pH-responsive modified magnetic nanoparticles for treatment of oily wastewater

Abstract

Unfortunately, with the expansion of oil industries, the damage caused by oil-in-water emulsions on the environment and people’s lives has increased. In this study, using different ratios of iron oxide nanoparticles and modified aniline (4:1 (Fe41), 1:1 (Fe11), and 1:4(Fe14)), demulsifiers were fabricated that could destabilize oil-in-water emulsions in three acidic, alkaline, and neutral environments. The characteristics of the made magnetic demulsifiers were evaluated by X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FE-SEM), etc. tests. The turbidity results, Fe41, indicate that the pH = 7 demulsifier can be reduced to below 100 NTU. Also, changing nanoparticles to modify the aniline ratio can improve the efficiency of demulsifiers in different environments. While, in pH = 4, the turbidity result for Fe41 is 153 NTU, the turbidity result for Fe14 is 114 NTU. Moreover, zeta potential, contact angle, and optical microscope tests were used to understand the demulsification mechanism better. These evaluations showed that in the neutral environment (pH = 7), electrostatic interactions play the most crucial role in the performance of the demulsifier. but in acidic conditions other parameters (such as pi-pi interaction) are also involved. Moreover, the magnetic property of the demulsifier made it possible to use this demulsifier up to four times.

Introduction

With the expansion of industrial activities, especially oil-based, a large amount of oily wastewater is produced^1,2. The oily wastewater properties is complex³. The addition of wastewater into water bodies can affect on contamination of aquatic ecosystems, decreasing water quality and human life^4,5. How to quickly and effectively deal with oil pollution has always attracted a lot of attention because it is a very common pollutant around the world that has irreparable effects on the environment⁶. Therefore, treating oil effluents, especially oil in water emulsions (o/w), is essential. These emulsions are relatively stable due to the appearance of natural surfactants in crude oil (asphaltene, resins, etc.)^7,8. There are several ways to remove stable emulsions such as electrical⁹, chemical, microwave irradiation, and methods¹⁰. However, each of those methods has advantages, but the chemical method has been used as an economical¹⁰. Chemical demulsifiers penetrate the oil and water interface film and also can interact with asphaltene¹¹. Both factors can affect emulsion stability.

Today, magnetic nanoparticles due to their environmental compatibility, excellent adsorption capacity, and low toxicity as the new materials for destabilizing oil in water emulsions have been considered by researchers¹². For this purpose, it can synthesize a composite with magnetic nanoparticles or be used as a core-shell, etc.¹².

Xu and his team used an amphiphilic, recyclable magnetic graphene oxide to destabilize the o/w emulsion, as well as the effect of different pH on the instability of the o/w emulsion¹³. In our previous work, it was studied graphene oxide with a core shell of Fe₃O₄@oliec acid as a recyclable demulsifier to separate oil from oily wastewater¹¹. Elmobarak et al. used Fe₃O₄ magnetite nanoparticles coated with a protective silica (SiO₂) shell to retrieve oil from o/w emulsions¹⁴. Wu and his team studied the effect of surface charge on demulsifier performance. They found that increasing the positive surface charge of the demulsifier improves its performance¹⁵. Peng et al. used magnetic nanorods to destroy the surfactant network around oil droplets¹⁶. Zhang and his team¹⁷ functionalized magnetic nanoparticles with highly positively charged quaternized chitosan (M-QC). This magnetic demulsifier was effective for oil droplets that stabilized with anionic surfactants. The electrostatic interaction has a critical role in the M-QC demulsification mechanism. Xu¹⁸ and et al. functionalized magnetic nanoparticle with chitosan (Fe₃O₄@CS), then grafted sodium oleate to Fe₃O₄@CS. They show that the electrostatic attraction interaction is important in the attached emulsified oil to magnetic demulsifier under neutral and acidic conditions by electrostatic attraction.

Various studies have been conducted on demulsification by iron oxide nanoparticles. However, in this study, by changing the ratio of iron oxide nanoparticles to modified aniline (it has a benzene ring and hydrophilic group that reacts with asphaltene), demulsifiers were made that can perform well in neutral acidic and alkaline environments. Also, the mechanism of demulsification in all three environments was studied by zeta potential, contact angle, and light microscopy. Moreover, to ensure the accuracy of synthesis and further characterization of the magnetic nanoparticle structure was employed X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermogravimetric (TGA), field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared spectroscopy (FTIR), vibrating-sample magnetometer (VSM), and the surface area of nanoparticles were measured with Brunauer-Emmett-Teller (BET) method. Bottle tests with turbidity measurement were used to evaluate the performance of magnetic demulsifiers in separating water from crude oil. Finally, the reusability of magnetic nanoparticles was studied.

Materials and methods

Materials

Crude oil was prepared from Iranian reservoir crude oil. Iron (III) chloride.6H₂O (FeCl₃.6H₂O, 98%), Iron (II) chloride.4H₂O (FeCl₂.4H₂O, 97%), Ammonium hydroxide (NH₄OH, 25%), N-phynyl-1,4-phenylenediamine (purity 99%), succinic anhydride, dichloromethane (CH₂Cl₂, 99%), and pure acetone were purchased from Sigma Aldrich company and used without purification.

Methods

Crude oil characterization

The crude oil used was obtained from the Oil Industry Research Institute. The interfacial tension (IFT) between crude oil and distilled water was measured with a Kruss K12 tensiometer. To determine the viscosity of crude oil was employed Anton paar. Also, for evaluation of the percentage of asphaltene in crude oil, the crude oil was mixed with n-heptane (1:40 v/v) then centrifuged at 2400 rpm for 10 min. The obtained sediment was washed with heptane and flirted. Finally, it dried in air at room temperature.

Nanoparticle characterization

Fourier transform infrared spectroscopy (FT-IR, Thermo Nicolet 100) in the range of 500–4000 cm^− 1 was used to describe and determine the functional groups. The surface morphology of nanoparticles was evaluated with a field emission scanning electron microscope (FESEM HITACHI S-4160 model). X-ray diffraction spectroscopy (XRD, Philips Xpert MPD) with Cu k radiation ( was used for the determination of crystal size and chemical phases of modified Fe₃O₄ nanoparticles. The behavior of nanoparticles against heat was investigated by thermal gravimetric test (model TG209F3 NETZSCH) in the range of 25 to 625 degrees Celsius. The water contact angle of the nanoparticles was obtained by a contact angle measuring device. The Zeta potential of nanoparticles was measured by a Malvern zetasize device. The magnetic properties of the modified Fe₃O₄ nanoparticles were measured at room temperature using a vibrating sample magnetometer (VSM, Kavir Magnetic Company). The specific surface area of the modified nanoparticles was measured using Brunauer – Emmett – Teller (BET- micropolitics TriStar II). Optical microscopic images of crude oil emulsion (O/W) were prepared to better evaluate the performance of nanoparticles. Also, for evaluation performance of modified aniline as an inhibitor of corrosion Fe₃O₄ nanoparticle was used inductively coupled plasma (dis, VISTA-PRO).

Synthesis of magnetic nanoparticle

Synthesis of aniline-COOH

This synthesis was performed using the co-precipitation method, which is summarized as follows: 0.721 g of N-phynyl-1,4-phenylenediamine and 3.5 g of succinic anhydride mixed in 30 ml of dichloromethane solvent, then the solution stirred for five hours. The obtained white residue was washed several times with dichloromethane. Finally, the resulting filter precipitate was dried under vacuum for 24 h¹⁹.

Preparation of Fe₃O₄ nanoparticles capped with aniline-COOH

To prepare Fe₃O₄@Anilin –COOH, 1.82 g of FeCl₂.4H₂O and 3.7 g of FeCl₃ .6H₂O were dissolved in 60 ml of distilled water under the influence of a mechanical stirrer. The solution was heated to 60 °C. Then 10 ml of ammonium was added to the reaction in three steps, followed by 400 mg of Aniline -COOH dissolved in 5 ml of acetone, and the reaction was continued for one and a half hours. Continued at 90 °C under a mechanical stirrer to produce a stable suspension. Finally, the resulting mixture was cooled and filtered at room temperature and dried in a vacuum for 12 h¹⁹.

Nanoparticles were prepared with different concentrations of aniline-COOH (Table 1).

Table 1.

Magnetite (Fe₃O₄) coated with different concentrations of aniline-COOH.

Nanoparticle	FeCl2 .4H2O (g)	FeCl3 .6H2O (g)	Aniline-COOH (g)
Fe41	0.8	1.6	0.4
Fe11	0.4	0.8	0.8
Fe14	0.2	0.4	1.6
Fe-0	0.8	1.6	0

Synthesis Fe₃O₄ nanoparticles

The magnetic nanoparticles were synthesized with a method described by TaKa¹⁹. Briefly, FeCl₂.4H₂O and FeCl₃.6H₂O were dissolved in distilled water and the solution was heated to 60 °C. Then it was added ammonium to the solution and the reaction continued for one and a half hours. Finally, magnetic particles were separated with a magnet and dried in the oven.

Asphaltene extraction

Asphaltene in crude oil was extracted from crude oil. Crude oil was mixed with normal heptane at a ratio of 1:40 and then refluxed for half an hour. The obtained black precipitate was separated, washed, and dried.

Oil in water emulsion preparation

To prepare the o/w emulsion, 5 ml of the crude oil and 95 ml of distilled water were mixed for 5 min and then sonicated for half an hour. This emulsion was stable for more than twelve hours.

Demulsification test

Bottle testing is a common way to measure the performance of a demulsifier in oil wastewater treatment^20–22 and water in oil emulsion^23,24. Magnetic nanoparticles were added to the stable emulsion and the bottle was shaken 100 times. Then the water was separated and introduced to the Turbidity meter.

Result and discussion

Characterization

Crude oil characterization

The various properties of crude oil are listed in Table 2. According to the API of crude oil, the crude oil that was used in this study is light oil. While the IFT value between water and aliphatic hydrocarbons is higher than 40 mN/m²⁵, the IFT value between crude oil and water is lower than aliphatic hydrocarbons can relate to the existence of the surface active material in crude oil such as asphaltene, resin, etc. The IFT value between crude oil and water was reported in this range²⁶.

Table 2.

The crude oil characterization.

Crude oil characterization	Value
API	33.8
IFT (mN/m) at room temperature	19.4
Viscosity at 305 K (cP)	2.9
Asphaltene (%)	3.3

Nanoparticle characterization

Since demulsifiers should be able to perform in acidic and neutral environments, Therefore, functionalization of Fe₃O₄ can help to reduce the dissolution of this corn in an acidic environment. For this purpose, modified aniline was used to functionalize the Fe₃O₄ nanoparticle surface. The ICP results showed that with the functionalization of magnetic nanoparticles, the concentration of Fe₃O₄ nanoparticles in the acidic environment (caused by dissolution) decreased from 44.4 ppm to 7.7 ppm.

Figure 1a shows the XRD patterns of different magnetic nanoparticles. The XRD spectrum of all nanoparticles confirms the synthesis of Fe₃O₄ in all samples (it is consistent with the reference card (JCPDS 3-0863) related to the magnetite spinel structure for Fe₃O₄). The average nanocrystal size was calculated using the Scherrer equation and presented in Table 3.

Fig. 1.

XRD pattern for all nanoparticles (a), VSM curve for all nanoparticles (b), TGA curve for Fe14 and Fe41 nanoparticles (c).

Table 3.

Average size and saturation magnetization values of nanoparticles.

Nanoparticle	Average crystal size* (nm)	Saturated magnetization (emu/g)	Average size** (nm)
Fe-0	11.75	59	18.3 ± 4.2
Fe41	10.66	62	38.5 ± 8.0
Fe11	10.27	58	38.3 ± 9.0
Fe14	8.11	45	35.7 ± 8.5

*By XRD method, **By FESEM

where D is the particle size, k is a constant (0.9), is the wavelength of the device, is half the peak width²⁷, and θ reperesent the Bragg angle²⁸.

Magnetic properties of modified Fe₃O₄ nanoparticles were analyzed by VSM (Fig. 1b). All nanoparticles had superparamagnetic behavior. The saturated magnetization (Ms) of nanoparticles is shown in Table 3. For Fe₃O₄ nanoparticles, Fe41 and Fe11 all have almost the same magnetic saturation. The magnetic saturation value for Fe14 is 45 emu/g^{− 1}, which is less than Fe₃O₄ (59 emu/g^{− 1}). The decrease in magnetic saturation for the Fe14 sample is related to the higher amount of modified aniline in the synthesis of this nanoparticle, with the complete coverage of the surface of the iron oxide nanoparticles, a magnetic dead surface has been created and the saturation magnetization has decreased. This trend was reported in several papers^11,29.

The behavior of Fe14 and Fe41 nanoparticles against heat was investigated using the TGA test in a nitrogen atmosphere (Fig. 1c). For both demulsifiers, the weight loss below 200 °C is related to the removal of adsorbed water. The second weight loss at 200 °C is attributed to removing oxygen groups placed in different positions of nanoparticles¹⁹. The weight loss occurred at about 400 °C, which is related to the breaking of the bond between aniline-COOH and iron oxide nanoparticles¹⁹. The fourth weight loss at 603 °C is attributed to the decomposition of the COO⁻coordinated group with Fe₃O₄ nanoparticles¹⁹. Also, as expected, the Fe14 sample has a more significant drop than the Fe41 sample. The TGA of Fe-0 was reported at Fig S1 in the supporting information. The result showed that the nanoparticle had a weight loss of about 6%.

The N₂ adsorption-desorption isotherm for Fe41 and Fe-0 were shown in Fig. 2a and the surface area nanoparticles were measured by BET-BJH method. The result of this method was presented that Fe41 and Fe-0 show type IV isotherm based on the IUPAC group³⁰, which shows mesoporous structure for nanoparticles. In addition, the nanoparticles show an H1-type hysteresis loop, which is related to narrow pore size distributions³⁰. The surface area of Fe-0 and Fe41 are 53.1 and 57.9 m²/g respectively. In another study, was reported that functionalized nanoparticles cause increased surface area³¹.

Fig. 2.

BET spectrum diagram of Fe41 (green) Fe₃O₄ (red) nanoparticles (a), FESEM image of Fe41 nanoparticle (b), FT-IR spectrum for all nanoparticles (c).

Figure 2b represents the FESEM image for Fe41 nanoparticles that indicate the size of particles is smaller than 100 nm. To investigate the morphology of the synthesized nanoparticles, the comparison of FESEM images between Fe-0 nanoparticles and modified Fe₃O₄ nanoparticles is shown in Fig S2 in supporting information. The particle size of all samples is smaller than 100 nm and they have spherical shape. It can be seen in Figure S2 that the Fe-0 has a smaller size than other nanoparticles.

Figure 2c shows the FT-IR spectrum of different nanoparticles. The infrared spectrum of Fe₃O₄ nanoparticles modified with various ratios of aniline-COOH shows the presence of a peak at 3441 cm^{− 1}, which is assigned to the stretching vibrations of the OH functional group²⁹. The peak at 1573 cm^{− 1} is for C-N stretching vibrations³⁰. The peak at 1402 cm^{− 1} is attributed to C = O stretching vibrations on the surface of the modified nanoparticles¹⁹. The peak at 566 cm^{− 1} represents the characteristic peak of the Fe-O stretching band^19,32.

The XPS measurements were used to investigate Fe₃O₄@Aniline –COOH by fitting the C1s, O1s, N1s, and Fe2p spectra (Fig. 3). The C1s and O1s showed an intense peak at 285 and 531 eV and N1s and Fe2p showed respectively peaks at 400 and 711 eV. The C1 XPS spectra showed two peaks at 284.3 and 285.6 related to C-H, C-C, and C=C functional groups and a peak at 287.2 related to C=O functional group³³. The appearance of the Fe2p (Fe2p_1/2 and Fe2p_3/2) confirmed the presence of Fe₃O₄ nanoparticles in the composition^33,34. The Fe2p_1/2 peak is fitted by one signal in 725 eV. The peak of Fe2p_3/2 consists of two peaks at 711 and 713 eV, representing Fe³⁺(oct) and Fe²⁺(oct)³³. The O1s XPS spectra showed two peaks at 530.2 and 532.3 eV relating to oxygen in Fe₃O₄ lattice and in carboxylate oxygen of coating materials on the surface of Fe₃O₄ confirmed that that modified aniline (aniline-COOH) linked by the oxygen atoms³³. The less intense peak of N1s XPS spectra consists of two peaks at 399 and 401.2 eV respectively can be related to nitrogen atoms in the -NH functional group and in iminium and radical cation amin (N⁺) groups³⁵.

Fig. 3.

XPS spectrum of Fe41 nanoparticles (a), C1s (b), O1s (c), Fe2p (d), and N1s of Fe41 (e).

Demulsification test

The bottle test was used for the evaluation demulsification performance of various nanoparticles. Different dosages of nanoparticles (from 100 to 650 ppm) were added to fresh o/w emulsion. The turbidity measurement results are given in Fig. 4a-c. Fe41 has the best performance (lowest turbidity) among all nanoparticles in the neutral environment. This demulsifier has reduced the turbidity of separated water below 100 NTU (which is a critical point). Also, by increasing the concentration up to 650 ppm, no overdose was observed, which indicates the non-formation of a stable film around the oil droplets by the demulsifier.

Fig. 4.

The remaining turbidity in the separated aqueous phase after demulsification in pH 7 (a), 4 (b), 8 (c). The optical microscope images of oil droplets in O/W emulsion before (d) after adding Fe41 nanoparticles (e).

To study the demulsification process, the morphological changes of the emulsion droplets were investigated before and after adding Fe41 nanoparticles (in pH = 7). It was observed using an optical microscope (Fig. 4d,e). As shown in Fig. 4d,e, for the emulsified oil effluent, oil droplets were uniformly dispersed in the water phase. After treatment by Fe41 nanoparticle, a minimal amount of oil droplets remained in the separated phase water. The demulsification of oil droplets from oily wastewater using Fe41 nanoparticles has led to a noticeable reduction of the oil content in the separated water. The picture of oil-in-water emulsion and separated water after adding a demulsifier is shown in Fig S3 in the Supporting Information.

In addition to the neutral environment, the performance of magnetic demulsifiers was also investigated in acidic and alkaline environments. In the acidic environment (Fig. 4b), the best performance belongs to Fe14 which has been able to reduce the turbidity up to 113 NTU. In this acidic environment (and of course in the alkaline environment), unlike the neutral environment, the phenomenon of overdose is seen.

In an alkaline environment, the best performance is related to Fe-0. Although the quality of the separated water in the alkaline environment is lower than in the other two environments, still the demulsifier has been able to destabilize the emulsion effectively (it has reduced the turbidity from above 800 to about 160 NTU).

Proposed mechanism

For a better understanding demulsification mechanism, was employed several experimental tests. The zeta potential of various nanoparticles (in different pHs) and oil droplets were measured. Moreover, the water contact angle nanoparticles and optical microscope were evaluated.

Figure 5a reveals that two of the nanoparticles have negative potential and two of the nanoparticles have positive potential (in a neutral environment). Also, the results showed that oil droplets in water have a negative potential, which is consistent with another study³⁶. Since the potential of oil droplets is negative, nanoparticles with positive potential can neutralize the electrostatic repulsion between the droplets and the droplets join each other. The turbidity measurement results also show that F41 (with the most positive potential) had the best result. On the other hand, the Fe14 nanoparticle with the highest amount of negative potential has the weakest demulsification performance (Fig. 5a). It seems at the neutral pH the zeta potential plays a vital role in demulsification. The contact angle of demulsifiers shows that all demulsifiers are hydrophilic (Fig. 5b), Therefore, this factor cannot play a key role in differentiating the performance of demulsifiers.

Fig. 5.

Zeta potentials of different nanoparticles at pH 4, 7, 8 and emulsified oil droplets (a), the contact angle of modified aniline and different nanoparticles (b), and the turbidity results for recycle demulsification test for Fe41 nanoparticles (c).

By changing the pH of the environment to acidic or basic, the demulsification mechanism changes. In the acidic environment, all nanoparticles have a positive potential, and the cause of this phenomenon probably can be attributed to the protonation of functional groups²⁹. The electrostatic interaction between all demulsifiers and oil droplets is attraction therefore, the role of other factors in demulsification becomes more critical. Fe14, which has the highest amount of modified aniline, has the highest pi-pi interaction between the demulsifier and asphaltene. Ren and his team showed that this interaction has a positive effect on demulsifier performance³⁷. Moreover, due to the present hetero atom in the asphaltene structure^38–40, modified aniline can react with asphaltene. Also, more presence of modified aniline helps to Fe14 more stable in acidic conditions than other demulsifiers.

In alkaline environments, the performance of all nanoparticles was severely reduced, and they could not separate water and oil. The cause of this phenomenon can be attributed to the electrostatic repulsion between oil droplets (with negative potential) and nanoparticles with negative potential in an alkaline environment.

In general, the reasons can be mentioned for reducing demulsifier performance in acidic and alkaline environments compared to neutral environments. In acidic conditions, asphaltene film is more complex, and the resin has a more significant effect on the hardness of the film in an alkaline environment¹⁰ .

Recycling test

Unlike surfactants, magnetic nanoparticles can be reused several times. For evaluation recycle capability of Fe41, this nanoparticle was isolated with an external magnet after the demulsification process. Then, it was washed with toluene several times to remove residual crude oil. The optimal pH value for recycling tests was around 7. Figure 5c presented the amount of turbidity of the water after demulsification with recycled nanoparticles. The water turbidity after 3 times recycled is reasonable and lower than that of other nanoparticles. The amount of turbidity remaining in the separated aqueous phase of the emulsion was 137 NTU after re-using the Fe 41 nanoparticle three times.

Conclusion

In this study, we have successfully destabilized oil-in-water emulsion in natural and acidic conditions. We find that changing magnetic nanoparticles to a modified aniline ratio can obtain a good demulsifier in acidic and neutral conditions. The results of the turbidity test indicate that Fe41 can reduce water turbidity under 100 NTU (in pH = 7). Although the removal of electrostatic repulsion between oil droplets plays an important role in destabilizing o/w emulsions, this study revealed that the demulsification mechanism can change with variations in the pH of the environment. While in the neutral environment, electrostatic interactions play the most crucial role in the instability of the emulsion, as the pH of the environment decreases to an acidic medium, other factor such as pi-pi interaction are more critical. Based on the obtained results in the alkaline environment (zeta potential and demulsification efficiency), it is recommended that future studies use materials for functionalizing the magnetic nanoparticles that can provide a positive or near-zero zeta potential under alkaline conditions.

Author contributions

Soheila Javadian: supervision, validation, resources, review & editing, Project administration, Funding acquisition. Aynaz Nobakht: interpretation, methodology, writing, data curation, investigation. S.Morteza Sadrpoor: writing, investigation, interpretation. Sedigeh Kiani: interpretation. Amir Hossein Saeedi Dehaghani: methodology.

Funding

The fund for this study has been provided by Tarbiat modares university.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.