Osmotic pressure as driving force for recovering ionic liquids from aqueous solutions

Journal Pre-proof Osmotic pressure as driving force for recovering ionic liquids from aqueous solutions Chang Liu, Yun-Peng An, Jing Yang, Bian-Bian Guo, Hao-Hao Yu, Zhi-Kang Xu PII:

S0376-7388(19)32657-2

DOI:

https://doi.org/10.1016/j.memsci.2020.117835

Reference:

MEMSCI 117835

To appear in:

Journal of Membrane Science

Received Date: 25 August 2019 Revised Date:

9 January 2020

Accepted Date: 10 January 2020

Please cite this article as: C. Liu, Y.-P. An, J. Yang, B.-B. Guo, H.-H. Yu, Z.-K. Xu, Osmotic pressure as driving force for recovering ionic liquids from aqueous solutions, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117835. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Graphical Abstracts

Osmotic Pressure as Driving Force for Recovering Ionic Liquids from Aqueous Solutions Chang Liua,#, Yun-Peng An a,#, Jing Yanga,*, Bian-Bian Guoa, Hao-Hao Yua, Zhi-Kang Xu a,* a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and Key

Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, Hangzhou 310027, China.

[Prepared as an article for publication in Journal of Membrane Science]

Osmotic Pressure as Driving Force for Recovering Ionic Liquids from Aqueous Solutions Chang Liua,#, Yun-Peng An a,#, Jing Yanga,*, Bian-Bian Guoa, Hao-Hao Yua, Zhi-Kang Xu a,*

a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and Key Laboratory

of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, Hangzhou 310027, China.

#

These authors equally contributed to this work.

1

Abstract: An emerging challenge occurs for recovering ionic liquids (ILs) because of their exponential growth in academic and industrial applications. We report for the first time to reclaim water-soluble ILs from their aqueous solutions by osmotically driving water to spontaneously permeate through semi-permeable membranes. Theoretical simulation indicates a weak hydrability of ILs and a tendency to form micelles in water. We thus demonstrate a significant difference in osmotic pressures between the aqueous solutions of ILs and inorganic salts. Four categories of broadly representative ILs are continuously recovered from their diluent aqueous solutions (~1 wt%) with semi-permeable membranes. Eventually, a record-high concentration up to 58~78 wt% is achieved in this work. Our strategy integrates superiorities of low energy consumption, universal applicability toward ILs, diversified selection of semi-permeable membranes, unattenuated water flux and high rejection to ions, hopefully providing new insights into the authentic “green” industrial applications of ILs.

Keywords: forward osmosis; ionic liquids; hydration behaviors; osmotic pressure; resources recovery.

2

1. Introduction Ionic liquids (ILs) are organic salts existing in a liquid state at ambient temperature with negligible vapor pressure, non-flammability, high thermal stability, excellent solvation ability, and wide electrochemical window [1]. Owning to their distinctive properties, ILs have been found great academic and industrial importance in a variety of fields including chemical synthesis [2-4], material processing [5-8], electrochemistry procedure [9-13], energy storage [14,15] and separation technology [16-22] as “green” alternatives to traditional volatile organic solvents. The past decades have witnessed an increasing role of ILs in chemical and pharmaceutical industries. A growing number of commercialization cases have created a rising market size with a compound annual growth rate (CAGR) around 9.35% from 2016 to 2024 [23]. For example, ILs have been industrially used as scavenging acid agents in alkylphenylphosphines synthesis [24], dissolution media for cellulose processing [25-29], electroplating liquids for metal deposition [30,31] and dehydration chemicals for anhydrous ethanol production [32,33]. It is worth stressing that an authentic “green” industrial application of ILs aims to construct a benign and closed-loop cycle in which ILs are hopefully recycled through the process to the maximum extent. This is, however, frustrated by lack of highly efficient technologies for reclaiming ILs. Moreover, the recycling of ILs can effectively reduce harms to our ecological environment because the majority of ILs used are not fully eco-friendly [34]. In particular, the aforementioned large-scale industrial processes inevitably produce a large quantity of ILs/water mixtures. Even though the recyclability and purification of hydrophobic ILs have been studied for those biphasic systems by supercritical carbon dioxide extraction [35] and super-ILs-philic filter separation [36,37], a central dilemma remains to separate and recover those hydrophilic ILs from their aqueous solutions. To this end, distillation can be used to remove water from the aqueous 3

solutions of ILs but it is high energy-consuming, especially when the initial concentration

is

very

low.

This

method

also

suffers

from

the

possible

thermal-degradation of ILs at high temperature [38]. Besides, pressure-driven membrane processes, such as nanofiltration and reverse osmosis [39-43], were evaluated to recover ILs from aqueous solutions. They still suffer from low retention to ILs, limited concentration of ILs being able to enrich, and high requirement of indispensable external driving energy. Therefore, it is greatly required to develop a facile, universal and low energy-consuming strategy to achieve highly efficient separation and recovery of ILs from aqueous solutions. Herein, we report for the first time to separate a variety of IL/water solutions and then reclaim ILs by virtue of the distinct difference in osmotic pressures between their aqueous solutions of ILs and inorganic salts. This strategy, with high efficiency and potential large-scale operation, is realized by the spontaneous forward osmosis (FO) of water across a semi-permeable membrane but the interception of ILs. The water permeation is induced by the intrinsic chemical structure of ILs other than additional energy supply. ILs are commonly composed of volumetrically asymmetric cation/anion pairs in which the cation bears long alkyl chains with a large volume to disrupt the lattice packing [44]. These large cations usually exhibit low charge densities in comparison to those inorganic salts with small ions, causing relatively weak electrostatic interaction with surrounding water molecules [45]. Such unfavorable IL-water interactions thus enhance the water activity, resulting in a lower osmotic strength than that of inorganic salts at the same concentration in their aqueous solutions. The difference in the osmotic pressures can spontaneously drive water to permeate through a selectively semi-permeable membrane from the ILs side to the inorganic salts side while ILs are retained. As a result, the aqueous solutions of ILs are continually condensed to achieve 4

effective separation and recovery of ILs from their aqueous solutions (Figure 1). On the basis of this philosophy, four categories of broadly representative ILs with imidazolium, pyridinium, phosphonium and ammonium cations (Figure 2a) have been continuously recovered from their diluent aqueous solutions (c = 0.05 mol·kg-1, ~1 wt%) using both commercially available and self-prepared semi-permeable membranes with a thin film composite structure. To ensure a favorable water flux and high separation efficiency, the saturated solutions of inorganic salts (NaCl and MgCl2) were used as the draw media to maximize the difference in osmotic pressures across the semi-permeable membranes. Ultimately, ILs in the aqueous solutions are able to be concentrated up to 58~78 wt% that is far exceeding all the reported maximum degrees of concentration in comparison to competing technologies such as distillation, electrodialysis, nanofiltration and reverse osmosis. This work thus may open a new era of innovation for recovering ILs from aqueous solutions with high universality and large-scale applicability in ILs-involved industries.

Figure 1. Schematic representation of the FO process driven by an osmotic pressure difference for ILs recovery. 5

2. Materials and methods 2.1 Materials ILs,

including

1-butyl-3-methylimidazolium

chloride

([C4mim]Cl,

97%),

1-hexyl-3-methylimidazolium chloride ([C6mim]Cl, 98%), 1-octy-3-methylimidazolium chloride ([C8mim], 97%), 1-decyl-3-methylimidazolium chloride ([C10mim], 95%), tetrabutylammonium chloride ([Bu4N]Cl, 97%), tetrabutylphosphonium chloride ([Bu4P]Cl, 96%) and 1-butyl-4-methylpyridinium chloride ([C4mpy]Cl, 98%), were purchased from Aladdin Industrial (China). NaCl (99.5%) and MgCl2 (98%) were bought from Sinopharm Chemical Reagent (China). The reverse osmosis membrane (ROm, BW30-4040) and the forward osmosis membrane (FOm, Aquaporin InsideTM) were commercially provided by Dow Chemical Company (USA) and Aquaporin A/S (Denmark), respectively. Anodic aluminum oxide (AAO) substrate (60 µm thick, with a mean pore size of 200 nm and a diameter of 47 mm) was purchased from Whatman (UK). Two batches of poly(vinylidene difluoride) (PVDF) with different molecular weights were bought from Solvay Solexis (Belgium, Mn = 110,000 g·mol-1, Solef 6010) and Shanghai 3F New Materials (China, Mn = 500,000 g·mol-1, FR904), respectively. m-Phenylenediamine (MPD, 99%) and trimesoylchloride (TMC, 98%) were acquired from Sigma-Aldrich (USA). Other chemicals, including dimethyl sulfone (DMSO2, 99.97%), ethanol (99.7%), dopamine hydrochloride (98%), CuSO4·5H2O (99%), H2O2 (30%), hexane (97%) were commerially obtained from Dakang Chemicals (China) or Sinopharm Chemical Reagent (China) and used as received without further purification. Deionized water with a resistivity of 18.2 MΩ prepared by ELGA LabWater system (France) was used for all experiments.

6

2.2 Computational methods DFT calculations. Geometry optimizations of all molecules and ions were performed by density functional theory (DFT) at the M06-2X level of theory [46] with 6-311G(d) basis set [47], including solvation energy corrections and Grimme’s D3 (zero-damping) dispersion corrections [48] (see Supporting Information for details). MD simulations. The simulation box consists of a cube box with a length of 72 Å containing several ion pairs and water molecules. Fully flexible molecular all-atom models for ILs were based on DFT calculations. To determine the parameters of the atomistic models, the optimized OPLS-AA force field was used to accurately describe the properties of ionic liquids [49,50]. Four-site water model TIP4P/Ew [51] was used to describe water molecules in all MD simulations (see Supporting Information for details).

2.3 Determination of osmotic pressure To determine the osmotic pressures of the aqueous solutions of ILs and inorganic salts (from 0 to 1.0 mol·kg-1), the following procedures were used: 1) freezing point osmometer (OM 806M, Löser Messtechnik, Germany) was used to measure osmolalities of the aqueous solutions; 2) the osmotic pressures of these solutions were thus calculated by Equation 1:

π =Cosm RT

(1)

where Cosm is osmolalities measured by the osmometer mentioned above, R is molar gas constant of 8.314 J·K-1·mol-1 and T is the absolute temperature. Note that osmolalities beyond the upper limit of freezing point osmometer (2.5 osm/kg) cannot be determined using this method. To further investigate the osmotic pressures of the solutions at high concentrations, OLI Analyzer Studio software (Version 3.1 OLI System Inc.) was used to calculate the osmotic pressures of MgCl2 and NaCl solutions with a concentration from 0 7

to 6.0 mol·kg-1, and osmotic pressures of [C4mim]Cl, [C6mim]Cl, [C8mim]Cl solutions with a concentration from 0 to 6.0 mol·kg-1 were also calculated by Equation 2:

π =

where MA is the molar mass of solvent,

RTM A φvm 1000V A

(2)

is the partial molal volume of solvent, v is the

stoichiometric number of the solute, m is the molality of solute and ϕ is osmotic coefficient [52]. The values of ϕ were obtained by fitting with Archer’s extension of Pitzer ion interaction model (see Supporting Information) using the data in literatures [53-55].

2.4 Fabrication of semi-permeable membranes Interfacial polymerization of MPD with TMC was conducted onto different porous substrates for fabricating semi-permeable membranes with a thin film composite structure. The upper side of AAO substrates was immersed in 5 mL of 30 g·L-1 MPD aqueous solution for 5 min and then dried in air for 30 min. The MPD solution saturated substrates was then soaked in 5 mL of 1.5 g·L-1 hexane solution of TMC for 2 min to form a polyamide selective layer on the AAO substrate. The resulting membrane was denoted as AAOm. The membranes were cured at 80 oC for 8 min, and then stored in ultrapure water before use. PVDF substrates with vertically oriented pores were prepared according to our previous work [56-57]. They were then pre-treated by CuSO4/H2O2 triggered mussel-inspiration to achieve surface hydrophilization according to our previous work [58]. After drying, they were used as substrates to carry out the interfacial polymerization to prepare PVDFm using the same procedures and conditions as those used for AAOm.

8

2.5 Characterization of membranes The surface and cross-sectional morphologies of semi-permeable membranes were observed by a field-emission scanning electron microscope (FESEM, Hitachi S4800, Japan) with an accelerating voltage of 10 kV. Transmission electron microscopy (TEM, Hitachi H7650, Japan) was used to visualize the morphology and measure the thickness of the skin layer of the semi-permeable membranes (accelerating voltage = 120 kV). More than 50 points in each TEM image were taken and analyzed with the ImageJ software for calculating the average value of thickness. Zeta potential of the membrane surface was measured by an electrokinetic analyzer (SurPASS AntonPaar, Austria) with KCl (1 mmol·L-1) solution as the electrolyte solution, and the pH was tuned by adding NaOH solution.

2.6 Evaluation of the membrane performances A laboratory cross-flow unit (Figure S1) was used to determine the FO performances of the semi-permeable membranes with an effective area of 4.91 cm2. Ultrapure water and 2.0 mol·kg-1 of NaCl solution were used as the feed solution and the draw solution, respectively. Both the feed solution and the draw solution were circulated on the two sides of the membrane at a cross-flow rate of 800 mL·min-1 driven by peristaltic pumps (Cole-parmer MasterFlux L/S, USA). The temperature was kept at 30 °C. The water flux was measured from the weight change on the feed solution side, which was monitored in real time with an electronic balance. The reverse salt flux was obtained based on the conductivity change on the feed solution side, which was measured in real time with a conductivity meter (Rex DDSJ-308A, Shanghai INESA Scientific Instrument, China). Nanofiltration experiments were conducted to determine the structure parameters of the membranes. Membranes were placed in a lab-scale cross-flow flat membrane filtration 9

setup, and 200 ppm NaCl solution was used as the feed solution. Tests were carried out under 0.8 MPa at 25 °C. Water flux (Jw) and salt rejection (R) were calculated according to Equation 3 and Equation 4, respectively: Jw =

V S × ∆t

 C  R =  1 − p  × 100%  Cf 

(3)

(4)

where V is the volume of the permeate solution, Cp and Cf are the concentrations of permeate and feed solution determined by solution conductivity, respectively. The structure parameter (Sp) can be calculated by Equation 5 and Equation 6 as follows: D  A × π draw + B  ln   F  A × π feed + B + F 

(5)

(1 − R ) × A × ( ∆P − ∆π ) Jw ,B = ∆P R

(6)

SP =

A=

where D is salt diffusion coefficient which equals to 1.61×10-9 m2/s for 200 ppm NaCl aqueous solution [59], F is water flux during cross-flow FO experiment, πdraw and πfeed are osmotic pressures of the draw solution and the feed solution during forward osmosis process, ∆P is the hydraulic pressure in the nanofiltration experiment, Jw is water flux, R is salt rejection and ∆π is osmotic pressure difference between the feed solution and the permeate solution during the nanofiltration test.

2.7 Evaluation of the ILs recovery process The ILs recovery process was carried out using a laboratory-made dead-end device with an effective membrane area of 4.91 cm2 (Figure S2). The testing membrane was held in the device with the skin layer facing the feed solution. Thirty milliliters of ILs aqueous solution with a concentration of 0.05 mol·kg-1 was used as the feed solution and 10

70 mL of saturated MgCl2 or NaCl solution was used as the draw solution. MgCl2 or NaCl was added in time to keep the draw solution saturated. Both the feed solution and the draw solution were magnetically stirred at 800 rpm to relieve concentration polarization. Water flux (F) was calculated according to Equation 7:

F =

∆V S × ∆t

(7)

where ∆V is the volume decrement of the feed solution, S is the effective membrane area, ∆t is the testing time. The concentration of ILs solution at a given time, ci, was calculated by Equation 8:

ci =

c0 × l 0 li

(8)

where c0 is the initial concentration (0.05 mol·kg-1) of the feed solution, l0 is the initial height of the feed solution in the dead-end FO device, and li is the height of the feed solution at a given time. The self-diffusion coefficient of the saturated MgCl2 and NaCl solutions were given by OLI Analyzer Studio software (USA). The total organic carbon content of each solution sample was measured by a total organic carbon analyzer (TOC, GE Sievers InnovOx ES, USA), and the final mass fraction of ILs (Cfinal) in the solutions was calculated by Equation 9: C final =

1000M ILw TOC × 100% 12n

(9)

where MIL is the molecular weight of IL, wTOC is the total carbon content determined by the TOC analyzer and n is the number of carbon atoms in the molecule of IL. The IL flux (FIL) can be calculated by the following Equation 10:

FIL =

mdraw MILwTOC 12n × S × t

(10)

where mdraw is the weight of draw solution after concentration process, S is the effective 11

area of FO membrane and t is the time period for the concentration process. The concentration

of

Na+

or

Mg2+

in

draw

solutions

was

determined

by

a

flame atomic absorption analyzer (AAS, Agilent Technologies, 200 Series AA 240 FS, USA), and the reverse salt flux (Fsalt) of whole process was calculated by Equation 11:

Fsalt =

nmfeed w salt fmetal S × t

(11)

where n is the dilution factor, mfeed is the weight of feed solution after the concentration process, wsalt is the mass fraction of metal ions in the feed solution measured by AAS, f metal

is the mass fraction of metal ions in the inorganic salts.

3. Results and discussion 3.1 Osmotic pressures of the aqueous solutions of ILs and inorganic salts governed by their hydration behaviors. The osmotic pressure of an aqueous solution is strongly governed by the hydration behaviors of the solute. Imidazolium ILs bearing different lengths of alkyl chain, including [C4mim]Cl, [C6mim]Cl, [C8mim]Cl and [C10mim]Cl (Figure 2a), were used as model examples to investigate their hydration behaviors. Note that the hydration behavior of their common anion, i.e., Cl-, does not change substantially with the counter ion and the solution concentration [60,61]. Therefore, we analysed the hydration of the cation moieties of inorganic salts and ILs in their aqueous solutions by the MD simulation. The hydration state of the ions can be assessed by calculating the radial distribution functions (RDFs) of water oxygen and hydrogen atoms with respect to cations (Figure 2 and Figure S3). Figure 2b shows that the Mg-O/H RDFs exhibit two sharp peaks, suggesting a regular orientation of water molecules towards Mg2+. The peak in the Mg-O RDF appears at a position closer to Mg2+ than that in the Mg-H RDF, 12

indicating that oxygen atoms point towards Mg2+ and hydrogen atoms orient far away from Mg2+. Note that the other wide peaks appeared for both Mg-O and Mg-H RDFs are due to the second hydration layers of Mg2+ in water (Figure 2b). For [C8mim]+, the nitrogen atom bonded with methyl group in imidazole ring is referred as the central atom to calculate the N-O/H RDFs. It is noticeable to see broad peaks, suggesting a typical random distribution of the water molecules surrounding the ILs cations. Besides, the peaks in both N-O RDF and N-H RDF appear at the same position, indicating that water molecules have no definite orientation toward [C8mim]+ ions (Figure 2c). According to the quantificational MD simulation, there are 6 water molecules coordinated with Mg2+ (Figure 2b, inset), and their orientated structures destroy the hydrogen bonds among the water molecules near the Mg2+ ions. While water molecules surrounding imidazolium cations form a cage-like structure through hydrogen bonding (Figure 2c, inset).

13

Figure 2. a) The chemical structures of representative ILs used in this work. b) Radial distribution functions between Mg2+ and oxygen/hydrogen atoms of water molecules (Mg2+ is the reference ion). Inset: the hydration structure and the hydration free energy of Mg2+. c) Radial distribution functions between nitrogen atom bonded with methyl group in [C8mim]+ and oxygen/hydrogen atoms of water molecules (N is the reference atom). Inset: the hydration structure and the hydration free energy of [C8mim]+. This cage-like hydration structure for the organic cations of ILs in their aqueous solutions suggests relatively weak interaction strength between the cation moieties and the water molecules, which is also reflected by the corresponding hydration free energies obtained by DFT simulations. Mg2+ has the lowest hydration free energy and followed by Na+ among the studied cations (Table S1). The hydration free energies of the alkyl-imidazolium cations are, however, much higher than those of the inorganic cations 14

(Table S1). This phenomenon can be explained by the suppressed charge density due to the electron delocalization of imidazole ring and hydrophobic alkyl chains in the studied ILs. The hypothesis is confirmed by the stepwise decline of hydration free energy when increasing the carbon number from 4 to 10 in the alkyl chain. In contrast, the inorganic salt cations have higher charge density than the alkyl imidazolium ones and thus are more likely to coordinate with water molecules. Obviously, ILs with organic cations show weaker hydratability compared to inorganic salts. Such difference in hydratability revealed by MD simulations and DFT calculations is inevitable to result in a conspicuous difference in their osmotic coefficients (Figure 3a) and consequent osmotic pressures (Figure 3b, 3c). MgCl2 solution always has the highest osmotic pressure among all aqueous solutions at a given concentration, followed by NaCl solution. Distinctly, the osmotic pressures of [C4mim]Cl, [C6mim]Cl, [C8mim]Cl and [C10mim]Cl solutions are lower than those of the inorganic salts when their concentrations are equal. It is also found that the osmotic pressure of ILs aqueous solution decreases with increasing the alkyl chain length in their cation moieties, exhibiting a consistent variation tendency of the hydration free energy with the alkyl chain length mentioned above.

15

Figure 3. a) Osmotic coefficients, b) measured osmotic pressures and c) calculated osmotic pressures of MgCl2, NaCl, and imidazolium ILs aqueous solutions with different concentrations. d) Osmotic pressure differences between the inorganic salt solutions and the ILs solutions with different concentrations.

Besides, the osmotic pressure rises sharply with salt concentration in the aqueous solution, especially in the case of MgCl2. The aqueous solutions of ILs, instead, show a mild increase in the osmotic pressure with increasing the solute concentration. For [C8mim]Cl and [C10mim]Cl with a relatively long alkyl chain, there is almost no increment in osmotic pressure with progressively increasing the content of ILs in the aqueous solutions. This difference in the osmotic pressure variation with concentration for ILs and inorganic salts in their aqueous solutions can be explained by the self-aggregation behavior of ILs in water [62-65]. The self-aggregation is an exclusive characteristic of ILs compared to inorganic salts, which is also verified by MD 16

simulations (Figure S4). The inherently amphiphilic [Cnmim]+ cations are able to form nano-sized micelles in water due to the considerable intermolecular hydrophobic interactions among the alkyl chains as well as the cation/π interactions especially when the solution is concentrated [66,67]. Taking [C4mim]+ and [C8mim]+ cations as examples, the cation-water RDFs between the last C atom of the hydrophobic alkyl chains and the O atom of water molecules are plotted at various concentrations of ILs (Figure S5). The distance between the butyl chains of [C4mim]+ and the water molecules keeps stable with increasing the concentration of ILs, while the water molecules are gradually away from the ends of the alkyl chains of [C8mim]+ with concentration, indicating well-formed micelles in its aqueous solution at high concentration [68]. Such micellization behavior is attributed to the enhanced hydrophobic interactions among the alkyl chains, which in turn to repel water molecules and lead to the formation of hydrophobic cores in the aqueous solution. It is clearly to see that [C4mim]+ cations are homogeneously distributed in water regardless of the IL concentration from MD simulations. Nevertheless, a heterogeneous distribution of [C8mim]+ is observed and the formed micelles are gradually obvious with increasing the concentration (Figure S5). The conductivity variations with ILs concentration also confirm the self-aggregation behavior of ILs in their aqueous solutions (Figure S6). The critical micelle concentration (CMC), i.e., the concentration at which the conductivity undergoes a sharp decline, decreases with increasing the length of hydrophobic alkyl chain. It is because the intermolecular interactions are enhanced among these alkyl substitutes and thus make the studied ILs easier to self-aggregate in water at lower concentration. Therefore, the osmotic pressure evolutions with concentration for the solutions of ILs with long alkyl chains (e.g., [C8mim]Cl and [C10mim]Cl) can be divided in two domains: i) one up to CMC, during which an obvious increase in osmotic pressure is observed, ii) the followed section where 17

the osmotic pressure only slightly increases with concentration (Figure 3b, 3c). The further augment in the apparent concentration of ILs after CMC only leads to a growing number of micelles in water, rather than increasing the solute number in the true solution. It is encouraging that the osmotic pressure of the ILs aqueous solutions grow slightly or even almost stop growing at their CMCs, which is very significant for concentrating and recovering ILs from their aqueous solutions. As we known, the osmotically driven FO process requires an enough osmotic pressure difference between the feed solution (FS) and the draw solution (DS) to pump water flow continuously throughout the semi-permeable membrane. Normally, the driving force in sea water desalination or resource recovery cases by FO is gradually declining during the whole process because DS is progressively diluted and FS is simultaneously concentrated. At last, the osmotic pressures for DS and FS will achieve an equilibrium, and thus the FO process terminates at this point. To realize a continuous FO operation in pilot plants or industrial applications, considerable efforts have been devoted to uninterruptedly pump fresh feed solutions and install a secondary instrument for reconcentrating DS either by RO or evaporation. In this way, an on-going osmotic pressure gradient can be maintained at the cost of energy-intensive consumption and complicated equipment [69]. It is very inspiring that the osmotic pressure of FS in our cases, i.e., the aqueous solutions of ILs, will undergo a marginal increase during the concentrating process. This behavior guarantees the progressive ILs recovery with a high and unattenuated water flux as long as we maintain a constant concentration of DS (e.g., constantly replenishing fresh brines to ensure a saturated state of the DS). Figure 3d summarizes the differences in osmotic pressures for the aqueous solutions of ILs and inorganic salts at the same concentration. There is a remarkable osmotic pressure difference if the aqueous solutions of ILs and inorganic salts with the same concentration 18

are used as FS and DS, respectively. It means even if the aqueous solutions of ILs are concentrated to achieve the same concentration as DS, the FO process is still able to proceed continuously. For example, the osmotic pressure difference between the aqueous solutions of [C8mim]Cl and MgCl2 even reaches up to 120 MPa at a highly concentrated state (6 mol·kg-1). It is worth noting that the osmotic pressure difference between the aqueous solutions of [C4mim]Cl and NaCl at 6.0 mol·kg-1 shows the lowest (12 MPa) (Figure 3d). Even so, this driving force is equivalent to that in mostly reported sea water desalination cases by FO process in which 2.0 mol·kg-1 of NaCl aqueous solution is used as DS and pure water as FS [70]. This key discovery opens the door to apply FO process for the recovery of ILs from their aqueous solutions using inorganic salts as DS because the large difference in osmotic pressures provides a sufficient driving force to concentrate ILs even in a highly concentrated state.

3.2 Osmotically driven concentration and recovery of ILs from their aqueous solutions. Four categories of semi-permeable membranes (ROm, FOm, AAOm and PVDFm) with a thin film composite structure have been used to concentrate the aqueous solutions of ILs via the FO process. The substrates of these semi-permeable membranes are elaborately chosen to cover a variety of porous morphologies. ROm and FOm exhibit typical sponge-like and finger-like pores, respectively, while AAOm and PVDFm possess vertically oriented pores with a diameter of 200 nm and 3~4 µm, respectively (Figure 4). The pure water permeability of the semi-permeable membranes follows the order of PVDFm > AAOm > FOm >> ROm (Figure S7). This order is reversed with the rank of their structure parameters (Sp), i.e., PVDFm < AAOm < FOm << ROm (Figure S8). A high Sp indicates a severe internal concentration polarization (ICP) effect due to 19

the high tortuosity of pores, resulting in a depressed water permeability (e.g., ROm). Besides, all the semi-permeable membranes have a polyamide skin layer with a thickness of 150~190 nm (Figure 4) and a surface zeta potential of -10~-20 mV at pH = 6 (Figure S9). These charged, dense and uniform skin layers will guarantee a high rejection to ions for the separation.

Figure 4. Surface and cross-sectional morphologies of the semi-permeable membranes by SEM and TEM. Scale bar = 200 nm.

In the ILs concentrating processes, the dilute aqueous solutions (initial concentration = 0.05 mol·kg-1) of four representative ILs (Figure 2a) and the saturated aqueous solution of MgCl2 or NaCl were separated by the aforementioned semi-permeable 20

membranes, being FS and DS respectively. All the processes were performed to the maximum extent at which the water flux is hardly detected by measuring the liquid level decline. The water flux variation was in-situ monitored during the concentrating process (Figure 5 and Figure S10). It is very interesting that the water fluxes in all recovery processes keep relatively stable with time during the whole concentrating period. Their dewatering rates do not fall into an obvious decline, even at the advanced concentration stage of ILs. For instance, the water flux still exceeds 23 L·m-2·h-1 when the aqueous solution of [C8mim]Cl is concentrated up to 2.4 mol·kg-1 using PVDFm as semi-permeable membrane and MgCl2 solution as DS. This water flux is almost equivalent to that of the initial concentration state (24 L·m-2·h-1) (Figure S11). It is well known the water flux is dependent on the osmotic pressure difference as well as the ICP effect which is related to the Sp of the semi-permeable membrane. For a given semi-permeable membrane used for concentrating ILs solutions, the water flux is thus proportional to the osmotic pressure difference between FS and DS. As mentioned above, the osmotic pressure difference remains relatively constant during the whole concentrating process due to the self-aggregation of ILs, resulting in a continuous enrichment of ILs with a stable water flux.

21

Figure 5. Water flux over time during the ILs recovery processes conducted with a) ROm, b) FOm, c) AAOm and d) PVDFm, respectively. Saturated MgCl2 solution (5.86 mol/kg) was used as DS; the initial concentration of ILs aqueous solutions is 0.05 mol·kg-1; a period of 30 h and 2.5 h is monitored for ROm and the other three membranes, respectively.

Since the water flux is able to be remained almost constant for hours, the average water flux is used to illustrate the FO performances during the concentrating process. Figure 6 demonstrates that the average water flux shows a stepwise increase with an order of ROm < FOm < AAOm < PVDFm, which is in consistence with their pure water permeabilities mentioned above (Figure S7). The vertically oriented pores in the substrates of AAOm and PVDFm allow more facile water transportation than those finger-like pores and sponge-like ones in the substrates of FOm and ROm (Figure 4). In 22

particular, the sponge-like pores with a very high Sp (12700 µm) in ROm cause a severe ICP effect and thus reduce the average water flux down to around 2.2 L·m-2·h-1 (saturated solution of NaCl as DS). In contrast, the large pore size and the highly penetrative pore structure of PVDFm cause an extremely low Sp of 100 µm (Figure S8) and thus a high average water flux. More specifically, the water flux with the aid of saturated NaCl solution as DS is able to reach up to 17.5 L·m-2·h-1, and saturated MgCl2 solution is able to dramatically withdraw water with a high flux of 24.0 L·m-2·h-1 when using PVDFm for the ILs recovery process (Figure 6).

Figure 6. Average water flux of the ILs recovery processes conducted with different semi-permeable membranes using saturated NaCl solution (6.14 mol/kg) or MgCl2 solution (5.86 mol/kg) as DS. The initial concentration of ILs aqueous solutions is 0.05 mol·kg-1.

Furthermore, saturated MgCl2 solution exhibits a more efficient drawing ability toward water (i.e., with higher water flux) than saturated NaCl solution using FOm, AAOm and PVDFm as semi-permeable membranes (Figure 6). This is attributed to the much larger osmotic pressure difference between a given IL solution and saturated 23

MgCl2 solution compared to the case of using saturated NaCl solution (Figure 3d). However, it seems confusing that the water flux of 1.5 L·m-2·h-1 achieved by DS of MgCl2 is a little lower than that of 2.2 L·m-2·h-1 realized by NaCl one when ROm is applied. It is calculated that the self-diffusion coefficients of Mg2+ (1.18×10-10 m2·s-1) , Cl- (2.35×10-10 m2·s-1) and H2O (3.07×10-10 m2·s-1) in the saturated solution of MgCl2 are lower than Na+ (7.5×10-10 m2·s-1), Cl- (1.05×10-9 m2·s-1) and H2O (1.31×10-9 m2·s-1) in the case of NaCl. The relatively low diffusion capability of ions and water molecules in the case of saturated MgCl2 solution as DS could cause a severe ICP effect, especially for those membranes with a high Sp (e.g., ROm), resulting in a depressed water flux. In contrast, the ICP effect for those membranes with relatively low Sp (e.g., FOm, AAOm and PVDFm) is not significant. The difference in osmotic pressures between DS and FS thus governs the water transportation across those membranes. Moreover, it is worthy to note that the water flux is approximately equivalent for concentrating various ILs using the same semi-permeable membrane and DS regardless of the type of target ILs, even though their osmotic pressures differ from each other (Figure 3b, 3c). To explain this behavior, it is reminded that the osmotic pressures are extremely higher for saturated MgCl2 and NaCl solutions than those for the aqueous solutions of ILs. It is thus reasonable to neglect the effect of IL type on the driving force because the osmotic pressure difference between FS and DS is high enough to spontaneously drive the FO process for concentrating different ILs. This result demonstrates that recovering ILs via FO process from their aqueous solutions is generally applicable to various ILs. Moreover, the bidirectional diffusion of ions across the semi-permeable membranes is investigated during the concentrating processes. Both IL flux (Figure S12a) and reverse salt flux (Figure S12b) are very low when either saturated NaCl or MgCl2 solution is used as DS. The total retentions to inorganic salts (Table S2) reach up to 99.7~99.9% for all 24

membranes. The ultimate retentions to ILs (Table 1) are in the range of 95.2% to 98.4% for FOm, AAOm and PVDFm, while those for ROm are slightly lower (88.7%~96.4%). Obviously, FOm, AAOm and PVDFm exhibit impressive inhibiting effects on the bidirectional diffusion of ions regardless of the types of ILs and inorganic salts, which is favorable to prevent the loss and contamination of ILs during the recovery processes. Nevertheless, it should be noted that ROm has a high Sp, leading to a relatively low water flux during the concentration process. It thus needs much longer time to finish the whole process compared to other three membranes, i.e., 36 h for ROm while 6 h for FOm, AAOm and PVDFm. The longer concentration time easily results in heavier ILs loss, so the total IL rejections are relatively low with the ROm. Therefore, semi-permeable membranes with a high structure parameter and subsequent low water flux seems not beneficial to obtain a high retention to ILs.

Table 1. Total retention to ILs (%) of the recovery processes conducted with different semi-permeable membranes using saturated NaCl solution (6.14 mol/kg) or MgCl2 solution (5.86 mol/kg) as DS. ROm

FOm

AAOm

PVDFm

NaCl

MgCl2

NaCl

MgCl2

NaCl

MgCl2

NaCl

MgCl2

[C4mim]Cl

88.7

90.0

96.1

96.2

95.4

95.2

95.5

95.8

[C6mim]Cl

91.5

92.9

97.4

97.2

96.6

96.6

97.3

97.3

[C8mim]Cl

93.7

94.5

97.9

97.9

97.6

97.4

97.9

97.8

[C10mim]Cl

94.5

95.2

98.2

98.0

98.1

98.0

97.6

97.7

[Bu4N]Cl

95.7

96.4

98.4

98.4

98.3

98.2

97.8

97.8

[Bu4P]Cl

95.4

96.1

98.3

98.3

98.4

98.3

97.3

97.4

[C4py]Cl

90.2

91.7

96.9

96.8

96.7

96.7

95.4

95.9

25

Figure 7. The final mass fractions (wt%) of ILs in the concentrated solutions when the ILs recovery processes conducted with different semi-permeable membranes completely terminated. Saturated NaCl solution (left) and saturated MgCl2 solution (right) were used as DS, respectively.

As the concentration goes on, FS becomes thicker and stickier until a limit point arrives to terminate the FO process. At this moment, the final mass fractions of ILs in the aqueous solutions are determined and compared in Figure 7. It is very interesting that the final mass fractions are able to achieve approximately 58 wt% to 78 wt% for all FO processes, demonstrating a high capability of our strategy to recover ILs. Table 2 summarizes the maximum degrees of concentrating ILs from their aqueous solutions using different techniques. FO exhibits an unprecedented capability to concentrate water-soluble ILs up to a high level of concentration among reported membrane processes, including distillation [71,72], electrodialysis [73,74] as well as pressure-driven nanofiltration (10~21 wt%) and reverse osmosis (22 vol%) [39-43]. Notably, the factor of hydrophobic alkyl chains in ILs makes a positive contribution to improve the final mass fraction of ILs. It is clear to see the raised final mass fraction of ILs with increasing the carbon number from 4 to 10. This behavior is due to the enhanced driving force to draw more quantity of water from the aqueous solutions of those ILs with long alkyl 26

substituents. It is noteworthy that we elaborately select the most representative ILs with the smallest cations for each family in this work. It is reasonable to popularize our strategy to reclaim a huge number of ILs even with longer alkyl substituents in cations because large-sized ions can be well rejected by the semi-permeable membranes, and also generate the enhanced osmotic pressure difference to impel water transportation across the membranes. Furthermore, saturated brines from salt-making and desalination industries conveniently provide a plentiful source of draw solutions.

Table 2. Comparison of state-of-the-art separation techniques for recovering ILs from their aqueous solutions. Technique

Initial concentration of ILs Final concentration of ILs

Ref

Vacuum membrane 20 wt%

65.5 wt%

[71]

5 wt%

50 wt%

[72]

distillation Direct contact membrane distillation 0.30 mol·L-1 (~ 5.24 wt%) 0.45 mol·L-1 (~ 7.86 wt%) Electrodialysis

[73]

6.90 g·L-1 (0.69 wt%)

4.46 g·L-1 (~ 0.45 wt%)

[74]

5 wt%

18.85 wt%

[39]

0.5 wt%

10 wt%

[40]

1.30 ~ 5.18 wt%

19.55 ~ 21.34 wt%

[41]

Nanofiltration

187.33 g·L-1 (~ 18.37

-1

5.9 g·L (~ 0.59 wt%)

[42] wt%)

5 vol%

30 vol%

[43]

Reverse osmosis

5 vol%

22 vol%

[43]

Forward osmosis

0.05 mol·kg-1 (~1 wt%)

57.61 ~ 78.38 wt%

This work

3. Conclusions 27

In summary, we demonstrate an osmotically driven strategy for highly efficient recovery of ILs from their dilute aqueous solutions. In comparison with inorganic salts, ILs possess relatively weak hydration strength with water and tend to form micelles with increasing the concentration. Benefiting from these unique hydration behaviors of ILs, a distinct and unattenuated difference in osmotic pressures between their aqueous solutions of ILs and inorganic salts can be easily achieved to spontaneously and continuously drive the FO process. The concentrating processes show high-performance recovery of ILs with an unprecedented high ultimate concentration up to 58~78 wt% initiated from a very diluent solution (~1 wt%), which outperforms all reported techniques including energy-greedy distillation and electrodialysis as well as pressure-driven nanofiltration and reverse osmosis. This proposed philosophy has been proved to be universal with respects to most of water-soluble ILs and commonly used semi-permeable membranes with a thin film composite structure. It is thus expected to boost the authentic “green” industrial application of ILs. ASSOCIATED CONTENT

Supporting

Information.

Detail

results

of

simulations,

characterization

and

performance of membranes during FO processing. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J. Yang), [email protected] (Z.-K. Xu)

Notes 28

The authors declare no competing financial interest.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No. 51803180 and 21534009), and the Open Foundation of Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Sci-Tech University (project number: YR2017002).

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35

36

Highlights

◊ The first time to separate ionic liquids/water solutions by forward osmosis process. ◊ Osmotic pressure is demonstrated for the aqueous solutions of ionic liquids. ◊ The aqueous solution of ionic liquids can be concentrated from 1 wt% to 78 wt%. ◊ The universality is evaluated in detail for recovering water soluble ionic liquids.

Author Statement The authors including Chang Liu, Yun-Peng An, Jing Yang, Bian-Bian Guo, Hao-Hao Yu and Zhi-Kang Xu declare no statements.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: