Sunlight assisted degradation of dye molecules and reduction of toxic Cr(VI) in aqueous medium using magnetically recoverable Fe₃O₄/reduced graphene oxide nanocomposite

Abstract

In view of the significant impact of magnetically recoverable catalysts in photocatalytic applications, Fe₃O₄/reduced graphene oxide (rGO) nanocomposite photocatalyst was synthesized by adopting an eco-friendly solution chemistry approach and has been characterized by high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and photoluminescence (PL) spectroscopy. Fe₃O₄/rGO nanocomposite is efficiently utilized towards photocatalytic degradation of carcinogenic and mutagenic cationic as well as anionic dye molecules namely methyl green (MG), methyl blue (MB) and rhodamine B (RhB) under direct sunlight irradiation. The Fe₃O₄/rGO nanocomposite also demonstrated excellent photocatalytic reduction of aqueous Cr(VI) solution to nontoxic aqueous Cr(III) solution of more than 96% within 25 min under sunlight irradiation. Moreover, reusability of the magnetically recovered photocatalyst was studied efficiently up to 10 cycles in the degradation process. The catalyst was also characterized after the degradation of the dye molecule and the particle size of the Fe₃O₄ nanoparticles on the rGO sheets remained unchanged. The present investigation focuses on the importance of the use of Fe₃O₄/rGO nanocomposite towards photocatalytic degradation of waste water containing organic dye pollutants and toxic Cr(VI), as an easily recoverable and reusable photocatalyst with potential for many environmental remediation applications.

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1. Introduction

Toxic pollutants like reactive dyes are being used in a plethora of products e.g. textile, paper, cosmetics, food and pharmaceuticals, fabric and plastic industries mostly because of their bright color and easy application.^1,2 Another water pollutant, Cr(VI) is also widely applied in various industries like leather tanning, steel production, electroplating, dye production industries, etc.^3,4 As a result these dye molecules and Cr(VI) are present in the environment as harmful entities because of their carcinogenic and mutagenic nature.^5–7 Wastewater containing dyes and Cr(VI) that is discharged from these industries pollutes the ground resources and poses severe carcinogenic effects to human health and the environment.⁸ Cr(VI) is highly water soluble and causes serious effects on human health like liver damage, pulmonary congestion, severe diarrhoea and vomiting.⁹ In this aspect, removal of dye molecules and Cr(VI) is extremely important. A large number of physico-chemical methods such as microfiltration, trickling filter, reverse osmosis, active sludge, chemical coagulation, adsorption, photocatalytic degradation are reported in the literature for the decolourization of dye molecules.^10,11 Reverse osmosis, ion exchange, cross-flow microfiltration and photocatalytic reduction are widely reported methods for the removal of Cr(VI).^12–15 Amongst the reported methods, photocatalytic degradation and photocatalytic reduction are widely used for the removal of dye molecules and other water contaminants because of the low cost involved and benign nature.¹⁶ Moreover, the photocatalytic process has great potential and high efficiency for removal of organic pollutants under direct natural sunlight irradiation in presence of solid catalyst.^17,18 Photocatalytic processes for dye degradation involves the initial absorption of photons by a molecule or the photocatalyst that results in the transfer of an electron from the valence band to the conduction band, thus creating an electron deficiency or hole (h⁺) in the valence band and simultaneously generating an electron (e⁻) in the conduction band. Hole can react with adsorbed water and generate hydroxyl (˙OH) radicals. ˙OH radicals can directly oxidize organic dye pollutants and convert them into inorganic harmless anions (NO₃⁻, SO₄²⁻) and other inorganic molecules like CO₂ and H₂O. In the photocatalytic reduction process, the electrons generated at the conduction band helps in the reduction of Cr(VI) to nontoxic Cr(III).^19–21

Development of non-precious metal and metal oxide nanoparticles supported on different templates to enhance the degradation process is a major challenge. Amongst templates used, carbonaceous materials namely active carbon, carbon black, carbon nanotube, graphene have wide applicability. Carbon based photocatalyst like carbon-doped TiO₂, carbon-coated TiO₂ etc. has been widely used.^22,23 Amongst carbonaceous support, the inexpensive crystalline material graphene, a unique honeycomb structure of 2D layer of sp²-hybridized carbon network, exhibits high surface area, unique mechanical and thermal properties as well as considerable zero-band gap semiconductor having high electrical conductivity. Here, both hole and electrons are charge carriers and mobility of electrons is very high and it also acts as a good supporting material for the synthesis of metal and metal oxide nanoparticles.^24,25 Owing to these advantages, graphene is widely used in various energy and environment remediation applications.²⁶ Over the last decades, researchers have developed several graphene-based semiconductor photocatalyst such as TiO₂/graphene, α-Fe₂O₃ nanorod/RGO, graphene–Au nanocomposite, CdS/ZnO/graphene, ZnO/graphene etc. towards the removal of water contaminates, both organic and inorganic pollutants to improve its activity.^21,27–32 However, the main problem associated with these catalysts is their recollection for further use. Therefore, researchers have tried to develop magnetically separable nanoparticles/nanocomposites for removal of dye molecules to overcome this problem. Amongst magnetic nanoparticles, Fe₃O₄ nanoparticles has attracted increasing attention because of their superparamagnetic behaviour, high surface-to-volume ratio, low toxicity, electrical conductivity, optical and chemical properties, etc. As such, the Fe₃O₄ nanoparticles have extensive applications in chemical, biological and medical fields as a magnetically separable catalyst.^33–35 However, Fe₃O₄ nanoparticles fabricated without any template tends to lose its photocatalytic activity towards degradation of organic dye molecules due to agglomeration which rapidly minimizes the active sites and chemical interaction between nanoparticles and dye molecules.^36,37 As such to overcome the separation problem, more recently researchers have developed magnetically separable graphene based green and inexpensive photocatalyst which are easily separable from the reaction mixture using an external magnetic field. Some of them include TiO₂–Fe₃O₄/graphene, Fe₃O₄/graphene, CdFe₂O₄/graphene, ZnFe₂O₄/graphene.^38–40 These nanocomposites exhibit maximum photocatalytic activity as the photogenerated charge instantaneously transfers from metal oxide to the graphene sheets which facilitate increased electron–hole pair separation as well as enhances the degradation process.³⁷ This phenomenon was utilized by Teok Peik-See et al. towards degradation of methylene blue dye using Fe₃O₄/rGO nanocomposite where 100% degradation efficiency was achieved in 1 h. However, the study focused only on selective degradation towards methylene blue.³⁷ In a different study carried out by Vinothkannan et al. for degradation of methylene blue mediated electron transfer process of BH₄⁻ ions, degradation efficiency of 95.18% was achieved by using Fe₃O₄/rGO nanocomposite. However, the major drawback associated in their method lies in the use of NaBH₄ in the degradation process.³⁹ Guo et al. reported the photo-Fenton degradation of the organic contaminates by using Fe₂O₃/GO photocatalyst under visible light irradiation.⁴¹ The reported catalyst possessed good stability, stable catalytic activity, easy separability and was efficient in a wide range of pH. In a separate study, Cu₂ZnSnS₄ nanoparticles was utilized by P. Kuch et al. towards photocatalytic reduction of Cr(VI) with 99.8% reduction efficiency.⁷ J. Yu and his co-authors explored hetero-p–n–CuO–ZnO nanocomposite device as efficient visible light driven photocatalyst towards reduction of Cr(VI).⁴² However, recollection of the catalyst is the main concern associated with these methods.

Keeping in view the shortcomings of the previous studies, we report in this study the photocatalytic degradation of potentially harmful dyes like methyl green (MG), methyl blue (MB), rhodamine B (RhB) by using magnetically separable Fe₃O₄/rGO nanocomposite. Structure and some chemical properties of these dye molecules are listed in the Table 1. We also report photocatalytic reduction of toxic aqueous solution of Cr(VI) to non-toxic aqueous Cr(III) solution by using the same nanocomposite under natural sunlight irradiation. Degradation of MG, MB and RhB by using Fe₃O₄/rGO nanocomposite is reported in this paper for the first time to the best of our knowledge. High degradation efficiencies of 99.31%, 98.97% and 87.13% are achieved for MG, MB and RhB, respectively. So far there is no report on the photocatalytic reduction of aqueous Cr(VI) solution (96.1%) using Fe₃O₄/rGO nanocomposite. The mechanism of photocatalytic dye degradation and proposed mechanism for the reduction of Cr(VI) is illustrated in the present investigation.

Table 1 Chemical structure and some properties of MG, MB and RhB dye molecules

Dyes	Chemical formula	Molar mass, g mol^−1	Ionic property
Methyl green		608.78	Cationic
Methyl blue		799.81	Anionic
Rhodamine B		479.01	Cationic

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2. Experimental

2.1 Materials

FeCl₂·4H₂O (>99%, Sigma-Aldrich, India), FeCl₃ anhydrous (>99%, Qualigens, India), graphite powder (<20 μm, Sigma-Aldrich), sulfuric acid (AR grade, Qualigens, India), hydrochloric acid (AR grade, Qualigens, India), H₂O₂ (30%, Qualigens, India), potassium permanganate (>99%, E-Merck, India), NaOH (99%, Qualigens, India), methyl green (LobaChemie, India), rhodamine B (reagent grade, Across organics, Belgium), methyl blue (microscopic reagent grade, LobaChemie, India) and K₂Cr₂O₇ (E-Merck, Germany).

2.2 Synthesis of Fe₃O₄/rGO nanocomposite photocatalyst

GO is the base material for the decoration of the Fe₃O₄ nanoparticles on the rGO sheets. GO sheets are synthesized from graphite powder.⁴³ The as synthesized GO was reduced to rGO sheets using ascorbic acid as a reducing agent adopting eco-friendly approach and further Fe₃O₄ nanopowder was synthesized on rGO sheets adopting chemical co-precipitation method.⁴⁴ The precursor for Fe₃O₄ nanoparticles and rGO sheets used in this synthesis are FeCl₂·4H₂O and GO, respectively. The reaction was carried out at pH 11. Here GO acts as an oxidizing agent and is reduced to rGO sheets and simultaneously Fe(II) is oxidized to Fe(II)(III).

2.3 Photocatalytic dye degradation and Cr(VI) reduction experiments

The photocatalytic degradation of MG, MB and RhB dye molecules was carried out considering different experimental conditions under direct natural sunlight irradiation. The photocatalytic experiments were executed on the days of bright sunny light in between 10 am to 2 pm in the winter session of Jorhat city, Assam, North-Eastern region of India and the average intensity of light was (50 ± 4) × 10³ lx (CHY 332, Digital Light Meter). M. Baruah et al. reported that, the average light intensity of sunlight of the Assam in the North Eastern region of India is found to be 3–6 kW h m⁻² d⁻¹.⁴⁵ Typically, for the photocatalytic experiments, the Fe₃O₄/rGO nanocomposite was dispersed in 50 mL aqueous suspension of dye solution and stirred in a 100 mL round bottom flask (RB). Initially the reaction mixture was magnetically stirred for 120 min before sunlight irradiation to ensure that it reaches the adsorption/desorption equilibrium. For the photocatalyst investigation, the reaction mixture was magnetically stirred for 120 min in presence of the Fe₃O₄/rGO nanocomposite under direct sun light irradiation. At a given time interval, 2.5 mL of the reaction mixture was collected and the Fe₃O₄/rGO photocatalyst was separated from the suspension using a bar magnet. The equilibrium concentration of the supernatant of each sample was monitored in a UV/visible spectrophotometer by measuring the intensity of absorbance at a fixed wavelength of the respective dyes such as 626 nm, 595 nm and 558 nm for MG, MB and RhB, respectively to determine the residual concentration of the dye solution. Percentage degradation of the dye molecules were calculated using the following equation.

Dye removal efficiency (%) = [1 − C_t/C_o] × 100 (1)

where, C_o is the initial dye concentration and C_t is the concentration of dye at time t, respectively. Similarly Cr(VI) reduction experiments were carried out under sunlight irradiation with same experimental conditions and the residual concentration of Cr(VI) in the supernatant was evaluated using a UV/visible spectrophotometer at a fixed wave length of 358 nm followed by evaluation of Cr(VI) reduction efficiency using eqn (1).

2.4 Characterization techniques

These synthesized materials were characterized by high resolution transmission electron microscopy (HRTEM) by using JEOL JEM 2100, Japan operated at an accelerating voltage of 200 kV. Magnetic properties were determined by vibrating sample magnetometer (VSM, Lake Shore 7410, USA). Raman spectroscopy measurement was carried out using a HR 800 Raman spectrophotometer (Jobin Yvon HORIBA, France) consisting of a He-Ne laser (632.8 nm) as the monochromatic radiation source and operating at 20 mW. X-ray photoelectron spectroscopic (XPS) measurements were carried out on a VG Micro Tech ESCA 3000 instrument in which Mg Kα (hν = 1253.6 eV) radiation was used as the source. A combined polynomial and Shirley type background function was used to deconvolute the spectra. Fluorescence measurements were executed on a F-2700, fluorescence spectrophotometer (HITACHI, Tokyo, Japan) at room temperature as the excitation source equipped with a 150 W xenon lamp (Ushio Inc., Japan). The residual concentration of the dye and chromium solutions were determined by a UV/visible spectrophotometer (SPECORD-200, Analytikjena, Germany) calibrated with standard samples. Percentages of carbon and hydrogen present in the Fe₃O₄/rGO nanocomposite was analysed using CHN Macro Determinator (Truspec; 630-100-300, ASTM D5373-14). Fe₃O₄ loading on the rGO sheets and dissolution of iron oxide from Fe₃O₄/rGO nanocomposite in solution after photocatalytic degradation was analysed by an atomic absorption spectrophotometer (Perkin Elmer, Analyst 200 Atomic absorption spectrophotometer). Before and after photodegradation, the photocatalyst was also characterized by X-ray diffraction (XRD) analysis using a Rigaku X-ray diffractometer (model: ULTIMA IV, Rigaku, Japan) with a scanning rate of 3° min⁻¹ in the 2θ value ranging from 5–100° with a Cu Kα X-ray radiation (λ = 1.54056 Å). Fourier transform infrared (FT-IR) spectra were recorded with 4 cm⁻¹ spectral resolutions using IR Affinity, Shimadzu, Japan FT-IR spectrophotometer equipped with a Shimadzu DRS-8000 DRIFT accessory and IR solution software.

3. Results and discussion

3.1 Structural characterization of the Fe₃O₄/rGO nanocomposite photocatalyst

The surface morphology and particle size of the synthesized Fe₃O₄/rGO nanocomposite was studied by HRTEM images and crystallinity was determined with selected area electron diffraction (SAED) pattern as shown in the Fig. 1a–f. The average size of the particles was found to be around 23 nm and a large number of these particles are spherical in nature. The lattice fringes are clearly seen in the Fig. 1e with an interplaner distance of 0.21 nm. It is clearly seen that the rGO sheets is decorated with Fe₃O₄ nanoparticles and different dots in the SAED pattern represents crystalline nature of the Fe₃O₄ nanoparticles. The crystalline lattice of Fe₃O₄ nanoparticles corresponds to the (440), (400), (311) and (220) lattice planes as shown in Fig. 1f.

Fig. 1 TEM study of Fe₃O₄/rGO nanocomposite: (a and b) low-magnified TEM images of nanoparticles on rGO, ((a) inset) VSM plot of the Fe₃O₄/rGO nanocomposite, (c) the particle size distribution curves, (d and e) HRTEM image of a single nanoparticle, (e) lattice fringes for (111) fcc plane of Fe₃O₄/rGO and (f) SAED pattern.

Fig. 2 shows the Raman spectra of Fe₃O₄/rGO nanocomposite. A small peak corresponding to Fe₃O₄ (magnetite) at 668 cm⁻¹ was observed in the spectra, because the laser irradiation causes the oxidation of Fe₃O₄ to α-Fe₂O₃ during the Raman measurement. The peaks at 217 and 487 cm⁻¹ correspond to the A_1g, whereas the peaks at 285, 392, 487 and 600 cm⁻¹ correspond to the E_g mode of α-Fe₂O₃ (hematite).^46–48 Generally a peak at around 1287 cm⁻¹ corresponding to interaction of two magnon scattering created from two antiparallel close spin states also appears.^48,49 Here, this two magnon scattering peak seems to be merging with the D band of rGO.

Fig. 2 Raman spectrum of the Fe₃O₄/rGO nanocomposite material.

The important information about the electronic structure and chemical composition of the product is explained by XPS as shown in Fig. 3. Fig. 3a shows the full scan XPS spectra of Fe₃O₄/rGO nanocomposite. The sharp peak at 285, 500 and 711 eV are associated with C 1s, O 1s and Fe 2p, suggesting the presence of C, O and Fe in the Fe₃O₄/rGO. Fig. 3b shows the high resolution spectra of Fe 2p spectrum, the two distinct peaks with binding energies of 711.1 eV belongs to Fe 2p_3/2 and 724.4 eV belongs to the Fe 2p_1/2 spin orbit peaks of Fe₃O₄, respectively, indicating the formation of a mixed oxide of Fe(II) and Fe(III). Fig. 3c shows the C 1s spectrum with three deconvoluted peaks at 283.8, 285.1 and 287.5 eV associated with C–C, C–O and C=O bond respectively. Fig. 3d shows the O 1s spectrum, the three peaks at 529.6, 531.2 and 532.7 eV belongs to Fe–O, C=O and C–O, respectively.^10,50,51

Fig. 3 (a) XPS survey spectrum of Fe₃O₄/rGO nanocomposite (b) the high-resolution Fe 2p spectrum (c) the high-resolution C 1s spectrum (d) the high-resolution O 1s spectrum.

The chemical constituents of the Fe₃O₄/rGO nanocomposite were evaluated by CHN analysis and shown in Table 2. It was observed that 13.80% carbon, 1.66% hydrogen, 32.14% oxygen and 52.40% of iron present in Fe₃O₄/rGO nanocomposite. The amount of the iron present (52.40%) on the Fe₃O₄/rGO nanocomposite is comparable with the reported literature.^52,53

Table 2 Elemental composition of rGO and Fe₃O₄/rGO nanocomposites

Materials	C	Elemental content (wt%)			C/O molar ratio
		H	O	Fe
rGO	91.90	2.35	5.70	—	15.98
Fe3O4/rGO nanocomposite	13.80	1.66	32.14	52.40	0.429

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The PL spectra of the nanomaterials (Fe₃O₄ nanopowder, Fe₃O₄/rGO nanocomposite and rGO sheets) yield three Gaussian like peaks at 695 nm, 698 nm and 705 nm, respectively (Fig. 4). However, the UV/visible absorption data of Fe₃O₄ nanopowder, Fe₃O₄/rGO nanocomposite and rGO sheets do not exhibit any peak in the above wavelength range. This indicates that the PL of the materials originates from the structure related defects rather than band-edge transition. PL spectra determines the migration of electron, electron–hole pair recombination rate and charge separation. The appearance of strong emission peaks in case of Fe₃O₄ nanopowder and Fe₃O₄/rGO nanocomposite also indicates size related quantization attributed to quantum confinement of the charge carriers of the nanoparticles in the restricted volume. The Fe₃O₄ nanopowder shows high PL intensities, which indicates high recombination rates of the excited electrons and holes due to narrow band gap value (1.4 eV).³⁷ This narrow band gap arises from the electrons of the d orbital due to which Fe₃O₄ exhibits high electrical conductivity. Higher PL intensity values indicate metallic nature of Fe₃O₄ nanopowder having lower charge carrier capacity of both electrons and holes. Fe₃O₄/rGO nanocomposite and rGO sheets exhibits lower PL intensities due to unique two dimensional hexagonal layer structures of sp²-hybridized carbon atoms of rGO exhibiting zero-band gap semiconductor and having high electrical conductivity in which both hole and electrons are charge carriers and the mobility of electrons is very high. Owing to these properties, rGO sheets can easily capture excited electrons from the conduction band of Fe₃O₄ through π-conjugation carbon network, as a result of which it suppresses the electron–hole pair recombination rate as well as increases the charge separation between conduction band and valence band in the Fe₃O₄/rGO nanocomposite.³⁷ Therefore, Fe₃O₄/rGO nanocomposite are expected to perform excellent photocatalytic activity towards degradation of dye molecules and reduction of Cr(VI).

Fig. 4 Photoluminescence spectra of Fe₃O₄ nanopowder, Fe₃O₄/rGO nanocomposite and rGO sheets.

The UV-visible diffuse reflectance (UV-vis DRS) spectra of rGO, Fe₃O₄ and Fe₃O₄/rGO nanocomposite are recorded and it exhibits the characteristic behaviour of semiconductor materials (shown in Fig. S1 in ESI†).

3.2 Photocatalytic dye degradation activity of Fe₃O₄/rGO nanocomposite

The photocatalytic activity of the Fe₃O₄/rGO nanocomposite was investigated using MG, MB and RhB as model dye molecules under direct natural sunlight irradiation. The result of the photodegradation study is shown in Fig. 5. Fig. 5a–c shows that the absorption spectra of the MG, MB and RhB in presence of Fe₃O₄/rGO nanocomposite under the direct sunlight irradiation at different time intervals. In presence of natural sunlight sources, 99.31% MG, 98.97% MB and 87.13% RhB dye degradation was achieved within 120 min irradiation at pH 5 under same catalytic conditions. Before sunlight irradiation, adsorption/desorption equilibrium was studied by keeping the dye suspension under magnetic stirring condition in dark. Only 18.3% for MG, 52.31% for MB, 12.34% for RhB adsorption was observed after 2 h treatment in dark for these dye molecules at pH 5 with initial dye concentration 0.1 mM and catalyst loading 0.5 g L⁻¹ (Fig. 5d). Dye degradation experiments were also carried out without addition of catalyst under sunlight irradiation. In absence of catalyst, 0 to 5% dye degradation was observed for all dye molecules within 4 h under sunlight irradiation (Fig. 5d). The photodegradation of the dye molecules in presence of the Fe₃O₄/rGO nanocomposite was investigated considering the different parameters like effect of catalyst loading, initial dye concentration, pH, addition of H₂O₂ and degradation kinetics.

Fig. 5 UV/visible spectral changes for the degradation of (a) MG, (b) MB, (c) RhB under direct sunlight irradiation (dye concentration = 0.1 mM, Fe₃O₄/rGO nanocomposite = 0.5 g L⁻¹, pH = 5), (d) photocatalytic degradation performance without addition of photocatalyst, photocatalyst in dark, photocatalyst under sunlight irradiation.

3.2.1. Effect of the catalyst loading. The loading of the Fe₃O₄/rGO nanocomposite photocatalyst on the degradation of MG, MB and RhB dye molecules was studied at a fixed concentration of the dye molecules (0.1 mM), at pH 5 under sunlight irradiation. Experiment was performed by varying the catalyst amount from 0.1 g L⁻¹ to 1 g L⁻¹. The results of catalyst loading on photodegradation are shown in the Fig. 6a. The amount of dye degradation increases when the loading of the catalyst increases from 0.1 g L⁻¹ to 0.5 g L⁻¹ and then decreases gradually. Therefore, optimum loading amount of the catalyst was considered as 0.5 g L⁻¹ (1.57 mmol of Fe₃O₄ loading in Fe₃O₄/rGO nanocomposite). This is due to increase in the number of active sites present in the catalyst as the amount of catalyst increases. This causes increase in adsorption of photons as well as dye molecules onto the catalyst. When amount of catalyst loading is further increased, an aggregation of the nanocomposite occurs for which the catalyst cannot clearly disperse and in turn amount of dye degradation decreases.

Fig. 6 Photodegradation efficiency of MG, MB and RhB dye molecules (a) catalyst loading (pH 5 and initial dye concentration 0.1 mM) (b) different initial concentration of MG, MB and RhB (catalyst loading 0.5 g L⁻¹ and pH 5) (c) effect of varying initial pH (dye concentration 0.1 mM and catalyst loading 0.5 g L⁻¹).

3.2.2. Effect of initial dye concentration. The effect of initial dye concentration on the photodegradation phenomenon of the selected model dye molecules was investigated by varying the concentration of dye molecules from 0.08 mM to 0.5 mM at a fix concentration of the photocatalyst of 0.5 g L⁻¹ at pH 5. It was observed that, the degradation efficiency decreased with increasing dye concentration due to unavailability of active sites on the photocatalyst. Fig. 6b and S2(a–c)† shows the dye degradation efficiency of MG, MB and RhB dye molecules with more than 98% when initial concentration dye is 0.1 mM. However, the degradation is found to decrease as the initial concentration of the dye molecules increases. Moreover, due to dimerization effect of RhB at higher concentration, only 87.13% degradation efficiency was achieved at pH 5.

3.2.3. Effect of initial pH. The pH of the suspension also plays an important role in the degradation of the MG, MB and RhB dye molecules. The degradation of all the three dye molecules was investigated in the pH range 3 to 11 at a fixed catalyst concentration and dye concentration (Fig. 6c). It was observed that maximum degradation efficiency appeared at pH 11 for MG (100%) and RhB (94.7%) i.e., degradation efficiency increased with increasing pH. However in case of MB anionic dye molecule, rate of degradation increases up to pH 5 and then decreases. The maximum degradation of the MB is 98.97%, achieved at pH 5 under same experimental conditions. The surface properties and the oxygen containing functionalities of the Fe₃O₄/rGO nanocomposite play an important role in the degradation of the dye molecules at different pH of the reaction medium. The point of zero charge of the Fe₃O₄/rGO nanocomposite is pH 5.5.⁴⁴ So, below pH 5.5, the surface charge of the Fe₃O₄/rGO nanocomposite remains positive, which decreases the availability of active sites in the photocatalyst for adsorption of the cationic dye molecules. However, in anionic dye molecules due to opposite surface charge present on the catalyst and the dye molecules, adsorption of dye molecules onto the photocatalyst occurs easily. At basic pH, surface hydroxyl groups are responsible for the degradation of the cationic dye molecules, which enhances the adsorption of dye molecules as well as facilitates the degradation process.⁵⁴

3.2.4. Effect of addition of H₂O₂ during the degradation process. The effect of addition of the H₂O₂ dosages on the degradation of MB dye molecule (concentration 0.2 mM) in presence of Fe₃O₄/rGO nanocomposite photocatalyst (0.5 g L⁻¹) under sunlight irradiation at pH 5 is illustrated in Fig. S3.† In the photocatalytic degradation process, H₂O₂ was generated due to oxidation of H₂O, however further H₂O₂ was generated during the degradation process by reduction of O₂ molecules, which act as an active free radical and enhances the degradation process. The efficiency of dye degradation increases on addition of 0.02 mM H₂O₂ solution in the reaction mixture.⁵⁵

3.2.5. Comparative photocatalytic dye degradation performance of Fe₃O₄ nanopowder, rGO sheet and Fe₃O₄/rGO nanocomposite. The comparative degradation of RhB dye molecule was studied using Fe₃O₄ nanopowder, rGO sheets and Fe₃O₄/rGO nanocomposite at 0.08 mM dye concentration and catalyst loading 0.5 g L⁻¹ at pH 5. It was observed from the Fig. 7 that, Fe₃O₄/rGO nanocomposite exhibits excellent photocatalytic activity towards the degradation of the selected dye molecules with almost 95% RhB dye degradation efficiency after 120 min of sunlight irradiation in comparison to Fe₃O₄ nanopowder and rGO sheets. However, in case of Fe₃O₄ nanopowder as photocatalyst, only 52.3% dye degradation efficiency is achieved for RhB dye molecules. In aqueous solution, Fe₃O₄ nanoparticles undergo agglomeration with neighbouring particles due to the magnetic interaction of nanoparticles towards each other.⁵⁶ Moreover, appearance of synergistic effect of Fe₃O₄ nanoparticles and rGO sheets facilitates increase of electron transfer efficiency and photogenerated charge transfer from Fe₃O₄ to rGO. Fe₃O₄/rGO nanocomposites have large numbers of active sites which causes maximum degradation efficiency as achieved by the Fe₃O₄/rGO nanocomposite. On the other hand, only 12.3% RhB dye degradation efficiency was found when rGO sheets was used as a photocatalyst which may be due to adsorption of dye molecules onto the rGO sheets.⁵⁷

Fig. 7 Degradation of RhB using Fe₃O₄/rGO nanocomposite, Fe₃O₄ and rGO (initial dye concentration 0.08 mM and, catalyst loading 0.5 g L⁻¹ and pH 5).

3.2.6. Degradation kinetics and investigation of intermediate and final products. Fig. S4 (in ESI†) shows the kinetics of the degradation process of MG, MB and RhB dye molecules in presence of Fe₃O₄/rGO nanocomposite photocatalyst. The kinetics study was carried out by varying the initial concentration of the dye molecules from 0.08 mM to 0.5 mM. The pseudo first order kinetic model is followed for all the dye molecules as represented by the Langmuir–Hinshelwood equation as follows.where, C_o represents the initial concentration (mM) of the dye before irradiation and C, the residual dye concentration, after irradiation at time t. K is the rate constant (min⁻¹) which is determined by executing linear regression. The kinetic parameters calculated for all three dye molecules are presented in Table 3.

Table 3 Parameters of pseudo first order kinetics model for the degradation of MG, MB and RhB using Fe₃O₄/rGO nanocomposite under sunlight irradiation

Dyes	Co (mM)	Degradation%	K (min^−1)	R^2
MG	0.1	99.31	0.048	0.9558
	0.2	94.65	0.024	0.9795
	0.3	87.23	0.022	0.9836
	0.5	78.94	0.017	0.9957
MB	0.1	98.97	0.030	0.9694
	0.2	93.26	0.024	0.9779
	0.3	87.82	0.022	0.9863
	0.5	73.54	0.015	0.9936
RhB	0.08	95.38	0.022	0.9749
	0.1	87.13	0.017	0.9306
	0.2	81.00	0.014	0.9634
	0.3	75.32	0.013	0.9768
	0.5	60.29	0.008	0.9851

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The intermediate and the final product in the MG, MB and RhB dye molecules were analysed using ion chromatography and HPLC analysis (details studies are illustrated in the Fig. S5 in ESI†).

3.3 Photocatalytic aqueous Cr(VI) solution reduction

The photocatalytic reduction of mutagenic aqueous Cr(VI) solution to nontoxic aqueous Cr(III) was investigated under direct sunlight irradiation by varying the initial Cr(VI) concentration as shown in Fig. 8. Before sunlight irradiation, the aqueous Cr(VI) suspension was magnetically stirred in dark and 38% Cr(VI) adsorption was observed after 2 h treatment (initial Cr(VI) concentration is 0.08 mM and catalyst loading is 25 mg in 50 mL suspension). Fe₃O₄/rGO nanocomposite can efficiently reduce Cr(VI), when irradiated under natural sunlight (Fig. 8a). It is observed that more than 96% Cr(VI) reduction completed within 25 min (Fig. 8b). Cr(VI) reduction follows pseudo first order kinetic model (Fig. 8c). Rate of the reaction was calculated and high rate constant values (0.1073, 0.096, 0.0875 and 0.0604 min⁻¹) were obtained using Langmuir–Hinshelwood equation (eqn (2)) for different initial Cr(VI) concentration. After completion of the reduction process, light yellow color of the solution of Cr(VI) is changed to colourless solution. Semiconductor materials play an important role in the reduction of the aqueous Cr(VI) solution to non-toxic aqueous Cr(III) solution. Zhao et al. recently reported the synthesis of the TiO₂/rGO nanocomposite using sol–gel method and utilized for the reduction of aqueous Cr(VI) under visible light irradiation and they have achieved 86.5% photoreduction efficiency.⁵⁸ Moreover, it is observed that the Fe₃O₄/rGO nanocomposite also acts as an efficient photocatalyst as compared to other reported photocatalyst for the reduction of environmentally pollutant aqueous Cr(VI) solution as represented in the Table 4.^59–63 The added advantage of the Fe₃O₄/rGO nanocomposite materials is its easy separability and reusability for several cycles which makes the nanocomposite favourable for use and well as sustainable.

Fig. 8 (a) UV/visible spectral changes for the reduction of Cr(VI) to Cr(III), (b) photocatalytic reduction% of Cr(VI), (c) Langmuir–Hinshelwood pseudo first order kinetic plots (catalyst loading 25 mg in 50 mL suspension) and (d) effect of pH on the photoreduction of Cr(VI).

Table 4 Comparison of the Cr(VI) reduction efficiency of Fe₃O₄/rGO nanocomposite with other graphene based photocatalyst

Nanocomposites	Photoreduction efficiency (%)	Light source	Irradiation time	References
TiO2/rGO	91	UV light	4 h	59
ZnO/rGO	96	UV light	4 h	60
CdS/rGO	90	Visible light	4 h	61
α-FeOOH nanorod/rGO	94	Visible light	3 h	62
GO coated coordination polymer nanobelt	85.42	Visible light	3 h	63
Fe3O4/rGO nanocomposite	<96	Sunlight	25 min	Present work

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3.3.1. Effect of pH on the photoreduction of Cr(VI). The pH of the reaction medium also play a key role in the Cr(VI) photoreduction process. Surface charge of the photocatalyst varies with the solution pH. The reduction efficiency of Cr(VI) decreases with increasing pH from 3 to 11 as depicted in Fig. 8d. At pH 3, more than 97% Cr(VI) reduction efficiency was achieved after 30 min under sunlight irradiation, whereas 82% reduction efficiency was observed at pH 11 under same experimental condition. The surface charge of Fe₃O₄/rGO nanocomposite becomes positive at acidic pH and negative at basic pH.⁴⁴ Cr(VI) is easily adsorbed onto the photocatalyst at lower pH due to strong electrostatic interaction between the positive surface charge of the photocatalyst and anionic chromate species such as HCrO₄⁻ and CrO₄²⁻. On the other hand, at basic pH the anionic Cr₂O₇²⁻ species repels away from the anionic surface of the photocatalyst. As a result, high reduction efficiency of Cr(VI) was achieved at lower pH. Due to the higher concentration of proton (H⁺) at lower pH, the rate of the reaction is enhanced and the reaction shifts to the right direction as a result of which high reduction efficiency of Cr(VI) was achieved.⁶⁴ The comparative photocatalytic reduction of aqueous Cr(VI) using Fe₃O₄/rGO nanocomposite, Fe₃O₄ nanopowder and rGO sheets were carried out to understand the reduction efficiency of these three nanomaterials as illustrated in the Fig. S6 in ESI.†

3.4 Mechanism of photodegradation and photo reduction phenomenon

3.4.1. Dye degradation mechanism. Due to indirect band gap (1.4 eV) present in the Fe₃O₄, it cannot absorb sunlight directly. Therefore, only 52.3% RhB dye degradation was observed upon use of Fe₃O₄ nanoparticles at pH 5 under sunlight irradiation. However, Fe₃O₄/rGO can absorb dye molecules effectively due to the presence of large surface size and phenyl ring structure of the rGO sheets which provides large surface sites when small particles of metal and metal oxide nanoparticles are anchored onto the graphene surface. Due to the presence of different oxygen containing functional groups such as hydroxyl, epoxy and carboxyl groups on the rGO sheets, it can electrically interact with the heterocyclic dye molecules. Appearance of π–π interaction between the aromatic rings of Fe₃O₄/rGO nanocomposite with heterocyclic dye molecules, i.e. electrostatic interaction between the π electrons of rGO sheets with π electrons of heterocyclic dye molecules facilitates the generation of hydroxyl radicals. Hydroxyl radicals are generally responsible for the degradation of dye molecules. Photocatalytic degradation highly depends on the concentration of photogenerated charge transfer. During the photocatalytic reaction, photogenerated charge is transferred from Fe₃O₄ to the rGO sheets. Fe₃O₄ nanoparticles are strongly anchored on to the rGO sheets and therefore under sunlight irradiation, easy migration of photo excited electrons from conduction band of the Fe₃O₄ nanoparticles to the rGO sheets occurs.³⁷

The schematic representation of the photocatalytic degradation of dye molecules are as shown in the Fig. 9. Photocatalytic processes involves the initial absorption of photons by Fe₃O₄/rGO nanocomposites under sunlight irradiation in aqueous medium resulting in transfer of an electron from the valence band to the conduction band of Fe₃O₄ thus creating an electron deficiency or hole (h⁺) in the valence band and simultaneously generating an electron in the conduction band that are of oxidizing and reducing equivalents (eqn (3)). Dye molecule absorbs photon light and undergoes photosensitization (eqn (4)). rGO acts as an electron carrier and therefore photogenerated electrons simultaneously transfers from the conduction band of Fe₃O₄ to the rGO sheets (eqn (5)). The electrons in the rGO sheets instantaneously capture the dissolved oxygen and generate O₂˙⁻ radicals (eqn (6)). In aqueous suspension, holes generate hydroxyl radicals (eqn (7)), which effectively degrades organic dye pollutants (eqn (8)).

Fe₃O₄ + hν → Fe₃O₄ (e⁻ + h⁺) (3)

Dyes + hν → (dyes)* (4)

Fe₃O₄/rGO + hν → Fe₃O₄ (h⁺) + rGO (e⁻) (5)

rGO (e⁻) + O₂ → rGO + O₂˙⁻ (6)

Fe₃O₄ (h⁺) + OH⁻ → Fe₃O₄ + OH˙ (7)

Dye molecules + OH˙ → degraded products (8)

Fig. 9 Schematic representation of the photocatalytic degradation of MG, MB and RhB dye molecules and Cr(VI) reduction using Fe₃O₄/rGO nanocomposite.

(Cl⁻, NO₃⁻, SO₄²⁻, CO₂⁻ and H₂O).

3.4.2. Dye degradation mechanism under addition of H₂O₂. Addition of 0.02 mM concentration of H₂O₂ facilitates the degradation process due to the generation of additional free radicals. However, further addition of H₂O₂ cannot help in degradation process, which suppresses the generation of OH˙.⁶⁵

H₂O₂ + OH˙ → H₂O + HO₂˙ (9)

Proposed mechanism of the photocatalytic degradation of MB in presence of H₂O₂ using Fe₃O₄/rGO nanocomposite is shown below. In eqn (10), transfer of electrons from the conduction band of Fe₃O₄ to the rGO sheets occurs. The electrons in the rGO sheets will undergo captured dissolved oxygen and generation of O₂˙⁻ radicals (eqn (11)). O₂˙⁻ radicals directly combine with H₂O₂ and generate OH˙ radicals (eqn (12)).

Fe₃O₄/rGO + hν → Fe₃O₄ (h⁺) + rGO (e⁻) (10)

O₂ + e⁻ → O₂˙⁻ (11)

H₂O₂ + O₂˙⁻ → OH˙ + OH⁻ + O₂ (12)

H₂O₂ + e⁻ → OH˙ + OH⁻ + O₂ (13)

OH˙ + dyes → degraded products (14)

Possible photo Fenton process may also occur during for the degradation of dye molecules under addition of H₂O₂ in the reaction mixture in presence of direct sunlight irradiation. Photo Fenton process is another way for the degradation of dye molecules within shorter time period. The photodegradation of the MB in presence Fe₃O₄/rGO nanocomposite and H₂O₂ under direct sun light irradiation enhance the efficiency and also completed with 80 min. This phenomenon is considered by the following plausible mechanism. In the Photo Fenton reaction, uses of H₂O₂ and ferrous ions (Fe²⁺) produces OH˙ radicals (eqn (15)). Regeneration of Fe²⁺ ions is achieved through phototreatment phenomenon (eqn (16)).^66,67

Fe²⁺ + H₂O₂ → Fe³⁺ + OH˙ + OH⁻ (15)

Fe³⁺ + H₂O + hν → Fe²⁺ + OH˙ + H⁺ (16)

3.4.3. Photocatalytic Cr(VI) reduction mechanism. A simple mechanism has been proposed for the photocatalytic Cr(VI) reduction as shown in stepwise manner (eqn (17)–(19)). Cr(VI) is adsorbed onto the Fe₃O₄/rGO nanocomposite due to the electrostatic interaction between the anionic chromate species and protonated Fe₃O₄/rGO nanocomposites and developed a Cr(VI) – complex (eqn (18)). Light adsorption capacity of Fe₃O₄/rGO nanocomposite was increased due to the presence of more active surface sites, which hinders the electron hole pair recombination and enhances the electron generation in the conduction band. When electrons are promoted from the valence band to the conduction band, electrons are easily transferred from the conduction band of Fe₃O₄ to the rGO sheets (eqn (17)). Electrons on the rGO sheets were simultaneously captured by Cr(VI) – complex that is present on the surface and consequently aqueous solution of Cr(VI) is reduced to nontoxic aqueous solution of Cr(III) (eqn (18) and (19)). Cr(III) species in turn are adsorbed onto the Fe₃O₄/rGO nanocomposite due to the electrostatic attraction between the cation of Cr(III) and dis-protonated surface of Fe₃O₄/rGO nanocomposite or Cr(III) is released to the solution by electrostatic repulsion of cationic Cr(III) species and protonated surface of Fe₃O₄/rGO nanocomposites (eqn (18) and (19)). The schematic representation of the photoreduction of Cr(VI) is shown in Fig. 9.

Fe₃O₄/rGO + hν → Fe₃O₄ (h⁺) + rGO (e⁻) (17)

Cr(VI)_adsFe₃O₄/rGO + 3e⁻ → Cr(III)_adsFe₃O₄/rGO (18)

Cr(III)_adsFe₃O₄/rGO → Cr(III) + Fe₃O₄/rGO (19)

3.5 Reusability of the Fe₃O₄/rGO nanocomposite photocatalyst

After completing the reaction, recycling of the catalyst is the most important parameter to determine the sustainability of the catalyst. The recyclability test was performed to evaluate the efficiency of the photocatalyst and we found that the Fe₃O₄/rGO catalyst can be used repeatedly to carry out degradation of MG, MB and RhB dye molecules (dye concentration and catalyst loading was taken to be 0.1 mM and 0.5 g L⁻¹ respectively at pH 5). After degradation, separation of the photocatalyst was performed using simple magnet and washed several times with hot water and dried in a hot air oven at 100 °C for 2 h. Then the photocatalyst was mixed with fresh dye solutions and degradation process was investigated under sunlight irradiation. Similarly degradation process was monitored up to 10 cycles. No significant loss of photocatalytic activity of Fe₃O₄/rGO nanocomposite was observed. However, slight loss of catalytic activity was found due to unavailability of the active surface site on the Fe₃O₄/rGO nanocomposite surface in every cycle. In the first cycle 98.31% of MG, 96.97% of MB and 86.32% of RhB dye degradation efficiency was observed. After 10 cycles, degradation efficiency reached upto 93.45.03%, for MG, 91.56% for MB and 83.07% for RhB dye molecules. Therefore, Fe₃O₄/rGO nanocomposite shows excellent reusability towards degradation of these dye molecules as shown in the Fig. 10.

Fig. 10 Reusability study of Fe₃O₄/rGO nanocomposite for photodegradation of MG, MB and RhB.

3.6 Dissolution studies

In this study dissolution of the Fe₃O₄/rGO nanocomposite during the photocatalytic degradation experiment at pH 3, pH 5 and pH 9 was investigated. The dissolution of Fe₃O₄/rGO nanocomposite was investigated by determining the amount of dissolved iron in the aqueous solution by atomic absorption spectrophotometer. It is observed that no dissolution of iron from Fe₃O₄/rGO nanocomposite during the photocatalytic degradation of dye molecules experiment at pH 5 and also pH 9. However, it was found that 0.032 mg iron was dissolved in the solution from Fe₃O₄/rGO nanocomposite at pH 3. The dissolution of the small amount of iron from the Fe₃O₄/rGO nanocomposite at pH 3 was observed due to the protonation of oxygen atom of Fe₃O₄. No loss of iron was observed at pH 5 and pH 9 due to decreases of the H⁺ ions concentration in the solution.⁶⁸

3.7 Characterization of Fe₃O₄/rGO nanocomposite after photocatalytic degradation

The stability of the photocatalyst was established by different characterization tools after performing the photocatalytic reaction. The XRD and FTIR analysis of Fe₃O₄/rGO nanocomposite was performed before and after degradation of the MG dye molecule (Fig. 11a). The XRD peaks at 2θ values of about 30.24°, 35.62°, 43.16°, 57.20°, and 62.88° of the Fe₃O₄ nanoparticles correspond to (220), (311), (400), (511) and (440) crystallographic planes respectively [JCPDS card no. 00-003-863]. The peaks due to these planes for Fe₃O₄/rGO nanocomposite remains at same 2θ values after MG dye degradation. It is noticed that crystalline morphology of Fe₃O₄/rGO nanocomposite remains intact even after degradation of the dye molecules. The FTIR spectra of Fe₃O₄/rGO nanocomposite is shown in the Fig. 11b. The band corresponding to 1585 and 1253 cm⁻¹ appears due to C–C stretching of the aromatic ring and C–O–H bending of the phenolic group in rGO sheets. The peak at 584 cm⁻¹ appears due to Fe–O bond stretching vibration. After photocatalytic degradation of the MG dye molecules, these significant peaks remain unchanged, indicating no change in the functional groups present on Fe₃O₄/rGO nanocomposite.

Fig. 11 Characterization Fe₃O₄/rGO nanocomposite after performing photocatalytic degradation of MG (a) XRD patterns and (b) FTIR spectra, before and after photodegradation.

The typical HRTEM images and SAED pattern of Fe₃O₄/rGO nanocomposite after photocatalytic dye degradation is shown in the Fig. 12a–f. It is clearly seen that Fe₃O₄ nanoparticles were decorated on the rGO sheets with an average particle size of around 21 nm analysed from the particle size distribution curve (Fig. 12c). Lattice fringe of 0.25 nm corresponding to the (311) plane of Fe₃O₄ nanoparticles is observed. SAED pattern indicates the crystalline nature of the Fe₃O₄ nanoparticles and the crystal lattice corresponding to the (220), (311) and (400) crystallographic planes are found to be persistent with the fresh catalyst. Therefore, it is clearly observed that morphology of the recovered catalyst remains unchanged even after photocatalytic dye degradation. The characterization of the Fe₃O₄/rGO nanocomposite photocatalyst after photodegradation is a very important study as it indicates the stability of the photocatalyst after reaction as well as the performance of the catalyst.

Fig. 12 TEM images of the Fe₃O₄/rGO nanocomposites after photocatalytic degradation of the dye molecules.

4. Conclusion

Magnetically separable Fe₃O₄/rGO nanocomposite as an efficient photocatalyst, capable of degradation of several cationic and anionic dye molecules under direct sunlight irradiation was investigated. Fe₃O₄ nanoparticles were incorporated in the rGO sheets, which extends the light absorption process and enhances the degradation pathways. The morphology of the Fe₃O₄/rGO photocatalyst remains unchanged after undergoing the photocatalytic degradation process. Fe₃O₄/rGO nanocomposite effectively degrades the harmful dye molecules and also efficiently reduces mutagenic aqueous solution of Cr(VI) to nontoxic aqueous solution of Cr(III) under natural sunlight irradiation. Another associated advantage is the magnetic separability of the Fe₃O₄/rGO photocatalyst after photocatalytic degradation for further practical applications. The stability of the Fe₃O₄/rGO photocatalyst is excellent and have great potential value for the organic dye molecules degradation as well as photocatalytic reduction of toxic Cr(VI) solution. Therefore, photocatalytic degradation and reduction using Fe₃O₄/rGO nanocomposite is a most convenient, suitable and low cost effective method for further treatment of industrial wastewater.

Acknowledgements

The authors are thankful to the Council of Scientific and Industrial Research for the financial support (Project MLP 6000 WP2) and Director, CSIR−NEIST, Jorhat, for his interest to carry out the work. We also gratefully acknowledge SAIF NEHU, Shillong for the HRTEM facility. GD and PB acknowledge DST, New Delhi, India for DST-INSPIRE Fellowship grant.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25035h

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