Fe3O4-NPs/orange peel composite as magnetic heterogeneous Fenton-like catalyst towards high-efficiency degradation of methyl orange

Magnetite nanoparticles (Fe3O4-NPs)/orange peel (MOP) composite was prepared via one-step in-situ co-precipitation method as magnetic heterogeneous Fenton-like catalyst. The properties of MOP were characterized by scanning electron microscopy, transmission electron microscopes, Brunauer–Emmett–Teller, X-ray diffraction, Fourier-transform infrared, thermogravimetric analysis and X-ray photoelectron spectroscopy technologies. Its Fenton-like catalytic responses towards removal of methyl orange (MO) were investigated, in which the effects of initial dye concentration, pH, temperature and hydrogen peroxide dosage were studied. The MO degradation ratio up to 98.0% was obtained within 20 min in optimized conditions. The catalyst showed excellent catalytic stability exhibiting nearly 90% degradation ratio in the 10th cycle within 20 min, whereas pure Fe3O4-NPs showed only 62.5% in this stage. Due to the stabilization of complexing orange peel hydroxyl to iron oxide in the composite and its magnetic separation property, MOP composite exhibits excellent Fenton-like catalytic performance, which offers great prospects for low-cost and high-efficiency organic dye wastewater treatment.


INTRODUCTION
Industrial water pollution is a serious threat to ecological environment and human health at present (Wang & Yang 2016;Naushad et al. 2019b;Alqadami et al. 2020;Khan et al. 2020). Wastewater from the printing and dyeing has large amounts of azo dyes like methyl orange (MO), which has metabolites that are often carcinogenic, teratogenic, mutagenic and biotoxic to living organisms (Jia et al. 2019). A number of recent studies on the removal of water polutants have been carried out, including biodegradation (Oliveira et al. 2020), physical adsorption (Ali et al. 2018;Ali et al. 2019;Naushad et al. 2019a;Abdul-Hameed & Al Juboury 2020;Al Juboury & Abdul-Hameed 2019), and catalytic oxidation or reduction degradation (Ahsan et al. 2019a(Ahsan et al. , 2019b(Ahsan et al. , 2019c(Ahsan et al. , 2020a. Among them, advanced oxidation processes, including UV-visible photolysis oxidation, catalytic wet air oxidation, ozone oxidation, electrochemical oxidation and Fenton oxidation, are considered to be a promising approach for the removal of refractory and toxic organic dyes (Chan et al. 2011;Tijani et al. 2014;Deng & Zhao 2015;Cheng et al. 2016;Ahsan et al. 2021). In Fenton oxidation, the strong oxidizing hydroxyl radicals (·OH, E ox ¼ 2.80 V) and hydroxyl oxygen radicals (·OOH, E ox ¼ 1.70 V) can be generated from decomposition of hydrogen peroxide (H 2 O 2 ) in the presence of ferrous ions (Fe 2þ ) (Babuponnusami & Muthukumar 2014). The advantage of the low cost and high reliability of Fenton processes is that organic dye molecules like MO, can be completely mineralized to carbon dioxide (CO 2 ) and water (H 2 O) in moderate conditions (Youssef et al. 2016). However, the classic homogeneous Fenton process has several disadvantages like a narrow pH range and the production of a large amount of iron sludge. To overcome these limitations, both noniron transition metal-based and iron-based catalysts for heterogeneous Fenton-like system have been investigated (Ahsan et al. 2020b(Ahsan et al. , 2020c. Iron-based heterogeneous Fenton-like catalyst is abundant in nature and can potentially be used for large-scale industrial dyeing wastewater treatment. Specifically, magnetite (Fe 3 O 4 ) nanoparticles (NPs) with Fe(II) and Fe(III) active sites have been good candidates for heterogeneous Fenton-like catalysts, because they are high-efficient and low-cost and can separate solid-state catalyst from the treated effluent with less post-treatment of ferric hydroxide (Fe(OH) 3 ) sludge (Zhang et al. 2009;Adyani & Soleimani 2019;Zheng et al. 2019). Nanoscaled Fe 3 O 4 with high surface/volume ratio can effectively enhance the reaction kinetics influenced by limited contact between reactant and catalyst in conventional heterogeneous Fenton-like systems (Wan et al. 2016). However, the magnetite NPs catalyst tends to aggregate and leach Fe element during multiple catalytic cycles due to its high surface energy and reaction activity, which reduces its Fenton catalytic performance (Tang & Wang 2018).
To overcome these drawbacks, the surface modification strategy to magnetic NPs can be adapted to change their surface states (Li et al. 2020). Some agents like organic acid derivatives (Wei et al. 2012;Fan et al. 2016) and silicate esters/organic amine (Abboud et al. 2015;Jia et al. 2019), polymer precursors (Abboud et al. 2015;Gamallo et al. 2018) and surfactants (Angamuthu et al. 2017) have been used to change the surface chemical groups of magnetite NPs or form a core-shell or yolk-shell structure (Cui et al. 2013;Lv et al. 2014;Du et al. 2017) so that the modified magnetite NPs can be stabilized instead of agglomerated. Although the developed Fenton-like catalyst can be obtained via surface modification technology towards higher catalytic efficiency at near neutral pH and more stable catalytic activity compared to nanoscaled Fe 3 O 4 , the corresponding preparation processes have been expensive and complex.
In the past few years, agricultural wastes and biomass residues have caused severe environmental problems due to their high chemical oxygen demand leading to the depletion of oxygen in natural water. After appropriate chemical treatment, these biomass sources become promising renewable carbon-rich materials (Titirici et al. 2007), which are widely used to produce low-cost biochar or their derivatives as absorbents due to the attainable porous structure and abundant surface groups (Chen et al. 2011;Gupta & Nayak 2012;Tan et al. 2016;Zou et al. 2016). One of the agricultural wastes from orange juice industry is orange peel (OP), which mainly consists of cellulose, hemicellulose, pectin (galacturonic acid), lignin and other low molecular weight compounds (Zapata et al. 2009). Abundant hydroxyl groups contribute to the anchoring effect for dispersing Fe oxide NPs in OP structure (Lafi et al. 2015;Shehzad et al. 2018), which makes it a low-cost and sustainable approach to developing a stable heterogeneous Fenton-like catalyst. Therefore, due to the enhanced stabilization of magnetite by OP, it can be used to reduce the reduction of Fe content in the catalyst during the Fenton-like process.
In this work, Fe 3 O 4 -NPs/OP composite (MOP) was developed in a one-step reaction to evaluate its feasibility as heterogeneous Fenton-like catalyst for the degradation of MO. After detailed characterization, the Fenton-like degradation of MO promoted by MOP in different conditions was performed to evaluate its catalytic reactivity, stability and separation properties. The reuse performance was also investigated during several catalytic cycles. MOP is a robust, high-efficiency Fenton-like catalyst with a very low-cost synthesis method in water treatment application.

Preparation of Fenton-like catalyst
The OP was sliced into small pieces and washed with deionized water twice, then dried in an air-oven at 70°C. These dry orange pieces were crushed into a powder which was filtered through 60 mesh screen (denoted as OP).
MOP was synthesized by co-precipitation method according to Gupta (Gupta & Nayak 2012), which was slightly modified. Briefly, 6.1 g FeCl 3 ·6H 2 O and 4.2 g of FeSO 4 ·7H 2 O were dissolved in 100 mL water and heated to 90°C in a round-bottomed flask. Then, 1 g OP powder was directly added into this flask. The mixture of 10 mL of ammonium hydroxide (26%) and 100 mL of water was prepared and dropwise added into the flask. The pH of the reaction system was adjusted to 10. The mixture was continuously stirred at 80°C for 30 min and then cooled to room temperature naturally. The black precipitate MOP was separated by centrifugation at 10,000 rpm for 10 min, washed to neutral pH with deionized water, dried at 50°C for 24 h and finally stored in a centrifuge tube for further use. The pure Fe 3 O 4 -NPs (denoted as MP) were also prepared in a similar way without adding OP powder.

Characterization methods
The products were characterized by scanning electron microscopy (SEM, JEOL-6701) and transmission electron microscopy (TEM; Tecnai G2 F30 S-TWIN) for morphology research. The specific surface area was calculated using Brunauer-Emmett-Teller (BET) method and obtained on ASAP 2,460 by N 2 adsorption-desorption method. X-ray diffraction (XRD) patterns were performed on DX-2700BH with Cu k α radiation at 30 kV and 25 mA. Fourier-transform infrared (FT-IR) spectra were recorded in the 4,000-400 cm À1 region by Nicolet Avatar 330 FT-IR at room temperature. The thermogravimetric analysis (TGA) was performed on SDT-Q600 at temperatures ranging from room temperature to 800°C at a heating rate of 10°C min À1 in air. The precise chemical composition of MOP was determined by inductively coupled plasma optical emission spectroscopy (ICPOES730, Agilent). The X-ray photoelectron spectroscopy (XPS) was measured using Thermo ESCALAB 250xi. The magnetic property of MOP was evaluated using a Quantum Design superconducting quantum interference device (SQUID VSM). The degradation of MO was monitored by the change of absorbance at maximum absorption wavelength using UV-visible spectrophotometer (UVmini-1240, Shimadzu).

Catalytic removal of MO
The heterogeneous Fenton-like catalytic reaction was evaluated by the degradation of MO. In a typical catalytic cycle, the reaction suspensions were prepared by adding a given amout of catalyst MOP (25 mg for catalyst concentration of 1 g/L) into a 50 mL centrifuge tube with 25 mL MO solution. The mixture was stirred and heated in a water bath at a specific temperature prior to adding H 2 O 2 (2 mL, 30%). Concentration samplings were taken at given time intervals during the reaction. The MOP catalyst was separated by applying magnetic field and the absorption sepctra of the supernatant MO solution was recorded at the maximum absorption wavelength. Meanwhile, the recovered catalyst was washed and purified with deionized water. After that, the recovered MOP catalyst was collected for cyclic use.

Characterization results
SEM and TEM photographs present the morphological characteristics of OP, MP and MOP. Figure 1(a) shows that OP has a complex structured surface morphology with numerous wrinkles. The energy-dispersive spectroscopy (EDS) analysis confirms that OP consists of abundant carbon, hydrogen, oxygen and little nitrogen, sulfur, potassium and calcium (Figure A.1 and  Table A.1, supplementary data) (Siles et al. 2016). According to TEM images of the prepared MP in Figure 1 where D hkl is the crystallite size in the direction perpendicular to the lattice planes, k is the Debye-Scherrer constant referring to the crystallite-shape factor (0.89), λ is the X-ray wavelength (0.15406 nm), β hkl is the radian value of full width at half maximum intenstiy of the diffration peak, and θ is the diffraction angle. The mean crystallite sizes of magnetite nanocrystalline of MOP and MP in the direction perpendicular to the lattice plane are calculated as shown in Table A.3. The calculated D 331 of MP is 14.1 nm (average) and of MOP is 16.1 nm (average), which is similar to the static average sizes of MP and MOP according to their TEM images ( Figure A.3).
The FT-IR spectra of OP, MP and MOP are shown in Figure 3, and the vibration absorption peak positions of the main functional groups are depicted as follows: -OH (absorption peak at 3,000-3,600 cm À1 ), C-H (absorption peaks at 2,939 and 1,384 cm À1 ), ester C ¼ O (absorption peak at 1,743 cm À1 ), aromatic C ¼ C (absorption peak at 1,651 cm À1 ), C-O-C (absorption peak at 1,071 cm À1 ). The broad and intense absorption peak at 3,418 cm À1 corresponds to the O-H stretching vibrations of cellulose, hemicellulose, pectin, lignin and absorbed water. The peaks observed at 2,939 cm À1 and 1,384 cm À1 can be attributed to C-H stretching and bending vibrations of methyl, methylene and methoxy groups. The peak at 1,743 cm À1 is the stretching vibration of C ¼ O bond due to non-ionic carboxyl groups (-COOH, -COOCH 3 ), and may be assigned to carboxylic acids or esters in pectin (Feng et al. 2011). The peak at 1,651 cm À1 can be assigned to the C ¼ C bond of benzene ring structure in lignin. The peak at 1,071 cm À1 refers to the glycosidic bond (C-O-C) mainly existing in cellulose, hemicellulose and pectin (Khan et al. 2019b). As a result, MOP retains most of the functional groups of OP due to its in-situ co-precipitation preparation method below 90°C.
The peak at 584 cm À1 assigned to Fe-O group (Lim et al. 2009) for MP and MOP indicates the presence of Fe oxide. FT-IR analysis shows the successful binding of Fe 3 O 4 to OP surface groups. The characteristic intense absorption peak of OP at 3,418 cm À1 (O-H stretch) becomes a significant broad absorption from almost 3,620 cm À1 to 2,500 cm À1 in the MOP infrared spectrum, which indicates the existance of intramolecular free hydroxyl group (3,650-3,580 cm À1 ) and the possible complexation interaction between Fe 3 O 4 -NPs and hydroxyl groups (3,400-3,200 cm À1 ), carboxyl hydroxyl groups (3,200-2,500 cm À1 ) of OP during the formation of MOP. Figure 4(a) shows the TGA curves of MOP and OP in air. The weight loss in Process I can be attributed to the water desorption in MOP and OP samples below 120°C. The total weight loss percentage is about 2.35% for MOP and 5.0% for OP, but the derivative thermogravimetry (DTG) peak of MOP appears at lower temperatures compared with OP, as shown in Figure 4(b) Section I. The intermolecular force between hydroxyl groups and water in OP seems to be more stable than that in MOP due to its possible complexation interaction of hydroxyl groups and Fe oxide.
According to their DTG curves in Figure 4(b) Section II and III, it is evident that both MOP and OP contain two oxidation processes: Process II begins at 121°C and is mainly caused by intramolecular dehydration, dissociation and oxidation of hemicellulose and cellulose; Process III begins at 293°C, which is mainly caused by combustion and carbonization of lignin (Zapata et al. 2009). In Process II, MOP shows better thermal stability than OP due to its DTG peak at a higher temperature, which indicates the binding force stabilization in the composite. Subsequently, the DTG peaks of MOP and OP appear nearly at the same temperature in Process III. The MOP and OP have the same thermal weight loss mechnism in Process III because the composite binding of OP and Fe oxide has been destroyed in this condition. Finally, the total weight loss percentage reaches up to 18.7% for MOP and 68.9% for OP at nearly 510°C. There is a broad weight loss peak above 510°C in the MOP DTG curve, which is probably attributed to the reaction between Fe oxide with pyrolytic carbon at high temperatures .
The weight loss of MOP is mainly attributed to OP oxygenolysis under the same decomposition condition and binding force between magnetite and OP is not considered. The apparent P value (magnetite weight percentage of MOP) can be obtained by Equation (2): where P MOP is the residual weight percentage of MOP and P OP is the residual weight percentage of OP at the same temperature with the similar heating rate. In Figure 5, the obtained apparent P value increases as the temperature rises from 150°C up to 240°C, which indicates that the thermal weight loss rate of MOP is lower than that of OP at the same temperature in first stage of Process II. It is worth noting that the apparent P value almost retains a constant value of 72.5% in the second half of Process II and in the whole Process III. This proves to be consistent with the previous hypothesis for this situation. The apparent P value decreases as the reaction between Fe oxide with pyrolytic carbon begins above 510°C. The Fe content of the MOP was found to be 55.0% by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis (76.0% for Fe 3 O 4 in MOP), which is similar to the above constant value of 72.5%.
To further identify the chemical characteristics of MOP, XPS analysis was performed and is shown in Figure 6. According to its wide-scan XPS spectrum (Figure 6(a)), MOP mainly consists of C, O, N, and Fe (H is not active for XPS analysis). According to C1s patterns of XPS in Figure 6(b), this peak can be divided into three simulated peaks of 284.7, 286.3 and 288.3 eV, which are assigned to the C in C-C, C-O and C ¼ O, respectively. For the O1s, patterns of XPS are also divided into three simulated peaks as shown in Figure 6(c). The peak at 529.9 eV is attributed to lattice oxygen (O 2À ) of Fe 3 O 4 -NPs. The peak at 531.4 eV is assigned to the O in hydroxyl groups (O-H). The peak of 532.7 eV may refer to the O in the oxygen bound species of C-O. The Fe2p peak can be further divided into eight peaks (Figure 6(d)), of which four simulated peaks at 710.1, 711.3, 723.0 and 724.6 eV are attributed to Fe 2þ 2p3/2, Fe 3þ 2p3/2, Fe 2þ 2p1/2 and Fe 3þ 2p1/2 and the other four peaks (grey lines) are assigned to the satellite peaks of Fe 2þ or Fe 3þ (Hu & Srinivasan 1999;Bhargava et al. 2007;Wilson & Langell 2014). These results show that the MOP catalyst contains Fe 2þ and Fe 3þ species (Fe 3þ /Fe 2þ peak area ratio is 2:1), which is consistent with the XRD results.
To quantitatively study the magnetic property of MOP, the magnetization curve was measured using vibrating sample magnetometry (VSM) at room temperature. The hystersis loop of MOP at 27°C is shown in Figure 7(a). The saturation magnetization of MOP is around 61.5 emu/g with a ultralow coercivity force of nearly 43 Oe, indicating the MOP exhibits a superparamagnetic property (Wei et al. 2012). MOP can be attracted easily by a magnet and removed from water within 1 min (Figure 7(b)), which is a simple and fast method for rapidly magnetically separating and recovering a catalyst from a Fenton reaction system. Therefore, MOP is a promising candidate for an efficient and cost-saving alternative Fenton-like catalyst for practical application.

Catalytic removal of MO
To evaluate the catalytic degradation of MO under different conditions in a Fenton-like system, the following situations should be considered: (1) reaction with only MOP catalyst and without H 2 O 2 (black diamond); (2) reaction with only H 2 O 2 and without MOP catalyst (red dot); (3) reaction with both MOP catalyst and H 2 O 2 (blue triangle). As shown in Figure 8(a), MO is non-degradable with only MOP added at 40°C for 120 min, when there is a slight absorption of MO by MOP (around 4% within 2 h). When only H 2 O 2 is added, the degradation ratio of MO is about 8% within 2 h, which is attributed to the lack of ·OH originating from the H 2 O 2 without catalyst. It barely promotes the degradation of MO. Specifically, when MOP and H 2 O 2 are simultaneously added, the efficiency of catalytic removal of MO reaches nearly 98% within 60 min and the aqueous solution in the reactor becomes nearly colourless.  The initial MO concentration is usually considered as one of the most critical factors affecting its degradation ratio. To elucidate the importance of initial dye concentration, 25 mL of 10, 20 and 30 ppm MO effluent solutions were investigated as shown in Figure A.5. The degradation ratio of MO decreases as the initial MO concentration increases. As the degradation of MO was increased by the MOP catalyst in the heterogeneous Fenton-like reaction, the percentages of the MO residue decreased to 50.7% (30 ppm), 18.6% (20 ppm) and 8.8% (10 ppm) within 20 min. More than 98% degradation ratio of MO within 60 min was observed for all three experimental conditions. The results indicate that the amount of ·OH generated by the MOP/H 2 O 2 Fenton-like process in this condition is sufficient for high concentrations of up to 30 ppm of MO dye. More detailed reaction conditions like pH value, temperature and H 2 O 2 dosage were subsequently investigated in this work.
The influence of pH on the degradation ratio of MO induced by the MOP catalyst is shown in Figure 8(b). It is observed that MO degradation ratio increases when the pH value changes from 3 to 1. Generally, MO degradation occurrs preferentially in an acidic condition for Fenton-like system. It has been reported that a very low pH restrains the degradation ratio of MO due to OH inactivation caused by hydrogen ion and dissolution of magnetite (Jia et al. 2019). Although the initial pH of 1 has shown better degradation in this reaction, it is likely because of protonation of abundant hydroxyl groups in the MOP which prevents the combination of H þ and ·OH. Therefore, the MO degradation ratio at pH ¼ 1.7 is almost 91.7% compared to with 96.6% for pH ¼ 1.0 within 40 min. A pH of 1.7 was chosen as the optimum pH condition for later degradation experiments.
Figure 8(c) shows the degradation ratio of MO at different temperatures. The MO degradation ratio increases from 13.4% to 98.0% within 20 min reaction time when the temperature increases from 25°C to 55°C at pH of 1.7. It has been proven that the enthalpy of the MO oxidative degradation is positive, which indicates that this process is endothermic. Therefore, the increase of temperature is beneficial to the degradation of MO for this heterogeneous Fenton-like reaction. However, the higher temperature needs more energy comsumption. Hence, the applicable temperature is considered to be 40°C in this article.
The effects of the H 2 O 2 dosage on the decomposition of MO was investigated at different volumes of H 2 O 2 being 0.5, 1 and 2 mL. Figure 8(d) shows that the degradation ratio of MO increased from 71.6% to 91.7% within 40 min. This is probably because more H 2 O 2 facilitates the increase of MO (30 ppm) degradation ratio. However, overdosed H 2 O 2 does not significantly promote the degradation of MO but rather leads to a wastage of oxidizing agent (Jia et al. 2019;Zhu et al. 2019).
It is well known that the ·OH and hydroxyl oxygen radicals (·OOH) are generated in the heterogeneous Fenton-like catalytic degradation of organics. To evaluate the role of radicals on Fenton-like mechanisms, ethanol and chloroform are used to serve as scavengers for ·OH radicals and ·OOH radicals, respectively. The first-order kinetic simulations of Fenton-like degradation of MO (30 ppm of 25 mL) with no scavenger, ethanol (∼18 mmol) and chloroform (∼18 mmol) were investigated at pH ¼ 1.7, 40°C, 2 mL H 2 O 2 (∼18 mmol) and additive amount of 25 mg MOP catalyst. The results show that the kinetic constants are 0.061 min À1 , 0.038 min À1 and 0.025 min À1 (Figure A.6), which indicates both ·OH and ·OOH radicals play important roles in the decomposition of MO at this stage.

Cycle stability of MOP catalyst for degradation of MO
To evaluate the stability of the MOP catalyst, cycle experiments of the catalytic degradation of MO were performed in 10 runs (MP as the control group). To maintain a constant amount of Fe oxide for catalytic degradation, 30 mg of MOP and 22 mg of MP were added to 25 mL MO solution (20 ppm) with 2 mL dosage of H 2 O 2 (30%) at pH 1.7 and 55°C. In this situation, each cycle lasted 20 min and the degradation ratio of MO was measured. At the end of each cycle, the used catalyst was magnetically separated and then washed twice with deionized water in an oscillating manner.
As shown in Figure 9, MOP shows a better cycle stability than MP after 10 cycles. In the fifth cycle, MP catalytic degradation ratio decreases to less than 90%. By contrast, nearly 90% degradation ratio can be obtained for MOP in the 10th cycle, whereas MP shows only 62.5% in this stage. According to SEM images of the original and recycled MOP (after 10 cycles) in Figure A.7, there is no obvious change in their microstructure and morphology. It is evident that the magnetite crystal structure remains unchanged according to their XRD patterns of MOP before and after catalytic reaction ( Figure A.8).
In addition, the FT-IR spectra of original MOP and recycled MOP (10 cycles) indicate that the main chemical groups of MOP do not changed after 10 cycles, as shown in Figure A.9. A slight decrease of infrared absorption peaks at 1,651 cm À1 (C ¼ C) 1,384 cm À1 (O-H) and 1,071 cm À1 (C-O-C) suggests that the benzene ring (lignin) and glycosidic bond (cellulose and hemicellulose) structures are partially damaged by oxidative ·OH in the catalytic Fenton process, which shows the sacrificial role of OP in MOP (Zubir et al. 2015). According to ICP-OES analysis (Table A.4), the Fe content of MOP increases from 55.0% (initial) to 66.6% (after five cycles catalysis) because the OP decomposition is greater than Fe leaching at this stage. The Fe content decreases to 61.0% after 10 cycles catalysis due to the increase in Fe leaching. All results demonstrate that the synthesized MOP is an excellent long cyclic stable heterogeneous Fenton-like catalyst.

CONCLUSIONS
MOP is fabricated by one-step in-situ co-precipitation method, which exhibits a high catalytic efficiency, superparamagnetic property and good catalytic stability for the degradation of MO by a heterogeneous Fenton-like reaction. The degradation ratio of MO (30 ppm for 25 mL) is 98.0% within 20 min at pH 1.7, 55°C, 2 mL H 2 O 2 and 25 mg MOP. MOP also demonstrates good cycle catalytic stability over 10 cycles, with nearly 90% of degradation ratio of MO (20 ppm). This is probably because of the sacrificial role of biomass (OP) in MOP during the Fenton-like reaction, which slows down the dissolution of iron from the solid-state catalyst. All these results indicate that the MOP is a potential heterogeneous Fenton-like catalyst for azo dye wastewater treatment.