidase mimic for MB removal by a Fenton-like reaction. Our results showed that nearly 100% of MB decolorization (99.78%) and 36% of MB mineralization could be obtained under optimal experimental conditions. The Fe₃O₄–MWCNT nanocomposites exhibited a good magnetic property with high saturation magnetization and could be quickly separated by a magnet after the degradation reaction was finished. In addition, high MB removal in the range from 88.13% to 98.68% could be obtained in the pH range from 1.0 to 10.0, suggesting high pH tolerance of the as-prepared composite catalyst. This feature is quite different from the traditional Fenton catalyst, which is only effective under pH < 3. Furthermore, the Fe₃O₄–MWCNT nanocomposites could be repeatedly used after a simple recycling treatment without any obvious loss of activity for MB degradation. Thanks to the high magnetic property, wide working pH range and durability, the Fe₃O₄–MWCNT magnetic nanocomposites have great potential for applications in water purification.

Acknowledgements

The financial support by the Natural Science Foundation of China (no. 21075099, 21275021 and 51378494) is acknowledged.

References

1. S. S. Azhar, A. G. Liew, D. Suhardy, K. F. Hafiz and M. D. Irfan, Am. J. Appl. Sci., 2005, 2, 1499–1503.
2. (a) S. Zhang, M. Zeng, J. Li, J. Li, J. Xu and X. Wang, J. Mater. Chem. A, 2014, 2, 4391–4397; (b) J. Cao, J. Li, L. Liu, A. Xie, S. Li, L. Qiu, Y. Yuan and Y. Shen, J. Mater. Chem. A, 2014, 2, 7953–7959.
3. M. Hirata, N. Kawasaki, T. Nakamura, K. Matsumoto, M. Kabayama, T. Tamura and S. Tanada, J. Colloid Interface Sci., 2002, 254, 17–22.
4. A. Ventura, G. Jacquet, A. Bermond and V. Camel, Water Res., 2002, 36, 3517–3522.
5. L. W. Lackey, R. O. Mines Jr and P. T. McCreanor, J. Hazard. Mater., 2006, 138, 357–362.
6. M. F. Sevimli and C. Kinaci, Water Sci. Technol., 2002, 45, 279–286.
7. O. D. Olukanni, A. A. Osuntoki, D. C. Kalyani, G. O. Gbenle and S. P. Govindwar, J. Hazard. Mater., 2010, 184, 290–298.
8. S. Venkata Mohan, N. Chandrasekhar Rao, K. Krishna Prasad and J. Karthikeyan, Waste Manage., 2002, 22, 575–582.
9. J. Feng, X. Hu and P. L. Yue, Water Res., 2006, 40, 641–646.
10. L. Zhou, Y. Shao, J. Liu, Z. Ye, H. Zhang, J. Ma, Y. Jia, W. Gao and Y. Li, ACS Appl. Mater. Interfaces, 2014, 6, 7275–7285.
11. L. Gao, J. Zhuang, L. Nie, J. Zhang, Y. Zhang, N. Gu, T. Wang, J. Feng, D. Yang, S. Perrett and X. Yan, Nat. Nanotechnol., 2007, 2, 577–583.
12. S. Zhang, X. Zhao, H. Niu, Y. Shi, Y. Cai and G. Jiang, J. Hazard. Mater., 2009, 167, 560–566.
13. J. Zhang, J. Zhuang, L. Gao, Y. Zhang, N. Gu, J. Feng, D. Yang, J. Zhu and X. Yan, Chemosphere, 2008, 73, 1524–1528.
14. N. Z. Wang, L. W. Da, Q. W. Ming, F. L. Zhi and Q. T. He, Ultrason. Sonochem., 2010, 17, 526–533.
15. (a) J. Jiang, J. Zou, L. Zhu, L. Huang, H. Jiang and Y. Zhang, J. Nanosci. Nanotechnol., 2011, 11, 4793–4799; (b) X. Zhang, M. He, J. Liu, R. Liao, L. Zhao, J. Xie, R. Wang, S. Yang, H. Wang and Y. Liu, Chin. Sci. Bull., 2014, 59, 3406–3412.
16. V. Vadivelan and K. V. Kumar, J. Colloid Interface Sci., 2005, 286, 90–100.
17. M. Rafatullah, O. Sulaiman, R. Hashim and A. Ahmad, J. Hazard. Mater., 2010, 177, 70–80.
18. B. Wang, J. Gong, C. Yang, G. Zeng and W. Zhou, China Environ. Sci., 2008, 28, 1009–1013.
19. L. Ai, C. Zhang, F. Liao, Y. Wang, M. Li, L. Meng and J. Jiang, J. Hazard. Mater., 2011, 198, 282–290.
20. Y. C. Lee, Y. S. Huh, W. Farooq, J. I. Han, Y. K. Ohd and J. Y. Park, RSC Adv., 2013, 3, 12802–12809.
21. Y. C. Lee, M. I. Kim, M. A. Woo, H. G. Park and J. I. Han, Biosens. Bioelectron., 2013, 42, 373–378.
22. X. Q. Zhang, S. W. Gong, Y. Zhang, T. Yang, C. Y. Wang and N. Gu, J. Mater. Chem., 2010, 20, 5110–5116.
23. G. Morales-Cid, A. Fekete, B. M. Simonet, R. Lehmann, S. Cardenas, X. Zhang, M. Valcarcel and P. Schmitt-Kopplin, Anal. Chem., 2010, 82, 2743–2752.
24. X. Liu, L. Pan, T. Lv, G. Zhu, T. Lu, Z. Sun and C. Sun, RSC Adv., 2011, 1, 1245–1249.
25. V. K. Gupta, S. Agarwal and T. Saleh, Water Res., 2011, 45, 2207–2212.
26. N. Wang, L. Zhu, M. Wang, D. Wang and H. Tang, Ultrason. Sonochem., 2010, 17, 78–83.
27. H. Liu, X. Z. Li, Y. J. Leng and C. Wang, Water Res., 2007, 41, 1161–1167.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07327d

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