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In situ repairing the large defects of macroporous ceramic membranes by polyelectrolyte-coated nanoparticles
Separation and Purification Technology 183 (2017) 318–326
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
In situ repairing the large defects of macroporous ceramic membranes by polyelectrolyte-coated nanoparticles Jie Li ⇑, Xinguo Si, Naixin Wang, Shulan Ji ⇑ Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China
a r t i c l e
i n f o
Article history: Received 16 January 2017 Received in revised form 31 March 2017 Accepted 1 April 2017 Available online 5 April 2017 Keywords: Nanohybrid multilayer membrane Nanofiltration Ceramic membranes Large defects Dynamic layer-by-layer assembly
a b s t r a c t Large defects have important impact on performance of separation membranes. In this paper, in order to reduce the large defects of the industrial ceramic supports, we prepared a nanohybrid separation membrane with a selective layer and a sub layer on the macroporous substrate with polyelectrolyte-coated nanoparticles. SEM, EDS and pore size analyses suggest that the although ZrO2 nanoparticles can migrate into the large defects to modify the membrane pores, the matching of pore-mouth size and diameter of NPs plays important role in the in situ modification of the substrates. The integrality of the as-prepared nanohybrid multilayer membrane was evaluated in the nanofiltration of dye solutions. The organicinorganic composite membranes perform high flux and retention for separation. Membranes were prepared to study the separation performance on different substrates and with different building blocks (polyelectrolyte molecule weights and polyelectrolyte molecule structure). We found that the poremouth size of substrates and the structure of building blocks mainly affected the structure and the nanofiltration performance of the composite membranes. Such assembly allow us to in situ modify the large defects of inorganic substrates while preparing the selective layer. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, organic–inorganic composite membranes has drawn more and more attention by providing a promising access to combine the superiorities of polymer membranes and inorganic membranes. However, developing organic–inorganic composite membranes with ideal structures and high performance remains a major challenge to date [1]. Modern organic–inorganic composite membranes usually are film composite membranes, fabricated by an inorganic support and a thin polymer selective layer. Though the performance of thin film composite membranes is improved compared with the traditional polymeric or ceramic membranes, reduction of the thickness selective layer is difficult because of the properties of the porous substrate [2]. The pore diameter of support should be smaller than the thickness of separation layer to make the membrane defect free. Therefore, the pore structure of the support plays an important role in the formation of a uniform and defect-free thin separation layer [3,4]. However, the industrial ceramic supports are usually with macro pores and large defects. Thus, a relatively thick selective layer has to be prepared on such supports to make the membranes defect free, which
⇑ Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (S. Ji). http://dx.doi.org/10.1016/j.seppur.2017.04.001 1383-5866/Ó 2017 Elsevier B.V. All rights reserved.
greatly reduce the flux [5,6]. As a result, the low flux of organicinorganic composite membranes obtained on industrial ceramic supports can hardly satisfy practical separation requirements. In contrast, a defect-free thin selective layer on a support with small or nano pore size is easy to build up, along with high flow resistance, low flux and higher separation factor. Therefore the ceramic support with uniform and proper pore size distribution is critical in order to achieve a balance between flux and separation factor. In order to prepare ceramic supports with proper pore size distribution, a sol-gel modification usually has to be used for the treatment of the supports [7]. By the sol–gel modification, a silica sol-gel layer can be formed on the macroporous support surface. In order to repair the possible defects of the previous sol-gel layer, the repeating of sol-gel modification was used, which at the same time reduced small pores and the mean pore size of the membrane. Then the water permeability of the modified support reduces sharply after modification (reduction by 90.3%) [8]. Moreover, repeating of heating treatment (500–800 °C) has to be used for the solgel. Therefore, we need to find an alternative method to the solgel to reduce only the large defects or the surface pore size (the pore-mouth size), with ease fabrication and low cost. Recently, a high loading nanohybrid membrane was developed with a layer-by-layer (LbL) self-assembly method, which incorporated single component NPs into both polyanion and polycation
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layers [9,10]. However, there are few reports on the preparation of nanohybrid membrane for the in situ modification of the porous substrates. In this paper, we reported on the organic-inorganic membranes that were prepared by means of dynamic LbL selfassembly of polyelectrolyte (PE)-coated ZrO2 nanocomposites. The ZrO2 NPs can migrate into the porous ceramic substrate to form a sub layer, which can in situ modify the large membrane defects and at the same time make the selective layer defect free. The integrality of the membranes were tested with the nanofiltration (NF) of dyes. Fig. 1 illustrates the procedure and the necessity of the process with the dynamic LbL self-assembly of PE-coated ZrO2 NPs. As shown in Fig. 1(a), self-assembly of the PE thin layer on macroporous supports without NPs would be incomplete since the PE molecular chain will infiltrate into the large pores but not covering them. By incorporating the NPs, a defect free selective layer could be achieved because the spherical NPs could repair the defects of the supports (Fig. 1(b)). And the building blocks could form a sub layer to in situ repair large pores (Fig. 1(b)). The nanohybrid sub layer was prepared for both the separation and the support of ceramic membrane, which could tune the pore size of the substrate and at the same time make the selective layer defect free. Moreover, the adsorption of polycation and polyanion with LbL assembly could joint the NPs and the substrate, forming the selective layers for the substrate in relatively low temperature. We found that the membranes showed a high performance in NF because of its thin top layer, sub layer and macroporous support layer composite structure. 2. Experiment 2.1. Materials Tubular ceramic membranes (internal diameter = 8 mm; outside diameter = 12 mm) were purchased from Foshan Science and Technology Factory. 3-Aminopropyl-trimethoxysilane (APTES, Mw is 179.29), branched poly(ethyleneimine) (PEI, Mw 60,000) were purchased from ACROS. Poly(sodium styrene sulfonate)
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(PSS, Mw 70,000 and 1,000,000), poly(diallyldimethyl ammonium chloride)(PDDA, Mw 100,000–200,000) and ZrO2 NPs (<100 nm) were purchased from Aldrich. Methanol and methyl blue (MB) were provided by Beijing Chemical Factory. 2.2. Preparation of PE-coated NPs The PE-coated NPs were prepared by the ZrO2 NPs coating with an outer PE layer. To obtain PDDA-coated ZrO2 NPs, ZrO2 NPs (0.4 g) were added to PDDA solution (0.94 wt%). The pH value was adjusted to 12.0. The preliminary PDDA-coated ZrO2 NPs were sonicated for 30 min and then centrifuged for 10 min at 10,000 rpm. Excess free PDDA chains from the surface of the NPs were replaced with water. This sonication and centrifugation procedure was repeated three times to form stable PDDA-coated ZrO2 NPs suspension. The PSS-coated ZrO2 NPs, PEI-coated ZrO2 NPs were prepared of following the same process. The pH values were adjusted to 6.0 and 10.0 respectively for PSS-coated ZrO2 and PEIcoated ZrO2 solutions. 2.3. Preparation of nanohybrid multilayers on tubular ceramic substrate membranes The tubular ceramic substrate was pre-treated with 2 g/L APTES in the mixture of ethanol solution (EtOH:H2O = 95:5) for 2 h at room temperature as reported previously [11,12]. The modified substrate was washed with deionized water and then dehydrated at 110 °C for 2 h in a vacuum oven. The pre-treated membranes were loaded into a 20 mm diameter tubular module made by polymethylmethacrylate. Both ends of the module were sealed with Teflon tube. The dynamic LbL self-assembly experiments were carried out using a cross-flow negative pressure filtration cell fabricated by our laboratory (Fig. 2) [9]. As shown in Fig. 2, the polycation-coated ZrO2 and polyanioncoated ZrO2 solutions were alternatively pumped into the inner channel of ceramic tube through separate tubing. On the shell side, in order to pull the PE-coated ZrO2 into the large pores or defects of
Fig. 1. Schematic diagram describing the modification of the inorganic substrates with nanoparticles.
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Fig. 2. Apparatus used for modification of the inorganic substrates with nanoparticles.
the substrate membranes, a water circulated vacuum pump was used to form a 0.09 MPa negative pressure. The assembly time was 30 min, and then rinsed with ultrapure water for 20 min and dried for 60 min at 40 °C. In order to study the matching between the diameter of NPs and the pore-mouth size of substrates, nanohybrid multilayers were built up on ceramic tube with small pore mouth size (Substrate-1) and large pore-mouth size (Substrate-2). 2.4. Dye removal experiment Dye removal experiment was carried out using a laboratory made cross-flow nanofiltration system [13]. The NF membrane was put into the membrane cell for nanofiltration and the effective area of the membrane was 20 cm2. The dye concentration of the feed solution is 100 mg/L. The dye solution was pressurized using a plunger pump at a pressure of 0.5 MPa. During the nanofiltration process, the concentrate was recirculated to the feed tank while permeate was collected in permeate vessel. The permeation flux J (L/m2 h MPa) was determined according to the following equation:
J¼
W AtDP
ð1Þ
where W (L) is the amount of permeate, DP (MPa) is the operation pressure, t (h) is a certain time for the nanofiltration, and A (m2) is the effective area of the membrane. And the solute retention rate R was calculated by:
Cp R ð%Þ ¼ 100 1 Cf
ð2Þ
where the Cp and Cf are the dye concentration in the permeate and feed solutions, respectively. The dye concentrations were analyzed by an ultraviolet–visible spectrophotometer (UV2800, Shanghai). 2.5. Characterization Organic-inorganic membranes were examined under a Hitachi S-4300 scanning electron microscope (SEM) attached to an energy-dispersive spectrometer (EDS). All samples were coated with gold in vacuum to increase their conductivity before observations. In order to obtain the pore size of organic-inorganic membranes, characterization was performed by mean pore size analyzer. Pore-mouth size (surface pore size) distribution of the ceramic tube before and after dynamic LbL assembly were mea-
sured using a GaoQ PSDA-20 porometer based on a modified bubble-point method [14].
3. Results and discussion Nanohybrid multilayers were built up on ceramic tubes with different pore-mouth size using the PE-coated NPs. The SEM photos of the surface and near-surface cross-section of membranes are shown in Fig. 3. Before dynamic LbL self-assembly, the surface of ceramic supports (substrate-1) is rough and uneven, with large pores and big particles (Fig. 3(a)). From the image of the membrane after self-assembly of PSS/PDDA with 2.5 layers, a more even plane is obtained although there are still some defects (Fig. 3(c)). For the (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane, the ceramic membrane surface is covered by a dense layer with the nanocomposites (Fig. 3(e)). In addition, ZrO2 particles can be found inside the pores of the ceramic substrate to form a sub layer (Fig. 3(f)). The above differences between PSS/PDDA and PSS-ZrO2/PDDA-ZrO2 are a result of the ZrO2 incorporation. On one hand, the dynamic LbL self-assembly of PSS-ZrO2/PDDA-ZrO2 led to an intrusion of zirconium into macroporous ceramic support to form a sub layer to repair the support. One the other hand, the nanocomposites could result in the formation of Zr-based nanohybrid selective layers to make the membrane defect free. As a comparison, nanohybrid multilayers were formed on a ceramic tube with larger poremouth size (Substrate-2). Although there is huge difference on the morphology of substrate-2, similar phenomena can be observed for these composite membranes (Fig. 3(g)–(l)). Fig. 3 (k) and (l) showed that the PE-coated NPs layers had been successfully built up on the ceramic substrates to in situ modify the large defects of substrate-2. And from Fig. 3 (l), a sub layer can be seen significantly after modification. In order to confirm the PE-coated nanoparticles can modify the large pores of the substrate, EDS measurements were performed. The changes of zirconium element composition through the cross-section were analyzed. As shown in Fig. 4(a) and (b), for the PSS-ZrO2/PDDA-ZrO2 membrane with substrate-1, the concentration of zirconium decreased while the aluminum increased before the 0.5 lm point. Since the ceramic substrate does not contain zirconium while the nanohybrid membrane does not contain aluminum, all the zirconium must arise from the selective PSSZrO2/PDDA-ZrO2 layer and aluminum from the supporting membrane. These observations led us to conclude that the thickness of the nanohybrid layers on substrate-1 is far less than 0.5 lm. As a comparison, for the PSS-ZrO2/PDDA-ZrO2 membrane with substrate-2 (Fig. 4(c) and (d)), the concentration of zirconium
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Fig. 3. SEM images of (a) inner surface of ceramic substrate-1, (b) cross session of ceramic substrate-1, (c) inner surface of (PSS/PDDA)2.5/ceramic membrane by substrate-1, (d) cross session of (PSS/PDDA)2.5/ceramic membrane by substrate-1, (e) inner surface of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane by substrate-1, (f) cross session of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane by substrate-1, (g) inner surface of ceramic substrate-2, (h) cross session of ceramic substrate-2, (i) inner surface of (PSS/ PDDA)2.5/ceramic membrane by substrate-2, (j) cross session of (PSS/PDDA)2.5/ceramic membrane by substrate-2, (k) inner surface of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane by substrate-2, (l) cross session of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane by substrate-2.
decreased while the aluminum increased before the 3 lm point. These observations led us to conclude that the thickness of the nanohybrid layers on substrate-2 is far less than 3 lm. From this analysis, we deduce that ZrO2 migrate into the ceramic substrate-2 pores more deeply because more zirconium was found there. Therefore the ZrO2 NPs are more likely to intruded into the substrate-2 pores while the ZrO2 NPs are mostly distributed near
the substrate-1 surface. This clearly proved that the NPs were successfully loaded into the pores especially the large defects of the membrane when the defects are larger than the sizes of the PEcoated ZrO2 NPs. For the pores that are less than the PE-coated ZrO2 NPs, NPs are mostly distributed near the substrate surface. To confirm the expectations of self-repairing by the incorporation of NPs, The pore-mouth size distribution of the ceramic tube
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Fig. 3 (continued)
before and after dynamic LbL assembly were performed in Table 1. As shown in Table 1, the largest and mean pore-mouth sizes of the ceramic substrate-1 were 2.5 lm and 1.1 lm, respectively. The max pore-mouth size of the (PSS/PDDA)2.5/ceramic membrane was 2.4 lm and the (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane was 1.4 lm, which reduces significantly. That means the large pores or defects decreased significantly after LbL modification (from 2.5 lm to 1.4 lm). However, the mean pore-mouth sizes only decreased from 1.0 lm to 0.9 lm. Therefore the dynamic LbL self-assembly will form a thin layer on the macroporous support surface. This behavior has the main function of the selfrepairing the possible defects of the macroporous substrate, rather than infinitely reduce small pore mouth size or the mean pore size of the membranes. Therefore, an asymmetrical structure was formed, which will be effective for improvement of membrane separation performance. As a comparison, ceramic substrate-2 with larger defects (dmax = 2.8 lm) was used for preparation of nanohybrid multilayers. Either mean pore-mouth size (dmean) or max pore-mouth size (dmax) of substrate-2 decreased after self-assembly. However, there is a difference for nanohybrid membranes on substrate-2. The mean pore-mouth size of substrate-2 decreased more significantly than substrate-1 while the max pore-mouth size decreased less than substrate-1. For example, the max pore-mouth size (dmax) of the substrate-1 decreased from 2.5 lm to 1.4 lm while that of substrate-2 decreased from 2.8 lm to 2.28 lm after PE coated NPs modification. And for mean pore-mouth size (dmean), substrate-1 decreased from 1.1 lm to 0.9 lm while that of substrate-2 decreased from 1.2 lm to 0.78 lm. The reason for this phenomenon is more ZrO2 distributed into the pores of the substrate-2 under dynamic pressure (Fig. 4) since the size of the ZrO2 (<100 nm) is much smaller than the substrate-2 poremouth size (dmax = 2.8 lm). This indicates that although most of ZrO2 NPs were able to repair the large defects of substrate, the matching of pore-mouth size and diameter of NPs plays important role in the in situ modification of the substrates.
The separation performance of the (PSS-ZrO2/PDDA-ZrO2)2.5/ ceramic membrane for nanofiltration was evaluated and shown in Fig. 5. The incorporation of ZrO2 NPs produced much higher retention in the dyes removal. The retention and flux of (PSSZrO2/PDDA-ZrO2)2.5/ceramic membrane towards nanofiltration of methyl blue (MB) can reach 98.9% and 109 L/m2 h MPa. As a control, using the (PSS/PDDA)2.5/ceramic membranes without NPs, the retention and flux for MB were 90.1% and 184 L/m2 h MPa. Better retention of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membranes can be attributed to the incorporation of NPs, which self-repaired the possible defects of the pore mouth. The max pore-mouth size decreased significantly from 2.4 lm to 1.4 lm (Table 1). Additionally, PSS as anionic polyelectrolyte has the same charge property with methyl blue. The methyl blue molecule can also be rejected through the charge repulsion. The (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane produced much higher retention in the dyes removal due to their unique structure. Furthermore, PSS/PDDA-ZrO2 and PSS-ZrO2/PDDA membranes were prepared as comparison to confirm the loading of NPs on the separation properties. It could be found that by tuning the charge properties, either polyanion or polycation layers could be incorporated with the ZrO2 NPs though the loading was limited [10]. Therefore, the retention of this kind of membranes was not as high as the PDDA-ZrO2/PSS-ZrO2 membranes. These results further suggest that when using dynamic LbL self-assembly of PEcoated NPs, the loading of the NPs plays an important role in the repairment of the NPs to the membrane pore mouth and in obtaining better structured organic-inorganic membranes. The high loading also can efficiently simplify the procedures by reducing process cycles of LbL. Moreover, the asymmetrical structure formed by PDDA-ZrO2/PSS-ZrO2 membrane (Fig. 3(f)) is extremely effective in reducing the mass transfer resistance and achieving higher separation performance. The effects of ceramic tube pore-mouth size on the separation performance of nanohybrid membranes were shown in Fig. 6. As shown in Fig. 6, after the incorporation of ZrO2, the retention and
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Fig. 4. Cross-sectional EDS analyses of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane. (a) Changes of zirconium element composition through the cross-section of (PSS-ZrO2/ PDDA-ZrO2)2.5/ceramic membrane with substrate-1, (b) changes of aluminum element composition through the cross-section of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane with substrate-1, (c) changes of zirconium element composition through the cross-section of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane with substrate-2, (d) changes of aluminum element composition through the cross-section of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane with substrate-2.
Table 1 Pore-mouth size of the various membranes. Substrate
Sample
Pore-mouth size (lm) dmax
dmean
Substrate-1
Ceramic membrane (PSS/PDDA)2.5/ceramic membrane (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane
2.5 2.4 1.4
1.1 1.0 0.9
Substrate-2
Ceramic membrane (PSS/PDDA)2.5/ceramic membrane (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane
2.8 2.4 2.28
1.2 0.8 0.78
flux of composite membrane with substrate-2 increased from 70.5% to 87.8%, while the flux decreased from 121 L/m2 h MPa to 84 L/m2 h MPa. As a control, after the incorporation of ZrO2 using the composite membranes with substrate-1, the retention and flux of nanohybrid membrane increased from 90.1% to 98.9%, while the flux decreased from 184 L/m2 h MPa to 109 L/m2 h MPa. The difference of can be attributed to the matching of pore-mouth size and diameter of NPs. As the SEM-EDS and pore-mouth size analyses
suggest that the ZrO2 NPs are mostly loaded on the near surface of substrate-1 while the NPs are more likely to migrate into the pores of substrate-2 membrane. Thus the NPs were able to modify most of the defects in substrate-1 to get a high retention of 98.9%. But for substrate-2, the NPs could only migrate into the large pores of the membrane to lead to a relatively low flux. Therefore the matching of pore-mouth size and diameter of NPs plays important role in the nanofiltration performance. In order to investigate the effect of the building blocks on the nanofiltration performance, nanohybrid multilayers prepared with different molecular weights (Mw) and molecular structure were also carried out. Table 2 shows that the composite membranes prepared from PDDA-ZrO2/PSS-ZrO2 with bigger Mw exhibited lower retention but higher fluxes. For example, the retention and flux of PDDA-ZrO2/PSS-ZrO2 with the PSS Mw of 70,000 were 98.9% and 109 L/m2 h MPa. For the PSS Mw of 1,000,000, the retention and flux of PDDA-ZrO2/PSS-ZrO2 were 86.7% and 314 L/m2 h Mpa. This is because the formation of nanohybrid layers on macroporous support was affected by the interaction of PE-NPs and support. The molecular weight has a remarkable influence on the level of entanglement [15]. Therefore, PE with low molecular weight is propi-
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Fig. 5. Effects of building blocks on dye retention.
Fig. 6. Effects of pore-mouth size of substrates on dye retention.
Table 2 Dye retention with membrane fabricated by different molecules of PSS. Membranes PDDA-ZrO2/PSS-ZrO2 PDDA-ZrO2/PSS-ZrO2
Molecular weight of PSS
Layer
1,000,000 70,000
2.5 2.5
Retention (%)
Flux (L/m2 h)
86.7 98.9
314 109
Table 3 Dye retention with membrane with membranes obtained from different charge concentrations (qc(PEI/PSS) = 0.714, qc(PDDA/PSS) = 0.0625). Membrane
Layer (n)
Retention (%)
Flux (L/m2 h Mpa)
PSS/PDDA PSS-ZrO2/PDDA-ZrO2 PSS/PEI PSS-ZrO2/PEI-ZrO2
2.5 2.5 2.5 2.5
90.1 98.9 90.8 99.1
184 109 214 125
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J. Li et al. / Separation and Purification Technology 183 (2017) 318–326 Table 4 Comparison of the dye removal performance in different membranes. Membrane
Substrate
Dye molecule
Retention (%)
Flux (L m2 h1 MPa1)
Pressure (MPa)
Reference
(PEI-GO)/PAA/PVA/GA ZIF-8/PSS CMCNa/PP PVDF/nanoclay/chitosan PDDA/PSS PSS-ZrO2/PDDA-ZrO2 PSS-ZrO2/PEI-ZrO2
PAN PAN PP PVDF Ceramic tube Ceramic tube Ceramic tube
Methyl Methyl Methyl Methyl Methyl Methyl Methyl
99.3 98.6 99.75 75 92 98.9 99.1
8.7 265 8.25 500 82.5 109 125
0.5 0.5 0.8 0.1 0.6 0.5 0.5
[16] [17] [18] [19] [13] This study This study
blue blue blue blue blue blue blue
GO: graphene oxide; PVA: polyvinyl alcohol GA: glutaraldehyde; PAN: polyacrylonitrile; PP: polypropylene; PVDF: poly(vinylidene fluoride); PE: Polyester.
tious to form small size PE-coated NPs, which are migrate into the pores of support. This can modify the substrate and decrease the defect of both the PDDA-ZrO2/PSS-ZrO2 boundary layer and the support membrane. A dynamic LbL self-assembly of PSS-ZrO2/PEI-ZrO2 nanohybrid multilayers were also built up on ceramic substrate to study the effect of the molecular structure on the separation properties. The effects of different polyelectrolyte pairs on the separation properties are shown in Table 3. As shown in Table 3, the incorporation of ZrO2 NPs into either PSS/PEI or PSS/PDDA composite membranes demonstrated improved separation performance than polymer composite membranes. For example, using the (PSS-ZrO2/ PEI-ZrO2)2.5/ceramic membrane, the retention and flux for MB were 99.1% and 125 L/m2 h MPa. As a control, for the (PSS/PEI)2.5/ ceramic membrane without NPs, the retention and the flux were 90.8 % and 214 L/m2 h MPa, respectively. Compared with (PSSZrO2/PEI-ZrO2)2.5/ceramic membrane, the (PDDA-ZrO2/PSSZrO2)2.5/ceramic membrane showed lower flux and retention. The retention and flux of (PDDA-ZrO2/PSS-ZrO2)2.5/ceramic membrane for MB were 98.9% and 109 L/m2 h MPa. As the charge concentration qc = 0.0714 (PSS/PEI) is higher than qc = 0.0625 (PSS/PDDA), the PSS/PEI is more effective in coating on the surface of ZrO2 NPs. That makes the sizes of PEI-coated ZrO2 NPs smaller than the PDDA-coated ZrO2 NPs [9]. Therefore, small size PSS-ZrO2/ PEI-ZrO2 was easier to go into the pores of the substrate and modify the defects. This further proved that for the macroporous substrates, in order to prepare a thin selective layer by LbL selfassembly, the modification of the supports by the building blocks is significantly important. Table 4 compares the nanofiltration performances of dye removal with various membranes reported in the literature. From Table 4, it can be found that most of the reported nanofiltration membranes use polymeric substrates. Compared with the membranes in Table 4, our ceramic tubular PSS-ZrO2/PEI-ZrO2/ceramic membrane showed excellent and comparable nanofiltration performances. Particularly, the membrane has a flux of 125 L/m2 h Mpa, which is higher than most of the reported nanofiltration membranes. And the membrane can perform very good retention of 99.1% for Methyl blue.
4. Conclusions In this study, the large defects of tubular ceramic substrates were successfully modified by dynamic self-assembly of nanohybrid multilayer membranes using building blocks of PE-coated ZrO2 NPs. The modified membranes were used for dye removal. We noted that the repairing of substrates with the PE-coated NPs was strongly dependent on the matching between the pore size of substrate and the diameter of NPs. And the repairing of substrates played important roles in the heterostructure and dye removal. The ZrO2 nanohybrid multilayers can form a dense selective layer and a sub layer to in situ repair the large defects of the pore mouth, giving much higher selectivity of the ceramic sub-
strate. In particular, the PSS-ZrO2/PEI-ZrO2 nanohybrid membrane assembled on the inside of ceramic tube had a flux of 125 L/m2 h Mpa, and a retention of 99.1% for methyl blue (0.5 Mpa), which are highly selective for dye removal. The separation performance was further optimized by studying effects of the building blocks, polyelectrolyte molecule weight, and polyelectrolyte molecule structure. Considering the ease of the manufacturability and the low cost of the fabrication, that the PE-coated NPs multilayers may be a promising candidate for in situ modifying ceramic substrate for practical applications. Acknowledgements The authors gratefully acknowledge the support of the National High Technology Research and Development Program of China (No. 2015AA03A062), National Natural Science Foundation of China (No. 21406006), and Science and Technology Program of Beijing Municipal Education Commission (KM201510005010). References [1] L. Cot, A. Ayral, J. Durand, C. Guizard, N. Hovnanian, A. Julbe, A. Larbot, Inorganic membranes and solid state sciences, Solid State Sci. 2 (2000) 313– 334. [2] Y. Lv, H.-C. Yang, H.-Q. Liang, L.-S. Wan, Z.-K. Xu, Novel nanofiltration membrane with ultrathin zirconia film as selective layer, J. Membr. Sci. 500 (2016) 265–271. [3] L. Li, C.W. Song, H.W. Jiang, J.S. Qiu, T.H. Wang, Preparation and gas separation performance of supported carbon membranes with ordered mesoporous carbon interlayer, J. Membr. Sci. 450 (2014) 469–477. [4] W. Wei, S.S. Xia, G.P. Liu, X.H. Gu, W.Q. Jin, N.P. Xu, Interfacial adhesion between polymer separation layer and ceramic support for composite membrane, AlChE J. 56 (2010) 1584–1592. [5] L. Wang, N. Wang, G. Zhang, S. Ji, Covalent crosslinked assembly of tubular ceramic-based multilayer nanofiltration membranes for dye desalination, AlChE J. 59 (2013) 3834–3842. [6] M. Samei, T. Mohammadi, A.A. Asadi, Tubular composite PVA ceramic supported membrane for bio-ethanol production, Chem. Eng. Res. Des. 91 (2013) 2703–2712. [7] Y.W. Chen, F.J. Xiangli, W.Q. Jin, N.P. Xu, Organic-inorganic composite pervaporation membranes prepared by self-assembly of polyelectrolyte multilayers on macroporous ceramic supports, J. Membr. Sci. 302 (2007) 78– 86. [8] R.J.R. Uhlhorn, M. Intveld, K. Keizer, A.J. Burggraaf, Synthesis of ceramic membranes. 1. Synthesis of non-supported and supported gamma-alumina membranes without defects, J. Mater. Sci. 27 (1992) 527–537. [9] J. Li, G. Zhang, S. Ji, N. Wang, W. An, Layer-by-layer assembled nanohybrid multilayer membranes for pervaporation dehydration of acetone–water mixtures, J. Membr. Sci. 415–416 (2012) 745–757. [10] G.J. Zhang, J. Li, S.L. Ji, Self-assembly of novel architectural nanohybrid multilayers and their selective separation of solvent-water mixtures, AlChE J. 58 (2012) 1456–1464. [11] N.X. Wang, G.J. Zhang, S.L. Ji, Y.Q. Fan, Dynamic layer-by-layer self-assembly of organic-inorganic composite hollow fiber membranes, AlChE J. 58 (2012) 3176–3182. [12] T. Wu, N.X. Wang, J. Li, L. Wang, W. Zhang, G.J. Zhang, S.L. Ji, Tubular thermal crosslinked-PEBA/ceramic membrane for aromatic/aliphatic pervaporation, J. Membr. Sci. 486 (2015) 1–9. [13] H. Tang, S. Ji, L. Gong, H. Guo, G. Zhang, Tubular ceramic-based multilayer separation membranes using spray layer-by-layer assembly, Polym. Chem. 4 (2013) 5621–5628. [14] J. Yu, X.J. Hu, Y. Huang, A modification of the bubble-point method to determine the pore-mouth size distribution of porous materials, Sep. Purif. Technol. 70 (2010) 314–319.
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
In situ repairing the large defects of macroporous ceramic membranes by polyelectrolyte-coated nanoparticles Jie Li ⇑, Xinguo Si, Naixin Wang, Shulan Ji ⇑ Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China
a r t i c l e
i n f o
Article history: Received 16 January 2017 Received in revised form 31 March 2017 Accepted 1 April 2017 Available online 5 April 2017 Keywords: Nanohybrid multilayer membrane Nanofiltration Ceramic membranes Large defects Dynamic layer-by-layer assembly
a b s t r a c t Large defects have important impact on performance of separation membranes. In this paper, in order to reduce the large defects of the industrial ceramic supports, we prepared a nanohybrid separation membrane with a selective layer and a sub layer on the macroporous substrate with polyelectrolyte-coated nanoparticles. SEM, EDS and pore size analyses suggest that the although ZrO2 nanoparticles can migrate into the large defects to modify the membrane pores, the matching of pore-mouth size and diameter of NPs plays important role in the in situ modification of the substrates. The integrality of the as-prepared nanohybrid multilayer membrane was evaluated in the nanofiltration of dye solutions. The organicinorganic composite membranes perform high flux and retention for separation. Membranes were prepared to study the separation performance on different substrates and with different building blocks (polyelectrolyte molecule weights and polyelectrolyte molecule structure). We found that the poremouth size of substrates and the structure of building blocks mainly affected the structure and the nanofiltration performance of the composite membranes. Such assembly allow us to in situ modify the large defects of inorganic substrates while preparing the selective layer. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, organic–inorganic composite membranes has drawn more and more attention by providing a promising access to combine the superiorities of polymer membranes and inorganic membranes. However, developing organic–inorganic composite membranes with ideal structures and high performance remains a major challenge to date [1]. Modern organic–inorganic composite membranes usually are film composite membranes, fabricated by an inorganic support and a thin polymer selective layer. Though the performance of thin film composite membranes is improved compared with the traditional polymeric or ceramic membranes, reduction of the thickness selective layer is difficult because of the properties of the porous substrate [2]. The pore diameter of support should be smaller than the thickness of separation layer to make the membrane defect free. Therefore, the pore structure of the support plays an important role in the formation of a uniform and defect-free thin separation layer [3,4]. However, the industrial ceramic supports are usually with macro pores and large defects. Thus, a relatively thick selective layer has to be prepared on such supports to make the membranes defect free, which
⇑ Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (S. Ji). http://dx.doi.org/10.1016/j.seppur.2017.04.001 1383-5866/Ó 2017 Elsevier B.V. All rights reserved.
greatly reduce the flux [5,6]. As a result, the low flux of organicinorganic composite membranes obtained on industrial ceramic supports can hardly satisfy practical separation requirements. In contrast, a defect-free thin selective layer on a support with small or nano pore size is easy to build up, along with high flow resistance, low flux and higher separation factor. Therefore the ceramic support with uniform and proper pore size distribution is critical in order to achieve a balance between flux and separation factor. In order to prepare ceramic supports with proper pore size distribution, a sol-gel modification usually has to be used for the treatment of the supports [7]. By the sol–gel modification, a silica sol-gel layer can be formed on the macroporous support surface. In order to repair the possible defects of the previous sol-gel layer, the repeating of sol-gel modification was used, which at the same time reduced small pores and the mean pore size of the membrane. Then the water permeability of the modified support reduces sharply after modification (reduction by 90.3%) [8]. Moreover, repeating of heating treatment (500–800 °C) has to be used for the solgel. Therefore, we need to find an alternative method to the solgel to reduce only the large defects or the surface pore size (the pore-mouth size), with ease fabrication and low cost. Recently, a high loading nanohybrid membrane was developed with a layer-by-layer (LbL) self-assembly method, which incorporated single component NPs into both polyanion and polycation
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layers [9,10]. However, there are few reports on the preparation of nanohybrid membrane for the in situ modification of the porous substrates. In this paper, we reported on the organic-inorganic membranes that were prepared by means of dynamic LbL selfassembly of polyelectrolyte (PE)-coated ZrO2 nanocomposites. The ZrO2 NPs can migrate into the porous ceramic substrate to form a sub layer, which can in situ modify the large membrane defects and at the same time make the selective layer defect free. The integrality of the membranes were tested with the nanofiltration (NF) of dyes. Fig. 1 illustrates the procedure and the necessity of the process with the dynamic LbL self-assembly of PE-coated ZrO2 NPs. As shown in Fig. 1(a), self-assembly of the PE thin layer on macroporous supports without NPs would be incomplete since the PE molecular chain will infiltrate into the large pores but not covering them. By incorporating the NPs, a defect free selective layer could be achieved because the spherical NPs could repair the defects of the supports (Fig. 1(b)). And the building blocks could form a sub layer to in situ repair large pores (Fig. 1(b)). The nanohybrid sub layer was prepared for both the separation and the support of ceramic membrane, which could tune the pore size of the substrate and at the same time make the selective layer defect free. Moreover, the adsorption of polycation and polyanion with LbL assembly could joint the NPs and the substrate, forming the selective layers for the substrate in relatively low temperature. We found that the membranes showed a high performance in NF because of its thin top layer, sub layer and macroporous support layer composite structure. 2. Experiment 2.1. Materials Tubular ceramic membranes (internal diameter = 8 mm; outside diameter = 12 mm) were purchased from Foshan Science and Technology Factory. 3-Aminopropyl-trimethoxysilane (APTES, Mw is 179.29), branched poly(ethyleneimine) (PEI, Mw 60,000) were purchased from ACROS. Poly(sodium styrene sulfonate)
319
(PSS, Mw 70,000 and 1,000,000), poly(diallyldimethyl ammonium chloride)(PDDA, Mw 100,000–200,000) and ZrO2 NPs (<100 nm) were purchased from Aldrich. Methanol and methyl blue (MB) were provided by Beijing Chemical Factory. 2.2. Preparation of PE-coated NPs The PE-coated NPs were prepared by the ZrO2 NPs coating with an outer PE layer. To obtain PDDA-coated ZrO2 NPs, ZrO2 NPs (0.4 g) were added to PDDA solution (0.94 wt%). The pH value was adjusted to 12.0. The preliminary PDDA-coated ZrO2 NPs were sonicated for 30 min and then centrifuged for 10 min at 10,000 rpm. Excess free PDDA chains from the surface of the NPs were replaced with water. This sonication and centrifugation procedure was repeated three times to form stable PDDA-coated ZrO2 NPs suspension. The PSS-coated ZrO2 NPs, PEI-coated ZrO2 NPs were prepared of following the same process. The pH values were adjusted to 6.0 and 10.0 respectively for PSS-coated ZrO2 and PEIcoated ZrO2 solutions. 2.3. Preparation of nanohybrid multilayers on tubular ceramic substrate membranes The tubular ceramic substrate was pre-treated with 2 g/L APTES in the mixture of ethanol solution (EtOH:H2O = 95:5) for 2 h at room temperature as reported previously [11,12]. The modified substrate was washed with deionized water and then dehydrated at 110 °C for 2 h in a vacuum oven. The pre-treated membranes were loaded into a 20 mm diameter tubular module made by polymethylmethacrylate. Both ends of the module were sealed with Teflon tube. The dynamic LbL self-assembly experiments were carried out using a cross-flow negative pressure filtration cell fabricated by our laboratory (Fig. 2) [9]. As shown in Fig. 2, the polycation-coated ZrO2 and polyanioncoated ZrO2 solutions were alternatively pumped into the inner channel of ceramic tube through separate tubing. On the shell side, in order to pull the PE-coated ZrO2 into the large pores or defects of
Fig. 1. Schematic diagram describing the modification of the inorganic substrates with nanoparticles.
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Fig. 2. Apparatus used for modification of the inorganic substrates with nanoparticles.
the substrate membranes, a water circulated vacuum pump was used to form a 0.09 MPa negative pressure. The assembly time was 30 min, and then rinsed with ultrapure water for 20 min and dried for 60 min at 40 °C. In order to study the matching between the diameter of NPs and the pore-mouth size of substrates, nanohybrid multilayers were built up on ceramic tube with small pore mouth size (Substrate-1) and large pore-mouth size (Substrate-2). 2.4. Dye removal experiment Dye removal experiment was carried out using a laboratory made cross-flow nanofiltration system [13]. The NF membrane was put into the membrane cell for nanofiltration and the effective area of the membrane was 20 cm2. The dye concentration of the feed solution is 100 mg/L. The dye solution was pressurized using a plunger pump at a pressure of 0.5 MPa. During the nanofiltration process, the concentrate was recirculated to the feed tank while permeate was collected in permeate vessel. The permeation flux J (L/m2 h MPa) was determined according to the following equation:
J¼
W AtDP
ð1Þ
where W (L) is the amount of permeate, DP (MPa) is the operation pressure, t (h) is a certain time for the nanofiltration, and A (m2) is the effective area of the membrane. And the solute retention rate R was calculated by:
Cp R ð%Þ ¼ 100 1 Cf
ð2Þ
where the Cp and Cf are the dye concentration in the permeate and feed solutions, respectively. The dye concentrations were analyzed by an ultraviolet–visible spectrophotometer (UV2800, Shanghai). 2.5. Characterization Organic-inorganic membranes were examined under a Hitachi S-4300 scanning electron microscope (SEM) attached to an energy-dispersive spectrometer (EDS). All samples were coated with gold in vacuum to increase their conductivity before observations. In order to obtain the pore size of organic-inorganic membranes, characterization was performed by mean pore size analyzer. Pore-mouth size (surface pore size) distribution of the ceramic tube before and after dynamic LbL assembly were mea-
sured using a GaoQ PSDA-20 porometer based on a modified bubble-point method [14].
3. Results and discussion Nanohybrid multilayers were built up on ceramic tubes with different pore-mouth size using the PE-coated NPs. The SEM photos of the surface and near-surface cross-section of membranes are shown in Fig. 3. Before dynamic LbL self-assembly, the surface of ceramic supports (substrate-1) is rough and uneven, with large pores and big particles (Fig. 3(a)). From the image of the membrane after self-assembly of PSS/PDDA with 2.5 layers, a more even plane is obtained although there are still some defects (Fig. 3(c)). For the (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane, the ceramic membrane surface is covered by a dense layer with the nanocomposites (Fig. 3(e)). In addition, ZrO2 particles can be found inside the pores of the ceramic substrate to form a sub layer (Fig. 3(f)). The above differences between PSS/PDDA and PSS-ZrO2/PDDA-ZrO2 are a result of the ZrO2 incorporation. On one hand, the dynamic LbL self-assembly of PSS-ZrO2/PDDA-ZrO2 led to an intrusion of zirconium into macroporous ceramic support to form a sub layer to repair the support. One the other hand, the nanocomposites could result in the formation of Zr-based nanohybrid selective layers to make the membrane defect free. As a comparison, nanohybrid multilayers were formed on a ceramic tube with larger poremouth size (Substrate-2). Although there is huge difference on the morphology of substrate-2, similar phenomena can be observed for these composite membranes (Fig. 3(g)–(l)). Fig. 3 (k) and (l) showed that the PE-coated NPs layers had been successfully built up on the ceramic substrates to in situ modify the large defects of substrate-2. And from Fig. 3 (l), a sub layer can be seen significantly after modification. In order to confirm the PE-coated nanoparticles can modify the large pores of the substrate, EDS measurements were performed. The changes of zirconium element composition through the cross-section were analyzed. As shown in Fig. 4(a) and (b), for the PSS-ZrO2/PDDA-ZrO2 membrane with substrate-1, the concentration of zirconium decreased while the aluminum increased before the 0.5 lm point. Since the ceramic substrate does not contain zirconium while the nanohybrid membrane does not contain aluminum, all the zirconium must arise from the selective PSSZrO2/PDDA-ZrO2 layer and aluminum from the supporting membrane. These observations led us to conclude that the thickness of the nanohybrid layers on substrate-1 is far less than 0.5 lm. As a comparison, for the PSS-ZrO2/PDDA-ZrO2 membrane with substrate-2 (Fig. 4(c) and (d)), the concentration of zirconium
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Fig. 3. SEM images of (a) inner surface of ceramic substrate-1, (b) cross session of ceramic substrate-1, (c) inner surface of (PSS/PDDA)2.5/ceramic membrane by substrate-1, (d) cross session of (PSS/PDDA)2.5/ceramic membrane by substrate-1, (e) inner surface of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane by substrate-1, (f) cross session of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane by substrate-1, (g) inner surface of ceramic substrate-2, (h) cross session of ceramic substrate-2, (i) inner surface of (PSS/ PDDA)2.5/ceramic membrane by substrate-2, (j) cross session of (PSS/PDDA)2.5/ceramic membrane by substrate-2, (k) inner surface of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane by substrate-2, (l) cross session of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane by substrate-2.
decreased while the aluminum increased before the 3 lm point. These observations led us to conclude that the thickness of the nanohybrid layers on substrate-2 is far less than 3 lm. From this analysis, we deduce that ZrO2 migrate into the ceramic substrate-2 pores more deeply because more zirconium was found there. Therefore the ZrO2 NPs are more likely to intruded into the substrate-2 pores while the ZrO2 NPs are mostly distributed near
the substrate-1 surface. This clearly proved that the NPs were successfully loaded into the pores especially the large defects of the membrane when the defects are larger than the sizes of the PEcoated ZrO2 NPs. For the pores that are less than the PE-coated ZrO2 NPs, NPs are mostly distributed near the substrate surface. To confirm the expectations of self-repairing by the incorporation of NPs, The pore-mouth size distribution of the ceramic tube
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Fig. 3 (continued)
before and after dynamic LbL assembly were performed in Table 1. As shown in Table 1, the largest and mean pore-mouth sizes of the ceramic substrate-1 were 2.5 lm and 1.1 lm, respectively. The max pore-mouth size of the (PSS/PDDA)2.5/ceramic membrane was 2.4 lm and the (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane was 1.4 lm, which reduces significantly. That means the large pores or defects decreased significantly after LbL modification (from 2.5 lm to 1.4 lm). However, the mean pore-mouth sizes only decreased from 1.0 lm to 0.9 lm. Therefore the dynamic LbL self-assembly will form a thin layer on the macroporous support surface. This behavior has the main function of the selfrepairing the possible defects of the macroporous substrate, rather than infinitely reduce small pore mouth size or the mean pore size of the membranes. Therefore, an asymmetrical structure was formed, which will be effective for improvement of membrane separation performance. As a comparison, ceramic substrate-2 with larger defects (dmax = 2.8 lm) was used for preparation of nanohybrid multilayers. Either mean pore-mouth size (dmean) or max pore-mouth size (dmax) of substrate-2 decreased after self-assembly. However, there is a difference for nanohybrid membranes on substrate-2. The mean pore-mouth size of substrate-2 decreased more significantly than substrate-1 while the max pore-mouth size decreased less than substrate-1. For example, the max pore-mouth size (dmax) of the substrate-1 decreased from 2.5 lm to 1.4 lm while that of substrate-2 decreased from 2.8 lm to 2.28 lm after PE coated NPs modification. And for mean pore-mouth size (dmean), substrate-1 decreased from 1.1 lm to 0.9 lm while that of substrate-2 decreased from 1.2 lm to 0.78 lm. The reason for this phenomenon is more ZrO2 distributed into the pores of the substrate-2 under dynamic pressure (Fig. 4) since the size of the ZrO2 (<100 nm) is much smaller than the substrate-2 poremouth size (dmax = 2.8 lm). This indicates that although most of ZrO2 NPs were able to repair the large defects of substrate, the matching of pore-mouth size and diameter of NPs plays important role in the in situ modification of the substrates.
The separation performance of the (PSS-ZrO2/PDDA-ZrO2)2.5/ ceramic membrane for nanofiltration was evaluated and shown in Fig. 5. The incorporation of ZrO2 NPs produced much higher retention in the dyes removal. The retention and flux of (PSSZrO2/PDDA-ZrO2)2.5/ceramic membrane towards nanofiltration of methyl blue (MB) can reach 98.9% and 109 L/m2 h MPa. As a control, using the (PSS/PDDA)2.5/ceramic membranes without NPs, the retention and flux for MB were 90.1% and 184 L/m2 h MPa. Better retention of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membranes can be attributed to the incorporation of NPs, which self-repaired the possible defects of the pore mouth. The max pore-mouth size decreased significantly from 2.4 lm to 1.4 lm (Table 1). Additionally, PSS as anionic polyelectrolyte has the same charge property with methyl blue. The methyl blue molecule can also be rejected through the charge repulsion. The (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane produced much higher retention in the dyes removal due to their unique structure. Furthermore, PSS/PDDA-ZrO2 and PSS-ZrO2/PDDA membranes were prepared as comparison to confirm the loading of NPs on the separation properties. It could be found that by tuning the charge properties, either polyanion or polycation layers could be incorporated with the ZrO2 NPs though the loading was limited [10]. Therefore, the retention of this kind of membranes was not as high as the PDDA-ZrO2/PSS-ZrO2 membranes. These results further suggest that when using dynamic LbL self-assembly of PEcoated NPs, the loading of the NPs plays an important role in the repairment of the NPs to the membrane pore mouth and in obtaining better structured organic-inorganic membranes. The high loading also can efficiently simplify the procedures by reducing process cycles of LbL. Moreover, the asymmetrical structure formed by PDDA-ZrO2/PSS-ZrO2 membrane (Fig. 3(f)) is extremely effective in reducing the mass transfer resistance and achieving higher separation performance. The effects of ceramic tube pore-mouth size on the separation performance of nanohybrid membranes were shown in Fig. 6. As shown in Fig. 6, after the incorporation of ZrO2, the retention and
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Fig. 4. Cross-sectional EDS analyses of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane. (a) Changes of zirconium element composition through the cross-section of (PSS-ZrO2/ PDDA-ZrO2)2.5/ceramic membrane with substrate-1, (b) changes of aluminum element composition through the cross-section of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane with substrate-1, (c) changes of zirconium element composition through the cross-section of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane with substrate-2, (d) changes of aluminum element composition through the cross-section of (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane with substrate-2.
Table 1 Pore-mouth size of the various membranes. Substrate
Sample
Pore-mouth size (lm) dmax
dmean
Substrate-1
Ceramic membrane (PSS/PDDA)2.5/ceramic membrane (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane
2.5 2.4 1.4
1.1 1.0 0.9
Substrate-2
Ceramic membrane (PSS/PDDA)2.5/ceramic membrane (PSS-ZrO2/PDDA-ZrO2)2.5/ceramic membrane
2.8 2.4 2.28
1.2 0.8 0.78
flux of composite membrane with substrate-2 increased from 70.5% to 87.8%, while the flux decreased from 121 L/m2 h MPa to 84 L/m2 h MPa. As a control, after the incorporation of ZrO2 using the composite membranes with substrate-1, the retention and flux of nanohybrid membrane increased from 90.1% to 98.9%, while the flux decreased from 184 L/m2 h MPa to 109 L/m2 h MPa. The difference of can be attributed to the matching of pore-mouth size and diameter of NPs. As the SEM-EDS and pore-mouth size analyses
suggest that the ZrO2 NPs are mostly loaded on the near surface of substrate-1 while the NPs are more likely to migrate into the pores of substrate-2 membrane. Thus the NPs were able to modify most of the defects in substrate-1 to get a high retention of 98.9%. But for substrate-2, the NPs could only migrate into the large pores of the membrane to lead to a relatively low flux. Therefore the matching of pore-mouth size and diameter of NPs plays important role in the nanofiltration performance. In order to investigate the effect of the building blocks on the nanofiltration performance, nanohybrid multilayers prepared with different molecular weights (Mw) and molecular structure were also carried out. Table 2 shows that the composite membranes prepared from PDDA-ZrO2/PSS-ZrO2 with bigger Mw exhibited lower retention but higher fluxes. For example, the retention and flux of PDDA-ZrO2/PSS-ZrO2 with the PSS Mw of 70,000 were 98.9% and 109 L/m2 h MPa. For the PSS Mw of 1,000,000, the retention and flux of PDDA-ZrO2/PSS-ZrO2 were 86.7% and 314 L/m2 h Mpa. This is because the formation of nanohybrid layers on macroporous support was affected by the interaction of PE-NPs and support. The molecular weight has a remarkable influence on the level of entanglement [15]. Therefore, PE with low molecular weight is propi-
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Fig. 5. Effects of building blocks on dye retention.
Fig. 6. Effects of pore-mouth size of substrates on dye retention.
Table 2 Dye retention with membrane fabricated by different molecules of PSS. Membranes PDDA-ZrO2/PSS-ZrO2 PDDA-ZrO2/PSS-ZrO2
Molecular weight of PSS
Layer
1,000,000 70,000
2.5 2.5
Retention (%)
Flux (L/m2 h)
86.7 98.9
314 109
Table 3 Dye retention with membrane with membranes obtained from different charge concentrations (qc(PEI/PSS) = 0.714, qc(PDDA/PSS) = 0.0625). Membrane
Layer (n)
Retention (%)
Flux (L/m2 h Mpa)
PSS/PDDA PSS-ZrO2/PDDA-ZrO2 PSS/PEI PSS-ZrO2/PEI-ZrO2
2.5 2.5 2.5 2.5
90.1 98.9 90.8 99.1
184 109 214 125
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J. Li et al. / Separation and Purification Technology 183 (2017) 318–326 Table 4 Comparison of the dye removal performance in different membranes. Membrane
Substrate
Dye molecule
Retention (%)
Flux (L m2 h1 MPa1)
Pressure (MPa)
Reference
(PEI-GO)/PAA/PVA/GA ZIF-8/PSS CMCNa/PP PVDF/nanoclay/chitosan PDDA/PSS PSS-ZrO2/PDDA-ZrO2 PSS-ZrO2/PEI-ZrO2
PAN PAN PP PVDF Ceramic tube Ceramic tube Ceramic tube
Methyl Methyl Methyl Methyl Methyl Methyl Methyl
99.3 98.6 99.75 75 92 98.9 99.1
8.7 265 8.25 500 82.5 109 125
0.5 0.5 0.8 0.1 0.6 0.5 0.5
[16] [17] [18] [19] [13] This study This study
blue blue blue blue blue blue blue
GO: graphene oxide; PVA: polyvinyl alcohol GA: glutaraldehyde; PAN: polyacrylonitrile; PP: polypropylene; PVDF: poly(vinylidene fluoride); PE: Polyester.
tious to form small size PE-coated NPs, which are migrate into the pores of support. This can modify the substrate and decrease the defect of both the PDDA-ZrO2/PSS-ZrO2 boundary layer and the support membrane. A dynamic LbL self-assembly of PSS-ZrO2/PEI-ZrO2 nanohybrid multilayers were also built up on ceramic substrate to study the effect of the molecular structure on the separation properties. The effects of different polyelectrolyte pairs on the separation properties are shown in Table 3. As shown in Table 3, the incorporation of ZrO2 NPs into either PSS/PEI or PSS/PDDA composite membranes demonstrated improved separation performance than polymer composite membranes. For example, using the (PSS-ZrO2/ PEI-ZrO2)2.5/ceramic membrane, the retention and flux for MB were 99.1% and 125 L/m2 h MPa. As a control, for the (PSS/PEI)2.5/ ceramic membrane without NPs, the retention and the flux were 90.8 % and 214 L/m2 h MPa, respectively. Compared with (PSSZrO2/PEI-ZrO2)2.5/ceramic membrane, the (PDDA-ZrO2/PSSZrO2)2.5/ceramic membrane showed lower flux and retention. The retention and flux of (PDDA-ZrO2/PSS-ZrO2)2.5/ceramic membrane for MB were 98.9% and 109 L/m2 h MPa. As the charge concentration qc = 0.0714 (PSS/PEI) is higher than qc = 0.0625 (PSS/PDDA), the PSS/PEI is more effective in coating on the surface of ZrO2 NPs. That makes the sizes of PEI-coated ZrO2 NPs smaller than the PDDA-coated ZrO2 NPs [9]. Therefore, small size PSS-ZrO2/ PEI-ZrO2 was easier to go into the pores of the substrate and modify the defects. This further proved that for the macroporous substrates, in order to prepare a thin selective layer by LbL selfassembly, the modification of the supports by the building blocks is significantly important. Table 4 compares the nanofiltration performances of dye removal with various membranes reported in the literature. From Table 4, it can be found that most of the reported nanofiltration membranes use polymeric substrates. Compared with the membranes in Table 4, our ceramic tubular PSS-ZrO2/PEI-ZrO2/ceramic membrane showed excellent and comparable nanofiltration performances. Particularly, the membrane has a flux of 125 L/m2 h Mpa, which is higher than most of the reported nanofiltration membranes. And the membrane can perform very good retention of 99.1% for Methyl blue.
4. Conclusions In this study, the large defects of tubular ceramic substrates were successfully modified by dynamic self-assembly of nanohybrid multilayer membranes using building blocks of PE-coated ZrO2 NPs. The modified membranes were used for dye removal. We noted that the repairing of substrates with the PE-coated NPs was strongly dependent on the matching between the pore size of substrate and the diameter of NPs. And the repairing of substrates played important roles in the heterostructure and dye removal. The ZrO2 nanohybrid multilayers can form a dense selective layer and a sub layer to in situ repair the large defects of the pore mouth, giving much higher selectivity of the ceramic sub-
strate. In particular, the PSS-ZrO2/PEI-ZrO2 nanohybrid membrane assembled on the inside of ceramic tube had a flux of 125 L/m2 h Mpa, and a retention of 99.1% for methyl blue (0.5 Mpa), which are highly selective for dye removal. The separation performance was further optimized by studying effects of the building blocks, polyelectrolyte molecule weight, and polyelectrolyte molecule structure. Considering the ease of the manufacturability and the low cost of the fabrication, that the PE-coated NPs multilayers may be a promising candidate for in situ modifying ceramic substrate for practical applications. Acknowledgements The authors gratefully acknowledge the support of the National High Technology Research and Development Program of China (No. 2015AA03A062), National Natural Science Foundation of China (No. 21406006), and Science and Technology Program of Beijing Municipal Education Commission (KM201510005010). References [1] L. Cot, A. Ayral, J. Durand, C. Guizard, N. Hovnanian, A. Julbe, A. Larbot, Inorganic membranes and solid state sciences, Solid State Sci. 2 (2000) 313– 334. [2] Y. Lv, H.-C. Yang, H.-Q. Liang, L.-S. Wan, Z.-K. Xu, Novel nanofiltration membrane with ultrathin zirconia film as selective layer, J. Membr. Sci. 500 (2016) 265–271. [3] L. Li, C.W. Song, H.W. Jiang, J.S. Qiu, T.H. Wang, Preparation and gas separation performance of supported carbon membranes with ordered mesoporous carbon interlayer, J. Membr. Sci. 450 (2014) 469–477. [4] W. Wei, S.S. Xia, G.P. Liu, X.H. Gu, W.Q. Jin, N.P. Xu, Interfacial adhesion between polymer separation layer and ceramic support for composite membrane, AlChE J. 56 (2010) 1584–1592. [5] L. Wang, N. Wang, G. Zhang, S. Ji, Covalent crosslinked assembly of tubular ceramic-based multilayer nanofiltration membranes for dye desalination, AlChE J. 59 (2013) 3834–3842. [6] M. Samei, T. Mohammadi, A.A. Asadi, Tubular composite PVA ceramic supported membrane for bio-ethanol production, Chem. Eng. Res. Des. 91 (2013) 2703–2712. [7] Y.W. Chen, F.J. Xiangli, W.Q. Jin, N.P. Xu, Organic-inorganic composite pervaporation membranes prepared by self-assembly of polyelectrolyte multilayers on macroporous ceramic supports, J. Membr. Sci. 302 (2007) 78– 86. [8] R.J.R. Uhlhorn, M. Intveld, K. Keizer, A.J. Burggraaf, Synthesis of ceramic membranes. 1. Synthesis of non-supported and supported gamma-alumina membranes without defects, J. Mater. Sci. 27 (1992) 527–537. [9] J. Li, G. Zhang, S. Ji, N. Wang, W. An, Layer-by-layer assembled nanohybrid multilayer membranes for pervaporation dehydration of acetone–water mixtures, J. Membr. Sci. 415–416 (2012) 745–757. [10] G.J. Zhang, J. Li, S.L. Ji, Self-assembly of novel architectural nanohybrid multilayers and their selective separation of solvent-water mixtures, AlChE J. 58 (2012) 1456–1464. [11] N.X. Wang, G.J. Zhang, S.L. Ji, Y.Q. Fan, Dynamic layer-by-layer self-assembly of organic-inorganic composite hollow fiber membranes, AlChE J. 58 (2012) 3176–3182. [12] T. Wu, N.X. Wang, J. Li, L. Wang, W. Zhang, G.J. Zhang, S.L. Ji, Tubular thermal crosslinked-PEBA/ceramic membrane for aromatic/aliphatic pervaporation, J. Membr. Sci. 486 (2015) 1–9. [13] H. Tang, S. Ji, L. Gong, H. Guo, G. Zhang, Tubular ceramic-based multilayer separation membranes using spray layer-by-layer assembly, Polym. Chem. 4 (2013) 5621–5628. [14] J. Yu, X.J. Hu, Y. Huang, A modification of the bubble-point method to determine the pore-mouth size distribution of porous materials, Sep. Purif. Technol. 70 (2010) 314–319.
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