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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 42862−42874

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Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl alcohol) as Specialized Microfibrous Li+ Adsorbents Lawrence A. Limjuco,†,∥,⊥ Grace M. Nisola,†,∥,⊥ Rey Eliseo C. Torrejos,†,∥,⊥ Jeong Woo Han,§ Ho Seong Song,§ Khino J. Parohinog,† Sangho Koo,‡ Seong-Poong Lee,*,† and Wook-Jin Chung*,† †

Department of Energy Science and Technology (DEST), Energy and Environment Fusion Technology Center (E2FTC) and Department of Chemistry, Myongji University, Myongji-ro 116, Cheoin-gu, Yongin City 17058, Gyeonggi Province, Republic of Korea § Department of Chemical Engineering, University of Seoul, Seoul 02504, South Korea ‡

S Supporting Information *

ABSTRACT: Crown ether (CE)-based Li+ adsorbent microfibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol cross-linking. The 14- to 16-membered CEs, with varied ring subunits and cavity dimensions, have two hydroxyl groups for covalent attachments to poly(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) cross-linking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in cross-linking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g−1 MF). GA cross-linking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE−GA−PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through Fourier transform infrared-attenuated total reflection, 13C cross-polarization magic angle spinning NMR, field emission scanning electron microscope, N2 adsorption/desorption, and universal testing machine. The MFs exhibited pseudo-second-order rate and Langmuir-type Li+ adsorption. At their saturated states, the MFs were able to use 90−99% CEs for 1:1 Li+ complexation, suggesting favorability of their microfibrous structures for CE accessibility to Li+. The MFs were highly Li+selective in seawater. Neopentyl-bearing CE was most effective in blocking larger monovalents Na+ and K+, whereas the dibenzo CE was best in discriminating divalents Mg2+ and Ca2+. Experimental selectivity trends concur with the reaction enthalpies from density functional theory calculations, confirming the influence of CE structures and cavity dimensions in their “size-match” Li+ selectivity. KEYWORDS: adsorption, cross-linking, crown ether, electrospinning, lithium recovery, microfiber, poly(vinyl alcohol) tions.4−12 These limitations motivated further research on other Li+ adsorbents. Crown ethers (CEs) are macrocyclic acid-stable14 ligands for “hard acid” cations (Mn+) like Li+.15 They exhibit Mn+ selectivity according to the so-called “size-match” relation imbued by their constrained cyclic forms.16 For a more exclusive uptake of the target Mn+, CE rings are rigidified17,18 or modified with blocking subunits19,20 to avoid the capture of unwanted Mn+ and alleviate the formation of high-order complexes.21 Lithium-selective CEs have competitive theoretical capacities of 17.7−23.1 mg g−1.22 The simplest Li+ capture systems have used neutral CEs dissolved in extractants,22 doped in supported ionic liquid phase,23 or blended in polymeric

1. INTRODUCTION Lithium (Li) is an “energy-critical element” whose shortage could significantly hamper the progress of emerging electronic and energy-related technologies.1,2 Currently, Li is being produced from ores, pegmatites, and brines.3 But surging Li demand has incited its recovery from other possible resources, such as seawater, bitterns, and industrial wastewater.3 There is no established technology for mining diluted lithium ions (Li+), but adsorption has been an attractive option.4 Majority of the reported Li+ adsorbents are inorganic lithium-ion sieves (LIS), such as λ-MnO2,4 H1.33Mn1.66O4,5 H1.6Mn1.6O4,6 HMn2O4,7 H2TiO3,8 and TiO2.9 Depending on their preparation, LIS capacities in diluted Li+ sources vary from 0.7 to 41 mg Li+ g−1. However, LIS exhibit slow internal Li+ diffusion due to their dense lattice structures.5,10−12 They are moderately stable in acidic environment, 13 and their fabrications require energy-intensive solid-state (SS) reac© 2017 American Chemical Society

Received: September 29, 2017 Accepted: November 22, 2017 Published: November 22, 2017 42862

DOI: 10.1021/acsami.7b14858 ACS Appl. Mater. Interfaces 2017, 9, 42862−42874

Research Article

ACS Applied Materials & Interfaces

Figure 1. Strategic preparation of CE/poly(vinyl alcohol) (PVA) MFs employing CE diols, electrospinning, and aerosol cross-linking.

microfibers.24 However, these unsecured immobilization techniques render the materials prone to CE loss during Li+ adsorption. With costly CEs, a more feasible strategy to produce regenerable Li+ adsorbents is to covalently link the CEs on solid supports. Improved studies have employed CEs modified with amine,25 vinyl, 26 allyloxy methyl, 27 carboxyl, 28,29 and hydroxyl (−OH)30−32 groups, which were polymerized,25,28,29 polymerimprinted on magnetite,27 silanized on silica,26,32 and tethered on multiwall carbon nanotube (MWCNT)31 or poly(glycidyl methacrylate).30 However, most of them have low CE loading or experienced substantial CE loss during fabrication.26−32 Furthermore, some were processed into resins, hydrogels, or films25,28−30 that have poor mass-transport properties, limited by internal diffusion.10,33 It is therefore important to develop a facile fabrication technique that would not only create covalent CE−matrix linkages but also minimize CE loss during fabrication and produce durable adsorbents with structures that promote Li+ adsorption. To achieve these, this study (Figure 1) demonstrates a very simple but highly effective method of preparing novel CE-based Li+ adsorbents through modified CE/matrix blending, electrospinning, and aerosol cross-linking. Among the modified CEs, dihydroxy CEs or CE diols were prepared due to their convenient synthesis routes.22 By melding the CE diols in a support with matching reactive −OH groups, CE−matrix linkage can be conveniently created through a common cross-linker. With its OH-rich backbone, poly(vinyl alcohol) (PVA) was the chosen matrix due to its excellent chemical property, mechanical stability, and processability.34,35 Meanwhile, glutaraldehyde (GA) was selected as a highly effective cross-linker that can render the PVA insoluble in

water34,36 and simultaneously create CE−GA−PVA linkages through acid-catalyzed acetalization.34−36 In effect, water-stable Li+ adsorbents with CEs covalently secured in PVA matrix are produced. Before GA cross-linking, the CE/PVAs were processed via electrospinning, a technique that can completely transform the blends into microfibers (MF) with near-zero wastage of the starting materials.37 Moreover, the narrow fiber diameters of MFs could significantly shorten the Li+ diffusion path10,38 and reduce the effect of internal diffusion.10,33,38 For the final step, aerosol cross-linking was carried out on the electrospun CE/ PVA as a new approach to rapidly deliver the acidified GA to the MFs while preventing CE leakage from the fibers. Unlike the conventional immersion method, aerosol cross-linking requires shorter reaction time, preserves the fibrous structure of the MFs, alleviates CE elution, and reduces the needed amount of GA. Aerosol cross-linking is still a widely unexplored technique, only recently employed in three-dimensional alginate scaffolds for bioengineering applications.39 Three CE-based Li+ MFs were fabricated containing different CE diols with unique dimensions and ring subunits. The MFs were thoroughly characterized to confirm the effectiveness of the preparation method. Adsorption/desorption and selectivity studies were carried out using simulated and seawater samples, respectively. Density functional theory (DFT) calculations were performed to elucidate the size-match Li+ selectivities of the CEs.

2. EXPERIMENTAL SECTION 2.1. Materials. All materials and reagents used in this study are listed in the Supporting Information (SI, p S-6). Purchased compounds were used without further purification. Real seawater samples were collected from Pyeongtaek, South Korea. 42863


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ACS Applied Materials & Interfaces 2.2. Synthesis and Characterization of CE Diols. Crown ether diols were synthesized using modified procedures from the literature,22,40−42 as detailed in Figure S1, SI. The synthesized CE diols were numerically denoted CE 1 (2HDB14C4), CE 2 (2HB2M15C4), and CE 3 (2HB16C4). Full compound names and characterizations by Fourier transform infrared (FTIR) spectroscopy (Varian Scimitar FTIR 2000) and 1H (400 MHz) and 13C (100 MHz) Fourier transform nuclear magnetic resonance (Varian, 400 MR FTNMR) are provided in Figures S2−S5, SI. 2.3. Electrospinning and Aerosol Cross-Linking of CE/PVA MFs. A CE/PVA dope is prepared by initially heating the CE diol in deionized (DI) water under constant agitation at 90 °C. After 1 h, PVA and nonionic surfactant polyoxyethylene octyl phenyl ether (Triton X-100, 0.35 wt %) were subsequently added to obtain 9 wt % PVA solution. The added amount of CE was fixed at 50 wt % (with respect to PVA amount), the highest loading, which produced consistent blend with PVA. The CE/PVA dope was stirred for another 9 h at 90 °C. Pure PVA dope was prepared similarly in DI water at 90 °C for 9 h. The dope is placed in a 12 mL syringe and then fed in the electrospinning apparatus (model: ESP200D/ESP100D, NanoNC Co., Ltd., South Korea) via a syringe pump (KD Scientific 750, South Korea). Electrospinning was carried out at 24−26 kV, and the fibers accumulated on a drum-roll-type collector (500 rpm) fixed 120 mm away from the nozzle. The details of electrospinning conditions are listed in Table S1, SI. Acid-catalyzed acetalization of MFs was performed via aerosol crosslinking using a 4 vol % glutaraldehyde solution in anhydrous acetone spiked with HCl (0.3 vol %). The collected MF (560 cm2) was divided into six parts, each fixed on a glass plate and then sprayed with 0.8 ± 0.03 mL of GA solution to sufficiently wet the fibers. The chemical cross-linking was completed at room temperature for 5 h. The MFs were vacuum-dried at 50 °C for 24 h to remove residual GA. The MF containing 2HDB14C4 was labeled as CE 1/PVA, 2HB2M15C4 as CE 2/PVA, and 2HB16C4 as CE 3/PVA. 2.4. CE/PVA MF Characterization. FTIR spectra of MFs were recorded on a VARIAN 2000 (Scimitar Series) using the standard attenuated total reflection method. The solid-state (SS) 13C NMR spectra with cross-polarization magic angle spinning (13C CP-MAS NMR, Zr rotor Øout = 4 mm, 12 kHz spinning rate) were acquired on a 400 MHz SS-NMR (9.4 T) spectrometer (AVANCE III HD, Bruker, Germany). The MFs were examined under a field emission scanning electron microscope (FE-SEM, II-EDS-EBSD, JEOL JSM-7000F, Japan). Fiber diameter histograms (n ≥ 100) were constructed from scanning electron microscope (SEM) images processed via ImageJ software. Nitrogen adsorption/desorption isotherms were measured at 77 K within relative pressure range (ppo−1) of 0.01−1.0 using Belsorpmini II (Bel Japan, Inc.). Specific surface areas (SAs) were acquired using the Brunauer−Emmett−Teller (BET) method, whereas the pore size distributions were calculated using the Cranston−Inkley model.43 True densities (ρtrue) were measured using liquid pycnometer, whereas the bulk densities (ρbulk) were determined through dimensional measurements (Mitutoyo IP65, Japan) and gravimetry (Ohaus Adventurer Balance, AR2140). Total porosities were estimated from % P = [1 − (ρbulk/ρtrue)] × 100.38 The mechanical properties of the MFs were evaluated via pull to break test using universal testing machine (LFPlus, Lloyd Instruments, Ametek Inc., U.K.) equipped with 1 kN load cell. Cut samples (20 mm × 50 mm) were preloaded with 0.02 kgf and analyzed at a cross-head speed of 50 mm min−1. 2.5. MF Adsorbent Stability Tests. The physical stability of cross-linked MFs was examined through elution tests by quantifying % PVA loss and % CE loss. For comparison, control samples were prepared by melding methylated CEs (mCEs) instead of CE diols with PVA. The mCEs are CE diols etherified with methyl iodide (CH3I). Without −OH groups, mCEs cannot cross-link with GA; hence, they are only physically confined in the cross-linked PVA matrix. The method for mCE synthesis is shown in Figure S6 (SI) with mCE characterization results in Figures S7−S10, SI. Dried MFs were carefully weighed before and after elution. The samples were agitated (200 rpm) in 50 mL of water (pH = 8) at 30 °C. After 48 h, the liquid samples were analyzed for PVA and CE concentrations, as described in

Section 2.8. Calculations for stability tests are detailed in SI (p S-18) with equations listed in Table S2, SI. Meanwhile, the swelling ratio (S) was calculated as S = (aw − a0)/a0 by comparing the areas (a = l × w) of dry (a0) and wet (aw) MFs.44 2.6. Adsorption Experiments. Adsorption experiments were performed at 30 °C. The MFs were prewashed in 0.5 M HCl and DI water until neutral pH to remove metal contaminants. The samples were vacuum-dried (50 °C, 24 h) and then weighed (mMF ≅ 30 mg). An adsorption run involves agitation of the MF in a V = 50 mL LiCl solution with known Li+ concentration (Co). The samples were agitated at 400 rpm to ensure minimal external mass-transport resistance.10 Liquid samples were collected either at time t or at equilibrium (t = 24 h) to determine their final Li+ concentrations, Ct or Ce, respectively (Section 2.8). Adsorption capacities qt (kinetics) or qe (equilibrium) were calculated using eq 1. Initial experiment was performed to observe the effect of pH on the adsorption behavior of the MFs (pH = 2−11, Co ≅ 7 mg Li+ L−1). Batch kinetics (pH = 8, Co ≅ 7 mg Li+ L−1, varied t) and adsorption isotherm (pH = 8, Co = 3−70 mg Li+ L−1) experiments were conducted to evaluate the Li+ adsorption performance of the MFs.24,27,36

C − Ce Co − C t × V (kinetics); qe = o × V (equilibrium) mMF mMF (1) The selectivity of the MFs toward Li+ was investigated using real Li+enriched seawater (actual pH = 8.32), which contains high levels of interfering cations (i.e., Na+, Mg2+, K+, and Ca2+). The MFs were evaluated through distribution coefficient (Kd) using eq 2, Li+ separation factors relative to other Mn+ (αLi+/Mn+) in eq 3, and concentration factor (CF) in eq 4. qt =

Kd =

(Co − Ce) ×V Ce × mMF

α Li+/Mn+ =

CF =

(2)

KdLi KdM

(3)

qe Ce

(4)

The recyclability of the MFs was tested through repeated adsorption/ desorption runs (Co ≅ 7 mg Li+ L−1, pH = 8). After adsorption, spent MFs were rinsed with DI water and regenerated using 50 mL of 0.5 M HCl to strip the Li+. After DI wash, the samples were gently pressed between Kimwipes and then used for the next cycle. After five cycles, the MFs were autopsied using FE-SEM. The stripping solutions were analyzed for eluted CEs using UV−vis spectrophotometry (Section 2.8). 2.7. Density Functional Theory (DFT) Calculations. The selectivity behavior of MFs toward Li+ was further elucidated via DFT. The Vienna ab initio simulation package was used for the planewave DFT calculations.45 The Perdew−Burke−Ernzerhof generalized gradient functional along with the projector-augmented wave method was used to describe the ionic cores.46,47 A plane-wave expansion with a cutoff of 650 eV was used with a 1 × 1 × 1 Monkhorst−Pack k-point sampling of the Brillouin zone. Total energy calculations were conducted using the residual minimization method for electronic relaxation, accelerated using Methfessel−Paxton Fermi-level smearing with a width of 0.2 eV. The geometries of the free CE and CE−Mn+ complexes were optimized in a 30 Å × 30 Å × 30 Å supercell. One or two electrons were removed from the atoms for the calculations of alkali and alkaline-earth-metal cations. 2.8. Analytical Methods. Metal-ion concentrations were quantified using inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500 series). Liquid samples were acid-digested (5 mL sample + 10 mL DI + 5 mL 60% HNO3) through microwave irradiation (MARS-5 CEM) and diluted before ICP-MS measurements. The pH values were recorded using a pH electrode (Z451 SI Analytics GmBH, Germany). Eluted PVA and CEs in stability test samples were determined using total organic carbon (TOC) analyzer 42864


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Figure 2. (a) Minimum-energy structures of free CE diols. (b) FTIR spectra of CE diols and pure CE diols cross-linked with GA (CE/GA materials).

chain (R = −CH2−CH2−CH2−CH2−). The reactions were easy to perform22 and afforded moderate to high yields (65− 85%) as confirmed by 1H and 13C NMR analyses (Figures S2− S5, SI). Optimized minimum-energy structures of the CEs from DFT calculations (Figure 2a) confirm that CE 1 has the smallest cavity (Øc = 1.23 Å vs CE 2 Øc = 1.34 Å and CE 3 Øc = 1.52 Å).22 Nonetheless, all had matching ring dimensions with the ionic size of Li+ (ØLi+ = 1.18 [IV]−1.52 Å [VI]), suggesting their suitability as Li+ ligands.49 The CE diols exhibited characteristic FTIR peaks (Figure 2b) from the O−H stretching of the hydroxyl groups (3402−3340 cm−1), from the C−H stretching (2930−2865 cm−1)34,50 and bending (1386−1361 cm−1) of the rings, from the CC stretching of benzene (1594−1452 cm−1),51 and from the C−O−C ether stretching of the rings (1257−1032 cm−1).24 3.2. Reactivity of CE Diols with GA. For GA to successfully cross-link both CE diols and PVA, the CE diols must also exhibit reactivity with GA. Ancillary tests were carried

(VCPH, Shimadzu, Japan) with air as carrier gas (200 kPa, 150 mL min−1). TOC calibration curves are shown in Figure S11, SI. CE diols and mCEs were selectively detected through their absorbances at λmax ≅ 275 nm (Figure S12, SI) using a UV−vis spectrophotometer (8453 Agilent). Their standard curves are provided in Figures S13 and S14, SI. Example UV−vis profiles of elution samples are also provided in Figure S15, SI.

3. RESULTS AND DISCUSSION 3.1. CE Diols. The CE diols were synthesized using modified procedures from the literature22,40−42 via intermolecular cyclization of various bis-epoxides with catechol (Figure S1, SI). All CE diols contain four ethereal oxygens available for Li+ complexation,30,31 benzo group(s) for cavity rigidification,17,18 and two −OH groups as attachment sites to the PVA matrix via GA cross-linking.31,48 CE 1 is a planar and symmetric 14-membered ring with two benzo groups, whereas CE 2 is a 15-membered macrocycle with a dimethyl-substituted tertiary carbon as its bulky alkyl moiety (R = −CH2C(CH3)2−CH2−). Meanwhile, CE 3 is a 16-membered ring with a flexible butyl 42865


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Figure 3. Stability tests and FTIR analysis of MFs. (a) Optical images of un-cross-linked and cross-linked CE/PVA samples. (b) % CE losses in CE/ PVA and control mCE/PVA MFs. (c) FTIR spectra of CE/PVA samples.

the lower reactivity of CE 1 could be due to the steric hindrances of its benzo groups.52 3.3. CE/PVA MF Preparation and Aerosol CrossLinking. During electrospinning, the entire dope solutions (12 mL) were successfully fabricated into MFs achieving a nearzero wastage of starting materials. Subsequent aerosol technique was highly beneficial as the cross-linking was completed at a shorter period and utilized minimal volume of GA/acetone solution. Previously, 1 g of PVA/inorganic adsorbent composite foam needed 1 L of 4 vol % GA solution.36,53 Meanwhile, immersion technique required 0.5−1 L of 0.6−5 vol % GA solutions to cross-link 1−2 g of PVA nanofiber for 24−48 h.44,54 In aerosol method, only 4.8 ± 0.05 mL of 4 vol % acidified GA/acetone aerosol was used to successfully cross-link 560 cm2 (2 g) of MF at room temperature for 5 h. The CE/PVA MFs were evaluated in terms of their (1) stability in water and (2) cross-link formation of GA with CE and PVA (Figure 3). When immersed in water, un-cross-linked MFs (Figure 3a) immediately disintegrated in contrast to the

out by removing the PVA component and allowing pure CE diols to cross-link with GA. The acid-catalyzed reactions afforded solid brown CE/GA products. FTIR results of the CE/GA samples (Figure 2b) reveal the CO stretching (1722−1705 cm−1) of unreacted GA,34,36 suggesting that not all aldehydes reacted with the CE diols. This is understandable because GA was added in excess with respect to the CEs (∼6 mmol mmol−1 diol). Similarly, the O−H stretching (3437− 3364 cm−1) and the doublets (2929−2862 cm−1) for C−H stretching were present in all samples. The characteristic peaks (P) were integrated and divided with those of pure CE diols to obtain integration ratios IP = ICE/GA/ICE diol (Table S3, SI) and determine if CE/GA cross-linking occurred. All CE/GA samples exhibit reactivity toward GA as indicated by the reduced OH groups (IP < 1) and increased C−H stretching (IP > 1). The broadened peaks at 1034−1106 cm−1 in CE/GA samples are also strong indications of acetal group formation.34,36,50 These results confirm the ability of all CE diols to cross-link with GA. The IP results suggest that CE 3 is most reactive to GA (details in p S-23, SI). On the other hand, 42866


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Figure 4. FE-SEM images of aerosol cross-linked MFs: (a) pure PVA, and CE/PVA samples containing: (b) CE 1, (c) CE 2, and (d) CE 3 (insets: average fiber diameters or Øave from histograms).

groups (3400−3294 cm−1)34,36 in PVA chains suggest that, although insoluble in water, the MFs remained hydrophilic. Hydrophilic matrices are desirable for a favorable water transport that could enhance Li+−adsorbent interaction.30,38 All MFs have no residual GA, except for CE 1/PVA, as evidenced by the peak from the aldehyde CO stretching (1720 cm−1).34,50 This is consistent with CE 1 being the least reactive of the three CE diols. Other familiar peaks were also present like the doublets of C−H stretching (2919−2912 and 2873−2860 cm−1) from the alkyl components of GA and CEs.34,50 Because evidence of CE−GA−PVA cross-linking is difficult to observe in FTIR alone, 13C CP-MAS NMR analysis was also carried out. The 13C CP-MAS NMR spectra provided some insights (details in p S-25, SI) into the nature of the cross-links formed.55−57 The results were partially resolved and predicted using MestRenova 11.0.255 due to the broad peaks of the spectra (Figures S16−S19, SI). Peak analyses (Tables S6−S9, SI) suggest several possible types of cross-links in the CE/PVA MFs. Type I involves acetal carbon formation from GA/PVA linkage whereas, Type II identifies acetal carbon formation from GA reacted with one OH-CE and one OH-PVA. Another possible form of linkage (III) involves only one OH of CE in GA cross-linking with its other OH group left unreacted. However, current results neither confirm nor rule out this type

highly opaque and stable cross-linked samples. The MFs experienced minimal swelling (Table S4, SI), especially the CE/ PVA samples (S ≅ 0.13). The cross-linked CEs rigidified the MFs by restricting the relaxation of PVA chains in water. The low S = 0.32 of pure PVA is also typical when a nonsolvent like acetone is used during GA cross-linking.44 Stability tests quantitatively confirm (Table S5, SI) the ability of aerosol GA cross-linking to covalently secure the CEs in the MFs. Gravimetric results reveal the substantial mass loss (27− 66% MF mass loss) in control mCE/PVA samples (Table S5, SI) due to the leakage of mCEs (13−95%) from the fibers (Figure 3b). This confirms that physical entrapment is ineffective to permanently confine the mCEs in the matrix. Furthermore, the substantial presence of physically bound mCEs inhibited GA from effectively cross-linking with PVA. Thus, all mCE/PVA also suffered from significant PVA losses (26−55%, Table S5, SI). Meanwhile, CE/PVA samples demonstrated negligible MF (0.99 >0.99

pH = 8.1; m ≅ 30 mg; Co = 7.4 mg L−1; S/L ratio = 0.60 g L−1; T = 30 °C.

Figure 6. Equilibrium adsorption experiments at varied Co = 3−70 mg L−1; m ≅ 30 mg; V = 50 mL; T = 30 °C; pH = 8.0. (a) Experimental qe of CE/PVA MFs; (b) adsorption capacity of CE in MF (qCE); (c) % CE utilization in MFs at varied Co, (d) CE/PVA performance compared to other reported CE-based Li+ adsorbents; *boundary line: ideal performance of an adsorbent at which 100% of the CEs were utilized for 1:1 complexation with Li+; #experimental qe of CE/PVA MFs; +experimental adsorption contribution of CEs in MFs defined in eq 7 as the first term qCE ·f ′CE.

saturated with Li+. Control pure PVA consistently showed negligible qe ≅ 0.56 mg g−1 at the highest Co = 70 mg L−1 in contrast to those of CE/PVA MFs (qe = 6.7−6.9 mg g−1). To fully evaluate the actual adsorption contributions of the MF components, a modified additive sorption model63 was adopted (eq 7), which expresses that the experimental qe (Figure 6a) is the sum of the adsorption contributions of the CE (qCE·f CE) and GA/PVA (qGA/PVA·(1 − f ′CE)). The CE adsorption contribution is the product of the adsorption capacity of the CE (qCE) and its actual mass fraction ( f ′CE) in MF (see p S-18 in SI for f ′CE calculation details). Meanwhile, [PVA + GA] contributions can be collectively estimated from the qe (0.99 >0.99 0.99 0.97

a pH = 8.0; m ≅ 30 mg; Co = 3−70 mg L−1; S/L ratio = 0.60 g L−1; T = 30 °C.

qe − [qGA/PVA ·(1 − f ′CE )] f ′CE

(8)

The CE/PVA MFs have relatively lower capacities than the LIS powders (Table S10, SI). This is understandable because these LIS are in their pure unsupported forms. Different LIS composites have remarkably different adsorption performances; some have superior and others have inferior capacities to CE/ PVA MFs (Table S10, SI). Although LIS are equally promising Li+ adsorbents, their high adsorption losses when fabricated as composites, 5 pH-inhibited Li+ adsorption,9,12 and slow adsorption kinetics (as mentioned earlier) often limit their adsorption performance. Moreover, given the remarkable differences between the LIS and CEs (i.e., nature, adsorption mechanism, synthesis, and fabrication), a more realistic comparison for CE/PVA MFs was carried out in detail with other reported CE-based Li+ adsorbents. Theoretical qe values of other reported CE-based Li+ adsorbents were estimated from their actual CE loading (Table S12, SI). An ideal adsorption performance for 1:1 CE/Li+ complexation is graphically represented by the boundary line (Figure 6d), wherein experimental qe ≅ theoretical qe at 100% CE utilization. The performances of other reported CE-based Li+ adsorbents (Table S12, SI) and CE/PVA MFs were plotted and classified according to their positions relative to the boundary line (Figure 6d). Adsorbents at the lower left region (group I) include CEs on SiO2 supports23,26 and ion-imprinted on magnetite27 and embedded in polymeric nanofiber matrix24 or macroporous beads.30 Although some of these adsorbents are along the boundary line, this group has limited adsorption capacity due to high CE loss during material fabrication (Table S12, SI).30 The adsorbent located above the boundary line (group II) experienced significant contribution of its support and hence does not necessarily reflect exclusive adsorption of Li+ by its CE component.32 Significant contribution of the matrix or support could impart nonselective Mn+ binding to the adsorbent, which could compromise its Li+ selectivity. Adsorbents far below the boundary line (group III) experienced either (1) CE capacity loss due to matrix/structural effects that limited the CE accessibility to Li+ or (2) low Li+ complexation with incompatible CEs.17,18,21 These materials include polymerized CEs processed into films25 or resins28 and CEs on MWCNTs.31 Meanwhile, experimental qe values of CE/PVA MFs along the boundary line suggest their nearly ideal performance (group IV). Being at the inflection of the blue curve suggests that the CE/PVA MFs have the most favorable properties for Li+ uptake among CE-based Li+ adsorbents reported in the literature. The exceptional performance of CE/PVA MFs highlights the importance of employing suitable CE diols with Li+-compatible cavity size and matrix like PVA, which has negligible adsorption contribution. These results also demonstrate that electro-

The qCE can be compared to the maximum theoretical capacities of pure CEs (qtheo) as listed in Table S11, SI. For 1:1 CE/Li+ complexation, qCE must approach qtheo = 21−22 mg g−1 CE, whereas for 2:1 complexation, qCE → qtheo = 11−12 mg g−1 CE at 100% CE utilization (i.e., Li+-saturated states). Thus, a valid comparison between qCE and qtheo can only be made if the CEs in the MFs were saturated with Li+ (Co ≥ 35 mg L−1). On the basis of Figure 6b, actual qCE value (eq 8) at Co > 35 mg L−1 approached ≅20 mg g−1, which is remarkably closer to the qtheo of 1:1 than the 2:1 complexation. Thus, the large discrepancies between qCE and qtheo at Co < 35 mg L−1 is mainly due to the unsaturated states of CEs given the limited availability of Li+ and not because of 2:1 complexation or inaccessibility of CEs to Li+. Furthermore, a 2:1 complexation requires close interaction and special orientation between two CEs.64 Thus, it is likely to occur only in systems with CEs that are free to move, like in liquid−liquid extraction.65 However, this is not the case in CE/PVA MFs because the neighboring CEs are distanced (l ≅ [MWCE/(ρtrue × f ′CE × NAvogadro)]1/3 × 100 ≅ 1.1−1.3 nm) and their movements are highly restricted by their covalent attachments to the matrix. The maximum qCE values strongly point out that 1:1 complexation was the predominant mechanism for Li+ uptake in CE/PVA MFs. Using 1:1 complexation as qtheo basis (Table S11, SI), % CE utilization = qCE/qtheo × 100 was calculated as shown in Figure 6c. At their highest qe values (Co = 70 mg L−1), 90.1−99.7% of the CEs in CE/PVA MFs were occupied by Li+. Interestingly, CE 3/PVA afforded the lowest % CE utilization. This can be associated to its denser fibers (Section 3.5), which rendered some of the CE 3 sites inaccessible to Li+. Nonetheless, overall results demonstrate that the nature of the MF structure was highly favorable for the high accessibility of the CEs for Li+. Nonlinear Langmuir (eq 9) and Freundlich (eq 10) isotherm models (Figure S25, SI) were used to estimate the maximum adsorption capacities of the CE/PVA MFs. In the equations, qm is the maximum adsorption capacity, whereas KL is the Langmuir constant, which pertains to the adsorption energy.27,66 The Freundlich constant KF is the adsorption capacity, whereas n is the adsorption intensity. Results (Table 3) reveal the Langmuir-type (r2 > 0.99) Li+ adsorption of the MFs.31,66 Their qm value (Table 3) ranged from 7.2 to 7.4 mg g−1, whereas that of pure PVA was only 0.6 mg g−1. qe =

samples

KF (mg g−1)

qmK LC e 1 + KLCe

qe = KFCe1/ n

(9) (10) 42870


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ACS Applied Materials & Interfaces Table 4. Li+ Separation Performance of CE/PVA MFs from Other Cations in Seawatera

a

cations

Co (mg L−1)

Co (mmol L−1)

Li+ Na+ Mg2+ Ca2+ K+

7.7 ± 0.3 10 171. 2 ± 39.0 1239.7 ± 8.1 177.4 ± 1.4 368.1 ± 5.2

1.12 442.42 51.01 4.43 9.42


7.7 ± 0.3 10 171. 2 ± 39.0 1239.7 ± 8.1 177.4 ± 1.4 368.1 ± 5.2

1.12 442.42 51.01 4.43 9.42


7.7 ± 0.3 10 171. 2 ± 39.0 1239.7 ± 8.1 177.4 ± 1.4 368.1 ± 5.2

1.12 442.42 51.01 4.43 9.42

Ce (mmol L−1)

qe (mmol g−1)

CE 1/PVA (2HDB14C4) 0.74 0.67 442.22 0.35 50.98 0.06 4.42 0.00 9.39 0.05 CE 2/PVA (2HB2M15C4) 0.72 0.65 442.33 0.14 50.95 0.09 4.42 0.01 9.40 0.02 CE 3/PVA (2HB16C4) 0.76 0.58 441.58 1.37 50.87 0.22 4.42 0.02 9.35 0.11

Kd (mL g−1)

α (Li+/Mn+)

CF × 10−3 (L g−1)

903.89 0.79 1.11 0.84 5.78

1.00 1134.38 815.79 1078.63 156.38

600.84 0.80 1.11 0.84 5.76

903.63 0.32 1.75 2.99 2.24

1.00 2786.12 516.94 302.11 402.64

585.26 0.32 1.75 2.99 2.24

765.16 3.10 4.27 3.77 11.93

1.00 246.87 179.01 203.07 64.16

520.58 3.09 4.26 3.76 11.84

pH = 8.32; m ≅ 30 mg; S/L ratio = 0.20 g L−1; T = 30 °C.

Figure 7. Elucidation of experimental selectivity trend using DFT calculation: (a) experimental αLi+/Mn+ and (b) reaction enthalpies for CE selectivity to Mn+ relative to Li+ by DFT.

Kd values, which were ≥765 for Li+ in contrast to the significantly lower values (Kd < 12) of other Mn+. Although Mg2+ has similar size to Li+, its free energy of hydration (ΔGh° = −1980 kJ mol−1) is 4 times that of Li+.12 Hence, it is more difficult to complex a dehydrated Mg2+ with the CEs than Li+. All CE/PVA MFs were able to concentrate Li+ CF = 520−600 times more efficiently than other Mn+. 3.8. Li+ Selectivity of CE Diols via DFT Calculations. DFT calculations were performed on the CE diols and Mn+ to further elucidate the selectivity performances of the CE/PVA MFs.46,47 Geometric distances of opposite ethereal oxygens (O−O) (Table S13, SI) and ethereal oxygen to Mn+(O−Mn+) (Table S14, SI) were measured from the minimum energy structures of (1:1) CE/Mn+ interactions before and after complexation (Figure S26, SI). Meanwhile, the exothermicities of exchange of reactions (ΔHn)67,68 were calculated (eq 11) to observe the selectivities of the CEs to the Mn+ relative to Li+ in a water-solvated system (n of surrounding H2O molecules = 6) (Tables S15−S17, SI).69

spinning and aerosol cross-linking are effective and practical techniques for producing specialized Li+ adsorbents. 3.7. Li+ Selectivity. The selectivity of the CE/PVA MFs was tested using seawater as sample feed solution. Competition of interfering Mn+ for CE binding could significantly reduce the Li+ adsorption capacity of the MFs. Among the major Mn+ present, Li+ has the smallest size similar to Mg2+ (1.44 Å); thus, both have matching sizes to the CEs. Meanwhile, other Mn+ like Na+ (2.04 Å), K+ (2.76 Å), and Ca2+ (2.00 Å) have larger diameters.12 The Mn+ were 22−1314 times more concentrated than Li+, with Na+ and Mg2+ being the most abundant species. Competition among the Mn+ for the CEs in the MFs was assessed in terms of their qe and Kd (eq 2). On the other hand, the Li+ selectivity of the MFs was evaluated in terms of αLi+/Mn+ (eq 3) and CF (eq 4).38 Results (Table 4) reveal that Li+ remained the most preferred cation by all CE/PVA MFs. Minimal CEs were lost to other competitive Mn+; their qe values in seawater were similar to those from pure Li+ solutions. This was also reflected in their 42871


Research Article


that negligible CE diol is lost during microfiber fabrication, whereas it provided a structure that is favorable to Li+ adsorption. Aerosol cross-linking was highly effective in stabilizing the CE/PVA MF in water while simultaneously linking the CEs to the PVA matrix via acetalization with GA. The technique required minimal cross-linking solution volume and relatively short cross-linking time. The CE−GA−PVA linkages ensured that no CEs were lost during adsorption/ desorption runs. The combined techniques produced CE/PVA MFs with better performances than other known CE-based Li+ adsorbents. By simply using other diol type of macrocyclic ligands with different cavity dimensions and blocking subunits, the presented fabrication strategy can be extended to the development of other specialized CE-based fibrous Mn+ adsorbents for the recovery of scarce precious metals or removal of highly toxic metal contaminants.

ΔHn = ΔH[Mn +(CE)] + ΔH[Li+(H 2O)n ] − ΔH[Li+(CE)] − ΔH[Mn +(H 2O)n ]

(11)

A reduction or expansion of the CE ring (Table S13, SI) suggests conformational change in its structure to accommodate the Mn+.17,22 In complexed forms, all CEs have either expanded or reduced rings depending on the Mn+. The least changes in CE cavities were observed in CE 1, which confirms that its ring was most effectively rigidified by the dibenzo units. On the other hand, the bulky subunit on one side of CE 2 resulted in its moderate conformational change, whereas the flexible butyl chain in CE 3 permitted the greatest movement of its ring.70 The O−M+ distances (Table S14, SI) varied according to the n+ M , which generally increased with the ionic size. The symmetric CE 1 showed the same O−Mn+ distances on both sides of its ring for all Mn+. On the other hand, CE 2 blocks larger Mn+ (i.e., Na+, K+) by repulsion at its bulky side (i.e., less stable complexes) or accommodates smaller Mn+ (i.e., Li+, Mg2+) closer to its bulky side (i.e., more stable complexes).22 Meanwhile, the butyl side of CE 3 exhibited better accommodation for Mg2+ than Li+. It also showed low blockage response to larger Mn+, suggesting its relatively lower sizematch selectivity than CE 1 and CE 2. The DFT-calculated ΔHn values (Tables S15 and S16, SI) for 1:1 complexation (Figure 7) indicate CE selectivities to different Mn+ values relative to Li+ (ΔHn = 0 kcal mol−1). A positive value suggests preference of the CE to Li+, whereas a negative value means favorability to the Mn+ (Table S17). The DFT results (ΔHn) of CE diols can be related to the experimental selectivity performance of CE/PVA MFs through αLi+/Mn+ (Figure 7). Consistent with the ΔHn trends, CE 2/PVA had the best ability to block larger Mn+, especially monovalent Na+ and K+, whereas CE 1/PVA was most effective to discriminate divalent cations like Mg2+ and Ca2+. CE 3/PVA exhibited the lowest selectivity due to its larger cavity size and flexible butyl subunit. Meanwhile, selection between CE 1/PVA and CE 2/PVA would depend on feed composition given their comparable adsorption capacities. Thus, for Na+-rich feed like seawater, CE 2/PVA is the more suitable adsorbent. 3.9. Recyclability. All CE/PVA MFs demonstrated consistent performance after several cycles of adsorption and desorption runs (Figure S27, SI). Their respective uptakes were consistent with qe obtained from batch experiments at Co ≅ 7 mg L−1 (Figure 5). The nearly 100% recovery from desorption runs confirms the effectiveness of using mild acid solution (0.5 M HCl) as Li+ stripping agent. The SEM images of used CE/ PVA MFs (Figure S28, SI) reveal similar morphological characteristics to pristine samples, suggesting physical stability of the MFs. Moreover, no eluted CEs were detected in the stripping solutions (Figure S29, SI). These results highlight the importance of covalently attaching the CEs to the PVA as it prevented CE leakage during adsorption/desorption operation. The regenerability of the CE/PVA MFs could offset the cost associated with CE synthesis, which is critical for the materialization of these specialized Li+ adsorbents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14858. CE diol and mCE synthesis protocol and characterizations, electrospinning conditions, details of MF stability tests, 13C CP/MAS NMR results and peak interpretation, morphology of un-cross-linked electrospun MFs, surface analysis and mechanical test results of MF samples, details of adsorption experiments, comparison of different Li+ adsorbents from the literature, density functional theory calculations, exothermicity of exchange of reactions, recycling experiments, and list of nomenclatures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (82)-31-330-6687. Fax: (82)-31-337-2902 (S.-P.L.). *E-mail: [email protected]. Tel: (82)-31-330-6687. Fax: (82)-31-337-2902 (W.-J.C.). ORCID

Jeong Woo Han: 0000-0001-5676-5844 Sangho Koo: 0000-0003-4725-1117 Wook-Jin Chung: 0000-0002-2503-4050 Author Contributions ⊥

L.A.L., G.M.N., and R.E.C.T. contributed equally to this work.

Author Contributions

All authors contributed in the preparation of the manuscript. L.A.L., G.M.N., and R.E.C.T. performed material synthesis, characterization, adsorption experiments, and data interpretation. J.W.H. and H.S.S. performed the DFT calculations. K.J.P. carried out adsorption experiments and water analyses. S.K. worked on the design and synthesis of CEs. S.-P.L. dealt with the concept design of the CE-based adsorbents and performed adsorption experiments. W.-J.C. worked on the concept design of the CE-based adsorbents and interpreted the results. Notes

The authors declare no competing financial interest. ∥ L.A.L., G.M.N., and R.E.C.T. are co-first authors.

4. CONCLUSIONS The combined use of CE diols, electrospinning, and aerosol cross-linking is an effective strategy to produce specialized CEbased Li+ adsorbents. The diols in CEs were used as covalent attachment sites to the PVA matrix. Electrospinning ensured

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and 42872


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ICT (Nos. 2016R1A2B1009221 and 2017R1A2B2002109) and the Ministry of Education (No. 2009-0093816).

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