Synthesis of MRGO Nanocomposites as a Potential Photocatalytic Demulsifier for Crude Oil-in-Water Emulsion

Abstract

Oil-in-water (O/W) emulsion has been a major concern for the petroleum industry. A cost-effective magnetite-reduced graphene oxide (MRGO) nanocomposite was synthesized to study the demulsification process of emulsion using said nanocomposite under solar illumination. Characterization data show that the magnetite was successfully deposited on reduced graphene oxide through redox reaction at varying loading amounts of magnetite. Demulsification of the O/W emulsion using MRGO nanocomposite shows that in general the demulsification efficiency was dependent on the loading amount of Fe₃O₄ on the RGO sheet. It was proposed that the surfactant hydroxyl groups have an affinity towards Fe₃O₄, which the loading amount was directly proportionate to available active site in Fe₃O₄. As the loading amount increases, charge recombination centers on the RGO sheet would increase, effectively affecting the charge distribution within MRGO structure.

1. Introduction

The oil and gas industry is one of the sectors with the heaviest industrialized resources in the world and the petroleum industry has long been faced with multitude of problems to improve the profitability and production. One of the issues is that the oily wastewater in the industries contains emulsion of water and oil liquids which was formed during the crude oil extraction process. Emulsions are classified into two classes, and the one focused in this work is oil-in-water (O/W) emulsion. O/W emulsion contains oil droplets encapsulated by the water matrix or dispersed water or surfactant forming an interfacial film at the oil and water interface which emulsifies the two immiscible liquids [1]. Emulsions have been a major concern in the petroleum industry as it affects the production and the pipeline system in the production [2]. Hence, in this work, a magnetite–carbon-based nanocomposite is studied for its efficiency and efficacy in demulsifying O/W emulsion.

Graphene, a revered material for its adsorption properties and in-plane conductivity, has shown excellent results in photocatalytic effect when composited with other materials [3]. Graphene oxide (GO) sheets has a π-π stacking within its sp² hybridized network and due to their large surface area and broad sp² network, they are promising material for organic compounds photodegradation [4] and demulsification [5]. However, separation of the material and recyclability has been an issue for graphene-based nanocomposites. To solve the separation issue, magnetite was suggested as Fe₃O₄ has shown excellent photocatalytic properties in organic compounds degradation [6] and the recyclability of the material has been promising [7]. Some previous studies [8,9,10] have shown that magnetite (Fe₃O₄) materials have favorable properties due to their low degree of cytotoxicity and good biocompatibility, especially during synthesis with surface coatings. In addition, magnetite has shown excellent adsorptive properties on organic compounds [11]. However, Fe₃O₄ has a strong tendency to agglomerate [12] and that is a major concern as it would inhibit the photocatalytic and adsorptive properties.

By combining both GO and Fe₃O₄ through deposition of magnetite on GO sheets through redox reaction, the magnetite-reduced graphene oxide (MRGO) nanocomposite shows improved stability and higher adsorption rate than pristine GO sheets and Fe₃O₄ nanoparticles [13]. The MRGO nanocomposite prevents both problems with the agglomeration of Fe₃O₄ nanoparticles and the restacking of graphene sheets [14], and by reducing GO to reduced graphene oxide (RGO), it enhances the charge separation properties of the material, suppressing charge recombination at Fe₃O₄ active sites [15]. Through the synergy of both materials, MRGO nanocomposites show improved organic compounds degradation under sunlight irradiation [16].

Furthermore, MRGOs promised easy separation of nanocomposites from the emulsion through magnetic means, and the reusability and recyclability of the MRGOs have shown promising results as well, indicating that the MRGOs can be a cost-effective material for demulsification usage [17,18]. A high demulsification factor was observed when the interfacial film of the oil–water emulsion was significantly disrupted [19]. Hence, in this work, the demulsification process of O/W emulsion using MRGO nanocomposite under solar irradiation is studied. To our knowledge, the use of MRGO as a photocatalytic demulsifier for crude oil-in-water emulsion has not been reported yet.

2. Materials and Methods

2.1. Material

Graphite powder (99.99%) was purchased from Sigma Aldrich, USA. Sulfuric acid (H₂SO₄, 98%), phosphoric acid (H₃PO₄, 85%), potassium permanganate (KMnO₄, 99.9%), hydrogen peroxide (H₂O₂, 30%), hydrochloric acid (HCl, 37%), ammonia solution (NH₄OH, 25%), Iron (II) chloride (FeCl₂·6H₂O), and surfactant Tween 60 were supplied by Merck, Germany. Tapis crude oil was obtained from Petronas Refinery at Melaka. Deionized water was used a Millipore Milli-Q water purification system with a resistivity of 18.2 MΩ cm⁻¹.

2.2. Synthesis of GO

GO was prepared and the magnetite-reduced graphene oxide was synthesized using an in situ chemical synthesis method [20]. An amount of 3 g of graphite flakes was oxidated with 400 mL of H₂SO₄, 40 mL of H₃PO₄ and 18 g of KMnO4 by using simplified Hummers’ method [21]. The chemicals were mixed by using a magnetic stirrer and stirred for 5 min. However, the mixture was stirred for 3 days to ensure complete oxidation of graphite. During the oxidation, the color of mixture changed from dark purplish green to dark brown. H₂O₂ solution was added into the mixture to stop the oxidation process and the color of mixture changed to bright yellow, indicating a high degree of oxidation of graphite. The mixture solution was washed with 1 M of HCl aqueous solution and deionized water alternatively by using centrifugation techniques. The graphite oxide solids were obtained and dried.

2.3. Synthesis of MRGO Nanocomposites

MRGO nanocomposites were synthesized using different mass ratio of graphene oxides to irons salts, with the ratio of 1:5, 1:10 and 1:20, and differentiating the MRGOs as MRGO-5, MRGO-10 and MRGO-20, respectively.

The synthesis route for all MRGO nanocomposites was strictly maintained to be the same and only deionized water was used during the whole synthesis and experimental procedure.

Firstly, 0.5 g of graphene oxide was added into 200 mL of deionized water, and the GO aqueous suspension was ultrasonicated for 10 minutes or until all GO was finely dispersed in the aqueous suspension. Once done, the GO suspension was placed onto a magnetic stirrer and the pH of the GO suspension was adjusted to about 11–12 using 25% ammonia solution added dropwise. Secondly, for MRGO-5, 2.5 g of FeCl₂ was dissolved in 100 mL of deionized water and added dropwise into the GO suspension. For MRGO-10 and MRGO-20, 5.0 g and 10.0 g of FeCl₂ were used, respectively.

The mixture solutions were then left for 12 h of stirring to ensure proper deposition of magnetite on GO to form MRGO nanocomposites. After 12 h, the black precipitation of the mixture solutions was extracted and washed with deionized water using a centrifuge. The centrifuge setting was 7000 rotations per minute with the rotation period of 10 minutes. The washing procedure was repeated for 3 times to wash out the extra ammonia present in the solution. The slurry was dried at 40 °C for 12 h, and the dried precipitation was grinded to obtain the powdered form of MRGO nanocomposites.

The entire process is repeated without GO to synthesize magnetite only through redox reaction in alkaline solution.

2.4. Preparation O/W Emulsion

The O/W emulsion is prepared by mixing 10 wt% of surfactant Tween 60, 10 wt% of Tapis crude oil and 80 wt% of deionized water. The physical and chemical properties of Tapis crude oil are tabulated in

Table 1. Physical properties of Tapis crude oil.

Parameter	Value
Density	0.827 g cm−3
Viscosity	0.028 Pa s
API gravity	43.0
Surface Tension (at 30 °C)	30.30 mN m−1
Interfacial Tension (at 30 °C)	28.80 mN m−1

and

Table 2. Chemical properties of Tapis crude oil.

Parameter	Value (wt%)
Saturates	60.00
Aromatics	25.00
Resins	13.50
Asphaltenes	1.50

, whereas Figure 1 shows the chemical formula of surfactant Tween 60. The mixed solution is then placed in a high-speed mixer for 10 utes to form the O/W emulsion.

2.5. Characterization of MRGOs

An X-ray diffractometer (XRD Brucker D8 advance diffractometer) was used to verify the complete synthesis of GO and MRGOs with Cu Kα radiation (λ = 1.5406 A) in the range of 2θ = 5°–80° with a step size 0.01°/step and exposure time 200 s/step. Then, Field-emission Scanning Electron Microscope (ZEISS Leo Supra 55, FESEM) was used to study the surface morphology of MRGO and verified the attachment of magnetite on the RGO sheets with an accelerating voltage of 5 kV. X-ray Photoelectron Spectroscopy (Thermo Scientific K-Alpha instrument, XPS) was used to study the composition information of GO and MRGOs. Measurements were used Al K_α (hv = 1468.6 eV) as the radiation source. Further on, Fourier-transform Infra-red Spectrometer (Perkin Elmer Spectrum BX, FTIR) was used to determine the surface bonding of the synthesized GO and MRGOs with the wavelength range of 400–4000 cm⁻¹. Raman spectroscopy (Horiba Jabin Yvon HR800, RAMAN) was used to characterize the structural properties of synthesized MRGOs with 514.5 nm laser. Lastly, a vibrating sample magnetometer (DMS Model 10, VSM) was used to evaluate the magnetic properties of the synthesized MRGOs.

2.6. Demulsification Test

The test samples were prepared by mixing 1 mL of prepared O/W emulsion and 0.05 g of MRGOs into 19.4 mL of deionized water filled in a capped bottle. A standard solution without MRGO was prepared as well. The prepared test bottle samples were sonicated at a high intensity for 10 mins; where the MRGOs settled down at bottom of the bottle, the bottle should be swirled by hand to ensure proper distribution and mixing of the materials. After sonication, the bottles were mounted onto a vertical bottle shaker with 500 oscillation/min, under solar lamp illumination with varying intensities (0, 200, 400, 600, 800 and 1000 W/m²) for 30 mins. The setup was as shown in Figure 2. Then, the bottles were removed from the shaker and placed on magnets for rapid settling of the MRGOs. The test samples were then left overnight on magnets for the mixture to stabilize at ambient conditions.

The demulsification capability of MRGOs was determined by the residual oil content in the extracted water above the settled MRGO in the test samples. The oil concentration in standard and demulsified samples were obtained and compared from the standard curve of UV–visible absorption spectra by measuring the absorbance intensity at 228 nm. The demulsification efficiency under varying solar intensities was calculated by the following equation:

Demulsification efficiency = (1−C_i/C_o) × 100%

where C_o is the initial concentration of oil content in the test sample and C_i represents the final concentration of oil content in the test sample.

3. Results and Discussion

3.1. XRD Analysis

Figure 3 displays the XRD patterns of GO, MRGO-5, MRGO-10 and MRGO-20; it shows the diffraction peak profiles of GO and MrGO from 5° to 75°. The characteristic peak of GO was observed at 10.3° corresponding to (001). However, for the MRGO nanocomposites, there are six main characteristic peaks were observed at 20°, 30.2°, 43.4°, 53.7° and 62.8°, which are corresponding to diffraction indices of (220), (311), (400), (422), (511) and (440) crystal planes of Fe₃O₄ [22]. Additionally, peak intensities of Fe₃O₄ were observed to strengthen with the increase in the loading ratio of Fe₃O₄ on GO sheets, suggesting the successful deposition of Fe₃O₄ on RGO sheet through redox reaction at varying ratios. At high GO loading ratio, GO may hinder the crystallization of Fe₃O₄, while at a high Fe₃O₄ loading ratio, the XRD patterns are similar to only nanoparticles, as seen in MRGO-20.

3.2. FESEM Analysis

Figure 4a shows the FESEM images of GO and Figure 4b–d show FESEM images of MRGO-5, MRGO-10 and MRGO-20. As observed from Figure 4a, GO sheets were associated with a rough and irregular wrinkled structure. This might be due to the oxidation of graphite where different oxygen functional groups were being introduced on the surface using the improved Hammer method. Figure 4b–d illustrate that all the Fe₃O₄ deposited on RGO sheets was spherical in shape and well dispersed across the sheets. By introducing Fe₃O₄ nanoparticles on the RGO sheets, the restacking of GO sheets was prevented [23] and the layering of RGO sheets can be well observed in Figure 4b–d. While Fe₃O₄ tends to aggregate, and the tendency would strengthen with increasing loading ratio [24], the deposition on RGO sheets significantly reduced the conglomeration of Fe₃O₄ nanoparticles in all MRGOs samples.

Moreover, as recorded in

Table 3. EDX analysis of GO and MRGOs.

	Element	Weight (%)	Atomic (%)	Atomic Ratio (Fe/c)
GO	C	50.34	57.45
	O	49.66	42.55
MRGO-5	C	23.27	38.77
	O	37.47	46.90	0.37
	Fe	39.26	14.33
MRGO-10	C	14.12	26.29
	O	39.42	55.07	0.71
	Fe	46.47	18.64
MRGO-20	C	9.74	20.85
	O	32.75	52.65	1.27
	Fe	57.51	26.50

, it was observed that as the loading ratio of Fe₃O₄ increases, the weight percentage of Fe increases proportionately as well. The atomic ratio of Fe to C for MRGO-5, MRGO-10 and MRGO-20 was 0.37, 0.71 and 1.27, respectively, increasing in an approximately two-fold manner. As the Fe content is more than C in MRGO-20, and as observed in Figure 4c, the synthesized MRGO was similar to magnetite in appearance which agreed with the XRD results.

3.3. Raman Analysis

Figure 5 shows the Raman analysis of GO, MRGO-5, MRGO-10 and MRGO-20. D-band signal and G-band signal of GO was observed at 1369 cm⁻¹ and 1585 cm⁻¹, respectively. On the other hand, Raman spectra of MRGO-5, MRGO-10 and MRGO-20 also exhibit a prominent D-band signal around 1353 cm⁻¹ and G band signal around 1586 cm⁻¹. D-band signals are related to the sp³ defects in GO sheets, while G-band signals are related to sp² in lane vibration of C-C bond in the RGO sheets [25]. As observed in Figure 5 and recorded in

Table 4. I_D/I_G ratio of GO, MRGO-5, MRGO-10 and MRGO-20.

ID/IG	GO	MRGO-5	MRGO-10	MRGO-20
	0.9625	1.0077	1.0181	1.0193

, the I_D/I_G ratio increased from 0.9625 (GO) to 1.0193 (MRGO-20) after the in situ chemical synthesis. The result indicated the presence of sp³ reduces within the sp² carbon network upon the reduction of GO [26], thereby confirming the formation of RGO sheets in MRGO nanocomposites. It was suggested that the D-band is in fact slightly higher than the G band signal for all MRGOs. Deposition of magnetite through reduction of GO has altered the structure of GO, resulting in higher number of sp³ defects in the RGO sheets [27]. Furthermore, a magnetite signature was observed around 200–450 cm⁻¹ for MRGO-20 [28], specifically at 217 cm⁻¹, 286 cm⁻¹ and 393 cm⁻¹, at which the signal at 393 cm⁻¹ was due to [OH] groups linked to Fe³⁺ on the magnetite surface [29].

The result strongly suggested the successful deposition of magnetite through redox reaction on the RGO sheets.

3.4. FTIR Analysis

Figure 6 shows the FTIR spectra for GO, MRGO-5, MRGO-10 and MRGO-20. The FTIR spectrum of GO had five characteristic peaks, namely, OH out-of-plane bending at 560 cm⁻¹, CO-O-CO at 1074 cm⁻¹, epoxy C-O at 1218 cm⁻¹, aromatic C=C stretching at 1577 cm⁻¹ and hydroxyl (-OH) at 3400 cm⁻¹. The presences of oxygenated functional groups at GO indicated that the graphite underwent oxidation. For MRGO composites, absorption spectrum observed at 3400 cm⁻¹ was attributed to the O-H stretching vibration of C-OH group in RGO, while the spectrum at 1577 cm⁻¹ was related to the C-OH bending in the RGO sheets [30]. Moreover, a peak was observed at 1220 cm⁻¹ suggesting the C-O stretching vibration of epoxy groups or rings in the RGO sheets, and the peak at 618 cm⁻¹ was the in-plane deformation vibration of aromatic aldehyde [31]. Notice that the 618 cm⁻¹ peak became more significant as the loading ratio of Fe₃O₄ increases, suggesting the deposition of magnetite disrupted the structure of the RGO plane. Lastly, the adsorption peak at 560 cm⁻¹ was depicted as the Fe-O vibrations in Fe₃O₄ [32], which increases in absorption intensity as the loading ratio of magnetite increases. This confirmed the interaction between magnetite and RGO sheets.

Table 5. FTIR analysis of GO and MrGOs.

Peak Value (cm−1)	Functional Groups	GO	MrGO
3400	Hydroxyl-OH	Yes	Yes
1577	Aromatic C=C stretching	Yes	N/A
1218	Epoxy C-O	Yes	Yes
1074	Anhydride group CO-O-CO	Yes	N/A
618	Fe-O	N/A	Yes
560	Fe-O/OH out-of-plane bending	OH out-of-plane bend	Fe-O

shows the comparison of functional groups between GO and MRGOs.

3.5. XPS Analysis

Figure 7a shows the wide scan XPS survey of MRGOs where C1s and O1s peaks at binding energies of 285 eV and 530 eV, respectively, were exhibited. As for the peak characteristic of Fe₃O₄, Fe2p_3/2 and Fe2p_1/2 peaks were observed at 711 eV and 725 eV, respectively [33]. In Figure 7c,d, deconvolution peaks of C1s show C-C (aromatic) bonds, C-O bonds, C=O bonds and π-π* satellite bonds signal at 285 eV, 287 eV, 289 eV and 291 eV, respectively [34,35]. A signal of π-π* satellite bonds at 291 eV indicates the successful reduction of GO sheets to RGO sheets, as RGO has more sp² hybridized zone due to the restoration from the reduction process [35]. The intensity of C1s peak decreases as the loading ratio of Fe₃O₄ increases and is deposited on the RGO sheets. Meanwhile, in Figure 7e,f, deconvolution peaks of O1s indicated Fe-O bonds, C=O bonds and C-O bonds at 530 eV, 531 eV and 533 eV, respectively. The intensity of Fe-O peak at 530 eV was observed to increase significantly for MRGO-20 as more Fe₃O₄ was deposited. Additionally, in both C1s and O1s deconvoluted peaks, C-O peaks were observed to decrease significantly in MRGO-20, which implied an increase in conductivity within the RGO sheets as more sp² bonds were restored, thereby enhancing its charge conductivity [36].

3.6. VSM Analysis

Figure 8 shows the VSM magnetization characteristic of MRGO-5 and MRGO-20. MRGO-5 and MRGO-20 exhibit magnetic saturation of 40.7 emu/g and 63.5 emu/g, respectively, inferring the susceptibility to magnetic field is proportionate to the concentration of Fe₃O₄ deposited on the RGO sheets. Further confirming the successful deposition of magnetite on GO sheets through redox reaction at varying ratios. Moreover, the very low coercivity value of the samples indicates that the MRGOs show soft magnetic material features, exhibiting superparamagnetic response and well isolated single domain particle properties [37]. Both have a significant degree of magnetization to allow them to be separated from the emulsion by a magnet after the demulsification process.

3.7. Demulsification Mechanism

Effective demulsification was proposed to be the disruption and breaking up of the interfacial film of Tween 60 between the oil and water interface. The Tween 60 structure has more hydroxyl functional groups, which have adsorptive affinity on Fe₃O₄ nanoparticles [7]. Hence, the demulsification efficiency of MRGOs was seen to increase with the loading ratio of magnetite on RGO sheet when there is no solar irradiance, indicating that Fe₃O₄ nanoparticles played an important role in demulsifying the prepared emulsion.

In Figure 9a, the demulsification efficiency of pure magnetite nanoparticles was about the same across the varying solar intensities, at 42–44% efficiency with a slight increment in line with the intensity. Pure Fe₃O₄ nanoparticles do not react with visible light wavelength range, hence it is postulated that the slight increment may be due to the following two reasons. (a) Photothermic properties of Fe₃O₄ due to visible light—near infrared spectrum range, which involves micro-heating at the Fe₃O₄ active sites [38]; (b) Fenton-like mechanism due to visible light—ultraviolet spectrum range, which involves the production of oxygen peroxide radicals, O₂ and hydroxyl radicals, OH, from the photocatalytic effect of MRGOs under the influence of solar irradiance [39,40], of which both radicals have high oxidative potential on organic compounds and enhance the demulsification efficiency.

In Figure 9b, the demulsification efficiency shows an increasing trend with the solar intensity. While the adsorptive capability of MRGO-5 was weak due to a low Fe₃O₄ loading amount, the MRGO nanocomposites show a photocatalytic effect in the demulsification process. The synergy of Fe₃O₄ with RGO sheets augmented the absorption wavelength range of the overall material and facilitated the photocatalytic properties of MRGOs [26]. At the optimum loading ratio of Fe₃O₄, magnetite reacted with the adsorbed hydroxyl group to produce hydroxyl radicals which would further disrupt the oil–water interfacial film of Tween 60 through oxidative reaction [26].

Meanwhile, in Figure 9c, it was expected that the increment in Fe₃O₄ loading amount would improve the demulsification efficiency by increasing the radicals’ production [41], but there was an optimum loading amount of Fe₃O₄ for the effective photocatalytic effect to occur [6]. While the adsorption capability of MRGO-10 has shown significant improvement, excess content of Fe₃O₄ introduced new charge recombination at Fe₃O₄, thus reducing the availability of charges within MRGOs [24,42]. That effectively disrupted the affinity of Fe₃O₄ toward hydroxyl group of Tween 60, decreasing the demulsification efficiency.

Figure 9d shows the demulsification efficiency of MRGO-20, at which it was not reactive toward solar irradiance; as magnetite excessively covered most of the RGO sheets, the MRGO nanocomposites behaved similarly to pure magnetite nanoparticles. However, due to the synergy with RGO which prevented the aggregation of Fe₃O₄ nanoparticles, it is suggested that there are more active sites of Fe₃O₄ available for adsorption as compared to pure Fe₃O₄ nanoparticles only. Hence, an enhanced demulsification capability independent of the presence of solar irradiance was observed.

4. Conclusions

Magnetite-reduced graphene oxide nanocomposites were successfully synthesized as verified in the characterization of MRGOs. In the demulsification test, the demulsification efficiency of MRGOs was observed to increase proportionately with the loading amount of Fe₃O₄ on RGO sheets. A positive photocatalytic effect under solar irradiance was observed only with a low loading amount of Fe₃O₄ on RGO sheets, such as in MRGO-5. Hence, it was concluded that while the demulsification efficiency was dependent on the loading amount of Fe₃O₄ on RGO sheets, only a low and optimal loading amount of Fe₃O₄ would show a positive photocatalytic effect.