Framework for Enhanced Ion Flux

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Materials and Interfaces

Mussel-inspired Monovalent Selective Cation Exchange Membranes Containing Hydrophilic MIL53(Al) Framework for Enhanced Ion Flux Jian Li, Junyong Zhu, Shushan Yuan, Xin Li, Zhijuan Zhao, Yan Zhao, Yuxin Liu, Alexander Volodine, Jiansheng Li, Jiangnan Shen, and Bart Van der Bruggen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00695 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Industrial & Engineering Chemistry Research

Mussel-inspired Monovalent Selective Cation Exchange Membranes Containing Hydrophilic MIL53(Al) Framework for Enhanced Ion Flux Jian Li a, Junyong Zhu a, Shushan Yuan a, Xin Li a, Zhijuan Zhao a,b, Yan Zhao a, Yuxin Liu c, Alexander Volodine d, Jiansheng Li c, Jiangnan Shen e*, Bart Van der Bruggen a,f* a

Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

b

University of Chinese Academy of Sciences, Beijing 100049, China

c

School of Environmental and Biological Engineering, Nanjing University of Science and Technology,

Nanjing, 210094, China d

Laboratory of Solid State Physics and Magnetism, KU Leuven, Celestijnenlaan 200d - box 2414, 3001

Leuven, Belgium e

Center for Membrane Separation and Water Science & Technology, Zhejiang University of Technology,

Hangzhou, 310014, China f

Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680,

Pretoria 0001, South Africa

* Corresponding author. E-mail address: [email protected] (J. Shen) [email protected] (B. Van der Bruggen)

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Industrial & Engineering Chemistry Research 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: The surface properties and structure of a membrane play a significant role in the ion selectivity during electrodialysis. Recently, polydopamine (PDA) and its related nanomaterials have emerged as promising materials to develop composite membranes for higher separation requirements. Previous research has shown that the codeposition of PDA and polyethylenimine (PEI) triggered by Cu2+/H2O2 could facilitate the transport of H+ while rejecting multivalent ions though electrostatic effects. However, the enhanced H+ flux by acid-base pairs is not applicable in Na+/Mg2+ system. Here we report a facile method to construct monovalent selective membranes through rapid codeposition of PDA/PEI and Mil(53)-Al, followed by cross-linking with trimesoyl chloride (TMC). The positive -NH2 allows to reject multivalent cations, while porous Mil(53)-Al can accelerate the migration of Na+. The surface morphology and physicochemical properties of the as-prepared composite membranes were studied by SEM, AFM and XPS analyses, and the electro-chemical properties were evaluated by EIS and current-voltage curves. The results demonstrated that a robust skin layer was formed on the commercial cation exchange membrane substrate, endowing the ion exchange membrane with an increased separation performance for multivalent ions. The monovalent selectivity and ion flux can be tuned by changing the ratio of Mil(53)-Al during the co-deposition process. A mass ratio of 0.2-0.4% (w/v) for Mil(53)-Al is the optimum protocol, yielding a membrane with a permselectivity of about 0.3 and an ion flux of about 22.0 and 0.6 mol·cm-2·s-1 for Na+ and Mg2+, respectively. At this condition, the PDA-coated membrane maintains a high monovalent selectivity with enhanced Na+ and Mg2+ flux in single salt solutions. This one-pot method to prepare PDA based membrane provides a new direction to prepare monovalent selective ion exchange membranes.

Keywords: Electrodialysis; MIL53(Al); Interfacial polymerization; Desalination; Monovalent selectivity

1. Introduction Electrodialysis (ED), one of the most economic and advanced separation processes, enables the continuous separation and concentration of brine water. During ED, ions are


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selectively transported from one compartment to another through ion exchange membranes (IEMs) by a direct current. In this way, the depletion of ions eventually results in a dilute compartment, while the adjacent compartment accumulates the ions, thus forming the concentrate compartment. Currently, ED is deemed one of the most competitive technologies for desalination when the feed solution is less than 5 g/L. Compared to other technologies such as multistage flash (MSF) evaporation and reverse osmosis (RO), ED has intrinsic advantages including selective desalination and enhanced water recovery 1. Furthermore, the reduced chemicals usage and low energy consumption can effectively diminish the cost of production. To date, electrodialysis has been applied in various applications like bio-refinery effluents 2-3, rare earth elements recycling

4

organic acid recovery

5-6

, and brackish water/wastewater

treatment 7. Nevertheless, in some cases, precipitation caused by scaling compounds (Ca2+, Mg2+, SO42-, CO32-) inevitably occurs in the concentrated compartment, which gives rise to a deleterious effect on the desalination performance 8. For a standard ion exchange membrane, the presence of multivalent ions can suppress the migration of monovalent ions by occupying the ion exchange transfer sites of the membrane

9-10

. Therefore, developing IEMs with the

ability to separate multivalent ions is urgently required. Furthermore, IEMs with monovalent selectivity could potentially expand the application scope of ED to disciplines like metallurgy 11

, sodium chloride production 12 and reverse electrodialysis 13. In consideration of exploiting the differences in ion valances and hydrated ionic radii,

many approaches have been proposed to design selective cation exchange membranes (CEMs) capable of separating multivalent ions from a mixed solution. Deposition of a thin charged surface layer is considered a promising way to prepare monovalent ion exchange membranes because the charged skin surface can increase the repulsion forces towards multivalent ions. Abdu

et

al.

14

reported

a

layer-by-layer

assembly

of

polyethylenimine

(PEI)/poly(styrenesulfonate) (PSS) bilayers on commercial CEMs (Astom, Japan); the perm-selectivity that was obtained is comparable with commercial monovalent CEMs while simultaneously a lower energy consumption is maintained. Wang et al.

15

fabricated a

monovalent CEM by preparation of a photo-induced self-polymerized chitosan layer. An enhanced monovalent selectivity was obtained for both the H+/Zn2+ and the Na+/Mg2+ system during the ED process. The leakage of Zn2+ and Mg2+ were reduced by 27.4% and 62.4%, ACS Paragon Plus Environment


respectively. Li et al.

16

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covalently bonded polyquaternium-7 on commercial CEMs; the

ensuing membranes exhibited similar Ca2+ and Mg2+ leakage compared to commercial monovalent CEMs, while the membrane resistance was smaller. However, all these fabrication processes require multiple steps, which limits their wide application. Alternatively, increasing the compactness of the functional layer on the membrane surface could be an alternative way to hinder the migration of multivalent ions. Nevertheless, the increased electrical resistance and the reduced current efficiency are the dilemmas that need to be conquered. Therefore, it is of interest to discover novel materials and methodologies to prepare monovalent CEMs. Dopamine, a kind of bio-glue, can form strong bonds with various inorganic/organic surfaces 17. In addition, the catechol groups of polydopamine (PDA) could react with thiols and amine groups though Michael addition or Schiff base reactions, which could be used to anchor other functional materials onto the surface 18. Inspired by the adhesive nature and film-forming ability of PDA, numerous efforts have been made to prepare PDA based membranes for various purposes

19-20

. Nevertheless, the inevitable aggregation of PDA during the

self-polymerization process leads to an uneven coating layer. Polyethylenimine (PEI), an amorphous amino-containing polymer, is able to reduce the self-aggregation of PDA through covalent connections between amino and catechol groups 21. Moreover, binding of PEI could shift the surface charge to positive 22, which may give rise to modified membranes with the potential for monovalent selectivity. To obtain a promising monovalent selective ion exchange membrane, the monovalent selectively is not the only factor to be taken into consideration. The membrane resistance, current efficiency and energy consumption are the key parameters to evaluate an electrodialysis process. MIL-53(Al), a sub-branch of metal-organic frameworks (MOFs), contains 1D diamond-shaped channels with pores of nanometer dimensions

23

. The

mesoporous structure enables a high mass transfer efficiency, and the hydrophilic characteristics facilitate a uniform dispersion of MIL-53(Al) in aqueous solution

24

. These

superiorities open up the possibility of preparing monovalent selective CEMs with enhanced ion flux by incorporating MIL-53(Al). In this study, novel organic–inorganic thin film composite (TFC) monovalent CEMs were fabricated by a fast co-deposition of PDA/PEI composites with MIL-53(Al). MIL-53(Al) was adopted as mesoporous component to maintain ACS Paragon Plus Environment

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a high ion flux while the positively charged PDA/PEI coatings could ensure the rejection of multivalent ions.

2. Materials and methods

2.1 Materials Membranes used in the experiments were commercial anion exchange membranes (AEM-80045) and cation exchange membranes (CEM-80050) purchased from Fujifilm Manufacturing Europe B.V (The Netherlands). Dopamine hydrochloride and polyethylenimine (PEI, 600 Da) were obtained from Sigma-Aldrich (Diegem, Belgium). Copper sulfate (CuSO4, 99%), sodium chloride (NaCl, ≥99.5%), magnesium chloride (MgCl2 ≥99.8%), sodium sulfate (Na2SO4 ≥ 99%), hydrochloric acid (HCl, 1M), TMC ( ≥ 98%), tris(hydroxymethyl) aminomethane (Tris, 99.8%) and hexane (anhydrous, 95%) were all purchased from Sigma-Aldrich. Basolite® A100 (Mil(53)-Al) used in this experiment was obtained from BASF.

2.2 Membrane preparation The fabrication process of monovalent selective CEMs is schematically shown in Figure 1. A CEM was first soaked in deionized water at room temperature until the membrane was fully hydrolyzed. Afterwards, the membrane was fixed on a lab-made membrane holder that only allows one side of membrane to contact the water solution. PEI (120 mg) was dissolved in a mixed solution with CuSO4 (39.9 mg) in a Tris buffer solution (50 mM, pH = 8.5). Then different mass ratios of MIL (53)-Al (0 mg, 10 mg, 20 mg, 30 mg) were added to the above solution followed by sonication to achieve a uniform dispersion of nanoparticles. Finally, 60 mg of dopamine hydrochloride was dispersed to the mixture followed by the addition of H2O2 (0.1 mL). The fresh solution was transferred to the holder to contact with membrane for 2 h. Subsequently, the aqueous solution on the membrane surface was replaced by an equal volume of 0.1 wt % TMC/n-hexane solution to perform the polymerization reaction for 2 min. After the ACS Paragon Plus Environment


excess organic solution was drained, the membrane surface was rinsed with water and n-hexane to remove the remaining chemicals. Finally, the prepared membranes were air-dried for further use.

Figure 1. Schematic diagram of the co-deposition process for preparing monovalent selective CEMs

2.3 Characterization and performance assessment A Philips Scanning Electron Microscope XL30 FEG (SEM, the Netherlands) was used to characterize the changes in surface morphology of the resultant membranes. The dried membrane samples were sputtered with gold before the characterization. SEM imaging was carried out at 10 kV. The surface roughness properties of the monovalent selective CEMs were measured using a Dimension FastScan AFM device (Bruker, Germany) with scan areas of 1 µm × 1 µm with the tapping mode under ambient conditions. The elemental composition of the membrane surface was measured by X-ray photoelectron spectroscopy (XPS, PHI Quantera II). The surface wetting nature of the membranes was measured at room temperature by measuring water contact angles with a contact angle system (OCA20, Dataphysics Instruments, Germany). Contact angles were measured using a circle fitting method by the drop shape analysis software for at least 5 times. The membrane resistance was measured with a Solartron Electrochemical System by electrochemical impedance spectroscopy (EIS) over a frequency range from 1 kHz to 1 MHz at 1 M NaCl and MgCl2 solution, respectively. The water uptake was based on the water retention inside the membrane. To determine


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the water uptake, the membrane was weighted at wet and moisture status. Water uptake was calculated by Eq. (1): WU =

Wwet − Wdry Wdry

× 100% (1)

where Wwet and Wdry are the mass of wet and dry membranes, respectively. The ion exchange capacity (IEC) of the prepared cation exchange membranes was determined by a titration method. The membrane was kept in a 1 M HCl solution to saturate exchange with H+, then liberate the H+ ions in a 1 M NaCl solution for 24 h. The released H+ ions in solution were titrated by 0.01 M NaOH solution using phenolphthalein as indicator. The IEC was calculated from the following equation:

IEC =

nH + Wdry

(2)

where nH is the concentration of H+ ions in milli-equivalent determined by NaOH and +

Wdry is the dry weight of the membrane (g). Diffusion dialysis was conducted using 1 M NaCl solution and distilled water at the feed side and permeate side, respectively. The effective membrane area was 13.84 cm2. Both compartments were continuously stirred to minimize concentration polarization. The conductivity of the permeate side was recorded every 10 min to determine the concentration changes through diffusion. The selectivity of the membranes was evaluated in a batch model electrodialysis set-up. In order to understand the transfer of monovalent and divalent cations in salt solutions by electrodialysis, single salt solutions were used in electrodialysis to evaluate the flux of ions in the presence of a direct current. There were three streams in the ED stack: the diluate, concentrate and the electrode rising solution. Both the concentrate and diluate compartment was filled with 150 ml 2 g/L NaCl or MgCl2 solution. The electrode rinsing solution was 1 L 20 g/L Na2SO4 with a constant flow rate of 30 L/h. Each compartment had an active membrane area of 19.6 cm2 with an o-ring to prevent leakage during the testing. The ED experiment was carried out at a current voltage of 20 V. The permselectivity of the membrane was measured on the same apparatus. A solution with 0.05 M MgCl2 and 0.5 M NaCl was used as the diluate compartment, and a 0.5 M NaCl



was used as the concentrate compartment. The concentration of Na+ and Mg2+ in the dilute chamber was measured by cation chromatography after 60 min. The cation flux through the membranes was calculated from the concentration change with time (dCi/dt) in the concentrate compartment according to the following equation: J C ( mol

2

cm s

)=

V d Ci ( ) A dt

(3)

where V is the volume (cm3) of the concentrate solution, A is the effective surface area of membrane (cm2). The perm-selectivity of the target membranes was calculated as follows: 2+

PNMg = a+

tMg2+ × CNa+ 2t Na+ × CMg 2+

(4)

where t Mg 2+ and t Na + are the transport number of Mg2+ and Na+. CMg 2+ and C Na + are the concentrations of Mg2+ and Na+ in the diluate compartment. The cations transport numbers can be calculated according to the concentration change of cations in the concentrated chamber during 60 min treatment as follows:

t=

F × ∆C I ×t

(5)

where ∆C is the equivalent cations concentration change in the concentrated chamber; I is the applied current; F is the Faraday constant and t is the time for electrodialysis.

3. Results and discussion

3.1 Surface morphology and chemical structure of the membrane The surface morphology of PDA/PEI based thin film nanocomposite (TFN) monovalent selective CEMs with different contents of MIL (53)-Al (0, 10, 20 and 30 mg) are shown in Figure 2. It can be observed that co-deposition with a PDA/PEI skin layer has no significant change on the surface morphology. After the addition of MIL (53)-Al, small rougher dots can be observed, indicating an inhomogeneous decoration of MIL (53)-Al. With increasing Mil(53)-Al loading to 0.4% w/v, many nodules were found. The nodules on the membrane surface are attributed to the aggregation of Mil(53)-Al. Despite the aggregation, the coverage


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becomes higher for the membrane with higher Mil(53)-Al content. However, too much Mil(53)-Al will result in a serious aggregation, which is difficult to deposit on the substrate surface. As a consequence, defects on the membrane surface can be found as indicated by the red circle in Figure 2 (PDA-mil-30), while no obvious Mil(53)-Al loading can be found in the blue circle. Therefore, the fabrication of PDA/PEI modified membrane with suitable Mil(53)-Al nanoparticles incorporation is particular important.

Figure 2. SEM images of (M-0) unmodified ion exchange membrane and TFC monovalent selective membrane surfaces. Nanoparticle loadings for PDA-0, PDA-mil-10, PDA-mil-20 and PDA-mil-30 are 0.0%, 0.2%, 0.4% and 0.6% (w/v), respectively (The scale bar represents 2 µm)

During the modification process, a tight skin facial layer is formed on the membrane surface through interactions such as π-π interaction, hydrogen bonding interaction, and electrostatic interaction. According to the three-dimensional AFM surface topography pictures presented in Figure 3, the PDA/PEI decorated membrane has no obvious changes on the surface morphology, which is in agreement with the SEM results. The surface root mean square height (Rsq) was similar after Mil(53)-Al decoration, whereas the maxium height (Rsz) increased from 15.4 nm to 23.0 nm (Table 1). The membrane surface with nanoparticles decoration largely improve the Rsz, meantime the Rsq is stable due to the low loading degree. A higher Mil(53)-Al coverage with more nanoparticles incorporation causes a further increase of



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the surface roughness. When the amount of decorated Mil(53)-Al reached 0.6% (w/v), a distinct increase of roughness was detected. This is in accordance with the aggregation of nanoparticles found in the SEM images.

Figure 3. AFM topography of primary membrane and modified membranes (a. M-0; b. PDA-0; c. PDA-mil-10; d. PDA-mil-20; e. PDA-mil-30)

Table 1 AFM surface roughness parameters of the pristine and modified membranes Membrane type

M-0

PDA-0

PDA-mil-10

DA-mil-20

DA-mil-30

Root mean square height (Rsq)

1.81 nm

1.86 nm

1.87 nm

1.99 nm

3.77 nm

Maximum height (Rsz)

15.7 nm

15.4 nm

23.0 nm

24.6 nm

31.6 nm

Typical XPS spectra of M-0, PDA-0, PDA-mil-20 are shown in Figure 4. All the membranes possessed C 1s, N 1s and O 1s characteristic peaks. In comparison with M-0 membranes, the emission peak for N 1s is more intensive for the modified membrane, which is assigned to the increased nitrogen content of PEI. For the PDA-mil-20 membrane, although the Al and Cu concentrations are too low to obtain Al 2s, Al 2p, Cu 2p1/2 and Cu 2p3/2 peaks, the elemental concentration given by Table 2 confirms the presence of Mil(53)-Al and Cu. For the method of PDA/PEI co-deposition, the thickness of the modified function layer is generally around 100 nm 25-26, which means a limited anchor effect while the size of the nanoparticles is large enough. As indicated by SEM and AFM results, no obvious aggregates with large size


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were observed. As a consequence, Mil(53)-Al nanoparticles with large size failed to be fixed on the membrane surface, ultimately leading to a low Al concentration.

Figure 4. XPS spectra of M-0, PDA-0 and PDA-mil-20 membranes



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Table 2 Atomic concentrations of C, N, O and Al for M-0, PDA-0 and PDA-mil-20 membranes Membrane M-0 PDA-0 PDA-mil-20

Element atomic concentrations (%) C 73.5 69.0 74.3

N 4.0 6.2 4.8

O 19.7 23.2 19.6

Al 0.0 0.0 0.3

Cu 0 0.1 0.1

3.2 Contact angle, ion exchange capacity and water uptake Water contact angle, ion exchange capacity (IEC) and water uptake were used to explore the effects of Mil(53)-Al nanoparticles. The commercial cation exchange membranes showed an initial water contact angle of 31°, which indicates the high hydrophilic properties of the primary CEMs (Figure 5). After decoration by PDA/PEI composites, the membrane surface became more hydrophobic as water contact angles increased to 77°. The decreased hydrophilicity can be rationalized by the more hydrophilic nature of the primary CEMs and the depletion of amine groups in PEI to react with acryl chloride groups in TMC 27. The addition of hydrophilic Mil(53)-Al nanoparticles to TFC membranes greatly enhances the surface hydrophilicity. However, a slight increase of the contact angle was observed when the decorated Mil(53)-Al concentration reached 0.6% (w/v). An increased concentration of Mil(53)-Al allows the aggregation of nanoparticles with increased size, which in turn leads to a more hydrophobic surface

28-29

roughness are closely related

. It has been shown that the contact angle and the surface

30

. Raising the initial concentration of Mil(53)-Al during the

preparation process increased the surface roughness; this could be another factor that contributes to the increase of the contact angle. The ion exchange capacity is responsible for the ionic conductivity of the membranes, while the water uptake can affect the transport behavior of ions across the membrane. After surface modification, both the IEC and water uptake exhibited a slight increase from 1.34 mmol/g to 1.38 mmol/g and 33.44% to 37.6%, respectively. Since PDA/PEI composites are positive charged via a synergetic effect of -NH2, -OH and -COOH groups, the change of IEC is not as obvious as the water uptake. For the M-0 and SPES-0 membranes, the surface composition was uniform while the surface of SPES-mil-10, SPES-mil-20 and SPES-mil-30 was heterogeneous, the strong affinity of Mil(53)-Al for water would give rise to hydrophilic ACS Paragon Plus Environment

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regions on the membrane surface. These hydrophilic regions formed around the cluster of chains lead to absorption of water and attraction of protons. Furthermore, the established pores of Mil(53)-Al can accommodate water molecules due to their relatively large sizes. The prepared membrane with Mil(53)-Al showed enhanced hydrophilicity, and a more positive charge density, indicating a strong potential use for monovalent separations.

Figure 5. Contact angle, IEC and water uptake of PDA/PEI modified membranes with Mil(53)-Al at different loadings (a. contact angle, b. IEC and water uptake)

3.3 Diffusion dialysis Diffusion dialysis referring to the effects of structural parameters with different Mil(53)-Al content would be useful to understand the diffusional ion-transport process. The diffusion of ions from the concentrate chamber to the diluate chamber caused an increase of the conductivity in the concentrated cell (Figure 6). For the diffusion experiments of primary membrane M-0, after 1 h self-diffusion, the conductivity of the concentrated compartment changed from 15.8 µs/cm to 258.8 µs/cm. The pristine interfacial polymerization between PDA/PEI and TMC limited the diffusion process. As a result, the diffusion of NaCl for the PDA-0 membrane becomes slower, and thus the conductivity change was reduced. Theoretically, the hydrated radius of Na+ is around 3.0 Å Mil(53)-Al pores (8.5 Å)

31-32

, which is smaller than the

33

. Therefore, the presence of Mil(53)-Al on the TFC surface can

provide extra space to enhance the Na+ diffusion process. Furthermore, the improvement of



IEC and water uptake could form ionic transfer pathways on the functional layer of the membrane surface, and facilitate the transport of ions. However, the diffusion was mitigated when the concentration increased to 0.4% (w/v). At this condition, ionic pathways on the membrane surface are occupied by the increase of additive particles and narrowed as space limiting factors

34

. When the Mil(53)-Al incorporation reached 0.6% (w/v), the high

concentration of Mil(53)-Al particles tends to agglomerate to form larger particles, thus large filler clusters were formed and Na+ can be easily transported. Although the conductivity variation in the concentrate compartment was not obvious during the diffusion experiments, the performance of the membranes after modification changed significantly, which was confirmed by the following characterizations.

Figure 6. NaCl diffusion process on the surface of TFC membranes with different Mil(53)-Al loadings

3.4 Eectrochemical properties of membranes Electrochemical impedance spectroscopy (EIS) is an important tool to enlighten electrochemical phenomena related to membranes, allowing to quantify the resistance of the membrane matrix. EIS results obtained in both NaCl and MgCl2 solutions are shown in Figure 7. However, the phenomena are very different. For the experiments conducted in NaCl solution, the increased resistance indicates that the presence of the PDA/PEI layer atop the CEMs


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hinders the ionic transport. A reduction of the membrane resistance was observed after introduction of Mil(53)-Al, because porous structures greatly facilitate the Na+ mitigation. As a consequence, Na+ permeating through the CEMs becomes easier and the membrane resistance is reduced. An increase in electrical resistance arising from the increasing Mil(53)-Al loadings implies that a hybrid membrane containing proper inorganic materials can significantly improve the membrane performance, but an excessive proportion of the inorganic materials leads to a high resistance 35. Such a variability in conductivity is in good agreement with the previous results of diffusion dialysis, confirming that an excess of Mil(53)-Al disrupts the ion transfer pathways in this particular system 35. For the EIS results conducted in MgCl2 solutions, two arcs appeared on the impedance spectrum. Typically, the geometric arch at low frequencies is determined by the ionic migration while the high frequencies of the Nyquist plot are the sum of the membrane resistance and the solution resistance 36. The total Ohmic resistance of the membrane with different Mil(53)-Al loadings was in this order: PDA-mil-30 < PDA-0 < PDA-mil-10 < PDA-mil-20 < M-0. It is exceptional that the resistance of M-0 is higher than PDA-0. The smaller resistance in these circumstances may be related to the enhanced shielding effect of double layer compression between the positive PDA/PEI modified layer and the negative CEMs, which reduced the electrostatic repulsion between the modified layer and Mg2+ 37. In addition, the great afﬁnity of Mg2+ for the functional groups of the CEMs gives the membrane a high electro conductance

38

. The only difference of the geometric arch at high

frequencies is the diameter of the additional arch, which indicates the formed functional layer on membrane surface can restrict the ions transport 39. Insight into the second diffusional arc at low frequencies, despite no significant difference can be observed concerning the diameter of arch, the slight reduction trend for membrane after modification confirms the restriction of the ion migration through the diffusion layer, electrical double layer and the CEMs.



Figure 7. Mil(53)-Al effect on EIS of membrane for NaCl and MgCl2 solutions

I–V curves of membranes modified with different Mil(53)-Al ratio were further investigated to characterize the limiting current density of the membranes. Consistent with classical I-V curves, three regions were explored. The first region at low current density represents the “Ohmic” region, where the electrical voltage was dominated by Ohm's law. The next region is the current-limiting region, where the current density provides the current necessary to transfer all the available ions. Therefore, the diluate side concentration close to the membrane surface is near zero and concentration polarization appears. The last region is the over-limiting current region, where water splitting and electroconvection occurs. The I–V curves of membranes with different Mil(53)-Al loadings are summarized in Figure 8. For the M-0 membrane, the limiting current density did not clearly appear within measuring conditions, as can be seen in Fig. 8. A membrane with low resistance could facilitate the transfer of ions, therefore, concentration polarization can be effectively avoided. By this means, the increased limiting current density ensured the application of ED under higher current density and thus an increased efficiency

40

. After modification by PDA/PEI, an obvious plateau appears. This

behavior corresponds to the formation of the modified layer, which enhances the concentration polarization. Moreover, the enhanced surface hydrophobicity can be another factor contributing to the increase of the plateau length

13, 41

. For the Mil(53)-Al incorporated

membrane, no apparent plateau can be found, which confirms the promotion effect of Mil(53)-Al for ion migration.


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Figure 8. Current voltage of M-0, PDA-0 and PDA-mil-20 membranes

3.5 Electrodialysis experiments The desalination performance of the membranes was first investigated by electrodialysis using single salt solutions (NaCl system and MgCl2 system, respectively). In these experiments, a constant-voltage strategy was applied. When the desalination process continues, the conductivity of the diluate compartment was recorded as the increase of the desalination time. During the experiments, the volume was stable for the diluate and concentrate compartments, thus the water flow across the membrane can be neglected. The flux of Na+ and Mg2+ reduced significantly after the interfacial polymerization of PDA/PEI, nevertheless, no distinct effect on total desalination rate can be observed. Since the current density tends to decrease with the increasing resistance of the diluate compartment, the concentration variation in the diluate compartment would be expected to be minimized. To better understand this specific process, the concentrations of Na+ and Mg2+ at 30 min were used to calculate the ion flux due to the large variations of concentrations in the first 30 min. As shown in Figure 9, higher Na+ and Mg2+ fluxes were obtained after introducing the porous Mil(53)-Al nanoparticles. The steric hindrance effect becomes much more obvious with increasing the Mil(53)-Al content, thus the flux of Na+ and Mg2+ was reduced. However, the reduction of the flux of Mg2+ was not obvious while the flux of Na+ was significantly reduced, which suggests a more obvious effect of the Mil(53)-Al content on Na+ with smaller hydrated radius.



Figure 9. Conductivity change of diluate compartment for different membranes at a. NaCl, b.MgCl2 suystems and c. ions flux

For the separation of monovalent from divalent cations, a comparason of monovalent and multivalent fluxes is presented in Figure 10. Likewise, PDA/PEI modified membranes exhibited a much lower Mg2+ flux than the untreated membrane, demonstrating the improvement of the steric hindrance effect. In contrast, the flux of Na+ is comparatively higher to compensate for the reduction in the migration current and the permselectivity of the modified membrane decreased from 1.26 to 0.37. By incorporating Mil(53)-Al nanoparticles, the flux of Na+ is slightly enhanced, while the Mg2+ flux is sustained. Improving the selectivity is typically at the expense of the flux of monovalent ions; Mil(53)-Al nanoparticles incorporation can be an alternative way to solve this problem. A further increase of the Mil(53)-Al content has no obvious effect on the perm-selectivity; however, in this case, both the Na+ and Mg2+ permeance increased. This proves the desalination contribution from Mil(53)-Al, indicating a higher current efficiency. For PDA-mil-30, the transfer resistance was further decreased and the voids facilitated the Mg2+ transfer from the interior of the membrane to the solution. In this case, Mg2+ with higher electrostatic interaction occupied the ion exchange transfer sites, resulting in a lower Na+ flux and a higher permselectivity. Furthermore,


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aggregates resulted in an uneven surface, so that the difference in PDA/PEI polymer thickness also contributed to the lower selectivity.

Figure 10. The ion flux and permselectivity of Na+/Mg2+ system during ED

4. Conclusions In conclusion, novel monovalent selective ion exchange membranes were fabricated with the potential for large scale application. Moreover, the usage of PDA/PEI solution can be performed in an economic and environmentally friendly way 19. Because of the high cationic charge density of PEI, an ultrathin PDA/PEI selective layer was constructed to reject multivalent ions while Mil(53)-Al nanoparticles would serve as an porous additive to enhance the ion flux. In particular, the permselectivity is maintained at a high level, demonstrating an excellent monovalent selectivity. This study can provide new insights into utilizing mussel-inspired materials for creating ion channels for various promising applications.

References (1) Vaselbehagh, M.; Karkhanechi, H.; Mulyati, S.; Takagi, R.; Matsuyama, H. Improved antifouling of anion-exchange membrane by polydopamine coating in electrodialysis process.

Desalination 2014, 332 (1), 126-133.



(2) Luiz, A.; Spencer, E.; McClure, D. D.; Coster, H. G.; Barton, G. W.; Kavanagh, J. M. Membrane selection for the desalination of bio-refinery effluents using electrodialysis. Desalination 2018, 428, 1-11. (3) Luiz, A.; McClure, D. D.; Lim, K.; Leslie, G.; Coster, H. G.; Barton, G. W.; Kavanagh, J. M. Potential upgrading of bio-refinery streams by electrodialysis. Desalination 2017, 2017, 415, 20-28. (4) Sadyrbaeva, T. Z. Separation of cobalt (II) from nickel (II) by a hybrid liquid membrane–electrodialysis process using anion exchange carriers. Desalination 2015, 365, 167-175. (5) Eliseeva, T.; Shaposhnik, V.; Krisilova, E.; Bukhovets, A. Transport of basic amino acids through the ion-exchange membranes and their recovery by electrodialysis. Desalination 2009,

241 (1-3), 86-90. (6) Luo, G.; Pan, S.; Liu, J. Use of the electrodialysis process to concentrate a formic acid solution. Desalination 2002, 150 (3), 227-234. (7) Ghaffour, N.; Missimer, T. M.; Amy, G. L. Technical review and evaluation of the economics of water desalination: current and future challenges for better water supply sustainability.

Desalination 2013, 309, 197-207. (8) Asraf-Snir, M.; Gilron, J.; Oren, Y. Gypsum scaling of anion exchange membranes in electrodialysis. J. Membr. Sci. 2016, 520, 176-186. (9) Liu, H.; Ruan, H.; Zhao, Y.; Pan, J.; Sotto, A.; Gao, C.; Van der Bruggen, B.; Shen, J. A facile avenue to modify polyelectrolyte multilayers on anion exchange membranes to enhance monovalent selectivity and durability simultaneously. J. Membr. Sci. 2017, 543, 310-318. (10) Firdaous, L.; Malériat, J. P.; Schlumpf, J. P.; Quéméneur, F. Transfer of monovalent and divalent cations in salt solutions by electrodialysis. Sep. Sci. Technol. 2007, 42 (5), 931-948. (11) Reig, M.; Vecino, X.; Valderrama, C.; Gibert, O.; Cortina, J. Application of selectrodialysis for the removal of As from metallurgical process waters: recovery of Cu and Zn. Sep. Sci. Technol. 2017. 2017 (12) Zhang, W.; Miao, M.; Pan, J.; Sotto, A.; Shen, J.; Gao, C.; Van der Bruggen, B. Separation of divalent ions from seawater concentrate to enhance the purity of coarse salt by electrodialysis with monovalent-selective membranes. Desalination 2017, 411, 28-37. (13) Güler, E.; van Baak, W.; Saakes, M.; Nijmeijer, K. Monovalent-ion-selective membranes for reverse electrodialysis. J. Membr. Sci. 2014, 455, 254-270. (14) Abdu, S.; Martí-Calatayud, M.-C. s.; Wong, J. E.; García-Gabaldón, M.; Wessling, M. Layer-by-layer modification of cation exchange membranes controls ion selectivity and water splitting. ACS Appl. Mater. Inter. 2014, 6 (3), 1843-1854. (15) Wang, M.; Jia, Y.-x.; Yao, T.-t.; Wang, K.-k. The endowment of monovalent selectivity to cation exchange membrane by photo-induced covalent immobilization and self-crosslinking of chitosan. J. Membr. Sci. 2013, 442, 39-47. (16) Li, J.; Zhou, M.-l.; Lin, J.-y.; Ye, W.-y.; Xu, Y.-q.; Shen, J.-n.; Gao, C.-j.; Van der Bruggen, B. Mono-valent cation selective membranes for electrodialysis by introducing polyquaternium-7 in a commercial cation exchange membrane. J. Membr. Sci. 2015, 2015, 486, 89-96. (17) Neto, A. I.; Cibrão, A. C.; Correia, C. R.; Carvalho, R. R.; Luz, G. M.; Ferrer, G. G.; Botelho, G.; Picart, C.; Alves, N. M.; Mano, J. F. Nanostructured Polymeric Coatings Based on Chitosan and Dopamine‐Modified Hyaluronic Acid for Biomedical Applications. Small 2014, 10


Page 20 of 23

Page 21 of 23 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


(12), 2459-2469. (18) Chien, H.-W.; Kuo, W.-H.; Wang, M.-J.; Tsai, S.-W.; Tsai, W.-B. Tunable micropatterned substrates based on poly (dopamine) deposition via microcontact printing. Langmuir 2012, 28 (13), 5775-5782. (19) Yang, H.-C.; Liao, K.-J.; Huang, H.; Wu, Q.-Y.; Wan, L.-S.; Xu, Z.-K. Mussel-inspired modification of a polymer membrane for ultra-high water permeability and oil-in-water emulsion separation. J. Mater. Chem. A 2014, 2 (26), 10225-10230. (20) Zhu, J.; Uliana, A.; Wang, J.; Yuan, S.; Li, J.; Tian, M.; Simoens, K.; Volodin, A.; Lin, J.; Bernaerts, K. Elevated salt transport of antimicrobial loose nanofiltration membranes enabled by copper nanoparticles via fast bioinspired deposition. J. Mater. Chem. A 2016, 4 (34), 13211-13222. (21) Yang, H.-C.; Pi, J.-K.; Liao, K.-J.; Huang, H.; Wu, Q.-Y.; Huang, X.-J.; Xu, Z.-K. Silica-decorated polypropylene microfiltration membranes with a mussel-inspired intermediate layer for oil-in-water emulsion separation. ACS Appl. Mater. Inter. 2014, 6 (15), 12566-12572. (22) Wang, J.; Zhu, J.; Tsehaye, M. T.; Li, J.; Dong, G.; Yuan, S.; Li, X.; Zhang, Y.; Liu, J.; Van der Bruggen, B. High flux electroneutral loose nanofiltration membranes based on rapid deposition of polydopamine/polyethyleneimine. J. Mater. Chem. A 2017, 5 (28), 14847-14857. (23) Ramsahye, N. A.; Maurin, G.; Bourrelly, S.; Llewellyn, P. L.; Loiseau, T.; Serre, C.; Férey, G. On the breathing effect of a metal–organic framework upon CO 2 adsorption: monte carlo compared to microcalorimetry experiments. Chem. Commun. 2007, (31), 3261-3263. (24) Pashley, R.; Rzechowicz, M.; Pashley, L.; Francis, M. De-gassed water is a better cleaning agent. J. Phys. Chem. B 2005, 109 (3), 1231-1238. (25) Lv, Y.; Yang, H.-C.; Liang, H.-Q.; Wan, L.-S.; Xu, Z.-K. Novel nanofiltration membrane with ultrathin zirconia film as selective layer. J. Membr. Sci. 2016, 500, 265-271. (26) Lv, Y.; Yang, H.-C.; Liang, H.-Q.; Wan, L.-S.; Xu, Z.-K. Nanofiltration membranes via co-deposition of polydopamine/polyethylenimine followed by cross-linking. J. Membr. Sci. 2015,

476, 50-58. (27) Yang, Z.; Huang, X.; Wang, J.; Tang, C. Y. Novel polyethyleneimine/TMC-based nanofiltration membrane prepared on a polydopamine coated substrate. Front. Chem. Sci. Eng. 1-10. (28) Lind, M. L.; Ghosh, A. K.; Jawor, A.; Huang, X.; Hou, W.; Yang, Y.; Hoek, E. M. Influence of zeolite crystal size on zeolite-polyamide thin film nanocomposite membranes. Langmuir 2009,

25 (17), 10139-10145. (29) Sotto, A.; Boromand, A.; Zhang, R.; Luis, P.; Arsuaga, J. M.; Kim, J.; Van der Bruggen, B. Effect of nanoparticle aggregation at low concentrations of TiO 2 on the hydrophilicity, morphology, and fouling resistance of PES–TiO 2 membranes. J. Colloid Interface Sci. 2011,

363 (2), 540-550. (30) Meiron, T. S.; Marmur, A.; Saguy, I. S. Contact angle measurement on rough surfaces. J.

Colloid Interface Sci. 2004, 274 (2), 637-644. (31) Tansel, B.; Sager, J.; Rector, T.; Garland, J.; Strayer, R. F.; Levine, L.; Roberts, M.; Hummerick, M.; Bauer, J. Significance of hydrated radius and hydration shells on ionic permeability during nanofiltration in dead end and cross flow modes. Sep. Sci. Technol. 2006,

51 (1), 40-47. (32) Kang, K. C.; Linga, P.; Park, K.-n.; Choi, S.-J.; Lee, J. D. Seawater desalination by gas hydrate process and removal characteristics of dissolved ions (Na+, K+, Mg 2+, Ca 2+, B 3+,



Cl− , SO 4 2− ). Desalination 2014, 353, 84-90. (33) Yang, C.-X.; Ren, H.-B.; Yan, X.-P. Fluorescent metal–organic framework MIL-53 (Al) for highly selective and sensitive detection of Fe3+ in aqueous solution. Anal. Chem. 2013, 85 (15), 7441-7446. (34) Nemati, M.; Hosseini, S.; Bagheripour, E.; Madaeni, S. Electrodialysis Heterogeneous Anion Exchange Membranes Filled with TiO2 Nanoparticles: Membranes' Fabrication and Characterization.

J. Membr. Sci. Res. 2015, 1 (3), 135-140. (35) Mistry, M. K.; Choudhury, N. R.; Dutta, N. K.; Knott, R.; Shi, Z.; Holdcroft, S. Novel organic− inorganic hybrids with increased water retention for elevated temperature proton exchange membrane application. Chem. Mater. 2008, 20 (21), 6857-6870. (36) Zhao, Z.; Shi, S.; Cao, H.; Shan, B.; Sheng, Y. Property characterization and mechanism analysis on organic fouling of structurally different anion exchange membranes in electrodialysis. Desalination 2018, 428, 199-206. (37) Mo, H.; Tay, K. G.; Ng, H. Y. Fouling of reverse osmosis membrane by protein (BSA): effects of pH, calcium, magnesium, ionic strength and temperature. J. Membr. Sci. 2008, 315 (1), 28-35. (38) Rodzik, I. Selectivity of Transport of Sodium, Magnesium, and Calcium Ions through a Sulfo-Cationite Membrane in Mixtures of Solutions of Their Chlorides. Russ. J. Electrochem. 2005, 41 (8), 888-891. (39) Zhao, Z.; Shi, S.; Cao, H.; Li, Y. Electrochemical impedance spectroscopy and surface properties characterization of anion exchange membrane fouled by sodium dodecyl sulfate. J.

Membr. Sci. 2017, 530, 220-231. (40) Ge, L.; Wu, B.; Li, Q.; Wang, Y.; Yu, D.; Wu, L.; Pan, J.; Miao, J.; Xu, T. Electrodialysis with nanofiltration membrane (EDNF) for high-efficiency cations fractionation. J. Membr. Sci. 2016, 498, 192-200. (41) Balster, J.; Yildirim, M.; Stamatialis, D.; Ibanez, R.; Lammertink, R. G.; Jordan, V.; Wessling, M. Morphology and microtopology of cation-exchange polymers and the origin of the overlimiting current. J. Phys. Chem. B 2007, 111 (9), 2152-2165.


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TOC


Framework for Enhanced Ion Flux

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