LiquidâLiquid Microextraction of Cu2+ from Water Using a New Circle

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Liquid-liquid micro-extraction of Cu2+ from water using a new circle micro-channel device Shuang Dai, Jianhong Luo, Jun Li, Xinhua Zhu, Yan Cao, and Sridhar Komarneni Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01888 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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Industrial & Engineering Chemistry Research

Liquid-liquid micro-extraction of Cu2+ from water using a new circle micro-channel device Shuang Dai,a Jianhong Luo,* abJun Li, aXinhua Zhu, aYan Cao, a Sridhar Komarneni,*b a

Department of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P.R. China.

E-mail:[email protected] b

Department of Ecosystem Science and Management and Materials Research Institute,Materials

Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. E-mail: [email protected]

ABSTRACT:In this work, to evaluate the new circle micro-channel device for the removal of Cu2+ from water, various parameters on the mass transfer effect and extraction efficiency were investigated by employing bis(2-ethylhexyl) phosphate (D2EHPA) as an extractant and kerosene as a solvent. The mechanism of Cu2+ extraction from water by D2EHPA(R2H2) is as follows：

Cu 2+ + 2.1(R2 H 2 )(o ) ⇔ (CuR2 ·2.2 RH )(o ) + 2 H (+o ) The contact time and micro-channel diameter were found to have great effect on the volumetric mass transfer coefficient ( K L a ). However, the temperature, the initial pH of water solution and the D2EHPA volume fraction of the extraction system nearly had no effect on the

K L a .The results of this study revealed that the new circle micro-channel may bean effective

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device for liquid-liquid micro-extraction as the extraction efficiency for Cu2+was more than 98% by three stage extraction under optimized conditions. Keywords: Liquid-liquid micro-extraction; Circle micro-channel device; Extraction efficiency; Mass transfer; Cu2+

1. INTRODUCTION Solvent extraction technology is widely used in industrial production because of the obvious advantages such as the simple operation, high extraction capacity and easy regeneration of organic phase. However conventional solvent extraction suffers from two disadvantages. It needs a long time to reach the extraction equilibrium, and it requires a high volume ratio of solvent/water below which extraction is poor. Furthermore, because of the nature of the mixing, sometimes there is the undesirable possibility of the formation of a third, colloidal phase which is difficult to eliminate. A new technique of liquid-liquid micro-extraction by micro-channels avoids these problems because micro-channels have some unique advantages1-9 such as high surface to volume ratio ensuring efficient intensified heat transfer rates to prevent thermal degradation, low inventory of chemical medicine that makes it possible to conduct reactions which involve hazardous and toxic substances, etc. In fact, micro-channels have been paid a lot of attention gradually by researchers in recent years10-20. Zhao et al. and Sarkar et al.21-22 investigated liquid-liquid two-phase flow patterns in micro-channels. Su et al.23-24 analyzed mass transfer intensification by gas agitation in a micro-channel and mass transfer characteristic in packed micro-channels. Raimondi et al.25 investigated liquid-liquid mass transfer in square micro-channel using 2D direct numerical simulations. Kashid et al.26 simulated the mass transfer in the liquid–liquid slug flow in a micro-channel by a computational fluid dynamics model. Plouffe et al.27 investigated the hydrodynamics of liquid-liquid two-phase flow and mass transfer characteristics in T-junction micro-channel. Benz et al.28 studied extraction efficiency in different extraction systems in the micro-channel when the total flow rate was determined. Aoki et al.29 studied the effect of mass transfer coefficient and extraction efficiency by the channel size and the flow rate on flow patterns in micro-channel.


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Geometric parameters of micro-channels are important factors for performance of a micro-channel in liquid-liquid micro-extraction process. Compared with the conventional extraction, the geometric parameters of micro-channels are often designed with a wide variety of shapes. In order to identify the relevant design parameters, many researchers have studied this topic and several reports appeared in the literature. Two kinds of stainless steel T-junction micro-channels, the opposing-flow and the cross-flow T-junction have been investigated experimentally to identify the mass transfer characteristics of immiscible fluids, the effects of the inlet configurations, the fluid’s inlet locations and the height and the length of the micro-channel by Zhao30. Kashid et al.31 investigated five different types of micro-channels, i.e., T-square micro-channel,

T-trapezoida

micro-channel,

Y-rectangular

micro-channel,

concentric

micro-channel and aterpillar micro-channel with different dimensions, cross sections and mixing elements to investigate their best performance in terms of mixing efficiency and energy consumption. Three different micro-channels such as a T-junction micro-channel, a serpentine micro-channel and a split-and-recombine micro-channels have been compared in experiments to identify the performance of micro-channels compared with conventional stage-wise extractors in liquid-liquid extraction process by Renjith32. Overall volumetric mass transfer coefficient( K L a ) influencing the flow regime in five different types of micro-channels was studied by Kashid33. Also, experiments to identify an optimal configuration that ensures high overall volumetric mass transfer coefficient were carried out by Darekar34 and the extraction process in PMMA slit-like micro-channels with four different configurations in different two-phase systems was investigated by Liu.35 Thus, there are numerous studies on the liquid-liquid micro-extraction process and here, this technology is investigated to extract Cu2+ from an aqueous solution. This basic study was done in order to find out its suitability for Cu2+ extraction from aqueous solution so as to determine the suitability of this technology for waste waters generated during industrial operations or other solutions that may be generated from dissolving components such as circuit boards to recover copper. Thus, the aim of this work is to experimentally evaluate a new circle micro-channel device for the liquid-liquid micro-extraction process of Cu2+.



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2.EXPERIMENTAL SECTION 2.1. Materials. The diluent in this work is kerosene from Luoyang Zhongda Chemical Co, Ltd., China (AR grade). D2EHPA was employed as an extractant produced by Luoyang Zhongda Chemical Co., Ltd., China (AR grade). Copper(II) sulfate pentahydrate was obtained from Tianjin City Bodi Chemical Co., Ltd., China (AR grade) and its aqueous solution was used as simulated waste water.

2.2. Liquid-liquid micro-extraction procedure. In order to investigate the effect of the diameter of micro-channel, three kinds of circle micro-channels whose diameters are 1mm, 1.5mm and 2mm were manufactured for this experiment. The organic phase and aqueous phase were pumped through the micro-channel by respective peristaltic pumps at the needed flow rate. In order to improve the accuracy of the experiment, peristaltic pumps were calibrated before starting the experiments. Super constant temperature trough was used to control the temperature.

2.3. Liquid-liquid micro-extraction experimental conditions. Various conditions used are listed below. Diluent: kerosene; D2EHPA volume content: 0.523; Phase ratio of aqueous to organic(A/O) =1:1; Initial Cu2+ concentration: 1000ppm; Initial pH of water: 4.55; Flow rates: 0.0152-0.0802mL/s; Reaction temperatures: 30-60℃.

2.4. Equipment and analysis. Figure 1 shows the schematic diagram of experimental setup used in the experiment. The aqueous phase and organic phase were fed into the micro-channel by two peristaltic pumps (YZ15, Baoding Lead Fluid Technology Co., Ltd.) and then a sample collection bottle was used to separate the oil-water mixture in the outlet of micro-channel. Extraction temperature was controlled by super constant temperature trough (601, JiangSu JinYi Instrument Technology Co., Ltd.). All experimental devices were connected by latex tube.


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Figure 1. Schematic diagram of experimental setup used in the present experiments: 1. Aqueous phase feed storage; 2. Organic phase feed storage; 3. Peristaltic pump A; 4. Peristaltic pump B; 5. Circle micro-channel; 6. Super constant temperature trough and 7. Sample collection bottle.

Figure 2. Schematic diagram of micro-channel: 1.Inlet A; 2.Inlet B and 3.Outlet C. The experiments were carried out in the micro-channels with a circular section of inner diameter of 1 mm and with a length 6 m. This equipment was home-built as can be seen in Figure 2. The channels had a T-junction at the inlet. The micro-channel had two inlets and an outlet. The organic and aqueous phases were pumped at inlet A and inlet B, respectively to achieve adequate mixing. The dispersion was generated quickly and heat transfer and mass transfer also took place quickly in the micro-channel during this process. After a few seconds, oil-water mixture from the outlet C was separated into two phases again in the sample collection bottle. The aqueous samples were analyzed for concentration of Cu2+ by the absorbance value at the



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wave length of 550nm36 using visible light spectrophotometry[UV-1100, Shanghai Mapada Instruments Co., Ltd.(China)].

3.RESULTS AND DISCUSSION The Equations 1- 4 given below are often used to analyze the experimental results. Equation 1 is used to quantify the percentage stage efficiency( η ( E ) ) and it is defined as follows:

η( E ) =

cM 0 , a − cM 1 , a cM 0 ,a − cM∗ 1 ,a

×100

(1)

Equation 2 is used to calculate percentage extraction(PE) and it is defined as follows:

PE =

cM 0 , a − cM 1 , a cM 0 , a

×100

(2)

Equation 3 is used to characterize the overall volumetric mass transfer coefficient in the extraction process( K Lα ), which is defined as follows:

K La =

(

Qo cM 0 ,o − cM 1 ,o

)

(3)

V∆ LMC

∆ LMC is log mean concentration difference defined by Equation (*)

∆ LMC

(c − c ) − (c − c ) = ln{(c − c ) / (c − c )} (K c − c ) − (K c − c ) = ln{(K c − c ) / (K c − c )} ∗ M 0 ,o ∗ M 0 ,o

M 0 ,o

d M 0 ,a

d M 0 ,a

M 0 ,o

M 0 ,o

M 0 ,o

∗ M 1 ,o ∗ M 1 ,o

M 1 ,o

M 1 ,o

d M1 ,a

d M1 ,a

(*)

M 1 ,o

M 1 ,o

Equation 4 is used for evaluating special extraction rate in Mixer(SER) and it is defined as follows:


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(c

S ER =

)

− cM 1,a Qo

M 0 ,a

(4)

V

SER indicates the number of moles extracted per unit time per unit volume of the contactor and is thus a measure of process intensification. Under the same extraction system, at the same extraction conditions, SER will be higher in smaller contractor volume and with shorter contact time.

As D2EHPA (R2H2) contains dissociable H+, the

3.1Effect of initial pH of water.

mechanism of extracting Cu2+ with D2EHPA perhaps is consistent with the cation exchange. In general, the extraction equilibrium mechanism of Cu2+ from water by D2EHPA can be described as follows:

n+

JM ( a ) + J

(n + x ) ( R H 2

2

)

2 (o)

⇔ ( MRn ·xRH ) J ( o ) + nJH + ( a )

where subscripts α and o represent aqueous phase and organic phase, respectively. When the activity coefficients of the compounds are certain, the apparent equilibrium constant can be defined as follows:

[(MR ·xRH ) ]( ) [H ]

+ nJ

Ki,x =

n

J

(n + x )

[M ] [R H ]( ) J

n+ J

(a )

If the species only exist in the form of

the form of

o

2

(a )

(5)

2

2 o

[(MR ·xRH ) ]( ) in organic phase and only exist in n

J

o

M 2 + in aqueous phase, the concentration of metal ions of outlet organic phase can be

described as follows:

cM1 ,o = J ·Ki, x ·c M ,a ·[R2 H J

1

 n+ x  J   2  2 (o )

]

[ ]

−nJ

· H + (a)

The above equation 6 can be turned into equation 7 as follows:


(6)


D = J ·K i , x ·c

J −1 M 1 ,a

·[R 2 H

 n+ x  J   2  2 (o )

]

[ ]

·H

+ − nJ

(7)

The following expression is obtained with logarithmic treatment on both sides of equation7:

lgD = lgJ ·K i,x ·c

J −1 M 1 ,a

·[R2 H

 n+ x  J   2  2 (o )

]

+ nJpH

(8)

When D2EHPA volume content remains constant, the relationship of lgD and the pH of extraction raffinate is shown in Figure 3. The slope of the equation is 1.67. Degree of polymerization can be calculated as 0.84, i.e, approximately equal to 1.

0.4 0.3 0.2

y = 1.665x - 2.54

0.1

lgD

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0.0 -0.1 -0.2 -0.3 1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

The pH of extraction raffinate

Figure 3. Determination of polymerization degree of copper ions extracted in D2EHPA kerosene solvent. Conditions: O/A=1:1,d=1mm,τ=78s and T=60℃

(lgD − 2 pH ) = lgK i , x + 1 + x  lg[R2 H 2 ](o ) 

2

（9）

Number of solvent molecules of extractant in the extraction complex is about 2.2 according


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to the equation 9 and from Figure 4. Meanwhile, the intercept of linear equation is -2.9388 (Fig. 4), which is apparently the extraction equilibrium constant.

-2.4 -2.6 -2.8

y = 2.1688x - 2.9388

-3.0

lgD-2pH

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-3.2 -3.4 -3.6 -3.8 -4.0 -0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

lg[R2H2](o)

Figure 4. Determination of number of solvent molecules of copper ions extracted in D2EHPA kerosene solvent. Conditions: O/A=1:1,d=1mm,τ=78s and T=60℃. Mechanism of extraction of copper ions from water by D2EHPA is as follows：

Cu 2+ + 2.1(R2 H 2 )(o ) ⇔ (CuR2 ·2.2 RH )(o ) + 2 H (+o ) Figure 5 shows that the percentage extraction of Cu2+ ions decreased with decreasing pH of water. Figure 6 shows the same variation trends of special extraction rate as with those of percentage extraction. Percentage extraction decreased from 71.44% to 37.17% as the initial pH of aqueous solution decreased from 4.55 to 1.54. As the cationic extractant D2EHPA contains dissociable hydrogen ions, hydrogen ions would be replaced when D2EHPA reacts with Cu2+. When the pH decreased from 4.55 to 1.54, the concentration of hydrogen ions increased by more than 1000 times and the increased concentration of hydrogen ions in the extraction system is not conducive to extract Cu2+ because of competition from protons. This competition clearly led to significant reduction in percentage extraction of Cu2+ ions. However, pH had little effect on the



mass transfer coefficient as can also be seen from Figure 6. Therefore, a suitable value of pH can be selected for the extraction process.

75 70 65 60

PE(%)

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55 50 45 40 35 1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Initial pH of water

Figure 5. Variation of percentage extraction of Cu2+ with the initial pH of water. Conditions: O/A=1:1, d=1mm,τ=78s and T=60℃


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0.0250

0.075

Overall volumetric mass transfer coefficient Special extraction rate

0.070 0.065

0.0245

SER(mol/m3/s)

0.060

KLa(s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.055 0.0240 0.050 0.045 0.0235

0.040 0.035

0.0230

0.030 1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Initial pH of water

Figure 6. Variation of K L a and special extraction rate of Cu2+ with the initial pH of water. Conditions: O/A=1:1, d=1mm, τ=78s and T=60℃

3.2 Effect of micro-channel diameter. According to Equations 10 and 11, Reynolds number increases with decreasing micro-channel diameter using the same contact time and therefore, the volumetric mass transfer coefficient and percentage extraction should improve in theory37.

Re (o ) =

Re(a ) =

du (o ) ρ(o ) µ (o )

du(a ) ρ(a ) µ( a )

(10)

(11)

Further experiments were carried out to verify the above theoretical conjecture. Experiments were conducted in three micro-channels which had diameters of 1.0mm, 1.5mm and 2.0mm, in order to study their performance in terms of percentage extraction, percentage stage efficiency, special extraction rate and volumetric mass transfer coefficient. Figure 7 shows the variation of percentage extraction and percentage stage efficiency as a function of the diameter of



micro-channels. Figure 8 shows the SER measured in different micro-channels and the effect of diameter on volumetric mass transfer coefficient. All the above measured parameters were found to increase with the reduction in micro-channel diameter. A stronger internal circulation was induced as the inner diameter decreased and therefore, the mass transfer area of the liquid-liquid, i.e., the two phases increased, the updating speed of two-phase interface was accelerated and convective mass transfer process was enhanced gradually. As a consequence, higher extraction efficiency and percentage stage efficiency were obtained in the micro-channel with smaller channel size. The mass transfer coefficient of total volume was improved remarkably.

73.0

Percentage extraction Percentage stage efficiency

1.000

72.0

0.995

71.5

0.990

71.0

0.985

η(E)

72.5

PE(%)

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70.5 0.980 70.0 0.975

69.5

0.970

69.0 1.0

1.2

1.4

1.6

1.8

2.0

The inner diameter of channel(mm)

Figure 7. Effect of diameter of micro-channels on percentage extraction and percentage stage efficiency. Conditions: O/A=1:1,pH=4.55 and T=60℃


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0.08 0.024


0.022

0.07

0.020 0.018

0.012 0.04

0.010 0.008

3

0.05

0.014

SER(mol/m /s)

0.06

0.016

KLa(s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.03

0.006 0.004

0.02

0.002 0.000

0.01 1.0

1.2

1.4

1.6

1.8

2.0

The inner diameter of the channel(mm)

Figure 8. Variation of K L a and special extraction rate with diameter of micro-channels. Conditions: O/A=1:1, pH=4.55 and T=60℃

3.3 Effect of flow velocity. As shown in Figure 9, the percentage extraction increased as contact time increased from 29s to 78s. When the flow velocity reached to a certain value, the extraction of Cu2+ decreased with the increasing flow velocity. The percentage stage efficiency showed the same trend with variation of percentage extraction. When the rotation speed of peristaltic pump decreased, the contact time increased in the micro-channel and the flow velocity of the organic phase and aqueous phase decreased. During this process, the dispersion would be limited and specific interfacial area would be reduced owing to the less energy available for generating dispersion. Reduced specific interfacial area tends to reduce the mass transfer while the increased contact time is beneficial to mass transfer. These two factors appear to almost balance each other. The effect of reduced specific interfacial area on percentage extraction and the percentage stage efficiency is greater than that of contact time with a low flow velocity. The percentage extraction and the percentage stage efficiency increased when the contact time increased from 29s to 78s. When the contact time reached to a certain value of 78s, the effect of contact time was greater than that of specific interfacial area and led to decreased percentage



extraction and the percentage stage efficiency.

Percentage extraction Percentage stage efficiency

73.0

1.03

72.5

1.02

72.0

1.01 1.00

71.0

0.99

η(E)

PE(%)

71.5

70.5 0.98 70.0 0.97 69.5 0.96 69.0 20

40

60

80

100

120

140

160

Contact time/s Figure 9. Variation of percentage extraction with contact time. Conditions: O/A=1:1, d=1mm, pH=4.55 and T=60℃

0.09 0.20

0.07

0.16

0.06

0.14

KLa(s )

0.12 0.05 0.10 0.04

0.08

0.03

0.06

0.02

0.04 0.02

0.01

0.00 20

40

60

80

100

120

Contact time/s


140

160

SER(mol/m /s)

0.18

3


0.08

-1

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Figure 10. Variation of K L a and special extraction rate with contact time. Conditions: O/A=1:1, d=1mm, pH=4.55 and T=60℃ Figure 10 shows the variation of special extraction rate with increasing contact time. It can be speculated that the value of SER will approach 0 at higher flow rates with shorter contact times. This may be attributed to higher specific interfacial area at higher flow rate or shorter contact time. Equations 2 and 4 can be combined to relate the PE and SER. The relation is given by Equation 12. For a given O/A ratio, specific extraction rate varied inversely with contact time but the SER increased with the reduction of contact time. Since the contact time in the micro-channels can be very high, SER in micro-channels at higher contact time turns out to be very small and probably equal to the effect of conventional extraction.

SER =

Q o PE cM 0 ,a 100 V

=

Q o PE cM 0 ,a

100(Q o + Q a ) τ

=

PE cM 0 ,a

(12)

100(1 + A/O) τ

Figure 10 also shows the variation of volumetric mass transfer coefficient, K L a with the increase in contact time. Compared with the high flow velocity, volumetric mass transfer coefficient is higher at the low flow velocity. The minimum values of K L a in micro-channels correspond to higher contact time. If micro-channels are used for carrying out solvent extraction using contact times typically used in conventional extractors at lower flow velocity, they do not offer any advantage over conventional extractors32. Percentage extraction and percentage stage efficiency decreases at higher flow velocity. Therefore, the velocity of micro-channel should be judiciously chosen. Optimum flow velocity of aqueous phase should be selected as 0.0302mL/s when the parameters are comprehensively considered. Compared to the different microchannels such as a T-junction microchannel, a serpentine microchannel and a split-and-recombine microchannel32, K L a in circle micro-channels was higher than that of other types of microchannel when the phase ratio was 1:1 and contact time was more than 30s. Plouffe et al27 investigated the mass transfer process of two curvature-based(SZ, TG) microchannels, the volumetric mass transfer coefficient of n-BuOH increased from 0.01 to



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0.05 as the total flow rate increased from 1mL/min to 15mL/min, mass transfer performance of SZ and TG microchannels were similar to that of circle microchannel. Volker Hessel and Safa Kutup Kurt et al38 studied liquid–liquid extraction system with microstructured coiled flow inverter which the volumetric mass transfer coefficient exceed 1 when the length of tube was 360mm and residence time was 2.8s. Because of different extraction conditions, It is not very accurate to determine whether the mass transfer performance of microchannel equipment was superior or not by analyzing mass transfer coefficients. In order to compare mass transfer performance of microchannel with coiled flow inverter and circle microchannel, further comparative experiments needed to do.

3.4 Effect of temperature. The extraction mechanism can be shown as follows: lgK i,x = lgD − 2 pH − 2.1lg [R2 H 2 ](o ) The extraction temperature is changed with the condition of constant [R2 H 2 ](o ) while lg D and pH are determined under the experimental conditions. K i , x under the corresponding temperature can be calculated. Linear relation of apparent extraction equilibrium constant and temperature is shown in Figure 11, which meets the Van’t Hoff equation as follows:

d ln K ∆H = dT RT 2


(13)

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-5.60

-5.65

-5.70

y = -0.8932x - 2.0578 lgKi,x

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5.75

-5.80

-5.85

-5.90 3.00

3.05

3.10

3.15 -3

3.20

3.25

3.30

-1

1/T×10 (K )

Figure 11. The relationship of apparent extraction equilibrium constant and temperature. Conditions: O/A=1:1, d=1mm, τ=78s and pH=4.55 Heat of reaction can be approximated as a constant with in a certain range of temperature and equation14 can be obtained by integration.

lg K i , x = −

∆H +C 2.303RT

(14)

The calculated value of heat of reaction is 17.10KJ, which means that the extraction reaction is an endothermic reaction. Percentage extraction increased as the reaction temperature increased as shown in Figure 12. The K L a and special extraction rate increased when the temperature of the extraction system increased as shown in Figure 13. However, volatility of extraction solvent may be severe under high temperature condition leading to severe gas-liquid coexistence in micro-channel. Thus, high temperature may lead to the loss of extraction solvent and reduce extraction efficiency.



74 73 72

PE(%)

71 70 69 68 67 30

35

40

45

50

55

60

T/ºC Figure 12. Variation of percentage extraction with temperature. Conditions: O/A=1:1, d=1mm, τ=78s and pH=4.55

0.075 Overall volumetric mass transfer coefficient Special extraction rate

0.0235

0.074 0.073

0.0230

0.0225 0.070 0.069

0.0220

0.068 0.067

0.0215

0.066 0.0210

0.065 30

35

40

45

50

55

60

T/ºC

Figure 13. Variation of K L a and special extraction rate with temperature. Conditions: O/A=1:1, d=1mm, τ=78s and pH=4.55


SER(mol/m3/s)

0.072 0.071

KLa(s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.5 Effect of the concentration of D2EHPA. Volume fraction of D2EHPA is a vital factor in the extraction process. To study the effect of D2EHPA volume fraction on the extraction process, different concentrations of extraction solvent, i.e., 0.3125mol/L, 0.625mol/L, 0.9375mol/L, 1.25mol/L and 1.5625mol/L were tested. The percentage extraction was found to be proportional to the concentration of extraction agent and the results would be seen in Figure 14. Figure 15 shows that the special extraction rate increased with the increase of the concentration of extractant and little variation of K L a with the concentration of extractant. However, when the D2EHPA concentration increases to a certain value, the extraction reaction reaches equilibrium and the extraction ratio will remain almost unchanged.

80

70

60

PE(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

40

30 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-1

c/molL

Figure 14. Variation of percentage extraction with concentration of the extractant. Conditions: O/A=1:1, d=1mm, pH=4.55, τ=78s and T=60℃.



0.030

0.08 Overall volumetric mass transfer coefficient Special extraction rate

0.029

0.07

0.028

0.06 0.05

3

0.026

SER(mol/m /s)

0.027

KLa(s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.04 0.025 0.03 0.024 0.02

0.023

0.01

0.022 0.021

0.00 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

c/molL-1

Figure 15. Variation of K L a and special extraction rate with concentration of extractant. Conditions: O/A=1:1,d=1mm, pH=4.55, τ=78s and T=60℃.

3.6 Effect of the extraction times on extraction under the optimized conditions. From the above results, the optimal conditions were determined to be as follows: the inner diameter of the channel is 1mm, the total flow velocity is 0.0604 mL/s, the initial pH of aqueous solution is 4.55, the reaction temperature is 60℃ and the D2EHPA volume fraction is 52.05%. More than 98% of extraction efficiency of Cu2+ could be attained by three stage extraction. It can be seen from Figure 16 that the extraction efficiency increased with the increase of extraction times. Liquid-liquid micro-extraction technology had been widely used in the many fields such as environmental science, separation science and biomedicine because of obvious advantages. Microfluidic solvent extraction (SX) of metal ions from particle-laden aqueous solutions was demonstrated as an alternative to conventional solvent extraction for a system of industrial interest by Craig39. Syouher et al40 studied a simple but effective micro-mixer-settler for nuclear solvent extraction and extraction rate of nearly 100% were observed during extraction process. Xu et al41 investigated the synthesis of copper nanoparticles by reduction of metal salt solutions with sodium


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borohydride in a T-shaped microfluidic device at room temperature. Zhao et al42 analyzed four aromatic amines in environmental water samples with a rapid and sensitive quantitative method applied in Liquid–liquid–liquid microextraction (LLLME) with hollow fibers in high-performance liquid chromatography (HPLC). The micro-extraction technology in this paper will be used in the purification of wet process phosphoric acid which contained a lot of metal impurities such as Cu2+, Fe3+ and Cr3+. A scale-up device about a new circle micro-channel will be designed and its characterization will be investigated in the next study. ”

100

95

90

PE(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


85

80

75

70 1.0

1.5

2.0

2.5

3.0

Extraction times

Figure 16. Variation of extraction efficiency with the extraction times. Condition:O/A=1:1,d=1mm,pH=4.55, τ=78s and T=60℃.

4. CONCLUSIONS Liquid-liquid extraction experiments were carried out by solvent extraction using D2EHPA in the circle micro-channel and led to the following conclusions:



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1) The optimal conditions for removing Cu2+ from aqueous phase were found to be as follows: The inner diameter of the channel is 1mm, the total flow velocity is 0.0604 mL/s, the initial pH of aqueous solution is 4.55, the reaction temperature is 60℃ and the D2EHPA volume fraction is 52.05%. 2) Mechanism of extraction of copper ions from copper sulphate solution by D2EHPA is：

Cu 2+ + 2.1(R2 H 2 )(o ) ⇔ (CuR2 ·2.2 RH )(o ) + 2 H (+o ) 3) Liquid-liquid extraction in the circle micro-channels was found to be an effective method to remove Cu2+ with an extraction efficiency of more than 98% by three stage extraction under the optimized conditions. 4) Volumetric mass transfer coefficient, K L a increased with reduction in contact time， decreased with the increase of micro-channel diameter and increased with the increase of temperature of the extraction system.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: 86-28-85468288. Fax: 86-28-85468288. Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENT The authors gratefully acknowledge financial support from the Applied Basic Research Programs of Science and Technology Commission Foundation of Sichuan Province(No. 2014JY0079), The People’s Republic of China.

NOMENCLATURE


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M = Metal ions

R2 H 2 = Existence as a dimer D2EHPA in organic phase

J = Polymerization degree of extraction complex

x = Number of solvent molecules of D2EHPA in extraction of complex

(MR 2 ·xRH )J ( o ) = Extraction complex in organic phase cM 0 ,o = Inlet concentration of the organic phase at the inlet, mol/m3

cM 1 ,o = Outlet concentration of the organic phase at the outlet, mol/m3 c M 0 , a = Concentration of the aqueous phase at the inlet, mol/m3

cM 1 ,a = Concentration of the aqueous phase at the outlet, mol/m3

cM∗ 0 ,o = Concentration of metal ion in the organic phase in equilibrium with the incoming aqueous phase, mol/m3

cM∗ 1 ,o = Concentration of metal ion in the organic phase in equilibrium with the outgoing aqueous phase, mol/m3

[H ] = The activity of hydrogen ion in aqueous phase D=

cM 1 ,o cM 1 ,a

PE = Percentage extraction

η ( E ) = Percentage stage efficiency K d = Distribution coefficient



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

K L a = Overall volumetric mass transfer coefficient, 1/s

S ER = Special extraction rate V = Total volume of extractor, m3

ν (a ) = Flow velocity of aqueous phase, m/s v(o ) = Flow velocity of organic phase, m/s

ρ (o ) = Density of organic phase, kg/m3 ρ (a ) = Density of aqueous phase, kg/m3 d= Diameter of micro-channel, mm

µ (o ) = Viscosity of organic phase, Pa·s µ (a ) = Viscosity of aqueous phase, Pa·s Qo = Flow rate of organic phase, mL·s-1 Qa = Flow rate of aqueous phase, mL·s-1

τ = Contact time based on the total flow velocity, s


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