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Preparation and characterization of thin-film composite membrane with nanowire-modified support for forward osmosis process.

Low ZX, Liu Q, Shamsaei E, Zhang X, Wang H - Membranes (Basel) (2015)

Bottom Line: In FO mode, the ratio of water flux to reverse solute flux was also improved.The result shows that Zn2GO4 nanowire may be used as a modifier to the substrate to improve the quality of the polyamide layer on the substrate to improve the flux and selectivity, but not as effective in reducing ICP.This work demonstrates that the incorporation of nanomaterials to the membrane substrate may be an alternative approach to improve the formation of polyamide skin layer to achieve better FO performance.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical Engineering, Monash University, Clayton VIC 3800, Australia.

ABSTRACT
Internal concentration polarization (ICP) in forward osmosis (FO) process is a characteristic problem for asymmetric thin-film composite (TFC) FO membrane which leads to lower water flux. To mitigate the ICP effect, modification of the substrates' properties has been one of the most effective methods. A new polyethersulfone-based ultrafiltration membrane with increased surface porosity and high water flux was recently produced by incorporating Zn2GeO4 nanowires. The composite membrane was used as a substrate for the fabrication of TFC FO membrane, by coating a thin layer of polyamide on top of the substrate. The substrate and the nanowires were characterized by a range of techniques such as SEM, XRD, and contact angle goniometry. The water permeability and molecular weight cut-offs (MWCO) of the substrate; and the FO performance of the TFC membrane were also determined. The Zn2GeO4-modified membrane showed ~45% increase in water permeability and NaCl salt rejection of 80% under RO mode. In FO mode, the ratio of water flux to reverse solute flux was also improved. However, lower FO flux was obtained which could be due to ICP. The result shows that Zn2GO4 nanowire may be used as a modifier to the substrate to improve the quality of the polyamide layer on the substrate to improve the flux and selectivity, but not as effective in reducing ICP. This work demonstrates that the incorporation of nanomaterials to the membrane substrate may be an alternative approach to improve the formation of polyamide skin layer to achieve better FO performance.

No MeSH data available.


Related in: MedlinePlus

(a) SEM images of the TFC FO membrane active layer; (b) Zn2GeO4/PES; (c) PES membrane cross section. The highlighted section indicates the thick pore wall.
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membranes-05-00136-f005: (a) SEM images of the TFC FO membrane active layer; (b) Zn2GeO4/PES; (c) PES membrane cross section. The highlighted section indicates the thick pore wall.

Mentions: The FO performance was determined by using 0.5 M, 1.0 M and 2.0 M NaCl solution as the draw solution, and DDI water as the feed solution. In FO mode (Figure 4), the modified membrane shows lower water flux and lower reverse solute flux, i.e., higher salt rejection, despite higher water permeability and salt rejection in RO mode. This indicates that incorporation of Zn2GeO4 nanowires on the membrane substrate does not effectively reduce the ICP as intended. On the contrary, the opposite result was obtained as shown in Figure 4a, where a non-linear relationship between water flux and NaCl concentration is obtained. This might be due to the thicker pore wall within the membrane matrix, which leads to higher tortuosity. Figure 5 shows the typical “ridge-and-valley” morphology of the polyamide layer and the cross-sectional view of the modified membrane. The increased ICP could be partly caused by the thick pore walls near the bottom matrix of the modified membrane (Figure 5b), which may also increase the tortuosity of the membrane. For the control membrane (Figure 5c), pore walls at the bottom matrix appear thinner and the widths of the finger-like pores are smaller. The results are also consistent with the structural parameter calculated from osmotic flux tests, where the S value of the modified membrane increased from 352 to 540 µm. The corresponding τ of the modified membrane is 6.58, as compared to 3.56 of the control membrane (Table 2). Despite lower FO flux, the overall ratio of the water flux to solute flux (Jw/Js) is improved (Table 2). This might be due to the changes in the surface physical and chemical property which improve the interfacial polymerization of the polyamide layer leading to the formation of the polyamide layer with higher permeability and salt rejection. It is widely established that the properties of the support layer such as hydrophilicity has an influence on the properties of the polyamide layer and the overall separation performance [33,34,35,36]. The Zn2GeO4 nanowires showed a contact angle of 30.6° (average of three measurements), indicating hydrophilic surface of the nanowires (Figure 1c). The modified membranes which showed slight improvement over surface hydrophilicity and surface porosity may facilitate MPD monomers adsorption within the porous substrate. The monomers eventually diffuse out from the pores and react with acid chlorides (TMC). Higher adsorption of MPD monomers may produce a more compact polyamide layer. On the other hand, the hydrophilic Zn2GeO4 nanowires may interact with MPD monomers, which may reduce the diffusion rate of MPD monomer during the interfacial polymerization. Since interfacial polymerization between MPD and TMC occurs predominantly in the organic phase, the slow diffusion may improve the stability of the polyamide layer on the support layer [34]. The existence of the nanowires may also affect the degree of crosslinking of the polyamide layer. The changes in the substrate properties and chemical interactions by incorporating Zn2GeO4 nanowires may be the reason to the formation of the higher quality of polyamide layer which increases the Jw/Js ratio of the TFC FO membrane.


Preparation and characterization of thin-film composite membrane with nanowire-modified support for forward osmosis process.

Low ZX, Liu Q, Shamsaei E, Zhang X, Wang H - Membranes (Basel) (2015)

(a) SEM images of the TFC FO membrane active layer; (b) Zn2GeO4/PES; (c) PES membrane cross section. The highlighted section indicates the thick pore wall.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4384095&req=5

membranes-05-00136-f005: (a) SEM images of the TFC FO membrane active layer; (b) Zn2GeO4/PES; (c) PES membrane cross section. The highlighted section indicates the thick pore wall.
Mentions: The FO performance was determined by using 0.5 M, 1.0 M and 2.0 M NaCl solution as the draw solution, and DDI water as the feed solution. In FO mode (Figure 4), the modified membrane shows lower water flux and lower reverse solute flux, i.e., higher salt rejection, despite higher water permeability and salt rejection in RO mode. This indicates that incorporation of Zn2GeO4 nanowires on the membrane substrate does not effectively reduce the ICP as intended. On the contrary, the opposite result was obtained as shown in Figure 4a, where a non-linear relationship between water flux and NaCl concentration is obtained. This might be due to the thicker pore wall within the membrane matrix, which leads to higher tortuosity. Figure 5 shows the typical “ridge-and-valley” morphology of the polyamide layer and the cross-sectional view of the modified membrane. The increased ICP could be partly caused by the thick pore walls near the bottom matrix of the modified membrane (Figure 5b), which may also increase the tortuosity of the membrane. For the control membrane (Figure 5c), pore walls at the bottom matrix appear thinner and the widths of the finger-like pores are smaller. The results are also consistent with the structural parameter calculated from osmotic flux tests, where the S value of the modified membrane increased from 352 to 540 µm. The corresponding τ of the modified membrane is 6.58, as compared to 3.56 of the control membrane (Table 2). Despite lower FO flux, the overall ratio of the water flux to solute flux (Jw/Js) is improved (Table 2). This might be due to the changes in the surface physical and chemical property which improve the interfacial polymerization of the polyamide layer leading to the formation of the polyamide layer with higher permeability and salt rejection. It is widely established that the properties of the support layer such as hydrophilicity has an influence on the properties of the polyamide layer and the overall separation performance [33,34,35,36]. The Zn2GeO4 nanowires showed a contact angle of 30.6° (average of three measurements), indicating hydrophilic surface of the nanowires (Figure 1c). The modified membranes which showed slight improvement over surface hydrophilicity and surface porosity may facilitate MPD monomers adsorption within the porous substrate. The monomers eventually diffuse out from the pores and react with acid chlorides (TMC). Higher adsorption of MPD monomers may produce a more compact polyamide layer. On the other hand, the hydrophilic Zn2GeO4 nanowires may interact with MPD monomers, which may reduce the diffusion rate of MPD monomer during the interfacial polymerization. Since interfacial polymerization between MPD and TMC occurs predominantly in the organic phase, the slow diffusion may improve the stability of the polyamide layer on the support layer [34]. The existence of the nanowires may also affect the degree of crosslinking of the polyamide layer. The changes in the substrate properties and chemical interactions by incorporating Zn2GeO4 nanowires may be the reason to the formation of the higher quality of polyamide layer which increases the Jw/Js ratio of the TFC FO membrane.

Bottom Line: In FO mode, the ratio of water flux to reverse solute flux was also improved.The result shows that Zn2GO4 nanowire may be used as a modifier to the substrate to improve the quality of the polyamide layer on the substrate to improve the flux and selectivity, but not as effective in reducing ICP.This work demonstrates that the incorporation of nanomaterials to the membrane substrate may be an alternative approach to improve the formation of polyamide skin layer to achieve better FO performance.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical Engineering, Monash University, Clayton VIC 3800, Australia.

ABSTRACT
Internal concentration polarization (ICP) in forward osmosis (FO) process is a characteristic problem for asymmetric thin-film composite (TFC) FO membrane which leads to lower water flux. To mitigate the ICP effect, modification of the substrates' properties has been one of the most effective methods. A new polyethersulfone-based ultrafiltration membrane with increased surface porosity and high water flux was recently produced by incorporating Zn2GeO4 nanowires. The composite membrane was used as a substrate for the fabrication of TFC FO membrane, by coating a thin layer of polyamide on top of the substrate. The substrate and the nanowires were characterized by a range of techniques such as SEM, XRD, and contact angle goniometry. The water permeability and molecular weight cut-offs (MWCO) of the substrate; and the FO performance of the TFC membrane were also determined. The Zn2GeO4-modified membrane showed ~45% increase in water permeability and NaCl salt rejection of 80% under RO mode. In FO mode, the ratio of water flux to reverse solute flux was also improved. However, lower FO flux was obtained which could be due to ICP. The result shows that Zn2GO4 nanowire may be used as a modifier to the substrate to improve the quality of the polyamide layer on the substrate to improve the flux and selectivity, but not as effective in reducing ICP. This work demonstrates that the incorporation of nanomaterials to the membrane substrate may be an alternative approach to improve the formation of polyamide skin layer to achieve better FO performance.

No MeSH data available.


Related in: MedlinePlus