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Fabrication of recyclable multi-responsive magnetic nanoparticles for emulsified oil-water separation
Journal Pre-proof Fabrication of recyclable multi-responsive magnetic nanoparticles for emulsified oilwater separation Ting Lü, Dongming Qi, Dong Zhang, Kejing Fu, Yi Li, Hongting Zhao PII:
S0959-6526(20)30340-1
DOI:
https://doi.org/10.1016/j.jclepro.2020.120293
Reference:
JCLP 120293
To appear in:
Journal of Cleaner Production
Received Date: 26 August 2019 Revised Date:
2 December 2019
Accepted Date: 26 January 2020
Please cite this article as: Lü T, Qi D, Zhang D, Fu K, Li Y, Zhao H, Fabrication of recyclable multiresponsive magnetic nanoparticles for emulsified oil-water separation, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120293. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Fabrication of recyclable multi-responsive magnetic nanoparticles for emulsified oil-water separation Ting Lü1, Dongming Qi2, Dong Zhang1, Kejing Fu1, Yi Li1, Hongting Zhao1* 1
Institute of Environmental Materials and Applications, College of Materials and
Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China 2
Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
Abstract: In order to promote the convenience and environmental friendliness of recycling process for the magnetic nanoparticles (MNPs) used in emulsified oil-water separation, a series of recyclable multi-responsive MNPs were successfully fabricated via grafting triple sensitive copolymers onto the silica coated ferroferric oxide (Fe3O4@SiO2) nanoparticles. It was found that the performance of fabricated MNPs was significantly influenced by three environmental parameters, namely pH value, temperature and ionic strength (IS) of water. The synthesized MNPs tended to assemble at oil droplet surface at a temperature less than lower critical solution temperature (LCST), lower pH level or higher IS, thereby facilitating removal of emulsified oil from water phase under external magnetic field. On the contrary, these MNPs could also reversibly desorb form oil droplet surface at a temperature higher than LCST, higher pH level or lower IS. Accordingly, the spent MNPs could be easily regenerated by just regulating solution temperature, pH or IS, rather than by washing with organic solvent, thereby minimizing the risks of secondary pollution. Recycling experiment showed that the well-designed MNPs could be reused up to five cycles without showing remarkable decline in separation efficiency. It’s noteworthy that the MNPs exhibited superior separation efficiency at high IS, which is typically encountered for real saline oily water system, thereby spent MNPs could be easily recycled by merely using deionized water due to the dramatical decrease of IS during their rinsing process, suggesting its applicability for practical use. It is 1
expected that the development of this multi-responsive and recyclable MNPs can potentially offer a promising, facile, cost-efficient and eco-friendly route for the emulsified oily wastewater treatment.
Keywords: Magnetic nanoparticles; multi-responsive; oil-water separation; recyclability *Corresponding author. E-mail: [email protected]
1. Introduction With the development of tertiary oil recovery, marine oil storage and transportation, machinery and food processing, a great quantity of oily wastewaters have been produced and led to severe environmental and ecological problems. Due to the usage of synthetic surfactants and existence of natural surfactants, significant part of oily wastewater exists in the form of oil-in-water emulsion (Bratskaya et al., 2006). These oil droplets become extremely stable because of the steric hindrance and electrostatic repulsive force between the oil droplets, and meanwhile its size is usually smaller than 20 µm; as a consequence, the emulsified oil is extremely difficult to be treated and separated from the aqueous environment. Generally, traditional methods such as gravity separation, flotation and centrifugation are ineffective for the emulsified oil removal (Xu et al., 2018). Although some flocculants or demulsifiers were reported to be used for effectively demulsifying the oil-in-water emulsion (Lü et al., 2019; Zhang et al., 2016a; Souza et al. 2017), the oil-water separation process was time-consuming
and
the
flocculant
or
demulsifier
could
not
be
recycled.
Superhydrophilic/underwater superoleophobic filtration materials have been successfully used to separate emulsified oil from emulsion (Zhao et al., 2016; Lv et al., 2017; Wang et al., 2016); however, this approach is limited by its complicated instrument setup and high energy consumption during its filtration process. Thus, there remains a great demand to fabricate 2
more cost-efficient materials and technologies for efficient separation of emulsified oil droplets from the water phase.
Adsorption is one of the important methods for the removal of different pollutants (AL-Othman et al., 2012; Naushad et al., 2015; Mironyuk et al., 2019) and magnetic adsorbent showed considerable attractive features (Ahamad et al., 2019, Alqadami et al., 2017). Recently, Fe3O4 magnetic nanoparticles (MNPs) have been paid more research attentions for emulsified oil-water separation (Peng et al., 2012; Liang et al., 2018; Mao et al., 2019). Since majority of emulsified oil droplets are hydrophobic and negatively charged (Bratskaya et al., 2006), hydrophobic or cationic modification is desirable for the nanoparticle assembly to the surface of oil droplet, thereby facilitating subsequent magnetic separation. Nonetheless, superhydrophobic MNPs are usually invalid owing to their poor water dispersibility. Theoretically, in order to achieve efficient oil-water separation, the key character is that the MNPs have excellent water dispersibility and are able to accumulate onto the emulsified oil-water interface. Accordingly, the desirable wettability of MNPs is amphiphilic; meanwhile, positive surface charge is also favorable for the nanoparticle sorption at oil droplet surface via electrostatic attraction. In light of the above theory, various Fe3O4 MNPs with satisfactory emulsified oil-water separation performance have been previously
reported
to
be
developed
by
modifying
with
polyvinylpyrrolidone
(Mirshahghassemi et al., 2019; Mirshahghassemi et al., 2017; Mirshahghassemi and Lead, 2015), oleic acid (Liang et al., 2014; Liang et al., 2015), polyethyleneimine (Lü et al., 2018a), demulsifier 5010 (Li et al., 2014), graphene oxide (Liu et al., 2017), expanded perlite (Xu et al., 2018), cyclodextrin (Zhang et al., 2016b), chitosan (Lü et al., 2018b; Lü et al., 2017a; Zhang et al., 2017), and so on. However, one of the major disadvantages is that the spent MNPs should be washed with organic solvents several times for their regeneration, resulting 3
in secondary pollution during the process of their cyclic utilization.
In order to improve the regeneration process, thermosensitive or pH-responsive MNPs were prepared and applied in emulsified oil-water separation (Chen et al., 2014; Wang et al., 2015). Similarly, in our previous studies, poly(N-isopropylacrylamide) (PNIPAM)-grafted MNPs and (3-aminopropyl)triethoxysilane (APTES)-coated MNPs were prepared and used for emulsified oil harvest (Lü et al., 2016; Lü et al., 2017b). Results indicated that the adsorption and desorption behaviors of MNPs on the oil droplet surface could be successfully regulated by temperature or pH. Specifically, the PNIPAM-grafted MNPs desorbed from oil droplet surface due to the dramatic hydrophobic transition of PNIPAM while the temperature was higher than LCST (32°C), and therefore could be recycled via hot water washing; while the APTES-coated MNPs desorbed from oil droplet surface because of the electrostatic repulsion under alkaline condition, and hence could be regenerated by washing with alkaline water. Although the thermosensitive or pH-sensitive MNPs avoided the usage of organic solvents during its regeneration process, the regeneration approach was single and not able to be improved or selected according to the character of specific oily wastewater. Consequently, there is a pressing need to design and prepare multi-responsive MNPs which can be recycled and regenerated via various approaches.
It is well known that the pH, temperature and salinity are the important parameters for the oily wastewaters. Accordingly, the main objective or content of our work is: (1) to fabricate a set of multi-responsive MNPs via graft copolymerization of thermosensitive monomer N-isopropylacrylamide (NIPAM), and ion and pH dual sensitive monomer sodium methacrylate (SMA) onto the surface of Fe3O4@SiO2 MNPs; (2) to evaluate their oil-water 4
separation efficiencies at different pH, temperature and ionic strength (IS) levels, and meanwhile delineate the mechanism of influence of the three parameters on separation efficiency; (3) to establish the regeneration approaches for the spent MNPs based upon changing pH, temperature and IS of water, and further examine the recycling performance of synthesized multi-responsive MNPs.
2. Experimental section
2.1. Synthesis of multi-responsive MNPs Multi-responsive MNPs was synthesized through a “grafting through” reaction. Firstly, a certain amount of NIPAM and SMA monomer was completely dissolved in water (100 mL) and
γ-methacryloxypropyl
triisopropoxidesilane
(MPS)-coated
Fe3O4@SiO2 MNPs
(Fe3O4@SiO2-MPS, 0.1 g) were then dispersed into the aqueous monomer solution. The resulting mixture was subsequently heated to 32°C in nitrogen atmosphere. Thereafter, aqueous solution of 2,2’-Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIBI) was introduced to start the graft copolymerization and the reaction lasted for 22 h under the protection of nitrogen. Table 1 shows a set of multi-responsive MNPs prepared with various monomer compositions. The synthesized responsive MNPs were collected and completely washed with deionized water for further usage. The synthesis procedure of Fe3O4 and Fe3O4@SiO2-MPS MNPs, all materials used for this work, and the characterization method were shown in the supporting information. Insert Table 1 here
5
2.2. Oil-water separation test The emulsion with containing 1.0 g/L of diesel oil was obtained via powerful sonication for the oil-water mixture. The pH of emulsion was adjusted by using HCl or NaOH solution, while the salinity was regulated by the addition of NaCl. The oil-water separation efficiency was determined at room temperature and the salinity of emulsion was kept at 0 mol/L unless otherwise mentioned. Herein, the pH, temperature and IS did not significantly affect the transmittance of the freshly prepared oil-in-water emulsion over the studied ranges. Various amounts of responsive MNPs were introduced into the freshly prepared emulsion, and its mixture was then shaken by turbine mixer for 5 min to let the MNPs assemble onto the diesel droplet surface. Subsequently, the MNPs coated diesel droplets were separated by using a magnet. The transmittance of water was examined to evaluate the performance of synthesized MNPs. 2.3. Recycle Test After oil-water separation, the multi-responsive MNPs could be regenerated by three different approaches, respectively: (1) the spent MNPs were rinsed by hot water (70°C) three times to eliminate the adhesional oil; (2) the spent MNPs were washed with alkaline water (pH=13) three times; (3) the spent MNPs were rinsed by room temperature deionized water three times. Regeneration approach could be selected based upon the actual situation. The recovered MNPs were then recycled and their cyclic utilization was carried out for five times to evaluate the recyclability.
6
3. Results and discussion 3.1. Characterizations of MNPs Typical TEM photographs of Fe3O4 (S0), Fe3O4@SiO2-MPS (S1) and multi-responsive MNPs (S6) are shown in Figure S1. Fe3O4 MNPs prepared via solvothermal method exhibited a globular shape with an average size of around 200 nm (Figure S1a), while Fe3O4@SiO2-MPS MNPs showed a core-shell structure with a thin and grey silica layer on surface (Figure S1b). After surface grafting with multi-responsive copolymer, the MNPs were inclined to attach with each other, and a layer of hazy polymer film was observed between the adjacent MNPs (Figure S1c). Figure S2 shows the XRD patterns of the synthesized MNPs. XRD peaks at 2θ = 30.2, 35.5, 43.1, 53.5, 57.3 and 62.8°corresponded to the inverse spinel structure of Fe3O4 (Liu et al., 2009). Meanwhile, approximate diffraction peaks are also observed for Fe3O4@SiO2-MPS and multi-responsive MNPs, suggesting that the crystal phase was unchanged during the surface coating and grafting.
The successful synthesis of multi-responsive MNPs could also be confirmed by the FTIR spectra (Figure 1a). Typical peak in relation to Fe-O stretching was observed at 582 cm-1 (Naushad et al., 2019a), and the peaks at 1640 and 1410 cm-1 were ascribed to the symmetric and asymmetric stretching vibrations of carboxylate group (Li et al., 2014), indicating part of trisodium citrate and sodium acetate coordinated with iron ion. After surface coating with silica layer, a new intense peak appeared at around 1088 cm-1 because of the Si-O-Si vibrations (Li et al., 2014). After surface graft copolymerization, the band at around 2900 cm-1 became relatively obvious due to the methyl and methylene vibrations (Naushad et al., 2019b). The band at 1547 cm-1 was assigned to N-H bending vibration (Zhang et al., 2016b), suggesting the successful grafting of PNIPAM; the band of carboxylate group in SMA unit 7
was overlapped with the carboxylate group in Fe3O4, and therefore the incorporation of SMA unit into grafted chains was unable to be verified by FTIR spectra. However, the successful introduction of SMA unit in grafted copolymer was confirmed by the zeta potential measurement at various pH levels (Figure 1b). Before the graft copolymerization, Fe3O4@SiO2-MPS MNPs were always negatively charged over the studied pH range due to the exposure of silica layer. After surface grafting of neutral PNIPAM chains, the zeta potential increased significantly although the MNPs were still negatively charged. When more SMA monomer was introduced into the graft copolymerization system, zeta potential of resulting MNPs declined gradually, suggesting that more SMA unit was grafted on nanoparticle surface. Insert Figure 1 here
Figure S3 shows the TGA curves of the synthesized MNPs. A thermal weight loss of 10.7% was observed for Fe3O4@SiO2-MPS MNPs (S1) from 25 to 800°C, owing to the dehydration and thermolysis of organic substance. After surface graft copolymerization, the weight loss of responsive MNPs (S2, S3, S4, S5 and S6) increased to 15.9%, 16.4%, 13.6%, 13.0% and 13.1%, respectively. The additional weight loss was assigned to the decomposition of surface-grafted polymer. On this basis, the grafting ratios were calculated be 5.2%, 5.7%, 2.9%, 2.3% and 2.4% for S2, S3, S4, S5 and S6, respectively. Meanwhile, it should be noted that the thermal decomposition behavior of S2 is different from that of S3, especially in the range of 350-550 oC. After introduction of SMA unit into the grafted chains, the polymer decomposed relatively slow between 350 and 550 oC, presumably due to the hydrogen bonding between the amide group of NIPAM units and carboxylic group of SMA units (Shieh et al., 2018). To achieve the goal of efficient oil-water separation, the MNPs should have good 8
water dispersibility and be able to be rapidly collected in the presence of magnetic field. Therefore, superparamagnetism and high saturation magnetization are very important for the synthesized MNPs. It was found that all MNPs showed negligible remanence and coercivity, suggesting the superparamagetism of Fe3O4 core (Figure 1c). The superparamagnetic behavior could be explained by the fact that each Fe3O4 microsphere prepared via solvothermal method was composed of many nano-sized primary particles, which are usually smaller than 30 nm (Wang et al., 2015). The saturation magnetization of bare Fe3O4 MNPs was determined to be 69.1 emu/g. After incorporation of silica shell and PNIPAM arms, the saturation magnetization was reduced to 42.9 emu/g (S1) and 33.6 emu/g (S2), respectively. Considering that the difference of grafting ratio was not significant between polymer-grafted MNPs, the saturation magnetization should be also close for all multi-responsive MNPs. Thus, the synthesized multi-responsive MNPs were suitable for removing emulsified oil droplets from aqueous media by applying magnetic field, just as shown in Figure 1(I, II, III).
3.2. Separation performance and recyclability of MNPs The grafted copolymer P(NIPAM-co-SMA) is sensitive to pH, temperature and IS, and therefore the impact of each of these three parameters on the oil-water separation performances of multi-responsive MNPs were investigated in detail as follow. 3.2.1. pH effect and regeneration via pH regulation In order to study the suitability of the pH range, the MNPs were tested to evaluate its performance at different pH levels (Figure 2a-e). PNIPAM has good interfacial activity and hence the PNIPAM-coated MNPs are able to efficiently assemble onto the emulsified oil droplet surface at various pH levels, thereby facilitating subsequent magnetic separation. As 9
shown in Figure 2a, the sample S2 exhibited superior oil-water separation performance when its dosage exceeded 200 mg/L, and the pH had little influence on the separation efficiency. However, after SMA unit was incorporated into the grafted copolymer chains, the MNPs became pH-sensitive and its separation efficiency decreased with pH rising; moreover, the effect of pH on its separation efficiency became more significant with increasing amount of SMA units (Figure 2b-e). For example, S3 was still effective at various pH levels (Figure 2b), S4 was efficient at low and medium pH levels (Figure 2c), while S5 and S6 were merely valid at low pH level (Figure 2d-e). The reason can be explained as follow. Ionization of SMA unit became remarkable at higher pH level, hence the MNPs became more hydrophilic and its charge repulsive force between nanoparticles and oil droplets increased (Lu et al., 2013; Gao et al., 2013); accordingly, the MNPs could not attach to the oil droplet with pH rising (Figure S4 (a-d)). In other word, the multi-responsive MNPs would desorb from the oil droplet surface under high pH condition. Consequently, the spent MNPs (S4) was rinsed with alkaline water for their regeneration. According to our previous study (Lü et al., 2018a), when the water transmittance reached 75%, the oil removal ratio exceeded 98%. As a result, the multi-responsive MNPs (800 mg/L) still retained efficient separation performance at medium pH level (pH 6.0) after five cycles despite the slightly decreased water transmittance (Figure 2f), which confirmed that water pH regulation was an effective approach for the regeneration of MNPs. Insert Figure 2 here After cycle use, the MNPs (S4) was further washed with alkaline water and deionized water respectively to remove the attached oil. Thereafter, various technologies were used to characterize the dried samples (S4) before and after cycle use, and the corresponding results were compared (Figure S5). As shown in Figure S5a, the peaks at around 2926 and 2852 cm-1 due to the methyl and methylene vibrations became more intensive relative to the peak at 10
1092 cm-1 (Si-O-Si vibrations) after cycle use; meanwhile, the C content of surface elemental composition increased from 66.89 to 84.79% according to the X-ray photoelectron spectrum (XPS) examination (Figure S5b), while the other elemental contents decreased after cycle use. These results suggested that a certain amount of oil still remained in the MNPs after cycle use, even though the MNPs were regenerated by washing with alkaline water and deionized water, respectively. The nitrogen adsorption-desorption isotherms curves of MNPs are shown in Figure S5(c, d). The absorbed quantity increased with increasing relative pressure and the surface area of sample S4 before use was calculated to be 45 m2/g. On the contrary, as shown in Figure S5d, the absorbed quantity decreased at first, presumably due to the volatilization of residual oil on MNPs after cycle use. As a result, the surface area could not be well calculated after cycle use; theoretically, its surface area was probably reduced because of the oil attachment. Moreover, it was found that the surface of MNPs became rougher after cycle use probably due to the residual oil pollution as shown in Figure S5 (e, f). In a word, the MNPs were gradually contaminated by oil during the cycle use, leading to the decrease in their interfacial activity; accordingly, the oil-water separation efficiency would be reduced to some degree after the continuous cycle use of MNPs. 3.2.2. Temperature effect and regeneration via temperature regulation Our previous study showed that temperature significantly affected the oil-water separation performance of PNIPAM-grafted MNPs (Lü et al., 2016). After incorporation of SMA unit, the MNPs also became pH-responsive. Accordingly, the influence of temperature on oil removal efficiency was investigated as a function of pH levels (Figure 3a-b). Herein, the dosage of MNPs in Figure 3a was 100, 125, 150, 150 and 150 mg/L for S2, S3, S4, S5 and S6, respectively, while the dosage of MNPs in Figure 3b was 100, 400, 400, 600 and 600 mg/L for S2, S3, S4, S5 and S6, respectively. At pH 4.0, the SMA unit was protonated and became relatively hydrophobic, therefore P(NIPAM-co-SMA) exhibited similar LCST as compared to 11
that of PNIPAM homopolymer (~32°C) (Lu et al., 2013; Gao et al., 2013). When the temperature was lower than LCST, a moderate increase in separation efficiency was observed due to the increasing hydrophobicity of NIPAM units with temperature rising. Nonetheless, the separation efficiency decreased significantly once the temperature was above LCST, due to the dramatic hydrophobicity transition of grafted copolymer (Chen et al., 2014; Lü et al., 2016). Accordingly, all synthesized multi-responsive MNPs exhibited optimal separation efficiency at around 30°C (Figure 3a). At pH 7.0, the SMA unit became ionized and hydrophilic. The LCST did not change significantly when a small amount of SMA unit was introduced, and hence the MNPs (S3 and S4) still showed optimal performance at around 30°C; however, the LCST was reported to increase to ~36°C at pH 7.0 when the SMA content increased to 10% (Gao et al., 2013), and the MNPs S5 remained efficient with temperature rising from 30 to 35°C (Figure 3b). Moreover, it is noteworthy that the demulsification efficiency of S5 was enhanced dramatically as temperature rising from 5 to 30 °C. This phenomenon indicated the hydrophobic interaction resulted from warming (< LCST) could to some extent surpass the charge repulsion between oil droplets and MNPs, thereby favoring the attachment of MNPs onto the oil droplet. Even increasing SMA content to 50%, the separation efficiency still increased slightly when the temperature reached 30°C, although the grafted copolymer became more hydrophilic and carried more negative surface charges. Inert Figure 3 here In short, the oil-water separation efficiency was enhanced at first with temperature rising but decreased dramatically once the temperature exceeded LCST. When SMA unit was protonated, the thermal-responsive behavior of the synthesized MNPs was similar and the optimal operating temperature ranged from 25 to 30°C; when more SMA units were ionized, the LCST increased accordingly and hence the MNPs could be potentially suitable to treat the oily wastewater with higher temperature. As discussed earlier, when the temperature exceeded 12
LCST, the MNPs would aggregate with each other and desorb from the emulsified oil droplet. Accordingly, the spent MNPs (S4) were rinsed with hot deionized water (70 oC) for its regeneration. Results showed that the MNPs (800 mg/L) remained high efficiency after five cycles at medium pH level (Figure 3c), indicating that water temperature regulation was an alternative approach for the regeneration of MNPs. 3.2.3. IS effect and regeneration via IS regulation Oily wastewater usually contains a lot of electrolytes, and therefore it is of great importance to examine the influence of IS on oil-water separation performance. It is evident in Figure 4a that the separation efficiency was significantly enhanced by increasing IS under various conditions, when the dosage of MNPs was kept at 143 mg/L. On the one hand, the ion partly shielded the surface negative charge as indicated by the zeta potential variation (Figure 4b), and therefore the charge repulsion was reduced significantly; on the other hand, the addition of salt also enhanced the hydrophobicity of the MNPs to some extent (Yang et al., 2006; Björkegren et al., 2017). Both of above-mentioned factors were favorable for the accumulation of MNPs on oil droplet surface (Figure S4(e)). Conversely, the MNPs would desorb from oil droplet surface when the ionic strength decreased dramatically (Figure S4(f)), for example during the rinsing process with deionized water. To verify this deduction, the spent S5 was rinsed with room temperature deionized water for regeneration and its reusability was evaluated toward treating an emulsion containing 0.1 mol/L of NaCl. It was found that S5 (143 mg/L) could be recycled and used for at least five times under medium pH condition (Figure 4c), suggesting that rinsing with room temperature deionized water was indeed a convenient and green approach for the regeneration of MNPs. Inert Figure 4 here As mentioned above, the separation performances of multi-responsive MNPs were 13
significantly influenced by environmental pH, temperature and IS. The synthesized MNPs were inclined to assemble at oil droplet surface at a temperature less than LCST, lower pH level or higher IS, thereby separating emulsified oil droplets from water phase by applying magnetic field. Meanwhile, the multi-responsive MNPs were also capable of reversibly desorbing form oil droplet surface at a temperature higher than LCST, higher pH level or lower IS, and hence the spent MNPs could be regenerated by three different approaches: (1) washing with hot water to remove the attached oil, and this approach (temperature regulation) could be taken into account when treating room temperature oily wastewater; (2) washing with alkaline water, and this approach (pH regulation) was more suitable for treating acidic and neutral oily wastewaters; (3) rinsing with room temperature deionized water, and this approach (IS regulation) was particularly applicable to treat oily wastewater with high salinity. These regeneration approaches were convenient, mild and eco-friendly, and could be selected based upon the actual situation. Besides, we speculate that the particle size, grafting chain length and grafting density also have an important impact on their separation efficiency and recyclability, which will be investigated in detail in our future research work.
4. Conclusions A series of recyclable multi-responsive MNPs with containing a magnetic core and a layer of triple sensitive copolymer brushes were successfully fabricated to remove emulsified oil from water phase. The magnetic core was prepared through a solvothermal method and followed by surface coating with silica layer, and the surface-tethered copolymer arms were produced through a grafting reaction. The oil-water separation efficiency of synthesized MNPs was enhanced at first with temperature rising but declined significantly once the temperature exceeded LCST, and declined monofonically with pH rising or IS decreasing over the studied 14
range. According to the character of oily wastewater, three different regeneration approaches for the spent MNPs were proposed, namely rinsing with alkaline water, hot water (70oC) and room temperature deionized water, respectively. Based on the three regeneration methods, the multi-responsive MNPs could be recycled at least five times without showing significant loss in separation efficiency. It is expected that this study can provide some new idea for developing advanced materials and green technologies for efficiently treating emulsified oily wastewaters.
Acknowledgements The authors greatly appreciate the support from the National Natural Science Foundation of China
(21878064),
and
the
Natural
Science
Foundation
of
Zhejiang
Province
(LZ18E030002).
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separation by using polyethylenimine-coated magnetic nanoparticles. J. Nanopart. Res. 20, 88. Lü, T., Zhang, S., Qi, D., Zhang, D., Zhao, H., 2018b. Enhanced demulsification from aqueous media by using magnetic chitosan-based flocculant. J. Colloid Interf. Sci. 518, 76-83. Lü, T., Luo, C., Qi, D., Zhang, D., Zhao, H., 2019. Efficient treatment of emulsified oily wastewater by using amphipathic chitosan-based flocculant. React. Funct. Polym. 139, 133-141. Lv, W., Mei, Q., Xiao, J., Du, M.; Zheng, Q., 2017. 3D multiscale superhydrophilic sponges with delicately designed pore size for ultrafast oil/water separation. Adv. Funct. Mater. 27, 1704293. Mao, X., Gong, L., Xie, L., Qian, H., Wang, X., Zeng, H., 2019. Novel Fe3O4 based superhydrophilic core-shell microspheres for breaking asphaltenes-stabilized water-in-oil emulsion. Chem. Eng. J. 358, 869-877. Mirshahghassemi, S., Lead, J. R., 2015. Oil recovery from water under environmentally relevant conditions using magnetic nanoparticles. Environ. Sci. Technol. 49, 11729-11736. Mirshahghassemi, S., Ebner, A. D., Cai, B., Lead, J. R., 2017. Application of high gradient magnetic separation for oil remediation using polymer-coated magnetic nanoparticles. Sep. Purif. Technol. 179, 328-334. Mironyuk, I., Tatarchuk, T., Naushad, M., Vasylyeva, H., Mykytyn, I., 2019. Highly efficient adsorption of strontium ions by carbonated mesoporous TiO2. J. Mol. Liq. 285, 742-753. Mirshahghassemi, S., Cai, B., Lead, J. R., 2019. A comparison between the oil removal capacity of polymer-coated magnetic nanoparticles in natural and synthetic environmental 18
samples. Environ. Sci. Technol. 53, 4426-4432. Naushad, M., ALOthman, Z. A., Awual, M. R., Alam, M. M., Eldesoky, G. E., 2015. Adsorption kinetics, isotherms, and thermodynamic studies for the adsorption of Pb2+ and Hg2+ metal ions from aqueous medium using Ti(IV) iodovanadate cation exchanger. Ionics. 21, 2237-2245. Naushad, M., Ahamad, T., Alothman, Z. A., Al-Muhtaseb, A. H., 2019a. Green and eco-friendly nanocomposite for the removal of toxic Hg(II) metal ion from aqueous environment: Adsorption kinetics & isotherm modeling. J. Mol. Liq. 279, 1-8. Naushad, M., Sharma, G., Alothman, Z. A., 2019b. Photodegradation of toxic dye using Gum Arabic-crosslinkedpoly(acrylamide)/Ni(OH)2/FeOOH nanocomposites hydrogel. J. Clean. Prod. 241, 118263. Peng, J., Liu, Q., Xu, Z., Masliyah, J., 2012. Synthesis of interfacially active and magnetically responsive nanoparticles for multiphase separation applications. Adv. Funct. Mater. 22, 1732-1740. Shieh, Y. T., Lin, P. Y., Kuo, S. W., 2018. Sequence length distribution affects the lower critical solution temperature, glass transition temperature, and CO2-responsiveness of N-isopropylacrylamide/methacrylic acid copolymers. Polymer. 143, 258-270. Souza, A. V., Mendes, M. T., Souza, S. T. S., Palermo, L. C. M., Oliveira, P. F., Mansur, C. R. E., 2017. Synthesis of additives based on polyethylenimine modified with non-ionic surfactants for application in phase separation of water-in-oil emulsions. Energ. Fuel. 31, 10612-10619. Wang, X., Shi, Y., Graff, R. W., Lee, D., Gao, H., 2015. Developing recyclable pH-responsive 19
magnetic nanoparticles for oil-water separation. Polymer. 72, 361-367. Wang, Z., Liu, G., Huang, S., 2016. In situ generated janus fabrics for the rapid and efficient separation of oil from oil-in-water emulsions. Angew. Chem. Int. Ed. 55, 14610-14613. Xu, H., Jia, W., Ren, S., Wang, J., 2018. Novel and recyclable demulsifier of expanded perlite grafted by magnetic nanoparticles for oil separation from emulsified oil wastewaters. Chem. Eng. J. 337, 10-18. Yang, F., Liu, S., Xu, J., Lan, Q., Wei, F., Sun, D., 2006. Pickering emulsions stabilized solely by layered double hydroxides particles: the effect of salt on emulsion formation and stability. J. Colloid Interf. Sci. 302, 159-169. Zhang, H., Duan, M., Fang, S., Zhai, L., Wang, X., 2016a. Preparation of a novel flocculant and its performance for treating acidic oily wastewater. RSC Adv. 6, 106102-106108. Zhang, J., Li, Y., Bao, M., Yang, X., Wang, Z., 2016b. Facile fabrication of cyclodextrin-modified magnetic particles for effective demulsification from various types of emulsions. Environ. Sci. Technol. 50, 8809-8816. Zhang, S., Lü, T., Qi, D., Cao, Z., Zhang, D., Zhao, H., 2017. Synthesis of quaternized chitosan-coated magnetic nanoparticles for oil-water separation, Mater. Lett. 191, 128-131. Zhao, Y., Zhang, M., Wang, Z., 2016. Underwater superoleophobic membrane with enhanced oil-water separation, antimicrobial, and antifouling activities. Adv. Mater. Interf. 3, 1500664.
20
Figure captions Figure 1 FTIR spectra (a), zeta potential at various pH levels (b) and magnetic hysteresis loops (c) of the synthesized MNPs; insert shows the oil-water separation process: (I) emulsion, (II) mixture of emulsion and S2, (III) after magnetic separation
Figure 2 Oil-water separation performances of the synthesized MNPs at various pH levels (a-e) and the reusability of sample S4 regenerated via pH regulation (f)
Figure 3 Effect of temperature on oil-water separation performance of the MNPs at various pH levels (a-b) and reusability of sample S4 regenerated via temperature regulation (c)
Figure 4 Effect of NaCl concentration on the oil-water separation performance (a) and zeta potential (b), and reusability of sample S5 regenerated by IS regulation (c)
Table 1 Recipes for the synthesis of multi-responsive MNPs
21
Table 1 Recipes for the synthesis of multi-responsive MNPs sample number
sample name
NIPAM (mol)
SMA (mol)
monomer ratio
S0
Fe3O4
/
/
/
S1
Fe3O4@SiO2-MPS
/
/
/
S2
responsive MNPs
0.024
0
1:0
S3
responsive MNPs
0.0234
0.0006
39:1
S4
responsive MNPs
0.0228
0.0012
19:1
S5
responsive MNPs
0.0216
0.0024
9:1
S6
responsive MNPs
0.012
0.012
1:1
Figure 1
22
Figure 2
Figure 3
23
Figure 4
Graphical abstract
24
Highlights
·Temperature, pH and IS-responsive MNPs were fabricated for oil-water separation. ·The MNPs exhibited superior performance at lower pH, lower temperature or higher IS. ·Spent MNPs could be regenerated by raising pH, raising temperature or decreasing IS. ·The MNPs could be recycled at least five times with negligible reduction in efficiency. ·The MNPs were particularly applicable to treat practical saline oily wastewater.
Declaration of Interest Statement
The authors declare that they have no conflict of interest.
1
Author Contributions Section
Author Contributions: Nanoparticles synthesis, T.L. and K.F.; nanoparticle characterization, T.L. and D.Q.; experimental data analysis, D.Z.; oil-water separation, K.F. and Y.L.; manuscript preparation, T.L.; experimental design and manuscript review, H.Z.
1
S0959-6526(20)30340-1
DOI:
https://doi.org/10.1016/j.jclepro.2020.120293
Reference:
JCLP 120293
To appear in:
Journal of Cleaner Production
Received Date: 26 August 2019 Revised Date:
2 December 2019
Accepted Date: 26 January 2020
Please cite this article as: Lü T, Qi D, Zhang D, Fu K, Li Y, Zhao H, Fabrication of recyclable multiresponsive magnetic nanoparticles for emulsified oil-water separation, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120293. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Fabrication of recyclable multi-responsive magnetic nanoparticles for emulsified oil-water separation Ting Lü1, Dongming Qi2, Dong Zhang1, Kejing Fu1, Yi Li1, Hongting Zhao1* 1
Institute of Environmental Materials and Applications, College of Materials and
Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China 2
Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
Abstract: In order to promote the convenience and environmental friendliness of recycling process for the magnetic nanoparticles (MNPs) used in emulsified oil-water separation, a series of recyclable multi-responsive MNPs were successfully fabricated via grafting triple sensitive copolymers onto the silica coated ferroferric oxide (Fe3O4@SiO2) nanoparticles. It was found that the performance of fabricated MNPs was significantly influenced by three environmental parameters, namely pH value, temperature and ionic strength (IS) of water. The synthesized MNPs tended to assemble at oil droplet surface at a temperature less than lower critical solution temperature (LCST), lower pH level or higher IS, thereby facilitating removal of emulsified oil from water phase under external magnetic field. On the contrary, these MNPs could also reversibly desorb form oil droplet surface at a temperature higher than LCST, higher pH level or lower IS. Accordingly, the spent MNPs could be easily regenerated by just regulating solution temperature, pH or IS, rather than by washing with organic solvent, thereby minimizing the risks of secondary pollution. Recycling experiment showed that the well-designed MNPs could be reused up to five cycles without showing remarkable decline in separation efficiency. It’s noteworthy that the MNPs exhibited superior separation efficiency at high IS, which is typically encountered for real saline oily water system, thereby spent MNPs could be easily recycled by merely using deionized water due to the dramatical decrease of IS during their rinsing process, suggesting its applicability for practical use. It is 1
expected that the development of this multi-responsive and recyclable MNPs can potentially offer a promising, facile, cost-efficient and eco-friendly route for the emulsified oily wastewater treatment.
Keywords: Magnetic nanoparticles; multi-responsive; oil-water separation; recyclability *Corresponding author. E-mail: [email protected]
1. Introduction With the development of tertiary oil recovery, marine oil storage and transportation, machinery and food processing, a great quantity of oily wastewaters have been produced and led to severe environmental and ecological problems. Due to the usage of synthetic surfactants and existence of natural surfactants, significant part of oily wastewater exists in the form of oil-in-water emulsion (Bratskaya et al., 2006). These oil droplets become extremely stable because of the steric hindrance and electrostatic repulsive force between the oil droplets, and meanwhile its size is usually smaller than 20 µm; as a consequence, the emulsified oil is extremely difficult to be treated and separated from the aqueous environment. Generally, traditional methods such as gravity separation, flotation and centrifugation are ineffective for the emulsified oil removal (Xu et al., 2018). Although some flocculants or demulsifiers were reported to be used for effectively demulsifying the oil-in-water emulsion (Lü et al., 2019; Zhang et al., 2016a; Souza et al. 2017), the oil-water separation process was time-consuming
and
the
flocculant
or
demulsifier
could
not
be
recycled.
Superhydrophilic/underwater superoleophobic filtration materials have been successfully used to separate emulsified oil from emulsion (Zhao et al., 2016; Lv et al., 2017; Wang et al., 2016); however, this approach is limited by its complicated instrument setup and high energy consumption during its filtration process. Thus, there remains a great demand to fabricate 2
more cost-efficient materials and technologies for efficient separation of emulsified oil droplets from the water phase.
Adsorption is one of the important methods for the removal of different pollutants (AL-Othman et al., 2012; Naushad et al., 2015; Mironyuk et al., 2019) and magnetic adsorbent showed considerable attractive features (Ahamad et al., 2019, Alqadami et al., 2017). Recently, Fe3O4 magnetic nanoparticles (MNPs) have been paid more research attentions for emulsified oil-water separation (Peng et al., 2012; Liang et al., 2018; Mao et al., 2019). Since majority of emulsified oil droplets are hydrophobic and negatively charged (Bratskaya et al., 2006), hydrophobic or cationic modification is desirable for the nanoparticle assembly to the surface of oil droplet, thereby facilitating subsequent magnetic separation. Nonetheless, superhydrophobic MNPs are usually invalid owing to their poor water dispersibility. Theoretically, in order to achieve efficient oil-water separation, the key character is that the MNPs have excellent water dispersibility and are able to accumulate onto the emulsified oil-water interface. Accordingly, the desirable wettability of MNPs is amphiphilic; meanwhile, positive surface charge is also favorable for the nanoparticle sorption at oil droplet surface via electrostatic attraction. In light of the above theory, various Fe3O4 MNPs with satisfactory emulsified oil-water separation performance have been previously
reported
to
be
developed
by
modifying
with
polyvinylpyrrolidone
(Mirshahghassemi et al., 2019; Mirshahghassemi et al., 2017; Mirshahghassemi and Lead, 2015), oleic acid (Liang et al., 2014; Liang et al., 2015), polyethyleneimine (Lü et al., 2018a), demulsifier 5010 (Li et al., 2014), graphene oxide (Liu et al., 2017), expanded perlite (Xu et al., 2018), cyclodextrin (Zhang et al., 2016b), chitosan (Lü et al., 2018b; Lü et al., 2017a; Zhang et al., 2017), and so on. However, one of the major disadvantages is that the spent MNPs should be washed with organic solvents several times for their regeneration, resulting 3
in secondary pollution during the process of their cyclic utilization.
In order to improve the regeneration process, thermosensitive or pH-responsive MNPs were prepared and applied in emulsified oil-water separation (Chen et al., 2014; Wang et al., 2015). Similarly, in our previous studies, poly(N-isopropylacrylamide) (PNIPAM)-grafted MNPs and (3-aminopropyl)triethoxysilane (APTES)-coated MNPs were prepared and used for emulsified oil harvest (Lü et al., 2016; Lü et al., 2017b). Results indicated that the adsorption and desorption behaviors of MNPs on the oil droplet surface could be successfully regulated by temperature or pH. Specifically, the PNIPAM-grafted MNPs desorbed from oil droplet surface due to the dramatic hydrophobic transition of PNIPAM while the temperature was higher than LCST (32°C), and therefore could be recycled via hot water washing; while the APTES-coated MNPs desorbed from oil droplet surface because of the electrostatic repulsion under alkaline condition, and hence could be regenerated by washing with alkaline water. Although the thermosensitive or pH-sensitive MNPs avoided the usage of organic solvents during its regeneration process, the regeneration approach was single and not able to be improved or selected according to the character of specific oily wastewater. Consequently, there is a pressing need to design and prepare multi-responsive MNPs which can be recycled and regenerated via various approaches.
It is well known that the pH, temperature and salinity are the important parameters for the oily wastewaters. Accordingly, the main objective or content of our work is: (1) to fabricate a set of multi-responsive MNPs via graft copolymerization of thermosensitive monomer N-isopropylacrylamide (NIPAM), and ion and pH dual sensitive monomer sodium methacrylate (SMA) onto the surface of Fe3O4@SiO2 MNPs; (2) to evaluate their oil-water 4
separation efficiencies at different pH, temperature and ionic strength (IS) levels, and meanwhile delineate the mechanism of influence of the three parameters on separation efficiency; (3) to establish the regeneration approaches for the spent MNPs based upon changing pH, temperature and IS of water, and further examine the recycling performance of synthesized multi-responsive MNPs.
2. Experimental section
2.1. Synthesis of multi-responsive MNPs Multi-responsive MNPs was synthesized through a “grafting through” reaction. Firstly, a certain amount of NIPAM and SMA monomer was completely dissolved in water (100 mL) and
γ-methacryloxypropyl
triisopropoxidesilane
(MPS)-coated
Fe3O4@SiO2 MNPs
(Fe3O4@SiO2-MPS, 0.1 g) were then dispersed into the aqueous monomer solution. The resulting mixture was subsequently heated to 32°C in nitrogen atmosphere. Thereafter, aqueous solution of 2,2’-Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIBI) was introduced to start the graft copolymerization and the reaction lasted for 22 h under the protection of nitrogen. Table 1 shows a set of multi-responsive MNPs prepared with various monomer compositions. The synthesized responsive MNPs were collected and completely washed with deionized water for further usage. The synthesis procedure of Fe3O4 and Fe3O4@SiO2-MPS MNPs, all materials used for this work, and the characterization method were shown in the supporting information. Insert Table 1 here
5
2.2. Oil-water separation test The emulsion with containing 1.0 g/L of diesel oil was obtained via powerful sonication for the oil-water mixture. The pH of emulsion was adjusted by using HCl or NaOH solution, while the salinity was regulated by the addition of NaCl. The oil-water separation efficiency was determined at room temperature and the salinity of emulsion was kept at 0 mol/L unless otherwise mentioned. Herein, the pH, temperature and IS did not significantly affect the transmittance of the freshly prepared oil-in-water emulsion over the studied ranges. Various amounts of responsive MNPs were introduced into the freshly prepared emulsion, and its mixture was then shaken by turbine mixer for 5 min to let the MNPs assemble onto the diesel droplet surface. Subsequently, the MNPs coated diesel droplets were separated by using a magnet. The transmittance of water was examined to evaluate the performance of synthesized MNPs. 2.3. Recycle Test After oil-water separation, the multi-responsive MNPs could be regenerated by three different approaches, respectively: (1) the spent MNPs were rinsed by hot water (70°C) three times to eliminate the adhesional oil; (2) the spent MNPs were washed with alkaline water (pH=13) three times; (3) the spent MNPs were rinsed by room temperature deionized water three times. Regeneration approach could be selected based upon the actual situation. The recovered MNPs were then recycled and their cyclic utilization was carried out for five times to evaluate the recyclability.
6
3. Results and discussion 3.1. Characterizations of MNPs Typical TEM photographs of Fe3O4 (S0), Fe3O4@SiO2-MPS (S1) and multi-responsive MNPs (S6) are shown in Figure S1. Fe3O4 MNPs prepared via solvothermal method exhibited a globular shape with an average size of around 200 nm (Figure S1a), while Fe3O4@SiO2-MPS MNPs showed a core-shell structure with a thin and grey silica layer on surface (Figure S1b). After surface grafting with multi-responsive copolymer, the MNPs were inclined to attach with each other, and a layer of hazy polymer film was observed between the adjacent MNPs (Figure S1c). Figure S2 shows the XRD patterns of the synthesized MNPs. XRD peaks at 2θ = 30.2, 35.5, 43.1, 53.5, 57.3 and 62.8°corresponded to the inverse spinel structure of Fe3O4 (Liu et al., 2009). Meanwhile, approximate diffraction peaks are also observed for Fe3O4@SiO2-MPS and multi-responsive MNPs, suggesting that the crystal phase was unchanged during the surface coating and grafting.
The successful synthesis of multi-responsive MNPs could also be confirmed by the FTIR spectra (Figure 1a). Typical peak in relation to Fe-O stretching was observed at 582 cm-1 (Naushad et al., 2019a), and the peaks at 1640 and 1410 cm-1 were ascribed to the symmetric and asymmetric stretching vibrations of carboxylate group (Li et al., 2014), indicating part of trisodium citrate and sodium acetate coordinated with iron ion. After surface coating with silica layer, a new intense peak appeared at around 1088 cm-1 because of the Si-O-Si vibrations (Li et al., 2014). After surface graft copolymerization, the band at around 2900 cm-1 became relatively obvious due to the methyl and methylene vibrations (Naushad et al., 2019b). The band at 1547 cm-1 was assigned to N-H bending vibration (Zhang et al., 2016b), suggesting the successful grafting of PNIPAM; the band of carboxylate group in SMA unit 7
was overlapped with the carboxylate group in Fe3O4, and therefore the incorporation of SMA unit into grafted chains was unable to be verified by FTIR spectra. However, the successful introduction of SMA unit in grafted copolymer was confirmed by the zeta potential measurement at various pH levels (Figure 1b). Before the graft copolymerization, Fe3O4@SiO2-MPS MNPs were always negatively charged over the studied pH range due to the exposure of silica layer. After surface grafting of neutral PNIPAM chains, the zeta potential increased significantly although the MNPs were still negatively charged. When more SMA monomer was introduced into the graft copolymerization system, zeta potential of resulting MNPs declined gradually, suggesting that more SMA unit was grafted on nanoparticle surface. Insert Figure 1 here
Figure S3 shows the TGA curves of the synthesized MNPs. A thermal weight loss of 10.7% was observed for Fe3O4@SiO2-MPS MNPs (S1) from 25 to 800°C, owing to the dehydration and thermolysis of organic substance. After surface graft copolymerization, the weight loss of responsive MNPs (S2, S3, S4, S5 and S6) increased to 15.9%, 16.4%, 13.6%, 13.0% and 13.1%, respectively. The additional weight loss was assigned to the decomposition of surface-grafted polymer. On this basis, the grafting ratios were calculated be 5.2%, 5.7%, 2.9%, 2.3% and 2.4% for S2, S3, S4, S5 and S6, respectively. Meanwhile, it should be noted that the thermal decomposition behavior of S2 is different from that of S3, especially in the range of 350-550 oC. After introduction of SMA unit into the grafted chains, the polymer decomposed relatively slow between 350 and 550 oC, presumably due to the hydrogen bonding between the amide group of NIPAM units and carboxylic group of SMA units (Shieh et al., 2018). To achieve the goal of efficient oil-water separation, the MNPs should have good 8
water dispersibility and be able to be rapidly collected in the presence of magnetic field. Therefore, superparamagnetism and high saturation magnetization are very important for the synthesized MNPs. It was found that all MNPs showed negligible remanence and coercivity, suggesting the superparamagetism of Fe3O4 core (Figure 1c). The superparamagnetic behavior could be explained by the fact that each Fe3O4 microsphere prepared via solvothermal method was composed of many nano-sized primary particles, which are usually smaller than 30 nm (Wang et al., 2015). The saturation magnetization of bare Fe3O4 MNPs was determined to be 69.1 emu/g. After incorporation of silica shell and PNIPAM arms, the saturation magnetization was reduced to 42.9 emu/g (S1) and 33.6 emu/g (S2), respectively. Considering that the difference of grafting ratio was not significant between polymer-grafted MNPs, the saturation magnetization should be also close for all multi-responsive MNPs. Thus, the synthesized multi-responsive MNPs were suitable for removing emulsified oil droplets from aqueous media by applying magnetic field, just as shown in Figure 1(I, II, III).
3.2. Separation performance and recyclability of MNPs The grafted copolymer P(NIPAM-co-SMA) is sensitive to pH, temperature and IS, and therefore the impact of each of these three parameters on the oil-water separation performances of multi-responsive MNPs were investigated in detail as follow. 3.2.1. pH effect and regeneration via pH regulation In order to study the suitability of the pH range, the MNPs were tested to evaluate its performance at different pH levels (Figure 2a-e). PNIPAM has good interfacial activity and hence the PNIPAM-coated MNPs are able to efficiently assemble onto the emulsified oil droplet surface at various pH levels, thereby facilitating subsequent magnetic separation. As 9
shown in Figure 2a, the sample S2 exhibited superior oil-water separation performance when its dosage exceeded 200 mg/L, and the pH had little influence on the separation efficiency. However, after SMA unit was incorporated into the grafted copolymer chains, the MNPs became pH-sensitive and its separation efficiency decreased with pH rising; moreover, the effect of pH on its separation efficiency became more significant with increasing amount of SMA units (Figure 2b-e). For example, S3 was still effective at various pH levels (Figure 2b), S4 was efficient at low and medium pH levels (Figure 2c), while S5 and S6 were merely valid at low pH level (Figure 2d-e). The reason can be explained as follow. Ionization of SMA unit became remarkable at higher pH level, hence the MNPs became more hydrophilic and its charge repulsive force between nanoparticles and oil droplets increased (Lu et al., 2013; Gao et al., 2013); accordingly, the MNPs could not attach to the oil droplet with pH rising (Figure S4 (a-d)). In other word, the multi-responsive MNPs would desorb from the oil droplet surface under high pH condition. Consequently, the spent MNPs (S4) was rinsed with alkaline water for their regeneration. According to our previous study (Lü et al., 2018a), when the water transmittance reached 75%, the oil removal ratio exceeded 98%. As a result, the multi-responsive MNPs (800 mg/L) still retained efficient separation performance at medium pH level (pH 6.0) after five cycles despite the slightly decreased water transmittance (Figure 2f), which confirmed that water pH regulation was an effective approach for the regeneration of MNPs. Insert Figure 2 here After cycle use, the MNPs (S4) was further washed with alkaline water and deionized water respectively to remove the attached oil. Thereafter, various technologies were used to characterize the dried samples (S4) before and after cycle use, and the corresponding results were compared (Figure S5). As shown in Figure S5a, the peaks at around 2926 and 2852 cm-1 due to the methyl and methylene vibrations became more intensive relative to the peak at 10
1092 cm-1 (Si-O-Si vibrations) after cycle use; meanwhile, the C content of surface elemental composition increased from 66.89 to 84.79% according to the X-ray photoelectron spectrum (XPS) examination (Figure S5b), while the other elemental contents decreased after cycle use. These results suggested that a certain amount of oil still remained in the MNPs after cycle use, even though the MNPs were regenerated by washing with alkaline water and deionized water, respectively. The nitrogen adsorption-desorption isotherms curves of MNPs are shown in Figure S5(c, d). The absorbed quantity increased with increasing relative pressure and the surface area of sample S4 before use was calculated to be 45 m2/g. On the contrary, as shown in Figure S5d, the absorbed quantity decreased at first, presumably due to the volatilization of residual oil on MNPs after cycle use. As a result, the surface area could not be well calculated after cycle use; theoretically, its surface area was probably reduced because of the oil attachment. Moreover, it was found that the surface of MNPs became rougher after cycle use probably due to the residual oil pollution as shown in Figure S5 (e, f). In a word, the MNPs were gradually contaminated by oil during the cycle use, leading to the decrease in their interfacial activity; accordingly, the oil-water separation efficiency would be reduced to some degree after the continuous cycle use of MNPs. 3.2.2. Temperature effect and regeneration via temperature regulation Our previous study showed that temperature significantly affected the oil-water separation performance of PNIPAM-grafted MNPs (Lü et al., 2016). After incorporation of SMA unit, the MNPs also became pH-responsive. Accordingly, the influence of temperature on oil removal efficiency was investigated as a function of pH levels (Figure 3a-b). Herein, the dosage of MNPs in Figure 3a was 100, 125, 150, 150 and 150 mg/L for S2, S3, S4, S5 and S6, respectively, while the dosage of MNPs in Figure 3b was 100, 400, 400, 600 and 600 mg/L for S2, S3, S4, S5 and S6, respectively. At pH 4.0, the SMA unit was protonated and became relatively hydrophobic, therefore P(NIPAM-co-SMA) exhibited similar LCST as compared to 11
that of PNIPAM homopolymer (~32°C) (Lu et al., 2013; Gao et al., 2013). When the temperature was lower than LCST, a moderate increase in separation efficiency was observed due to the increasing hydrophobicity of NIPAM units with temperature rising. Nonetheless, the separation efficiency decreased significantly once the temperature was above LCST, due to the dramatic hydrophobicity transition of grafted copolymer (Chen et al., 2014; Lü et al., 2016). Accordingly, all synthesized multi-responsive MNPs exhibited optimal separation efficiency at around 30°C (Figure 3a). At pH 7.0, the SMA unit became ionized and hydrophilic. The LCST did not change significantly when a small amount of SMA unit was introduced, and hence the MNPs (S3 and S4) still showed optimal performance at around 30°C; however, the LCST was reported to increase to ~36°C at pH 7.0 when the SMA content increased to 10% (Gao et al., 2013), and the MNPs S5 remained efficient with temperature rising from 30 to 35°C (Figure 3b). Moreover, it is noteworthy that the demulsification efficiency of S5 was enhanced dramatically as temperature rising from 5 to 30 °C. This phenomenon indicated the hydrophobic interaction resulted from warming (< LCST) could to some extent surpass the charge repulsion between oil droplets and MNPs, thereby favoring the attachment of MNPs onto the oil droplet. Even increasing SMA content to 50%, the separation efficiency still increased slightly when the temperature reached 30°C, although the grafted copolymer became more hydrophilic and carried more negative surface charges. Inert Figure 3 here In short, the oil-water separation efficiency was enhanced at first with temperature rising but decreased dramatically once the temperature exceeded LCST. When SMA unit was protonated, the thermal-responsive behavior of the synthesized MNPs was similar and the optimal operating temperature ranged from 25 to 30°C; when more SMA units were ionized, the LCST increased accordingly and hence the MNPs could be potentially suitable to treat the oily wastewater with higher temperature. As discussed earlier, when the temperature exceeded 12
LCST, the MNPs would aggregate with each other and desorb from the emulsified oil droplet. Accordingly, the spent MNPs (S4) were rinsed with hot deionized water (70 oC) for its regeneration. Results showed that the MNPs (800 mg/L) remained high efficiency after five cycles at medium pH level (Figure 3c), indicating that water temperature regulation was an alternative approach for the regeneration of MNPs. 3.2.3. IS effect and regeneration via IS regulation Oily wastewater usually contains a lot of electrolytes, and therefore it is of great importance to examine the influence of IS on oil-water separation performance. It is evident in Figure 4a that the separation efficiency was significantly enhanced by increasing IS under various conditions, when the dosage of MNPs was kept at 143 mg/L. On the one hand, the ion partly shielded the surface negative charge as indicated by the zeta potential variation (Figure 4b), and therefore the charge repulsion was reduced significantly; on the other hand, the addition of salt also enhanced the hydrophobicity of the MNPs to some extent (Yang et al., 2006; Björkegren et al., 2017). Both of above-mentioned factors were favorable for the accumulation of MNPs on oil droplet surface (Figure S4(e)). Conversely, the MNPs would desorb from oil droplet surface when the ionic strength decreased dramatically (Figure S4(f)), for example during the rinsing process with deionized water. To verify this deduction, the spent S5 was rinsed with room temperature deionized water for regeneration and its reusability was evaluated toward treating an emulsion containing 0.1 mol/L of NaCl. It was found that S5 (143 mg/L) could be recycled and used for at least five times under medium pH condition (Figure 4c), suggesting that rinsing with room temperature deionized water was indeed a convenient and green approach for the regeneration of MNPs. Inert Figure 4 here As mentioned above, the separation performances of multi-responsive MNPs were 13
significantly influenced by environmental pH, temperature and IS. The synthesized MNPs were inclined to assemble at oil droplet surface at a temperature less than LCST, lower pH level or higher IS, thereby separating emulsified oil droplets from water phase by applying magnetic field. Meanwhile, the multi-responsive MNPs were also capable of reversibly desorbing form oil droplet surface at a temperature higher than LCST, higher pH level or lower IS, and hence the spent MNPs could be regenerated by three different approaches: (1) washing with hot water to remove the attached oil, and this approach (temperature regulation) could be taken into account when treating room temperature oily wastewater; (2) washing with alkaline water, and this approach (pH regulation) was more suitable for treating acidic and neutral oily wastewaters; (3) rinsing with room temperature deionized water, and this approach (IS regulation) was particularly applicable to treat oily wastewater with high salinity. These regeneration approaches were convenient, mild and eco-friendly, and could be selected based upon the actual situation. Besides, we speculate that the particle size, grafting chain length and grafting density also have an important impact on their separation efficiency and recyclability, which will be investigated in detail in our future research work.
4. Conclusions A series of recyclable multi-responsive MNPs with containing a magnetic core and a layer of triple sensitive copolymer brushes were successfully fabricated to remove emulsified oil from water phase. The magnetic core was prepared through a solvothermal method and followed by surface coating with silica layer, and the surface-tethered copolymer arms were produced through a grafting reaction. The oil-water separation efficiency of synthesized MNPs was enhanced at first with temperature rising but declined significantly once the temperature exceeded LCST, and declined monofonically with pH rising or IS decreasing over the studied 14
range. According to the character of oily wastewater, three different regeneration approaches for the spent MNPs were proposed, namely rinsing with alkaline water, hot water (70oC) and room temperature deionized water, respectively. Based on the three regeneration methods, the multi-responsive MNPs could be recycled at least five times without showing significant loss in separation efficiency. It is expected that this study can provide some new idea for developing advanced materials and green technologies for efficiently treating emulsified oily wastewaters.
Acknowledgements The authors greatly appreciate the support from the National Natural Science Foundation of China
(21878064),
and
the
Natural
Science
Foundation
of
Zhejiang
Province
(LZ18E030002).
References
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20
Figure captions Figure 1 FTIR spectra (a), zeta potential at various pH levels (b) and magnetic hysteresis loops (c) of the synthesized MNPs; insert shows the oil-water separation process: (I) emulsion, (II) mixture of emulsion and S2, (III) after magnetic separation
Figure 2 Oil-water separation performances of the synthesized MNPs at various pH levels (a-e) and the reusability of sample S4 regenerated via pH regulation (f)
Figure 3 Effect of temperature on oil-water separation performance of the MNPs at various pH levels (a-b) and reusability of sample S4 regenerated via temperature regulation (c)
Figure 4 Effect of NaCl concentration on the oil-water separation performance (a) and zeta potential (b), and reusability of sample S5 regenerated by IS regulation (c)
Table 1 Recipes for the synthesis of multi-responsive MNPs
21
Table 1 Recipes for the synthesis of multi-responsive MNPs sample number
sample name
NIPAM (mol)
SMA (mol)
monomer ratio
S0
Fe3O4
/
/
/
S1
Fe3O4@SiO2-MPS
/
/
/
S2
responsive MNPs
0.024
0
1:0
S3
responsive MNPs
0.0234
0.0006
39:1
S4
responsive MNPs
0.0228
0.0012
19:1
S5
responsive MNPs
0.0216
0.0024
9:1
S6
responsive MNPs
0.012
0.012
1:1
Figure 1
22
Figure 2
Figure 3
23
Figure 4
Graphical abstract
24
Highlights
·Temperature, pH and IS-responsive MNPs were fabricated for oil-water separation. ·The MNPs exhibited superior performance at lower pH, lower temperature or higher IS. ·Spent MNPs could be regenerated by raising pH, raising temperature or decreasing IS. ·The MNPs could be recycled at least five times with negligible reduction in efficiency. ·The MNPs were particularly applicable to treat practical saline oily wastewater.
Declaration of Interest Statement
The authors declare that they have no conflict of interest.
1
Author Contributions Section
Author Contributions: Nanoparticles synthesis, T.L. and K.F.; nanoparticle characterization, T.L. and D.Q.; experimental data analysis, D.Z.; oil-water separation, K.F. and Y.L.; manuscript preparation, T.L.; experimental design and manuscript review, H.Z.
1