Catalytic oxidation of 4-chlorophenol with magnetic Fe₃O₄ nanoparticles: mechanisms and particle transformation

Abstract

Magnetite (Fe₃O₄) is usually inert and when combined with metal catalysts or enzymes it forms a composite that exhibits both magnetism and catalytic activity. However, it has been reported that Fe₃O₄ nanoparticles have intrinsic peroxidase-like activity. In this study, super paramagnetic Fe₃O₄ nanoparticles with a diameter of about 30 nm were synthesized using self-designed experimental devices under mild conditions. Moreover, 4-chlorophenol (4-CP), which is a priority pollutant that widely exists in the environment but is recalcitrant towards chemical and biological degradation, was used as a model compound to test the catalytic activity of the synthesized Fe₃O₄ nanoparticles and analyse the mechanisms for 4-CP removal. Besides, surface analysis techniques, such as SEM, XRD and Raman spectroscopy, were used to investigate the transformation of the nanoparticles and further verify the interaction between the nanoparticles and 4-CP. The results revealed that the synthesized Fe₃O₄ nanoparticles show high catalytic activity even after being used several times, and acidic conditions are favourable for the dechlorination of 4-CP. However, 4-CP could also be degraded under neutral and alkali conditions. In the process 4-CP was transformed to formic acid, acetic acid and other byproducts. Adsorption tests indicated that the adsorption process does not play an important role in 4-CP removal, but it occurs between 4-CP and Fe₃O₄. The surface morphology of the Fe₃O₄ nanoparticles changed a lot and the reactive sites on the surface increased, which resulted in the higher activity of the particles after being used. The crystal structure of the nanoparticles did not change, suggesting the role of Fe₃O₄ nanoparticles as catalysts. Moreover, Raman spectra reflected that the adsorption and catalytic oxidation were surface reaction processes. It is proposed that the hydroxyl radical produced during the reaction is the main cause for the degradation of 4-CP. The reaction of H₂O₂ with ferrous to produce hydroxyl radical is the initial step, and is very important for the overall process.

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1. Introduction

As important chemical raw materials, chlorophenols have been widely used in pesticides, medicine, dyes and synthetic materials.¹ The exposure to chlorophenols poses a serious problem to the environment. As a result, chlorophenols are tagged as hazardous and top priority pollutants by the USEPA (Environmental Protection Agency) due to their toxicity, carcinogenicity, and intractability.^2,3 In addition, chlorophenols are highly toxic and recalcitrant towards chemical and biological degradation in the environment. Therefore, the treatment of chlorophenols has been one of the hot topics in the field of environmental science and engineering.

In past decades, zero-valent iron has been used for groundwater and wastewater remediation.^4,5 Many studies show that zero-valent iron has the capacity to remove refractory organic pollutants.^6,7 However, surface passivation seriously decreases the activity and capacity of zero-valent iron in terms of removing pollutants.^8–10 Due to their high specific surface area and high surface reactivity, nanoscale zero-valent iron particles have attracted extensive attention.^11,12 In recent years, studies mainly focus on the reduction mechanism of iron nanoparticles. However, Fe₃O₄ nanoparticles, which are the main corrosion products of nanosized zero-valent iron particles, receive little attention.

Generally, Fe₃O₄ is inert and is used as a magnetic carrier for catalysts or enzymes. Recently, it was reported that Fe₃O₄ nanoparticles have intrinsic peroxidase-like activity, and their combination with catalysts is not necessary.¹³ In our previous study, it was found that Fe₃O₄ is the main product of iron nanoparticles when they react with pentachlorophenol.¹⁴ In addition, the removal efficiency of 4-chlorophenol (4-CP) is improved when iron nanoparticles are used for a period of time.¹² This indicates that some product in the system contributes to the removal of 4-CP. In this study, the role of nanosized Fe₃O₄, which is a product of nanosized iron, was studied. 4-CP was used as the model compound to test the catalytic activity of synthesized Fe₃O₄ nanoparticles and analyse the mechanisms for 4-CP removal. The results would provide an important complement for the current reduction dechlorination mechanism of zero-valent iron nanoparticles, and would be helpful in the application of nanosized iron particles.

2. Materials and methods

2.1 Chemicals and materials

4-Chlorophenol (4-CP, ≥99%, reagent) was provided by Tianjin Jinke Fine Chemical Industry Research Institute. Ferric sulfate (Fe₂(SO₄)₃, 99%, reagent) was purchased from Nankai Fine Chemical Factory. Ferrous sulfate (FeSO₄·7H₂O, 99.0–101.0%) was supplied by Shenyang Reagent Factory. Methanol (99%, reagent), ethanol (99.7%, reagent), hydrogen peroxide (30%, reagent), sulfuric acid (98%, reagent) and sodium hydroxide were obtained from the Beijing Chemical Factory. Argon gas (Ar, 99.99%) was supplied by Beijing Aolin Gas Company. All the chemicals were of reagent grade and were used without further purification.

2.2 Preparation of nanoparticles

Fe₃O₄ nanoparticles were synthesized using the Massart hydrolysis method.¹⁵ Briefly, a 100 mL mixed solution of 0.07 mol L⁻¹ Fe₂(SO₄)₃ and 0.07 mol L⁻¹ FeSO₄ was added dropwise into a four-necked flask containing 100 mL 1 mol L⁻¹ NaOH aqueous solution at 80 °C. The reaction was carried out in an Ar atmosphere. Fe₃O₄ nanoparticles were produced via the following reaction:

Fe²⁺ + 2Fe³⁺ + 8OH⁻ → Fe₃O₄ + 4H₂O (1)

The synthesized Fe₃O₄ nanoparticles were deposited under an Ar atmosphere, then washed three times with deionized water, and dried in a vacuum dryer.

The morphology of the synthesized Fe₃O₄ nanoparticles was observed using a scanning electron microscope (SEM, Hitachi S-4500) and a transmission electron microscope (TEM, Hitachi H-7650B). The crystal structure of the as-prepared nanoparticles and transformation products were characterized using X-ray powder diffraction (XRD, D8-advance) on a Rigaku D/max-RB X-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm). Magnetic properties were determined using a vibrating sample magnetometer (VSM, 730T).

Besides, a microscopic confocal Raman spectrometer (RM2000, Renishaw) was used to investigate the transformation of the nanoparticles and verify the interaction between the nanoparticles and 4-CP.

2.3 Experimental procedure

Batch experiments were conducted in 50 mL flasks containing 15 mL solution, in which 4-CP, H₂O₂ and the synthesized Fe₃O₄ nanoparticles were added with initial concentrations of 20 mg L⁻¹, 1.0‰ and 2 g L⁻¹, respectively. The flasks were sealed with sealing films and placed on a rotary shaker (TZ-2EH, Beijing Wode Company). The rotation speed and the temperature of the reaction were set to 150 rpm and 30 °C, respectively. Samples were withdrawn from various test groups at predetermined time intervals and then filtered with a 0.22 μm filter film.

In the adsorption test, Fe₃O₄ nanoparticles (100 mg) were loaded into a 250 mL conical flask containing 100 mL of an aqueous solution of 4-CP. The initial concentration of 4-CP was 20 mg L⁻¹. The conical flask was placed on the same rotary shaker as mentioned above. In the reciprocating experiment, 4-CP solutions were withdrawn for detection and new 4-CP solutions were added into the flask. Moreover, the particles were maintained in the flask all the time. The initial concentration of 4-CP in each new solution was 20 mg L⁻¹ and the volume of the solution was 100 mL.

2.4 Analysis

4-CP and its byproducts were quantified using an Agilent 1100 HPLC (Shanghai Agilent Ltd) equipped with a C¹⁸ column and an L-4000 UV-vis detector. The mobile phase for 4-CP consisted of 60% methanol and 40% water distilled three times. The flow rate was 1 mL min⁻¹ and the detector wavelength was 280 nm for 4-CP. Chlorine ions were quantified using a DX-100 ion chromatogram (IC, DIONEX Company, Germany). The operational conditions were as follows: eluent, 3.5 mM Na₂CO₃/1.0 mM NaHCO₃; eluent flow, 1.2 mL min⁻¹; sample loop volume, 250 μL; and run time, 6 min. Deionized water was used as the blank.

3. Results and discussion

3.1 Characterization of Fe₃O₄ nanoparticles

The morphology of the Fe₃O₄ nanoparticles synthesized is shown in Fig. 1. As seen in the SEM image, the samples are circular particles and relatively uniform. Most particles have a diameter of about 30 nm (the statistical data is shown in Fig. A1 in the ESI†). As seen in the TEM image, the fine structure of the sample is fairly uniform.

Fig. 1 The morphology of the synthesized Fe₃O₄ nanoparticles: (a) SEM image; (b) TEM image.

As shown in the XRD pattern (Fig. 2), the peak position and relative intensity of the as-prepared particles are consistent with that of A.R. Fe₃O₄ particles. All the diffraction peaks can be assigned to the (220), (311), (400), (422), (511) and (440) planes of an Fe₃O₄ structure, and the three lines are (311), (440) and (511) (JCPDS no. 26-1136). The diffraction peaks broaden as the particle size decreases, from pattern a for A.R. Fe₃O₄ particles to pattern b for the synthesized sample. No other peaks were detected in the XRD pattern, indicating the high purity of the sample.

Fig. 2 XRD pattern of A.R. Fe₃O₄ particles and the synthesized nanoparticles.

The magnetic property of the as-prepared particles was also measured. As seen in the magnetization curve (Fig. A2 in ESI†), the saturation magnetization was 50 emu g⁻¹, and the remanence and coercivity was zero. This indicated that the synthesized particles are superparamagnetic and that they could be recovered with an external magnetic field. Actually, in our experiments, the particles were separated from the solution using a magnet.

3.2 Adsorption performance of Fe₃O₄ nanoparticles

The adsorption test showed that 4-CP that was removed through the adsorption process was rather limited (Fig. 3). Moreover, not more than 10% of 4-CP was removed from solution by the synthesized nanoscale Fe₃O₄ particles. It is generally acknowledged that specific surface area will increase with a decrease in particle size. In addition, the adsorption of heavy metals by nanoscale Fe₃O₄ particles has received great attention.¹⁶ This test showed a different result, which could be due to the different surface properties of the particles. As is known, 4-CP is an electron acceptor. There is an extra electron in Fe₃O₄, but Fe₃O₄ is not a good electron donor. As a result, direct electron transfer is not very easy between 4-CP and Fe₃O₄.¹⁷ On the other hand, Fe₃O₄ is magnetic and 4-CP is dipole-like, thus there should be some magnetic attraction between Fe₃O₄ and 4-CP. However, the removal efficiency of 4-CP did not resolve this issue.

Fig. 3 Adsorption of 4-CP with the synthesized Fe₃O₄ nanoparticles.

To understand the surface property of the as-prepared Fe₃O₄ nanoparticles and the interaction between 4-CP and Fe₃O₄, the Raman spectrum of Fe₃O₄ nanoparticles before and after adsorption was determined. As shown in Fig. 4, there is a strong peak at 665 cm⁻¹ for both the samples, which is the characteristic peak of Fe₃O₄.¹⁸ Moreover, in the spectrum of the sample after the reaction, there were other two weak peaks at 380 cm⁻¹ and 295 cm⁻¹ which may have been produced by the molecular vibration of 4-CP adsorbed on the surface of the Fe₃O₄ particles. The signal from the Raman spectrum indicated that there was some interaction between 4-CP and Fe₃O₄ particles, although the interaction was very weak. It is speculated that physical adsorption occurred between 4-CP and Fe₃O₄ particles, and the low amount of adsorption led to the weak peak.¹⁹

Fig. 4 Raman spectrum of Fe₃O₄ nanoparticles before and after adsorption.

3.3 Removal of 4-CP

It is well known that pH value has a significant effect on the formation of hydroxyl radicals (˙OH) and removal of pollutants in Fenton/Fenton-like systems. The removal of 4-CP with the synthesized Fe₃O₄ nanoparticles at different pH values was tested in this study. As shown in Fig. 5, the compound was completely removed within 4 h in acidic conditions. On the contrary, the removal rate at pH = 7.0 or 8.0 was about 40% even though the reaction was extended to 30 h. Therefore, the degradation rate of 4-CP at low pH values was absolutely higher than that at high pH values. This is similar to the Fenton systems.^20,21 Different from the conventional Fenton system, the organic pollutant could still be degraded to a certain extent in neutral and alkaline conditions. At a low pH value, the results can be attributed to the following factors: (1) the high oxidation capacity of hydroxyl radicals (˙OH), which is responsible for the removal and oxidation of chlorophenol; (2) more hydroxyl radicals (˙OH) were generated via Fenton reactions;²² (3) less adsorption of iron hydroxides on the surface of particles;²³ and (4) relative stability of H₂O₂.²⁴

Fig. 5 Removal of 4-CP at different pH values.

Based on the previous studies, the removal mechanism was involved for dechlorination in the system;^25–27 thus, chlorine ions in the system were determined. Moreover, a similar phenomenon was attained from the perspective of chlorine ion release. A significant higher dechlorination rate was obtained in acidic conditions compared to neutral and alkaline conditions (Fig. 6) and the dechlorination rate showed a trend of growth in acidic conditions. In particular, when 4-CP was completely removed after reacting for 4 h, the dechlorination rate was only 48.8%, and then dechlorination continued. When the time was extended to 30 h, the dechlorination rate was 83.9%.

Fig. 6 Dechlorination of 4-CP at different pH values.

Comparing the dechlorination rate of 4-CP (Fig. 6) with the removal rate (Fig. 5) at different pH values, apparently the dechlorination rate was lower than the removal rate in each system, which suggests that there were other processes involved in the removal of 4-CP in addition to dechlorination. On the one hand, it could be physical processes such as adsorption and volatilization; on the other hand, chlorine compounds may be produced. As determined in Section 3.2, the physical adsorption contributed very little to the removal of 4-CP. Besides, the volatilization of 4-CP is so weak that the volatilization process could be neglected. Therefore, the suspect was likely to be chlorine compounds, which will be elaborated in Section 3.6.

3.4 Removal of 4-CP by reused Fe₃O₄ nanoparticles

To study whether the residual particles had the catalysis ability to remove 4-CP, a series of tests were carried out at different pH values. As shown in Fig. 7(a), 4-CP can be completely removed in 0.5 h with reused Fe₃O₄ particles in repeated experiments. This indicates that the synthesized Fe₃O₄ nanoparticles still have excellent catalysis activity even after being used for a few times. When the initial pH value was 8, the removal rate was obviously lower (Fig. 7(b)). However, the results showed a similar trend when the Fe₃O₄ particles were reused. The removal of 4-CP was enhanced when Fe₃O₄ nanoparticles were reused in the systems. It was speculated that the reaction sites were changed on the surface of Fe₃O₄ nanoparticles, which improved the oxidation of 4-CP. Besides, long time reaction made a significant contribution to the morphological change of the Fe₃O₄ nanoparticles (details are demonstrated in Section 3.5).

Fig. 7 Removal of 4-CP with reused Fe₃O₄ nanoparticles: (a) initial pH = 5.0; (b) initial pH = 8.0.

3.5 The transformation of Fe₃O₄ nanoparticles

The morphology of Fe₃O₄ nanoparticles underwent little change after reaction without hydrogen peroxide and they were still observed as evenly dispersed small particles (Fig. A3 in ESI†). However, the observation totally changed when hydrogen peroxide was added to the system. Although Fe₃O₄ remained as scattered small particles for a short period, the nanoparticles developed chain and flower structures as the reaction proceeded, i.e. they formed a rough surface. As shown in Fig. 8, the particles were eroded, aggregated, and finally large flakes were generated. The erosion points were the sources of the surface active sites of the Fe₃O₄ particles. In other studies, some surface defects of nanoscale iron were also formed, which could be used as active sites in the process of dechlorination.^20,28

Fig. 8 SEM images of the Fe₃O₄ nanoparticles after reaction with H₂O₂ present. (a)–(c) show the particles after reacting for different periods. (d) The amplification of image (c).

The XRD pattern revealed that the composition of the Fe₃O₄ nanoparticles underwent little change before and after the reaction (Fig. 9) and this result further confirms that the Fe₃O₄ nanoparticles act as a catalyst in the system. There was no impurity peak detected, which indicates that no new solid matter was produced, and the phase of the nanoparticles did not change during the reaction process.

Fig. 9 XRD pattern of Fe₃O₄ after reacting with 4-CP (with H₂O₂ present).

To investigate the changes in surface properties of the particles, Raman spectra of the Fe₃O₄ nanoparticles before and after the reaction were obtained. Similar to Fig. 4, strong peaks (Fe₃O₄) were detected at 665 cm⁻¹ for all the samples (Fig. 10). Besides, weak peaks were detected at 358 cm⁻¹ and 488 cm⁻¹ after the sample was used once, which might be due to 4-CP and its products. However, the peaks were not detected in the sample after it was used twice. This revealed that there were certain interactions, but very weak physical adsorption occurred between the surface of the nanoparticles and chlorophenol. Moreover, no were peaks detected in the sample after reacting for 30 h without H₂O₂, except for the peak at 665 cm⁻¹, which was different from the previous result (Fig. 4). This meant that the peaks for the adsorption of 4-CP on the surface of the Fe₃O₄ particles disappeared after a long time reaction. This result confirmed that the physical adsorption occurs between 4-CP and the Fe₃O₄ particles, and desorption could occur easily. This is a dynamic process, thus the weak peaks could only be detected sometimes.

Fig. 10 Raman spectra of Fe₃O₄. Nanoparticles before and after the reaction: (a) after being used once; (b) after being used twice; (c) without H₂O₂; and (d) before the reaction.

3.6 The mechanism of 4-CP removal

Based on the abovementioned analysis, it can be inferred that the catalytic oxidation of 4-CP by Fe₃O₄ nanoparticles is a surface reaction process. The Fenton-like reaction between Fe₃O₄ nanoparticles and hydrogen peroxide occurs as follows:

Fe²⁺ + H₂O₂ + H⁺ → Fe³⁺ + H₂O + ˙OH (2)

Fe²⁺ + ˙OH → Fe³⁺ + OH⁻ (3)

H₂O₂ + ˙OH → H₂O + ˙O₂H (4)

Fe³⁺ + H₂O₂ → Fe²⁺ + H⁺ + ˙O₂H (5)

˙O₂H + Fe³⁺ → Fe²⁺ + ˙O₂ + H⁺ (6)

˙O₂H → ˙H + ˙O₂ (7)

˙OH + 4-CP → Cl⁻ + intermediates (8)

˙OH + 4-CP → Cl–R (9)

Reaction (2) plays a significant role in the 4-CP removal due to the generation of a hydroxyl radical and it also explains the fact that acidic conditions are conducive for chain reactions, which greatly improve the oxidation rate of 4-CP.

In Section 3.3, chlorine ion was detected. This explains (in eqn (8)) the fact that the chlorine atom of 4-CP was attacked by a hydroxyl radical, which led to the substitution of chlorine atom located in the para-position of 4-CP.^20,29,30 As a result, chlorine ions were released into the solution. Besides, the dechlorination rate was absolutely lower than the removal rate, which suggests that the chlorine compounds were generated through different reaction pathways.^20,30 Moreover, the byproducts in the solution might affect the dechlorination of 4-CP by Fe₃O₄ nanoparticles.³¹ Fig. 11 demonstrates the mechanism of catalytic oxidation of 4-CP by Fe₃O₄ nanoparticles.

Fig. 11 Illustration of the mechanism of catalytic oxidation of 4-CP by Fe₃O₄ nanoparticles.

Moreover, there was an obvious decline of pH value in each solution system over time in the abovementioned studies (Fig. A4 in ESI†), which indicates that the amount of acidic matter generated during the reaction is in accordance with a published study.²⁰ In fact, formic acid and acetic acid were detected in the system, which indicate that the carbon–carbon double bond of the benzene ring was broken down, which made the formation of acidic matter possible during the reaction.^20,32

In addition, intermediates were also detected from the HPLC spectrum (Fig. A5 in ESI†).

4. Conclusions

(1) The removal rate and dechlorination rate of 4-CP in acidic conditions were absolutely higher than that in neutral and alkaline conditions. However, 4-CP could also be degraded under neutral and alkali conditions.

(2) The synthesized Fe₃O₄ nanoparticles showed high catalytic activity even after they were used several times. In particular, the activity was improved after they were used.

(3) The surface morphology of the Fe₃O₄ nanoparticles changed a lot and the reactive sites on the surface increased, which resulted in the higher activity of the particles after they were used.

(4) The adsorption process did not play an important role in 4-CP removal, but it did occur between 4-CP and Fe₃O₄. Hydroxyl radical produced through a Fenton reaction made an outstanding contribution to the degradation of 4-CP.

Acknowledgements

This study was supported by the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (Grant No. 11XNK016), which are greatly acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10433e

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