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MgCl2 separation
Accepted Manuscript A positively charged composite nanofiltration membrane modified by EDTA for LiCl/MgCl2 separation Wei Li, Chang Shi, Ayang Zhou, Xiao He, Yawei Sun, Jinli Zhang PII: DOI: Reference:
S1383-5866(17)30705-0 http://dx.doi.org/10.1016/j.seppur.2017.05.044 SEPPUR 13757
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
3 March 2017 25 May 2017 25 May 2017
Please cite this article as: W. Li, C. Shi, A. Zhou, X. He, Y. Sun, J. Zhang, A positively charged composite nanofiltration membrane modified by EDTA for LiCl/MgCl2 separation, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.05.044
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A positively charged composite nanofiltration membrane modified by EDTA for LiCl/MgCl2 separation Wei Li*, Chang Shi, Ayang Zhou, Xiao He, Yawei Sun, Jinli Zhang Collaborative Innovation Center of Chemical Science and Chemical Engineering (Tianjin), School of Chemical Engineering &Technology, Tianjin University, Tianjin 300350, People’s Republic of China.
Abstract For efficient recovery of lithium from brines, a composite nanofiltration (NF) membrane with positively charged skin layer (PA-B) was first synthesized via interfacial polymerization between branched poly(ethylene imine) (BPEI) and trimesoyl chloride (TMC) on the support of crosslinked polyetherimide, and the separation performance was assessed for simulative brine of mixed LiCl/MgCl2 solution. To improve the permselectivity of Li+ against Mg2+, then we adopted EDTA to modify the as-prepared positively charged PA-B NF membrane. The optimal EDTA modified NF membrane PA-B2-E3 showed the separation factor S Li,Mg about 9.2 for the LiCl/MgCl2 mixed solution with a Mg2+/Li+ mass ratio of 24, with excellent stability within 36h filtration, which would have promising applications in the recovery of Li from brine.
Keywords: Nanofiltration; positively charged; EDTA; recovery Li from brine; Modification
1. Introduction Lithium, as the lightest metal which owns the highest redox potential value and specific heat capacity on the earth, has numerous applications in batteries, ceramics and special glass industry, nuclear industry, pharmaceutical industry and other fields [1]. A previous report [2] had shown that nearly 70% of the worldwide Li source reserves exists in brines (about 21.6 Mt). The recovery of Li from brines has been a trend in the Li recovery industry. With the rapid increase of the demands for lithium resources and the deepening of the development for recovery Li from brine, a new challenge arose as well. The majority brines have large amount of magnesium (Mg2+/Li+ mass ratio higher than 8), consequently the traditional precipitation method is invalid for the recovery of Li from brine with high Mg2+/Li+ mass ratio. While other technologies used in the recovery Li from high Mg2+/Li+ ratio brine like solvent extraction [3-8] and lithium ion-sieve (LIS) [9-13] lead to the production of a great deal of wastewater. In addition, solvent extraction and LIS technology need two steps such as extraction/ stripping and adsorption/ desorption, which reinforce the complexity of recovery Li from brine. Therefore, effective recovery Li from high Mg2+/Li+ ratio brine has been an area of intense investigation. Membrane separation processes, such as nanofiltration (NF) [14-19]
and electrodialysis (ED) [20-23], were applied to recover Li and could avoid the environmental problems and state change of Li. Recently, Xu et al[24] indicated that NF membranes could combine with ED process by replacing cation exchange membrane due to its separation capability for monovalent and multivalent ions. Therefore, NF membrane is more promisingly attractive for the recovery of Li from brine and some articles have reported the application of NF membranes for Li recovery. Wen et al [14] first reported the study on recovering of lithium chloride from diluted brine by NF using the negatively charged Desal-5 DL membrane (GE Osmonics, polysulfone and polyester as support layer, modified poly(piperazineamide) as skin layer) and the separation factor SLi,Mg was about 3.5. Yang et al [15] chose the spiral-wound Desal DK membrane element (GE Osmonics, polysulfone as support layer, aromatic polyamide as skin layer) with negative charge for the lithium recovery and the separation factor SLi,Mg was 2.6 for simulated brine with a Mg2+/Li+ mass ratio of 24 under 1.0 MPa. Somrani et al [16] studied the separation of lithium from salt lake brines by NF (NF90, DOW) and LPRO (XLE, DOW), and reported almost 100% rejection for Mg2+ whereas only 15% for Li+ using NF90 for the diluted Tunisian Salt Lake brine with Mg2+/Li+ mass ratio of 50. Bi et al [17] chose the spiral-wound DK-1812 model for the lithium recovery, and reported the separation factor SLi,Mg was 42 with the 6.0 g/L feed which has Mg2+/Li+ mass ratio of 40 at 0.8 MPa. Sun et al
[18] chose Desal DL-2540 model for the lithium recovery from brine. The separation factor SLi,Mg was near 3.3 and the rejection of Mg2+ was about 65% for the simulated West Taijiner brine (Mg2+/Li+=64) under 3.0 MPa. NF membranes show the potential in Li recovery from brine with high mass ratio of Mg2+/Li+. Up to now, majority of commercially available NF membranes ordinarily hold a negatively charged polyamide skin layer, furthermore there are few report on Li recovery from brine by positively charged NF membranes. Basing on Li’s report [19], it is possible that positively charged membranes have better performance for selective separation of LiCl/MgCl2 owing to the Donnan exclusion. They primarily synthesized a hollow fiber NF membrane with a positively charged surface via interfacial
polymerization
(IP)
between
1,4-Bis(3-aminopropyl)
piperazine and trimesoyl chloride (TMC). The as-prepared membrane holds a separation factor SLi,Mg of 2.6 which is higher than that (2.1) of the
negatively
charged
commercial
NF90
membrane
(DOW,
copolymerization of 1,3-phenylenediamine and trimesoyl chloride as skin layer) under 0.3 MPa for 2000 ppm feed solution (Mg2+/Li+=20). Owing
to
its
large
amount
of
amino
groups,
branched
polyethyleneimine (BPEI) can be used as a aqueous phase monomer of IP to react with acyl chloride monomers [25] and finally make the surface of NF membrane positively charged as result of the amino hydrolysis
[26,27]. Ethylenediaminetetraacetic acid (EDTA) has the capability to complex with divalent cations with a high affinity constant. Much work so far has been focused on the removal of divalent cations by EDTA functionalized materials with various of forms, such as silica gel [28], nanoparticle [29,30] and nanocomposites [31,32]. EDTA also can be used in membrane process for the removal of divalent cations by being added to the solution to complex with divalent cations [33,34]. Therefore, it is possible to synthesise EDTA functionalized NF membrane for efficient recovery Li from brine with high mass ratio of Mg2+/Li+ by the generation of EDTA-Mg (II) complexes. In this article, we first synthesized a positively charged NF membrane via IP between TMC and BPEI on the support of crosslinked polyetherimide and assessed the separation performance for simulative brine of mixed LiCl/MgCl2 aqueous solution. In order to enhance the selectivity of such membrane, then we modified the positively charged NF membrane with EDTA and finally confirmed that the EDTA modified positively charged NF membrane can effectively recover Li+ from feed solution which has a Mg2+/Li+ ratio of 24.
Scheme 1. (a) PA membrane obtained from cross-linked polyetherimide support via interfacial polymerization between amine groups on the top layer and TMC; (b) PA-B membrane obtained via interfacial polymerization with BPEI; (c) PA-B-E membrane obtained via EDTA-modification.
2. Experimental 2.1. Materials and reagents Polyetherimide (Ultem 1000) was provided by Saudi Basic Industries Corporation. Before using, it was dried under 120 oC for at least 12h. The polypropylene (PP) non-woven fabric was supplied by Teda filters Co., Ltd. TMC were provided by Heowns Biochem Technology Co., Ltd. BPEI (Mw=1800 Da) were provided by Aladdin Reagent Co., Ltd. Organic solvents including dimethylacetamide (DMAc), methanol and n-hexane were provided by Kermel Chemical Reagent Co., Ltd. LiCl and Ethylenediaminetetraacetic acid disodium salt (EDTA) were provided by Tianjin Yuanli Chemical Reagent Co., Ltd. Other inorganic salts including MgCl2, MgSO 4, Na2SO4 and NaCl were provided by Sinopharm Chemical Reagent Co., Ltd. All the reagents were
analytical grade and were not purified further. Deionized (DI) water was utilized during all the experiments in this paper. 2.2. Fabrication of NF membranes 2.2.1 Polyamide composite NF membrane (PA) The polyetherimide ultrafiltration membranes as microporous support were obtained via the non-solvent induced phase inversion (NIP) method. First, dried polyetherimide was dissolved in DMAc for a 22 wt% solution under stirring for 6 h at 60 oC. The solution was set aside for 24 h to remove air bubbles. A 150 µm knife gap was utilized for casting on polypropylene non-woven fabric at room temperature, followed by the treatment in water coagulation bath for 60 minutes. Then the obtained polyetherimide membrane was crosslinked in a methanol solution of 5% (w/v) EDA for 1h after its top surface was wiped out by filter paper. After a 3h dipping in DI water, the crosslinked membrane was covered by a n-hexane solution of 0.1% (w/v) TMC at 25.0 ± 1.0 °C for 1 min, followed by the annealing treatment at 70 °C for 5 min. The prepared polyamide NF membrane was washed with DI water, and denoted as PA. 2.2.2 Positively charged composite NF membranes (PA-Bx) PA-Bx membranes were prepared via the interfacial polymerization of BPEI on the crosslinked PEI membrane. The n-hexane solution of 0.1% (w/v) TMC was poured on the top of the crosslinked PEI membrane for 1 min at 25.0 ± 1.0 °C, after removing the excess TMC solution and being
vaporized for 1 min, a x% (w/v) BPEI aqueous solution was poured on the surface of the above membrane for 5 min. Then, being experienced the annealing treatment at 70 °C for 5 min, the prepared positively charged NF membrane was washed with DI water, and indicated as PA-Bx. 2.2.3 EDTA-modified positively charged NF membrane (PA-Bx-Ey) Before the annealing treatment of the above PA-Bx NF membrane, a y% (w/v) EDTA aqueous solution was poured on its toplayer for 10 min to prepare the EDTA-modified NF membrane. Then the similar annealing treatment was carried out at 70 °C for 5 min, the obtained membrane was soaked in DI water overnight to eliminate the free EDTA on the membrane. Finally the obtained membrane was denoted as PA-Bx-Ey. 2.3. Characterization of NF membranes 2.3.1 Chemical structure characterization To characterize the structure and functional groups of the composite NF membranes, attenuated total reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR, FTS-6000, Bio-Rad) was utilized in the region of 4000–700 cm-1. Every spectrum was recorded with the resolution of 4 cm-1 for 32 scans. To examine the chemical compositions of the top layer in NF membranes,
X-ray
photoelectron
spectroscopy
(XPS,
PHI5000VersaProbe, ULVAC-PHI Inc.) was utilized with the 45° electron
emission angle and AlKα as radiation source. 2.3.2 Morphology characterization To observe the top view and cross-sectional morphologies of the composite NF membranes, scanning electron microscope (SEM, LEO 1530VP) was used. All the membrane samples were pre-treated by following steps before SEM characterization: 1) being dried at 30°C for more than 12 h; 2) being fractured in liquid nitrogen for the cross-section morphologies; 3) being sputter-coated with gold. To analyze surface roughness of the composite NF membrane surface, Atomic force microcopy (AFM, Agilent 5500) was utilized with tapping mode of 10 µm × 10 µm. The root mean square roughness (Rms) and average roughness (Ra) were calculated to evaluate the surface roughness. 2.3.3 Static contact angle characterization To confirm the hydrophilicity of the composite NF membrane surface, static contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co.) was applied. To evaluate the mean value, each membrane sample was assessed at least five different locations. All the membrane samples were dried at 30°C more than 12 h before contact angle measurements, and the pH of the droplet was about 5.5. 2.3.4 Zeta potential characterization The zeta potential measurements were carried out by electrokinetic
analyzer for solid surface analysis (SurPASS, Anton Paar GmbH). The 1 mmol/L KCl solution was utilized as the background and 0.1 mol/L of HCl and NaOH were utilized to adjust the pH. Finally, the Helmholtz–Smoluchowski equation was utilized to determine the zeta potential. 2.3.5 Molecular weight cut off (MWCO) and pore size distribution (PSD) The pore size analysis of MWCO value and PSD were examined to further confirm the structure of membranes. A series of filtration tests for solutions which were consisted of neutral solutes such as ethanol, glycerol, glucose, saccharose and raffinose were carried out under 1.0 MPa. Total organic carbon analyzer (TOC, multi N/C®3100, Analytik Jena) was utilized to measure the concentrations of ethanol and glycerol. The concentrations of glucose, saccharose and raffinose were measured by High Performance Liquid Chromatography (HPLC, Agilent). The pore distribution evaluation was carried out according to the method reported previously [35,36]. 2.4 Performance tests Permeate flux and rejection are major parameters to evaluate the separation performance of NF membranes, which were measured in the cross-flow filtration flat-plate membrane module (Filter and Membrane Technology Co., Ltd.). The effective area of each tested sample is 24 cm2. Separate conditions were 25 °C and 1.0 MPa for all experiments. The
membranes were stabilized for at least 2h before collecting the permeate solution. Each test was carried out at least three times. The separation capability of Li+ and Mg2+ was tested using simulated brine which was consisted of LiCl/MgCl2 salt mixture solution with Mg2+/Li+ mass ratio about 24 and the concentration of Li+ about 0.1 g L−1. Permeate flux F (L m-2 h-1), was defined by Eq. (1). F=
·
(1)
Where V (L) represents the permeate solution volume, t (h) represents the operation time and A (m2) represents the effective area of membrane. Rejection (R) was defined by Eq. (2). R = 1 −
× 100%
(2)
Where Cp and Cf refer to the concentrations in the permeate solution and the feed, respectively. To further investigate the permselectivity of membranes for the simulated brine (LiCl/MgCl2), the separation factor S was defined by Eq. (3). Separation factor of Li over Mg: , =
, / , , / ,
(3)
Where CLi,p and CLi,f represents the concentration of Li+ in the permeate and feed solution, respectively. CMg,p and CMg,f represents the concentration of the Mg2+ in the permeate and feed solution, respectively. When SLi,Mg = 1, magnesium and lithium do not separate; when SLi,Mg> 1,
Li+ penetrates the membrane preferentially and becomes more concentrated when SLi,Mg is higher. The separation performance for single inorganic salt aqueous solution was also measured with 2g L-1 as the initial concentration, including LiCl, NaCl, MgCl2, MgSO4, and Na2SO4. A conductivity meter (DDS-307A, Rex) was utilized to examine the salt concentration. The selectivity and rejection of the simulated brine (LiCl/MgCl2) were characterized by titration. The amount of Mg2+ was characterized by EDTA chelate titration. The amount of Cl- was characterized by AgNO3 titration. The concentration of Li+ was calculated according to the conservation of charge. The average values with standard deviation presented in this paper were evaluated from at least three samples. 3. Results and discussion 3.1. Physico-chemical characteristics 3.1.1. Characterization of chemical structure
Fig. 1. FTIR-ATR spectra of NF membranes including PA, PA-B1, PA-B2, PA-B3, PA-B4.
The chemical structures of the positively charged membranes PA-Bx were characterized by the ATR-FTIR spectra. As shown in Fig. 1, the bands at 1647 cm-1 and 1546 cm-1 are due to the amide groups, and the band at 3278 cm-1 corresponds to the amine groups [37,38]. The amidation reaction is occurred during the crosslinking and interfacial polymerization. The amount of acyl chloride group is less than that of the terminal amine group of crosslinked polyetherimide support membranes and amine groups of BPEI, therefore the relative intensities of peaks at 1647 and 1546 cm-1 present few changes among PA and PA-B. The band at 1273 cm-1 is attributed to the symmetric stretching of Ar-O-Ar in polyetherimide support [39], while the band at 3278 cm-1 is due to amine groups. In order to confirm the existence of BPEI, the relative intensity ratio of I3278/I1273, was calculated. The ratio I3278/I1273 is calculated as 0.39
for PA, and increases in the order: 0.39 (PA-B1) <0.46 (PA-B2) <0.48 (PA-B3) < 0.49 (PA-B4), which is consistent with the order of BPEI concentration.
Fig. 2. Deconvoluted C1s, N1s and O1s XPS spectra for membranes including PA, PA-B2 and PA-B2-E3.
Fig. 2 displays the deconvoluted C1s, N1s and O1s XPS spectra for three kinds of NF membranes. The PA membrane has four carbon species located respectively at 284.5, 285.4, 287.6 and 288.2 eV, corresponding to C-C, C-N, N-C=O and O-C=O species respectively, besides three nitrogen species at 399.1, 399.6 and 400.5 eV due to C-N, N-C=O and N-H respectively, and two oxygen species at 530.9 and 531.8 eV due to C=O and -OH respectively [39]. The PA-B2 membrane has the similar carbon species with PA membrane except for O-C=O species, three
nitrogen species at 398.9, 399.6 and 401.1 eV due to C-N, N-C=O, -NH3+ respectively, the oxygen-related peaks at 531 eV due to N-C=O [40]. It is illustrated that for PA-B2 membrane the amine groups of BPEI indeed react with the O-C=O groups of TMC to generate polyamide structure, consequently there is no carboxylic group detected in the surface of PA-B2. In addition, the obviously stronger C-N bond of PA-B2 is due to the presence of BPEI. In the case of EDTA-modified membrane, PA-B2-E3, O1s XPS spectra show the peaks at 530.8 and 531.8 eV due to C=O and -OH respectively, which are the typical groups of EDTA. Meanwhile, PA-B2-E3 membrane shows the peak of -NH3+ species, similar as that in PA-B2 membrane, suggesting the positive charges on the surface. The element content on the top layers of composite NF membranes were also analyzed by XPS spectra. As listed in Table 1, the PA membrane includes 77.39% carbon, 8.61% nitrogen and 14.00% oxygen, while the PA-B2 membrane possesses 72.68% carbon, 15.59% nitrogen and 11.74% oxygen respectively. For the PA-B2-E3 membrane, it consists of 71.72% carbon, 15.60% nitrogen and 12.68% oxygen before separation and consists of 72.52% carbon, 11.94% nitrogen, 14.42% oxygen and 1.12% magnesium after separation. The lower O/N ratio of PA-B2 membrane is attributed to the covalently bonding BPEI on the surface, comparing with PA.
According to XPS spectra for the EDTA-modified membrane PA-B2-E3, the O/N ratio is a little higher than that of PA-B2, which is caused by the high oxygen content of EDTA. After separation of LiCl/MgCl2 mixed solution, the used PA-B2-E3 membrane shows certain amount of magnesium, confirming that EDTA-modified surface can adsorb Mg2+. Table 1 The elementary compositions on the surface of fresh membranes PA, PA-B2, PA-B2-E3 and the used PA-B2-E3
Element content (at%) Membranes C
N
O
Mg
O/N
PA
77.39
8.61
14.00
0.00
1.63
PA-B2
72.68
15.59
11.74
0.00
0.75
PA-B2-E3
71.72
15.60
12.68
0.00
0.81
Used PA-B2-E3
72.52
11.94
14.42
1.12
1.21
3.1.2 Characterization of morphology
Fig. 3. SEM images of composite NF membranes including PA, PA-B2 and PA-B2-E3.
SEM images were measured to compare the difference of top view and cross-section of these composite membranes. There are great difference among the top view of three kinds of membranes. The PA-B2 membrane has lots of wrinkles and the PA-B2-E3 membrane seems to be the smoothest. Several articles [41,42] have reported conceptual model of TFC membrane formation via interfacial polymerization, suggesting that in the case of larger pores, the migration of amine monomer toward the organic solution is faster, which can push and twist the initially formed nascent ultrathin cross-linked film and consequently forming the ridge and valley structure. In our work, TMC solution was first poured on the surface of support membrane, followed by the addition of the BPEI solution. Therefore, the migration of BPEI would be rapid due to the
much larger contact area. Owing to the bigger molecular size and flexible aliphatic chain of BPEI, it reacts with acyl chloride groups, resulting in lots of wrinkles on the surface. A uniform asymmetric structure composed of a finger-like porous support and a top skin layer is appeared in the cross-section images for these composite membranes. As the manufacture steps go on, the skin layer becomes thicker, the thickness value for the membrane PA, PA-B2 and PA-B2-E3 is respectively 101, 112 and 135 nm.
Fig. 4. AFM images and morphological statistics for PA, PA-B2 and PA-B2-E3 membranes.
AFM analysis was carried out to compare the surface roughness of PA, PA-B2 and PA-B2-E3. As shown in Fig. 4, the RMS values of the membranes PA, PA-B2, PA-B2-E3 are respectively 17.5 ± 3.8 nm, 12.4 ± 0.1 nm and 6.7 ± 1.1 nm. While the Ra values of the membranes PA, PA-B2, PA-B2-E3 are respectively 11.5 ± 1.9 nm, 8.9 ± 0.5 nm and 4.8 ± 0.9 nm. Similar trends of Rms and Ra are shown in Fig. S1(Supplementary Information) for three kinds of NF membranes with the smaller scan size of 3 µm. Variation of RMS and Ra values indicates
that both the interfacial polymerization and modification make the surface of membranes smoother, which is in accord with the top view SEM images. 3.1.3. Characterization of hydrophilicity
Fig. 5. Static water contact angles for PA, PA-Bx and PA-B2-Ey membranes.
The hydrophilicity of NF membranes, an important property affecting the flux and antifouling property of membranes, was evaluated by the values of static contact angles. As shown in Fig. 5, the static contact angle shows a reduce trend after modification, indicating that BPEI and EDTA make the membranes more hydrophilic than the PA membrane. And the hydrophilicity of composite NF membranes PA-B2 and PA-B2-E3 is attributed to the terminal amino and carboxyl groups enriched surface.
3.1.4. Characterization of zeta potential
Fig. 6. Zeta potentials of the composite membranes
Moreover, the surface charge of PA, PA-B2, PA-B2-E1, PA-B2-E3 and PA-B2-E5 membranes was examined by zeta potential test. As shown in Fig. 6, PA membrane has the isoelectric point about pH=4.5. When the pH is higher than 4.5, PA membrane is negatively charged. Experienced the interfacial polymerization with BPEI, the PA-B membranes are positively charged under the solution pH of 5.5, which is due to the large amounts of amines on the surface of PA-B2 can be protonated and form positive groups [43, 44]. In the case of EDTA-modified membranes PA-B-E, the surface still keeps positively charged, however, the variation of zeta potential of PA-B2-E membranes is not linear associated with the concentration of EDTA. Comparing with PA-B2-E1 and PA-B2-E5, the
PA-B2-E3 membrane presents the highest zeta potential, which is consistent with the separation performance of LiCl/MgCl2 mixed solution. With much higher concentration of EDTA, the amines group of BPEI located on the membrane surface are consumed by the reaction with the carboxyl groups of EDTA, therefore, the zeta potential of PA-B2-E5 becomes lower than that of PA-B2-E3. 3.1.5. Characterization of pore size
Fig. 7. MWCO and pore size distribution of PA, PA-B2 and PA-B2-E3 membranes.
The pore size in terms of MWCO, pore size distribution and mean effective pore radius (rp) was assessed to compare the difference of the composite NF membranes. The MWCO is respectively 298, 292 and 278 Da for the membranes PA, PA-B2 and PA-B2-E3, with the individual rp is 0.22, 0.21 and 0.20 nm, respectively. The somewhat decreasing value of rp is reasonable owing to the multistep manufacture including the covalent bonding BPEI and the subsequent EDTA modification. 3.2. Separation performance of composite NF membranes
3.2.1. Effects of BPEI concentration for PA-Bx membranes
Fig. 8. Separation performance of NF membranes without EDTA modification as a function of BPEI concentration: (a) rejection of Cl-, Li+ and Mg2+ ; (b) separation factor S Li,Mg and flux (conditions:1.0 MPa, pH = 5.5, C Mg2+= 2.4 g/L, Mg2+/Li+= 24).
As shown in Fig. 8(a), the PA membrane shows the rejection of 85.4% for Mg2+, 81.5% for Cl-, and 45.0% for Li+, with the flux about 6.3 L m-2 h-1. For the PA-Bx membranes, as the BPEI concentration rises from 1% (w/v) to 2% (w/v), the rejections for both Mg2+ and Cl- increase, i.e., from 82.2% to 89.8%, and from 78.3% to 85.7%. While further increasing BPEI concentration, these rejections decrease. Similar trend is observed for the rejection of Li+. The PA-B2 membrane shows the optimal performance with the rejection of 89.8% for Mg2+ and 35.4% for Li+. Fig. 8(b) displays the flux and the separation factor. PA-B2 has the largest SLi,Mg value about 6.5, much higher than that of PA (3.8), while the flux of PA-B2 is 5.8 L m-2 h-1, a little lower than that of PA membrane (6.3 L m-2 h-1). Previously, Yang et al [15] reported a separation factor of 2.6 and a flux of about 27 L m-2 h-1 using Desal DK membrane(negatively charged) with the feed of Mg/Li=24 and CLi+=0.1 g/L.
The zeta potential of PA-B membranes display an overall decreasing trend as the BPEI concentration rises, as shown in Fig. S2 (Supplementary Information). It is worthy to note that within the pH range of 4~9, the PA-B2 membrane has the relative stable positive charges. With lower BPEI concentration (PA-B1), some amine groups of BPEI would react with TMC, while the rest would afford the surface positive charge. With much higher BPEI concentration (PA-B3, PA-B4), the excess amine groups of BPEI would react competitively with TMC, resulting the poor connection between TMC and the terminal amine groups on the crosslinked polyetherimide support and consequently lower rejection. It is suggested that the selective separation capacity of LiCl/MgCl2 using the PA-B membranes is probably controlled by not only Donnan effect but also steric effect. 3.2.2. Effects of EDTA concentration for PA-B2-E membranes
Fig. 9. Separation performance of EDTA-modified NF membranes as a function of EDTA concentration:(a) rejection of Cl-, Li+ and Mg2+; (b) separation factor SLi,Mg and flux (conditions:1.0 MPa, pH near 5.5, C Mg2+= 2.4 g/L, Mg2+/Li+= 24).
As shown in Fig. 9(a), for the PA-B2-Ey membranes, as the EDTA
concentration rises from 1% (w/v) to 3% (w/v), the rejection for Mg2+ increases from 61.1% to 92.3%, while for Cl- from 57.2% to 88.4%. Under much higher concentration of EDTA, the rejections for Mg2+ and Cl- decrease. The rejection for Li+ also shows the rise-tandem-fall trend as a function of EDTA concentration. Fig. 9(b) displays the flux and the separation factor. PA-B2-E3 has the largest SLi,Mg value near 9.2, which increases by about 40% comparing with that of PA-B2 (6.5). It is known that EDTA is capable of chelating with Mg2+, thus, the efficiency of selective separation of Li+ and Mg2+ using PA-B2-E3 is enhanced significantly, while the flux of PA-B2-E3 (6.0 L m-2 h-1) is comparable to that of PA-B2 (5.8 L m-2 h-1). Comparing with the previous work [15], EDTA-modified membranes PA-B-E can enhance greatly the separation selectivity of Li+ against Mg2+, which is probably due to the adsorption of Mg2+ by the EDTA-modified surfaces. 3.2.3. Long-term filtration test
Fig. 10. Separation performance of the PA-B2-E3 membrane for 36 h filtration: (a) the rejection of Cl-, Li+ and Mg2+; (b) the separation factor SLi,Mg and flux (conditions: 1.0 MPa, pH = 5.5, C Mg2+= 2.4 g/L, Mg2+/Li+= 24).
To investigate the separation stability of the optimal PA-B2-E3 membrane, a 36 h filtration test was conducted. Fig. 10(a) shows that the rejections of Cl- and Mg2+ maintain relatively stable values around 86.5% and 91.9% respectively within 36 h. The rejection of Li+ exhibits random fluctuations owing to the relatively low concentration of Li+. As a result, the SLi,Mg value in Fig. 10(b) shows the same trend with Fig. 10(a) that is around 12.7±0.9. At the same time, the flux changes a little within 36h filtration. The results indicate that the EDTA-modified membranes possess the stable separation capacity for Mg2+ and Li+, which has promising application in the recovery of Li from brine with high Mg2+/Li+ mass ratio. 3.2.4. Separation performance for inorganic salts aqueous solution
Fig. 11. Separation performance of the PA-B2-E3 membrane for 2 g/L inorganic salts aqueous solution (the separation conditions: 1.0 MPa, pH near 5.5).
The performance of the PA-B2-E3 membrane for different single inorganic salts aqueous solution such as Na2SO4, MgSO 4, NaCl, MgCl2 and LiCl was further assessed. As shown in Fig. 11, the rejection of salt decreases in the order of MgSO 4 (91.7%) >MgCl2 (84.6%) >Na2SO4 (83.1%) >NaCl (69.1%) >LiCl (68.1%), which has little difference with the general positively charged membranes governed by Donnan exclusion (MgCl2 >MgSO4 >Na2SO 4 >NaCl). The reason is perhaps that the Donnan exclusion and chelation have a synergy effect on the rejection of inorganic salts for the EDTA-modified membrane. Considering MgSO 4 has a higher molecular weight than MgCl2，the amount of Mg2+ in 2 g/L MgSO4 aqueous solution is less than in MgCl2 aqueous solution. Therefore, the existence of EDTA on the surface of the PA-B2-E3
membrane shows a more significant effect on the rejection of MgSO4, which causes the different rejection order of MgCl2 and MgSO4 using EDTA-modified PA-B2-E3 membrane, comparing with the general positively charged membrane. 4. Conclusion A positively charged nanofiltration membrane used for recovery Li from LiCl/MgCl2 mixed solution was firstly prepared via IP between BPEI and TMC on the support of crosslinked polyetherimide. Then separation performance for LiCl/MgCl2 mixed solution of as-prepared positively charged NF membranes was assessed. The prepared membranes exhibited an increased separation factor. In order to enhance the separation selectivity for Li+/Mg2+, we adopted EDTA which is a common chelating agent as the monomer to modify the positively charged NF membrane. The as-prepared NF membrane modified by EDTA exhibits good separation performance for LiCl/MgCl2 mixed solution. Characterizations including ATR-FTIR, XPS and zeta potential, etc., proved that TMC can react with BPEI to generate the amide bonding structures of PA-B NF membranes via interfacial polymerization on the crosslinked polyetherimide support, and subsequent EDTA-modified membranes PA-B-E can enhance greatly the separation selectivity of Li+ against Mg2+, which is probably due to the adsorption of Mg2+ by the EDTA-modified surfaces. The optimal EDTA modified NF membrane
PA-B2-E3 showed the separation factor SLi/Mg about 9.2 and the flux 6.0 L m-2 h-1 for the simulated brine (LiCl/MgCl2) with a Mg2+/Li+ mass ratio of 24. Further, the PA-B2-E3 membrane showed excellent stability for 36 h filtration, suggesting that such EDTA modified positively charged NF membrane would have promising applications in the recovery of Li from brine. Acknowledgements This work was supported by National key research and development program (2016YFC1201503), NSFC (21576206, 21621004) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R46).
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Highlights A positively charged nanofiltration membranes were synthesized via interfacial polymerization between branched poly(ethylene imine) and trimesoyl chloride on the crosslinked polyetherimide. EDTA was used for modification of the positively charged NF membrane. The EDTA modified membrane shows a higher separation factor and comparable flux compared with the membrane without modification. The EDTA modified composite NF membranes show stable separation performance for LiCl/MgCl2 mixed solution.
S1383-5866(17)30705-0 http://dx.doi.org/10.1016/j.seppur.2017.05.044 SEPPUR 13757
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
3 March 2017 25 May 2017 25 May 2017
Please cite this article as: W. Li, C. Shi, A. Zhou, X. He, Y. Sun, J. Zhang, A positively charged composite nanofiltration membrane modified by EDTA for LiCl/MgCl2 separation, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.05.044
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A positively charged composite nanofiltration membrane modified by EDTA for LiCl/MgCl2 separation Wei Li*, Chang Shi, Ayang Zhou, Xiao He, Yawei Sun, Jinli Zhang Collaborative Innovation Center of Chemical Science and Chemical Engineering (Tianjin), School of Chemical Engineering &Technology, Tianjin University, Tianjin 300350, People’s Republic of China.
Abstract For efficient recovery of lithium from brines, a composite nanofiltration (NF) membrane with positively charged skin layer (PA-B) was first synthesized via interfacial polymerization between branched poly(ethylene imine) (BPEI) and trimesoyl chloride (TMC) on the support of crosslinked polyetherimide, and the separation performance was assessed for simulative brine of mixed LiCl/MgCl2 solution. To improve the permselectivity of Li+ against Mg2+, then we adopted EDTA to modify the as-prepared positively charged PA-B NF membrane. The optimal EDTA modified NF membrane PA-B2-E3 showed the separation factor S Li,Mg about 9.2 for the LiCl/MgCl2 mixed solution with a Mg2+/Li+ mass ratio of 24, with excellent stability within 36h filtration, which would have promising applications in the recovery of Li from brine.
Keywords: Nanofiltration; positively charged; EDTA; recovery Li from brine; Modification
1. Introduction Lithium, as the lightest metal which owns the highest redox potential value and specific heat capacity on the earth, has numerous applications in batteries, ceramics and special glass industry, nuclear industry, pharmaceutical industry and other fields [1]. A previous report [2] had shown that nearly 70% of the worldwide Li source reserves exists in brines (about 21.6 Mt). The recovery of Li from brines has been a trend in the Li recovery industry. With the rapid increase of the demands for lithium resources and the deepening of the development for recovery Li from brine, a new challenge arose as well. The majority brines have large amount of magnesium (Mg2+/Li+ mass ratio higher than 8), consequently the traditional precipitation method is invalid for the recovery of Li from brine with high Mg2+/Li+ mass ratio. While other technologies used in the recovery Li from high Mg2+/Li+ ratio brine like solvent extraction [3-8] and lithium ion-sieve (LIS) [9-13] lead to the production of a great deal of wastewater. In addition, solvent extraction and LIS technology need two steps such as extraction/ stripping and adsorption/ desorption, which reinforce the complexity of recovery Li from brine. Therefore, effective recovery Li from high Mg2+/Li+ ratio brine has been an area of intense investigation. Membrane separation processes, such as nanofiltration (NF) [14-19]
and electrodialysis (ED) [20-23], were applied to recover Li and could avoid the environmental problems and state change of Li. Recently, Xu et al[24] indicated that NF membranes could combine with ED process by replacing cation exchange membrane due to its separation capability for monovalent and multivalent ions. Therefore, NF membrane is more promisingly attractive for the recovery of Li from brine and some articles have reported the application of NF membranes for Li recovery. Wen et al [14] first reported the study on recovering of lithium chloride from diluted brine by NF using the negatively charged Desal-5 DL membrane (GE Osmonics, polysulfone and polyester as support layer, modified poly(piperazineamide) as skin layer) and the separation factor SLi,Mg was about 3.5. Yang et al [15] chose the spiral-wound Desal DK membrane element (GE Osmonics, polysulfone as support layer, aromatic polyamide as skin layer) with negative charge for the lithium recovery and the separation factor SLi,Mg was 2.6 for simulated brine with a Mg2+/Li+ mass ratio of 24 under 1.0 MPa. Somrani et al [16] studied the separation of lithium from salt lake brines by NF (NF90, DOW) and LPRO (XLE, DOW), and reported almost 100% rejection for Mg2+ whereas only 15% for Li+ using NF90 for the diluted Tunisian Salt Lake brine with Mg2+/Li+ mass ratio of 50. Bi et al [17] chose the spiral-wound DK-1812 model for the lithium recovery, and reported the separation factor SLi,Mg was 42 with the 6.0 g/L feed which has Mg2+/Li+ mass ratio of 40 at 0.8 MPa. Sun et al
[18] chose Desal DL-2540 model for the lithium recovery from brine. The separation factor SLi,Mg was near 3.3 and the rejection of Mg2+ was about 65% for the simulated West Taijiner brine (Mg2+/Li+=64) under 3.0 MPa. NF membranes show the potential in Li recovery from brine with high mass ratio of Mg2+/Li+. Up to now, majority of commercially available NF membranes ordinarily hold a negatively charged polyamide skin layer, furthermore there are few report on Li recovery from brine by positively charged NF membranes. Basing on Li’s report [19], it is possible that positively charged membranes have better performance for selective separation of LiCl/MgCl2 owing to the Donnan exclusion. They primarily synthesized a hollow fiber NF membrane with a positively charged surface via interfacial
polymerization
(IP)
between
1,4-Bis(3-aminopropyl)
piperazine and trimesoyl chloride (TMC). The as-prepared membrane holds a separation factor SLi,Mg of 2.6 which is higher than that (2.1) of the
negatively
charged
commercial
NF90
membrane
(DOW,
copolymerization of 1,3-phenylenediamine and trimesoyl chloride as skin layer) under 0.3 MPa for 2000 ppm feed solution (Mg2+/Li+=20). Owing
to
its
large
amount
of
amino
groups,
branched
polyethyleneimine (BPEI) can be used as a aqueous phase monomer of IP to react with acyl chloride monomers [25] and finally make the surface of NF membrane positively charged as result of the amino hydrolysis
[26,27]. Ethylenediaminetetraacetic acid (EDTA) has the capability to complex with divalent cations with a high affinity constant. Much work so far has been focused on the removal of divalent cations by EDTA functionalized materials with various of forms, such as silica gel [28], nanoparticle [29,30] and nanocomposites [31,32]. EDTA also can be used in membrane process for the removal of divalent cations by being added to the solution to complex with divalent cations [33,34]. Therefore, it is possible to synthesise EDTA functionalized NF membrane for efficient recovery Li from brine with high mass ratio of Mg2+/Li+ by the generation of EDTA-Mg (II) complexes. In this article, we first synthesized a positively charged NF membrane via IP between TMC and BPEI on the support of crosslinked polyetherimide and assessed the separation performance for simulative brine of mixed LiCl/MgCl2 aqueous solution. In order to enhance the selectivity of such membrane, then we modified the positively charged NF membrane with EDTA and finally confirmed that the EDTA modified positively charged NF membrane can effectively recover Li+ from feed solution which has a Mg2+/Li+ ratio of 24.
Scheme 1. (a) PA membrane obtained from cross-linked polyetherimide support via interfacial polymerization between amine groups on the top layer and TMC; (b) PA-B membrane obtained via interfacial polymerization with BPEI; (c) PA-B-E membrane obtained via EDTA-modification.
2. Experimental 2.1. Materials and reagents Polyetherimide (Ultem 1000) was provided by Saudi Basic Industries Corporation. Before using, it was dried under 120 oC for at least 12h. The polypropylene (PP) non-woven fabric was supplied by Teda filters Co., Ltd. TMC were provided by Heowns Biochem Technology Co., Ltd. BPEI (Mw=1800 Da) were provided by Aladdin Reagent Co., Ltd. Organic solvents including dimethylacetamide (DMAc), methanol and n-hexane were provided by Kermel Chemical Reagent Co., Ltd. LiCl and Ethylenediaminetetraacetic acid disodium salt (EDTA) were provided by Tianjin Yuanli Chemical Reagent Co., Ltd. Other inorganic salts including MgCl2, MgSO 4, Na2SO4 and NaCl were provided by Sinopharm Chemical Reagent Co., Ltd. All the reagents were
analytical grade and were not purified further. Deionized (DI) water was utilized during all the experiments in this paper. 2.2. Fabrication of NF membranes 2.2.1 Polyamide composite NF membrane (PA) The polyetherimide ultrafiltration membranes as microporous support were obtained via the non-solvent induced phase inversion (NIP) method. First, dried polyetherimide was dissolved in DMAc for a 22 wt% solution under stirring for 6 h at 60 oC. The solution was set aside for 24 h to remove air bubbles. A 150 µm knife gap was utilized for casting on polypropylene non-woven fabric at room temperature, followed by the treatment in water coagulation bath for 60 minutes. Then the obtained polyetherimide membrane was crosslinked in a methanol solution of 5% (w/v) EDA for 1h after its top surface was wiped out by filter paper. After a 3h dipping in DI water, the crosslinked membrane was covered by a n-hexane solution of 0.1% (w/v) TMC at 25.0 ± 1.0 °C for 1 min, followed by the annealing treatment at 70 °C for 5 min. The prepared polyamide NF membrane was washed with DI water, and denoted as PA. 2.2.2 Positively charged composite NF membranes (PA-Bx) PA-Bx membranes were prepared via the interfacial polymerization of BPEI on the crosslinked PEI membrane. The n-hexane solution of 0.1% (w/v) TMC was poured on the top of the crosslinked PEI membrane for 1 min at 25.0 ± 1.0 °C, after removing the excess TMC solution and being
vaporized for 1 min, a x% (w/v) BPEI aqueous solution was poured on the surface of the above membrane for 5 min. Then, being experienced the annealing treatment at 70 °C for 5 min, the prepared positively charged NF membrane was washed with DI water, and indicated as PA-Bx. 2.2.3 EDTA-modified positively charged NF membrane (PA-Bx-Ey) Before the annealing treatment of the above PA-Bx NF membrane, a y% (w/v) EDTA aqueous solution was poured on its toplayer for 10 min to prepare the EDTA-modified NF membrane. Then the similar annealing treatment was carried out at 70 °C for 5 min, the obtained membrane was soaked in DI water overnight to eliminate the free EDTA on the membrane. Finally the obtained membrane was denoted as PA-Bx-Ey. 2.3. Characterization of NF membranes 2.3.1 Chemical structure characterization To characterize the structure and functional groups of the composite NF membranes, attenuated total reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR, FTS-6000, Bio-Rad) was utilized in the region of 4000–700 cm-1. Every spectrum was recorded with the resolution of 4 cm-1 for 32 scans. To examine the chemical compositions of the top layer in NF membranes,
X-ray
photoelectron
spectroscopy
(XPS,
PHI5000VersaProbe, ULVAC-PHI Inc.) was utilized with the 45° electron
emission angle and AlKα as radiation source. 2.3.2 Morphology characterization To observe the top view and cross-sectional morphologies of the composite NF membranes, scanning electron microscope (SEM, LEO 1530VP) was used. All the membrane samples were pre-treated by following steps before SEM characterization: 1) being dried at 30°C for more than 12 h; 2) being fractured in liquid nitrogen for the cross-section morphologies; 3) being sputter-coated with gold. To analyze surface roughness of the composite NF membrane surface, Atomic force microcopy (AFM, Agilent 5500) was utilized with tapping mode of 10 µm × 10 µm. The root mean square roughness (Rms) and average roughness (Ra) were calculated to evaluate the surface roughness. 2.3.3 Static contact angle characterization To confirm the hydrophilicity of the composite NF membrane surface, static contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co.) was applied. To evaluate the mean value, each membrane sample was assessed at least five different locations. All the membrane samples were dried at 30°C more than 12 h before contact angle measurements, and the pH of the droplet was about 5.5. 2.3.4 Zeta potential characterization The zeta potential measurements were carried out by electrokinetic
analyzer for solid surface analysis (SurPASS, Anton Paar GmbH). The 1 mmol/L KCl solution was utilized as the background and 0.1 mol/L of HCl and NaOH were utilized to adjust the pH. Finally, the Helmholtz–Smoluchowski equation was utilized to determine the zeta potential. 2.3.5 Molecular weight cut off (MWCO) and pore size distribution (PSD) The pore size analysis of MWCO value and PSD were examined to further confirm the structure of membranes. A series of filtration tests for solutions which were consisted of neutral solutes such as ethanol, glycerol, glucose, saccharose and raffinose were carried out under 1.0 MPa. Total organic carbon analyzer (TOC, multi N/C®3100, Analytik Jena) was utilized to measure the concentrations of ethanol and glycerol. The concentrations of glucose, saccharose and raffinose were measured by High Performance Liquid Chromatography (HPLC, Agilent). The pore distribution evaluation was carried out according to the method reported previously [35,36]. 2.4 Performance tests Permeate flux and rejection are major parameters to evaluate the separation performance of NF membranes, which were measured in the cross-flow filtration flat-plate membrane module (Filter and Membrane Technology Co., Ltd.). The effective area of each tested sample is 24 cm2. Separate conditions were 25 °C and 1.0 MPa for all experiments. The
membranes were stabilized for at least 2h before collecting the permeate solution. Each test was carried out at least three times. The separation capability of Li+ and Mg2+ was tested using simulated brine which was consisted of LiCl/MgCl2 salt mixture solution with Mg2+/Li+ mass ratio about 24 and the concentration of Li+ about 0.1 g L−1. Permeate flux F (L m-2 h-1), was defined by Eq. (1). F=
·
(1)
Where V (L) represents the permeate solution volume, t (h) represents the operation time and A (m2) represents the effective area of membrane. Rejection (R) was defined by Eq. (2). R = 1 −
× 100%
(2)
Where Cp and Cf refer to the concentrations in the permeate solution and the feed, respectively. To further investigate the permselectivity of membranes for the simulated brine (LiCl/MgCl2), the separation factor S was defined by Eq. (3). Separation factor of Li over Mg: , =
, / , , / ,
(3)
Where CLi,p and CLi,f represents the concentration of Li+ in the permeate and feed solution, respectively. CMg,p and CMg,f represents the concentration of the Mg2+ in the permeate and feed solution, respectively. When SLi,Mg = 1, magnesium and lithium do not separate; when SLi,Mg> 1,
Li+ penetrates the membrane preferentially and becomes more concentrated when SLi,Mg is higher. The separation performance for single inorganic salt aqueous solution was also measured with 2g L-1 as the initial concentration, including LiCl, NaCl, MgCl2, MgSO4, and Na2SO4. A conductivity meter (DDS-307A, Rex) was utilized to examine the salt concentration. The selectivity and rejection of the simulated brine (LiCl/MgCl2) were characterized by titration. The amount of Mg2+ was characterized by EDTA chelate titration. The amount of Cl- was characterized by AgNO3 titration. The concentration of Li+ was calculated according to the conservation of charge. The average values with standard deviation presented in this paper were evaluated from at least three samples. 3. Results and discussion 3.1. Physico-chemical characteristics 3.1.1. Characterization of chemical structure
Fig. 1. FTIR-ATR spectra of NF membranes including PA, PA-B1, PA-B2, PA-B3, PA-B4.
The chemical structures of the positively charged membranes PA-Bx were characterized by the ATR-FTIR spectra. As shown in Fig. 1, the bands at 1647 cm-1 and 1546 cm-1 are due to the amide groups, and the band at 3278 cm-1 corresponds to the amine groups [37,38]. The amidation reaction is occurred during the crosslinking and interfacial polymerization. The amount of acyl chloride group is less than that of the terminal amine group of crosslinked polyetherimide support membranes and amine groups of BPEI, therefore the relative intensities of peaks at 1647 and 1546 cm-1 present few changes among PA and PA-B. The band at 1273 cm-1 is attributed to the symmetric stretching of Ar-O-Ar in polyetherimide support [39], while the band at 3278 cm-1 is due to amine groups. In order to confirm the existence of BPEI, the relative intensity ratio of I3278/I1273, was calculated. The ratio I3278/I1273 is calculated as 0.39
for PA, and increases in the order: 0.39 (PA-B1) <0.46 (PA-B2) <0.48 (PA-B3) < 0.49 (PA-B4), which is consistent with the order of BPEI concentration.
Fig. 2. Deconvoluted C1s, N1s and O1s XPS spectra for membranes including PA, PA-B2 and PA-B2-E3.
Fig. 2 displays the deconvoluted C1s, N1s and O1s XPS spectra for three kinds of NF membranes. The PA membrane has four carbon species located respectively at 284.5, 285.4, 287.6 and 288.2 eV, corresponding to C-C, C-N, N-C=O and O-C=O species respectively, besides three nitrogen species at 399.1, 399.6 and 400.5 eV due to C-N, N-C=O and N-H respectively, and two oxygen species at 530.9 and 531.8 eV due to C=O and -OH respectively [39]. The PA-B2 membrane has the similar carbon species with PA membrane except for O-C=O species, three
nitrogen species at 398.9, 399.6 and 401.1 eV due to C-N, N-C=O, -NH3+ respectively, the oxygen-related peaks at 531 eV due to N-C=O [40]. It is illustrated that for PA-B2 membrane the amine groups of BPEI indeed react with the O-C=O groups of TMC to generate polyamide structure, consequently there is no carboxylic group detected in the surface of PA-B2. In addition, the obviously stronger C-N bond of PA-B2 is due to the presence of BPEI. In the case of EDTA-modified membrane, PA-B2-E3, O1s XPS spectra show the peaks at 530.8 and 531.8 eV due to C=O and -OH respectively, which are the typical groups of EDTA. Meanwhile, PA-B2-E3 membrane shows the peak of -NH3+ species, similar as that in PA-B2 membrane, suggesting the positive charges on the surface. The element content on the top layers of composite NF membranes were also analyzed by XPS spectra. As listed in Table 1, the PA membrane includes 77.39% carbon, 8.61% nitrogen and 14.00% oxygen, while the PA-B2 membrane possesses 72.68% carbon, 15.59% nitrogen and 11.74% oxygen respectively. For the PA-B2-E3 membrane, it consists of 71.72% carbon, 15.60% nitrogen and 12.68% oxygen before separation and consists of 72.52% carbon, 11.94% nitrogen, 14.42% oxygen and 1.12% magnesium after separation. The lower O/N ratio of PA-B2 membrane is attributed to the covalently bonding BPEI on the surface, comparing with PA.
According to XPS spectra for the EDTA-modified membrane PA-B2-E3, the O/N ratio is a little higher than that of PA-B2, which is caused by the high oxygen content of EDTA. After separation of LiCl/MgCl2 mixed solution, the used PA-B2-E3 membrane shows certain amount of magnesium, confirming that EDTA-modified surface can adsorb Mg2+. Table 1 The elementary compositions on the surface of fresh membranes PA, PA-B2, PA-B2-E3 and the used PA-B2-E3
Element content (at%) Membranes C
N
O
Mg
O/N
PA
77.39
8.61
14.00
0.00
1.63
PA-B2
72.68
15.59
11.74
0.00
0.75
PA-B2-E3
71.72
15.60
12.68
0.00
0.81
Used PA-B2-E3
72.52
11.94
14.42
1.12
1.21
3.1.2 Characterization of morphology
Fig. 3. SEM images of composite NF membranes including PA, PA-B2 and PA-B2-E3.
SEM images were measured to compare the difference of top view and cross-section of these composite membranes. There are great difference among the top view of three kinds of membranes. The PA-B2 membrane has lots of wrinkles and the PA-B2-E3 membrane seems to be the smoothest. Several articles [41,42] have reported conceptual model of TFC membrane formation via interfacial polymerization, suggesting that in the case of larger pores, the migration of amine monomer toward the organic solution is faster, which can push and twist the initially formed nascent ultrathin cross-linked film and consequently forming the ridge and valley structure. In our work, TMC solution was first poured on the surface of support membrane, followed by the addition of the BPEI solution. Therefore, the migration of BPEI would be rapid due to the
much larger contact area. Owing to the bigger molecular size and flexible aliphatic chain of BPEI, it reacts with acyl chloride groups, resulting in lots of wrinkles on the surface. A uniform asymmetric structure composed of a finger-like porous support and a top skin layer is appeared in the cross-section images for these composite membranes. As the manufacture steps go on, the skin layer becomes thicker, the thickness value for the membrane PA, PA-B2 and PA-B2-E3 is respectively 101, 112 and 135 nm.
Fig. 4. AFM images and morphological statistics for PA, PA-B2 and PA-B2-E3 membranes.
AFM analysis was carried out to compare the surface roughness of PA, PA-B2 and PA-B2-E3. As shown in Fig. 4, the RMS values of the membranes PA, PA-B2, PA-B2-E3 are respectively 17.5 ± 3.8 nm, 12.4 ± 0.1 nm and 6.7 ± 1.1 nm. While the Ra values of the membranes PA, PA-B2, PA-B2-E3 are respectively 11.5 ± 1.9 nm, 8.9 ± 0.5 nm and 4.8 ± 0.9 nm. Similar trends of Rms and Ra are shown in Fig. S1(Supplementary Information) for three kinds of NF membranes with the smaller scan size of 3 µm. Variation of RMS and Ra values indicates
that both the interfacial polymerization and modification make the surface of membranes smoother, which is in accord with the top view SEM images. 3.1.3. Characterization of hydrophilicity
Fig. 5. Static water contact angles for PA, PA-Bx and PA-B2-Ey membranes.
The hydrophilicity of NF membranes, an important property affecting the flux and antifouling property of membranes, was evaluated by the values of static contact angles. As shown in Fig. 5, the static contact angle shows a reduce trend after modification, indicating that BPEI and EDTA make the membranes more hydrophilic than the PA membrane. And the hydrophilicity of composite NF membranes PA-B2 and PA-B2-E3 is attributed to the terminal amino and carboxyl groups enriched surface.
3.1.4. Characterization of zeta potential
Fig. 6. Zeta potentials of the composite membranes
Moreover, the surface charge of PA, PA-B2, PA-B2-E1, PA-B2-E3 and PA-B2-E5 membranes was examined by zeta potential test. As shown in Fig. 6, PA membrane has the isoelectric point about pH=4.5. When the pH is higher than 4.5, PA membrane is negatively charged. Experienced the interfacial polymerization with BPEI, the PA-B membranes are positively charged under the solution pH of 5.5, which is due to the large amounts of amines on the surface of PA-B2 can be protonated and form positive groups [43, 44]. In the case of EDTA-modified membranes PA-B-E, the surface still keeps positively charged, however, the variation of zeta potential of PA-B2-E membranes is not linear associated with the concentration of EDTA. Comparing with PA-B2-E1 and PA-B2-E5, the
PA-B2-E3 membrane presents the highest zeta potential, which is consistent with the separation performance of LiCl/MgCl2 mixed solution. With much higher concentration of EDTA, the amines group of BPEI located on the membrane surface are consumed by the reaction with the carboxyl groups of EDTA, therefore, the zeta potential of PA-B2-E5 becomes lower than that of PA-B2-E3. 3.1.5. Characterization of pore size
Fig. 7. MWCO and pore size distribution of PA, PA-B2 and PA-B2-E3 membranes.
The pore size in terms of MWCO, pore size distribution and mean effective pore radius (rp) was assessed to compare the difference of the composite NF membranes. The MWCO is respectively 298, 292 and 278 Da for the membranes PA, PA-B2 and PA-B2-E3, with the individual rp is 0.22, 0.21 and 0.20 nm, respectively. The somewhat decreasing value of rp is reasonable owing to the multistep manufacture including the covalent bonding BPEI and the subsequent EDTA modification. 3.2. Separation performance of composite NF membranes
3.2.1. Effects of BPEI concentration for PA-Bx membranes
Fig. 8. Separation performance of NF membranes without EDTA modification as a function of BPEI concentration: (a) rejection of Cl-, Li+ and Mg2+ ; (b) separation factor S Li,Mg and flux (conditions:1.0 MPa, pH = 5.5, C Mg2+= 2.4 g/L, Mg2+/Li+= 24).
As shown in Fig. 8(a), the PA membrane shows the rejection of 85.4% for Mg2+, 81.5% for Cl-, and 45.0% for Li+, with the flux about 6.3 L m-2 h-1. For the PA-Bx membranes, as the BPEI concentration rises from 1% (w/v) to 2% (w/v), the rejections for both Mg2+ and Cl- increase, i.e., from 82.2% to 89.8%, and from 78.3% to 85.7%. While further increasing BPEI concentration, these rejections decrease. Similar trend is observed for the rejection of Li+. The PA-B2 membrane shows the optimal performance with the rejection of 89.8% for Mg2+ and 35.4% for Li+. Fig. 8(b) displays the flux and the separation factor. PA-B2 has the largest SLi,Mg value about 6.5, much higher than that of PA (3.8), while the flux of PA-B2 is 5.8 L m-2 h-1, a little lower than that of PA membrane (6.3 L m-2 h-1). Previously, Yang et al [15] reported a separation factor of 2.6 and a flux of about 27 L m-2 h-1 using Desal DK membrane(negatively charged) with the feed of Mg/Li=24 and CLi+=0.1 g/L.
The zeta potential of PA-B membranes display an overall decreasing trend as the BPEI concentration rises, as shown in Fig. S2 (Supplementary Information). It is worthy to note that within the pH range of 4~9, the PA-B2 membrane has the relative stable positive charges. With lower BPEI concentration (PA-B1), some amine groups of BPEI would react with TMC, while the rest would afford the surface positive charge. With much higher BPEI concentration (PA-B3, PA-B4), the excess amine groups of BPEI would react competitively with TMC, resulting the poor connection between TMC and the terminal amine groups on the crosslinked polyetherimide support and consequently lower rejection. It is suggested that the selective separation capacity of LiCl/MgCl2 using the PA-B membranes is probably controlled by not only Donnan effect but also steric effect. 3.2.2. Effects of EDTA concentration for PA-B2-E membranes
Fig. 9. Separation performance of EDTA-modified NF membranes as a function of EDTA concentration:(a) rejection of Cl-, Li+ and Mg2+; (b) separation factor SLi,Mg and flux (conditions:1.0 MPa, pH near 5.5, C Mg2+= 2.4 g/L, Mg2+/Li+= 24).
As shown in Fig. 9(a), for the PA-B2-Ey membranes, as the EDTA
concentration rises from 1% (w/v) to 3% (w/v), the rejection for Mg2+ increases from 61.1% to 92.3%, while for Cl- from 57.2% to 88.4%. Under much higher concentration of EDTA, the rejections for Mg2+ and Cl- decrease. The rejection for Li+ also shows the rise-tandem-fall trend as a function of EDTA concentration. Fig. 9(b) displays the flux and the separation factor. PA-B2-E3 has the largest SLi,Mg value near 9.2, which increases by about 40% comparing with that of PA-B2 (6.5). It is known that EDTA is capable of chelating with Mg2+, thus, the efficiency of selective separation of Li+ and Mg2+ using PA-B2-E3 is enhanced significantly, while the flux of PA-B2-E3 (6.0 L m-2 h-1) is comparable to that of PA-B2 (5.8 L m-2 h-1). Comparing with the previous work [15], EDTA-modified membranes PA-B-E can enhance greatly the separation selectivity of Li+ against Mg2+, which is probably due to the adsorption of Mg2+ by the EDTA-modified surfaces. 3.2.3. Long-term filtration test
Fig. 10. Separation performance of the PA-B2-E3 membrane for 36 h filtration: (a) the rejection of Cl-, Li+ and Mg2+; (b) the separation factor SLi,Mg and flux (conditions: 1.0 MPa, pH = 5.5, C Mg2+= 2.4 g/L, Mg2+/Li+= 24).
To investigate the separation stability of the optimal PA-B2-E3 membrane, a 36 h filtration test was conducted. Fig. 10(a) shows that the rejections of Cl- and Mg2+ maintain relatively stable values around 86.5% and 91.9% respectively within 36 h. The rejection of Li+ exhibits random fluctuations owing to the relatively low concentration of Li+. As a result, the SLi,Mg value in Fig. 10(b) shows the same trend with Fig. 10(a) that is around 12.7±0.9. At the same time, the flux changes a little within 36h filtration. The results indicate that the EDTA-modified membranes possess the stable separation capacity for Mg2+ and Li+, which has promising application in the recovery of Li from brine with high Mg2+/Li+ mass ratio. 3.2.4. Separation performance for inorganic salts aqueous solution
Fig. 11. Separation performance of the PA-B2-E3 membrane for 2 g/L inorganic salts aqueous solution (the separation conditions: 1.0 MPa, pH near 5.5).
The performance of the PA-B2-E3 membrane for different single inorganic salts aqueous solution such as Na2SO4, MgSO 4, NaCl, MgCl2 and LiCl was further assessed. As shown in Fig. 11, the rejection of salt decreases in the order of MgSO 4 (91.7%) >MgCl2 (84.6%) >Na2SO4 (83.1%) >NaCl (69.1%) >LiCl (68.1%), which has little difference with the general positively charged membranes governed by Donnan exclusion (MgCl2 >MgSO4 >Na2SO 4 >NaCl). The reason is perhaps that the Donnan exclusion and chelation have a synergy effect on the rejection of inorganic salts for the EDTA-modified membrane. Considering MgSO 4 has a higher molecular weight than MgCl2，the amount of Mg2+ in 2 g/L MgSO4 aqueous solution is less than in MgCl2 aqueous solution. Therefore, the existence of EDTA on the surface of the PA-B2-E3
membrane shows a more significant effect on the rejection of MgSO4, which causes the different rejection order of MgCl2 and MgSO4 using EDTA-modified PA-B2-E3 membrane, comparing with the general positively charged membrane. 4. Conclusion A positively charged nanofiltration membrane used for recovery Li from LiCl/MgCl2 mixed solution was firstly prepared via IP between BPEI and TMC on the support of crosslinked polyetherimide. Then separation performance for LiCl/MgCl2 mixed solution of as-prepared positively charged NF membranes was assessed. The prepared membranes exhibited an increased separation factor. In order to enhance the separation selectivity for Li+/Mg2+, we adopted EDTA which is a common chelating agent as the monomer to modify the positively charged NF membrane. The as-prepared NF membrane modified by EDTA exhibits good separation performance for LiCl/MgCl2 mixed solution. Characterizations including ATR-FTIR, XPS and zeta potential, etc., proved that TMC can react with BPEI to generate the amide bonding structures of PA-B NF membranes via interfacial polymerization on the crosslinked polyetherimide support, and subsequent EDTA-modified membranes PA-B-E can enhance greatly the separation selectivity of Li+ against Mg2+, which is probably due to the adsorption of Mg2+ by the EDTA-modified surfaces. The optimal EDTA modified NF membrane
PA-B2-E3 showed the separation factor SLi/Mg about 9.2 and the flux 6.0 L m-2 h-1 for the simulated brine (LiCl/MgCl2) with a Mg2+/Li+ mass ratio of 24. Further, the PA-B2-E3 membrane showed excellent stability for 36 h filtration, suggesting that such EDTA modified positively charged NF membrane would have promising applications in the recovery of Li from brine. Acknowledgements This work was supported by National key research and development program (2016YFC1201503), NSFC (21576206, 21621004) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R46).
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Highlights A positively charged nanofiltration membranes were synthesized via interfacial polymerization between branched poly(ethylene imine) and trimesoyl chloride on the crosslinked polyetherimide. EDTA was used for modification of the positively charged NF membrane. The EDTA modified membrane shows a higher separation factor and comparable flux compared with the membrane without modification. The EDTA modified composite NF membranes show stable separation performance for LiCl/MgCl2 mixed solution.