From metal–organic frameworks to magnetic nanostructured porous carbon composites: towards highly efficient dye removal and degradation

Abstract

A magnetic nanostructured porous carbon material (γ-Fe₂O₃/C) was easily synthesized using a microwave-enhanced high-temperature ionothermal method with an iron terephthalate metal–organic framework-MIL-53(Fe), as a template. The structure, morphology, magnetic properties, and porosity of γ-Fe₂O₃/C were characterized by powder X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy, vibrating sample magnetometry, and Brunauer–Emmett–Teller surface area analysis. The obtained porous carbon materials (γ-Fe₂O₃/C), possessed a high specific surface area (397.2 m² g⁻¹) and pore volume (0.495 cm² g⁻¹). The adsorption properties were tested by removal of malachite green (MG) from an aqueous solution. After reaching adsorption equilibrium, the maximum adsorption capacity was 499 mg g⁻¹ at 30 °C, and reached 863 mg g⁻¹ at 60 °C. The excellent magnetism (20.10 emu g⁻¹) provided an ideal magnetic-separation performance. Analysis of the sorption kinetics and isotherms showed that these sorption processes were better fitted to the pseudo-second-order and Langmuir equations than pseudo-first-order and Freundlich equations. Various thermodynamic parameters, such as ΔG^θ, ΔS^θ, and ΔH^θ, were also calculated, and indicated that the present system was spontaneous and endothermic. It was further demonstrated that γ-Fe₂O₃ showed powerful photocatalytic activity for the degradation of MG under sunlight in the presence of H₂O₂.

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1. Introduction

Water pollution has become a serious issue for ecosystems with the development of modern society. As one of the main pollutants in wastewater, dyes are used to colour the final products of industries such as textiles, rubber, food, paper and plastics, etc. The fabrication of these products involves the formation of wastewater that contains dyes. Most of the dyes not only reduce the water quality, but also significantly impact the health of human beings because they are toxic, mutagenic, and carcinogenic.¹ Among the dyes used, cationic dyes are more toxic than anionic dyes because they can easily interact with cytomembranes, which are negatively charged, and can enter cells and concentrate in the cytoplasm.² Malachite green (MG), which is a cationic triphenylmethane dye, is abundant in wastewater and is difficult to degrade because of its complex aromatic structure.³ Therefore, the development of a method for removing MG from wastewater is extremely important.

Up to now, various techniques including photo-degradation,⁴ bio-degradation,⁵ and adsorption⁶ have been used to remove MG from wastewater. Currently, adsorption is the most common treatment for effective dyestuff removal because it is simple to operate, high performing and cost-effective. In the past years, numbers of adsorbents have been reported to eliminate dyes from industrial or agricultural wastewater, such as carbon-based nanomaterials,⁷ agricultural wastes,⁸ inorganic materials,^9–11 and polymers.⁶

Owing to their unique structure and properties, nanostructured porous carbon materials (NPCMs), which have different allotropes and microtextures, have been used as adsorbents for gas adsorption/storage^12–14 and adsorption of environment pollution.¹⁵ NPCMs can be prepared by numerous methods including chemical vapor decomposition,¹⁶ electrical arc,¹⁷ and templating as well as chemical- or physical-activation methods¹⁸ and so on. Recently, metal–organic frameworks (MOFs), which are typically inorganic–organic hybrids with tunable pore sizes and functionalities, have been demonstrated as novel potential templates for the synthesis of NPCMs. MOF-5,¹⁹ zeolitic imidazolate framework-8 (ZIF-8),²⁰ and isoreticular metal–organic frameworks^21–23 have been successfully used as templates and precursors for fabricating NPCMs. However, difficult separation has limited their application. In order to overcome this weakness, unprecedented research efforts have been focused on preparing of micro- or nano-crystals magnetic porous carbon and exposing to improve their environmental applications.^24,25 For instance, Cui et al.²⁵ prepared hybrid γ-Fe₂O₃/C hollow spheres by one-step hydrothermal method, and this material exhibits superior capacities for heavy metal removal, but the Brunauer–Emmett–Teller (BET) surface area was low (62.6 m² g⁻¹). So as to synthesise magnetic porous carbon with high BET surface area, the research efforts have been focused on MOFs once again. Lee et al.¹⁵ reported a one-pot synthesis of magnetic particle-embedded porous carbon composites from MOFs; however, the samples were carbonized under very harsh conditions (i.e., temperatures as high as 600–1000 °C, extremely anaerobic environments), the BET surface area was low (204 m² g⁻¹). Qiu et al.²⁶ prepared MOF-derived γ-Fe₂O₃/C using a microwave (MW)-enhanced high temperature ionothermal method; although the BET surface areas were as high as ∼800 m² g⁻¹, the magnetism at that condition (4.12–9.15 emu g⁻¹) was insufficient for magnetic separation, and the dyes adsorptive capacity was relative low.

Here, we demonstrate the synthesis of γ-Fe₂O₃/C through the carbonization of Fe-based MOFs using an MW-enhanced high-temperature ionothermal method. The results show that the obtained γ-Fe₂O₃/C composite possesses relative high surface area (397.2 ± 7.4 m² g⁻¹), excellent magnetic characteristics (20.10 emu g⁻¹) for magnetic separation, and high dye adsorption capacity (499 mg g⁻¹, 30 °C). We also found that the prepared magnetic NPCM has excellent photocatalytic activity for the degradation of MG under sunlight in the presence of H₂O₂ and can be recycled from the reaction media after the catalytic reactions.

2. Experimental

2.1 Reagents and materials

1,4-Dicarboxybenzene (H₂BDC), ferric chloride hexahydrate (FeCl₃·6H₂O), furfuryl alcohol (FA), and zinc chloride (ZnCl₂) were purchased from Aladdin Chemical Reagents Co., Ltd (Shanghai, China). MG, N,N-dimethylformamide (DMF), and hydrofluoric acid (HF, ≥40.0%) were purchased from Sinopharm Chemical Reagents Co., Ltd (Shanghai, China). Water was filtered and deionized using a Milli-Q Millipore system (Milford, MA, USA). All chemicals used were at least of analytical grade and were not further purified before use.

2.2 Preparation of magnetic nanoporous carbons (γ-Fe₂O₃/C)

Firstly, MIL-53(Fe) was synthesized according to a previously reported procedure.²⁷ Typically, 1.0812 g of FeCl₃·6H₂O, 0.6646 g of H₂BDC, and 20 μL of hydrofluoric acid was dissolved in 20 mL of DMF. The mixture was vigorously stirred for about 10 min until the solid completely dissolved and then loaded into a Teflon-lined autoclave, which was then sealed and placed in an oven at 150 °C for 48 h. After cooling slowly to room temperature, a light yellow product was obtained by centrifugation and further rinsed with water and ethanol to remove any unreacted H₂BDC. The yellow solid was collected by centrifugation at 8000 rpm for 5 min and evacuated in vacuo at 150 °C for 24 h.

1.0 g of the dried MIL-53(Fe) was soaked in 20 mL of FA, the mixture was pumped at ∼10 kPa for about 2 h and then stirred at atmospheric pressure for 24 h in order to enable FA to fully infiltrate into the pores of MIL-53(Fe). The FA/MIL-53(Fe) composite was collected by centrifugation and washed with ethanol to remove any compounds that were adhered to the outer surface of MIL-53(Fe). Subsequently, the FA/MIL-53(Fe) composite was dried at 60 °C for 1 h to evaporate the ethanol and then heated at 80 °C for 3 h to polymerize FA in the MIL-53(Fe) pores. The dried composites and ZnCl₂ (1 : 7, w/w) were uniformly mixed by quick grinding, followed by a reaction in a microwave reactor (600 W, Galanz, WD800, Shenzhen, China) for 3 min. The obtained γ-Fe₂O₃/C composites were consecutively washed with 900 mL of deionized water, 100 mL of diluted HCl (0.01 mol L⁻¹), and 40 mL of ethanol, then separated using an external magnet and finally dried at 70 °C in vacuum.

2.3 Characterization of the adsorbents

Power X-ray diffraction (PXRD) was performed on a RigakuD/max2500v (RigakuD, Japan) using Cu Kα radiation (40 kV, 150 mA) from 0 to 90°. X-ray photoelectron spectroscopy (XPS) was obtained using a Thermo-ESCALAB 250XI (Thermo, USA) instrument with unmonochromated Al Kα 1486.6 eV radiation. FT-IR spectra of γ-Fe₂O₃/C and γ-Fe₂O₃/C–MG were collected in the range of 400–4000 cm⁻¹ using a Perkin Elmer 2000 (Perkin Elmer, USA). Vibrating sample magnetometer (MPMS-XL-7, Quantum Design, USA) was applied to measure the magnetic properties of the synthesized materials. Scanning electron microscopy (SEM) was performed on a NovaTM NanoSEM 430 (FEI, USA). Transmission electron microscopy (TEM) was performed on a Tecnai G20 (FEI, USA). The surface areas and pore size distributions of γ-Fe₂O₃/C were measured by nitrogen adsorption and desorption at 77 K using a 3Flex surface characterization analyzer (Micromeritics, USA). Surface areas of the samples were determined using the BET method and pore-size distributions and pore volumes were analyzed using the Barrett–Joyner–Halenda (BJH) method.

2.4 Adsorption procedure

γ-Fe₂O₃/C was dried overnight at 100 °C in vacuum before being applied as an adsorbent. A stock solution of MG (2000 mg L⁻¹) was prepared by dissolving MG in deionized water and stored at 4 °C in the dark before use. Working solutions were prepared by dilution of the stock solution with deionized water. Batch experiments were carried out to measure the adsorption characteristics of MG on γ-Fe₂O₃/C. An exact amount of γ-Fe₂O₃/C composite (10 mg) was placed into a 10 mL glass vessel, followed by the addition of 5 mL of MG solution. The glass vessel containing the mixture solution was fixed on a vortex generator for about 3 min and then placed into a water bath. After 24 h adsorption, the suspensions were separated using an external magnet, and the residual concentration of the MG solution was determined using a calibration curve prepared at the corresponding wavelength (614 nm) using a double-beam Cary 60 UV-visible spectrometer (Agilent, USA). The MG uptake at equilibrium, q_e (mg g⁻¹), was calculated as follows:

The adsorption efficiency was calculated as follows:

where C₀ and C_e are the initial and equilibrium MG concentrations (mg L⁻¹), respectively, V is the volume of MG solution (L), and M is the weight of γ-Fe₂O₃/C (g).

2.5 Desorption and degradation procedure

In this study, γ-Fe₂O₃/C (10 mg) and 5 mL of 500 mg L⁻¹ MG solution were subjected to multiple cycles to reduce the adsorbent cost. After completion of each cycle, the adsorbents were separated using an external magnet and washed thoroughly with methanol until the solution became limpid. Finally, γ-Fe₂O₃/C was separated using a magnet, dried at 70 °C in vacuo, and recycled for the next run.

Sunlight-driven photocatalytic degradation of MG in the presence of H₂O₂ over γ-Fe₂O₃/C was carried out in a 10 mL glass vessel.

Typically, 10 mg γ-Fe₂O₃/C was added into the glass vessel containing 5 mL of 500 mg L⁻¹ MG solution. After 5 min of adsorption–desorption equilibrium in the dark, 20 μL of H₂O₂ was added to the glass vessel, which was then fixed onto a vortex generator. The suspension was exposed to solar light; the photocatalytic experiments were performed in June to July 2014 between 11 a.m. and 2 p.m.

3. Results and discussion

3.1 Characterization of the prepared γ-Fe₂O₃/C

Structures of the as-synthesized MIL-53(Fe) and γ-Fe₂O₃/C composites were characterized by PXRD. As shown in Fig. 1a, samples showed diffraction patterns identical to the simulated patterns, indicating that the structure of MIL-53(Fe) was well preserved and the complexes were pure. From Fig. 1b, we can determine that the composites contain γ-Fe₂O₃/C or Fe₃O₄, as showed presence of the diffraction peaks of the (220), (311), (400), (422), (511), and (441) planes.²⁸ Those diffraction peaks marked by triangles were indexed to spinel ZnO (JCPDS card no. 36-1451). However, the PXRD peaks corresponding to the crystalline phases of carbon were not very clear in the PXRD patterns. Therefore, Raman spectroscopy was used to further investigate the nature of carbon within γ-Fe₂O₃/C (Fig. 1c). Two distinctive peaks were observed at 1335 and 1592 cm⁻¹, which corresponded to the D and G bands, respectively. In detail, the D band commonly refers to breathing vibrations of sp² rings, which is characterized by A_1g symmetry that is disallowed in graphite or the double-resonance Raman process in disordered carbon, whereas the G band is ascribed to graphite in-plane vibrations with E_2g symmetry.²⁹

Fig. 1 PXRD patterns of (a) simulated MIL-53(Fe) and as-synthesized MIL-53(Fe) sample; (b) as-synthesized γ-Fe₂O₃/C. (c) Raman spectroscopy of γ-Fe₂O₃/C. (d) FT-IR spectra of γ-Fe₂O₃/C (a) before and (b) after adsorption of MG.

Chemical structures of the as-synthesized γ-Fe₂O₃/C before and after adsorption of MG were characterized by FT-IR (Fig. 1d). The FT-IR spectra showed an adsorption band at about 530 cm⁻¹, which represented Fe–O stretching in the γ-Fe₂O₃/C sample, and indicated that the sample included maghemite. After adsorption of MG into γ-Fe₂O₃/C, two bands appeared at 1318 and 1586 cm⁻¹, which corresponded to the γ_C–N and δ_N–H in MG, respectively, and indicated that MG was adsorbed into γ-Fe₂O₃/C.

It was difficult to clearly distinguish the γ-Fe₂O₃/C and Fe₃O₄ phases from the PXRD pattern as they exhibit similar peaks. Thus, the chemical compositions of the products were further analyzed by XPS to clearly demonstrate that the magnetic composition was γ-Fe₂O₃ (Fig. 2a). A typical survey spectrum of γ-Fe₂O₃/C is depicted in Fig. 2a, and showed the presence of O, C, Zn, and Fe elements. The high-resolution spectrum of Fe is shown in Fig. 2b: the XPS spectra exhibit the peaks at 710.5 eV and 724.3 eV, which are the characteristic peaks of Fe 2p_3/2 and Fe 2p_1/2 oxidation states. And there are two obvious shake-up satellite structures at the higher binding energy sides of both main peaks, which were characteristic of γ-Fe₂O₃.^30,31 From Fig. 2c, it can be observed that two characteristic peaks arising at 1021.8 and 1045.0 eV corresponding to orbitals of Zn 2p_3/2 and Zn 2p_1/2 respectively, confirm the existence of ZnO.³²

Fig. 2 XPS spectra of as-prepared γ-Fe₂O₃/C: (a) survey spectrum, (b) Fe(2p) binding-energy spectrum, and (c) Zn(2p) binding-energy spectrum. (d) Magnetization curves of γ-Fe₂O₃/C composites at room temperature.

Meanwhile, the saturation magnetization of the composites were performed in the applied magnetic field sweeping from −20 to 20 kOe at room temperature. The M–H curve measurements of the composites recorded in Fig. 2d showed that the saturation magnetization value of the sample was 20.10 emu g⁻¹; this value demonstrated that the composite has strong magnetism.

To further characterize the porosity of γ-Fe₂O₃/C, nitrogen-adsorption experiments were performed at 77 K; the resultant porosity properties are summarized in Table 1. The nitrogen adsorption–desorption isotherms and the corresponding pore size distribution of γ-Fe₂O₃/C are shown in Fig. 3. As is evident from Fig. 3a, γ-Fe₂O₃/C exhibited a type IV isotherm with a distinct hysteresis loop at relatively high P/P₀, suggesting a mesoporous structure. The pore-size distributions of the as-prepared γ-Fe₂O₃/C samples were calculated from the desorption branch of the N₂ isotherm using the BJH model, and are shown in Fig. 3b and the insets of Fig. 3b. As is evident from Table 1, the BET surface area of γ-Fe₂O₃/C measured from the nitrogen isotherms was 397.2 ± 7.4 m² g⁻¹, which was higher than those of the previously reported magnetic porous carbons,^15,23,33 but lower than some porous carbons^26,34,35 because of the negative effects of the γ-Fe₂O₃ and ZnO in the magnetic composites.

Table 1 Properties of the γ-Fe₂O₃/C samples for N₂-adsorption experiments at 77 K

Sample	SBET^a (m^2 g^−1)	Vp^b (cm^3 g^−1)	Dp^c (nm)
a Note: BET surface area.b Note: total pore volume.c Note: average pore diameter.
γ-Fe2O3/C	397.2 ± 7.4	0.495	6.29

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Fig. 3 N₂ adsorption–desorption isotherms and pore-size distribution of the as-synthesized γ-Fe₂O₃/C composites.

The morphology, microstructures, and porous characteristics of as-prepared γ-Fe₂O₃/C were investigated by SEM and TEM (Fig. 4). From the SEM images shown in Fig. 4a–c, it was clear that γ-Fe₂O₃/C retained the morphology of MIL-53(Fe). The TEM and HRTEM images shown in Fig. 4d–f provided evidence of the structures of the composite and the size range of γ-Fe₂O₃ nanoparticles (∼5 nm). The magnified high-resolution transmission electron microscopy (HRTEM) image in Fig. 4f displays almost completely disordered lattice spacing, which indicated the non-crystalline/amorphous nature of the material. This observation was also supported by the selected area electron diffraction pattern (Fig. 4f), which featured vague and broad diffraction rings.³⁶ Hence, the HRTEM results were in good accordance with the PXRD patterns and Raman spectra.

Fig. 4 (a–c) SEM and (d–f) TEM images of the γ-Fe₂O₃/C composites.

3.2 Adsorption isotherms of MG on γ-Fe₂O₃/C

The sorption capacities of MG on as-prepared γ-Fe₂O₃/C were determined via equilibrium sorption experiments (Fig. 5). To evaluate the maximum adsorption capacities of MG more scientifically, two-parameter models, the Freundlich and Langmuir model, were selected for this study. The Langmuir model (eqn (S1) and (S2)†), which provides a basic view of adsorption, is usually valid for surfaces with a finite number of identical sites.³⁷ The Freundlich model (eqn (S3) and (S4)†) is used to describe adsorption onto heterogeneous surfaces with different functional groups on the surface and several adsorbent–adsorbate interactions.³⁸

Fig. 5 Standard adsorption curve for MG onto γ-Fe₂O₃/C at 30 °C. The concentrations of MG were 500, 600, 700, 800, 900, 1000, 1100, 1200 mg L⁻¹.

To study the applicability of the Langmuir and Freundlich isotherms for dye adsorption onto γ-Fe₂O₃/C, linear plots of C_e/q_e against C_e and log q_e versus log C_e were plotted (Fig. 6). Table 2 summarizes the obtained Langmuir and Freundlich parameters. The applicability of these isotherm equations was compared via the R² value. From Fig. 6 and Table 2, it was evident that the Langmuir isotherm (R² = 1.000) provided a better fit than the Freundlich model (R² = 0.870); this suggests that the better description of the observed adsorption equilibrium behavior of MG onto γ-Fe₂O₃/C is the Langmuir adsorption isotherm. Thus, adsorption may be assumed to occur at homogeneous binding sites on the surface of the adsorbents up to monolayer coverage.

Fig. 6 (a) Langmuir plots and (b) Freundlich plots of the isotherms for MG adsorption onto γ-Fe₂O₃/C at 30 °C.

Table 2 Comparison of the coefficients of the isotherm parameters for MG adsorption onto γ-Fe₂O₃/C at 30 °C^a

System	qe,exp (mg g^−1)	Langmuir isotherm			Freundlich isotherm
		qe,cal (mg g^−1)	KL	R^2	KF	1/n	R^2
a Notes: qe,cal, calculated adsorption capacity; qe,exp, experimental adsorption capacity.
MG	499	500	1.25	1.000	317.09	0.1013	0.870

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Compared with other adsorbents presented in the literature^9,40–44 (as shown in Table 3), γ-Fe₂O₃/C presented satisfactory adsorption capacity and can be considered as an alternative for the removal of reactive dyes from aqueous effluents.

Table 3 Comparison of the adsorption capacities of various adsorbents for MG

Adsorbent	qe (mg g^−1)	Temperature (°C)	References
γ-Fe2O3/C	499	30	This work
γ-Fe2O3/C	863	60	This work
MIL-100(Fe)	266	30	9
Cd(OH)2–NW–AC	19.0	25	40
AP-g-PAA	352.11	35	41
SnO2–NP–AC	142.87	27	42
Red mud	336.4	25	43
Activated carbon	27.78	27	44

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3.3 Thermodynamics of the adsorption of MG

The adsorption of MG was studied at 30, 40, 50, and 60 °C to determine the thermodynamic parameters; the results are shown in Fig. 7a. It was found that the adsorption capacity of MG increased at higher temperatures. To further elucidate the adsorption mechanism, the thermodynamic parameters, standard free-energy change (ΔG^θ, kJ mol⁻¹), enthalpy change (ΔH^θ, kJ mol⁻¹), and entropy change (ΔS^θ, J mol⁻¹ K⁻¹) for the adsorption of MG onto γ-Fe₂O₃/C were determined based on the following equations:³⁹

ΔG^θ = −RT ln K₀ (4)

where K₀ is the adsorption equilibrium constant, R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), and T is the temperature (K).

Fig. 7 (a) Adsorption isotherms for MG onto γ-Fe₂O₃/C at different temperatures. (b) Effect of contact time on the adsorption of MG onto γ-Fe₂O₃/C (10 mg) at different initial concentrations of MG at 30 °C.

The value of ln K₀ at a certain temperature was obtained by plotting ln(q_e/C_e) against q_e and extrapolating qe to zero based on eqn (4) (Fig. 8a). The values of ΔS^θ and ΔH^θ were calculated from the slope and intercept of the Van't Hoff linear plot of log K versus T⁻¹ (Fig. 8b). The negative values of ΔG^θ and positive values of ΔH^θ (Table 4) showed the spontaneity and endothermic nature of adsorption, respectively. The positive values of ΔS^θ reflected the increased randomness at the solid/solution interface during the adsorption of MG.

Fig. 8 (a) Plots of ln(q_e/C_e) versus q_e at various temperatures. (b) Plots of ln K₀ versus T⁻¹ for the adsorption of MG.

Table 4 Kinetic parameters for the adsorption of MG onto γ-Fe₂O₃/C at 30 °C

C0 (mg L^−1)	qe,exp (mg g^−1)	Pseudo-first-order kinetic constant			Pseudo-second-order kinetic constant
		qe,cal (mg g^−1)	k1 (min^−1)	R^2	qe,cal (mg g^−1)	k2 (g mg^−1 min^−1)	R^2
500	249	47	0.00619	0.867	252	4.00 × 10^−4	1.000
600	298	68	0.00574	0.881	300	3.60 × 10^−4	1.000
700	346	106	0.00554	0.923	352	1.80 × 10^−4	1.000
800	394	147	0.00494	0.963	403	1.08 × 10^−4	1.000
900	441	237	0.00444	0.997	457	5.24 × 10^−5	0.999

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3.4 Kinetics for the adsorption of MG

As is well-known, adsorption kinetics are important for the prediction of the rate at which contamination can be removed by γ-Fe₂O₃/C. From Fig. 7b, it was clear that the adsorption capacity significantly increased as the initial concentration of MG increased. To elucidate the mechanism of the adsorption of MG onto the adsorbent, characteristic constants of sorption were determined using pseudo-first-order and pseudo-second-order equations (Fig. 9).

Fig. 9 (a) Plots of pseudo-first-order kinetics for the adsorption of MG at 30 °C. (b) Plots of pseudo-second-order kinetics for the adsorption of MG at 30 °C.

The parameters obtained by linear regression are reported in Table 5. The pseudo-first-order model did not fit the data well, as evident from the low R² values. Furthermore, when using the pseudo-second-order model, the estimated q_e values (Table 5) were in good agreement with the experimental q_e,exp values. These results indicated that the MG adsorption system displayed pseudo-second-order kinetics over the entire adsorption period.

Table 5 Thermodynamic parameters for the adsorption of MG^a

T (K)	qe,exp (mg g^−1)	ln K0	ΔG^θ (kJ mol^−1)	ΔH^θ (kJ mol^−1)	ΔS^θ (kJ mol^−1 K^−1)
a Note: K0 is the adsorption equilibrium constant.
303.15	499	10.91	−27.51	31.68	195.70
313.15	638	11.34	−29.52	—	—
323.15	729	11.87	−31.89	—	—
333.15	863	12.01	−33.27	—	—

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3.5 Effect of the system pH on MG uptake

The pH value of the dye solution is recognized as an important factor in the adsorption process, which influences the surface charge, dissociation of functional groups on the active sites, degree of ionization of the adsorbents, and dye chemistry. Fig. 10 shows the variations in the removal of dye at different system pH values. The experimental data indicated that the pH has little influence on the adsorption of MG onto γ-Fe₂O₃/C. The higher adsorption of MG on γ-Fe₂O₃/C may probably attributed to the π–π interactions with the aromatic moiety of the MG. While the π–π interactions have been less affected by pH. Therefore the pH does not have any effect on the adsorption of MG.

Fig. 10 Effect of system pH on the adsorption of MG (1000 mg L⁻¹) onto γ-Fe₂O₃/C (10 mg) at 30 °C.

3.6 Regeneration and reproducibility

Repeated availability is an important factor for adsorbents. Desorption and regeneration of MG-loaded γ-Fe₂O₃/@C were achieved using methanol.⁹ The good reusability of γ-Fe₂O₃/C (Fig. 11a) also demonstrated the potential of γ-Fe₂O₃/C for the repeated availability is an important factor for adsorbents. Desorption and regeneration of MG-loaded γ-Fe₂O₃/C were adsorption and removal of MG. With an increased number of cycles, the MG removal efficiency decreased, which may be caused by MG that remained in the pores of the material.

Fig. 11 (a) Regeneration cycles and removal efficiency. (b) Photocatalytic degradation cycles and removal efficiency.

Of course, we also have taken the possible error of the adsorption capacity brought by the high concentrations and small volume into consideration. We have investigated the larger volume with lower concentration of MG impact of the adsorption capacity, and the results are shown in Fig. S1.† From Fig. S1,† it was clear that the adsorption capacity (491 mg g⁻¹) is almost entirely the same as the high concentrations and small volume.

The reproducibility of the synthetic method is confirmed by preparing this magnetic nanoporous carbons in triplicate at the same condition, and measuring their adsorption capacity. The data of the adsorption capacity of MG were shown in Fig. S2.† And the experiment result indicates this method is relatively stable.

3.7 Degradation

Iron–oxo clusters^4,45,47 are perfect catalysts for dye-degradation under light irradiation in the presence of H₂O₂. This could explain the photoactive characteristic of γ-Fe₂O₃/C. Fe(III) in γ-Fe₂O₃/C catalyzes the decomposition of H₂O₂ to produce ˙OH radicals by the Fenton-like reaction as follows:

Fe(III) + H₂O₂ → Fe(II) + H₂O˙ + H⁺ (6)

Fe(II) + H₂O₂ → Fe(III) + ˙OH + OH⁻ (7)

However, ZnO^30,46 in γ-Fe₂O₃/C could also act as a catalyst for dye degradation. H₂O₂, as an efficient scavenger, could capture the photoinduced electrons in excited ZnO to form ˙OH radicals as follows:

ZnO + hv → h⁺ + e⁻(ZnO) (8)

H₂O₂ + e⁻(ZnO) → ˙OH + OH⁻ (9)

As expected, the above processes could cooperatively contribute to the activation of H₂O₂ by γ-Fe₂O₃/C to produce more ˙OH radicals, thus greatly enhancing the degradation efficiency of MG.

On the basis of the obtained results in the Fig. S3,† using similar dosages of the catalyst, γ-Fe₂O₃/C performed much more than the bare γ-Fe₂O_3. This suggests that the porous carbon supported on the γ-Fe₂O₃ could improve the degradation reaction rate and aid the degradation. The main reason may be the porous carbon can adsorb the MG from water and narrow MG and the catalyst.⁴⁷

The recyclability of γ-Fe₂O₃/C was also checked by repeated cycles of catalytic degradation of MG (Fig. 11b). The results in Fig. 11b showed that the maximum degradation degrees of MG were close to 100% after six cycles, indicating that the γ-Fe₂O₃/C catalyst was very stable and could be used for repeated treatment of MG dye. Meanwhile, γ-Fe₂O₃/C almost completely retained its excellent magnetism after six recycles.

4. Conclusions

In summary, we reported a safe, controllable, and rapid method for the synthesis of a nanostructured porous carbon material (γ-Fe₂O₃/C) with a high specific surface area, high pore volume, and excellent magnetism. The adsorptive capacity for dye was measured using a cationic triphenylmethane dye (MG). The adsorption conformed to pseudo-second-order kinetics. It was found that dye adsorption onto γ-Fe₂O₃/C was well-fitted to a Langmuir isotherm. The maximum adsorption capacity was 499.15 mg g⁻¹ at 30 °C and reached 863.15 mg g⁻¹ at 60 °C. The high adsorption capacity and reusability make γ-Fe₂O₃/C promising as a novel adsorbent for the adsorption and removal of MG from aqueous solution. Finally, γ-Fe₂O₃/C showed powerful photocatalytic activity for the degradation of MG under sunlight in the presence of H₂O₂.

Acknowledgements

The financial support from the National Natural Science Foundation of China (no. 21365005), Guangxi Natural Science Foundation of China (2012GXNSFAA053024, 2014GXNSFGA118002), and State Key Laboratory Cultivation Base for the Chemistry and Molecular Engineering of Medicinal Resources, Ministry of Science and Technology of China (CMEMR2014-A09) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15942j

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