A facile method for emulsified oil-water separation by using polyethylenimine-coated magnetic nanoparticles
Oil spills and oily wastewater discharges from ships and industrial activities have serious impacts on the environment and human health. In this study, a class of easy-to-synthesize polyethylenimine (PEI)-coated Fe3O4 magnetic nanoparticles (MNPs) was successfully synthesized via a one-step coprecipitation method. The synthesized PEI-coated Fe3O4 MNPs were characterized by using multiple technologies and applied in emulsified oil-water separation for the first time. It was found that the PEI effectively tuned the surface charge and wettability of MNPs. As a result, the PEI-coated MNPs could successfully assemble at the oil-water interface and promote the coalescence of oil droplets, thereby facilitating the subsequent magnetic separation. Results showed that the oil-water separation performance was superior and enhanced with the increase of ionic strength. Recycling experiment indicated that the PEI-coated MNPs could be reused up to six times without showing a significant decrease in separation efficiency. All of these results suggested that the PEI-coated MNP could potentially be used as a class of promising nanomaterials for emulsified oil-water separation.
KeywordsMagnetic nanoparticles Polyethylenimine Wettability Oil-water separation Recyclability
In recent years, a significant amount of emulsified oily wastewaters is produced in industrial processes involving petroleum, machining, pesticides, pharmaceuticals, essential oils, and flavors (Liu et al. 2017; Zhang et al. 2016a). Emulsified oily wastewaters can cause serious environmental problems; thus, many efforts have been focused on emulsified oil-water separation. Since the emulsified oil droplets are very small (less than 20 μm) and stable (Zhu and Guo 2016; Duan et al. 2015), it is inefficient to separate them from aqueous media by using conventional methods, such as flotation, centrifugation, chemical coagulation, and adsorption separation (Lü et al. 2017).
Nanotechnology is an increasingly important field of technology. Fe3O4 magnetic nanoparticles (MNPs) are attractive for their inherent low toxicity and good magnetic property (Yang et al. 2017; Tempesti et al. 2014). Recently, the application of MNPs in solving oil-water separation problems has been paid more and more attentions. For example, magnetic superhydrophobic particles were prepared to achieve floating oil-water separation (Chen et al. 2013; Yu et al. 2015a, b). A majority of superhydrophobic nanoparticles are incapable of separating emulsified oil due to their poor dispersibility in aqueous media (Zhang et al. 2016b), although a breakthrough on using spiky superhydrophobic particles to achieve demulsification was reported recently (Chen et al. 2018). In order to efficiently separate the emulsified oil, one of the most important features of MNPs is that they are able to disperse in aqueous phase and further transfer to the oil-water interface (Zhang et al. 2016b). As a consequence, it is important for these MNPs to have surface amphiphilicity. The surface wettability of MNPs can be modified by surface adsorption or chemical grafting. For example, poly(N-isopropylacrylamide) and poly(2-dimethylaminoethyl methacrylate)-grafted MNPs were fabricated to harvest emulsified oil (Chen et al. 2014; Lü et al. 2016; Wang et al. 2015). It was found that complete oil-water separation could be achieved by controlling the temperature or pH of emulsions. Li et al. prepared demulsifier 5010-coated MNPs to remove the emulsified oil droplets (Li et al. 2014). Xu et al. (2018) synthesized amphiphilic and magnetic expanded perlite for separating emulsified oil from aqueous media. Moreover, cyclodextrin-grafted MNPs could not only separate emulsified oil droplets from aqueous media but also remove emulsified water droplets from oil phase (Zhang et al. 2016b). However, the synthesis methods for these MNPs are multistep and need organic solvent, making scale-up difficult and expensive.
Recently, oleic acid (OA)-coated (Liang et al. 2014, 2015) or polyvinylpyrrolidone (PVP)-coated Fe3O4 MNPs (Palchoudhury and Lead 2015; Mirshahghassemi et al. 2017) were synthesized via a one-step method. The synthesized MNPs exhibited good oil-water separation performance. Nevertheless, these oil-water separation processes were time-consuming, since the mixing of MNPs and oily wastewater involved sonication or several hours of shaking to obtain a satisfactory separation effect. Therefore, there remains a need to develop more efficient MNPs and better understand their demulsification behaviors. It is well known that silica and laponite are very hydrophilic; however, polyethylenimine (PEI) effectively tuned its surface wettability, and hence, the resulting hybrid nanoparticle could efficiently emulsify the oils (Williams and Armes 2012, 2014; Williams et al. 2014). Actually, using PEI alone could also emulsify some oils (e.g., sunflower oil), implying its interfacial activity (Williams and Armes 2012). Meanwhile, PEI is also a kind of cationic water-soluble polyelectrolyte due to the protonation of amine groups. Therefore, the PEI can also regulate the surface charge of MNPs. Consequently, the PEI-coated Fe3O4 MNPs have great potential for sorption at oil droplets surface, thereby facilitating the magnetic separation of emulsified oil from aqueous media.
Up till now, several methods have been reported to prepare PEI-coated Fe3O4 MNPs. For example, PEI-coated Fe3O4 MNPs were successfully synthesized by using a one-step solvothermal method (Wang et al. 2013; Jiang et al. 2016). However, this method required organic solvent and high temperature, which limited its industrial production. Moreover, a multistep synthesis method was also used. Fe3O4 MNPs were usually synthesized via a coprecipitation process, followed by surface coating with citric acid and PEI (Wang et al. 2009). In this study, the PEI-coated Fe3O4 MNPs were synthesized via a one-step coprecipitation process. This method was facile and its reaction condition was mild. Moreover, although the PEI-coated MNPs have been applied in biological field and heavy metal adsorption, to our best knowledge, this is the first report of using PEI-coated Fe3O4 MNPs for emulsified oil removal. Accordingly, the synthesized MNPs were characterized by using various technologies and their demulsification efficiencies were investigated in detail as a function of dosage, pH value, ionic strength, oil type, and recyclability.
Iron(III) chloride hexahydrate (FeCl3·6H2O), iron(II) chloride tetrahydrate (FeCl2·4H2O), PEI (branched, MW = 70,000), toluene, and sodium hydroxide (NaOH) were purchased from Aladdin Chemistry (Shanghai, China). Hydrochloric acid (HCl) was supplied by Zhejiang Sanying Chemical Reagent Co., Ltd. Ethanol was purchased from Hangzhou Gaojing Fine Chemical Co., Ltd. These chemicals were of analytical grade and used without further purification. The deionized water was used throughout the experiment. A commercially available diesel was obtained from Sinopec, while soybean oil and olive oil were obtained from a local supermarket.
Nanoparticle synthesis and characterization
PEI-coated Fe3O4 MNPs were synthesized using a one-step coprecipitation method. Firstly, 1.0 g of PEI was dissolved in 250 mL of 1.5 mol/L sodium hydroxide solution that was heated in a glass reactor to 80 °C while purging with nitrogen for 30 min. Subsequently, 10.81 g of FeCl3·6H2O and 3.98 g of FeCl2·4H2O were dissolved in 50 mL of 0.5 mol/L HCl solution that was then added dropwise to the reactor while vigorous stirring in a nitrogen gas environment. Afterward, the black ferrofluid composed of Fe3O4 nanoparticles was further heated to 90 °C and aged for 60 min. The obtained PEI-coated Fe3O4 nanoparticles were washed with deionized water several times and dispersed in water for further use. Herein, naked Fe3O4 MNPs were prepared in the absence of PEI.
Sample morphology was examined by transmission electron microscopy (TEM; JSM-1200EX, Japan). The number-averaged particle size and size distribution were estimated by counting at least 200 particles in TEM images. Powder X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (D8 Discover, Germany). Fourier transform infrared (FTIR) spectra (Nicolet 6700, USA) were recorded between 4000 and 400 cm−1. Thermogravimetric analysis (TGA) was performed using a thermal analyzer (TGA/DSC 1, Mettler Toledo, Switzerland) with a heating rate of 10 °C/min to 800 °C. The Brunauer-Emmett-Teller (BET) method (Autosorb IQ, USA) was utilized to calculate the specific surface areas, pore size, and volume. Hydrodynamic diameter and surface zeta potentials of the nanoparticles were measured by using a Malvern Nanosizer instrument (Zetasizer Nano ZS90, Malvern Instruments Company, UK). Water contact angle measurements were performed on a DSA-20 contact angle goniometer (Kruss, Germany) by the sessile drop method at room temperature. The magnetic nanoparticles were firstly compressed into a circular sheet with a diameter of ~ 7 mm at 10 MPa. A droplet of deionized water (2 μL) was carefully placed on the surface of sheet using a microsyringe. Photographs of the droplet were then recorded after equilibrium has been reached. Typically, five parallel samples were prepared to test the water contact angle and the average of five readings was taken to evaluate the wettability of MNPs. Magnetic properties were measured using a physical property measurement system (PPMS-9, Quantum Design, USA) at room temperature. Microstructure of the MNPs and emulsified oil wastewater mixtures were observed by means of a digital optical microscope (KH-7700; Hirox, Japan).
Oil-water separation tests
Oil-in-water emulsion containing 1000 mg/L of oil was prepared by powerful sonication for 2 min. This emulsion was stable, and no significant phase separation occurred within 3 weeks. Herein, diesel oil was used throughout the separation test except as otherwise indicated. A certain amount of MNPs was added to the emulsion, and the mixture was shaken at 1000 rpm for 5 min at room temperature by using a turbine mixer (Vortex-Genie 2; Scientific Industries, USA). Upon applying an external magnetic field, the MNP-coated oil droplets were moved to the vial wall. The water transmittance was then determined to assess the separation efficiency. Every oil-water separation test was repeated for three times to determine the averaged transmittance. The water transmittance was recorded by a UV-vis spectrometer (UV-2600; Shimadzu, Japan) at a wavelength of 610 nm. After oil-water separation, the spent MNPs were washed with ethanol three times to remove the attached oil. The regenerated MNPs were redispersed in water and then reused in the next cycle of oil-water separation. Herein, in order to estimate the residual oil content, a calibration curve was established via measuring the water transmittance of emulsion at various oil concentrations.
Results and discussion
Characterization of MNPs
Oil-water separation process
Under acidic condition, the PEI-coated MNPs were positively charged, while the oil droplets were negatively charged (Lü et al. 2017). Accordingly, both charge attraction and interfacial activity favored the sorption of PEI-coated MNPs at the oil droplet surface, thereby promoting the coalescence of emulsified oil droplets. As a result, larger droplets with a size of around 25 μm appeared and the size of the resulting flocs exceeded 100 μm (Fig. 6b). Under both neutral and alkaline conditions, the size of flocs seemed to be relatively small (Fig. 6c, d). At this time, both PEI-coated MNPs and oil droplets were negatively charged; hence, the electrostatic interaction was repellent. However, due to their interfacial activity, the PEI-coated MNPs could still successfully attach to the oil droplets and flocculate the oil droplets through bridging. In conclusion, the PEI-coated Fe3O4 MNPs could effectively flocculate the emulsified oil droplets, thereby facilitating subsequent magnetic separation.
Oil-water separation performance
In this study, PEI-coated MNPs were successfully synthesized via a facile one-step method, and their oil-water separation performances were then evaluated. The synthesized PEI-coated MNPs could successfully adsorb to the oil droplet surface and further flocculate the oil droplets, thereby facilitating the magnetic separation. The separation capacity was estimated to be > 13 mg of diesel oil/mg of MNPs. Moreover, the separation efficiency of PEI-coated MNPs declined somewhat with pH rising, while significantly enhanced with the introduction of electrolyte. Recycling experiment indicated that the PEI-coated MNPs could be reused up to 6 cycles without showing a significant loss in separation efficiency. These results suggested that PEI-coated Fe3O4 MNPs could provide a simple but powerful tool to remove emulsified oil from an aqueous environment.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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