Fe₃O₄@SiO₂ nanoparticles as a high-performance Fenton-like catalyst in a neutral environment

Abstract

Advanced oxidation processes (AOP) have been widely applied in water treatment. However, traditional Fenton reactions based on a Fe²⁺–H₂O₂ system requires an acidic environment and generates a large amount of Fe³⁺ ions. Herein, we reported that magnetic Fe₃O₄ core–SiO₂ shell nanoparticles (Fe₃O₄@SiO₂ NPs) could be used as a Fenton-like catalyst for the decomposition of H₂O₂, resulting in the decoloration of methylene blue (MB). Fe₃O₄@SiO₂ NPs had much higher activity than bare Fe₃O₄ cores, suggesting the coating of SiO₂ enhanced the catalytic activity. Most importantly, the best performance of Fe₃O₄@SiO₂ NPs was observed at neutral pH values. A higher temperature facilitated the diffusion of MB in solution, and thus, promoted the decoloration efficiency. The radical reaction nature was reflected by the electron spin resonance spectrum and the significant inhibition of the decoloration in the presence of the radical scavenger tertiary butanol. Fe₃O₄@SiO₂ NPs could be magnetically separated and partially regenerated after the decoloration. The implication for the applications of Fe₃O₄@SiO₂ NPs in water treatment is discussed.

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Introduction

Water pollution is one of the most concerning environmental problems that limits the development of human society nowadays.¹ Organic pollutants cause serious pollution to water environments, including dyes, antibiotics, pesticides, oil and other small organic molecules. To decontaminate the pollution, many technologies, such as advanced oxidation process (AOP), adsorption, activated sludge method and electrolysis, are developed and applied.^2–5 Of particular interest and importance is the AOP method, which produces highly active radicals to oxidize the electron-enriched organic pollutants.^4,5 AOP can handle diverse pollutants, especially for those not suitable for activated sludge, so AOP has been widely adopted in laboratory and industry.

Fenton reaction is the most studied and used AOP method, where H₂O₂ is decomposed under the catalysis of Fe²⁺.^6,7 Unfortunately, Fe²⁺–H₂O₂ system has several drawbacks that hinder its applications. The stoichiometric amounts of iron are added to the system, which consequently are oxidized into Fe³⁺ ions and require further treatment to get removed. The optimal condition for the production of hydroxyl radicals is pH 3, thus large quantities of acid are essential and a neutralization step is required afterward.

To these regards, heterogeneous Fenton-like catalysts based on iron or other metals are extensively researched to overcome the aforementioned drawbacks of Fe²⁺–H₂O₂ system.^8–15 Many studies have demonstrated that under sonication or irradiation, Fe₃O₄ nanoparticles (NPs) could decontaminate many organic pollutants.^16,17 Very recently, we showed that upon coating Fe₃O₄ NPs with a thin layer of carbon, Fe₃O₄ NPs catalyzed the Fenton-like reaction effectively without external energy supply.¹⁸ However, the carbon coated Fe₃O₄ NPs only worked well in acidic environment. Therefore, designing suitable core–shell structure of Fe₃O₄ NPs as Fenton-like catalyst in neutral environment becomes the major challenge.

In this study, we reported the preparation of silica coated Fe₃O₄ NPs (Fe₃O₄@SiO₂ NPs) as high-performance Fenton-like catalyst in neutral environment for the decoloration of methylene blue (MB). Silica was deposited on Fe₃O₄ NPs by the hydrolysis of tetraethyl orthosilicate (TEOS). The decoloration efficiency of MB was measured. Influencing factors were investigated to optimize the parameters, where Fe₃O₄@SiO₂ NPs showed even higher performance at neutral pH than under acidic conditions. The magnetic separation and regeneration were also performed. The implication to the applications of Fe₃O₄@SiO₂ NPs as Fenton-like catalyst in water treatment is discussed.

Experimental

Materials

TEOS was bought from Jinshan Chemical Reagent Co., Ltd, China. FeCl₂ was purchased from Damao Chemical Reagent Co., Ltd, China. FeCl₃ was bought from Bodi Chemical Engineering Co., Ltd, China. MB was obtained from Sinopharm Chemical Reagent Co., Ltd, China. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Sigma Co., America. They were used without further purification. Other chemicals were of analytical grade.

Preparation of Fe₃O₄@SiO₂ NPs

FeCl₂·4H₂O (0.131 g) and FeCl₃·6H₂O (0.269 g) were added to 50 mL water and the pH was adjusted to 12 under vigorous stirring. The mixture was further stirred for 1 h to allow the full precipitation. After the co-precipitation, Fe₃O₄ NPs were collected and washed with deionized water for three times. The coating of SiO₂ layer was achieved by the hydrolysis of TEOS in the presence of NH₃·H₂O. Briefly, Fe₃O₄ NPs (0.1 g) were added to 1 mL of water, and then 10 mL of isopropanol was added. The mixture was sonicated for 10 min before the addition of TEOS. The mass ratios of Fe₃O₄ NPs : TEOS were in the range of 10 : 1–1 : 10. The mixture was shaken at 100 rpm under 35 °C for 5 h. The obtained Fe₃O₄@SiO₂ NPs were washed by water for three times and alcohol twice. The final product was dried under vacuum overnight.

Transmission electron microscopy (TEM, JEM-200CX, JEOL, Japan), X-ray photoelectron spectroscopy (XPS, Kratos, UK), Brunauer–Emmett–Teller (BET) technique (ASAP2010, Micromeritics, USA), infrared spectrometer (IR, Magna-IR 750, Nicolet, USA) and magnetometer (MPMS XL-7Tesla, Quautum Design, USA) were adopted to characterize Fe₃O₄@SiO₂ NPs.

Decoloration of MB

To decolorize MB, 20 mL of MB (pH 6.5, 50 mg L⁻¹) was incubated with 20 mg of Fe₃O₄@SiO₂ NPs for 2 h (100 rpm at 35 °C) on a thermostat shaker (CHA-S, Jintan Hankang Electronic Co., China), during which the adsorption of MB reached the equilibrium. Before the addition of H₂O₂, 50 μL of the supernatant was collected for the absorbance (A₀) measurements (664 nm) on a UV-Vis spectrometer (UV1800, PGeneral, China). Then, 1.5 mL of H₂O₂ was added to the mixture and incubated. At each time interval, 50 μL of the supernatant was collected for absorbance (A_t) measurements. The decoloration efficiency was calculated as (1 − A_t/A₀) × 100%. The decoloration efficiency of Fe₃O₄@SiO₂ NPs of different compositions was also measured. For comparison, the activity of Fe₃O₄ NPs was evaluated following the same protocol. As a control, the protocol was performed without adding catalyst. The chemical oxygen demand (COD) was determined using Hach reagent (low range 3–150 mg L⁻¹) on Hach DR900. The kinetic constant k of decoloration was calculated following eqn (1).

−ln(C/C₀) = kt (1)

Influencing factors

To investigate the influence of H₂O₂, 20 mg of Fe₃O₄@SiO₂ NPs was used to decolorize 20 mL of MB (pH 6.5, 50 mg L⁻¹) at 35 °C in the presence of different volumes of H₂O₂.

To investigate the influence of catalyst amount, Fe₃O₄@SiO₂ NPs of different amounts and 1.5 mL of H₂O₂ were used to decolorize 20 mL of MB (50 mg L⁻¹) of different pH values (3.5–8.5) at 35 °C.

To investigate the influence of pH, 20 mg of Fe₃O₄@SiO₂ NPs and 1.5 mL of H₂O₂ were used to decolorize 20 mL of MB (50 mg L⁻¹) of different pH values (3.5–8.5) at 35 °C. Similarly, the influence of pH on the performance of Fe₃O₄ core was investigated.

To investigate the influence of temperature, 20 mg of Fe₃O₄@SiO₂ NPs and 1.5 mL of H₂O₂ were used to decolorize MB (20 mL, pH 6.5, 50 mg L⁻¹) at different temperature (0–45 °C).

To investigate the influence of inhibitor, 20 mg of Fe₃O₄@SiO₂ NPs and 1.5 mL of H₂O₂ were used to decolorize 20 mL of MB (pH6.5, 50 mg L⁻¹) at 35 °C in the presence of different amounts of tertiary butanol.

Magnetic separation and regeneration

To recycle Fe₃O₄@SiO₂ NPs, the used Fe₃O₄@SiO₂ NPs were magnetically separated with a magnet. The separated Fe₃O₄@SiO₂ NPs were washed with deionized water by stirring for 20 min. After washing for three times, the recycled Fe₃O₄@SiO₂ NPs were dried and the catalytic activity was determined following aforementioned protocol. The procedure was repeated to reach the cycles of 6. Separately, the used Fe₃O₄@SiO₂ NPs were reduced by vitamin C (10 mg mL⁻¹) for 2 h. After washing, the reduced Fe₃O₄@SiO₂ NPs were subjected to activity measurement as aforementioned.

Electron spin resonance (ESR) assay

For ESR assay, 100 μL sample was collected from the reaction solution (Fe₃O₄@SiO₂ NPs/H₂O₂ system) at 5 min and immediately mixed with 20 μL of 0.2 mol L⁻¹ DMPO to form DMPO–OH adduct. The ESR spectrum was obtained on a JEOLJES FA200 facility with microwave bridge (receiver gain: 1 × 10⁵; modulation amplitude: 2 Gauss; microwave power: 4 mW; modulation frequency: 100 kHz).

Results and discussion

Characterization of Fe₃O₄@SiO₂ NPs

Fe₃O₄@SiO₂ NPs were spherical particles with diameters of 20–40 nm. The core–shell structure could be roughly distinguished in Fig. 1a, where the resolution was limited by the magnetic interference to reach a clear verification of the core–shell structure. The IR spectrum in Fig. 1b was similar to those of typical SiO₂ materials. The peak at 3419 cm⁻¹ was attributed to the antisymmetric stretching vibration of –OH. The peak at 1641 cm⁻¹ was attributed to the bending vibration of –OH. The peak at 1031 cm⁻¹ was attributed to the antisymmetric stretching vibration of –O–Si. The chemical components were further analyzed by XPS. There were 24.5 wt% of Si and 34.6 wt% of Fe in Fe₃O₄@SiO₂ NPs.

Fig. 1 TEM (A) and IR (B) spectrum of Fe₃O₄@SiO₂ NPs.

The BET measurement indicated that the specific surface area of Fe₃O₄@SiO₂ NPs was 124.3 m² g⁻¹, and the total pore volume was 0.405 cm³ g⁻¹. The surface area of Fe₃O₄@SiO₂ NPs was larger than that of Fe₃O₄ core (112.7 m² g⁻¹). The large surface area and the amorphous SiO₂ would benefit the pre-concentration of pollutants around Fe₃O₄@SiO₂ NPs, because amorphous SiO₂ is good adsorbent for many pollutants. The N₂ adsorption/desorption isotherm curve followed type IV (Fig. 2a), suggesting the nature of adsorption hysteresis. The magnetic property measurement found that the saturated magnetization was 57 emu g⁻¹, weaker than that of bulk Fe₃O₄ (92.8 emu g⁻¹),¹⁹ which could be attributed to the small size of NPs.²⁰ The magnetic hysteresis loop indicated that Fe₃O₄@SiO₂ NPs was ferromagnetic (Fig. 2b). The outstanding magnetic property allowed the magnetic separation of Fe₃O₄@SiO₂ NPs after the water treatment.

Fig. 2 Nitrogen adsorption isotherm (A) and magnetic hysteresis loop (B) of Fe₃O₄@SiO₂ NPs.

Catalytic activity of Fe₃O₄@SiO₂ NPs

The catalytic activity of Fe₃O₄@SiO₂ NPs was reflected by the decoloration capability of MB. We optimized the parameters of the preparation protocol. As shown in Fig. 3a, different initial ratios of TEOS : Fe₃O₄ led to different catalytic efficiency. The optimal ratio was 1 : 4, with a decoloration efficiency of 91%. The corresponding COD decreased from 101 mg L⁻¹ to 25 mg L⁻¹ after decoloration. The kinetic constant k at this ratio was calculated as 0.020 min⁻¹ (Fig. S1†). This was competitive to those of the high-performance nanocatalysts in the literature.^21–23 For example, Hsieh et al. reported the k value of FePt NPs–H₂O₂–MB system was 0.0033–0.023 min⁻¹ at pH 5.5.²¹ When more TEOS was added, the decoloration efficiency decreased. A possible explanation could be that more TEOS induced a compact coating of Fe₃O₄ core, which hindered the diffusion of MB toward the Fe₃O₄ surface. Beyond that, too much SiO₂ might also block the electron-transfer from iron to MB. On the other hand, when less TEOS was added, the pre-concentration effect of SiO₂ shell was not dominating. Correspondingly, the promotion in catalytic activity was less.

Fig. 3 Decoloration of MB in Fe₃O₄@SiO₂–H₂O₂ system at neutral pH. (A) The effect of initial Fe₃O₄ : TEOS mass ratio on the decoloration; and (B) the comparison between Fe₃O₄@SiO₂ NPs and bare Fe₃O₄ cores.

Nevertheless, coating SiO₂ was very effective in enhancing the catalytic activity of Fe₃O₄. Although bare Fe₃O₄ cores had similar specific surface area, bare Fe₃O₄ NPs had no catalytic activity in the decomposition of H₂O₂ at near neutral pH (Fig. 3b). The kinetic constant k was calculated as 0.00020 min⁻¹, suggesting the much slower decoloration kinetics (Fig. S1†). Bare Fe₃O₄ NPs only worked at pH 3.5 (k = 0.0046 min⁻¹) and 8.5 (k = 0.0041 min⁻¹) in our experiments (Fig. S2†), where the kinetics were still much lower than that of Fe₃O₄@SiO₂–H₂O₂ system. The comparison clearly indicated that SiO₂ coating was vital for the high performance of Fe₃O₄@SiO₂ NPs. SiO₂ at least had two important effects on Fe₃O₄@SiO₂ NPs. Firstly, amorphous SiO₂ could adsorb MB, which led to the pre-concentration of MB around Fe₃O₄ cores.^24,25 When radicals were generated, more radicals could reach MB before the self-extinction. Another effect might be that SiO₂ facilitated the electron-transfer from iron to MB.^26–28 Collectively, these effects resulted in the high catalytic activity of Fe₃O₄@SiO₂ NPs in neutral environment.

Influencing factors

We investigated the influencing factors for Fe₃O₄@SiO₂ NPs. As indicated in Fig. 4a, more H₂O₂ was favorable in promoting the decoloration efficiency. Adding 1.5 mL and 2.0 mL of H₂O₂ showed the highest decoloration efficiency. To save H₂O₂, we recommended 1.5 mL H₂O₂. More Fe₃O₄@SiO₂ NPs also benefited the decoloration (Fig. 4b). 20 mg of Fe₃O₄@SiO₂ NPs showed competitive activity to 40 and 60 mg. Thus, 20 mg was used through our experiments. It was reasonable that more H₂O₂ and Fe₃O₄@SiO₂ NPs promoted the decoloration from the views of kinetics and equilibrium.

Fig. 4 Influence of H₂O₂ (A) and Fe₃O₄@SiO₂ NPs (B) on the decoloration of MB in Fe₃O₄@SiO₂–H₂O₂ system.

In the evaluations of pH, Fe₃O₄@SiO₂ NPs showed very high activity at near neutral pH values (Fig. 5a). From pH 4.5 to pH 8.5, the decoloration efficiencies were all over 80%. At pH 6.5 the decoloration achieved 90% and the ratio was 94% at pH 7.5. The results indicated an important merit of Fe₃O₄@SiO₂ NPs that Fe₃O₄@SiO₂ NPs could be used in neutral environment without external energy supply.²³ It was somehow surprising that Fe₃O₄@SiO₂ NPs showed less activity at pH 3.5, which was the optimal pH for traditional Fe²⁺–H₂O₂ system. This might imply that the fundamental mechanism of the catalysis was changed. Similar phenomena were reported in literature, too.^29,30 However, the mechanism of such changes still requires future investigations.

Fig. 5 Influence of pH (A) and temperature (B) on the decoloration of MB in Fe₃O₄@SiO₂–H₂O₂ system.

In the evaluations of temperature, the decoloration performance of Fe₃O₄@SiO₂ NPs showed a temperature-dependent manner (Fig. 5b). Obviously, higher temperature benefited the decoloration. This was consistent with our previous observation of Fe₃O₄@C NPs, where higher temperature facilitated the diffusion of MB and generation of radicals.¹⁸ It should be noted that the decoloration efficiencies at 35 °C and 45 °C were very close, although the decoloration efficiency reached the maximum much faster.

The presence of radical scavenger (tertiary butanol) inhibited the decoloration. Mechanistically, the decoloration was achieved by the attack of radicals generating during the decomposition of H₂O₂. As shown in Fig. 6a, the ESR spectrum confirmed the presence of ˙OH radicals upon the catalysis of Fe₃O₄@SiO₂ NPs. The ESR spectrum in the presence of Fe₃O₄@SiO₂ NPs displayed a 4-fold characteristic peak of the typical DMPO–OH adduct with an intensity ratio of 1 : 2 : 2 : 1. When the radicals were eliminated before they reached the pollutants, the decontamination would be blocked. To this regard, we suggested that radical scavengers should be avoided during the decontamination. Again, SiO₂ coating was much better than C coating.¹⁸ In our previous report, only 100 μL of tertiary butanol could completely inhibit the decoloration. In this study, the performance of Fe₃O₄@SiO₂ NPs was only inhibited by 13%. Even when 2 mL tertiary butanol was added, the decoloration efficiency was retained as 39%. A possible explanation could be that the affinity of MB to Fe₃O₄@SiO₂ NPs was stronger than that of tertiary butanol, which resulted in the accumulation of MB rather than tertiary butanol around Fe₃O₄@SiO₂ NPs. Therefore, the radicals reached MB before the scavenging by tertiary butanol. Further investigations on the mechanism are highly encouraged.

Fig. 6 (A) DMPO spin-trapping ESR spectrum of ˙OH radicals in Fe₃O₄@SiO₂–H₂O₂ system; (B) influence of tertiary butanol on the decoloration of MB in Fe₃O₄@SiO₂–H₂O₂ system.

Separation and regeneration

After the decoloration, Fe₃O₄@SiO₂ NPs could be magnetically separated with an external magnet. The used Fe₃O₄@SiO₂ NPs were washed with deionized water to remove most remnant MB. The regenerated Fe₃O₄@SiO₂ NPs was used directly after the wash or dried for storage. The catalytic activity of recycled Fe₃O₄@SiO₂ NPs was determined (Fig. 7). At cycle one, the catalytic activity of Fe₃O₄@SiO₂ NPs was well retained without obvious loss. Thereafter, the recycling led to moderate decrease of activity of Fe₃O₄@SiO₂ NPs. The decrease became slow when the cycle number reached 3. Despite the decrease of catalytic activity, the results indicated that Fe₃O₄@SiO₂ NPs could be partially regenerated after the Fenton-like reaction. Thus, Fe₃O₄@SiO₂ NPs were more environment-friendly than traditional Fe²⁺–H₂O₂ system.

Fig. 7 Recycling of Fe₃O₄@SiO₂ NPs after the decoloration of MB.

There might be a possibility that Fe₃O₄ core was oxidized during the treatment and the oxidation of Fe₃O₄ resulted in the activity loss. To exclude this, we reduced the recycled Fe₃O₄@SiO₂ NPs with Na₂S₂O₃ (Fig. 7). The reduction did not improve the regeneration, implied that the oxidation of Fe₃O₄ was not the main reason of the activity loss. Similar phenomena were observed when vitamin C and hydrazine hydrate were used as the reducer (data not shown). Herein, we speculated that the loss of catalytic activity might be due to the destruction of some vulnerable domains of Fe₃O₄@SiO₂ NPs. After 3 cycles, the relative stable sites survived and the catalytic activity became almost constant.

Conclusions

In summary, Fe₃O₄@SiO₂ NPs was prepared as high-performance Fenton-like catalyst for the decoloration of MB, where neutral pH was the optimal condition. SiO₂ coating is crucial in promoting the catalytic activity and shifting the optimal pH toward neutral. Higher temperature was preferred and the radical scavengers should be avoided in the applications of Fe₃O₄@SiO₂ NPs. The magnetic separation of Fe₃O₄@SiO₂ NPs after the decoloration made the process environment-friendly. We hope that our results will stimulate more interests on the applications of high-performance nanocatalysts for the environmental remediation.

Acknowledgements

We acknowledge financial support from Science & Technology Department of Sichuan Province (Pillar Program no. 2013FZ0060), the China Natural Science Foundation (no. 21307101), and the National Training Programs of Innovation and Entrepreneurship for Undergraduates (no. X201410656008).

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Footnote

† Electronic supplementary information (ESI) available: decoloration kinetics constants; catalytic performance of Fe₃O₄ core under different pH values. See DOI: 10.1039/c4ra10207j

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