Novel anion exchange membrane for concentration of lithium salt in hybrid capacitive deionization

Desalination 452 (2019) 279–289

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Novel anion exchange membrane for concentration of lithium salt in hybrid capacitive deionization

T

Anna Siekierka , Marek Bryjak ⁎

Wroclaw University of Science and Technology, Department of Polymer and Carbon Materials, Wyb. S. Wyspiańskiego 27, 50-370 Wroclaw, Poland

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Poly(vinylidene fluoride) Ethylenediamine Hybrid capacitive deionization

Novel anion exchange membranes that blocks the co-ions effect and improve the desorption step in hybrid capacitive deionization (HCDI) process have been presented. The membranes were prepared by chemical modification of poly(vinylidene fluoride) by ethylene diamine. To confirm ion exchange character and electrical properties of obtained membranes such methods as FTIR, SEM and EIS were applied. Transport phenomenon was described by Arrhenius and Eyring-Polanyi equations. On their bases, taking into account desorption efficiency, membrane modified for 24 h was selected for the further investigation. Its application to the HCDI system allowed to obtain desorption efficiency at 96% level with over 0.9 current efficiency. The salt adsorption capacity of the system reached over 30 mg per 1 g of electrode. For these reasons the evaluated membrane could be considered as perspective material for salt concentration with electro driven process.

Abbreviations: A, the active surface area (cm2); aLiCl, the salt activity (arb.uni.); C, concentration of HCl during DD process (mol/dm3); ca, the molar concentration of HCl (mmol/dm3); cb, the molar concentration of NaOH (mmol/dm3); Cf, the final salt concentration during HCDI process (mg/dm3); ci, the concentration of ion (g/ dm3); Cin, the initial salt concentration during HCDI process (mg/dm3); Co, the initial concentration of HCl during DD process (mol/dm3); Ea, the energy activation (cal/mol); F, faraday's number (F = 96485C/mol); fi, the active LiCl coefficient; h, the Planck constant (h = 6626·10−34 m2/kg·s); I, the ionic strength; J, the DD flux (mol/m2s); k, the rate mass transfer coefficient (m/s); kB, the Boltzmann constant (kB = 1,3806 · 10−23 J/K); L, thickness of membrane sample (cm); mads,des, the mass of adsorbed or desorbed salt during HCDI process (g); mak.el., the mass of both active electrodes material; md, the weight of the dry membrane (g); Q, the charge stored for charging step (C); R, the gas constant (R = 8,3144 J/K·mol); Rb, the bulk resistance (Ω); S, the effective area of DD process (cm2); T, temperature (K); t, time of adsorption during HCDI process (s); tDD, the time of DD process (s); V, the volume of circulating solution during HCDI process (dm3); va, the volume of HCl (dm3); vb, the volume of NaOH (dm3); VDD, the volume of compartment of DD process (cm3); WH2O, water uptake (gH2O/g); zi, the charge number of ion; ZIEC, ion exchange capacity (mmol/g); ZN, nitrogen content (mmol/dm3); ΔG‡, the Gibbs energy of activation (J/mol); ΔH‡, the enthalpy of activation (J/mol); ΔS‡, the entropy of activation (J/K·mol); η, desorption efficiency during HCDI process (%); κ, the transmission coefficient of the Eyring-Polanyi equation; λ, current efficiency (arb.uni.); σ, proton conductivity (S/cm) ⁎ Corresponding author. E-mail address: [email protected] (A. Siekierka). https://doi.org/10.1016/j.desal.2018.10.009 Received 10 July 2018; Received in revised form 17 September 2018; Accepted 3 October 2018 0011-9164/ © 2018 Elsevier B.V. All rights reserved.

Desalination 452 (2019) 279–289

A. Siekierka, M. Bryjak

1. Introduction

2.2. PVDF-EDA fabrication

The hybrid capacitive deionization is an emerging electromembrane assisted process for selective removal of ions from aqueous solutions [1–3]. The cell of HCDI is formed by one selective electrode and one counter electrode covered by anion or cation exchange membrane. Affinity towards particular element is ensured by using a selective electrode that can be made from specific adsorbents like lithium‑manganese‑titanium oxide (LMTO) [1–3], nickel hexacyanoferrate (NiHCF) [4], manganese oxide [5], sodium manganese oxide (NMO) [6,9], lithium manganese oxide (LMO) [7] or with activated carbon modified by inorganic oxides [8,10], silver [11] or conductive polymers [12]. As opposed to selective action of applied materials, wrapping of counter electrode by ion exchange membranes is needed to reduce the co-ions effect [13]. Ions adsorbed in the adsorption step are being removed from selective electrode and, in the same time, are being captured by the counter electrode. In consequence the desorption process is difficult to control. To prevent this undesirable phenomenon a counter electrode is wrapped with ion exchange membranes. Hence, the ion exchange membranes have the critical function on controlling the efficiency of ions recovery. For lithium capturing, the primary cell of HCDI is comprised of lithium selective adsorbent as the cathode and activated carbon electrode wrapped with anion-exchange membrane as the composite anode. Poly(vinylidene fluoride), PVDF, is well known as a highly versatile polymer with an excellent balance between its comprehensive properties and numerous applications. It has many advantages, such as low permittivity, wide frequency response, flexibility, low level of resistance lost, easy fabrication, biocompatibility and cost effectiveness [14]. PVDF is one of the most widely used fluoropolymers in such membrane technologies as microfiltration, ultrafiltration, membrane bioreactor, membrane distillation, water reclamation, gas separation, recovery of biofuels or ion exchange processes [15,16]. To apply PVDF as the functional membrane for electromembrane applications, like hybrid capacitive deionization, the polymer material has to be modified. Using some chemical reactions, it is possible to obtain a wide range of functional membranes containing chemical groups that enhance antifouling phenomenon and have pH-sensitivity or ion-selectivity character [16]. The main objective of this study was to obtain PVDF membrane with good mechanical properties, chemically stable and with high concentration of anion-groups onto its surface. It was done by reacting PVDF with ethylenediamine (EDA). According to the chemical structure of poly(vinylidene fluoride) the addition of EDA can cause two reactions: (1) direct cross-linking via intrachain dehydrofluorization [17] and (2) indirect cross-linking via intrachain dehydrofluorization followed by the Michael addition [23]. The details on reaction mechanisms are discussed in Result and discussion section. In this paper, we present preparation method of novel anion exchange membrane for hybrid capacitive deionization and concentration of lithium salts. The first objective of the work was to show synthetic paths of poly(vinylidene fluoride) modification that led to preparation of anion exchange membrane. The second objective was to test new membrane for extraction of lithium salt in the HCDI process.

An appropriate amount of PVDF was dissolved in DMF (96 h at room temperature). The films of 0.20 ± 0.05 mm thickness were cast from 15% PVDF/DMF solution onto a glass plate and dried overnight in a vacuum dryer. The obtained PVDF films were immersed in EDA (100% concentration, 20 mL of volume) and kept for 3, 8 and 24 h. In such way, the obtained anion-exchange membranes were encoded as PVDFEDA3, PVDF-EDA8 and PVDF-EDA24, respectively. After modification membranes were rinsed with DI water and ethanol, and kept in 40% aqueous solution of ethanol. 2.3. PVDF-EDA characterization 2.3.1. Scanning electron microscope The scanning electron microscope (SEM, Tescan Vega3 SB) was used to determine the morphology of obtained PVDF-EDA membranes. All samples were gold-coated with 7 nm thick layer. 2.3.2. Fourier transform infrared spectroscopy To identify presence of amine groups into polymer films Fourier transform infrared spectroscopy in the range of 4000–400 cm−1 (Vertex 70 vacuum spectrometer equipped with the horizontal ATR device) was used. For each analysis, 64 scans were collected. The FTIR spectra were recorded for dry membranes. 2.3.3. Conductivity measurements Proton conductivity was measured on membranes that were immersed in DI water for 24 h. The conductivity was determined by means of impedance spectroscopy using a Solatron SI 1260 workstation at AC amplitude of 500 mV. Oscillation frequencies were swept logarithmically from 10−1 Hz to 10−6 Hz. Samples were evaluated in twoprobe configuration [18]. The active area of membrane was 2.25 cm2. The EIS spectra were analysed using ZView 2 software. The high-frequency data of Nyquist plot corresponded to combination of bulk resistance and capacitance of the polymeric film-electrode system. Proton conductivity of the samples was calculated using the following equation [20]:

=

L Rb A

(1)

where σ is the proton conductivity (S/cm), L the thickness (cm) of the polymer films, A the active surface area between polymeric films and the Rb the bulk resistance (Ω) calculated from the Nyquist plot. 2.3.4. Surface wettability Contact angles of such probing liquids as water, formamide and diiodomethane were measured at 25 °C by means of goniometer PG-X (Fibro System AB). The results were given as the average of 10 independent measurements for each liquids. 2.3.5. The analytical section gH O 2.3.5.1. Water uptake. Water uptake WH2O [ g2 was determined from Eq. (2):

2. Experimental section

WH2 O =

2.1. Materials

(m w

md ) md

(2)

where mw is the weight of swollen membrane and md is the weight of dry membrane.

Poly(vinylidene fluorine) (PVDF) with molecular weight of 180,000 g/mol and ethylenediamine (EDA) were supplied by SigmaAldrich. N,N-dimethylformamide (DMF), chloric acid, lithium chloride, sodium hydroxide and ethanol (96%) were purchased from Avantor Performance Materials, Poland S.A. Deionized water (DI) was delivered from RO Water Purification Systems Millipore (14,4 MΩ/cm2).

2.3.5.2. Ion-exchange capacity. Ion-exchange capacity (ZIEC) was estimated by means of acid-base titration method [18]. Membrane sample was placed into an Erlenmeyer flask and 50 mL of 0.1 M NaOH solution was added. The membrane was kept in the solution for 24 h at room temperature. After that time, 10 mL of solution was taken and 280

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titrated with 0.1 M HCl solution. The ion-exchange capacity ZIEC was calculated from Eq. (3):

ZIEC =

(cb vb

ca va) md

electrodes, one covered with anion-exchange membrane and 200 μm thick spacer. As a cathode spinel lithium‑manganese‑titanium oxide (LMTO) with 5 wt% of TiO2 was mounted [33]. The characterization of LMTO can be found elsewhere [21]. As an anode activated carbon of YP-50F was used. The HCDI system was biased by DC power supply from Konrad KD3005D Digital-Control DC Power Supply and controlled by DC Electronic Load BK Precision 8601. All of experiments were conducted in constant current mode with different current density that ranged from 10 A/m2 to 30 A/m2. The charging and discharging steps are tested for the same values of current density but polarization of electrodes was changed for sorption and desorption. To monitor the conductivity of LiCl solution (20 mM LiCl/dm3) the CRC-501 multimeter was applied. Each tests were carried out for 10 min at 25 °C. To detect pH in stripping and feeding solutions the CRC-501 multimeter was used. The electrode and HCDI stack configuration are summarized in Table 2. Both electrodes were prepared by mixing 90 wt% of lithium adsorbent or activated carbon, with 10 wt% of PVC dissolved in DMF (3.5 wt% solution of polymer). Paste was ultrasonicated for 30 min at room temperature. Finally, slurry was cast on the graphite foil and electrode of 70–80 μm was formed by a casting knife. The evaporation of solvent was carried out by 24 h in vacuum dryer. The prepared electrodes were kept in DI water. Before measurement electrode were activated three times in charge-discharge process to remove any contaminants.

mmol g

(3)

where cb and ca are the molar concentrations of the NaOH and HCl, respectively, vb is the volume of NaOH taken for the titration, va is the volume of HCl used for the titration of the NaOH solution and md is the weight of the dry membrane. 2.3.5.3. Nitrogen content. Nitrogen content (ZN) was determined by Kjeldahl's method after mineralization of the sample (about 200 mg) in concentrated sulphuric acid with copper and potassium sulphates [19]. 2.3.5.4. Diffusion dialysis. The DD process was carried out in a twocompartment cell divided by flat membrane with active area at 4.91 cm2. Each of compartment was stirred by a magnetic bar rotating with 200 rpm. The feeding (0.1 M HCl) and stripping (DI water) solutions, 35 mL each, were placed on both sides of the membrane. The experiments were conducted for 30 min and flux, J (mol/m2s), was calculated according to Eq. (4):

J=

V dC S dt

(4)

where V is the volume of the compartment, S is the effective area of the membrane and t is the time of DD. The mass transport thought anion exchange membranes is expressed by Fick's first law and given by J = kΔC, where ΔC (mol/dm3) is the concentration gradient and k (m/s) is the rate mass transfer coefficient. Finally, after integration the following expression was used:

C ln = Co

S k t V

2.3.6.2. Process metrics. For all experiments, the metrics as salt adsorption capacity (SAC, mg/g) and average salt adsorption rate (ASAR, mg/g/s) were calculated:

SAC =

(Cin

(5)

where Co (mol/dm3) is initial concentration of HCl. C is equal to ΔC and is defined as ΔC = Co − Ct, where Ct is the concentration of salt in time.

ASAR =

Cf ) V (6)

mak . el SAC t

(7) 3

3

where, Cin (mg/dm ) and Cf (mg/dm ) are initial and final salt concentration, V (dm3) is the solution volume, and m (g) is the mass of both active electrodes material, and t (s) is time of adsorption. The operational parameter used for evaluation of the HCDI stack

2.3.6. Hybrid capacitive deionization 2.3.6.1. HCDI stack. To study recovery of lithium chloride, a laboratory electrodialyzed FT-ED-100-4 was used. The stack was composed of two

Fig. 1. SEM images and aggregate size distribution for investigated membrane in comparison with pristine PVDF. 281

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was current efficiency λ (arb.uni.) given by:

=

3.1.3. Analytical characterization 3.1.3.1. Chemical grafting. To characterize the aminated PVDF membranes their nitrogen content, ion-capacity and water uptake were investigated. In Fig. 4A one can see the change of nitrogen contents with time of modification. It was found that with progress of time of ethylenediamine action, the PVDF was extensively modified. Moreover, the presence of nitrogen into polymer matrix was linearly related to ion-exchange capacity (Fig. 4B) and nitrogen content exponentially affected water uptake reaching almost 30% for PVDFEDA24 (Fig. 4C).

mads, des

(

Q F

M

)

(8) −1

where F is the Faraday's number, F = 96,485 (C·mol ), Q is a charge stored for charging steps and it can be calculated by integrating the current and time of charging, mads,des is the mass of adsorption and desorption, respectively. 3. Result and discussion

3.1.3.2. Surface energetics. For characterization of membrane surface energetics the contact angle measurements for DI water, DIM and FA were performed. Based on results shown in Fig. 4F, the surface polarity, as well as base and acid components of polar contribution were calculated. Wu's protocol was used for calculation of dispersive and polar components [27] while the procedure proposed by van Oss, Chaudhury and Good [28,29] served for determination of acid and base components. The polarity was calculated as a share of polar component in total surface energy. The data in Fig. 4E show that when surface polarity reached 30% the flux stabilized. Hence, the flux was controlled by surface polarity to some extent. This phenomenon could be related to values of base and acid components shown in Fig. 4G and H, respectively. The base component was depended on nitrogen content in the PVDF matrix and increased from 1 mJ/m2 to 41.1 mJ/m2. In the same time, reduction of acid component, associated with growing contents of nitrogen, was observed. Hence, the balance between acidbase constituents played a general role in controlling of ion transportation through investigated membranes.

3.1. Membranes characterization 3.1.1. Membranes morphology The surface of PVDF-EDA membrane was determined by SEM images (Fig. 1). It can be noted that the morphology have been changed during amination procedure. The aggressive environment of ethylene diamine caused visible increase of pore number on surface. Surprisingly, PVDF-EDA24 membrane contained more smaller pores that pristine membrane. It could be suggested that PVDF films was destroyed by EDA and some aggregates were deposited on the surface. 3.1.2. Membrane chemistry Infrared spectra for pristine PVDF, PVDF-EDA3, PVDF-EDA8, PVDFEDA24 are shown in Fig. 2A. For modified PVDF, the adsorption bands could be assigned to secondary amine groups at 3025 cm−1 (Fig. 2B) while strong band at 1640 cm−1 to secondary amine (Fig. 2C). A shoulder at 1720 cm−1 was attributed to C]N stretching [22]. The CH2 bending mode expected to appear at 1450 cm−1 (Fig. 2A) was found at 1405 cm−1. Therefore, the exposition of PVDF films to EDA caused dehydrofluorination and crosslinking [22]. The strong bending vibration at 800 cm−1 was identified to C]C group and was specific for vinylidene compositions. Expected mechanism of reaction is shown in Fig. 3. During the reaction some unsaturated bonds were created and addition of ethylenediamine according to Michael route appeared immediately [23]. Hence, the reaction between PVDF and EDA resulted in alterations of surface character [22].

3.1.3.3. Transportation though membranes. To evaluate the transport phenomenon though membranes the diffusion dialysis process was applied. In the conducted tests the mass transport coefficient at 25 °C, 30 °C and 32 °C, and permeate flux were calculated. According to data presented in Fig. 4D the PVDF-EDA24 membrane was the most efficient for dialysis process. The PVDF-EDA24 was characterized by the highest nitrogen content that and its ion-exchange capacity (see Fig. 2B). Taking into account the flux of Cl− during the DD process, membrane PVDF-EDA24 was selected to the HCDI process. The relationship

Fig. 2. The wide (A) and narrow (B) (C) infrared scans. 282

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Fig. 3. Mechanism of nucleophilic addition ethylenediamine into poly(vinylidene fluoride) chain.

between ln(C/C0) and t is shown in Fig. 5A. Good linearity allowed to calculate mass transfer coefficient, k. The values of k at 25 °C were as follows 5.51 10−6, 2.99 10−6, 4.74 10−7 and 7.92 10−9 for PVDFEDA24, PVDF-EDA8, PVDF-EDA3 and pristine PVDF, respectively. Anion transport resulted of all mechanisms involved in passage of the solute through membrane. The rejection of Cl− by pristine PVDF was related to porous structure of these films and was not affected by presence of ionic functionalities. Hence, the energy activation for interaction between eNH3 and Cl− in modified membranes could give an insight to the transport mechanism of through the membrane. The energy activation was calculated from linearized form of Eq. (9):

where ΔG‡ is the Gibbs energy of activation, kB is Boltzmann constant, h is Planck's constant and κ is the transmission coefficient. The transmission coefficient is often assumed to be equal one as it reflects what fraction of the flux proceeds without recrossing the transition state. Hence, transmission coefficient equal to one means that the fundamental no-recrossing assumption of transition state theory holds perfectly. Now, the Eyring-Polanyi equation can be rewritten as:

k=

ln

For calculation of Gibbs energy and state functions of complex eNH3 and Cl− the Eyring-Polanyi equation was applied. The general form of the Eyring-Polanyi equation is show below [30]:

k=

kB T e h

G‡ RT

S‡ H‡ R e RT

(11)

and its linear form looks as follows:

(9)

Ea

k = Ae RT

kB T e h

k = T

H‡ 1 k + ln B + R T h

S‡ R

(12)

where ∆H‡ is the enthalpy of activation, ΔS‡ is the entropy of activation and T is the temperature in K. The results from linearization of Eqs. (9) and (12) are summarized in Table 3. At the beginning the activation energy for ion transport through

(10)

Fig. 4. Analytical characterization of modified PVDF films: effect of modification time on nitrogen content (A), contact angles of water, DIM and FA (F); effect of nitrogen content on ion-exchange capacity (B), water uptake (C), flux (D), base component (G) and acid component of surface tension (H); Effect of surface polarity on volumetric flux (E). Lines added to eye guide only. 283

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groups into the polymeric chains. When the effect caused by porosity was excluded and the membranes exhibited zero net-charge on their surface, the parameter controlling transport of Cl− ions was activation energy of complex NH3-Cl formation. Generally, the lower activation energy of complexation results in faster transportation of Cl−. Hence, the activation energy for PVDF-EDA24 enhanced the transport of chloride anions. What is shown in Fig. 5A. Eyring-Polanyi equation allowed to calculate ΔH‡, ΔS‡ and ΔG‡. The value of ΔS‡ provides clue on molecularity of the rate determining step, i.e. the reactants affinity. Positive values suggest that entropy increases upon achieving the transition state and indicate a dissociation mechanism of the complex. Negative values of ΔS‡ indicate an association mechanism in which two reactants form an activated complex. Together with entropy of activation, the Gibbs energy of activation could be considered as indicators of the process self-sufficiency. For modified membrane the requirements for self-sufficiency were met for ΔS‡ > 0 and ΔG‡ < 0. It meant that all modified membranes were able to transfer anions though their bodies. Different response was obtained for unmodified PVDF film. The requirements for spontaneity were unfulfilled and polymer matrix did not promote the anion complexation. The change of Gibbs energy of activation can be equal to the work the system can offer. Hence, the value of ΔG‡ should be equal to electrical work that can be collected. This fact is essential for application of PVDF-EDA membrane in electrical systems and can be helpful for energy storage.

Fig. 5. Linearization of experimental data of HCl transport (A), and plot of ln(k/ T) vs 1/T according to Eyring-Polanyi equation (B). Table 1 Characteristic of activated carbon YP-50F [21,34,35]. Sample

SBET [m2/g]

VT [cm3/g]

VDR [cm3/g]

VDR/VT

Lo [nm]

AC-YP-50F

1660 OeC (at%) 12

0.752 O]C (at%) 4.6

0.594 CeC, CeH (at%) 83

0.79 1.13 96.24 Precursor and conditions

Carbon (wt%)

3.1.3.4. Membrane conductivity. Proton conductivity of investigated membranes was evaluated in-plate and through-plane directions by means of impedance spectroscopy. The results are presented in Fig. 6. The in-plane conductivity of modified membranes was 0.024mS/cm for PVDF-EDA3 and it increased to 0.042 mS/cm for PVDF-EDA24. The increase of 73% could be attributed to presence of amine groups in the polymer matrix [24] as well as to material nanostructure [25,26]. Therefore, the roughness of modified membranes (see SEM image in Fig. 1) could affect the proton conductivity. The film through-plane conductivity was 1.45 · 10−5 mS/cm and 4.54 · 10−5 mS/cm for PVDFEDA3 and PVDF-EDA24 membranes and that relationship was consistent with water uptake caused mostly by presence of amine groups (see FTIR spectra in Fig. 2). That last relation is shown in Fig. 4C where water uptake increases exponentially with contents of nitrogen.

Coconut; NaOH activation

Table 2 General parameters of HCDI. General parameters of HCDI Acell Φv T Lch M Lel mel,wo mel,co tads tdes Lmem

Electrode geometric surface area (9 cm × 4 cm) Water flowrate Temperature Thickness flow channel (100% open) Number of electrodes calls Electrode thickness Mass of working electrode (LMTO) Mass of counter electrode (AC) Time of charging step during HCDI Time of discharging step during HCDI Anion-exchange membranes thickness

36 4 25 200 1 ~150 0.185 0.185 600 600 ~70–80

cm2 dm3/h °C μm Pair μm g g s s μm

3.2. Concentration of lithium ions in hybrid capacitive deionization 3.2.1. Selection of PVDF-EDA membrane To determine the best anion exchange membrane for concentration

Table 3 Enthalpy of activation, entropy od activation and Gibbs energy of activation for investigated anion exchange membranes determined by Eyring-Polanyi equation and energy activation by Arrhenius equation. Sample

ΔH‡ [J/mol]

ΔS‡ [J/K·mol]

ΔG‡ [J/mol]

Ea [cal/mol]

PVDF-EDA24 PVDF-EDA8 PVDF-EDA3 Pristine PVDF

263 297 299 400

1.60 1.64 1.42 1.27

−224 −202 −135 13

74 83 83 108

modified PVDF membranes was evaluated according to Eq. (9). The concentration of Cl− was measured at different temperatures in the range of 25–32 °C. The date was plotted as lnk versus 1/T to obtain the activation energy from the slope of the curves (Fig. 5B). The calculated activation energies for complex formation eNH3Cl in membranes were listed in Table 1. According to them, the minimum activation energy for passage of Cl− was obtained for PVDF-EDA24 and it was few times lower that for other investigated membranes. This observation is directly connected to a modification factor and incorporation the amino

Fig. 6. PVDF-EDA3, PVDF-EDA8, PVDF-EDA24 proton conductivity measured at 25 °C after immersion in water DI by 24 h. 284

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Fig. 7. The changing of relative concentration of lithium chloride during (A) adsorption and (B) desorption steps. The change of current efficiency during charging and discharging steps over the resistance of membrane (C). Desorption efficiency in relation to ion exchange capacity of membranes (D).

of lithium chloride, the HCDI process was carried out. The cell of HCDI was comprised of LMTO specific adsorbent and composite electrode made of activated carbon and covered by investigated membrane. All of experiments were performed under the same conditions (CLiCl = 20 mM, j = 10 A/m2, tads = tdes = 10 min, Vfeed = 0.1 dm3). The results are summarized in Fig. 7. The effectiveness of removal of lithium salt from feeding solution reached the maximum for PVDF-

EDA24 and PVDF-EDA8 membranes. The SAC values were maintained at 31.1, 32.2, 24.5 and 8.8 mg/g for PVDF-EDA24, PVDF-EDA8, PVDFEDA3 and pristine PVDF, respectively. By comparing these factors one can conclude that the modified membrane fostered complexation of amine and chloride species and through this effect higher ratio of chloride ions was exchanged on anode and lithium cations on LMTO cathode. For system with pristine PVDF the promoted reaction did not

Fig. 8. The pH fluctuation during the HCDI process for different current density (A), voltage changes vs time for different current density condition (B) and change of conductivity (C). Applied membrane: PVDF-EDA24. 285

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take place and sole adsorption on the surface was responsible for controlling the salt recovery. To verify suitability of investigated membranes in the HCDI system, the effect of ions transportation was evaluated. This phenomenon can be observed in Fig. 7A and B where we compered action of obtained membranes during adsorption and desorption steps. Based on desorption date the desorption efficiency η was calculated and related to ion exchange capacity (Fig. 7D). One more metrics used to characterize the HCDI process was current efficiency [31]. The metrics was calculated by dividing the mass of adsorbed salt per charges passed thought the system in both adsorption and desorption steps. Having a look at Fig. 7C one realizes that current efficiency depends on the membrane resistance. The values of resistance were as follows 2.76, 1.11 and 0.88 Ω/cm2 for PVDF-EDA3, PVDF-EDA8 and PVDF-EDA24, respectively. It was concluded that the most efficient membrane was PVDF-EDA24 that had the lowest resistance. Hence, for further investigation this membrane was selected. 3.2.2. Concentration of LiCl by HCDI To identify the system stability the pH and voltage fluctuations were evaluated. The pH fluctuation for various current density is shown in Fig. 8A. It is seen that pH was stable for the experiments conducted with 10 A/m2. The same conclusion can be withdrawn for voltage fluctuation (Fig. 8B). For low salt concentration it can be expected generation of OH− ions (2H2O + 2e− ↔ H2(g) + 2OH−). However reduction of carbon can compensate presence of OH− ions (C + H2O ↔ CO2(g) + 4H+ + 4e−). It was shown that for process running in capacitive mode (formation of EDLs in porous materials), the changes of pH are low and could be negligible [36–38]. The changes of conductivity for tested solutions are shown in Fig. 8C. It is evident that conductivity reached the same level for all of tested current densities. Hence, the HCDI process was not assisted by formation of OH− ions. The effect of applied current density as a driving force on exhaustive extraction of lithium chloride was investigated for current density of

Fig. 10. The repeatability of the HCDI process: (A) desorption efficiency, (B) current efficiency for desorption, (C) current efficiency for adsorption (D) SAC for desorption step (E), SAC for adsorption step for 20 cycles of 1st step of lithium extraction. Conditions: current density = 10 A/m2, tads = tdes = 10 min, Φ = 4 dm3/h, membrane: PVDF-EDA24.

Fig. 9. Extraction performance of HCDI hybrid with PVDF-EDA24membrane. Effect of current density on: SAC (A), desorption efficiency (B), current efficiency (C). LiCl concentration effect (D). Line are added to eye guide only. 286

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10, 20 and 30 A/m2 (see Fig. 9). All tests were conducted in three step cascade: 1st step - 100 cm3 of feeding solution and 25cm3 of stripping solution, 2nd step - 100 cm3 of feed after the 1st step and 25cm3 of strip after the 1st step, 3rd step - 100 cm3 of feed after 2nd step and 25 cm3 of strip after the 2nd step. In Fig. 9A the effect of current density on the SAC metrics is shown. When higher current density was applied the SAC values raised. Such phenomenon is well known and described by Porada et al. [13]. However, for 10 and 20 A/m2 density, salt adsorption capacity decreased for 2nd and 3rd steps. That could be caused by reduction of salt concentration and decrease of ionic strength. The ionic strength is expressed as:

I=

1 2

stripping solution was carried out. It meant we tried to extract maximum lithium chloride and concentrate it in the stripping solution. Current efficiency for desorption and adsorption steps are presented in Fig. 11A and B, respectively. After 6th step, the drop of current efficiency was observed. It was probably caused by significant decrease of salt concentration in the solution. The effect is seen in Fig. 11E. Despite the low adsorption value after 6th step the desorption efficiency achieved level above 0.8 for all steps. Hence, when not so large amount of LiCl was adsorbed, desorption was still completed. What was more, with significantly low SAC value after 6th step the concentration of LiCl in stripping solution increased (Fig. 11C). This phenomenon suggested that the desorption process was active even if the stripping solution had higher salt concentration than the feed. It could be rationalized that ‘pumping’ of the salt from electrodes to concentrated solution took place. The critical point of the conducted studies was to find desalination ratio. After 10th step the LiCl concentration in stripping solution was 72 mM. Comparing it to initial concentration, 20 mM, it can be said that HCDI with PVDF-EDA24 membrane allowed to concentrate lithium chloride with 3.5 factor. The final evaluation of the HCDI process by PVDF-EDA24 membrane showed significant improvement of the process in comparison to configuration without anion-exchange membrane. The modified Ragone plot was used to show the optimal conditions and to select the best HCDI system [32]. The modified Ragone plots for system with or without PVDF-EDA24 membrane as well as for the system with commercial AMX-408 Neosepta membrane are presented in Fig. 12A, B and C, respectively. It is seen that application of anion exchange membrane resulted in the SAC value of ~30–40 mg/g. Opposite situation are observed for system without membrane, where SAC for 1st step was ~30 mg/g. However for 2nd and 3rd cycle configuration lost its

n

ci z i2 i=1

(13)

and can affect LiCl activity coefficient according to

log fi =

0.5z i2 I

(14)

The above equations show that dilution causes a decrease of ionic strength and an increase of activity coefficient (fi). For 10 A/m2 current density, the ionic strength decreased after each step and reached 0.012, 0.011, 0.010, 0.009 values for initial concentration, and after 1st, 2nd and 3rd step, respectively. In the same sequence, the activity of lithium chloride (aLiCl) was 0.022, 0.020, 0.018 and 0.016. Hence, ionic strength had a direct impact on salt adsorption capacity causing decrease of process efficiency. Absolutely different behaviour was observed for process carried out with 30 A/m2. Here, the SAC had the same value for each of three steps and it was not dependent of ionic strength. In such situation, polarization of the electrode with higher current density allowed to trap the ions nearby electrode surface that resulted in higher SAC value. The effect of current density on desorption is shown in Fig. 9B. It can be noted that the highest η was observed for 10 A/m2 in the 1st step. This phenomenon can be explained by possible blocking of electrode volume. However, the most spectacular effect was observed for current efficiency (Fig. 9C). It is clear that the most effective use of current was observed for process conducted at 10 A/m2. This was caused by relative high SAC in comparison to 30 A/m2. Moreover, the current efficiency felt down with steps and took value of 0.92, 0.87, 0.83 for adsorption and 0.88, 0.82, 0.75 for desorption in 1st, 2nd and 3rd step, respectively. The current efficiency for process carried out at 10 A/m2 had three times higher value that for 20 A/m2 and 30 A/m2. The next evaluated factor was concentration of LiCl in stripping solution. The data are presented in Fig. 9D for each step and for each current density. It is seen that concentration of lithium chloride was almost the same for processes conducted at 10 and 20 A/m2. For 30 A/ m2 the concentration of LiCl was higher. As mentioned above it was possible to entrap lithium ions on the surface of electrode. The examination of current efficiency pointed at 10 A/m2 as the best current density for the HCDI process. One of the most important factors that determine the CDI process is the repeatability of the operation. To evaluate it, 20 repetitions of the 1st step process were compared. The results of desorption efficiency are shown in 10 A. The average value of η was kept at 91.5%. This fact clearly shows that PVDF-EDA24 membrane prevented co-ions re-adsorption [13]. Additionally, the current efficiency and salt adsorption capacity for 20 cycles were measured. As it can be seen the fluctuation of salt removal factor (Fig. 10D) had the same character as current efficiency for desorption (Fig. 10B). The same conclusion can be withdrawn for adsorption operation (Fig. 10C and E). On that base, one can conclude that either adsorption or desorption processes with PVDFEDA24 membrane were stable over time and their current efficiency reached the value of 0.85 and 0.80, respectively. The second studied feature of the HCDI system was to maximize extraction of lithium salt. For that purpose, the 10 steps cascade process with the reuse of feeding solution in the next step and collecting

Fig. 11. The exhaustive extraction of LiCl in the HCDI process: (A) current efficiency for desorption, (B) current efficiency for adsorption, (C) concentration of stripping solution occurring in desorption steps, (D) the desorption efficiency and (E) the SAC for 10 steps of HCDI process. Conditions: current density = 10 A/m2, tads = tdes = 10 min, Φ = 4 dm3/h, membrane: PVDFEDA24. 287

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A. Siekierka, M. Bryjak

Fig. 12. Modified Ragone plot for system with PVDF-EDA24 (A), without it (B) and with AMX-408 membrane (C). Comparison of desorption efficiency (D) and current efficiency (E).

capacity of 21% and 64%, respectively. Hence, application of anion exchange membrane allowed to released salt from the HCDI cell with higher efficiency. Application of PVDF-EDA24 membrane affected the desorption efficiency factor. In Fig. 11D we can see that the system with PVDFEDA24 membrane allowed to desorb 96%, 95% and 92% for 1st, 2nd and 3rd step, respectively. Based on these data it can be concluded that configuration with PVDF-EDA24 was characterized by high salt adsorption capacity and the rate of adsorption was repeatable. Moreover, the ability to adsorb and desorb high quantity of salt is manifested by current efficiency for adsorption and desorption (Fig. 12E). The odd behaviour for system with AMX-408 Neosepta was seen for the desorption step, where the current efficiency had ~0.3 which is also improved by the desorption efficiency (Fig. 12D). The following reasons advocate for use of PVDF-EDA membrane: i) to obtain higher desorption rate [13], ii) to get membrane for concentration of salts, and iii) to use the HCDI system in a cyclic mode. When PVDF-EDA24 membrane wrapped anode, it was possible to obtain the desorption efficiency of 96% with over 0.9 current efficiency. Hence, this membrane can be used in electro-driven membrane processes for desalination and for recovery of some salts (at least lithium chloride).

• •

Acknowledgement This work was financially supported by National Science Centre, Poland, Grant No. 2017/25/N/ST5/01330. References [1] A. Siekierka, M. Bryjak, Hybrid Capacitive Deionization With Anion-Exchange Membranes for Lithium Extraction, E3S Web of Conferences, 22 201700157, , , https://doi.org/10.1051/e3sconf/20172200157. [2] A. Siekierka, J. Wolska, M. Bryjak, W. Kujawski, Anion-exchange membranes in lithium extraction by means of capacitive deionization system, Desalin. Water Treat. 75 (2017) 331–341, https://doi.org/10.5004/dwt.2017.20431. [3] M. Bryjak, A. Siekierka, J. Kujawski, K. Smolińska-Kempisty, W. Kujawski, Capacitive deionization for selective extraction of lithium from aqueous solutions, J. Membr. Sep. Technol. 4 (2016) 110–116, https://doi.org/10.6000/1929-6037. 2015.04.03.2. [4] S. Porada, A. Shriastava, P. Bukowska, P.M. Biesheuvel, K.C. Smith, Nickel hexacyano-ferrate electrodes for continuous cation intercalation desalination of brackish water, Electrochim. Acta 255 (2017) 369–378, https://doi.org/10.1016/j. electacta.2017.09.137. [5] S. Hand, R.D. Cusick, Characterizing the impacts of deposition techniques on the performance of MnO2 cathodes for sodium electrosorption in hybrid capacitive deionization, Environ. Sci. Technol. 51 (20) (2017) 12027–12034, https://doi.org/ 10.1021/acs.est.7b03060. [6] S. Kim, H. Yoon, D. Shin, J. Lee, J. Yoon, Electrochemical selective ion separation in capacitive deionization with sodium manganese oxide, J. Colloid Interface Sci. 506 (2017) 644–648, https://doi.org/10.1016/j.jcis.2017.07.054. [7] D.-H. Lee, T. Ryu, J. Shin, J.C. Ryu, K.-S. Chung, Y.H. Kim, Selective lithium recovery from aqueous solution using a modified membrane capacitive deionization system, Hydrometallurgy 173 (2017) 283–288, https://doi.org/10.1016/j. hydromet.2017.09.005. [8] C.-S. Fan, S. Ya, H. Liou, C.-H. Hou, Capacitive deionization of arsenic-contaminated groundwater in single-pass mode, Chemosphere 184 (2017) 924–931, https://doi.org/10.1016/j.chemosphere.2017.06.068.

4. Conclusions

• The modification PVDF films by EDA runs according to Michael • • •

10 A/m2 current density when anode is wrapped with PVDF-EDA24 membrane. The PVDF-EDA24 membrane allows to run process with 96% desorption efficiency By application PVDF-EDA24 it is possible to concentrated lithium chloride with 3 times factor.

addition reaction and leads to preparation of excellent anion exchange membrane with high amount of amino groups that participate in complexation of chloride ions. The best chemical and electrical properties were obtained for PVDFEDA24 membrane. The PVDF-EDA24 membrane is capable to block the co-ions effect during desorption step in the HCDI process and allow to get over 30 mg/g salt of adsorption capacity. The most effective and current efficient HCDI process appears for

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