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Demulsification of oleic acid-coated magnetite nanoparticles for cyclohexane-in-water nanoemulsions Jiling Liang, Haiping Li, Jingen Yan, and Wan Guo Hou Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501169m • Publication Date (Web): 12 Aug 2014 Downloaded from http://pubs.acs.org on August 16, 2014
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Graphical Abstract
Oleic acid-coated magnetite nanoparticles were used as magnetic demulsifiers in cyclohexane-in-water nanoemulsions. Demulsification efficiency was influenced by the wettability of the magnetic nanoparticles.
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Figures
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Fig 1. TEM images of bare Fe3O4 and Fe3O4@OA sample S4.
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Fig 2. XRD patterns of bare Fe3O4 and Fe3O4@OA samples.
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Fig 3. FT-IR spectra of OA, bare Fe3O4 and Fe3O4@OA samples.
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Fig 4. TG/DTG curves for bare Fe3O4 and Fe3O4@OA samples.
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Fig 5. Magnetization curves of bare Fe3O4 and Fe3O4@OA sample S4. Inset shows
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photographs of the (A) nanoemulsion, (B) S4 dispersed nanoemulsion, and (C)
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S4 nanoemulsion after demulsification using a hand magnet.
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Fig 6. Effect of magnetic samples dosage (CS) on the demulsification efficiency (ED) for the nanoemulsion. Fig 7. Effect of wettability of magnetic samples on their demulsification efficiency (ED) at CS = 30.0 g·L-1.
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Fig 8. Effect of pH on demulsificasion efficiency (ED) of S4 sample at CS=40.0 g·L-1.
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Fig 9. Effect of salt concentration on demulsificasion efficiency (ED) of sample S3 at
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CS = 20.0 g·L-1. Fig 10. Demulsification efficiency of sample S4 during subsequent cycles at CS = 40.0 g·L-1.
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Fig 1. TEM images of bare Fe3O4 and Fe3O4@OA sample S4.
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Fig 2. XRD patterns of bare Fe3O4 and Fe3O4@OA samples.
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Fig 3. FT-IR spectra of OA, bare Fe3O4 and Fe3O4@OA samples.
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Fig 4. TG/DTG curves for bare Fe3O4 and Fe3O4@OA samples.
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Fig 5. Magnetization curves of bare Fe3O4 and Fe3O4@OA sample S4. Inset shows
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photographs of the (A) nanoemulsion, (B) S4 dispersed nanoemulsion, and (C)
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S4 nanoemulsion after demulsification using a hand magnet.
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Fig 6. Effect of magnetic samples dosage (CS) on the demulsification efficiency (ED) for the nanoemulsion.
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Fig 7. Effect of wettability of magnetic samples on their demulsification efficiency (ED) at CS = 30.0 g·L-1.
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Fig 8. Effect of pH on demulsificasion efficiency (ED) of S4 sample at CS=40.0 g·L-1.
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Fig 9. Effect of salt concentration on demulsificasion efficiency (ED) of sample S3 at CS = 20.0 g·L-1.
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Fig 10. Demulsification efficiency of sample S4 during subsequent cycles at CS = 40.0 g·L-1.
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Table Captions Table 1. Characterization of the magnetic nanoparticles
Table 1. Characterization of the magnetic nanoparticles. AO
Sample
RO/M (g·g-1)
(g·g-1)
S0 S1 S2 S3 S4 S5 S6
/ 0.04 0.12 0.19 0.48 0.96 1.92
/ 0.03 0.08 0.11 0.11 0.11 0.13
(mg·m-2)
Dh (nm)
/ 0.34 0.93 1.29 1.15 1.23 1.53
11.3 13.2 13.5 13.6 12.1 12.9 13.6
ao (nm ·molecule-1)
θW (º)
2
/ 0.87 0.52 0.37 0.42 0.37 0.31
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pH 6.3
pH 11.5
27 47 76 113 95 110 124
16 35 49 110 96 111 125
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Demulsification of oleic acid-coated magnetite nanoparticles for
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cyclohexane-in-water nanoemulsions
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Jiling Liang a, Haiping Li b, Jingen Yan c, Wanguo Hou a, d *
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a
Environment Research Institute, Shandong University, Jinan 250100, P.R. China.
8
b
National Engineering Technology Research Center for Colloidal Materials, Shandong University, Jinan 250100, P.R. China
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c
257237, P.R. China
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Gudong Oil Production Factory, Shengli Oilfield Company, SINOPEC, Dongying
d
Key Laboratory of Colloid & Interface Chemistry (Ministry of Education), Shandong University, Jinan 250100, P.R. China.
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* To whom correspondence should be addressed
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Tel.: +86 531 88365460
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Fax: +86 531 88564750
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E-mail:
[email protected] 20 21
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Abstract
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Oleic acid (OA) coated-magnetite (Fe3O4) nanoparticles, denoted Fe3O4@OA, were
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synthesized by co-precipitation in the presence of varying contents of OA. The Fe3O4@OA
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nanoparticles were characterized by X-ray diffraction, transmission and scanning electron
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microscopies, Fourier transform-infrared spectroscopy, thermal gravimetric-differential
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thermal gravimetric analyses and vibrating sample magnetometry. Increasing the OA
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content during preparation resulted in an increase of OA coating amount (AO, in units of g
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OA/g Fe3O4) on the Fe3O4 surface, before reaching an equilibrium value. The resulting
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magnetic nanoparticles were nearly spherical, and of size ∼ 12−14 nm. OA molecules
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formed a single layer coating on the Fe3O4 surface. The AO and area occupied by a single
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OA molecule at saturation coating were estimated to be 0.11 g·g-1 (1.22 mg·m-2) and 0.37
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nm2, respectively. The Fe3O4@OA nanoparticles were applied in the demulsification of a
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cyclohexane-in-water nanoemulsion, under an external magnetic field. The effects of AO,
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demulsifier dosage, pH and electrolytes on the demulsification efficiency (ED) were
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investigated. The ED increased and then decreased with increasing AO, which was attributed
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to a change in wettability of the magnetic nanoparticles. A maximum ED of ~98% was
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observed at a ~90º contact angle between water and the magnetic nanoparticles. The ED was
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independent of pH and electrolyte (NaCl or CaCl2) concentration, under the studied
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conditions. The magnetic demulsifier exhibited excellent stability after reuse over six
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cycles. Fe3O4@OA nanoparticles are effective for oil-water multiphase separation, and
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treating oily wastewater.
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Keywords: Magnetic nanoparticle, nanoemulsion, demulsification, oleic acid, magnetite
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1. Introduction
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Magnetic nanoparticles (MNPs) functionalized by organic and/or inorganic substrates
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have attracted much interest, because of their potential in high-density data storage (1),
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catalysis (2, 3), contrast enhancement in magnetic resonance imaging (4), drug delivery (5,
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6) and wastewater purification (7, 8) applications. Magnetite (Fe3O4) is an ideal magnetic
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material because of its low cytotoxicity and good biocompatibility (9, 10). Applying
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functional MNPs in oil-water multiphase separation has received increased recent attention
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(11-14). This is because of their response to external magnetic fields, and easy separation
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from multiphase systems under an external magnetic field. MNPs with interfacial activity
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and dispersibility can accumulate at oil-water interfaces and/or within dispersed droplets,
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imparting their magnetic properties on the dispersed droplets. Under an applied magnetic
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field, the magnetically tagged droplets can rapidly coalesce, and be isolated from the
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continuous phase (11a). MNPs are commonly modified with active substances (surfactants
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or polymers), to improve their interfacial activity and dispersibility. Peng et al. (11) grafted
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ethyl cellulose on the surface of Fe3O4 nanoparticles, to impart the MNPs with interfacial
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activity. The resulting interfacial-active and magnetically responsive ethyl cellulose-grafted
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Fe3O4 could be used as a magnetic demulsifier to remove water droplets from
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water-in-heavy naphtha or water-in-diluted bitumen emulsions, by an external magnetic
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field. Lemos et al. (12) fabricated a magnetic amphiphilic composite of hydrophobic carbon
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nanotubes/nanofibers growing on a hydrophilic surface of MgSi fibers and carbon-coated
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Fe nanoparticles, by chemical vapor deposition. The magnetic amphiphilic composite was
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used to separate oil droplets from a biodiesel-in-water emulsion. Li et al. (14) modified
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magnetite nanoparticles using a polyether polyol demulsifier commonly used in the oil
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industry. The resulting magnetic demulsifer could be used to remove oil droplets from an
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oil-in-water (O/W) emulsion, by an external magnetic field. Magnetic demulsifiers can be 3
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recovered by magnetic separation, and be reused with or without regeneration (11b).
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Previous studies (11, 12, 14) have demonstrated the potential of interfacial-active MNPs in
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oil-water multiphase separation and oily wastewater treatment.
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Significant amount of oily wastewater are generated in industrial processes involving
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petroleum, pesticides, pharmaceuticals, essential oils and flavors (12, 14, 15). Oily
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wastewater is an ecological hazard, and therefore should be treated before discharge. Many
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efforts have been focused on treating oily wastewater (16-23), most of which are in the
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form of O/W emulsions and highly stable due to the presence of interfacial-active
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substances. Thus, they are hard to treat, especially those containing droplet sizes < 20 µm
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(19). Techniques to treat oily wastewater include flotation (16), chemical coagulation
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coupled with flotation (17), chemical and electrochemical demulsification (18), membrane
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separation (19), microwave demulsification (20), freeze/thaw treatment (21), combined
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demulsification
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demulsification with tailored magnetic demulsifiers provides an alternative, eco-friendly
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and efficient strategy for treating oily wastewater (11-14). However, few studies have
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investigated this possibility (12, 14) and there remains a need to develop more magnetic
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demulsifiers, and better understand their magnetic demulsification behaviors.
and
reverse
osmosis
(22),
and
biotechnology
(23).
Magnetic
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In the current study, oleic acid (OA)-coated magnetite (Fe3O4) nanoparticles, denoted
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Fe3O4@OA, were synthesized by co-precipitation in the presence of OA. The Fe3O4@OA
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nanoparticles were applied in the demulsification of a cyclohexane-in-water (O/W)
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nanoemulsion, under an external magnetic field. The effects of the OA coating amount,
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demulsifier dosage, pH and electrolyte on the oil removal efficiency from the nanoemulsion
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were investigated. While the synthesis and characterization of Fe3O4@OA nanoparticles
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has received much attention (24-32), to the best of our knowledge this is the first report of
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Fe3O4@OA nanoparticles being applied to degrade nanoemulsions. This work improves the
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understanding of the magnetic demulsification behavior.
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2. Experimental
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2.1. Materials
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Cyclohexane, ethanol, chloroform, sodium hydroxide and Sudan III were purchased
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from Damao Chemical Reagent Co., P. R. China. FeCl3·6H2O, FeSO4·7H2O, NaCl,
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CaCl2·2H2O and ammonium hydroxide (25-28 wt% NH3 in water) were purchased from
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Sinopharm Chemical Reagent Co., P. R. China. Tween 60 and OA were purchased from
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Kermel Chemical Reagent Co., P. R. China. All chemicals were of analytical grade
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except Tween 60 (chemical pure) and were used as received. Deionized water was
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obtained from a Hitech-Kflow water purification system (Hitech, P. R. China).
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2.2. Preparation Fe3O4@OA nanoparticles
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Fe3O4@OA nanoparticles were prepared by chemical co-precipitation, using a
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modified version of a procedure reported by Yang et al. (27). Briefly, 5.56 g (0.020 mol) of
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FeSO4·7H2O and 11.60 g (0.043 mol) of FeCl3·6H2O were dissolved in 350 mL of
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deionized water under a flow of N2. The solution was heated to 80 °C and stirred
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vigorously. A total of 20 mL of ammonium hydroxide was added rapidly, and the resulting
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suspension was stirred vigorously for 10 min. A given volume (0.20–10 mL) of OA was
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added, and the mixture was maintained at 80 °C for 60 min. The mixture was cooled to
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room temperature naturally. The black product was collected using a magnet, and
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thoroughly washed with ethanol and deionized water to remove excess OA. The resulting
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Fe3O4@OA nanoparticles were dried under vacuum at 60 ºC for 12 h.
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The mass ratio of OA to expected Fe3O4 (RO/M) of the raw materials was designed to
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be tuned in the range of 0.04−1.92 g·g-1. The Fe3O4@OA samples obtained at a RO/M of
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0.04, 0.12, 0.19, 0.48, 0.96 and 1.92 g·g-1 were denoted as S1, S2, S3, S4, S5 and S6,
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respectively (Table 1). 5
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For comparison, bare Fe3O4 nanoparticles (denoted as S0) were synthesized by the
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same process but in the absence of OA.
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2.3. Preparation of cyclohexane-in-water nanoemulsions
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A mixture of 10 wt.% cyclohexane, 10 wt.% Tween 60 and 80 wt.% deionized water
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was stirred using a GJ-2S high mixing machine (Qingdao Haitongda Dedicated Instrument
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Co., P. R. China) at 9500 rpm for 20 min, yielding the cyclohexane-in-water mother
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nanoemulsion.
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The mother nanoemulsion was diluted by 1/10 with deionized water, and then used for
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demulsification tests. This tested nanoemulsion had a mean droplet size of 262 nm (Fig. S1,
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Supporting Information) and was very stable. No significant change in droplet size was
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observed by dynamic laser light scattering (DLS) analysis during 10 days.
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The effects of pH and electrolytes (NaCl and CaCl2) on the demulsification efficiency
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of the Fe3O4@OA nanoparticles were investigated. For the effect of pH, the mother
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nanoemulsion was diluted with deionized water whose pH had been previously adjusted
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using NaOH. For the effect of electrolytes, the mother emulsion was diluted with brine.
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2.4. Demulsification tests
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The demulsification ability of the Fe3O4@OA nanoparticles was determined by
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measuring the residual oil content of nanoemulsion after settling on a hand magnet. A given
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amount (0.050–0.400 g) of Fe3O4@OA nanoparticles and 10 mL of freshly prepared
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nanoemulsion were thoroughly mixed in a 40 mL glass vial. The mixture was shaken in a
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THZ-82 thermostatic water bath shaker (Wuhan Grey Mo Lai Detection Equipment Co., P.
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R. China) at 240 cycles·min-1 for 3 h at 25 °C. Oil removal kinetic tests indicated that 3 h of
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shaking was sufficient to achieve equilibrium (Fig. S2, Supporting Information). The solid
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material was then removed by a 5000 Gs NdFeB magnet (Zibo Dry Magnetic Industry
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Science and Technology Co., P. R. China). The residual oil content of the liquid phase was
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measured by monitoring the absorbance at 523 nm using a 8453 UV-Vis spectrometer
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(Hewlett-Packard Co., P. R. China), and compared with a standard curve obtained from a
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series of standard nanoemulsions with different oil contents. The demulsification efficiency
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(ED) was calculated from the measured residual oil content by
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ED (%) = [(C0 – Ce)/C0] × 100
(1)
where C0 and Ce were the initial and residual oil contents of the liquid phase, respectively.
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Each sample was tested three times, and reported ED values are averages of the three
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repeats. For comparison, blank tests were performed for nanoemulsions in the absence of
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demulsifier. The relative error of the demulsification tests was S2 > S6 >
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S1 > bare Fe3O4. Thus, ED increased and then decreased with increasing AO, with an AO of
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1.15 mg·m-2 (S4) exhibiting the highest ED. This indicated that the OA coating density on
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the Fe3O4 surface played an important role in the demulsification. The bare Fe3O4
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nanoparticles exhibited some demulsification of the nanoemulsion, demonstrating that the
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nanoparticles possessed limited interfacial activity. The OA coating on the Fe3O4 surface
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increased the interfacial activity of the magnetic nanoparticles, thus increasing their
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degradation capacity.
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The variation of ED with AO could be explained by the change in the wettability of the
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Fe3O4@OA samples (39, 40). Fe3O4@OA nanoparticles with a θW of ~90° reportedly
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strongly accumulate at the oil–water interface, with more hydrophilic particles entering the
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water phase and more hydrophobic particles entering the oil phase (28). Pickering
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emulsions using Fe3O4@OA nanoparticles as an emulsifier are less stable when the θW of
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the nanoparticles approaches 90° (28). In the current study, the θW of the bare Fe3O4 was
