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Nano-void membranes embedded with hollow zwitterionic nanocapsules for superior desalination performance Zhijuan Sun, Qian Wu, Changhuai Ye, Wei Wang, Liuchun Zheng, Fengkai Dong, Zhuan Yi, lixin xue, and Congjie Gao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00060 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019
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Nano Letters
Nano-void membranes embedded with hollow zwitterionic nanocapsules for superior desalination performance Zhijuan Sun 1, Qian Wu 1, Changhuai Ye 2, Wei Wang 1, Liuchun Zheng 3, Fengkai Dong 1, Zhuan Yi 1, Lixin Xue 1*, and Congjie Gao 1 1.Ocean College, Zhejiang University of Technology, Hangzhou, Zhejiang Province 310014, China 2.College of Materials Science and Engineering, Donghua University, Shanghai 201620, China 3.Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing 100190, China *Corresponding author. Email:
[email protected] Abstract In order to lower the capital and operational cost of desalination and wastewater treatment processes, nanofiltration (NF) membranes need to have high water permeation, ionic rejection while also maintaining stable performance through antifouling resistance. Recently, Turing-type reaction conditions [Science 2018, 360, 518-521] and sacrificed metal organic frame (MOF) nanoparticles [Nat. Commun. 2018, 9, 2004] have been reported to introduce nano-voids into thin-film composite (TFC)
polyamide (PA) NF membranes for improved performance. Herein, we report a one-step fabrication of thin-film nanocomposite membranes (TFNM) with controllable nano-voids in the polyamide layer by introducing hollow zwitterionic nanocapsules (HZNCs) during interfacial polymerization. It was found that embedding HZNCs increases the membrane internal free volume, external surface area and hydrophilicity, thus enhancing the water permeation and antifouling resistance without trading off the rejection of multi-valent ions. For example, water permeation of the NF membranes embedded with about 19.0 wt% of HZNCs (73 L m-2 h-1) increased by 70% relative to the value of the control TFC NF membrane without HZNCs (43 L m-2 h-1). This increase comes while also maintaining 95% rejection of 1
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Na2SO4. Further, we also determined the effect of the mass loading of HZNCs in the top surface of TFC NF membranes on the membrane performance. This work provided a direct and simple route to fabricate advanced desalination membranes with superior separation performance. Key Words: Nanofiltration membranes, Hollow nanocapsules, High-flux, Desalination,
Zwitterionic polymers, Thin-film nanocomposite membranes Nanofiltration (NF) is an ideal water treatment technology for low-energy, high-throughput desalination applications. To lower the capital and operational cost of the desalination and wastewater treatment processes, great efforts have been put into developing NF membranes with higher water permeation, rejection and antifouling resistance simultaneously.1-5 However, it is still a challenge to build NF membranes with further improved water permeation, keeping high rejection rates for multi-valent ions and antifouling resistance. Introducing nano-voids into the polyamide (PA) separation layers can boost up the free volume and surface area needed for higher water permeation,6, 7 but the size and distribution of the nano-voids introduced need to be finely controlled to avoid the loss of ion rejections and antifouling resistance. For example, aquaporins (AQPs) had been incorporated into biomimetic membranes to facilitate water permeation using nano-void water channels in the desalination processes8 ． Inter-phase boundary nano-voids had also been built into mixed matrix NF membranes via novel nanofillers with various structures and characteristics
9, 10
including carbon-based nanoparticles
(NPs)11-22, metal organic frame (MOF) materials,23-25 and metal oxide nanoparticles 26, 27.
However, the nano-voids from the boundary spaces were generally with limited
free volume and difficult to be tuned in size and shape for improved desalination performance. For example, Lee et al. 15 reported the addition of Graphene Oxide (GO) tended to decrease the mechanical strength and ionic rejection of the PA NF membranes. Additions of zwitterionic nanogels28
tended to decrease water flux,
mechanical property and ionic rejection of Thin-film composite (TFC) membranes due to the formation of uncontrollable large cracks. Therefore, it is very challenging in building nano-voids in NF membranes for better desalination performance. Microcapsules have been incorporated into membranes for liquid separation, 2
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such as in self-healing membranes and polymer electrolyte membranes29,
30,
while
nanocapsules have been incorporated into mixed-matrix membrane for gas separation applications, especially for CO2 capture31-33. However, hollow nanocapsules with controllable void size and shape have not been embedded into PA active layers of TFC NF membranes for performance improvement up to now. Recently, progresses have been made in developing nano-voids with larger free volume in TFC NF membranes. For example, Jin and her co-workers5 had reported the formation of TFC NF membranes with embedded nano-voids under crumpled polyamide layers using metal-organic framework nanoparticles (ZIF-8) as sacrificial templates. Zhang and Gao et al.4 have reported their route to generate nano-voids from Turing-type nano-bubbles or channels in polyamide membranes. However, these processes were complicated and the nano-voids generated were hard to manage. Herein, we report a one-step fabrication of a TFC NF membrane with controllable nano-voids in the polyamide layer by introducing hollow zwitterionic nanocapsules during interfacial polymerization, where the size and shape of the free volume introduced could be controlled by the size and shape of the hollow cores of the nanocapsules while the rejection and antifouling resistance of the formed membranes could be regulated by the shell structures of the hollow core-shell nanocapsules used.34 , so the water permeation and anti-fouling resistance of TFC NF membranes may be enhanced without sacrificing ionic rejections. We believe these could be a direct and simple route to fabricate advanced desalination membranes with superior separation performance.
Results As described in detail in supporting information (S2.3 and Scheme S1), the hollow zwitterionic nanocapsules (HZNCs) with core-shell structure were synthesized by inverse RAFT (Reversible Addition-Fragmentation Transfer ） mini-emulsion interfacial polymerization35-39 using 3-[Dimethyl-[2-(2-methylprop-2-enoyloxy) ethyl] azaniumyl propane-1-sulfonate (SBMA) as monomer. The nanostructure and average diameter of the formed HZNCs were determined by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and dynamic light scattering(DLS), and 3
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the results are shown in Figure 1 and Figure S1. Different from non-hollow zwitterionic nanoparticles (ZNps) shown in Figure 1B, HZNCs (Figure 1A) showed distinguished core-shell structures with an average diameter of 102.6 nm and internal nano-void cores ranged from about 50 to 80 nm in diameter. In water, HZNCs were swollen to have an increased average diameter of 118.9 nm as shown in Figure S1B. As reported before, the core size and the shell property of HZNCs can be easily tuned for better by adjusting the RAFT polymerization conditions35. The free volume created by their nano-void cores may serve as water permeating short-cut channels when the HZNCs are embedded in NF membranes.
(A)
(B)
Figure 1 TEM images of HZNCs(A) and non-hollow ZNPs (B) The process for fabricating the thin-film nanocomposite membranes with HZNCs is shown in Scheme 1. Thin-film nanocomposite membranes containing the HZNCs were prepared by interfacial polymerization using piperazine (PIP) and trimesoyl chloride (TMC), and HZNCs were added to hexane as oil phase. The mass loading of HZNCs in the top surface of TFC NF membranes relative to PA may be estimated by the average S/N elemental content data obtained from XPS chemical analysis40. The background S from polysulfone supporting layer detected from the top surface of the control TFC NF membrane contributed to a low residual surface S/N mass ratio value of 0.0089 while the additional S from HZNCs loaded on the surfaces of TFC NF membranes increased the S/N mass ratio to various levels. Since the S/N mass ratio 4
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from pure HZNCs was 1.25, the mass loading (wt%) of HZNCs in the top surface of TFC
NF
membranes relative
to
PA
may
be
roughly
calculated
from
((S/N-0.0089)/1.25) × 100% as listed in Table S1. For example, when the adding amounts of HZNCs were 200,400, 600 and 800 mg/L, the mass loading of HZNCs in the top surface layer of TFC NF membrane with HZNCs were calculated to be 10.4, 13.0, 19.0 and 36.4 wt%, respectively. Additional details of XPS chemical analysis for TFC NF membranes with different mass loading of HZNcs are also included in Supporting Information.
Scheme 1 Schematic diagram for preparing thin-film nanocomposite membranes with HZNCs and the water channels in membrane matrix The desalination performance of the thin-film nanocomposite membranes (TFNM) containing HZNCs was evaluated compared with the TFNM incorporated with non-hollow zwitterionic nanoparticles (ZNPs) with the same average diameter. As shown in Table.1, the NF membrane with about 19.0 wt% of HZNCs relative to the PA exhibited the highest water flux for Na2SO4 solution (73 L m-2 h-1), which is 40% increase from the value of the NF membrane with the non-hollow ZNPs (58.8 L m-2 h-1) and 70% increase from the value of the control TFC NF membrane without HZNCs (43.2 L m-2 h-1), respectively. These increased comes even with slightly increased rejection for Na2SO4, from 92.1% to 94.7%. 5
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Table 1 ．Separation performance of TFN membranes containing HZNCs, ZNPs and control TFC NF
Solute
TFNM with HZNCs
TFNM with ZNPs
Control TFC NF
Flux
Flux
Flux
(L m-2 h-1)
Rejection (%)
(L m-2 h-1)
Rejection (%)
(L m-2 h-1)
Rejection (%)
Na2SO4
73+ 4
94.7+1.3
59+ 3
92.3+2.6
43+ 4
92.1+1.9
NaCl
99+ 2
38.2+1.6
87+ 3
36.8+3.5
63+ 3
32.7+2.8
MgSO4
70+ 5
93.4+2.3
59+2
90.7+2.9
42+ 3
91.5+1.4
TEM images (Figure 2E.F) confirmed that HZNCs, incorporated inside the thin separation layer of NF membrane, had created free volume as nano-void bubbles. Across these HZNCs, water molecules may transport from the surface of PA layer into the inner void of HZNCs, and then go out from the other side of HZNCs, as shown in Scheme1. HZNCs and their nano-void cores had provided a lot of “short-cut”s for water transportation in NF membranes.41-43 That is to say, more straight and effective water channels have been built by incorporating HZNCs into the NF membrane as shown in Scheme 1, where the shells of HZNCs could also contribute to the effective rejection to the di-valent salts for good ionic rejection rates obtained. Therefore，it is suggested that the superior separation performance of TFN NF membranes with HZNCs comes from the existence of nano-void free volume, which serves as “short-cut” for water permeation in addition to the interfacial spaces between particles and polyamide membrane matrix. SEM and TEM results in Figure 2A-D have indicated that HZNCs have been embedded in the separation layer and visible from the top surface, resulting in greatly increased concavity and convexity of the membranes. AFM results in Figure 3 showed the roughness of the thin-film NF membranes containing HZNCs increases from 12.1±0.3nm to 29.5±0.3 nm compared with the control TFC NF membranes. The high water flux across the membranes also partially resulted from the increased specific surface area of the thin-film NF membranes with increased concavity and convexity.1 Therefore, the performance improvement comes also, in part, from the same mechanism observed for the 6
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Turing-type membranes4 with nano-bubbles, where the crumpled surface texture of the membrane simply allows for water flux directly into the meso-pores of the support membrane by creating an interfacial gap between the PA and support.
Figure 2 (A) Top-down, (C) cross-section SEM morphology and (E) high-resolution TEM cross-section morphology for the control TFC NF membrane; (B) Top-down, (D) cross-section SEM morphology and (F) high-resolution TEM cross-section morphology thin-film nanocomposite membranes containing HZNCs
7
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Figure 3 AFM images of surface morphology of the control TFC NF membrane (a) and thin-film nanocomposite membranes containing HZNCs (b) The effects of loading levels of HZNCs on the performance of TFN NF membranes were also investigated. Since the zwitterionic shells of the hollow nanocapsules are super-hydrophilic, TFNMs with HZNCs became more and more hydrophilic from inside to surfaces with the increasing mass loading of HZNCs44-46. As shown in Figure S2, the water contact angle decreased rapidly from 62.58° to 32.87° when the mass loading of HZNCs increased from 0 to 19.0 wt% in the top surface of TFC NF membranes relative to PA, contributing to large effects on the water permeation through the membrane. As shown in Figure 4A, water flux of Na2SO4 solution increased from 63 L m-2 h-1 to 99 L m-2 h-1 when the mass loading of HZNCs increased from 0-19.0 wt% when more water permeation free volume were built into the membranes was discussed before. However, the water flux was decreased slightly when the mass loading of HZNCs was further increased from 19.0-36.4 wt%. At low mass loading (
