Int. J. Environ. Sci. Technol. DOI 10.1007/s13762-017-1276-4
ORIGINAL PAPER
Cu (II) removal intensification using Fe3O4 nanoparticles under inert gas and magnetic field in a microchannel O. Jafari1 • M. Rahimi1 • F. Hosseini Kakavandi1 • N. Azimi1
Received: 20 May 2016 / Revised: 24 December 2016 / Accepted: 6 February 2017 Ó Islamic Azad University (IAU) 2017
Abstract In the present study, Cu (II) ions removal from aqueous solution was intensified by exciting magnetic nanoparticles under inert gas, magnetic field and combination of these two mixing methods in a T-type microchannel. The flow patterns and liquid–liquid twophase mass transfer were studied in three different magnet distances from mixing channel (3, 6 and 10 mm) and also in the presence of different inert gas flow rates (1, 3 and 5 mL/min). Depending on the mixing method and the flow rate of both phases, several distinct flow patterns were observed including slugs, droplet, parallel and dispersed flows. The performances of mixing techniques for mass transfer enhancement based on relative removal efficiency ratio (k) and mass transfer coefficient ratio (c) were compared with simple layout (without nanoparticles, magnetic field and inert gas). The results showed that simultaneous using of inert gas and magnetic field can drive the nanoparticles as mixer. Liquid–liquid mass transfer with 27–62% enhancement in E and 235–285% in KLa compared with plain one was observed. Keywords Copper extraction Micromixer Magnetic field Fe3O4 nanoparticles Inert gas List of symbols Caq,in Concentration of propionic acid in the inlet of the aqueous phase (mol/L)
Caq,out Caq
E E0 KLa KLa Qaq Qor T tm V
Concentration of propionic acid in the outlet of the aqueous phase (mol/L) Equilibrium concentration of the propionic acid in the aqueous phase (mol/L) Extraction efficiency (–) Extraction efficiency of simple layout (without any mixing factor) (–) Volumetric mass transfer coefficient (1/s) Volumetric mass transfer coefficient of simple layout (without any mixing factor) (1/s) Aqueous phase volume flow rate (m3/s) Organic phase volume flow rate (m3/s) Temperature (K) Residence time of mixture of two phase (s) Total volume of mixing channel (m3)
Greek letters k Relative removal efficiency ratio (–) c Relative mass transfer coefficient ratio (–) Subscripts aq Aqueous phase or Organic phase in Inlet m Mixture of the liquid–liquid two-phase mass transfer out Outlet
Introduction Editorial responsibility: B.B. Huang. & M. Rahimi
[email protected] 1
CFD Research Center, Chemical Engineering Department, Razi University, Taghe Bostan, Kermanshah, Iran
Nowadays, water pollution by heavy metals is a very important industrial problem with serious environmental concerns (Malkoc and Nuhoglu 2005; Salmasi and Tavassoli 2006; Mahvi et al. 2013; Karami 2013). The heavy metals, such as Cu (II), Cd (II), Hg(II), Zn (II), Pb(II) are among the most common toxic pollutants in
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surface waters and in industrial wastewater (Cay et al. 2004). Recently, increasing interest has been focused on the removal of Cu (II) from aqueous media, due to its harmful effects on human health and other living organisms (Jing et al. 2014; Yuezhong et al. 2015). Human activities such as mining, metal melting, using of domestic and industrial sewage sludge in agricultural lands, using copper as fungicides and pesticides cause water pollution (Xu et al. 2006). There are many physicochemical techniques for treatment of water containing Cu (II) ions including precipitation, ion exchange, reverse osmosis, electrochemical treatment, evaporation, biological methods, adsorption and liquid extraction (Onundi et al. 2010; Meterfi et al. 2012; Lin et al. 2005). However, some of these methods are not economical and are often inefficient, especially at the low concentrations. For examples, chemical precipitation requires a large amount of chemicals to decrease metals to an acceptable level for discharge. Other drawbacks of chemical precipitation technique are its excessive sludge production that requires further treatment, poor settling, slow metal precipitation, the aggregation of metal precipitates and the long-term environmental influences of sludge disposal (Barakat 2011; Aziz et al. 2008). The disadvantage of ion exchange method is that it cannot handle concentrated metal solution as the matrix gets easily fouled by organics and other solids in the wastewater. In addition, ion exchange is nonselective and is highly sensitive to the pH of the solution (Barakat 2011). Reverse osmosis has high operation and maintenance costs for dilute solutions and is subject to fouling (Lin et al. 2005). Adsorption using activated carbons has been found to be insufficient although it is widely applied for the removal of organic pollutants (Lin et al. 2005; Reed et al. 1994). Therefore, attempt for finding a low cost and an efficient method for Cu (II) ions removal from aqueous media is certainly required. Solvent extraction is one of the effective techniques employed for the recovery and separation of metal ions such Cu (II) from aqueous solutions (Wei et al. 2003; Chang et al. 2010). This method of Cu (II) removal has great capabilities particularly when enough extracting agents and diluents are used and it is simply based on the transfer of a solute from an aqueous phase to an organic one. Elimination of Cu (II) ions from aqueous solution by liquid–liquid extraction has increasingly received much attention in recent years as a safe and effective method (Lemos et al. 2012). Indeed, the separations that can be achieved by this method are simple, convenient and rapid to perform; they are clean as much as the small interfacial area certainly precludes any phenomena analogous to the undesirable co-precipitation encountered in precipitation separations. A further advantage of solvent extraction method lies in the convenience of subsequent analysis of the extracted species (Kumar 2014).
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In the liquid–liquid extractors with large size, significant amount of solvent is necessary due to lack of efficient mixing. In addition, the large size conventional processes are time-consuming and expensive. Therefore, the study on the reducing the size of these systems can be interesting. Microstructure devices are appropriate equipments for the process intensification (Rahimi et al. 2014), and using these devices has become an effective technique for liquid–liquid mass transfer (Zhao et al. 2007; Jovanovic et al. 2011). Micromixers offer many advantages, such as heat and mass transfer enhancement, extremely large surface-to-volume ratio, reduction in reagent consumption, short transport path and stable laminar flow even at high shear rates (Su et al. 2009; Azimi et al. 2015). Many researchers tried to use the microdevices for solvent extraction, and some good results have been obtained. Tamagawa and Muto (2011) developed a slug flow in a Y-junction microchannel for the solvent extraction of Cs?. Their results showed that Cs? extraction rate was significantly increased with the slug flow microreactor, compared with conventional batch extraction. Priest et al. (2012) evaluated the solvent extraction of copper in a microfluidic system, and high efficiency and extraction rates have been reported. They stated that the microfluidic solvent extraction proved to be suitable for process intensification. Yang et al. (2013) performed a study on the Cu2? extraction accompanying a chemical reaction under different operating conditions in a T-junction microchannel. It was found that the copper extraction process in the microchannel is under the control of reaction intrinsic kinetics and mass transfer, depending on the characteristics of the reaction and the fluid hydrodynamics. Darekar et al. (2014) used a T-junction serpentine microchannel and a split and recombine microchannel for the solvent extraction of zinc. Their study showed the possibility of intensification of liquid–liquid extraction processes by using microchannel contactors. Singh et al. (2015) compared the performance of three different microchannels and conventional stage wise-extractors for liquid–liquid extraction using water-succinic acid-n-butanol system as a standard phase system. All mentioned works related to liquid–liquid mass transfer in microchannels were implied that the mixing is not a trivial task owing to operating under low Reynolds number flows. Fluid actuation in the microchannels is one of the efficient methods for increasing the mixing and mass transfer rate in these devices (Azimi et al. 2015). Mixing intensification can be achieved by chaotic advection and creation of turbulences in fluid. In order to enhance the mixing in microtubes and micromixers, inert gas (Su et al. 2009; Assmann and Von Rohr 2011) or nanoparticles (Azimi et al. 2015; Olivier et al. 2014; Hajiani and Larachi 2013; Chang et al. 2013; Wu et al. 2013) separately can be used. Assmann and Von Rohr (2011) analyzed the effect of
Int. J. Environ. Sci. Technol.
using inert gas for agitation in a liquid extraction system in a microstructured device. It was found that adding the inert gas increases the mass transfer rate at high flow velocities. Su et al. (2009) used an inert gas to break up parallel flow in microchannel and achieve much higher liquid–liquid mass transport coefficients compared with the layout without inert gas. In our previous work (Azimi et al. 2015), liquid–liquid two-phase mass transfer between an organic and aqueous phases has been investigated in a Y-type microchannel, whereas this process intensified by Fe3O4 nanoparticles under magnetic field. It was revealed that mass transfer coefficient and extraction efficiency were increased because of stimulation of magnetic nanoparticles under magnetic field. From studies reported in the literature (Azimi et al. 2015; Olivier et al. 2014; Hajiani and Larachi 2013; Chang et al. 2013; Wu et al. 2013), it can be concluded that the main reason for this enhancement is as a result of stretching and folding fluid elements due to interactions between excited nanoparticles under magnetic field. Therefore, the movement of magnetic nanoparticles can lead to an increase in the interfacial area of fluid and consequently amplify the mixing and the mass transfer rate. In this work, the main objective is to propose new methods for efficient mixing including, the use of inert gas, magnetic nanoparticles, magnetic field and the simultaneous use of them for heavy metal removal. Gas injection caused mixing and more turbulences in the direction of flow. The high dispersion of aqueous and organic phases in microchannel can be obtained by gas agitation. Indeed, in microscale space, the microchannel can be considered as microstirred tank, and the gas as microagitator (Su et al. 2009). In addition, using a magnet besides the mixing channel to stimulate the nanoparticles caused the mixing perpendicular to the flow direction. Applying magnetic field next to the mixing channel causes actuation of magnetic suspension and inducing secondary and convective flow that results in intensive mixing and so mass transfer enhancement (Azimi et al. 2015). The removal efficiency (E) and mass transfer coefficient (KLa) were determined to study the amount of Cu (II) removal. In addition, the effects of these mixing techniques on the flow patterns under different inert gas flow rates, distance of magnet from mixing channel and organic–aqueous phases flow rates have been investigated.
Materials and methods Materials The materials used in the present work were prepared from following suppliers: Copper sulfate pentahydrate (Merck,
C99.6% purity), D2EHPA (Merck, C99% purity), and sodium acetate trihydrate (sigma, C99% purity). In addition, the deionized water was used for preparation of aqueous solution. The kerosene was provided from Tehran Refinery Company, Iran. Nanostructured Fe3O4 with diameter of 25 nm (purity, 99.5%) were purchased from US Research Nanomaterials Inc. (Houston, TX). Methods The initial concentration of the Cu (II) was 1000 ppm in aqueous solution, 1:1 organic to aqueous phase ratio and initial pH = 5.5 of aqueous phase. The organic phase containing 3% (v/v) of D2EHPA as the solvent that dissolved in kerosene as diluents. The copper ions have been removed from the aqueous phase based on reactive extraction by D2EHPA as solvent and adsorption on Fe3O4 nanoparticles. The aqueous phase was prepared by dissolving copper sulfate pentahydrate in acetate buffer media. Density and viscosity of aqueous phase are 995 kg/ m3 and 0.895 mPa s, respectively. In addition, the amounts of these physical properties for organic phase are 789.8 kg/ m3 and 1209 mPa s, respectively. Acetate buffer solution with concentration of 0.2 M was prepared by dissolving sodium acetate trihydrate in deionized water. In order to evaluate the effect of superficial velocity on the mass transfer coefficient and extraction efficiency of Cu (II), the experiments were carried out at different flow rates. Organic and aqueous phases flow rate range was from 1.875 to 6 mL/min, and the gas flow rate of 1–5 mL/min was examined. In all tests, the flow rate of aqueous and organic phases is equal. After the phase separation, filtration and dilution, the concentration of Cu (II) in the samples of aqueous phase was determined by atomic absorption spectroscopy (AA-680 Shimadzu). Each one of the experiments was done three times to ensure the accuracy and reproducibility of the results obtained. Preparation of Fe3O4 nano-suspension In order to prepare the suspension containing the nanoparticles, Fe3O4 nanoparticles with the little concentration of 0.008 (w/v) are dispersed in the organic phase. In order to prevent the Fe3O4 nanoparticles from settling, the prepared nano-suspension of Fe3O4 nanoparticles was homogenized with ultrasonic homogenizer (Hielscher UP400S, Germany). The sample was sonicated at a frequency of 24 kHz and a nominal power of 400 W at the controlled temperature of 293–298 K. The temperature of suspension during the sonication was controlled by surrounding the suspension container by water–ice cooling bath. It took one hour to prepare a stabilized nanomagnetic fluid. Although, even after 24 h, the nano-suspension was
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stable and exhibits no significant sedimentation under static condition but for preventing possible sedimentation, for each test a new suspension was prepared and used immediately. Experimental setup A schematic diagram of the experimental setup used in the present work is shown in Fig. 1. Air was injected from middle inlet channel, and aqueous and organic streams enter coaxially from the two opposing inlet channels by two high-precision syringe pumps that they work continuously. Three phases begin to contact in the junction and along the mixing channel, then leave through the outlet. The real photograph of T-type micromixer with three inlet channels is illustrated in Fig. 2. The micromixer was fabricated from glass tubes, and the intersection angle of the two liquid inlet channels was 180°. An inlet channel was inserted at the middle of two other inlet channels for gas injection. The length of each three inlet channels and outlet channel was 25 and 70 mm, respectively. In addition, the outer diameter of all channels was 1 mm and inner diameter of all inlets and outlet channels were 800 lm. Fe3O4 nanoparticles were excited by subjecting mixing channel to a static magnetic field. A permanent magnet, which it was placed next to the mixing channel, was used to generate the static magnetic field. Permanent magnet was a cube with the dimensions of 60 mm–12 mm–5 mm and an intensity of 1200 G. In order to divert the Fe3O4
nanoparticles toward the magnetic field, permanent magnet was located at the side of aqueous phase entrance at three different distances from the mixing channel. In order to observe generated flow pattern in the micromixer, the real images of fluid flow inside the mixing channel were recorded using an electronic microscope (640 9 480 pixels and with magnification 10 9 300). In addition, blue dye was added to the aqueous phase to observe flow patterns in various volumetric flow rates in the micromixer. Each one of the mentioned experiments was repeated three times to ensure the accuracy of the obtained results.
Theory In the present liquid–liquid mass transfer process, Cu (II) is transferred from water to the organic phase containing solution of D2EHPA, kerosene and Fe3O4 nanoparticles. Aqueous and organic phases are two immiscible phases. The removal efficiency is the ratio of transported Cu (II) to the maximum possible transferable Cu (II) and could be defined as follows (Azimi et al. 2015; Su et al. 2010; Mondal et al. 2010): E¼
Caq;in Caq;out Caq;in Caq;out
ð1Þ
where Caq;in and Caq;out are the concentrations of Cu (II) at the inlet and outlet of the aqueous phase, respectively. Caq;out is the equilibrium concentration of the Cu (II) in
Fig. 1 Schematic view of the experimental setup: (a) air cylinder (b) syringe pumps, (c) microchannel, (d) digital microscope, (e) magnet, (f) laptop
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Int. J. Environ. Sci. Technol. Fig. 2 Real and schematic images of micromixer and details of its geometry
the aqueous phase. Since the initial concentration of Cu in the aqueous phase and also the amount of solvent in organic phase are constant in all tests, in order to obtain the value of Caq;out , equal volumes of the two phases were added to a closed vessel equipped with a stirrer and mixing was performed for a long time. During the mixing, the copper concentration in the aqueous phase was measured several times. When Cu concentration remained almost constant, the equilibrium concentration was determined. The overall volumetric mass transfer coefficient (KLa) was used to evaluate the mass transfer characteristics. Since the interfacial area between phases is not measurable, it was obtained from the experimental results. KLa was calculated using the following equations (Yang et al. 2013; Assmann and Von Rohr 2011): ! Caq;in Caq;out 1 KL a ¼ ln ð2Þ tM Caq;out Caq;out tM ¼
V Qor þ Qaq þ Qg
ð3Þ
in which tM is the residence time of mixture of three phases, and V is the total volume of the mixing channel. In
addition, Qor, Qaq and Qg are flow rate of organic, aqueous and gas phases, respectively. In order to investigate the effectiveness of mixing methods on improving the mass transfer, relative mass transfer coefficient ratio (c) and removal efficiency ratio (k) based on simple layout (without inert gas, nanoparticles and magnetic field) were defined as follows (Azimi et al. 2015): c¼
K L a K L a0 K L a0
ð4Þ
k¼
E E0 E0
ð5Þ
where KLa0 and E0 are mass transfer coefficient and removal efficiency of the layout without any mixing factor at the same liquid phase flow rate, respectively.
Results and discussion When there are any acetate ions in the aqueous solution, the mechanism of the reaction between copper and D2EHPA is as follows (Van de Voorde et al. 2005; Ihm et al. 1988; Huang and Tsai 1991; Belkhouche et al. 2005):
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Fig. 3 Flow patterns observed at various flow rates of aqueous, organic and gas phases. a (Qaq = Qor = 1.875 and Qg = 0 mL/min), b (Qaq = Qor = 6 and Qg = 0 mL/min), c (Qaq = Qor = 1.875 and
Qg = 1 mL/min), d (Qaq = Qor = 6 and Qg = 1 mL/min), e (Qaq = Qor = 1.875 and Qg = 5 mL/min) and f (Qaq = Qor = 6 and Qg = 5 mL/min)
þ Cu2þ aq þ ðH2 R2 Þor $ ðCuR2 Þor þ 2Haq
increases the possibility of contact between the D2EHPA as solvent and copper ions.
ð6Þ
where H2R2 indicatives the dimeric form of D2EHPA. According to Eq. (6), in the reaction between copper ions and solvent molecules, the copper ions exchange with H? of H2R2 and then, H? ions enter the aqueous medium. The concentration of H? in the aqueous solution increases with time, which has not benefit to reaction represented by Eq. (6). Adding acetate ions cause the aqueous phase buffered, and H? ions cannot be prevented to the forward reaction. The extraction mechanism in the presence of acetate ions can be given as follows (Belkhouche et al. 2005; Sella and Bauer 1998): Cu2aq þ CH3 COO þ ðH2 R2 Þor $ ðCuCH3 COO ðHR2 ÞÞor þ Hþ aq
ð7Þ
Since the CH3COO– is a Lewis base group, Cu (II) was extracted from acetate buffer medium as CuCH3COO(HR2), where the acetate anion participates in the creation of the copper complex. The purpose of evaluation of various mixing methods includes (liquid–liquid extraction only, with inert gas, with nanoparticle, with inert gas ? nanoparticles and with inert gas ? nanoparticle ? magnetic field), finding a way to enhance the interface between the two phases and thus
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The effect of inert gas In order to illustrate the effect of inert gas on the flow pattern of organic and aqueous phase flows, real images of the flow regime were prepared as shown in Fig. 3. In these images, the flow regimes for two different flow rates of gas (1 and 5 mL/min) were qualitatively compared with the case without inert gas. This comparison was performed at low flow rates of aqueous and organic phases (left column) and also at high flow rates of these phases (right column). Figure 3a shows the slug flow regime for the low flow rate of aqueous and organic phases. As can be seen, the aqueous phase in relatively large slugs is surrounded by the organic phase (continuous phase). The slug flow of the gas–immiscible liquids is observed in Fig. 3b at the low flow rate of inert gas (1 mL/min). In this case, aqueous and organic phases have been in contact with each other in the space between the gas slugs. It can be seen in Fig. 3c that the slugs of aqueous phase have been broken by addition of inert gas. These small droplets have high surface-to-volume ratio and can increase the interface between the aqueous and organic phases. It can be also seen that the slugs of gas phase
Int. J. Environ. Sci. Technol. 1 simple layout Qg=1 (ml/min) Qg=3(ml/min) Qg=5 (ml/min)
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(b) Fig. 4 Effect of inert gas flow rate on a Cu (II) removal efficiency, b mass transfer coefficient
caused the organic and aqueous phases to be in contact with each other at smaller volumes. In other words, besides breaking the aqueous phase, it causes dividing the continuous phase into small spaces between the slugs, which has a positive effect on mass transfer. When the flow rate of water and organic phases is high (6 mL/min), parallel flow with smooth interface was established in the microchannel (Fig. 3d). When gas with low flow rate was injected into the system, the aqueous phase passes through the center of the tube as wavy annular flow (Fig. 3e). It can be observed that the injection of gas leads to a disorder pattern in parallel flow that it helps to further mixing. In addition, it can say that the interfacial area between the two immiscible liquid phases has also increased. If the gas flow rate increases, mixing of aqueous–organic phases will be so high that it is difficult to distinguish phases (Fig. 3f). The mass transfer characteristics, in terms of E and KLa, in the studied systems are given in Fig. 4. In these figures, the mass transfer characteristics were compared in three different flow rates of inert gas (1, 3 and 5 mL/min). The removal efficiency was increased by increasing the aqueous flow rate to 3 mL/min and then decreased after it. In
fact, when the flow rate of aqueous and organic phases is too low, although the residence time is high, but sufficient mixing was not undertaken between two liquid phases and effective mass transfer area is small. Once the flow rate of liquid phases is slightly increased (3 mL/min), the mixing quality is improved and thus the removal of Cu (II) from aqueous solutions has increased. With further increase in flow rate of phases, due to decrease in residence time, there is no enough opportunity to achieve high mass transfer efficiency. In addition, the results showed that the injection of inert gas generally increases the mass transfer coefficient, compared with the case that the inert gas is not implemented. In this comparison, the highest rate of removal efficiency in all of range of the aqueous phase is attributed to the case when the middle rates of inert gas (3 mL/min) have been applied to the system. The maximum enhancement in the copper removal efficiency (k), obtained at the aqueous flow rate of 4.29 mL/min was about 28% for injection of inert gas compared with the layout without it. Adding gas to the system has two opposite effects on the mass transfer characteristics. As can be seen in the real photographs of the flow regimes, the mixing performance and effective surface of mass transfer between the two liquid phases increased with increase in the flow rate of inert gas. Therefore, it can be expected that mass transfer to the organic phase increased. On the other hand, by increasing the flow rate of inert gas, the residence time of two liquid phases in the mixing channel is reduced according to Eq. (3). Therefore, enough time would not exist to transfer the more copper ions to another phase. The effect of gas injection on the improvement in the mixing overcomes its effect on the residence time. Figure 4a also shows that the effect of adding the inert gas on removal efficiency is more evident at the low flow rates of aqueous and organic phases. In addition, Fig. 4b indicates that the total mass transfer coefficient increased with increase in the flow rate of the inert gas. In other words, the mass transfer driving force is increased with increasing the gas velocity due to the decline in the residence time according to Eq. (2). It also observed that in the case of low gas flow rate and also in the case without inert gas, the mass transfer coefficient firstly increased with a gentle slope and then remained constant. However, when the gas flow is increased, KLa at start increased as function of aqueous flow rate and after reaching to a certain value declined again. According to Eq. (2), the difference in the trend of mass transfer coefficient changes can be attributed to the influence of the gas flow rate on the residence time and on the outlet concentration of copper in the aqueous phase. KLa is the driving force of mass transfer and it is a function of residence time and when the flow rate of
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Fig. 5 Flow patterns observed in the presence of nanoparticles (0.008 w/v) at various flow rates of aqueous, organic and gas phases. a (Qaq = Qor = 1.875 and Qg = 0 mL/min), b (Qaq = Qor = 6 and
Qg = 0 mL/min), c (Qaq = Qor = 1.875 and Qg = 1 mL/min), d (Qaq = Qor = 6 and Qg = 1 mL/min), e (Qaq = Qor = 1.875 and Qg = 5 mL/min) and f (Qaq = Qor = 6 and Qg = 5 mL/min)
organic and aqueous phases are low, residence time is high. In this condition, efficient mixing is not performed and the driving force of mass transfer is low. By injecting gas into microchannel according to Eq. 3, residence time is reduced and thus the mass transfer driving force increases. If injected gas flow rate is high (3 ml/min and above), can further improve the mixing between liquid phases. However, since phases there are enough time for mass transfer at low flow rates of aqueous and organic, under good mixing, the possibility of collision between the solvent and reactant molecules increases and thus makes it possible to achieve the high values of mass transfer coefficient.
phase (in Fig. 5a) are broken and smaller slug are formed (Fig. 5b). With increasing the gas flow rate (5 mL/min), very fine droplets of aqueous phase are dispersed in the oil phase and more efficient mixing between the two liquid phases is visible (Fig. 5c). Moreover, at high flow rates of liquid phases (6 mL/min) injecting the inert gas (1 mL/ min) leads to change the flow regime of the parallel (Fig. 5d) to wavy annular (Fig. 5e). Therefore, the aqueous–organic interface is increased, which plays an important role in enhancing the mass transfer performance. So that, by reaching the gas flow rate to the higher value (5 mL/min), the boundaries between the liquid phases are almost indistinguishable (Fig. 5f). High effective mixing in this system can be attributed to the simultaneous presence of the gas and nanoparticles in the system. In addition, whatever the gas velocity is high, the effect of nanoparticles on the mixing of two liquid phases is more evident. It is because of the fact that the inert gas is a major factor in the movement and distribution of nanoparticles in the mixing channel. There are two reasons for why addition of nanoparticles is effective for copper ions removal. The first one returns to movement of nanoparticles between phases that improve the mixing. Furthermore, by leaving the nanoparticles from the organic phase, higher amount of solvent transmitted to the aqueous phase which makes the possibility of collision between the solvent and solute. In this case, the reaction between solvent and solutes is not
The effect of inert gas and nanoparticles In another part of this work, simultaneous effect of gas flow rate and nanoparticles have been studied on the flow regime and mass transfer. For this purpose, the aqueous phase was in contact with the organic phase containing Fe3O4 nanoparticles in three different flow rates of gas. In all experiments, the concentration of nanoparticles in the organic phase was equal to 0.008 (w/v). The real photographs of the flow regime for high (5 mL/min) and low flow rate (1 mL/min) of inert gas and the without gas phase are shown in Fig. 5. Similar to Fig. 3, when the flow rate of liquid phases is low (1.875 ml/min), by adding an inert gas with low flow rate (1 mL/min), long slugs of aqueous
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The effect of magnetic field
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(b) Fig. 6 Effect of inert gas flow rate on a Cu (II) removal efficiency and b mass transfer coefficient in the presence of nanoparticles [concentration of NP = 0.08 (w/v)]
limited only to the interface between the two liquid phases, but it can be done everywhere of the mixing channel. The second reason is due to slight adsorption of copper ions by Fe3O4 nanoparticles. For this case, the equilibrium concentration of Cu (II) in aqueous phase (Caq ) was determined and the values of E and KLa were calculated based on it, as are shown in Fig. 6. The trends of removal efficiency and mass transfer coefficient were similar to the previous section (without the nanoparticles in the system). However, the curves were shift to higher values. The maximum improvements in the removal efficiency (k) of this method were approximately 18 and 43% based on the case without inert gas in the presence and absence of nanoparticles, respectively, which they were obtained in the gas and liquid (aqueous or oil) flow rate of 3 and 6, respectively. This increase in the mass transfer performance can be attributed to the effect of nanoparticles on the mixing and changing the flow patterns.
In order to induce the forced motion of nanoparticles to achieve proper mixing in the microchannel, the static magnetic field was applied on the mixing channel using a permanent magnet. The effect of magnetic field intensity on the mixing quality was assessed by placing the magnet at three different distances (3, 6 and 10 mm) from the mixing channel. In order to visualize the effect of magnetic field, the real photographs of the flow regimes are illustrated in Fig. 7. When the flow rate of liquid phases is low (1.875 mL/min) and the magnet is far from the channel (10 mm), a bubble flow regime occurs in the microchannel (Fig. 7a). In the case that the magnet is placed close to the wall of the mixing channel (3 mm), one can see that the droplets are broken down and dispersed as tiny droplets in the organic phase (Fig. 7b). The interface between phases is increased with decrease in droplets diameter that helps to liquid–liquid mass transfer enhancement. In addition, Fig. 7c, d illustrates the schematic and real images of flow patterns for high flow rates (6 mL/min) of the aqueous and organic phases in the presence of the magnet with the distance of 3, 10 mm from the mixing channel. It can be seen that when the magnet was located at the distance of 10 mm, nanoparticles have not a great impact on the mixing enhancement. In this case, such as the case without magnetic field (Fig. 5d), the flow regime is parallel. However, when the magnet was placed close to the micromixer, an efficient mixing between two immiscible phases was established. The empirical data of mass transfer between two liquid phases as removal efficiency and total mass transfer coefficient are given in Fig. 8a, b. In these diagrams, the influences of magnetic field on improving the mass transfer were compared with the case that it is not applied on the system. The results showed that for the cases that magnetic field was used, the values of E and KLa are higher than those of layout without magnetic field. This means that applying the magnetic field at each three distances has positive effect on the mixing of two liquid phases. In the presence of the magnetic field, the maximum enhancement of the removal efficiency (k), obtained at the magnet distance of 6 mm at the aqueous flow rate of 6 mL/ min, which was approximately 23 and 49% compared with layout without magnetic field (only nanoparticles) and without nanoparticles, respectively. The lowest mass transfer performance is attributed to the condition that magnet has a maximum distance (10 mm) from the mixing channel. The curve in this case has almost an overlap with the case of the absence of the magnetic field. In other words, at this position of magnet, the magnetic field has a little ability to divert nanoparticles and increase in the mass transfer efficiency. Due to the high flux density of magnetic field at the very close distance from the mixing channel,
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Fig. 7 Flow patterns observed in the presence of magnetic field at various flow rates of aqueous and organic phases. a (Qaq = Qor = 1.875 mL/min and d = 10 mm), b (Qaq = Qor = 1.875 mL/
min and d = 3 mm), c (Qaq = Qor = 6 mL/min and d = 10 mm), d (Qaq = Qor = 6 mL/min and d = 3 mm)
nanoparticles accumulate on the inner wall of the channel and its concentration among the liquid phases is low. In this case, the ability of nanoparticles is decreased to make effective mixing and absorption of copper ions. Therefore, their impact on the improvement in mass transfer is reduced. As a result, finding a suitable distance between the magnet and the mixing channel is very important to achieve the higher efficiency of Cu (II) removal.
Simultaneous effect of inert gas and magnetic field
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Since the inert gas and magnetic field have positive effect on the increasing the movement of nanoparticles and improving the mass transfer, the combination of these two parameters was studied in order to find a more efficient system. These experiments were carried out at the most appropriate operating condition including the flow rate of 3 mL/min for inert gas and the magnet distance of 6 mm.
Int. J. Environ. Sci. Technol.
obtained in the aqueous flow rate of 6 mL/min. Furthermore, the results indicate that the removal efficiency of Cu (II) was reduced with increasing the flow rate due to the reduction in the residence time. Figure 9b shows that the total mass transfer coefficient increased with a steep slope and then decreased with a little slope.
1 0.95 0.9 0.85
E
0.8 0.75
Comparison of the effect of mixing techniques on mass transfer rate
0.7 0.65 0.6 0.55 0.5 1.5
2.5
Just NP
d=3 mm
d=6 mm
d=10 mm
3.5
4.5
5.5
6.5
Q aq(ml/min)
(a) 12
10
Just NP
d=3 mm
d=6 mm
d=10 mm
Kla
8
6
4
2
0 1.5
2.5
3.5
4.5
5.5
6.5
Q aq(ml/min)
(b) Fig. 8 Effect distance of the magnetic field on a Cu (II) removal efficiency and b mass transfer coefficient [concentration of NP = 0.08 (w/v)]
The results of the flow regime as well as E and KLa (Fig. 9) have been reported. As can be seen, in the low and high flow rates of two liquid phases, the phases are completely mixed, so that chaotic flow patterns were observed in both cases. The simultaneous use of these two factors (the addition of gas and magnetic field) leads to combine the two types of vertical flow (by the magnetic field) and axial flow of nanoparticles (by the inert gas). In addition, the results of E and KLa plots confirmed the observations of real images. It can be observed from Fig. 9a that the copper removal efficiency is high in all range of aqueous flow rates. Moreover, the highest enhancement in the removal efficiency (k) of this case was 34% in comparison with the case with only nanoparticles. However, it was 62% compared with the case without any mixing factor such as gas, Fe3O4 nanoparticles and magnetic field. These values were
In order to compare the ability of the methods used to intensify the mixing and mass transfer, the optimum relative removal efficiency ratio (k) and mass transfer coefficient ratio (c) were obtained according to Eqs. (4) and (5) at each aqueous flow rate. The results are tabulated in Table 1. As can be seen in this table, k value for each system has a positive amount. This means that all applied techniques are useful and provide more efficient mass transfer rather than simple layout. The results also showed that c is more at high aqueous (and oil) flow rates (4.29 and 6 mL/min). In other words, the importance of using these methods to improve mass transfer at high flow rates is more evident. Moreover, this table shows that the results related to the effects of various mixing approach on c are similar to those of k. However, in cases where the inert gas exists in the system, c at low flow rate of liquid phases (1.875 mL/ min) is more than that of high flow rate (6 mL/min). This may be due to the effect of adding inert gas on the reduction in residence time and the enhancement of mixing that more comes into sight at low rates of liquid phases. As it is seen, the simultaneous application of inert gas and magnetic field caused a dramatic increase in driving force ratio (c [ 230%) at all studied flow rates of aqueous and organic phases. Thus, according to the results obtained from comparison between various used mixing methods, it can be said that the concurrent application of both aggravating factor (magnetic field and inert gas) greatly improves the mass transfer performance. An enhancement of 235–285% compared with plain layout on the average in KLa and 27–62% in E was obtained.
Conclusion Removal of Cu (II) ions from aqueous medium by solvent extraction method was performed in a T-shaped microchannel. In order to finding a proper mixing technique that can be effective, low-cost and easy to use, different methods for mixing of two immiscible liquid phases and high efficiency extraction of Cu (II) ions were investigated. These methods were based on adding inert gas at three different flow rates (1–5 mL/min) and locating a magnet at the different distances (3–10 mm) from the
123
Int. J. Environ. Sci. Technol. Fig. 9 Effect of both inert gas and of the magnetic field on a Cu (II) removal efficiency and b mass transfer coefficient [concentration of NP = 0.08 (w/v)] 1 0.95 0.9 0.85
E
0.8 0.75 0.7 0.65
simple layout
0.6
Qg=3 ml/min& d=6 mm
0.55
Just NP 0.5
1.5
2.5
3.5
4.5
6.5
5.5
Q aq(ml/min)
(a) 18 16 14 12
Kla
10 8 6 4
Just NP
2
Qg=3 ml/min& d=6 mm simple layout
0 1.5
2.5
3.5
4.5
5.5
6.5
Q aq(ml/min)
(b) mixing channel and also using the Fe3O4 nanoparticles with constant concentration of 0.008 (w/v). The key issue in this study is intensification of the mixing and mass transfer by the presence of magnetic nanoparticles under the condition of using simultaneously the magnetic field and inert gas injection. In order to find the most effective method for high mass transfer rate, the performance of each case was discussed by comparing the removal
123
efficiency (E) and the total mass transfer coefficient (KLa). The performance of these methods was determined by investigation of the removal efficiency and mass transfer coefficient enhancements. Based on this, enhancement of copper removal (k), for gas injection layout was 28% compared with the case without it. Furthermore, this amount was increased to 43 and 18% for the case with the simultaneous presence of nanoparticles and gas compared
Int. J. Environ. Sci. Technol. Table 1 Comparison of the best performance of different mixing methods at each aqueous flow rate Mixing method
Qaq = Qorg (mL/min)
Qg (mL/ min)
d (mm)
k (%)
c (%)
Inert gas injection
1.87
3
–
21.67
187.96
3
3
–
15.38
121.86
4.29
3
–
27.82
127.47
6
3
–
21.74
72.54
1.87
–
–
9.54
21.24
3
–
–
6.50
16.41
4.29
–
–
17.90
38.19
Adding Fe3O4 nanoparticles
6
–
–
20.97
36.46
Inert gas injection (with nanoparticles)
1.87
3
–
21.95
190.10
3
3
–
15.94
125.67
4.29
3
–
29.85
137.72
6
3
–
42.87
138.04
Applying magnetic field
1.87
–
6
22.97
65.49
3 4.29
– –
6 6
20.66 33.83
76.91 92.98
6
–
6
48.59
109.25
Simultaneous application of inert gas and magnetic field
1.87
3
6
31.30
284.57
3
3
6
26.72
260.36
4.29
3
6
44.35
252.38
6
3
6
62.11
236.64
with the case without and with the nanoparticles in the system, respectively. These enhancements reach to 49 and 23% when the magnetic field applied to the system. In addition, the combination of best operating conditions including inert gas with flow rate of 3 mL/min and the magnetic field with 6 mm distance from mixing channel was examined to improve the mixing and removal of Cu (II). In this case, the greatest relative removal efficiency ratio (k) was 62 and 34% and also maximum mass transfer coefficient ratio (c) was 285 and 217% based on base layout (without any mixing factor) and the case with presence only nanoparticles, respectively. From this study, it can be concluded that the simultaneous use of magnetic field and inert gas as mixing agents on the system containing the magnetic nanoparticles is an efficient way to achieve higher efficiency of mixing and mass transfer. In this study, both technologies were combined in a microchannel due to several inherent advantages such as high surface-to-volume ratio ensuring intensified mass transfer rates, low inventories ensuring possibility of carrying out reactions involving toxic and hazardous chemicals and compact size for industrial scale usage. As far as it is practical to control the rate of fluid flow pass through microscale systems easily, it can be quite confident about the use of this technique in industrial scale by
parallel cascading many of these microscale devices. Therefore, the integration of these two methods can be practical and efficient and has the potential of industrial applications. Acknowledgements The authors would like to thank the Nanotechnology Initiative Council of Iran for providing the financial support to carry out this work.
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