Facile synthesis of low-cost biomass-based γ-Fe₂O₃/C for efficient adsorption and catalytic degradation of methylene blue in aqueous solution

Abstract

In this study, a novel and low-cost method was developed for synthesizing magnetic porous γ-Fe₂O₃/C as an adsorbent and catalyst for removal of methylene blue (MB) from aqueous solution. Using FeCl₃·6H₂O and bagasse as the precursor, the biomass-based material was synthesized rapidly using a microwave heating process. Fe ions were introduced to achieve both adsorption and catalytic degradation by the Fenton reaction. Characterization by BET, XRD, SEM, and TEM indicated that the prepared γ-Fe₂O₃/C material exhibited homogeneous mesoporosity with a BET surface area, mesoporous volume, and saturation magnetization of 764.116 m² g⁻¹, 0.54 cm³ g⁻¹, and 17.1 emu g⁻¹, respectively. The material exhibited excellent adsorption and catalytic degradation abilities for removal of MB from aqueous solution (up to 352.96 mg g⁻¹), with complete degradation of MB within 30 min. Moreover, γ-Fe₂O₃/C had good stability over 5 cycles. Therefore, this biomass-based γ-Fe₂O₃/C material can be used as a highly effective and low-cost adsorbent and catalyst and has great potential application in clean-up processes for environmental pollutants.

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1. Introduction

With the development of society, water pollution has become increasingly serious. The most serious pollution is derived from colored organic dye contamination. Pollution sources include a variety of manufacturing, printing, and dyeing industries. Industrial production consumes as much as 7 × 10⁵ t of colored organic dyes each year,^1,2 and thus large amounts of colored pollutants are produced. Organic dyes are difficult to break down under natural conditions because of their stability, and can cause great harm to the environment.³ For these reasons, investigation of effective purification methods for removal of organic dye pollutants from solution is important for the protection of human health and social and economic stability.

Simple and rapid methods to prepare inexpensive materials for removal of environmental pollutants are required to meet the increasingly stringent environment standards. There are many methods to remove environmental pollutants. In particular, adsorption and degradation methods have attracted considerable attention due to the excellent adsorption capacity of adsorbents and efficient catalytic performance of catalysts, respectively.^4,5 However, there are concerns to do with the preparation of such materials.

Activated carbon (AC) is a typical effective absorbent that is widely used for the removal of organic compounds from wastewater due to its large specific surface area and developed porous structure.^6–8 However, traditional AC materials are difficult to recycle because the traditional separating method is inconvenient.⁹ Fortunately, this problem can overcome by using magnetic separation methods, which can provide efficient and rapid pollutant removal from wastewater.¹⁰

Moreover, readily available and low-cost precursor materials have been studied, such as biomass and agricultural by-products or waste, to reduce the cost of AC.¹¹ Functional porous materials are widely used in environmental clean-up processes.^12–15 Many inexpensive carbon precursors have been reported, such as rosa fruits, rice husks, coffee waste residue, peanut shells, grape stalks, waste wood shavings, and corncobs.^16–19 Most of these materials are produced by carbonization and activation, which requires carbonization in a muffle furnace or other instrument under the suitable conditions, such as air and carbon dioxide, followed by impregnation of the precursor with an activation medium and then heat treatment at a high temperature under an inert atmosphere. In contrast, using microwave heating as the heating source, imparts the synthesis of AC materials with green characteristics, as this technique provides fast carbonization and high efficiency heating.²⁰

Microwave heating is a convenient method that can realize the carbonization process under mild conditions due to the uniqueness of its heating process.²¹ Microwave heating is also a uniform and efficient heating approach that can greatly reduce the synthesis time and provide better carbonization results.²⁰ This process has already been successfully used in the synthesis of various biomass-based materials. For example, a porous biomass-based adsorbent prepared by the microwave heating approach with banana as a precursor possessed a favourable methylene blue (MB) adsorption capacity.¹⁵ Meanwhile, the catalytic degradation of colored pollutants in water clean up processes has attracted much attention due to the efficient catalytic performance. The most extensively used degradation method is the Fenton process, which is an advanced oxidation process (AOP) that uses Fe²⁺ and H₂O₂ to generate a powerful oxidant.⁵ This process has received considerable attention in the cleanup of wastewater due to the highly activity and stability of the catalyst.²² Moreover, the Fenton process has also successfully been used in the preparation of porous magnetic carbon materials.²³ Therefore, the discovery a rapid and simple method to prepare biomass-based activated carbon materials, which have both good adsorption and degradation performance, has great application value.

In this work, we used the waste bagasse as an AC precursor, and introduced Fe ions by a one-step method by steeping the precursor in a FeCl₃·6H₂O aqueous solution. The porous γ-Fe₂O₃/C material was prepared by a microwave heating process within 4 min. The porous material has excellent properties that combine both adsorption and catalytic degradation by the Fenton reaction. When used as an adsorbent to remove MB from aqueous solution, γ-Fe₂O₃/C exhibited an excellent adsorption ability of 352.96 mg g⁻¹. In addition, the material realized the catalytic degradation of MB in the presence of H₂O₂ and NH₂OH,⁵ which can further decrease the time of the MB removal. However, to the best of our knowledge, there are few previous reports on the synthesis of activated biomass-based porous carbon materials that combine carbonization via the microwave heating approach with the catalytic degradation process. Therefore, this effective and inexpensive bagasse-based γ-Fe₂O₃/C material has a tremendous competitive advantage and great application value in cleanup processes for environmental pollutants.

2. Experimental

2.1 Reagents and materials

Zinc chloride (ZnCl₂), ferric chloride hexahydrate (FeCl₃·6H₂O), 5-sulfosalicylic acid dihydrate (C₇H₆O₆S·2H₂O), and hydrogen peroxide (H₂O₂) were supplied by Aladdin (Shanghai, China). Hydroxylamine (NH₂OH) was supplied by Energy Chemical (Shanghai, China). Sodium hydroxide (NaOH), hydrochloric acid (HCl), and methylene blue (MB) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of these chemicals were of at least analytical reagent grade and used without further purification. MB stock solutions (1000 mg L⁻¹) were prepared and further diluted to the required concentrations to examine the adsorption capacity and the adsorption process. Ultrapure water was obtained from a Millipore purification system (Bedford, MA, USA) and used to prepare all aqueous solutions.

2.2 Instrumentation

X-ray diffraction (XRD) patterns of the γ-Fe₂O₃/C material were recorded using a D/max 2550 VB/PC diffractometer (Rigaku, Japan). X-ray photoelectron spectroscopy (XPS) data were obtained with a Thermo ESCALAB 250XI electron spectrometer (Thermo, USA) using 150 W Al Ka radiation. Fourier transform infrared (FT-IR) spectra (4000–400 cm⁻¹) in KBr were obtained using a PE Spectrum One FT-IR spectrometer (PE, USA). The morphologies and microstructures of γ-Fe₂O₃/C were characterized by field emission scanning electron microscopy (SEM) (NoVaTM Nano SEM 430, FEI, USA) and transmission electron microscopy (TEM) (Tecnai G20, FEI, USA). The specific surface areas of γ-Fe₂O₃/C were calculated using the Brunauer–Emmett–Teller (BET) method. Nitrogen adsorption and desorption at 77 K was carried out using an adsorption instrument (3Flex, Micromeritics, USA), and the pore structures, pore size distributions, and pore volumes were analyzed by the Barrett–Joyner–Halenda (BJH) method. The magnetization curves were measured at 300 K under a magnetic field (in the range of −20 and 20 kOe) using an MPMS-XL-7 magnetometer (Quantum Design, USA). Raman spectra were obtained using an inVia spectrometer (Renishaw, UK). Thermogravimetric analysis (TGA) was performed with a LABSYS evo TG-DSC/DTA instrument (Setaram Instrumentation, France) under air at a heating rate of 20 °C min⁻¹.

2.3 Preparation of γ-Fe₂O₃/C

Bagasse was gathered from a local fruit market (Guilin, China), and then dried overnight at 80 °C in oven and ground into 60/200 meshes by herbal medicine grinding machines (FW-135, TianJin, China). Then a certain quantity of bagasse powder (1.000 g) and FeCl₃·6H₂O (0, 0.500, 1.000, 1.500, and 2.000 g) was immersed in ultrapure water at 70 °C in thermostated water bath (DHG-9036A, Shanghai, China) for 24 h. The sample was strained and there served solids were dried at 70 °C in oven. After that, ZnCl₂ was added to the Fe loaded bagasse powder (the mass ratio of ZnCl₂ and Fe loaded bagasse powder was 1 : 4, g g⁻¹). Then the mixture of ZnCl₂ and Fe loaded bagasse powder were uniformly grind and reacted in a microwave reactor (600 W, Glanz WD800BL23, Shenzhen, China) for 2–6 minutes. The obtained solid was washed with ultrapure water, ethyl alcohol, and hydrochloric acid three times, respectively. Finally, the material was isolated from aqueous solution using an external magnetic field and dried in a vacuum oven at 80 °C for 24 h.

2.4 Adsorption and degradation experiments

2.4.1 Adsorption experiments. The adsorption experiments employed MB as an organic pollutant in water. The prepared γ-Fe₂O₃/C (0.010 g) was added to 5 mL of MB aqueous solution with an initial concentration of 50 to 800 mg L⁻¹ at a constant temperature of 30 °C (pH 7.0). After various adsorption times and magnetic separation, the equilibrium concentration was measured at 665 nm using a UV-vis spectrophotometer (Cary 60, Agilent, USA).

The effects of other conditions, such as initial concentration, FeCl₃·6H₂O and bagasse ratio (g g⁻¹), microwave carbonization time, pH value, and contact time, were also studied. The adsorption quantity at equilibrium (q_e, mg g⁻¹) was calculated using the following relationship:

where q_e (mg g⁻¹) is the amount of dye adsorbed on γ-Fe₂O₃/C at equilibrium. C₀ (mg L⁻¹) and C_e (mg L⁻¹) are the initial and equilibrium concentrations of MB. V (L) is the volume of the solution, and M (g) is mass of the adsorbent.

2.4.2 Degradation experiments. First, γ-Fe₂O₃/C (10 mg) was added into the glass vessel containing 10 mL of 100 mg L⁻¹ MB solution, and the material was uniformly dispersed in the aqueous with vortex mixer. After 5 min of reaching the adsorption–desorption equilibrium, 5–20 mmol L⁻¹ NH₂OH and 1–7 mmol L⁻¹ H₂O₂ were added to the glass vessel, and the mixture was then placed in an incubator shaker (120 rpm) at 30 °C. During the degradation process, a certain amount of the supernatant was removed at fixed time intervals after magnetic separation. The absorbance was measured using a UV-vis spectrophotometer, and finally the supernatant was recycled to maintain the overall volume of the solution.

3. Results and discussion

3.1 Synthesis and characterization of γ-Fe₂O₃/C

As illustrated in Scheme 1, bagasse were rich in cellulose, and thus contains a large number of ⁻OH and ⁻COOH groups, which would be a good carbon framework source and can strongly interact with Fe³⁺. After dipping bagasse in Fe³⁺ aqueous solution for 48 h at 70 °C with the mass ratios of Fe-based bagasse was 0.5 : 1 (g g⁻¹). The bagasse-Fe precursor was obtained after filtration and drying of the solid. After adding ZnCl₂ and carbonization by microwave heating for 4 min, Fe³⁺ was oxidized to γ-Fe₂O₃, as described in our previous studies.^24,25 Finally, the obtained powder (from the optimal preparation conditions) was further processed by drying and grinding. Then, the following data was come from the material above.

Scheme 1 Synthesis of γ-Fe₂O₃/C.

As shown in Fig. 1(a) XRD patterns of the γ-Fe₂O₃/C materials match well with the Standard PDF card of γ-Fe₂O₃. The crystalline structures of γ-Fe₂O₃/C were characterized using a powder XRD technique. The main characteristic peaks of γ-Fe₂O₃ appeared around 33.2°, 35.56°, 43.26°, 54.18°, 57.16° and 62.64°, which correspond to the (220), (311), (400), (422), (511), and (440) planes (JCPDS card no. 39-1346). In addition to the standard characteristic peaks of γ-Fe₂O₃, there were several additional peaks (marked with “●” and “■”) in the XRD patterns, which could be attributed to the spinel lattice of ZnO and ZnCl₂.²⁶ This could be because of these species cannot be washed out by using 0.01 M HCl and 0.01 M NaOH.

Fig. 1 XRD patterns of simulated Fe₂O₃ and γ-Fe₂O₃/C.

However, it was difficult to clearly distinguish the γ-Fe₂O₃ and other iron oxide phase from the XRD pattern as they exhibit similar peaks. Thus, the chemical compositions of the pre-preparation material were further analyzed by XPS to clearly demonstrate that the magnetic composition was γ-Fe₂O₃.

A typical survey spectrum of γ-Fe₂O₃/C was depicted in Fig. 2(a), and showed the presence of O, C, Zn, and Fe elements. The high-resolution spectrum of Fe was shown in Fig. 2(b): the XPS spectra exhibited the peaks at 711.0 eV and 724.5 eV, which were the characteristic peaks of Fe 2p_3/2 and Fe 2p_1/2 oxidation states. And there were two obvious shake-up satellite structures at the higher binding energy sides of both main peaks, which were characteristic of γ-Fe₂O₃.^27,28 As show in Fig. 1(c), it can be observed that two characteristic peaks arising at 1022.3 and 1045.4 eV corresponding to orbitals of Zn 2p_3/2 and Zn 2p_1/2 respectively, confirmed the existence of ZnO.²⁹ The O 1s peak in Fig. 2(d) can be deconvoluted into three peaks by the XPS peak fitting program. The peaks at 530.5 and 532.9 eV can be attributed to O in γ-Fe₂O₃ and ZnO respectively,^30,31 and the peak at 531.7 eV should be assigned to surface oxygen, which can be described as surface oxygen species “O⁻”.³²

Fig. 2 XPS spectra of the as-prepared γ-Fe₂O₃/C: (a) survey spectrum; (b) high-resolution Fe 2p binding energy spectrum; (c) high-resolution Zn 2p binding energy spectrum; (d) high-resolution O 1s binding energy spectrum.

Fig. S1(a)† showed the TGA curves of γ-Fe₂O₃/C. The main weight-loss stage was due to destruction of the carbon structures of the samples at temperature up to about 270 °C. These measurements indicated that the carbon content of γ-Fe₂O₃/C was 16%. The magnetization of γ-Fe₂O₃/C was determined by MPMS measurements. In Fig. S1(b),† The saturation magnetization (MS) value of γ-Fe₂O₃/C-1 and -2 was 17.1 emu g⁻¹ and 145.6 emu g⁻¹. Using an external magnetic field, the magnetization of γ-Fe₂O₃/C was adequate to realize rapid separation of the adsorbent from solution.

The N₂ adsorption–desorption isotherms of γ-Fe₂O₃/C-1 and -2 was shown in Fig. 3(a). This material exhibits type II isotherms, which indicated that this material was mesoporous. The BET surface areas and pore volumes of γ-Fe₂O₃/C-1 and -2 were 746.116 m² g⁻¹, 114.782 m² g⁻¹, and 0.54 cm³ g⁻¹, 0.12 cm³ g⁻¹, respectively (Table S1†). The data showed that γ-Fe₂O₃/C-1 had larger specific surface area with the FeCl₃ impregnation ratio was 0.5 : 1. Thus, these parameters also indicate that the prepared γ-Fe₂O₃/C possesses excellent surface performance properties. Owing to the high surface areas, γ-Fe₂O₃/C will inevitably have a satisfactory adsorbing capability.

Fig. 3 (a) N₂ adsorption–desorption isotherm of γ-Fe₂O₃/C 1 and 2, and (b) the corresponding pore diameter distribution analysis (the FeCl₃ impregnation ratio of 1 and 2: 0.5 : 1, and 2 : 1; the carbonization time of 1 and 2: 4 min). (c) Raman spectrum of γ-Fe₂O₃/C. (d) FT-IR spectra of γ-Fe₂O₃/C with MB, γ-Fe₂O₃/C, and MB.

Using the BJH method to estimate the poresize of γ-Fe₂O₃/C-1 and -2, as shown in Fig. 3(b), the maximum pore diameter of γ-Fe₂O₃/C occurred at approximately 2.81 and 3.41 nm. This result also suggested that γ-Fe₂O₃/C is a mesoporous material.

The structure of the carbon phase of γ-Fe₂O₃/C was investigated using Raman spectroscopy. As show in Fig. 3(c), two broad peaks with similar intensities appeared at about 1357 and 1598 cm⁻¹, and were assigned to the defect sp³ carbon band (D band) and stretching vibrations of basal graphite layers (G band), respectively.^33,34 In addition, the information from XRD also indicated that the carbon structure in this sample was amorphous. The adsorption of MB by this sample was also investigated using FT-IR, as shown in Fig. 3(d). The characteristic peaks at 1139, 1354 and 1548 cm⁻¹ are typical of C–S bond, C–N bond, and C=N bond stretching, respectively, and indicate that MB was successfully adsorbed on γ-Fe₂O₃/C.

The morphology and microstructure of the materials was characterized by SEM and TEM. The SEM images (Fig. 4) showed that the surface of material has amount of porous structures between the particles. The TEM images (Fig. 5) indicated that there were ultrafine γ-Fe₂O₃ particles on the material, which was beneficial for magnetic separation.

Fig. 4 SEM images of γ-Fe₂O₃/C.

Fig. 5 TEM images of γ-Fe₂O₃/C.

The elemental mapping of γ-Fe₂O₃/C revealed the distribution of the four elements within the structures (Fig. 6). In particular, Fe was uniformly distributed in γ-Fe₂O₃/C.

Fig. 6 Elemental mapping of the homogenous dispersion of (a) original images; (b) O elements; (c) Fe elements; (d) C elements in γ-Fe₂O₃/C.

Considering the results supported by FT-IR, SEM, TEM and XRD et al., we can conclude that the porous γ-Fe₂O₃/C material that consists of γ-Fe₂O₃ was successfully synthesized.

3.2 Adsorption of MB

MB was employed as a model pollutant to evaluate the adsorption performance of the prepared γ-Fe₂O₃/C adsorbents. γ-Fe₂O₃/C (0.010 g) was added to 5 mL of MB aqueous solution with an initial concentration of 50, 100, 200, 400, or 800 mg g⁻¹ at pH 7.0. The adsorption experiments were carried out at 303 K in a thermostated water bath. The factors that influence the adsorption process, such as the proportion of impregnation, carbonization time, initial dye concentration, and the contact time, were studied. As shown in Fig. 7, the adsorbed amount of MB significantly increased over the initial 8 h.

Fig. 7 Effect of contact time on the adsorption of MB onto γ-Fe₂O₃/C (10 mg) at different initial concentrations of MB at 30 °C.

Moreover, when the initial concentration was increased, the adsorbed amount of MB increased, which indicates the excellent adsorption capacity at high MB concentrations in aqueous solution. The proportion of FeCl₃ and the microwave carbonization time also influenced the adsorption quantity. In Fig. S2(a),† the adsorption quantity decreased with an enhancement of the FeCl₃ impregnation ratio. The data showed that the FeCl₃ impregnation ratio had greatly influenced the composite structure/porosity. In contrast, the magnetic performance improved at higher FeCl₃ impregnation ratios (Fig. S1(b)†). Therefore, after considering the influence of the adsorption capacity in comparison with the magnetic performance, an impregnation proportion of 0.5 : 1 (g g⁻¹) was chosen.

The time of microwave heating process directly determined the construction of the carbon material.²¹ With the appropriate microwave treatment processing time, the material rapidly produced well-developed hole structures (Fig. S2(b)†). However, the adsorption capacity was significant decreased at extended processing times. It might be that excessive microwave heating destroys the structure of active carbon. Therefore, we chose a processing time of 4 min.

The pH value of the adsorption system can greatly influence the charge of both the surface of the adsorbent and the adsorbate. Therefore, it was necessary to the impact of different pH values on the adsorbed amount of MB. As shown in Fig. S3,† the adsorbed amount increased with the increasing of pH. It maybe that cationic MB competes with H₃O⁺ for the adsorption sites when the pH value was low. With increasing pH, the number of negatively charged adsorption site increases, which was beneficial for the adsorption of cationic MB. Thus, this process ultimately resulted in an increase of the adsorbed amount of MB.

The composition of practical wastewater is complicated, as it may be acidic or alkaline. There might be limits to the practical usage of this kind of material, as the most common synthetic iron oxides are not resistant to extreme acid–base conditions. Therefore, the acid–base resistance properties of the material were very important for practical application. As a result, we examined the stability of γ-Fe₂O₃/C by adding sulfosalicylic acid to the supernatant of various acidic and alkali solutions in which γ-Fe₂O₃/C had been soaked for 24 h. As shown in the inset of Fig. S3,† only the strongly acidic condition (pH = 1.0) appeared slightly purple after addition of salicylic acid. We calculated that the dissolved ion concentration of Fe³⁺ in this solution was 1.874 mg L⁻¹. This small amount of dissolved Fe³⁺ indicates that γ-Fe₂O₃/C was stable in most acid–base conditions, which was greatly advantageous for the removal of actual organic dyes.

The experimental data for the adsorption of MB were further analyzed using a pseudo-second-order adsorption kinetic model.

where q_e and q_t are the amounts of adsorbate adsorbed on the adsorbent (mg g⁻¹) at equilibrium and at a given time t (min), respectively, and k₂ is the rate constant for pseudo-second-order adsorption (g mg⁻¹ min⁻¹).

Fig. S4† shows pseudo-second-order kinetics plots for absorption of MB at different initial concentrations. The adsorption kinetics data were well described by the pseudo-second-order model (R² > 0.997). As shown in Table S2,† the experimental measured values were basically the same as the calculated values. The results showed that the overall adsorption process was controlled by the chemical adsorption.¹⁵

In order to reduce costs, the adsorbents in actual production processes are always recycled. Therefore, the ability to reuse the adsorbent is also an important aspect when determining the properties of an adsorbent. Moreover, recycling can maximize the efficiency of an adsorbent. Although the removal rate of MB decreases with recycling (Fig. S5†), a rate of over 85% was still achieved after five cycles. The reduction in the adsorption performance might be due to the repetitive washing and drying processes, which contributed to the loss of adsorbent. And MB could be difficult to clean completely using ethyl alcohol which adsorbed in the smaller holes of γ-Fe₂O₃/C. In addition, ZnO in γ-Fe₂O₃/C could also act as a catalyst for dye degradation.^27,35 ZnO in γ-Fe₂O₃/C could cooperatively contribute to the removal of MB by γ-Fe₂O₃/C, thus enhancing the degradation efficiency of MB. As show in Fig. 7-red circles, there was some MB degradation with the γ-Fe₂O₃/C. This phenomenon further decreased the recyclability of γ-Fe₂O₃/C.

3.3 Catalytic degradation of MB

The Fenton process is an AOP that uses Fe²⁺ and H₂O₂ to generate a powerful oxidant.²² This process can be used to oxidize macromolecules, such as refractory organics, into low-toxicity and non-toxic small molecules. The Fenton process is a type of deep oxidation technology that can produce ˙OH free radicals through the reaction between Fe ions and H₂O₂,³⁶ which can oxidize toxic and refractory organic compounds to achieve removal of contaminants. Meanwhile, NH₂OH is used to restore Fe³⁺ generated during homogeneous catalysis of the system in which H₂O₂/Fe is used as a reducing agent.³⁷ This system is suitable for the oxidation of organic wastewater, such as biological refractory wastewater, or general chemical oxidation.

In H₂O₂/Fe catalytic degradation of organic matter, pH has a great influence on the degradation efficiency. Therefore, determining the effect of pH on the degradation efficiency is necessary. As shown in Fig. S6(a),† the degradation rate of MB was fast at low pH values, with complete degradation of MB within 30 min at pH 3.0. With increasing pH values, the degradation efficiency gradually decreased. At pH 9.0, the extent of waste degradation was reduced considerably, with only 63.9% of MB removed within 40 min.

A similar phenomenon is known for the homogeneous catalytic degradation of organic matter. At higher pH values, H₂O₂ becomes unstable due to the increased concentration of ⁻OH in the solution. Thus, the amount of ˙OH produced at the surface of the γ-Fe₂O₃/C is reduced, and the degradation rate and extent of degradation are decreased.³⁸

The influence of different amounts of the catalytic components, such as the reductant, catalyst, and H₂O₂, were evaluated. As shown in Fig. S6(b),† with an increasing concentration of H₂O₂, the degradation efficiency was greatly enhanced. It might be the system cannot produce enough free radicals when the concentration of H₂O₂ was low. However, this does not mean that a higher concentration of H₂O₂ will shorten the degradation time because the reaction between the produced free radicals and H₂O₂ would be inhibited at higher concentrations of H₂O₂.³⁸ The reaction equation was as follows:

H₂O₂ + ˙OH → H₂O + ˙OOH

˙OOH + ˙OH → H₂O + O₂

In the H₂O₂/Fe homogeneous catalysis system, we used NH₂OH as the reductant to allow the Fenton reaction to proceed, and reduce the narrow pH range of the reaction. In this study, Fe³⁺ was efficiently transformed into Fe²⁺ following addition of, which can speed up the catalytic rate. With the increasing of the NH₂OH concentration, the degradation rate was obviously improved (Fig. S6(c)†). Further investigation showed that NH₂OH had little influence on the catalytic degradation when its concentration was less than 10 mmol L⁻¹. The amount of the catalyst directly affects the time it takes to reach catalytic equilibrium and the removal rate of MB. As shown in Fig. S6(d),† by increasing the amount of γ-Fe₂O₃/C, the catalytic degradation time was reduced to 10 min and the catalytic efficiency was close to 99%. This improvement is because the increased catalyst dosage can provide more H₂O₂ active sites, and thus produce more ˙OH to degrade MB.

Moreover, as the distances between each molecule were reduced, the production of free radicals can result in faster oxidation of MB. The structure of the γ-Fe₂O₃/C catalyst reduced the probability of reaction between the free radicals and other molecules, so that the catalytic efficiency was further enhanced. The catalytic degradation of MB can be completed within 30 min with the combined γ-Fe₂O₃/C, NH₂OH, and H₂O₂ system. In contrast, the removal rate was not as high when γ-Fe₂O₃/C alone was used. However, γ-Fe₂O₃/C was superior to other materials for the catalytic degradation of MB, with greatly shortened times for the removal of organic dyes. The data show that γ-Fe₂O₃/C has great potential application in the removal of organic dyes (Fig. 8). Compared with other adsorbents presented in the literature^{13,15,18,19,22,23,26} (as shown in Table S3†), γ-Fe₂O₃/C presented satisfactory adsorption capacity and degradation efficiency in this work, which can be considered as an alternative for the removal of reactive dyes from aqueous effluents.

Fig. 8 Catalytic properties of different materials with MB (100 mg L⁻¹) at 30 °C and pH = 7 (the dosages of catalyst: 10 mg; C_NH2OH: 10 mmol L⁻¹; C_H2O2: 3 mol L⁻¹).

We also surveyed the recycling performance of the γ-Fe₂O₃/C material in the degradation process. The great degradation performance of the materials was maintained after five reuse cycles (Fig. S7†). However, the addition of NH₂OH and H₂O₂ might harm the catalyst as these reagents may produce organic acids during the degradation process. To inspect the stability of γ-Fe₂O₃/C during the catalytic reaction, we obtained the impregnation liquid after 5 catalysis cycles, and then, after removal of H₂O₂, added 10 wt% sulfosalicylic acid. As shown in the insert of Fig. S6,† there were no significant change in the color, even at this low pH value, and thus the estimated amount of dissolved Fe ions in aqueous solution was very low. This data indicated that γ-Fe₂O₃/C has excellent stability after several reuse cycles.

4. Concluding remarks

In summary, we developed a novel and inexpensive method to synthesize biomass-based γ-Fe₂O₃/C for the efficient removal and degradation of dyes in aqueous solution using a microwave heating approach. The obtained porous γ-Fe₂O₃/C material showed excellent magnetism, a high specific surface area, and a high pore volume. In addition, γ-Fe₂O₃/C had superior MB adsorption capacities (up to 352.96 mg g⁻¹ at 30 °C) and excellent catalytic degradation ability for MB (complete degradation within 30 min in the presence of H₂O₂ and NH₂OH). Thus, with this novel and inexpensive method a bagasse-based porous γ-Fe₂O₃/C material was obtained that has great potential for the clean-up of environmental pollutants.

Acknowledgements

The financial support from the National Natural Science Foundation of China (21365005), Guangxi Natural Science Foundation of China (2014GXNSFGA118002) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24900k

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