Magnetite–reduced graphene oxide nanocomposite as an efficient heterogeneous Fenton catalyst for the degradation of tetracycline antibiotics†

Jaimy Scaria ^ab and Puthiya Veetil Nidheesh *^ab
aEnvironmental Impact and Sustainability Division, CSIR-National Environmental Engineering Research Institute, Nagpur, Maharashtra, India. E-mail: pv_nidheesh@neeri.res.in; nidheeshpv129@gmail.com
^bAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad-201002, India

First published on 29th April 2022

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Water impact Magnetite nanoclusters supported on reduced graphene oxide are able to degrade tetracyclines, an emerging contaminant, which are among the most frequently consumed broad-spectrum antibiotics for bacterial infections in humans and livestock, by a heterogeneous Fenton process, from water media. Reusability and reduced iron leaching properties make the material a promising Fenton catalyst. Significant degradation of the pollutant from real sewage was observed.

Introduction

Tetracyclines (TCs) are among the most frequently consumed broad-spectrum antibiotics for bacterial infections in humans and livestock.^1,2 The poor metabolization in organisms results in environmental contamination and a wide transport range across multiple environmental compartments. Excessive consumption of TCs, a class of emerging contaminants, leads to antibiotic resistance, endocrine disruption, and carcinogenicity.^3–5 This urges the up-gradation of conventional water treatment systems via adopting advanced treatments like advanced oxidation processes (AOPs).^6–8 One of the promising AOPs for the degradation of emerging contaminants is heterogeneous Fenton oxidation. The heterogeneous Fenton process using an insoluble solid iron source^9–12 can overcome the bottlenecks associated with the homogeneous Fenton process, such as a narrow working pH range, sludge generation, and lack of recovery of reactive agents.^13–15

Magnetite (Fe₃O₄) is a form of iron oxide with the inverse spinel structure (Fe(III))_tet[Fe(II)Fe(III)]_octO₄. The Fe(III) cations and Fe(II) cations share octahedral voids. Magnetite is the most suitable iron oxide for the heterogeneous Fenton process due to benefits such as direct involvement of Fe(II) in the Fenton reaction,^16,17 the possibility of Fe(III)/Fe(II) conversion in the same structure,¹⁸ magnetic properties,^19,20 possibility to easily modify different ferrites by replacing cations with other transition metals,¹⁶ and higher electron mobility.²¹

Magnetite composites with clay, zeolite, carbon materials, polymers and other supporting materials exhibited higher Fenton-like activity than bare magnetite.¹⁵ Graphene is an exceptional carbon-based supporting material with sp² carbon atoms arranged in a two-dimensional honeycomb structure.^22,23 A long-range π electron cloud provides graphene with excellent properties such as high specific surface area,²⁴ and mechanical, thermal, and electrical properties.²⁵ Despite this, pristine graphene is a hydrophobic, inert material, which limits its application in water treatment while graphene oxide (GO), the highly oxidized form of graphene, possesses improved hydrophilicity and excellent reactant interaction.^26,27 In spite of these benefits, GO has lower carrier mobility and conductivity.²⁸ Thus, to retain the graphene-like properties in GO, the polar oxygen functional groups were eliminated to an extent, yielding reduced graphene oxide (rGO).²⁹ Thus, by providing active sites and facilitating electron transfer, rGO can serve as an excellent supporting material.^30,31

In this study, a magnetite–rGO nanocomposite was prepared using a facile solvothermal method with ethylene glycol as the sole reductant. This investigation confirms the reductant ability of ethylene glycol to simultaneously reduce GO to rGO, and FeCl₃ to Fe₃O₄. The heterogeneous Fenton activity of graphene supported magnetite has only been reported in a few papers.^32–35 To the best of our knowledge, no studies on the heterogeneous Fenton activity of magnetite–rGO on tetracycline (TC) removal have been published. This work would contribute to the development of heterogeneous Fenton catalysts with a predominant surface based Fenton mechanism over the often reported homogeneous Fenton process by metal oxide leaching. This work also reveals the possibility of surface based pollutant degradation, even when pollutant sorption onto the catalyst surface is absent. The benefits of the magnetite–rGO composite were evaluated by studying the degradation of TC at a wide pH range. Furthermore, the catalyst reusability, solar photo-Fenton activity, and TC degradation in domestic wastewater were also scrutinized to broaden the range of applications for the prepared catalyst.

Experimental section

Materials

Graphite powder pract. (100 microns, 99.5%) was received from S D Fine Chemicals, India. Tetracycline hydrochloride (≥95%, European Pharmacopoeia HPLC assay) was purchased from Sigma-Aldrich. Potassium permanganate (KMnO₄, ≥99.0%), silver nitrate (AgNO₃, EMPARTA, ≥99.5%), hydrogen peroxide (H₂O₂, 30% w/w), anhydrous iron(III) chloride (FeCl₃ anhydrous, ≥98%), ethylene glycol (HOCH₂CH₂OH, ≥99.0%), sodium azide (NaN₃, ≥99%), 1,10-phenanthroline monohydrate (C₁₂H₈N₂·H₂O, ≥99.5%), sodium acetate trihydrate (CH₃COONa·3H₂O 99.0–101.0%), t-butanol ((CH₃)₃OH, ≥99%, Emplura), sodium hydroxide pellets (NaOH ≥97%), hydrochloric acid (H₂SO₄, 36.5–38%), sulfuric acid (H₂SO₄, EMPARTA, 95.0–98.0%), and hydroquinone (C₆H₄(OH)₂, 99%) were purchased from Merck. Polyethylene glycol-400 (99.9%) and potassium iodide (KI, 99%) were purchased from Qualigens, India and ethanol (C₂H₅OH, 99.5%) was purchased from Ancott, India. All the chemicals or solvents were of analytical or HPLC quality and were used directly without any purification.

Synthesis of the magnetite–rGO composite

Catalyst characterization

The crystal structure of the as-synthesized materials was characterized by X-ray powder diffraction (XRD, Rigaku Japan,) using a 3 kW advanced X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). The functional groups present in the sample were confirmed by Fourier transform infrared spectroscopy (FTIR, Bruker, Germany). FTIR spectra were recorded in the region between 400 and 4000 cm⁻¹. Raman spectroscopy was performed using a laser Raman microscopy (Horiba Jobin Yvon). The morphology observations were done on a scanning electron microscope (SEM, FEI Quanta 200) and a field-emission scanning electron microscope (FE-SEM, JSM-7600F Schottky) at a working voltage of 15 kV. The elemental composition was characterized using an energy-dispersive spectrometer (SEM-EDS). A Shimadzu UV-vis spectrometer was used to obtain the UV-vis absorption spectra of TC samples. The magnetic properties of the catalysts were analysed at room temperature using a vibrating sample magnetometer (Microsense, EV7 VSM, USA). Thermogravimetric analysis (TGA) was conducted on a Perkin-Elmer Pyris TGA instrument with heating at 10 °C min⁻¹ under a nitrogen flow (20 mL min⁻¹). X-ray photoelectron spectroscopy (XPS) was conducted to measure the surface electronic states using an AXIS Supra Kratos Analytical XPS system with an Al K source (1486.6 eV) operated with a power of 600 W.

Heterogeneous Fenton experiments

TC solutions were prepared in DDW using 0.1% ethanol. These solutions were stored in a refrigerator at 4 °C, and used within two days. The heterogeneous Fenton reactions were carried out in a 500 mL beaker with mechanical stirring. In a typical experiment, the required amount of catalyst was added to a 300 mL solution of a known concentration of TC. The mixture was then stirred for 30 min to achieve adsorption–desorption equilibrium. Adding a known amount of hydrogen peroxide (30% w/w) triggered a Fenton-like reaction. The reacted solution was collected at regular intervals of 30 min to determine the residual TC concentration. In pH-specific experiments, 0.1 M HCl and 0.1 M NaOH were used to adjust the solution pH. Experiments were carried out in duplicate to ensure accuracy.

Reaction analysis

After magnetic separation and filtration, the TC concentration was determined using a Shimadzu UV-vis spectrophotometer at the TC maximum absorption wavelength of 357 nm. Moreover, the instrument calibration was performed at different TC concentrations to achieve acceptable reproducibility. The TOC values of TC samples were determined using a TOC V_CPH, Shimadzu.

The degradation efficiency was calculated using eqn (1)

Degradation = C/C0	(1)

where C₀ is the initial TC concentration and C is the TC concentration in the selected time interval.

Point of zero charge (pH_PZC) determination

The pH drift method was used to determine the surface charge of MG10 in aqueous media.³⁸ In five conical flasks, 50 mL of 0.01 M NaCl solution was taken, and the initial pH was adjusted to 2, 4, 6, 8, and 10, respectively, using 0.1 M HCl/0.1 M NaOH. Then, 150 mg of MG10 was added to each conical flask, and shaken at 200 rpm for 18 hours. The catalyst was then removed, and the final pH of each conical flask was determined. The final pH (y-axis) vs. initial pH (x-axis) is depicted graphically, as well as a linear plot of initial pH = final pH. The pH_PZC is determined as the point where both curves intersect.

Intermediate and by-product identification using HR-LC-MS analysis

A quadrupole time-of-flight (Q-TOF) mass spectrometer (6550 iFunnel Q-TOF, Agilent Technologies, USA) connected to an Agilent UPLC system (1290 Infinity UHPLC system) was used to identify the intermediate degradation products. The samples were run through an ultrahigh performance liquid chromatography (UHPLC) system equipped with a photodiode array (PDA) detector linked to a mass spectrometer. A C-18 column (3 μm particle size, 100 mm length, and 2.1 mm diameter: Hypersil GOLD) was used for sample separation. The mobile phases were 0.1% formic acid (A) and acetonitrile (B), and the column temperature was set to 40 °C. Gradient elution was programmed as shown in Table S1.† The analyses were carried out for 30 min at a flow rate of 0.3 mL min⁻¹. The HR-LC-MS system was used with the positive electrospray ionization (+ESI) mode of the quadrupole spectrometer for ionization in the m/z range of 150–600 u. The mass spectrometer was operated using the following settings: nebulizer temperature: 250 °C; capillary voltage: 3.5 kV, gas flow rate:13 L min⁻¹, and pressure: 35 psi.

Total iron and Fe²⁺ quantification

The total iron released under the optimum reaction conditions was analyzed using ICP-OES (iCAP 6300 DUO, Thermo Scientific, USA). The Fe²⁺ quantification was done by spectrophotometric quantification with the 1,10-phenanthroline method.³⁹

Results and discussion

Preliminary studies were conducted to determine the best catalyst among the synthesized magnetite, MG2, MG5, MG10, and MG20. The experimental conditions and the results are given in Fig. 8(a). Based on the observation, MG10 was the best heterogeneous Fenton catalyst for TC degradation and was chosen for further characterization studies.

Characterization of the magnetite–rGO composite

Fig. 2 shows the XRD patterns of graphite, GO, rGO, magnetite, and MG10. From Fig. 2a, a sharp diffraction peak at 26.5° indicates the peculiar (002) plane of graphite, with an interlayer spacing of 0.34 nm. When oxidized to GO using the modified Hummers' method, this peak diminishes, and the well-defined (001) crystal plane of GO appears at 9.8°. Along with this, there is a weak diffraction of (100) at 42.5°. When graphite is converted to GO, the interlayer spacing increases to 0.89 nm, implying the incorporation of numerous oxygen functionalities into the graphite layers. Using Scherrer's crystallite size determination, the average thickness of stacked graphite sheets was calculated from the full width at half maximum (FWHM) of the (001) diffraction peak (constant as 0.9). This value is determined as 9 nm. Moreover, the interlayer spacing (d(001)) was calculated using Bragg's equation and was found to be 0.89 nm, which can be taken as the layer-to-layer distance in the GO structure. Based on its stacking structure, GO can have a few layers (≈10 layers). When GO is solvothermally reduced and rGO is formed, a broad (002) peak reappears and the (001) peak of GO is removed. The (002) peak at 24.1°, with an interlayer spacing of 0.37 nm, indicates the graphitic structure re-ordering through solvothermal reduction.

Fig. 2b shows the XRD data for magnetite and MG10. The XRD diffraction peaks of magnetite are well synchronized with the standard XRD data for JCPDS no. 19-0629. Diffraction peaks were found at 18.42°, 30.26°, 35.65°, 37.25°, 43.32°, 53.69°, 57.24°, 62.84°, 71.25°, 74.35°, 75.49°, and 79.36° which correspond to the (111), (220), (311), (222), (400), (422), (511), (440), (620), (533), (622), and (444) diffraction planes respectively. As per the crystal size determination, the magnetite spheres are 33.1 nm in diameter. The catalyst composite MG10, on the other hand, has all of the magnetite peaks at a slightly lower diffraction angle as compared to magnetite (18.36°, 30.21°, 35.58°, 37.20°, 43.27°, 53.58°, 57.21°, 62.83°, 71.21°, 74.36°, 75.41°, 79.40°). This indicates the electronic transfer between magnetite and rGO sheets. The determined crystal size (26 nm) of MG10 is smaller than that of magnetite. Moreover, the rGO peaks are diminished in MG10, and a weak peak broadening at around 25° (inset of Fig. 2b) corresponding to the (002) plane of rGO (ref. 32) is found, indicating that the rGO layers are well covered with magnetite spheres.

It is difficult to distinguish between maghemite and magnetite using XRD results alone as both possess similar lattice parameters. FTIR analysis can provide solid evidence. In the FTIR data (Fig. 3a), the peak found at 580 cm⁻¹ corresponds to the Fe–O stretching mode of magnetite, and is unlike the maghemite peak (630 cm⁻¹).^40,41 The functional groups present in the samples are also indicated by FTIR analysis. As shown in Fig. 3a, FTIR analysis of GO shows the presence of many oxygen functional groups such as –OH, C–H, –CO, –CC, C–O–C/C–O–H, and –C–O at around 3419.6 cm⁻¹, 2923.8 cm⁻¹, 1689.6 cm⁻¹, 1622 cm⁻¹, 1230/1365.5 cm⁻¹, and 1053 cm⁻¹ respectively. Magnetite shows mainly –OH peak at 3431.2 cm⁻¹, and two Fe–O peaks at 1644.3 cm⁻¹ and 584.4 cm⁻¹ respectively. During composite formation (MG10), the Fe–O peaks have a lower intensity, with a red shift from 584.4 cm⁻¹ to 588.3 cm⁻¹. On investigating the fingerprint region of the FTIR spectra (1800–800 cm⁻¹) (Fig. 3b), it is clear that peaks corresponding to oxygen functionalities were absent in MG10. This indicates that GO has been reduced to rGO and that magnetite has been adequately bound to rGO.⁴²

The Raman spectra of GO and rGO further affirm the reduction of GO to rGO. As shown in Fig. 3c, two distinct D and G bands of carbonaceous materials occur at 1343 cm⁻¹ and 1584 cm⁻¹, respectively. Along with these, a broad peak at around 3000 cm⁻¹, shows the 2D peak, which is present in both GO and rGO. The I_d/I_g values of GO and rGO were 1.82 and 1.78 respectively. The lowering of I_d/I_g after chemical reduction indicates rGO attained fewer defects and more sp² structures through chemical reduction.⁴³ In the case of the catalyst composite, MG10, the characteristic peak at 650 cm⁻¹ (ref. 44 and 45) and the absence of other iron oxide peaks indicate the possibility of pure magnetite formation. The prominent peak at 1418 cm⁻¹ indicates the highly defective rGO sheets, where the D and G bands may converge.

The SEM images of GO and magnetite are given in Fig. S1.† Magnetite possesses spherical structures (Fig. S1a†). The wrinkles on GO sheets are evident from the SEM image (S1b†). The FE-SEM images of the MG10 composite are shown in Fig. 4. As shown in Fig. 4a and b, the magnetite particles are rough, spherical structures enveloping the rGO sheets. From Fig. 4b and c, it is clear that the magnetite particles are nanosized spheres, which aggregate to form spherical clusters of around 342.7 nm. A similar result is reported by Shen et al.³⁵Fig. 4d shows the size distribution diagram using imageJ software, and non-linear curve fitting using the Gaussian function. It shows that the size of the magnetite particles can vary from a few nm to 450 nm, with an average of 342.7 nm. Moreover, the SEM-EDS analysis (Fig. 4e) shows the presence of carbon, oxygen, and iron, without any other impurity elements, which shows the purity of the as-prepared magnetite–rGO composite.

The mean crystallite size of magnetite in MG10 determined from XRD (26 nm) is smaller than the average size (342.7 nm) obtained from FE-SEM images. Since the size revealed by the XRD data is similar to that of the smallest crystallites, the size discrepancy between XRD and FE-SEM calculations indicates that MG10 has a polycrystalline structure.⁴⁶

The magnetic hysteresis characteristics of magnetite nanoclusters and the MG10 composite are depicted in Fig. 5. The hysteresis loops of both samples were nearly saturated at higher fields, implying that the magnetite nanoparticles are well above the critical superparamagnetic size of ∼20 nm.⁴⁷ This correlates with the average particle size determined by FE-SEM analysis of 342.7 nm. It is also possible to conclude that the magnetite nanoclusters are in a multi-domain regime because the majority of the particles are larger than the critical single domain size of 128 nm (ref. 47) for Fe₃O₄. The magnetite and MG10 saturation magnetization (Ms) values were found to be 75.3 emu g⁻¹ and 56.4 emu g⁻¹, respectively. The significantly high saturation magnetisation correlates with the pure crystalline phase of magnetite, as confirmed by XRD analysis. The presence of non-ferrimagnetic rGO sheets in MG10 may account for the approximately 25% reduction in Ms compared to bare magnetite. Both samples have very low coercivity (Hc = 87.89 Oe for magnetite and 44 Oe for MG10) and remanence (Mr = 6.3 emu g⁻¹ for magnetite and 3.11 emu g⁻¹ for MG10), indicating that the synthesized material has excellent soft magnetic characteristics. Because of their high saturation magnetization and soft magnetic properties, magnetite and the MG10 composite are ideal for separation from reaction solutions using an external magnet (inset of Fig. 5).

The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of magnetite and MG10 are shown in Fig. 6. Both magnetite (Fig. 6a) and MG10 (Fig. 6b) exhibit no weight loss below 120 °C, implying the absence of absorbed water molecules.⁴⁸ Magnetite nanoclusters demonstrated excellent thermal stability. Only a 3.46% weight loss was observed in the temperature range of 550–650 °C (decomposition temperature 600 °C from the DTA curve). In contrast to magnetite, a slight weight loss of 1.56% is observed in the MG10 composite at temperatures ranging from 201–349 °C, which can be attributed to the removal of oxygen functional groups in rGO as CO, CO₂, and H₂O vapor.^49,50 The decomposition of the carbon skeleton can be confirmed from the gradual weight loss found in the temperature range 465–681 °C (decomposition temperature 550 °C from the DTA curve) corresponding to a weight loss of 5.86%.^51,52 Furthermore, at higher temperatures, no weight loss was observed. This demonstrates that magnetite and the magnetite–rGO composite are both highly thermally stable catalysts.

The chemical composition of MG10 was revealed in detail by XPS studies.⁴⁹ Strong peaks in the full scan spectrum indicated the presence of carbon, oxygen, and iron components in MG10, with binding energies of 283 eV (C 1s), 530 eV (O 1s), and 709 eV (Fe 2p), respectively (Fig. 7a). In the Fe 2p spectrum (Fig. 7b), the peaks for Fe 2p_3/2 and Fe 2p_1/2 are seen at 709.6 eV and 723.2 eV, respectively, as well as the absence of a satellite peak at 719 eV, which is also suggestive of the development of a pure Fe₃O₄ phase in the MG10 matrix.^46,53,54 When the binding energy peaks of O 1s are deconvoluted, the peak at 528.1 eV is attributed to magnetite lattice oxygen,⁵⁵ whereas the peak at 530.2 eV is attributed to residual oxygen in rGO sheets (Fig. 7c).⁵⁵ Different peaks were found in rGO sheets at 283.2 eV and 285.7 eV, corresponding to non-oxygenated CC/C–C in aromatic rings and residual oxygen (C–O of epoxy and alkoxy groups, respectively) (Fig. 7d).⁵⁶ As illustrated in Fig. 7e, according to the curve fit, the Fe 3p peak for Fe²⁺ in MG10 is located at 54 eV and has an FWHM of 3.6 eV. Meanwhile, the Fe 3p peak for Fe³⁺ is located at 58.98 eV and has an FWHM of 7.3 eV. When the deconvoluted Fe 3p spectrum is analysed using the Fe³⁺ and Fe²⁺ peak parameters, obtaining Fe²⁺:Fe³⁺ = 3.6:7.3, MG10 is found to clearly follow the stoichiometric expression of magnetite (Fe²⁺:Fe³⁺ = 1:2).⁵³

The magnetite–rGO composite synthesis through the solvothermal method is a single step, easy to scale up synthesis strategy to yield a magnetically separable Fenton catalyst. In this procedure, GO was easily dispersed in ethylene glycol through ultrasonication. Owing to the presence of abundant surface functional groups, the stable GO colloidal solution is available for the anchorage of metal groups. When FeCl₃ is added into this system, the molecules attach to the reactive and anchoring sites of GO, which allows nucleation and further growth of magnetic particles. The appropriate stirring in the presence of a stabilizing agent (sodium acetate) and surfactant (polyethylene glycol) can prevent the agglomeration of particles. When this solution is subjected to solvothermal treatment, the Fe³⁺ ions will convert to Fe₃O₄ and seed growth to nanoclusters occurs on the support. Meanwhile, GO undergoes reduction to chemically reduced graphene. As nucleation and seed growth occur on the carbon support, intimate interaction between rGO and Fe₃O₄ can be obtained, which can help in the charge carrier transfer mechanism. On the other hand, the reduction of GO to rGO was achieved without using any harmful chemicals like hydrazine hydrate, which adds to the green synthesis strategy. The growth of Fe₃O₄ spheres on the carbon support can lower its aggregation tendency; thus, more reactive sites will be available for Fenton activity.

Tetracycline degradation

Effect of H₂O₂ concentration on TC degradation

The effect of H₂O₂ concentration on TC degradation was investigated. When H₂O₂ is not present in the system, the chances of MG10 adsorptive removal of TC were evaluated. Adsorptive removal is insignificant, as only 5% adsorptive removal is observed after 3 h. As a result, the presence of H₂O₂ is necessary for TC removal. As shown in Fig. 8b, increasing the concentration of H₂O₂ from 50 to 150 mM results in a significant improvement of TC degradation. This could be because H₂O₂ is the sole source of ˙OH, and a more efficient reaction with the catalyst is possible when there are enough H₂O₂ molecules present. When the concentration of H₂O₂ was increased from 150 mM to 250 mM, the rate of TC degradation declined. This is due to the fact that when there is an excess of H₂O₂, H₂O₂ consumes ˙OH, and less reactive HO₂˙ radicals will be formed (eqn (2)). Furthermore, because there are fewer catalyst active sites available, H₂O₂ does not decompose effectively into ˙OH.³⁴ As a result, high H₂O₂ concentrations may result in less TC removal.

H2O2 + HO˙ → H2O + HO2˙	(2)

Effect of catalyst dosage on TC degradation

The removal of TC at various catalyst dosages was investigated. According to Fig. 9a, the degradation rates of TC were 76.9%, 80.2%, and 76.2% after 150 min in the presence of 100 mg L⁻¹, 300 mg L⁻¹, and 500 mg L⁻¹ MG10, respectively. It was discovered that increasing the MG10 dosage from 100 mg L⁻¹ to 300 mg L⁻¹, improved TC degradation to some extent (Fig. 9a). This is due to the fact that the concentration of Fe(II) and Fe(III) increases with increasing dosage. Thus more surface sites would be available for the decomposition of H₂O₂. However, by increasing the MG10 dosage from 300 mg L⁻¹ to 500 mg L⁻¹, the TC degradation was reduced. This could be due to the radical scavenging by excessive Fe(II) (eqn (3)), resulting in a drop in ˙OH concentrations.⁵⁷

Fe(II) + HO˙ → Fe(III) + OH−	(3)

The optimal MG10 (mg L⁻¹):H₂O₂ (mM) ratio is determined as 2:1. As a result, various multiples of this fixed ratio were investigated (Fig. 9b). It is clear that a 100 (mg L⁻¹):50 (mM) ratio is insufficient for TC degradation. The plots of 300 (mg L⁻¹):150 (mM) and 500 (mg L⁻¹):250 (mM) show that increasing the catalyst dosage and H₂O₂ concentration, simultaneously, results in a significant increase in the initial 60 min of heterogeneous Fenton activity. Following that, TC removal is stabilised at 80%, and further removal is found to be difficult. As a result, the optimum ratio of MG10:H₂O₂ is 300 (mg L⁻¹):150 (mM).

Conclusions

A one-pot solvothermal strategy was used to successfully synthesise ferrimagnetic magnetite–rGO composites. The XRD, FTIR, Raman spectroscopy, XPS, and FE-SEM results showed the simultaneous conversion of FeCl₃ to magnetite and reduction of GO to rGO, as well as magnetite anchorage on rGO sheets. Among the various GO weight ratios tested, using 10% GO precursor resulted in a better Fenton active catalyst. Within 150 min, this composite removed 80% of TC. The high saturation magnetization and stability over four cycles of operation allow easy magnetite–rGO catalyst separation and reusability, as evidenced from the literature.^73,74 In contrast to previous research, the contribution of adsorption and the homogeneous Fenton process to the removal mechanism was found to be negligible. The heterogeneous Fenton process, mediated by radicals such as ˙OH, O₂˙⁻, and surface bound ˙OH, is responsible for the significant removal efficiency. The HR-LC-MS study showed the possible mineralization by ring opening. Although good Fenton activity is observed at pH 3–10, mineralization is adequate only at acidic pH. Solar irradiation substantially improved the TC removal and lowered the required reaction time by one-third. On evaluating the application of the magnetite–rGO heterogeneous system in domestic wastewater, TC removal was 49%, and TOC reduction was 47% within 150 min. The application of magnetite–rGO heterogeneous Fenton catalysts in real field applications needs further validation for catalyst poisoning, pollutant mineralization at neutral pH, rGO release into the water, and toxicity profiles.

Author contributions

Jaimy Scaria: investigation, methodology, formal analysis, validation, and writing – original draft. P. V. Nidheesh: conceptualization, supervision, and writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to express their gratitude to the University Grants Commission (UGC-MANF)-India for their financial support. The authors would also like to thank the Sophisticated Analytical Instrument Facility (SAIF), the Central Surface Analytical Facility (ESCA Lab), the Department of Energy Science and Engineering, the Centre for Research in Nanotechnology & Science (CRNTS) – Indian Institute of Technology – Bombay, India, and the Advance Centre for Material Science, Indian Institute of Technology – Kanpur, India, for their assistance with various analyses. We would like to express our gratitude to Mr. Ansaf V. Karim for his appropriate support at various stages of the work. We would like to thank Mr. Prakash Karipoth and Mr. Mohit Aggarwal of the Indian Institute of Technology Bhubaneswar, as well as Dr. Laiju A. R. of the National Institute of Technology, Uttarakhand, for their assistance with data interpretation.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ew00019a

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