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Liquid-Liquid Extraction from Frozen Aqueous Phases Enhances Efficiency with Reduced Volumes of Organic Solvent Kensuke Yanagisawa, Makoto Harada, and Tetsuo Okada ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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ACS Sustainable Chemistry & Engineering

Liquid-Liquid Extraction from Frozen Aqueous Phases Enhances Efficiency with Reduced Volumes

of Organic Solvent

Kensuke Yanagisawa, Makoto Harada, and Tetsuo Okada*

Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551,

Japan

Phone and fax: +81-3-5734-2612

Email: [email protected]

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ACS Sustainable Chemistry & Engineering 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

Abstract

Liquid-liquid extraction is widely employed for the separation and analysis of mixtures of

compounds, both in the laboratory and in industry. Although a large volume of organic solvent is

required to maximize extraction efficiency, the principles of sustainability and green chemistry

dictate that the consumption of organic solvents should be minimized. This paper proposes a new

liquid-liquid extraction method, realizing high extraction efficiencies with only a small volume of

organic solvent. Since extraction efficiency is a function of the ratio of organic and aqueous solvent

volumes, attaining high extraction efficiency usually requires a large volume of the organic phase. In

general, it is difficult to increase this ratio by reducing the volume of the aqueous phase, except by

heating—which damages, and potentially decomposes, the solutes. However, if the aqueous phase is

frozen, a freeze-concentrated solution containing the solutes is formed; the volume of the

freeze-concentrated solution can be controlled by altering the main solute concentration and

temperature. This paper demonstrates the extraction of organic molecules and metal chelates by

freezing the aqueous phase. In some cases, freezing reduces the volume of the organic phase to a

twentieth of that required to achieve the same extraction ratio without freezing.

Keywords: Two phase partition, Freeze solvent extraction, Ice, High extraction ratio, Metal chelates.

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Introduction

Liquid-liquid extraction is an efficient separation method, which is widely used in laboratory studies and industrial processes.1-4 Neutral species, such as organic molecules and metal chelates, are

typically considered extractable into an organic phase; however, this concept of solvent extraction has been significantly extended to include proteins5 and ions.6-7 An aqueous two-phase system that avoids organic solvents has been applied to bioseparation.8-9 Extraction into room-temperature ionic liquids has also received attention, because these are considered green solvents.10-14 In some cases,

mechanisms such as ion-exchange, which are not involved in extractions to the usual molecular solvents, have improved extraction efficiency.12 Thus, a number of attempts have been made to avoid

or reduce organic solvent consumption in widely used liquid-liquid extractions. However, these

alternatives are not environment-friendly, often because of the low biodegradability of the substitute solvents.15-16 Additionally, organic solvents have a number of other advantageous properties, e.g.

solute solubility, polarity, and density. Therefore, a route to minimize the amount of organic solvent

required, while maintaining these flexibilities in properties is desired.

Regardless of solvent type, extraction relies on the distributive law, which describes the

distribution of a solute between mutually immiscible phases. When a solute is distributed between

two phases, its extraction ratio (E) from the aqueous phase (W) to the organic phase (O) is expressed

by Eq. (1).

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E=

DVO Dϕ = DVO + VW Dϕ + 1

(1)

where D, VO, and VW are the distribution ratio of the solute and the volumes of O and W, respectively, and ϕ (= VO/VW) is the volumetric ratio of the phases. Increasing ϕ is a universal approach to achieving a high value of E; this is usually achieved using a large volume of the organic phase.

However, using a large volume of the organic phase is undesirable, particularly for industrial processes, because of environmental and cost considerations. An alternative way to gain a large ϕ is to reduce the volume of the aqueous phase. Ultrafiltration, for example, has been applied to macromolecular samples for this purpose.17 Evaporation of water is undesirable because it may

cause significant loss or decomposition of the solutes. Thus, a method which reduces the volume of

the aqueous phase without losing or damaging the solutes is required.

In this paper, we propose a new method for solvent extraction, which can provide high phase-ratio (ϕ) without increasing the volume of the organic phase; that is, freeze-solvent extraction. Frozen aqueous salt solutions usually form eutectic systems; when the temperature is higher than the

eutectic temperature and the solute concentration is below the eutectic concentration, the system

forms a mixture of ice and a freeze concentrated solution (FCS). In previous work, we developed ice

chromatography, in which ice/frozen solutions were used as the stationary phase for liquid chromatography,18 and then used for chiral separation,19-20 adsorption-partition control,21 and detection of a thin liquid layer on the ice surface.22 In addition, some phenomena specific to frozen

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solutions were found through these measurements, e.g. enhanced crown ether complexation,23 accelerated reaction kinetics,24 suppressed hydrophobic interactions,25 etc. Thus, freezing not only

causes the thermodynamically expected results, but also shows unpredicted aspects.

Freeze-solvent extraction is based on the concentration of solutes in the FCS. As discussed

below, the volume of the FCS can be predicted from the phase diagram of the system. Since freezing

is performed by lowering the temperature, there is only a low risk of sample decomposition or

contamination. Thus, it is an effective method of solute enrichment, though there are only a few reports of successful use of freeze concentration as an analytical strategy.26 Here, we demonstrate the

basic concept and effectiveness of freeze-solvent extraction.

Experimental Section

Solvent extraction experiments were performed using 20-mL vials.

In extraction under

unfrozen conditions, the phase ratio was unity. The extraction of an organic compound from a frozen aqueous phase to an organic phase was performed in the following way. A solute,

p-nitrophenol (pNP) or m-nitrobenzyl alcohol (mNBA), was dissolved in aqueous sodium chloride.

Ice particles were then prepared from the solution in one of two ways. In the first method, fine ice

particles were prepared by nebulizing the solution and introducing the mist into liquid nitrogen. The

ice particles were then sieved to select the particles with diameters between 75 µm and 150 µm. In

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the second method, coarse ice particles were prepared by flowing the solution through a bundle of

fused silica capillaries (75 µm internal diameter); the resultant droplets were immersed into liquid

nitrogen. The diameter of these particles ranged from 1.6 mm to 2.0 mm. In a low-temperature

reactor, 4-5 g of the ice particles was added to 8.0 mL of organic solvent, pre-cooled to −35 ºC. Ice

particles were well immersed and dispersed in an organic solvent when the solvent density was close

to that of ice. A 1:1 mixture of hexane and 1,2-dichloroethane (DCE) was used as the organic phase

for this reason. The ice/organic mixtures were maintained at −3.0 ºC, in a refrigerator, for 3 h. The

concentration of the solute in the organic phase was measured with a UV-Vis spectrophotometer.

Extraction from an organic phase to a frozen aqueous phase was performed by a similar method.

A solute-containing organic phase was precooled, and aqueous NaCl was frozen to prepare ice

particles of 75 µm to 150 µm in diameter; the ice particles were then added to the organic phase. The

resulting mixture was kept at -3.0 ºC, and the concentration of the solute in the organic phase was

spectrometrically determined.

For the extraction of the metal chelates, the organic phase was a 7:3 mixture of hexanol and DCE, containing 1.0 mM of 8-hydroxylquinoline (HQ). The aqueous phase contained metal ions (Fe2+, Ni2+, or Zn2+) at a concentration of 50 µM. The solution pH was adjusted with glycine (pH 2.31), acetate (pH 4.16), and phosphate buffer (pH 5.57) during the extractions of Fe2+, Ni2+, and Zn2+, respectively. In frozen solutions, pH was measured using a low-temperature pH electrode (InLab®

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Cool, Mettler Toledo, Switzerland). After a reverse extraction into 0.1 M aqueous HCl, the

concentration of the metal ion extracted into the organic phase was determined by atomic absorption

spectrometry.

Results and Discussion

Overview of extraction from a frozen aqueous phase

In this work, extraction was carried out from aqueous NaCl. Figure S1 shows the phase diagram

of the NaCl/water system; at temperatures between the eutectic point (-21.3 °C) and the melting i ) is below the eutectic point, the FCS coexists with ice when the overall concentration of NaCl (NaCl FCS concentration (4.2 M). The concentration of NaCl in the FCS (NaCl ) depends only on temperature i FCS i and is independent of NaCl . Thus, NaCl is concentrated in the FCS by a factor of α = NaCl /NaCl .

Minor components are dissolved in the original solution, they are also concentrated in the FCS upon

freezing unless they are removed by specific process, i.e. adsorption on the ice surface, the

distribution into the ice phase, deposition; these rarely occur, except at high concentrations. The concentration of the minor components in the FCS (sFCS ) is equal to αsi , where si is the initial concentration of a minor solute in the original solution before freezing.

Although the accurate volume of the FCS can be calculated using the lever rule, taking the

density of the FCS into account, the following approximation is useful. When an aqueous phase of a

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volume of VW is frozen, the volume of the FCS, VFCS, is given by Eq. (2). VFCS = VW / α

(2)

Freezing reduces the volume of the aqueous phase to 1/α of its original value. Thus, when a solute is extracted from the FCS into an organic phase with a volume of VO, the extraction ratio is given by Eq. (3).

=

O

O

i

=

=

i

NaCl FCS

(3)

NaCl

where, ViW is the initial volume of the aqueous phase before freezing. Thus, for any value of D, E approaches unity as α→∞. Figure 1 shows the theoretical relationship between Eϕ=1 and α at various D values. At D = 5, Eϕ=1 approaches unity at α ≥ 10. Even at D = 0.1, Eϕ=1 reaches 0.9 when α = 100. Thus, we can efficiently extract a solute into an organic phase even when its extraction would be

poor through the usual method. In the following sections, Eϕ=1 values, measured at −3.0 °C, are i FCS discussed and α was varied by changing NaCl . In the frozen conditions, NaCl is 890 mM at this i temperature, regardless of NaCl . Therefore, the aqueous phase has a constant ionic strength of 890

mM of NaCl.

Solvent extraction of organic compounds

In this study, pNP and mNBA were selected as model solutes, because they have different log P values 2.17 and 1.21, respectively.27 We considered that these solutes are suitable for studying

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the distribution properties in the present system, because the different extraction behaviors are i for pNP at −3.0 °C. The expected. Figure 2 shows the relationship between Eϕ=1 and NaCl

extraction of pNP from a 890 mM aqueous NaCl solution provides a basis for discussing the

efficiency of the frozen extraction. The extraction ratio, Eϕ=1, of pNP from 890 mM NaCl solution is 0.43 and, thus, D = 0.75. The solid green curve represents the prediction made by Eq. (3). The points

in Figure 2 show the experimental results obtained with the coarse (1.6 – 2.0 mm diameter, red

circles) and fine (75 – 150 µm diameter, blue circles) ice particles. As predicted by Eq. (3), Eϕ=1 i increases as NaCl decreases (increasing α); however, the change in Eϕ=1 clearly depends on the size

of ice particles. For the coarse ice particles, Eϕ=1 increases to 0.57 at cNaCli = 200 mM and then i decreases with further decreases in NaCl . In contrast, for the fine particles, Eϕ=1 increases with i i decreasing NaCl , following the prediction of Eq. (3). The highest Eϕ=1, 0.85, is reached at NaCl =

50 mM. Thus, Eϕ=1 from the frozen state is twice as high as that from the liquid state. To attain this extraction ratio in a single extraction from an unfrozen solution, ϕ would need to be 7.6. Thus, freezing reduces the volume of the organic phase required to achieve this extraction ratio by a factor

of 7.6. i The different dependences of Eϕ=1 on NaCl at different ice particle sizes suggest that part

of the FCS in the coarse particles is not in contact with the organic phase, but is confined in the

interstitial spaces between the ice crystals. In contrast, the agreement between the theoretical (Eq.

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(3)) and experimental results for the fine particles suggest full contact between the FCS and the

organic phase. For efficient extraction of solutes into the organic phase, contact between the FCS

and the organic phase should be maximized. For the fine ice particles, the contact was assessed by

measuring the solute partition from the organic phase to the FCS; the results, for pNP, are plotted as

open squares in Figure 2. The Eϕ=1 values from the organic phase agree well with those from the FCS, indicating that, for the fine particles, the FCS is in complete contact with the organic phase.

Figure 3 shows the results of the freeze extraction of mNBA using fine ice particles. Similar to the i extraction of pNP, Eϕ=1 increases with decreasing NaCl ; for the extraction from 890 mM aqueous i solution Eϕ=1 was 0.49, this increased to 0.81 at NaCl = 50 mM. The extraction ratios from the FCS

are close to those from the organic phase. Thus, high extraction ratios from frozen aqueous phases

can be achieved without increasing the volume of the organic phase.

The extraction kinetics strongly depends on the diffusivity of a solute in the system. In the

present scheme, the diffusion path length is equal to the radius of an ice particle, i.e. 37.5 - 75 µm for fine ice particles. Since the typical droplet size in liquid/liquid extraction is in the mm range,28 the

diffusion distance in the ice particle is < 1/10 of that in usual liquid/liquid extraction. In contrast, the

diffusion coefficient (D) in the present condition (890 mM NaCl at -3.0 °C) is 0.4 of that at room temperature (see details in the supporting information). The diffusion time is given by l2/(2D), where

l is the diffusion distance. Thus, the diffusion time in the present scheme is < 1/40 of that in

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liquid/liquid extraction at room temperature. Thus, from a kinetic viewpoint, a use of fine ice particle

overcomes a kinetic disadvantage coming from low temperature operation.

Solutes may be precipitated in a frozen solution when concentrated in the FCS. The solubility of pNP in 890 mM aqueous NaCl at −3.0 °C was determined to be 39 mM, suggesting that pNP is

precipitated when its concentration in the FCS exceeds this solubility limit. This situation is, for example, encountered when 1 mM aqueous pNP is frozen with α > 40. Effects of solute precipitation on the extraction ratio in freeze extraction were studied under the condition, where the pNP

concentration in the FCS exceeds 39 mM. In the experiments, the pNP concentration in the original solution was varied, but α was kept constant at 20. Results are summarized in Figure S2. pNP is dissolved in the FCS in the low concentration range (two plots in the figure), whereas it should be

precipitated at high concentrations. Interestingly, the pNP concentration in the organic phase linearly

increases with increasing the initial concentration of pNP before freezing, suggesting that pNP was

distributed in the organic phase even though it is precipitated in the FCS. Solidified pNP is expelled

from ice crystals similar to the FCS and deposited on the surface of ice particles or in the FCS. When

the ice particles are in contact with the organic phase, solidified pNP is readily dissolved in the

organic phase. Thus, the precipitation of a solute by freezing does not cause problems in the present

scheme. In the present case, since extraction is performed at −3.0 °C, the limited solute solubility in an

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organic solvent can be a practical problem. The temperature dependence of pNP solubility in the

organic phase was studied. The solubility in 1:1 hexane/DEC is 118 mM at -25 °C and 55 mM at -3.0 °C. The loading capacity at −3.0 °C is almost half of that at room temperature. We should

design and handle freeze extraction systems taking the solubility limit well into consideration.

Since the partition coefficient is a function of temperature, the separation of two solutes is affected by temperature. According to the literature,29 Kd values for pNP, p-toluene, and p-hydroxymethylbenzoate in the n-octanol/water system are 122.9, 125.4, and 101.3 at 20 °C and

216.2, 173.0, and 217.9, respectively. These values suggest that the order of the extractability is

altered by changing temperature, indicating that some separations can be better at lower

temperatures but others may become worse. In freeze solvent extraction, this should also be taken

into consideration.

Figure 4 shows the dependences of the partition ratios (Kd) of mNBA and neutral pNP on i NaCl . The Kd values were calculated from the data shown in Figures 2 and 3. The dissociation

constant of pNP in an 890 mM NaCl(aq) solution at −3.0 °C was measured by spectrometry as Ka = 10-8.34 (Figure S3). This Ka value was used to correct the distribution ratios to estimate Kd of the neutral species of pNP; in the liquid state, Kd was 0.76 for pNP and 0.96 for mNBA. Interestingly, Kd i is not constant, but decreases with decreasing NaCl for both pNP and mNBA. This decrease in Kd is i particularly obvious when NaCl ≤ 200 mM. Electrochemical studies of the FCS indicated that the

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dissolution of reduced species of viologen derivatives is enhanced by freezing aqueous NaCl i decreases.25 This may originate from the solutions, and the extent of dissolution increases as NaCl

perturbed ice/FCS interface, which facilitates the structuring of the water by hydrogen bonding. A

simulation study indicated that the dissolution of hydrophobic solutes is facilitated in highly structured water.30 The results shown in Figure 4 are consistent with enhanced dissolution in the FCS

of compounds that have inherently poor solubilities in water. This phenomenon is of scientific

interest but is undesirable from the viewpoint of solvent extraction, as the extraction ratio is lowered i as NaCl decreases. It should be noted that, despite this disadvantageous phenomenon, the measured

Eϕ=1 increases through freezing the aqueous solutions.

Extraction of metal ions as chelates

To demonstrate the efficiency of freeze-extraction, the extraction of metal ions, from frozen

solutions, into an organic phase containing an appropriate chelating agent was also studied. The widely used chelating agent 8-hydroxyquinoline (HQ) was employed for these studies.31 Extraction of the metal ions Fe2+, Ni2+, and, Zn2+ was studied using hexanol-DCE as the organic phase. The

extraction of metal ions as chelates strongly depends on the pH of the aqueous phase; therefore, an

appropriate buffer was added to the aqueous phase to keep the pH constant. Direct pH measurements

of the frozen solutions confirmed that freezing does not impact the pH. The concentration of the

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buffer was set at one hundredth of that of NaCl so that the aqueous phase followed the NaCl/water

phase diagram. In this extraction, the distribution ratio for the divalent metal ions (M2+) is given by Eq. (4).

M =

[HQ]$

HQ

[%& ]

(4)

where β is the overall formation constant of the metal chelate (MQ2), KdMQ2 is the partition ratio of MQ2, DHQ is the distribution ratio of HQ, and K2 is the acid dissociation constant of HQ into Q- in water. Thus, DM increases with decreasing [H+]W and increasing [HQ]O. Because a large excess of HQ was added in the organic phase, [HQ]O can be regarded as constant in most cases; exceptions to this assumption are discussed below. When the pH of the aqueous phase is constant and [HQ]O can be considered constant, DM will also be constant. Under such conditions, the results of the

freeze-extraction can be predicted similarly to those for organic molecules. When the metal ion

concentration is very high, FCS pH should decrease by the progress of extraction. Also, the

distribution of HQ to the FCS affects FCS pH. A buffer capacity should be appropriately adjusted

based on the concentrations of metal ions in the FCS and the HQ concentration in the organic phase.

The results of the extractions of these metal ions from frozen aqueous NaCl are summarized in

Figure 5. The solution pH values were chosen to provide an Eϕ=1 of 0.2 - 0.3 from the liquid solutions, because the effect of freeze extraction is clearly seen at this ratio. For this reason, pH values of 5.6, 4.2, and 2.31 were selected for the extraction of Zn2+, N12+, and Fe2+, respectively. For

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i decreases. The maximum Eϕ=1 all the metal ions, as with the organic solutes, Eϕ=1 increases as NaCl

values for Fe2+, Ni2+, and, Zn2+ are 2.9, 5.6, and 2.6 times larger than those for the liquid solutions, respectively. For Zn2+, the Eϕ=1 values in the frozen conditions almost agree with those predicted for the liquid condition, as shown by the curve in Figure 5. In contrast, the Eϕ=1 values for the frozen conditions for Fe2+ and Ni2+ are always higher than the predictions. The departures from the

predictions appear to increase as the aqueous solution becomes more acidic. The concentration of H2Q+ in the aqueous phase increases with decreasing pH. The dissociation constant of H2Q+ (K1) in an 890 mM NaCl solution at −3.0 °C was spectrometrically determined. Figure S4 shows the pH dependence of absorbance, which gives K1 = 10-5.45. At low pH, an increase in the H2Q+ concentration in the aqueous phase leads to a decrease in [HQ]O, as shown by Eqs. (5) and (6).

HQ = [%

[HQ]$

[HQ]$ HQ ini

)O HQini

where O

(5)

& ( ' ] [HQ] [' ]

=

HQ

HQ

(6)

is the initial concentration of HQ in the organic phase (1 mM). When the pH decreases,

[H2Q+] increases and DHQ decreases. This results in a decrease in the quotient of Eq. (6), i.e., [HQ]O decreases; thus, according to Eq. (4), DM also decreases. However, when the aqueous solution is frozen, the volume of the aqueous liquid phase decreases (i.e. in Eq. (6), α increases), suppressing the decrease in [HQ]O. Thus, freezing enhances the extraction of metal chelates from acidic aqueous

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solutions. The experimental and theoretical Eϕ=1 values were corrected to account for this effect, as shown by the blue symbols and curves in Figure 5. Although this correction increases the Eϕ=1 values—particularly for the extraction of Fe2+, which was performed at the lowest pH—the

experimental values are still higher than the predictions. Although the detailed mechanism behind

this discrepancy has not been elucidated, this improvement in extraction efficiency is beneficial for

the application of freezing-extraction.

Temperature affects the extraction equilibrium (thermodynamics) involving chelate formation, its partition, etc. For the extraction of the Cu2+-HQ chelate, the standard enthalpy change of the entire extraction equilibrium (∆H°) was reported as < -10 kJmol-1.32 When the temperature decreases

from 298 K to 270 K, the equilibrium constant increases by a factor of 1.5 for the reaction of ∆H° = -10 kJmol-1. Thus, from an equilibrium viewpoint, this temperature decrease is advantageous for

solvent extraction. The temperature decrease is unfavorable for the extraction of metal chelates from kinetic perspectives. For Ni2+-HQ chelate, the activation enthalpy was reported to be 23.9 kJmol-1,33

indicating that the reaction rate constant is lowered by a factor of 0.36 when temperature decreases

from 298 K to 270 K. However, the short diffusion path length in the present system compensates

for the kinetic disadvantage as discussed above.

The separation between metal ions by the present method is worse than usual solvent

extraction because the high extractability at lower pH makes mutual separation difficult. However,

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since the reversed extraction can be performed in a usual way, metal ions are separated in this step.

Freeze-extraction is a method that enables the efficient transfer of metal ions into an organic phase

of a smaller volume.

Conclusion

The present work has indicated that extraction from frozen aqueous phases allows a significant

reduction in the volume of organic solvent required, without impairing extraction efficiency. This

holds true for extraction not only of organic compounds, but also of metal chelates. Figure 6

summarizes the ratios of organic to aqueous phases required for extraction from a liquid aqueous

phase to attain the same performance as Eϕ=1 in freeze-extraction. Freeze-extraction allows substantial reductions in the volume of the organic phase in all cases. Thus, when the aqueous phase

is frozen, higher extraction ratios can be achieved using a smaller volume of organic solvent. Low

temperature extractions have the additional advantages of low risk of solute decomposition and

reduced volatilization of organic solvents. Because the present approach produces varying results, as

shown by the error bars in Figures 2, 3, and 5, this scheme is not suitable for quantification purposes;

however, high extractability with a small volume of organic solvent is advantageous for separation

purposes.

In the present paper, the concentration ratio, α, was controlled through the NaCl concentration.

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However, this control is possible with any other additives, such as HCl, HNO3, alcohols, and sugars. Thus, if the presence of a particular ion or molecule is inappropriate for extraction (e.g. Cl- acts as

interferent for metal chelate formation), we can select other main component containing no interferents. It should be noted that α and the concentration of the main component in the FCS cannot be independently controlled in any case. Also, a sample may contain unknown matrixes.

Although the concentration ratio cannot be controlled if the matrix concentration is very high,

solutes can be concentrated in the FCS by freezing. Thus, the principle of freeze extraction is

applicable to such cases, and the existence of the matrix does not cause a practical problem though the quantitative discussion is difficult because α is unknown. In industrial processes, continuous liquid-liquid extraction is utilized for efficient solute

partition to an organic phase. Frozen aqueous phase cannot be employed in continuous flow extraction. Instead, ice particles packed in a column can be used for a chromatographic process.18, 34

This operation should allow us to design efficient industrial extraction processes. However, we

should pay attention to ice sintering, which may hinder efficient extraction. In this paper, we used ice

particles prepared in liquid nitrogen to quantitatively demonstrate the efficiency of freeze extraction.

For practical purposes, simpler protocols than used herein are available, e.g., freezing two-phase

mixtures while mixing or shaking. In such cases, incorporation of an appropriate surfactant assists

downsizing of the mixed phases and enhances extractability. Hence, various modifications of freeze

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extraction are possible.

Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science.

Supporting Information Estimation of a ratio of the diffusion coefficient at −3.0 °C to that at 25.0 °C, phase diagram of the NaCl/water system, freeze solvent extraction of pNP from concentrated aqueous solutions, pH dependence of absorbance of pNP at 400 nm in an 890 mM aqueous NaCl solution, and pH dependence of absorbance of 8-hydroxyquinoline at 360 nm in an 890 mM aqueous NaCl solution.

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Page 25 of 31

1

5 1 0.5

0.8

D = 0.1

0.6

Eϕ=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


0.4

0.2

0

0

20

40

60

80

100

α Figure 1. Extraction ratios predicted by Eq. (1) as a function of concentration ratio (α) for selected values of D.

25



1 0.9 0.8

fine ice particles

Eϕ=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

Page 26 of 31

0.7 0.6 0.5

coarse ice particles 0.4

0

200

400 i

c

NaCl

600

800

1000

/ mM

Figure 2. Relationship between Eϕ=1 and cNaCli for the freeze-extraction of pNP at -3.0 °C, using fine and coarse ice particles. The organic phase is 1:1 (V/V) hexane/DCE. The solid circles represent

extraction from the frozen aqueous phase; the open squares represent extraction from the organic

phase; the green curve shows theoretical values of Eϕ=1 for a liquid solution of 890 mM of NaCl at -3.0 °C. Coarse and fine ice particles have diameters of 1.6 mm – 2 mm and 75 µm –150 µm,

respectively.

26


Page 27 of 31

1 0.9 0.8

Eϕ=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


0.7 0.6 0.5 0.4 0

200

400 i

c

NaCl

600

800

1000

/ mM

Figure 3. Relationship between Eϕ=1 and cNaCli for the freeze-extraction of mNBA at -3.0 °C, using fine ice particles. The organic phase is 1:1 (V/V) hexane/DCE. The solid circles represent extraction

from the frozen aqueous phase; the open squares represent extraction from the organic phase; the

green curve shows the theoretical values of Eϕ=1 for a liquid solution of 890 mM NaCl at -3.0 °C.

27



1.2

mNBA 1

pNP

0.8

Kd

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

Page 28 of 31

0.6 0.4 0.2 0

0

200

400 i

c

NaCl

600

800

1000

/ mM

Figure 4. Variation of Kd values for pNP and mNBA with cNaCli, as determined from the extraction data shown in Figures 2 and 3.

28


Page 29 of 31 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


Figure 5. Relationship between Eϕ=1 and cNaCli for the freeze-extraction of Fe2+-, Ni2+-, and Zn2+-8-hydroxyqunoline (HQ) chelates. The aqueous phase pH values were 2.31, 4.16, and 5.57 for Fe2+, Ni2+, and Zn2+, respectively. The red circles and curves assume constant [HQ]o; the blue circles and curves use [HQ]o after correction for the partition of HQ into the FCS.

29



25

Ni2+ 20

Fe2+ W

15 VO/ V

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

10

pNP

5

0

mNBA 0

100 200 300 400 500 600 700 800 i

c

NaCl

/ mM

Figure 6. Phase ratios required for liquid extraction to achieve the extraction ratios obtained in the

freeze extraction. The extraction data were taken from Figures 2, 3, and 5.

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Page 30 of 31

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For Table of Contents Use Only

Synopsis Freezing of an aqueous phase allows us to reduce the consumption of organic solvents and to attain high extractability.

31


LiquidâLiquid Extraction from Frozen Aqueous ... - ACS Publications

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