Nanosorbcats of methylene blue on novel Fe₂O₃ nanorods for photocatalytic water oxidation

Abstract

The development of dye-loaded nanosorbcats can make best use of their advantages in removing dyes from wastewater, also bypassing their disadvantages. Herein, functionalized Fe₂O₃ nano-rods (pfa@Fe₂O₃) were synthesized by using tween-80 induced assembly of pyrrol-2-methanoic acid (pfa) and iron(III) salts, and characterized by FT-IR, XRD, EPR, and SEM. The pfa@Fe₂O₃ nanorods’ adsorption capacity for methylene blue (MB) was 450 mg g⁻¹ compared to only 39 mg g⁻¹ for α-Fe₂O₃. Moreover, the kinetics to reach equilibrium were approximately 0.5 h and the pfa@Fe₂O₃ nanorods can be used as efficient recyclable adsorbents for MB. In particular, the MB loaded MB@pfa@Fe₂O₃–water system could also be used as dioxygen generator when irradiated with red LED light (10 W). The light induced intramolecular charge transfer between catalysts and photosensitizers was more efficient in the MB@pfa@Fe₂O₃/H₂O system, and it exerted higher activity for water oxidation than the traditional catalyst/Ru(bpy)₃²⁺ system. Control experiment results suggested that the α-Fe₂O₃ can’t be used as LED light driven catalysts for water oxidation. In conclusion, the MB@pfa@Fe₂O₃ nanorods were efficient recyclable nanosorbcats for LED-light-driven dioxygen generation using water as oxygen source.

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Introduction

Effluents of colored wastewater from the textile industry contain many toxic and organic dyes which can cause severe environmental damage that affects aquatic ecosystems. Dyes are also known to be carcinogenic and toxic, thus causing serious environmental problems that are mutagenic for aquatic organisms.¹ The removal of dyes from the colored effluent has therefore attracted increasing research attention.² Physical and chemical technologies such as adsorption,^3–6 membrane separation,^7–9 catalytic oxidation,^10,11 etc. have been reported to date. Among these, the adsorption techniques have proved to be an effective process for removal of non-biodegradable dyes from wastewater. High sludge production and post-treatment of toxic sludge are major problems in large-scale applications.^12,13 In addition, catalytic oxidation generates highly reactive radicals that drive degeneration of new discolored species in the wastewater. There are some problems in terms of recovery of catalysts from the wastewater and high cost due to usage of large amounts of oxidants. Moreover, the toxicity of the discolored species to aquatic organisms is another problem. From the view of environmental effects iron oxide nanoparticles are considered to be naturally occurring and their applications for wastewater treatment have focused on using them as adsorbents for removal of contaminants or as catalysts for photocatalytic degradation of contaminants.^14,15 For example, cysteamine modified iron oxide nanoparticles were able to degrade a model organic dye in water.¹⁶ Also, groundwater treatment sludge was transferred to magnetic particles for the adsorption of methylene blue.¹⁷ Fe₂O₃ nanoparticles have recently been reported as nanoadsorbents and catalysts (nanosorbcats) for methylene blue (MB) adsorption and oxidation.¹⁸

Water oxidation catalysts (WOCs) based on iron are highly attractive due to their abundance, greenness, and low-cost. Most of the iron(III) based complexes have been reported as catalysts for water oxidation at low pH using Ce^IV and IO₄⁻ as oxidants,^19,20 and also as light-driven water oxidation catalysts under basic conditions in the presence of Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine).^21–23 The development of dye-loaded nanosorbcats can make the best use of the advantages and disadvantages of the removal of dyes from the wastewater. However, research on the dye-loaded iron oxide nanoparticles (as nanosorbcats) for water oxidation has been rarely reported. We previously found that the cobalt(II) complex of 1H-pyrrol-2-yl-methanone (PPM) could catalyze water oxidation without extra electron accepters.²⁴ We therefore proposed that 1H-pyrrol-2-methanic acid assembling with nano iron(III) oxide may lead to new types of light-driven super-molecular nanoparticles for efficient dye adsorption and formation of nanosorbcats for LED light driven dioxygen generation using water as oxygen source. Herein, we report the synthesis of economic iron oxide nanorods as effective adsorbents of dyes (MB). Regeneration of the adsorbent was easily carried out by an extraction process. In particular, the MB-loaded nanoadsorbents were subsequently upgraded as valuable recyclable catalysts (nanosorbcats) for the LED-light-driven water oxidation reaction.

Materials and methods

Materials

Pyrrol-2-methanoic acid (pfa) and methylene blue (MB) were used as analytical grade reagents (Aladdin Reagent Co. Ltd., Shanghai) without further purification. α-Fe₂O₃ was synthesized as reported.²⁵ All the other chemicals used for synthesis were analytical grade reagents (Sinopharm Chemical Reagent Co. Ltd., China), used without further purification. Water was purified with a Millipore Milli-Q system.

Characterization

The IR spectra were recorded on a Niconet-470 spectrophotometer in the range of 4000–400 cm⁻¹. The electronic absorption spectra were recorded in the 200–800 nm regions using a Varian Cary 50-Bio UV-vis spectrophotometer. SEM was carried out on a JEOL JSM-7001F field emission scanning electron microscope at an acceleration voltage of 15 kV. The content of iron in the nanoparticles was determined using an ICP-AES instrument (HORIBA Jobin Yvon, Longjumeau Cedex, France). X-Band EPR spectra were recorded on a Bruker Biospin GmbH instrument at room temperature.

Synthesis of pfa@Fe₂O₃

Ferric chloride hexahydrate aqueous solution (0.25 M, 20 ml) was added into the pyrrol-2-methanoic acid (pfa) aqueous solution (0.1 M, 50 ml) with Tween-80 (5% v/v). The mixture was stirred for 15 min under room temperature, followed by its transfer into a reaction kettle and further heating for 6 h at 160 °C. After removal of the solvent by centrifugation, the product was washed with ethanol three times and dried in vacuum to give a black solid product. The yield was as follows: 892 mg (83%). IR (KBr, cm⁻¹): 3263, 2956, 2924, 2853, 1697.

Adsorption experiments

Adsorption experiments for MB using α-Fe₂O₃ and pfa@Fe₂O₃ were carried out by mixing 10.0 mg of α-Fe₂O₃ and pfa@Fe₂O₃ in MB solution (10.0 ml). After the mixture was stirred for some time, the mixture was separated and analyzed. The absorption of MB solutions was measured by UV-vis absorption spectra to monitor absorption at λ_max = 664 nm. The concentration of MB solution was determined by linear regression equation obtained by plotting the calibration curve for MB over a range of concentrations. Adsorption efficiency for MB by α-Fe₂O₃ or pfa@Fe₂O₃ was calculated using the following expression:

q_t = C₀ − C_t

where q_t was the MB (mg l⁻¹) adsorption quantity at t min. C₀ and C_t were the initial concentrations of MB (mg l⁻¹) in the solution and t min, respectively.

Water oxidation experiments

Water oxidation experiments, catalyzed by MB loaded pfa@Fe₂O₃ (MB@pfa@Fe₂O₃) or pfa@Fe₂O₃/Ru^II(bpy)₃²⁺, were carried out by mixing the MB@pfa@Fe₂O₃ (or pfa@Fe₂O₃/Ru^II(bpy)₃²⁺) with water at 298 K under red (or blue) LED light (10 W) irradiation. The MB@pfa@Fe₂O₃ nanorods were prepared by adding pfa@Fe₂O₃ into the MB solutions for some time and then the product were separated and dried in an oven. The volume of O₂ was measured by direct methods via connection of the reaction vessel with a U-tube to a calibrated microburet to collect the released gas. Oxygen evolution was monitored by gas chromatography using a thermal conductivity detector (GC-TCD).

Result and discussion

Synthesis and characterization of pfa@Fe₂O₃

The pfa@Fe₂O₃ nanorods were synthesized using surfactant template (tween-80) induced assembly of the pyrrol-2-methanoic acid (pfa) and iron(III) salts by hydrothermal reaction at 433 K. The stretching frequency of –COOH (approximately 1706 cm⁻¹) in the IR spectrum of pfa@Fe₂O₃ (Fig. 1) indicated the existence of a pyrrol-conjugated carboxyl group, which shifted from 1720 cm⁻¹ to 1706 cm⁻¹ (free –COOH) due to coordination of carboxylic acid and iron(III) atoms. The main peaks at 228, 285, 400, 636 and 1336 cm⁻¹ in the resonance Raman spectra of pfa@Fe₂O₃ (Fig. 2) were assigned to α-Fe₂O₃,¹⁶ which was very similar to pure α-Fe₂O₃. In addition, the peak at 1571 cm⁻¹ was assigned to Fe–OOC–, indicating the coordination of pyrrol-2-methanoic acid with iron(III) atom. Moreover, these results were entirely consistent with those from the X-ray diffraction spectrum (XRD) analysis of pfa@Fe₂O₃ (Fig. 3). The diffraction peaks at 2θ = 24°, 33°, 36°, 41°, 43°, 49°, 54°, 58°, 62°, 64°, 72°, and 77° were assigned to α-Fe₂O₃, indicating that the main structure of pfa@Fe₂O₃ was similar to α-Fe₂O₃. The XRD pattern displayed broad peaks at around 2θ = 25° at the same time, which was due to the diffraction of amorphous pyrrol-2-methanoic acid. These results revealed that the pfa@Fe₂O₃ had been successfully synthesized. Furthermore, the EPR spectra for the pfa@Fe₂O₃ nanorods (Fig. 4) showed a peak at g = 4.3, which was attributed to rhombic high-spin Fe(III) species, while the peak at g = 2.0 was more consistent with an organic radical.²⁶ The SEM image (Fig. 5) indicated that the pfa@Fe₂O₃ nanoparticles were rod-like in shape. Based on the above results, it can be concluded that the synthesized pfa@Fe₂O₃ were rhombic rod-like Fe₂O₃ nanoparticles in which the iron(III) was coordinated with pyrrol-2-methanoic acid.

Fig. 1 (a) FT-IR spectrum of MB; (b) FT-IR spectrum of pfa@Fe₂O₃; (c) FT-IR spectrum of MB@ pfa@Fe₂O₃.

Fig. 2 Resonance Raman spectrum of pfa@Fe₂O₃.

Fig. 3 X-Ray diffraction of pfa@Fe₂O₃.

Fig. 4 X-Band EPR spectrum of pfa@Fe₂O₃. Experimental conditions: frequency: 9.851 GHz, power: 1.185 mW, modulation frequency: 100 kHz, modulation amplitude: 1 G, time constant: 20.480 ms.

Fig. 5 SEM image of pfa@Fe₂O₃.

Adsorption capacity of pfa@Fe₂O₃ to MB

The adsorption capacity of pfa@Fe₂O₃ was compared with that of α-Fe₂O₃. It was obviously found that the adsorption capacity (MB/adsorbent) for the pfa@Fe₂O₃ (450 mg g⁻¹) was approximately 12 times higher than that of the α-Fe₂O₃ (39 mg g⁻¹). Moreover, the time required to reach the equilibrium was approximately 0.5 h (Fig. 6). The adsorbed amount of MB showed no obvious increase with increasing time, which was different from α-Fe₂O₃, in which the adsorbed amount of MB gradually increased with increasing time. The pfa@Fe₂O₃ also showed excellent removal capacity for MB in the solution, removing 98.4% of the MB (initial concentration, 100 mg l⁻¹) from aqueous solution at the adsorbent dosage of 1000 mg l⁻¹ (10 mg adsorbent in 10 ml MB solution), which was much higher than that of the α-Fe₂O₃ (only 19.5% for MB). As shown in Fig. 7, the adsorption capacity of pfa@Fe₂O₃ increased and was then constant with increasing concentration of the MB solution. The achieved adsorption capacity was approximately 450 mg g⁻¹, thus indicating that the increase of the initial dye concentration contributed to enhancement of the driving force at the solid–liquid interface (ion exchange, electrostatic attraction and chemical association) until the adsorption sites were saturated. In addition, the adsorption capacity of pfa@Fe₂O₃ decreased with increasing amount of the pfa@Fe₂O₃, thus suggesting that the increase of the adsorbent may not have been conducive for the driving force at the solid–liquid interface until the adsorption sites were saturated. Moreover, compared to other reported adsorbents such as Fe₂O₃/CNT, Fe₃O₄/CNT and Fe/ordered mesoporous carbon,²⁷ the pfa@Fe₂O₃ nanorods showed a relatively high adsorption capacity for adsorption of the MB. This indicated that the pfa@Fe₂O₃ nanorods were greatly superior to the α-Fe₂O_3, and could be used as an efficient and promising adsorbent for the decontamination of wastewater containing a cationic dye.

Fig. 6 Adsorption capacity of pfa@Fe₂O₃ (10 mg) to MB (200 mg l⁻¹, 10 ml) during 3 h, T = 298 K.

Fig. 7 Adsorption capacity of pfa@Fe₂O₃ (10 mg) at different concentrations of MB (10 ml), T = 298 K.

In order to further investigate the characteristics of the adsorption process, the pseudo second-order kinetic model was used to fit the experimental data obtained from the batch experiments as follows:

where, q_t and q_e were the concentrations of MB at time t (min) and equilibrium, k₂ was the pseudo second-order reaction rate constant. Fig. 8 and Table 1 display kinetic curves for the adsorption of MB 200 mg l⁻¹ by the pfa@Fe₂O₃ (10 mg) at 299 K using the pseudo second-order kinetic model. An equation was used as follows:
R² = 0.99777. The data in Fig. 8 showed that the MB degradation curve was well fitted by the pseudo second-order kinetics with high values of the regression coefficients. As seen in the above equation, q_e = 199 mg g⁻¹, k₂ = 8.69 × 10⁻⁶ mg g⁻¹ h⁻¹. These results indicated that the pfa@Fe₂O₃ showed excellent removal capacity for the MB in the solution.

Fig. 8 The pseudo second order kinetics of pfa@Fe₂O₃ adsorbing MB.

Table 1 The adsorption kinetics data at different times

Time/h	qt/mg g^−1	t/qt/h g mg^−1
0	0	0
0.5	170.4	0.00294
1.5	182.2	0.00823
2	191.2	0.01046
2.5	196.2	0.01274
3	196.8	0.01524

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MB@pfa@Fe₂O₃ as nanosorbcats for water oxidation

Water oxidation catalyzed by nanoparticles was carried out at room temperature under irradiation with red LED light (10 W) and blue LED light (10 W), respectively. For comparison, the photocatalytic activity of MB@pfa@Fe₂O₃, pfa@Fe₂O₃ and MB@recycled pfa@Fe₂O₃ were investigated under identical conditions. During the experimental process, we found that there was almost no evolution of O₂ in pfa@Fe₂O₃, indicating that MB as photosensensitizer was an essential condition for photocatalytic water oxidation. It was interesting that both MB@pfa@Fe₂O₃ and MB@recycled pfa@Fe₂O₃ show a similar activity of water oxidation, which prove the better adsorption reusability of pfa@Fe₂O₃ from the others under identical conditions (Fig. S3†). As shown in Table 2 and Fig. S1,† the MB@pfa@Fe₂O₃/H₂O system showed much higher activity with TON of 2.17 and TOF of 2.79 h⁻¹, while the TON and TOF values for the pfa@Fe₂O₃/Ru(bpy)₃Cl₂/Na₂S₂O₈ system were 0.87 and 1.30 h⁻¹, respectively. These results clearly demonstrated that the catalyst created by the MB@pfa@Fe₂O₃ was more active than the traditional oxidation system (catalyst/Ru^II(bpy)₃Cl₂/Na₂S₂O₈). There are two main reasons for this: firstly, the light absorption ability for methylene blue is better than Ru(bpy)₃Cl₂, thus the MB@pfa@Fe₂O₃/H₂O system can use light energy more efficiently than the catalyst/Ru(bpy)₃Cl₂ system. Secondly, catalysts and photosensitizers were combined in the MB@pfa@Fe₂O₃/H₂O system, so the light induced intramolecular charge transfer between the catalysts and photosensitizers in the MB@pfa@Fe₂O₃/H₂O system more efficiently than in the catalyst/Ru(bpy)₃Cl₂ system. In comparison, the TON and TOF from the MB@pfa@Fe₂O₃ irradiated by the blue LED light were only 0.99 and 0.99 h⁻¹, respectively (Fig. S2,† Table 2). It can therefore be concluded that the MB@pfa@Fe₂O₃/H₂O system exerted higher activity when irradiated by the red LED light due to the maximum absorption capacity for the MB, which was at 664 nm. Furthermore, the MB@pfa@Fe₂O₃ also exerted high activity for water oxidation in aqueous solution conditions (Fig. 9), which indicated that the MB@pfa@Fe₂O₃/H₂O could be used as a promising dioxygen generator or oxidant in hypoxic conditions using water as oxygen source. Moreover, the control experiments suggested that α-Fe₂O₃ couldn’t be LED-light driven catalysts for water oxidation. Hence, the pfa@Fe₂O₃ had higher activity for water oxidation than α-Fe₂O₃.

Table 2 Water oxidation activity of MB@pfa@Fe₂O₃ and pfa@Fe₂O₃/Ru^II(bpy)₃²⁺

Catalysts	TON	TOF/h^−1
a Irradiated with red LED light.b Irradiated with blue LED light. TON = nO2 (μmol)/nFe (μmol), TOF = TON/time (h).
MB@pfa@Fe2O3^a	2.17	2.79
pfa@Fe2O3/Ru^II(bpy)3^2+^b	0.87	1.30
MB@pfa@Fe2O3^b	0.99	0.99

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Fig. 9 Water oxidation activity of MB@pfa@Fe₂O₃ (10 mg) in aqueous solution at 298 K under irradiation of red LED light (10 W).

Adsorption reusability and possible mechanism

The recyclability and reusability of an adsorbent are very important for its practical applications. In this study, it was easy to recycle and reuse the pfa@Fe₂O₃. The regeneration of pfa@Fe₂O₃ after MB adsorption was conducted by flushing 3 times with ethanol and then drying in an oven. Fig. 10 shows the performance of the recycled pfa@Fe₂O₃. After three times of MB adsorption, the adsorption capacity was approximately 92% of the initial adsorption capacity. FT-IR spectra from the pfa@Fe₂O₃, MB@pfa@Fe₂O₃ (the first cycle) were also performed and obvious shifts were observed at 1596 and 1385 cm⁻¹ for C=N (and C=C) after adsorption by the pfa@Fe₂O₃, and also for C–N bonds in the heterocycle of MB and to 1436 cm⁻¹ for CH₂ deformation vibration in the benzene ring (Fig. 1c) which corresponded to the attachment of the MB on the surface of the pfa@Fe₂O₃ by π–π stacking interaction between the aromatic backbone of the MB and rhombic skeleton of pfa@Fe₂O₃, since the MB is an ideally planar molecule. Besides, the Ar–N deformation vibration and stretching vibrations for C=S and C–S were broadened with significant decreases in intensity, and they shifted to 1120 cm⁻¹ and 1103 cm⁻¹, respectively, suggesting that the sulfur atom from the C–S group could be used as hydrogen-bonding acceptor to form intramolecular hydrogen bonds with the H–N_pyrrole of pfa@Fe₂O₃, since the MB is a cationic dye which can be adsorbed easily by electrostatic forces on the negatively charged surfaces.

Fig. 10 Adsorption reusability of pfa@Fe₂O₃ (10 mg) in MB solution (200 mg l⁻¹, 10 ml) at 298 K.

Photocatalytic reusability

It is important for a photocatalyst to be stable and reusable for water oxidation. To investigate the stability of the MB@pfa@Fe₂O₃, recycling of the MB@pfa@Fe₂O₃ was conducted by flushing 3 times with water then drying in an oven. The recycled MB@pfa@Fe₂O₃ nanoparticles were then reused as photocatalyst. The TON and TOF were maintained as approximately 2 (Fig. 11). Furthermore, EPR data demonstrated that there was no significant change between the initial MB@pfa@Fe₂O₃ and recycled MB@pfa@Fe₂O₃, proving the stability of the MB@pfa@Fe₂O₃.

Fig. 11 Photocatalytic reusability of MB@pfa@Fe₂O₃ (10 mg) in water at 298 K and irradiation with red LED light (10 W).

Conclusions

The pyrro-2-methanic acid modified Fe₂O₃ nano-rods (pfa@Fe₂O₃) were successfully synthesized and characterized. It was found that the MB could be absorbed onto the pfa@Fe₂O₃ through hydrogen bonding and π–π stacking interaction between the aromatic backbone of the MB and rhombic skeleton of the pfa@Fe₂O₃. Both the absorption capacity and absorption rate were larger than those of the α-Fe₂O₃. The pfa@Fe₂O₃ could also be used as efficient reusable adsorbents for methylene blue (MB). In particular, the pfa@Fe₂O₃ and MB were combined into MB@pfa@Fe₂O₃ and this was an efficient recyclable nanosorbcat for LED-light driven dioxygen generation using water as oxygen source. Our results demonstrated that the conversion of efficient nanoadsorbents and valuable recyclable nanosorbcats can be an economic path for the treatment of colored waste water.

Acknowledgements

Financial support of the National Science Foundation of China (21271090, 21571085), the Youth Science Foundation of Jiangsu Province (BK20130485) and the Highly Qualified Professional Initial Funding of Jiangsu University (12JDG052) is acknowledged.

Notes and references

1. H. S. Rai, M. S. Bhattacharyya, J. Singh, T. K. Bansal, P. Vats and U. C. Banerjee, Removal of dyes from the effluent of textile and dyestuff manufacturing industry: A review of emerging techniques with reference to biological treatment, Crit. Rev. Environ. Sci. Technol., 2005, 35, 219.
2. V. K. Gupta and Suhas, Application of low-cost adsorbents for dye removal, J. Environ. Manage., 2009, 90, 2313.
3. T. Yao, S. Guo, C. F. Zeng, C. Q. Wang and L. X. Zhang, Investigation on efficient adsorption of cationic dyes on porous magnetic polyacrylamide microspheres, J. Hazard. Mater., 2015, 292, 90.
4. S. Eftekhari, A. Habibi-Yangjeh and S. Sohrabnezhad, Application of AlMCM-41 for competitive adsorption of methylene blue and rhodamine B: Thermodynamic and kinetic studies, J. Hazard. Mater., 2010, 178, 349.
5. C. H. Huang, K. P. Chang, H. D. Ou, Y. C. Chiang, E.-E. Chang and C. F. Wang, Characterization and application of Ti-containing mesoporous silica for dye removal with synergistic effect of coupled adsorption and photocatalytic oxidation, J. Hazard. Mater., 2011, 186, 1174.
6. C. T. Frijters, R. H. Vos, G. Scheffer and R. Mulder, Decolorizing and detoxifying textile wastewater, containing both soluble and insoluble dyes, in a full scale combined anaerobic/aerobic system, Water Res., 2006, 40, 1249.
7. L. Wang, J. P. Zhang and A. Q. Wang, Removal of methylene blue from aqueous solution using chitosan-g-poly(acrylicacid)/montmorillonite superadsorbent nanocomposite, Colloids Surf., A, 2008, 322, 47.
8. Y. M. Zhou, S. Y. Fu, H. Liu, S. P. Yang and H. Y. Zhan, Removal of methylene blue dyes from wastewater using cellulose-based superadsorbent hydrogels, Polym. Eng. Sci., 2011, 51, 2417.
9. L. Yao, L. Z. Zhang, R. Wang, S. R. Chou and Z. L. Dong, A new integrated approach for dye removal from wastewater by polyoxometalates functionalized membranes, J. Hazard. Mater., 2016, 301, 462.
10. Y. Liu, L. Yu, Y. Hu, C. Guo, F. Zhang and X. W. Lou, A magnetically separable photocatalyst based on nest-like γ-Fe₂O₃/ZnO double-shelled hollow structures with enhanced photocatalytic activity, Nanoscale, 2012, 4, 183.
11. N. Anand, K. H. P. Reddy, T. Satyanarayana, K. S. R. Rao and D. R. Burri, A magnetically recoverable γ-Fe₂O₃ nanocatalyst for the synthesis of 2-phenylquinazolines under solvent-free conditions, Catal. Sci. Technol., 2012, 2, 570.
12. Y. Fan, H. J. Liu, Y. Zhang and Y. Chen, Adsorption of anionic MO or cationic MB from MO/MB mixture using polyacrylonitrile fiber hydrothermally treated with hyperbranched polyethylenimine, J. Hazard. Mater., 2015, 283, 321.
13. A. X. Yan, S. Yao, Y. G. Li, Z. M. Zhang, Y. Lu, W. L. Chen and E. B. Wang, Incorporating polyoxometalates into a porous MOF greatly improves its selective adsorption of cationic dyes, Chem.–Eur. J., 2014, 20, 6927.
14. N. N. Nassar and A. Ringsred, Rapid adsorption of methylene blue from aqueous solutions by goethite nanoadsorbents, Environ. Eng. Sci., 2012, 29, 790.
15. S. Singh, K. C. Barick and D. Bahadur, Functional oxide nanomaterials and nanocomposites for the removal of heavy metals and dyes, Nanomater. Nanotechnol., 2013, 3, 1.
16. C. J. Jones, S. Chattopadhyay, N. I. Gonzalez-Pech, C. Avendano, N. Hwang, S. Soo Lee, M. J. Cho, A. Ozarowski, A. Prakash, J. T. Mayo, C. Yavuz and V. L. Colvin, A novel, reactive green iron sulfide (sulfide green rust) formed on iron oxide nanocrystals, Chem. Mater., 2015, 27, 700.
17. L. W. Hou, L. G. Wang, S. Royer and H. Zhang, Ultrasound-assisted heterogeneous Fenton-likedegradation of tetracycline over a magnetite catalyst, J. Hazard. Mater., 2016, 302, 458.
18. A. El-Qanni, N. N. Nassar, G. Vitale and A. Hassan, Maghemite nanosorbcats for methylene blue adsorption and subsequent catalytic thermo-oxidative decomposition: Computational modeling and thermodynamics studies, J. Colloid Interface Sci., 2016, 461, 396.
19. W. C. Ellis, N. D. McDaniel, S. Bernhard and T. J. Collins, Fast water oxidation using iron, J. Am. Chem. Soc., 2010, 132, 10990.
20. J. L. Fillol, Z. Codola, I. Garcia-Bosch, L. Gomez, J. J. Pla and M. Costas, Efficient water oxidation catalysts based on readily available iron coordination complexes, Nat. Chem., 2011, 3, 807.
21. C. Panda, J. Debgupta, D. D. Díaz, K. K. Singh, S. S. Gupta and B. B. Dhar, Homogeneous photochemical water oxidation by Biuret-Modified Fe-TAML: Evidence of FeV(O) intermediate, J. Am. Chem. Soc., 2014, 136, 12273.
22. J. A. Bau, E. J. Luber and J. M. Buriak, Oxygen evolution catalyzed by nickel–iron oxide nanocrystals with a nonequilibrium phase, ACS Appl. Mater. Interfaces, 2015, 7, 19755.
23. R. D. L. Smith, M. S. Prévot, R. D. Fagan, S. Trudel and C. P. Berlinguette, Water oxidation catalysis: Electrocatalytic response to metal stoichiometry in amorphous metal oxide films containing iron, cobalt, and nickel, J. Am. Chem. Soc., 2013, 135, 11580.
24. J. J. Xue, Q. Y. Chen, X. L. Xu, S. J. He, P. X. Wu, L. L. Qu and C. Y. Zhu, Inorg. Chem. Commun., 2014, 47, 168.
25. L. Xu, J. X. Xia, L. G. Wang, J. Qian, H. M. Li, K. Wang, K. Y. Sun and M. Q. He, α-Fe₂O₃ cubes with high visible-light-activated photoelectrochemical activity towards glucose: Hydrothermal synthesis assisted by a hydrophobic ionic liquid, Chem.–Eur. J., 2014, 20, 2244.
26. Q. Zhang, D. G. John and R. G. Christian, C–H oxidation by H₂O₂ and O₂ catalyzed by a non-heme iron complex with a sterically encumbered tetradentate N-donor ligand, Inorg. Chem., 2013, 52, 13546.
27. S. Y. Zhu, S. Fang, M. X. Huo, Y. Yu, Y. Chen, X. Yang, Z. Geng, Y. Wang, D. J. Bian and H. L. Huo, A novel conversion of the groundwater treatment sludge to magnetic particles for the adsorption of methylene blue, J. Hazard. Mater., 2015, 292, 173.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16049b

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