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Magnetic nano-Fe3O4 stabilized Pickering emulsion liquid membrane for selective extraction and separation
Chemical Engineering Journal 288 (2016) 305–311
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Magnetic nano-Fe3O4 stabilized Pickering emulsion liquid membrane for selective extraction and separation Zhaoyun Lin a, Zhe Zhang b, Youming Li a, Yulin Deng b,⇑ a b
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road, Guangzhou, Guangdong 510640, PR China School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA
h i g h l i g h t s Pickering emulsion was prepared with oleic acid-coated Fe3O4 nanoparticles. W/O/W Pickering emulsion liquid membrane (PELM) was used for extraction. The oil phase and particulate stabilizer can be recovered and repeatedly used. The extraction efficiency of over 86% was achieved.
a r t i c l e
i n f o
Article history: Received 25 September 2015 Received in revised form 1 November 2015 Accepted 4 November 2015 Available online 11 December 2015 Keywords: Pickering emulsion Nano-Fe3O4 Membrane Phenol Extract efficiency Recycle
a b s t r a c t Emulsion liquid membrane (ELM) is a fast, effective and highly selective method for chemical separation, extraction and water treatment. However, an effective and economic method for demulsification after extraction process without changing the oil phase chemistry is still a great challenge, which restricts the commercial application of ELM. Hereby, for the first time, we reported a W/O/W Pickering emulsion liquid membrane (PELM) system in which the internal water in oil emulsion can be simply demulsified by magnetic or centrifugation force without causing obvious change in oil phase chemistry. Practically, we selected 4-methoxyphenol as extracting target in this report, and its extraction efficiency of over 86% was achieved by using surface modified Fe3O4 nanoparticle as the emulsifier. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Emulsion liquid membrane has been well studied in separation, extraction and wastewater treatment application [1–6]. Although ELM is featured by simplicity, high selectivity, rapid extraction and low energy consumption, the commercial application of ELM has been limited due to instability of emulsions and the difficulty of demulsification after extraction [7–12]. In order to recover the internal phase and reuse the oil phase, the internal emulsion must be broken up after extraction. Because most emulsions are stabilized by surfactant, the demulsification after extraction is very difficult. Chemical demulsification method is not acceptable because the oil phase cannot be reused after chemical demulsification. Physical demulsification methods, such as electrical, ultrasonication, and thermal treatment, have been reported [13–15], but
⇑ Corresponding author. E-mail address: [email protected] (Y. Deng). http://dx.doi.org/10.1016/j.cej.2015.11.109 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
the high energy consumption and low efficiency restricted the commercial application of ELM technology. Particle-stabilized emulsion, or Pickering emulsion, has attracted great attentions due to their remarkable stability [16– 21]. The solid particles irreversibly adsorbed at oil–water interface or existing in continuous phase can provide a steric hindrance between emulsion droplets, preventing the collision and coalescence among droplets [22–25]. Magnetic Pickering emulsions, which is stabilized by magnetic particles is one of such systems [26–31]. Meanwhile, magnetic emulsion can also be easily demulsified by quickly attracting the particle emulsifier from the droplet interface using external magnetic field. Phenol is an organic pollutant which poses a risk to the environment and human health even at low concentration [32,33]. Extraction of phenols and its substituted compounds from wastewater is a very urgent request nowadays. 4-Methoxyphenol (4-MP) is one of phenolic compounds that extensively used as polymerization inhibitors, ultraviolet inhibitors, antioxidants, chemical intermediates. In this study, we described a new class of ‘‘Pickering emulsion
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liquid membrane”, which used W1/O/W2 Pickering emulsion to selectively extract 4-MP from wastewater (as is shown in Scheme 1). Experimentally, oleic acid coated nano-Fe3O4 particles were synthesized and used as the stabilizer and emulsifier [34,35]. Sodium hydroxide solution (NaOH) was used as the internal phase (W1), corn oil was used as the external oil phase for water-in-oil emulsion. Tri-n-butyl phosphate (TBP) was used as the capture agent and mixed with corn oil. The feed phase (W2) was prepared with 300 ppm of 4-Methoxyphenol with a pH value of 1.0–2.0. After emulsification, the emulsion was immediately dispersed into feed phase and formed water in oil in water emulsion. The influences of emulsion stability, internal concentration, carrier concentration, and volume ratio of internal phase on the PELM extraction performance were studied. Reutilization of magnetic particles and oil phase in repeated PELM test was also studied. This research suggests that PELM has tremendous potential as a new class of emulsion liquid membrane for separation and extraction applications.
2. Experimental 2.1. Preparation and characterization of oleic acid modified nanoFe3O4 particles FeCl36H2O and FeSO47H2O were dissolved in 100 mL deionized water according to the molecular proportion of 2:1 and stirred for half an hour in a three-neck flask. Then 2 mol L 1 NH3H2O solution was added to adjust the pH of aqueous dispersion to 11.0–12.0. The temperature was raised to 65 °C and mixed for 2 h. 30 mL oleic acid was added to the solution and the temperature of the solution was raised to 80 °C. The pH of the mixture was then adjusted with dilute H2SO4 solution to 6.0–7.0. The precipitates were washed repeatedly by deionized water and acetone to remove excess oleic acid. Finally, the particles were dried in vacuum oven at 60 °C overnight. Infrared (IR) spectroscope was used to monitor structural changes. The size distribution was tested by a particle size analyzer (Brookhaven instruments corporation, 90 plus) using an automatic process with a 10-run method.
2.2. Preparation of Pickering emulsion stabilized by nano-Fe3O4 particle and extraction procedure The w/o emulsion was prepared with the following formulation: a certain qualities of nano-Fe3O4 and TBP were dispersed into 10 mL corn oil with ultrasonic treatment for 30 min. 5 mL of NaOH solution was added dropwise to the oil phase, and the mixture was homogenized at 10,000 rpm for 5 min. After preparation, the emulsion was immediately poured into standard cylindrical sample bottles at room temperature for further observation. 2.3. PELM experiments After preparation, the emulsion was dispersed into feed phase and continuously stirred at ambient temperature with a low rotational speed of 200 rpm for certain time interval. Then the mixture was poured into a separatory funnel, and the concentrations of 4-MP in the lower phase were tested at characteristic wavelength of 222 nm and 287 nm using an Agilent 8453 UV–visible Spectroscopy. 2.4. Recovery experiments The lower aqueous phase and upper emulsion phase was collected respectively. The lower phase was purified with magnet and filtered to get particles, and the upper phase was broken under the magnetic force using a 1T magnet. Then the particles, water and oil from the broken emulsion were separately collected. Thoroughly washed with acetone and distilled water, the collected particles were dried under vacuum at 60 °C for 12 h. The recycled particles and oil phase were used for preparing new Pickering emulsion. 3. Results and discussions 3.1. Characterization of nano-Fe3O4 particles The spectra of oleic acid-coated nano-Fe3O4 particles (Fig. 1a) exhibited the characteristic oleic acid band at 2922 and
Scheme 1. Schematic illustration of the Pickering emulsion liquid membrane process.
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100
(a) Intensity
80
1409cm-1 2922cm
-1
1456cm
2853cm-1
Mean Diam. 402.2nm PDI 0.279
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60
40
-1
1708cm-1
20
565cm-1 3500
3000
2500
2000
1500
1000
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Wavenumber (cm-1)
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Fig. 1. (a) Infared (IR) spectra and (b) the size distribution of oleic acid-coated nano-Fe3O4 particles.
2853 cm 1, which were the characteristic asymmetric and symmetric CAH stretching vibrations. And the peak at 1409 cm 1 was assigned to CAH bending vibrations. The C@O band at 1708 cm 1 and OAH band at 1456 cm 1 for the ACOOH group were absent, confirming the oleic acid was chemically bound to the Fe3O4 nanoparticles [36]. Peak at 565 cm 1 was assigned to FeAO lattice vibrations. The mean particle diameter of nanoFe3O4 particles measured by a particle size analyzer was 402 nm with a polydispersibility index of 0.279 (Fig. 1b). 3.2. Effect of emulsion stability The mass transfer interfaces are shown in Scheme 2: the membrane is consisted of carrier (TBP), corn oil and nano-Fe3O4 particulate emulsifier. The 4-MP in external water phase diffuses to the O/W2 interphase and reacts with the carrier TBP in the oil phase to form oil soluble complex of [(CH3OPhOHTBP)org]. The complex then diffuses across the oil membrane to reach the internal W1/O interface, and reacts with NaOH to form a water insoluble salt as shown as follows: CH3OPhOHTBP + NaOH = CH3OC6H5ONa + H2O + TBP. Sodium phenolate cannot diffuse back into the feed phase through liquid membrane [11] because it is insoluble in the oil phase. As a result, 4-MP is selectively extracted by the carrier from external phase to internal phase. The stability of Pickering emulsion plays an important role in the extract process. The influence of nano-Fe3O4 particle
Scheme 2. Schematic diagram of mass transfer interfaces.
concentration on the extraction of 4-MP from external phase was studied, ranging from 0.1–1.0 wt% (w/w, related to aqueous phase). As we can see from Fig. 2a and b, the emulsions were all stable in 3 h, and had no obvious phase separation. After 3 d, all emulsion creamed with clear water layer, and fw decreased with increasing particle concentration initially but then increased. As we can see from Fig. 2c, the extract efficiency increased with the concentration of nano-Fe3O4 particles to the highest at 0.5 wt% and decreased thereafter. According to the stabilizing mechanism mentioned above, the emulsions become more stable with the increasing concentration of nano-Fe3O4 particles by covering more emulsion interface, resulting in an increase in the extract efficiency. However, with further increasing nano-Fe3O4 concentration beyond full coverage of the emulsion droplets, extra nano-Fe3O4 particles were dispersed in continuous phase and some of the particles might form aggregates on W/O interface, which definitely affected the stability of emulsion and impeded the transfer process, and the extraction efficiency decreased.
3.3. Effect of internal phase concentration The stability of emulsion with varied NaOH concentration and dosage of emulsifier was investigated. Under the condition of CFe3O4 = 0.5 wt%, Row = 2 (volume ratio of oil to water) and u = 4% (volume fraction of carrier to oil phase), the emulsion was stable without creaming or phase separation over 3 h as the increasing of NaOH concentration from 0.1 mol L 1 to 0.8 mol L 1. However, the emulsion with 1.0 mol L 1 and 2.0 mol L 1 NaOH, all had separated water on the bottom, indicating the unstability of the emulsion. After settling for 3 days, all emulsion had aqueous phase separated and the volume fraction of water increased with increasing CNaOH (Fig. 3a). When the volume ratio of external phase and emulsion was fixed at 10, extract efficiency showed an increasing tendency firstly and then decreased with the increasing of CNaOH (Fig. 3b). As is known, emulsion stability is the key parameter for a successful ELM procedure, therefore, the leakage of emulsion would affect the extraction efficiency. When the NaOH concentration is low, the emulsion is stable, nevertheless, there is not enough NaOH to react with 4-MP in internal phase. Hence, the extract efficiency increases from 0.1 mol L 1 to 0.8 mol L 1 with stable emulsion. When the NaOH concentration is high, the liquid membrane is unstable, so the extract efficiency decreases.
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(a)
12
55
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fw-3h fo-3h fw-3d fo-3d Extract Efficiency (%)
Volume fraction (%)
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Fig. 2. (a) The stability of Pickering emulsions stored at room temperature versus different nano-Fe3O4 particles concentration for 3 h (left) and 3 d (right), and (b) fw represents dispersed water fraction (v/v, related to bulk volume), and fo represents dispersed oil fraction (v/v, related to bulk volume) within 3 h and 3 d respectively. (c) Extract efficiency with different nano-Fe3O4 particles concentration.
Fig. 3. (a) The stability of Pickering emulsion stored at room temperature versus different concentration of NaOH within 3 d. fw (▲) represents dispersed water fraction (v/v, related to bulk volume), and fo (.) represents dispersed oil fraction (v/v, related to bulk volume). The illustration is digital image of Pickering emulsion after settling for 3 d. (b) Extract efficiency of emulsion stabilized with different concentration of NaOH.
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90
100
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80
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Extract efficiency (%)
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20 0
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Fig. 4. Extract efficiency with different carrier concentration (a) and (b) treatment ratios.
(a)
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Extract efficiency (%)
80
60
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0
1
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cycle time Fig. 5. (a) Demulsification of collected emulsions after extraction and (b) extract efficiency with recovered particles and oil phase.
3.4. Effect of carrier concentration and treatment ratios The effects of carrier concentration and treatment ratios on the extraction were studied at constant conditions with Row = 2, CFe3O4 = 0.5 wt% and CNaOH = 0.5 mol L 1. It was found that carrier
concentration was the most effective factor in the extraction process. In Fig. 4a, extract efficiency was plotted as function of varied TBP concentration. It was observed that the increasing TBP concentration enhanced extract efficiency, since more extractable [(CH3OPhOHTBP)org] would be formed and the transport speed
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was enhanced. However, the increasing volume of carrier also changed the chemistry of the organic phase, which might affect the stability of emulsion so the extract efficiency slightly decreased. As is shown in Fig. 4, the extraction was very fast at the concentration of 30% (volume ratio of TBP to oil phase) TBP. The extract efficiency reached the highest to 86% in 2 min when Rew (volume ratio of emulsion to feed phase) was 1:5, while the extract efficiency reached to the highest to 78% in 3 min when Rew was 1:10. Thereafter, the extract efficiency decreased slightly and stayed at about 80% and 75% respectively, which also reflected the evolution of stability of emulsion. In extraction process, the volume of external was fixed at 30 ml and the total content of 4-MP in the feed phase was 9 mg. When Rew increased from 3:30 to 6:30, the removal efficiency was continuously improved. With the increasing Rew, the interface area between external water and oil phase increased, which could help the reaction of phenol and TBP. Furthermore, the increase of internal NaOH concentration could cause a fast release of phenol to inside water droplets by forming oil insoluble phenolic salt. In conclusion, the removal efficiency of 4-MP would be improved with increasing Rew. Similarly, in the case of lower Rew, the extract efficiency would be lower. 3.5. Recyclability of particles and oil phase After extraction, some big emulsion droplets at the upper layer of the mixture were collected and a 1T cylindrical magnet was used to remove the nano-Fe3O4 particles from the emulsion surface. Then emulsion was broken up, oil phase and aqueous phase were separated within 30 min (Fig. 5a). The collected oil phase and nano-Fe3O4 particles were mixed with new sodium hydroxide (0.5 mol L 1), and re-emulsified (Row = 2, CFe3O4 = 0.5 wt%). The recyclability of the emulsion was investigated over 3 times and no significant decline in the extraction efficiency was observed (Fig. 5b). The slight reduction of extract efficiency was observed, which may due to the loss of the carrier due to the partial dissolution of carrier in water.
4. Conclusions For the first time, W/O/W Pickering emulsion liquid membrane was used for removing 4-MP from wastewater. Oleic acid-coated nano-Fe3O4 particles were synthesized and used as emulsifier to stabilize water in oil Pickering emulsion. The experimental results indicated that the stability of the primary Pickering emulsion was a key parameter and TBP concentration was the most effective factor. To make a green approach, corn oil was selected as organic solvent and showed good extract efficiency. When TBP concentration was 30%, extract efficiency achieved 86% with 0.5 wt% magnetic nano-Fe3O4. A pronounced difference between this approach and traditional emulsion liquid membrane is the absence of surfactant in system, consequently, easily breaking of the emulsion under external magnetic force. Traditionally, demulsifiers have been used to destabilize and break unwanted emulsions after extraction, which results in difficulty in reusing the oil phase. The magnetically controlled systems can easily recover emulsifier, oil phase and find application in a wide range of industrial wastewater treatment process. Moreover, in principle, Pickering emulsion liquid membrane concept can also be extended to non-magnetic nanoparticles, in which, the demulsification can be achieved by centrifugation. Because the particles and oil phase can be easily collected and reused, PELM can minimize the environmental burden, reduce cost and consumption of materials.
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Magnetic nano-Fe3O4 stabilized Pickering emulsion liquid membrane for selective extraction and separation Zhaoyun Lin a, Zhe Zhang b, Youming Li a, Yulin Deng b,⇑ a b
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road, Guangzhou, Guangdong 510640, PR China School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA
h i g h l i g h t s Pickering emulsion was prepared with oleic acid-coated Fe3O4 nanoparticles. W/O/W Pickering emulsion liquid membrane (PELM) was used for extraction. The oil phase and particulate stabilizer can be recovered and repeatedly used. The extraction efficiency of over 86% was achieved.
a r t i c l e
i n f o
Article history: Received 25 September 2015 Received in revised form 1 November 2015 Accepted 4 November 2015 Available online 11 December 2015 Keywords: Pickering emulsion Nano-Fe3O4 Membrane Phenol Extract efficiency Recycle
a b s t r a c t Emulsion liquid membrane (ELM) is a fast, effective and highly selective method for chemical separation, extraction and water treatment. However, an effective and economic method for demulsification after extraction process without changing the oil phase chemistry is still a great challenge, which restricts the commercial application of ELM. Hereby, for the first time, we reported a W/O/W Pickering emulsion liquid membrane (PELM) system in which the internal water in oil emulsion can be simply demulsified by magnetic or centrifugation force without causing obvious change in oil phase chemistry. Practically, we selected 4-methoxyphenol as extracting target in this report, and its extraction efficiency of over 86% was achieved by using surface modified Fe3O4 nanoparticle as the emulsifier. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Emulsion liquid membrane has been well studied in separation, extraction and wastewater treatment application [1–6]. Although ELM is featured by simplicity, high selectivity, rapid extraction and low energy consumption, the commercial application of ELM has been limited due to instability of emulsions and the difficulty of demulsification after extraction [7–12]. In order to recover the internal phase and reuse the oil phase, the internal emulsion must be broken up after extraction. Because most emulsions are stabilized by surfactant, the demulsification after extraction is very difficult. Chemical demulsification method is not acceptable because the oil phase cannot be reused after chemical demulsification. Physical demulsification methods, such as electrical, ultrasonication, and thermal treatment, have been reported [13–15], but
⇑ Corresponding author. E-mail address: [email protected] (Y. Deng). http://dx.doi.org/10.1016/j.cej.2015.11.109 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
the high energy consumption and low efficiency restricted the commercial application of ELM technology. Particle-stabilized emulsion, or Pickering emulsion, has attracted great attentions due to their remarkable stability [16– 21]. The solid particles irreversibly adsorbed at oil–water interface or existing in continuous phase can provide a steric hindrance between emulsion droplets, preventing the collision and coalescence among droplets [22–25]. Magnetic Pickering emulsions, which is stabilized by magnetic particles is one of such systems [26–31]. Meanwhile, magnetic emulsion can also be easily demulsified by quickly attracting the particle emulsifier from the droplet interface using external magnetic field. Phenol is an organic pollutant which poses a risk to the environment and human health even at low concentration [32,33]. Extraction of phenols and its substituted compounds from wastewater is a very urgent request nowadays. 4-Methoxyphenol (4-MP) is one of phenolic compounds that extensively used as polymerization inhibitors, ultraviolet inhibitors, antioxidants, chemical intermediates. In this study, we described a new class of ‘‘Pickering emulsion
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liquid membrane”, which used W1/O/W2 Pickering emulsion to selectively extract 4-MP from wastewater (as is shown in Scheme 1). Experimentally, oleic acid coated nano-Fe3O4 particles were synthesized and used as the stabilizer and emulsifier [34,35]. Sodium hydroxide solution (NaOH) was used as the internal phase (W1), corn oil was used as the external oil phase for water-in-oil emulsion. Tri-n-butyl phosphate (TBP) was used as the capture agent and mixed with corn oil. The feed phase (W2) was prepared with 300 ppm of 4-Methoxyphenol with a pH value of 1.0–2.0. After emulsification, the emulsion was immediately dispersed into feed phase and formed water in oil in water emulsion. The influences of emulsion stability, internal concentration, carrier concentration, and volume ratio of internal phase on the PELM extraction performance were studied. Reutilization of magnetic particles and oil phase in repeated PELM test was also studied. This research suggests that PELM has tremendous potential as a new class of emulsion liquid membrane for separation and extraction applications.
2. Experimental 2.1. Preparation and characterization of oleic acid modified nanoFe3O4 particles FeCl36H2O and FeSO47H2O were dissolved in 100 mL deionized water according to the molecular proportion of 2:1 and stirred for half an hour in a three-neck flask. Then 2 mol L 1 NH3H2O solution was added to adjust the pH of aqueous dispersion to 11.0–12.0. The temperature was raised to 65 °C and mixed for 2 h. 30 mL oleic acid was added to the solution and the temperature of the solution was raised to 80 °C. The pH of the mixture was then adjusted with dilute H2SO4 solution to 6.0–7.0. The precipitates were washed repeatedly by deionized water and acetone to remove excess oleic acid. Finally, the particles were dried in vacuum oven at 60 °C overnight. Infrared (IR) spectroscope was used to monitor structural changes. The size distribution was tested by a particle size analyzer (Brookhaven instruments corporation, 90 plus) using an automatic process with a 10-run method.
2.2. Preparation of Pickering emulsion stabilized by nano-Fe3O4 particle and extraction procedure The w/o emulsion was prepared with the following formulation: a certain qualities of nano-Fe3O4 and TBP were dispersed into 10 mL corn oil with ultrasonic treatment for 30 min. 5 mL of NaOH solution was added dropwise to the oil phase, and the mixture was homogenized at 10,000 rpm for 5 min. After preparation, the emulsion was immediately poured into standard cylindrical sample bottles at room temperature for further observation. 2.3. PELM experiments After preparation, the emulsion was dispersed into feed phase and continuously stirred at ambient temperature with a low rotational speed of 200 rpm for certain time interval. Then the mixture was poured into a separatory funnel, and the concentrations of 4-MP in the lower phase were tested at characteristic wavelength of 222 nm and 287 nm using an Agilent 8453 UV–visible Spectroscopy. 2.4. Recovery experiments The lower aqueous phase and upper emulsion phase was collected respectively. The lower phase was purified with magnet and filtered to get particles, and the upper phase was broken under the magnetic force using a 1T magnet. Then the particles, water and oil from the broken emulsion were separately collected. Thoroughly washed with acetone and distilled water, the collected particles were dried under vacuum at 60 °C for 12 h. The recycled particles and oil phase were used for preparing new Pickering emulsion. 3. Results and discussions 3.1. Characterization of nano-Fe3O4 particles The spectra of oleic acid-coated nano-Fe3O4 particles (Fig. 1a) exhibited the characteristic oleic acid band at 2922 and
Scheme 1. Schematic illustration of the Pickering emulsion liquid membrane process.
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Fig. 1. (a) Infared (IR) spectra and (b) the size distribution of oleic acid-coated nano-Fe3O4 particles.
2853 cm 1, which were the characteristic asymmetric and symmetric CAH stretching vibrations. And the peak at 1409 cm 1 was assigned to CAH bending vibrations. The C@O band at 1708 cm 1 and OAH band at 1456 cm 1 for the ACOOH group were absent, confirming the oleic acid was chemically bound to the Fe3O4 nanoparticles [36]. Peak at 565 cm 1 was assigned to FeAO lattice vibrations. The mean particle diameter of nanoFe3O4 particles measured by a particle size analyzer was 402 nm with a polydispersibility index of 0.279 (Fig. 1b). 3.2. Effect of emulsion stability The mass transfer interfaces are shown in Scheme 2: the membrane is consisted of carrier (TBP), corn oil and nano-Fe3O4 particulate emulsifier. The 4-MP in external water phase diffuses to the O/W2 interphase and reacts with the carrier TBP in the oil phase to form oil soluble complex of [(CH3OPhOHTBP)org]. The complex then diffuses across the oil membrane to reach the internal W1/O interface, and reacts with NaOH to form a water insoluble salt as shown as follows: CH3OPhOHTBP + NaOH = CH3OC6H5ONa + H2O + TBP. Sodium phenolate cannot diffuse back into the feed phase through liquid membrane [11] because it is insoluble in the oil phase. As a result, 4-MP is selectively extracted by the carrier from external phase to internal phase. The stability of Pickering emulsion plays an important role in the extract process. The influence of nano-Fe3O4 particle
Scheme 2. Schematic diagram of mass transfer interfaces.
concentration on the extraction of 4-MP from external phase was studied, ranging from 0.1–1.0 wt% (w/w, related to aqueous phase). As we can see from Fig. 2a and b, the emulsions were all stable in 3 h, and had no obvious phase separation. After 3 d, all emulsion creamed with clear water layer, and fw decreased with increasing particle concentration initially but then increased. As we can see from Fig. 2c, the extract efficiency increased with the concentration of nano-Fe3O4 particles to the highest at 0.5 wt% and decreased thereafter. According to the stabilizing mechanism mentioned above, the emulsions become more stable with the increasing concentration of nano-Fe3O4 particles by covering more emulsion interface, resulting in an increase in the extract efficiency. However, with further increasing nano-Fe3O4 concentration beyond full coverage of the emulsion droplets, extra nano-Fe3O4 particles were dispersed in continuous phase and some of the particles might form aggregates on W/O interface, which definitely affected the stability of emulsion and impeded the transfer process, and the extraction efficiency decreased.
3.3. Effect of internal phase concentration The stability of emulsion with varied NaOH concentration and dosage of emulsifier was investigated. Under the condition of CFe3O4 = 0.5 wt%, Row = 2 (volume ratio of oil to water) and u = 4% (volume fraction of carrier to oil phase), the emulsion was stable without creaming or phase separation over 3 h as the increasing of NaOH concentration from 0.1 mol L 1 to 0.8 mol L 1. However, the emulsion with 1.0 mol L 1 and 2.0 mol L 1 NaOH, all had separated water on the bottom, indicating the unstability of the emulsion. After settling for 3 days, all emulsion had aqueous phase separated and the volume fraction of water increased with increasing CNaOH (Fig. 3a). When the volume ratio of external phase and emulsion was fixed at 10, extract efficiency showed an increasing tendency firstly and then decreased with the increasing of CNaOH (Fig. 3b). As is known, emulsion stability is the key parameter for a successful ELM procedure, therefore, the leakage of emulsion would affect the extraction efficiency. When the NaOH concentration is low, the emulsion is stable, nevertheless, there is not enough NaOH to react with 4-MP in internal phase. Hence, the extract efficiency increases from 0.1 mol L 1 to 0.8 mol L 1 with stable emulsion. When the NaOH concentration is high, the liquid membrane is unstable, so the extract efficiency decreases.
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(a)
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Fig. 2. (a) The stability of Pickering emulsions stored at room temperature versus different nano-Fe3O4 particles concentration for 3 h (left) and 3 d (right), and (b) fw represents dispersed water fraction (v/v, related to bulk volume), and fo represents dispersed oil fraction (v/v, related to bulk volume) within 3 h and 3 d respectively. (c) Extract efficiency with different nano-Fe3O4 particles concentration.
Fig. 3. (a) The stability of Pickering emulsion stored at room temperature versus different concentration of NaOH within 3 d. fw (▲) represents dispersed water fraction (v/v, related to bulk volume), and fo (.) represents dispersed oil fraction (v/v, related to bulk volume). The illustration is digital image of Pickering emulsion after settling for 3 d. (b) Extract efficiency of emulsion stabilized with different concentration of NaOH.
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cycle time Fig. 5. (a) Demulsification of collected emulsions after extraction and (b) extract efficiency with recovered particles and oil phase.
3.4. Effect of carrier concentration and treatment ratios The effects of carrier concentration and treatment ratios on the extraction were studied at constant conditions with Row = 2, CFe3O4 = 0.5 wt% and CNaOH = 0.5 mol L 1. It was found that carrier
concentration was the most effective factor in the extraction process. In Fig. 4a, extract efficiency was plotted as function of varied TBP concentration. It was observed that the increasing TBP concentration enhanced extract efficiency, since more extractable [(CH3OPhOHTBP)org] would be formed and the transport speed
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was enhanced. However, the increasing volume of carrier also changed the chemistry of the organic phase, which might affect the stability of emulsion so the extract efficiency slightly decreased. As is shown in Fig. 4, the extraction was very fast at the concentration of 30% (volume ratio of TBP to oil phase) TBP. The extract efficiency reached the highest to 86% in 2 min when Rew (volume ratio of emulsion to feed phase) was 1:5, while the extract efficiency reached to the highest to 78% in 3 min when Rew was 1:10. Thereafter, the extract efficiency decreased slightly and stayed at about 80% and 75% respectively, which also reflected the evolution of stability of emulsion. In extraction process, the volume of external was fixed at 30 ml and the total content of 4-MP in the feed phase was 9 mg. When Rew increased from 3:30 to 6:30, the removal efficiency was continuously improved. With the increasing Rew, the interface area between external water and oil phase increased, which could help the reaction of phenol and TBP. Furthermore, the increase of internal NaOH concentration could cause a fast release of phenol to inside water droplets by forming oil insoluble phenolic salt. In conclusion, the removal efficiency of 4-MP would be improved with increasing Rew. Similarly, in the case of lower Rew, the extract efficiency would be lower. 3.5. Recyclability of particles and oil phase After extraction, some big emulsion droplets at the upper layer of the mixture were collected and a 1T cylindrical magnet was used to remove the nano-Fe3O4 particles from the emulsion surface. Then emulsion was broken up, oil phase and aqueous phase were separated within 30 min (Fig. 5a). The collected oil phase and nano-Fe3O4 particles were mixed with new sodium hydroxide (0.5 mol L 1), and re-emulsified (Row = 2, CFe3O4 = 0.5 wt%). The recyclability of the emulsion was investigated over 3 times and no significant decline in the extraction efficiency was observed (Fig. 5b). The slight reduction of extract efficiency was observed, which may due to the loss of the carrier due to the partial dissolution of carrier in water.
4. Conclusions For the first time, W/O/W Pickering emulsion liquid membrane was used for removing 4-MP from wastewater. Oleic acid-coated nano-Fe3O4 particles were synthesized and used as emulsifier to stabilize water in oil Pickering emulsion. The experimental results indicated that the stability of the primary Pickering emulsion was a key parameter and TBP concentration was the most effective factor. To make a green approach, corn oil was selected as organic solvent and showed good extract efficiency. When TBP concentration was 30%, extract efficiency achieved 86% with 0.5 wt% magnetic nano-Fe3O4. A pronounced difference between this approach and traditional emulsion liquid membrane is the absence of surfactant in system, consequently, easily breaking of the emulsion under external magnetic force. Traditionally, demulsifiers have been used to destabilize and break unwanted emulsions after extraction, which results in difficulty in reusing the oil phase. The magnetically controlled systems can easily recover emulsifier, oil phase and find application in a wide range of industrial wastewater treatment process. Moreover, in principle, Pickering emulsion liquid membrane concept can also be extended to non-magnetic nanoparticles, in which, the demulsification can be achieved by centrifugation. Because the particles and oil phase can be easily collected and reused, PELM can minimize the environmental burden, reduce cost and consumption of materials.
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