Studies on membrane stability and extraction of ciprofloxacin from aqueous solution using pickering emulsion liquid membrane stabilized by magnetic nano-Fe2O3

Journal Pre-proof Studies on Membrane Stability and Extraction of Ciprofloxacin from aqueous solution using Pickering Emulsion Liquid Membrane Stabilized by Magnetic Nano-Fe2 O3 Ahmed A. Mohammed, Mohammed A. Atiya, Maad A. Hussein

PII:

S0927-7757(19)31035-0

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124044

Reference:

COLSUA 124044

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

7 August 2019

Revised Date:

26 September 2019

Accepted Date:

28 September 2019

Please cite this article as: Mohammed AA, Atiya MA, Hussein MA, Studies on Membrane Stability and Extraction of Ciprofloxacin from aqueous solution using Pickering Emulsion Liquid Membrane Stabilized by Magnetic Nano-Fe2 O3 , Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124044

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Studies on Membrane Stability and Extraction of Ciprofloxacin from aqueous solution using Pickering Emulsion Liquid Membrane Stabilized by Magnetic Nano-Fe2O3

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Ahmed A. Mohammeda, Mohammed A. Atiyab, Maad A. Husseina

Environmental Engineering Department, University of Baghdad, Baghdad, Iraq.

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Al-Kawarizmi College of Engineering, University of Baghdad, Baghdad, Iraq.

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E-mail: [email protected], [email protected], [email protected].

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Graphical abstract

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Corresponding author: Maad A. Hussein, E-mail: [email protected]

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Abstract Significant features of the Pickering emulsion liquid membrane (PELM) as innovative trend of the traditional emulsion liquid membrane (ELM) such as simple operation, fast process, high stability and easy de-emulsification process have gained extensive responsiveness in the recent years. PELM consists of an n-heptane as the diluent, nano-Fe2O3 particles as a stabilizing agent, tri-butyl-phosphate (TBP) as an extractant and HCl acid in the internal phase that are used to remove ciprofloxacin (CIP) from wastewater. This paper comprises

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simultaneous studies of emulsion stability and extraction efficiency through various parameters, including homogenizer speed, emulsification time, nanoparticles concentration, external phase pH, internal to membrane volume ratio, internal phase concentration and

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extractant concentration. The results confirm that PELM is a very effective technique to

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extract more than 98% of CIP without significant emulsion breakage after a contact time of 10 mins. The recyclability of the membrane phase and nanoparticles are also examined, and

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the results showed that the extraction efficiency was approximately the same after five times.

1. Introduction

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Keywords: Pickering emulsion; Ciprofloxacin; Stability; Fe2O3 nanoparticles; Extraction.

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Antibiotics is the main sources of drug pollution that are released into the surrounding environment from pharmacies, pharmaceutical industries and human excretions [1]. Although,

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the low antibiotic concentration in the aqueous environment (1-100 ppb), its presence in human drink water may cause diarrhea, headaches, nausea, tremors, vomiting, etc. [2-4]. The fluoroquinolone antibiotic ciprofloxacin (CIP) is considered an effective antibiotic for the treatment of bacterial toxicities in humans and animals. CIP has high water solubility under different pH values, and it is very difficult to be degraded in natural circumstances [5]. It's presence in the aqueous environment even at low 2

concentrations can cause an increased concern over a long-term influences on the human health, and also threaten the functionality of an ecosystem [6-8]. Ciprofloxacin may not be removed efficiently by the traditional wastewater treatment methods; therefore, a powerful Pickering emulsion liquid membrane (PELM) technique is being advanced to eliminate CIP from wastewaters. PELM is a modified type of the traditional Emulsion liquid membrane technique. ELM was first discovered by Norman Li [9], it is still considered by many researchers to be organic and

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an efficient technique in the removal and recovery of numerous valuable

inorganic contaminants from aqueous solutions than other wastewater treatment methods such as chemical precipitation, ion exchange [10], biosorption [11], ozonation and advance

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oxidation [12], adsorption [13-17], pressure-driven membrane processes and electrochemical processes [18, 19]. In general, PELM is a three-phase dispersion system, two miscible liquid

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aqueous phases (external and internal) and one immiscible organic phase (membrane). The

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emulsion is created by homogenizing the aqueous internal phase and organic oil phase with nanoparticles as a stabilizing agent to produce water-in-oil (w/o) Pickering emulsion. Emulsion stabilized by nanomaterials, has gained extensive interest due to their notable

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stability [20-23]. These nanoparticles are irreversibly adsorbed at the water-oil interface to stabilize the emulsion droplet and prevent the breakage and coalescence processes.

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PELM has some evident advantages over the traditional wastewater treatment methods

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such as high removal and recovery efficiency, extraction and re-extraction (stripping) in the same time and same system, low costs, low toxicity, high mass transfer interfacial area and higher permeate flux [24-26]. The powerful advantage of PELM stabilized by magnetic nanoFe2O3 particles is the higher stability with various experimental conditions, and the magnetic PE can easily de-emulsified after the extraction process by applying an external magnetic field to rapidly attract the particle emulsifier from the emulsion surfaces. This can

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significantly enhance the reutilization of the membrane phase for additional extraction processes and can reduce the consumption of energy. PEs are not always milky white but sometimes gray, black [27, 28], or reddish (brown) [29] depending on the type of nanoparticles used. Free magnetic nanoparticles without surface modification cannot make the w/o Pickering emulsion and the oil and water phases remains separated from each other. Therefore, they must be modified with carboxylic acid (RCOOH) compounds such as oleic

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acid to increase their hydrophobicity [30]. In this work, magnetic α-Fe2O3 nanoparticles coated with oleic acid were used to stabilize the Pickering emulsion. The magnetic property of α-Fe2O3 was proved by previous studies [31-34], these nanoparticles exhibit unique

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nanoscale properties such as being more environmentally friendly, biocompatible, low toxicity, biodegradable and the most stable iron oxide [35, 36]. Oleic acid is a familiar surface

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acting agent in stabilizing colloids and nanoparticles since it can strongly bond the magnetite

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nanoparticles by interaction with its carboxyl group and have higher affinity to the superfine magnetite surface [37-39]. This is essential for creating monodisperse and highly uniform magnetic nanoparticles. Lin et al., [40, 41] reported that magnetic nano-Fe3O4 modified with

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oleic acid can successfully stabilize the Pickering emulsion for selective extraction and separation of 4-methoxyphenol of efficiency over 86%. Perumal et al., [42] studied the use of

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amphiphilic silica nanowires as stabilizing agents with the addition of 10 to 40 mL of ethanol

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as a surfactant in creating the PELM, the process was very efficient for removing more than 99% of Cr (VI). While Selman and Mohammed [43] used magnetic nano-Fe2O3 without any modification as a co-stabilization agent with 2% span80 as a surfactant for the extraction of lead (II) from the water solution, the extraction efficiency of 97% within a 8 mins contact time and a breakage percentage of 0.3% was achieved.

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To the best of our knowledge, none of the earlier studies have reported the use of magnetic nano-Fe2O3 coated with oleic acid for creating Pickering emulsions, and the extraction of antibiotic CIP from aqueous solution by ELM or PELM method have not been studied yet by any literatures. This study is aimed to investigate the stability and extraction capacity of PELM for the removal of ciprofloxacin from the water solution through different experimental factors, including homogenizer speed, emulsification time, nanoparticle concentration, internal to membrane phase volume ratio, internal phase concentration,

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extractant concentration and pH of the external solution. De-emulsification of the PE and the recyclability of the oil phase are also studied to determine its capacity to extract CIP. 2. Chemicals and experimental methods

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2.1. Chemicals

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Magnetic nano-iron oxide (Fe2O3) as a stabilizing agent, Oleic acid and Tri-butylphosphate (TBP) as an extractant were purchased from Sigma-Aldrich (Merck, Germany).

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The diluent n-heptane, sodium hydroxide (NaOH), and hydrochloric acid (HCl) were obtained from (Thomas beaker, India). Antibiotic ciprofloxacin was obtained from Samarra

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pharmaceutical industry (Iraq). All the chemicals were used without extra purification. All laboratory experiments were operated at an ambient room temperature of 20±2 °C.

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2.2. Experimental methods

2.2.1. Surface modification of nano-Fe2O3 with oleic acid

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Fe2O3 nanoparticles were bonded by oleic acid through constant heating and stirring

[30, 40, 44, 45], and a certain quantities were dispersed in 200 mL methanol by ultrasonication. 20 mL of oleic acid was then added dropwise while constantly stirring at 80 °C, and the pH of the mixture was then adjusted to 6.5-7.5. The Fe2O3 nanoparticles coated with oleic acid precipitates were filtrated through filter paper, and then repeatedly washed

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with distilled water and acetone to eliminate excess oleic acid. The collected particles were dried overnight at 70°C. 2.2.2. Pickering emulsion preparation Twenty-five milliliters of the membrane phase which consist of a certain amounts of Fe2O3 nanoparticles as stabilizing agents and TBP as an extractant were dispersed in the diluent n-heptane and homogenized by ultrasonication for 20 mins. The internal aqueous phase was formulated by taking the appropriate volume of acidic solution in the distilled

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water and adding it dropwise to the membrane solvent, while the system was being homogenized using a high speed SR30 homogenizer (Korea) for an appropriate speeds and times.

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2.2.3. Extraction process

Before preparing the PE, 250 mL of the external solution was prepared in a 400 mL

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beaker by dissolving a specific quantity of CIP in distilled water to produce 100 mg/L initial

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concentration. The emulsion was then added into the external solution while stirring the system and samples were taken at certain operation times. All samples were filtrated using a filter syringe of 0.22 µm. The obtained results in all experiments were in terms of CIP

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concentration extracted from the external solution. Table 1 summarizes the operating

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conditions of all experiments.

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2.2.4. Extraction mechanism CIP molecules is transported from the external aqueous phase into the internal

aqueous phase via mass transfer across the interfaces by making a complex with the extractant TBP as shown in Fig. 1. The complex diffuses through the organic membrane phase from the external-membrane interface towards the internal-membrane interface. De-complexation process was occurred at the internal-membrane interface releasing the CIP, in which a

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stripping agent in the internal droplet of the emulsion stripped the CIP molecule. The extractant diffuses back into the external-membrane interface to make another complex. The extraction and re-extraction process mechanisms are described in the following equations: 1. TBP in the membrane phase reacts with H+ present in the external phase. [TBP(org.) ] + [H + (aq.) ] + [Cl− (aq.) ] = [TBPH + Cl− (org.) ]

(1)

2. At external-membrane interface, TBP reacts with CIP to produce CIP-TBP complex. 2[TBPH + Cl− (org.) ] + [C17 H18 FN3 O8 (aq.) ] = [(TBPH)2 C17 H18 FN3 O8 (org.) ] + 2[Cl− (aq.) ]

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(2)

3. At internal-membrane interface, the CIP-TBP complex breaks and regenerated the TBP.

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[(TBPH)2 C17 H18 FN3 O8 (org.) ] + 2[OH − (aq.) ] = 2[TBP(org.) ] + [H2 O(aq.) ] + [C17 H18 FN3 O8 (aq.) ]

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(3)

Fig. 1. Extraction mechanisms of PELM system.

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2.3. Analysis and calculations The concentrations of CIP in the external phase were determined by using a ultraviolet spectrophotometer UV (ThermoSpectronic, USA). The standard method and calibration curve were used to determine the concentration of the CIP. The mean droplet diameters of the emulsions were determined using an Olympus optical microscope equipped with a camera and ImageJ analyzer software program. Extraction efficiency was calculated as follows: 𝐶𝐼,𝐶𝐼𝑃 −𝐶𝐼𝐼,𝐶𝐼𝑃 𝐶𝐼,𝐶𝐼𝑃

* 100

(4)

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%E=

Where 𝐶𝐼,𝐶𝐼𝑃 : signifies the CIP initial concentration in aqueous external solution.

𝐶𝐼𝐼,𝐶𝐼𝑃 : signifies the CIP concentration in aqueous external solution at certain time period.

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The droplet diameter of w/o emulsion can be defined as Sauter mean diameter (d32)

∑ 𝑛 𝑑3

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and can be calculated as follows [46]: 𝑉

d32= ∑𝑖 𝑛𝑖 𝑑𝑖2 = 6 𝐴 𝑖

(5)

𝑖 𝑖

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Where di, ni: are the droplet diameter and number belonging to the ith class V, A: refers to the total volume and area of the dispersed phase, respectively.

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The emulsion breakage (%ξ) is the percentage ratio of the internal phase volume leaked into the external phase by splitting (VS) to the initial internal phase volume (VI),

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whereas VS is determined by the mass balance from the external phase measure before and after contact [43, 47-49]. 𝑉𝑆

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𝑉𝐼

VS = VE

∗ 100

10−𝑝𝐻0 −10−𝑝𝐻 𝑖𝑛𝑡 10−𝑝𝐻 − 𝐶𝐻 +

(6) (7)

+ where VE is the initial volume of external phase, 𝐶𝐻𝑖𝑛𝑡 + is the initial H concentration in

the internal phase, pH0 is the initial pH of the external phase and pH is the external phase pH after a certain time. 8

3. Results and discussion 3.1. Characterizations of nano-Fe2O3 particles

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3.1.1. FTIR characterization The Fourier transformed infrared spectroscopy of magnetic nano-Fe2O3 particles coated with oleic acid is shown in Fig. 2. The spectra showed a characteristic bands of oleic

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acid at 2921 and 2850 cm-1, that are symmetric and asymmetric characteristic of C ̶ H stretching vibrations. The peaks at 1633 cm-1 and 1450 cm-1 exhibited a bands of the C=O and

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O ̶ H, respectively. While the peak at 1403 cm-1 was assigned to C ̶ H bending vibrations. This

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confirms that oleic acid was chemically bonding the Fe2O3 nanoparticles [40, 45]. The Fe ̶ O

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lattice vibrations were assigned at peak 535 cm-1.

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90 70 60

1633cm-1

50 1403cm-1

40

1450cm-1

30

2851cm-1

20 10

535cm-1

0 2800

2200

1600

1000

400

Wave number (cm-1)

Fig. 2. FTIR analysis of nano-Fe2O3 particles coated with oleic acid. 9

transmittance (%)

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2921cm-1

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3.1.2. XRD characterization The X-Ray Diffraction (XRD) patterns of Fe2O3 nanoparticles obviously indicate the formation of hematite according to the ICDD card (33-0664) with the hexagonal structure [50]. The peaks detected at 2θ of 24°, 33°, 41°, 49.5° and 54° can be assigned to the (012), (104), (110), (113), (024), (116) and (018) crystalline structures of Fe2O3 [51]. All of the predicted peaks can be clearly indexed to the hexagonal structure of XRD pattern of sample: (1) S–1, –Fe2O3 nanoparticles; (2) S–2, –Fe2O3 mesoporous hollow micro-spheres; (3) S–4, –

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Fe2O3 micro-cubes; (4) S–5, –Fe2O3 nano-rods; (5) –FeOOH nano-rods (S–5 before annealing). The impurity in the structure of formed Fe2O3 nanoparticles may be the cause of other observed sharp peaks. The typical XRD pattern for α–Fe2O3 (hematite) is shown in Fig.

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Fig. 3. XRD analysis of α-Fe2O3 nanoparticles.

3.1.3. Measurements of SEM

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Fig. 4 shows the structural characteristics of the hematite nanoparticles such as particle size and morphology, which were scrutinized by using a scanning electron microscope (SEM), Czech Republic. All the particles are nearly spherical in shape, the

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average sizes of the hematite particles were in the range 400-1000 nm.

Fig. 4. SEM image of α-Fe2O3 particles.

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3.2. Effect of homogenizer speed

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One of the most important parameters that are considered to have significant influences on the PELM stability and the whole extraction process is the homogenizer speed.

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The emulsion stability is a key-essential factor in PELM process, due to lower stability leading to breaking the emulsion and a very high stability causing the emulsion to be very problematic to break by any mechanical means. Homogenizing speed of the emulsion constituent was examined in the range as mentioned in Table 1. The results presented in Fig. 5 shows that the PE stability increases and breakage decreases with increasing the homogenizer speed from 3000 to 12700 rpm which gave a minimum breakage percentage of 11

0.063 % and the extraction efficiency increased from 78.3 to 98.8%. This is due to the internal droplets becoming smaller in size at higher rotation speeds in which the d32 decreased from 18.5 µm at 3000 rpm to 4.31 µm at 12700 rpm, which lead to heightens in the droplets surface area and increases the rate of solute transfer. Selman and Mohammed [43] obtained the same activity in the extraction of Pb2+ from aqueous solution using magnetic nano-Fe2O3 as costabilizing agent, while Lin et al., [40] create the nano-Fe3O4 Pickering emulsion with a homogenizer speed of 10000 rpm for the extraction of 4-Methoxyphenol. Higher

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homogenizer speed (at 19700 rpm) led to a decreasing the PE stability (breakage percent to 2%) and the extraction efficiency to 87.5%. Homogenizer speed less than 12700 rpm led to a reduction in the PE stability and increasing in the breakage percentage to 8%, due to the

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larger droplet size and the coalescence phenomenon occur in short time. As a result, homogenizer speed of 12700 rpm was selected as the optimal speed for further experiments.

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Table 2 summarizes the extraction efficiency, breakage percentage and sauter mean diameter

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%E

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Breakage %

6 5 4 3

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0

5000

% Breakage

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100 90 80 70 60 50 40 30 20 10 0

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% Extraction Efficiency

at different emulsification speeds and times.

2 1 10000

15000

20000

0 25000

Homogenizer speed (rpm)

Fig. 5. The effect of homogenizer speed on the extraction efficiency and membrane breakage (emulsification time: 7 mins, Fe2O3: 0.7 (%w/v), 0.1M HCl, internal to membrane ratio: 1, TBP: 6% (v/v), external pH: 8, mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

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3.3. Effect of emulsification time The required time to create a stable PE is a very important condition to give suitable time to encapsulate the aqueous internal phase inside the oil phase. The stability of PE with different emulsification times was examined in the range of 5 to 15 mins. Results plotted in Fig. 6 shows that the highest CIP extraction efficiency of about 99% occurred within a 7 mins emulsification time, which offers higher emulsion stability with a minimum breakage and a sauter mean diameter of 0.06% and 3.85 µm, respectively. While inadequate emulsification

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time (at 5 mins), the breakage and extraction efficiency were 0.5% and 91.7%, respectively. This occurred due to a big droplet size of 10.41 µm, in which the coalescence phenomenon occurs easily. Further increasing of the emulsification time to 10 and 15 mins, the breakage

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starts to increase to 0.13 and 2%, while the extraction efficiency decreased to 95.7 and 83.3%,

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respectively. This happens due to the internal shearing was high, thus causing a very high number of emulsion globules by unit volume, which facilitates their diffusion into the external

100 96 94 92

% Breakage

1

90 88

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2 1.5

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% Extraction Efficiency

98

2.5

%E

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throughout this study.

0.5

86

% Breakage

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solution [52]. Therefore, an emulsification time of 7 mins was considered to be the best time

0

84 82

-0.5 4

5

6

7

8

9

10

11

12

13

14

15

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Emulsification time (mins)

Fig. 6. The effect of Emulsification time on the extraction efficiency and membrane breakage (homogenizer speed: 12700 rpm, Fe2O3: 0.7 (%w/v), 0.1M HCl, internal to membrane ratio: 13

1, TBP: 6% (v/v), external pH: 8, mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

3.4. Effect of magnetic nano-Fe2O3 particles concentration The concept of Pickering emulsion liquid membrane involves the addition of nanoparticles to the membrane phase to highly stabilize the emulsion globules. Without these Nano-materials, dispersing the aqueous water phase (internal) into the oil (membrane) phase

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is impossible, and Pickering emulsion cannot be created. Therefore, nano-Fe2O3 particles modified with oleic acid were used in this study in the range of 0.1 to 1 (%w/v) to examine their effect upon PE stability in extracting CIP. Fig. 7 clarifies that the emulsion stability is

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significantly enhanced by increasing the concentration from 0.1% (breakage of about 13%) to 0.7% (no significant breakage) while the extraction efficiency was 98.8% at concentration of

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0.7% compared to 50.8% at concentration of 0.1%. This is due to an increase in the concentration of nanoparticles results in more nanoparticles covering the emulsion interfaces

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[40, 43], and thereby enhances the stability and CIP extraction efficiency, this also led to lowering the surface tension of the membrane phase [53]. At a higher concentration of Fe2O3

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nanoparticles pf 1%, the breakage of the PE increased to 0.2% and the extraction efficiency decreased to about 95%, which is because of the extra nanoparticles could be dispersed in the

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external phase and some of the particles can create aggregate on the PE interface. This causes high mass transfer resistance for the transport of CIP. Hence, nano-Fe2O3 concentration of 0.7

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%w/v was identified as the best value in this study.

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100

%E

14

% Breakage

12 10

80

8 60 6 40

% Breakage

% Extraction Efficiency

120

4

20

2

0

0 0

0.2

0.4

0.6

0.8

1

1.2

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Nano-Fe2O3 particles (%w/v)

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Fig. 7. The effect of nano-Fe2O3 particles concentration on the extraction efficiency and membrane breakage (homogenizer speed: 12700 rpm, emulsification time: 7 mins, 0.1M HCl, internal to membrane ratio: 1, TBP: 6% (v/v), external pH: 8, mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

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3.5. Effect of external phase pH

The pH of the external phase plays an essential role in the CIP extraction process, and

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it can also influence the stability of the membrane phase, since high or low pH values can accelerate the de-emulsification process of the PE droplets. Experiments were carried out at

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acidic, neutral and basic pH values in the range of 2 to 9 and the results are showm in Fig. 8. It is obviously that CIP extraction is highly affected by the pH of the external phase. At high

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acidity (pH= 2), the extraction efficiency was only about 9%, while the breakage percentage was at its highest about 30%. This might be due to higher H+ concentrations (lower pH)

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causing a destabilization process to the PE, while the dropping in the extraction efficiency is due to the neutral extractant (TBP) which cannot sufficiently make a complex with CIP molecule at this pH region. When raising the external phase pH to a more neutral condition (pH= 6), the extraction efficiency increases to more than 71% and the breakage percentage reduces to a 0.05% within 10 mins mixing time. While in the case of pH=8, CIP extraction efficiency reached the optimal value of 98.85% and the emulsion breakage remained 15

unaffected. Moreover, at a pH of 9 the extraction efficiency began to reduce slightly to about 89% and the breakage increased to 1.26%. This can be explained by the protons being released as a result of anion exchange reaction [54], and also an increasing pH causes a formation of other species. For further experiments, it was very suitable to maintain the

100 90 80 70 60 50 40 30 20 10 0

35 30

20

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Breakage %

% Breakage

25

%E

15 10 5 0

-5

1

3

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% Extraction Efficiency

external phase pH at 8.

5

7

9

11

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External phase pH

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Fig. 8. The effect of external phase pH on the extraction efficiency and membrane breakage (homogenizer speed: 12700 rpm, emulsification time: 7 mins, Fe2O3: 0.7 (%w/v), 0.1M HCl, internal to membrane ratio: 1, TBP: 6% (v/v), mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

3.6. Effect of extractant concentration

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Extractants (or Carriers) play an important role in the mass transport processes across the liquid membrane [55]; first they create a complex with the species at the external-

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membrane interface and when this complex is broken at the internal-membrane interface, the extractant goes back to create another complex. Also, they have considerable effects on the instability of the membrane phase. Tri-n-butyl phosphate (TBP) was used in this study as an extractant in the range of 0% (no extractant) to 10% (v/v). The results shown in Fig. 9 indicate that CIP transport efficiency was no higher than 7% in the absence of the extractant (concentration of 0%); this reveals that although CIP is an organic compound, it's not soluble 16

enough in the diluent n-heptane to be transported into the internal phase. Increasing the extractant concentration to 6% (v/v) results in an optimal extraction efficiency of 98.8% and the emulsion breakage was not significantly affected (about 0.06%). Further increasing in the TBP concentration led to an increase in the leakage of the internal solution to the aqueous external phase (breakage of 3.2% at 10% (v/v)), and consequently, the efficiency of extraction decreased to 79.8%. This was due to competitive adsorption and reaction of the TBP with the magnetic nano-Fe2O3 particles at the membrane interfaces in which they have opposite

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behaviors. In addition, an increase in extractant concentration in the membrane solvent raised the interfacial tensions, leading to increase the membrane phase viscosity and creation of larger sized PE globules that can enhance the swelling phenomenon [56]. Zhao et al., [24]

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also found the same behavior in which more than 99% of chromium (III) can be extracted at TBP concentration of 7%. Thus, a TBP concentration of 6% (v/v) was used for further

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3.5 3

2

2

1.5 1 0.5

%E Breakage %

4

% Breakage

2.5

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100 90 80 70 60 50 40 30 20 10 0

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% Extraction Efficiency

experiments throughout this work.

6

8

10

0

12

TBP Concentration% (v/v)

Fig. 9. The effect of TBP concentration on the extraction efficiency and membrane breakage (homogenizer speed: 12700 rpm, emulsification time: 7 mins, Fe2O3: 0.7 (%w/v), 0.1M HCl, internal to membrane ratio: 1, external pH: 8, mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

3.7. Effect of internal phase concentration 17

In order to advance the stability of PE and minimize the releasing of the internal solution outside the emulsion globules, and as the extraction step is necessary at the externalmembrane interface; the re-extraction step is also required at the internal-membrane interface to fully transport the CIP. HCl was used as the stripping agent in the internal phase to strip and re-extract the CIP solute from the CIP-TBP complex, thus concentration was varied from 0.01 to 0.25 M. The breakage percentages were reduced and the extraction efficiencies were increased as shown in Fig. 10, from 6.3% and 66.8% to 0.06% and 99.8%, respectively; by

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raising the concentration of HCl from 0.01 to 0.1 M, and the Nano-emulsion was stable for a total of 24 hours without any phase separation. This is due to the main driving force in the ELM technique is the variation in the amount of H+ ions between the two aqueous phases

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[49]. Furthermore, raising the acidity of the internal phase (above 0.1M) partially increasing the emulsion breakage, and led to a more releasing of the internal constituent into the external

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feed phase and thus declining the extraction efficiency. This could be due to the higher HCl

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concentrations causing loss of the coating properties of the nano-Fe2O3 particles and some of the particles were dissolved into the internal phase and had not yet acted as a stabilizing agent. Razo-Lazcano et al., [57] also found the same behavior and results by using HCl as

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internal solution for the re-extraction of Chlorpheniramine from aqueous solution. As a result,

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this work.

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0.1 M of HCl that gives the lowest breakage and higher extraction percentage was chosen in

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7

%E Breakage %

6 5 4 3 2 1

% Breakage

% Extraction Efficiency

100 90 80 70 60 50 40 30 20 10 0

0 -1 -2 0

0.05

0.1

0.15

0.2

0.25

0.3

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HCl Concentration (M)

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Fig. 10. The effect of HCl concentration on the extraction efficiency and membrane breakage (homogenizer speed: 12700 rpm, emulsification time: 7 mins, Fe2O3: 0.7 (%w/v), internal to membrane ratio: 1, TBP: 6% (v/v), external pH: 8, mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

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3.8. Effect of the internal to membrane phase volume ratio

Sufficient amounts of the internal to membrane phase volume lead to a reduction in

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the thickness of the PE interface and improving the solute transport through the membrane phase. The internal to membrane phase volume ratio varied from 1:3 to3:1 in order to

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examine the influence on PE stability and the extraction of the CIP, Profiles of the extraction efficiency and PE breakage are presented in Fig. 11. Changing the ratio from 1:3 to 1:1 by

ur

equalizing the volumes of the membrane and internal phases, led to decreasing the membrane leakage from 9 to 0.06%, and increasing the extraction of CIP from about 70 to 99%. This

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behavior is due to that at a low volume ratio of 1:3, too much oil phase volume led to create a thicker and more viscous emulsion wall which obstruct the diffusing of the internal phase inside the oil membrane. The decrease in the extraction efficiency is due to less stripping agents available to re-extract the solute from the membrane phase. A larger droplet size and higher membrane surface tension are produced by increasing the volume of membrane phase, therefore the w/o emulsion droplets are difficult to be dispersed [46]. Further increasing the 19

volume ratio from 1:1 to 3:1, by increasing the volume of the internal phase compared to the volume of the membrane phase led to obvious weakening in the emulsion stability (2.52% breakage percentage), and the extraction efficiency decreased to about 75%. This decrease is due to insufficient membrane phase volume causing the internal droplet cannot be completely entrapped and hence tends to leak outside the emulsion bubble into the external phase; similar performances of this volume ratio were also observed by previous studies [58-61]. Thus, the internal to membrane phase volume ratio of 1:1 was selected as an optimum ratio to achieve a

na

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lowest membrane breakage and highest CIP extraction efficiency.

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Fig. 11. The effect of internal to membrane phase volume ratio on the extraction efficiency and membrane breakage (homogenizer speed: 12700 rpm, emulsification time: 7 mins, Fe2O3: 0.7 (%w/v), 0.1M HCl, TBP: 6% (v/v), external pH: 8, mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

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3.9. Effect of mixing time

The time required to obtain the highest extraction of the CIP from the aqueous

solution is the period in which the external phase and the Pickering emulsion remain in contact with each other while the system is stirring. The optimum mixing time should be selected, for a better operation of PELM. The mixing time was examined through this study in the range of 2 to 15 mins. Under the optimum experimental conditions as shown in Fig. 12, 20

more than 50% of the CIP extraction efficiency occurs within the first 2 mins and extraction efficiency remains to increase and reached the optimal level at 10 mins. However, longer contact time of 12 or 15 mins, caused a slight decrease in the efficiency of extraction. This could be due to the increase in PE breakage, and thus escaping of the extracted CIP molecules

100 90 80 70 60 50 40 30 20 10 0

0.12

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0.08

% Breakage

0.1

0.06 0.04 0.02 0

0

2

4

6

8

-p

% Extraction Efficiency

from the internal solution and going back to the external phase.

10

12

14

16

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Time (mins)

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Fig. 12. Extraction and breakage process versus mixing time (homogenizer speed: 12700 rpm, emulsification time: 7 mins, Fe2O3: 0.7 (%w/v), 0.1M HCl, internal to membrane ratio: 1, external pH: 8, mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

3.10. Recyclability of the membrane and nanoparticles materials

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Recycling the Fe2O3 nanoparticles and the membrane phase in another extraction

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process is an important feature in PELM process from a point of economic and environmental protection view. The nanomaterials, oil phase and extractant are cost effective materials and disposing them into the environment can cause undesirable pollution. After completing the extraction process, the emulsion and the clear external phase were separated by a separating funnel. Then the external clear water solution was removed and the emulsion phase was deemulsified applying an external magnetic force to separate the magnetic Fe2O3 nanoparticles

21

from the emulsion surface [41, 45]. The emulsion was then allowed to separate for 60 mins to oil phase and aqueous internal phase. The collected solid particles were then washed with acetone and distilled water and dried at 70 °C for 12 hr. The obtained membrane phase without utilizing any further extractant and Fe2O3 nanoparticles was reused under the optimum experimental conditions: homogenizer speed of 12700 rpm, 7 mins emulsification time, internal to membrane ratio was 1, 0.1 M HCl internal phase, 100mg/L of CIP at pH of 8, 250 rpm mixing speed and external to emulsion ratio was 5/1. As shown in Fig. 13 the

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emulsion recyclability was successful for five times, the extraction efficiency was approximately the same, and the emulsion breakage was no more than 0.1 %. However, in the sixth reusing process, the extraction efficiency began to drop. This could be due to that the

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membrane phase and especially the extractant material being saturated with CIP molecules by continuous recycling processes and the loss of the extractant due to the partial dissolution in

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the water phase. Lin et al., [41] also found that magnetic Fe3O4 nanoparticles are efficiently

97.895

97.883

3 95.237

95.188 2.5 %E

0.063

0.063

0.0793

Breakage %

83.653

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First reuse

1.5

1 1 0.0793

0.0999

0.5

0.0999

70

Fresh

2

% Breakage

80

98.528

na

90

98.848

ur

% Extraction Efficiency

100

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recycled for 3 times in creating the PE for the extraction and separation of 4-Methoxyphenol.

0 Second reuse

Third reuse

Forth reuse

Fifth reuse

Sixth reuse

Reuse processes

Fig. 13. The effect of recycling processes on the extraction efficiency and membrane breakage (homogenizer speed: 12700 rpm, emulsification time: 7 mins, Fe2O3: 0.7 (%w/v), 0.1M HCl, internal to membrane ratio: 1, external pH: 8, mixing speed: 250 rpm for 10 mins, external to emulsion ratio: 5/1).

22

4. Conclusions The current study investigates the use of α-Fe2O3 nanoparticles coated with oleic acid as a stabilizing agent in creating Pickering emulsion for the removal of ciprofloxacin from wastewater. PELM proved to be a very efficient and stable process for the extraction of CIP, and can be de-emulsified easily using an external magnetic field to separate the magnetic nanoparticles from the emulsion surface. The highest extraction efficiency of 98.85% and minimum emulsion breakage of 0.06% within 10 mins mixing time were achieved at the

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optimal operating conditions: 12700 rpm homogenizer speed, 0.7 (%w/v) nano-Fe2O3 particles concentration, 6% (v/v) TBP concentration at emulsification time of 7 mins and 0.1 M HCl in the internal phase. In addition, the membrane phase and nanoparticles were

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effectively reused to create another PE to extract CIP for five times with approximately the

lP

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same extraction efficiency.

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(2018) 1-8.

30

Table 1 Operating parameters for ELM system. Ranges

Homogenizer speed (rpm)

3000, 5800, 12700, 19700

Emulsification Time (mins)

5, 7, 10, 15

Nano-Fe2O3 concertation (%w/v)

0.1, 0.3, 0.5, 0.7, 1

External phase pH

2, 3, 3.8, 5, 6

Extractant concentration% (v/v)

0, 2, 4, 6, 8, 10

HCl concentration (M)

0.01, 0.05, 0.1, 0.2, 0.25

Internal to membrane ratio

1:3, 2:3, 3:3, 3:2, 3:1

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Parameter

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Table 2

Extraction efficiency, breakage percentage and sauter mean diameter at different

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emulsification speeds and times.

Homogenizer speed (rpm) 5800

12700

19700

5

7

10

15

Efficiency %

78.292

90.973

98.848

87.479

91.676

98.848

95.696

83.349

Breakage %

8.006

2.518

0.063

1.999

0.501

0.063

0.125

1.999

d32 (µm)

18.5

8.22

4.31

9.21

10.41

3.85

4.31

9.27

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3000

Emulsification time (mins)

31