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Comment on Precisely Tunable Ion Sieving with an Al13–Ti3C2Tx Lamellar Membrane by Controlling Interlayer SpacingClick to copy article linkArticle link copied!
- Tobias Foller*Tobias Foller*Email: [email protected]School of Materials Science and Engineering, University of New South Wales Sydney, Kensington NSW 2052, AustraliaMore by Tobias Foller
- Rakesh Joshi*Rakesh Joshi*Email: [email protected]School of Materials Science and Engineering, University of New South Wales Sydney, Kensington NSW 2052, AustraliaMore by Rakesh Joshi
This publication is licensed for personal use by The American Chemical Society.
Recently, Zhuet al. published an interesting article entitled “Precisely Tunable Ion Sieving with an Al13–Ti3C2Tx Lamellar Membrane by Controlling Interlayer Spacing”. They show a robust way to control the interlayer spacing of Ti3C2Tx membranes (TM), which is beneficial for ion selectivity. (1)
The authors show that by intercalating Al13 ions, the swelling of TM is largely suppressed, which is reflected by an increase in ion rejection of ion species with larger hydrated radii. It is shown that for TM with intercalated Al13, the Al3+ ions are rejected. In original TMs, they can pass through. This is a good indication that the intercalated membrane was improved toward rejection of species with smaller hydrated radii. (2) However, smaller ions such as K+ and Na+ still show a permeation rate (∼10–2 mol m–2 h–1) on same order of magnitude as that of the original and intercalated membranes.
It is now surprising that the authors claim a NaCl rejection rate of 99% in forward osmosis (FO). In the methods part of the article, it is described that the rejection rate was calculated with the NaCl concentration in the feed side Cf and draw side Cd, similar to methods reported earlier: (2)
A simple thought experiment with the setup described by Zhuet al. and others (2) underlines the major problem of this methodology. Let us assume two containers separated by a nonselective, semipermeable barrier. One side, the feed side, f, is filled with a NaCl solution having an initial, i, concentration Cf,i,NaCl. The other side, the draw side, d, is filled with a sucrose solution, S, with an initial, i, concentration Cd,i,S ≫ Cf,i,NaCl, while Cd,i,NaCl = 0. During the experiment, water will be drawn to the draw side. Similarly, NaCl will also diffuse into the draw side, due to the lack of salt selectivity by the barrier. The final, F, NaCl concentration of the draw and feed side will be equal: Cd,F,NaCl = Cf,F,NaCl = 0.5 × Cf,i,NaCl. However, using eq 1 will still result in a salt rejection of 50% regardless of the inability of the nonselective membrane to separate salt and water:
Even though the reported membranes are probably not completely nonselective, the problem in calculating the rejection rate with eq 1 becomes clearer with the following calculation. It may be noted that the permeation rate is derived from experiments with 0.5 M NaCl solution and no sucrose in the draw side. Thus, the actual permeation rate for the FO experiments was probably lower. However, this permeation rate can still illustrate the problems of the methodology. If Cd is measured only 1 h after initializing the experiment, only a small amount of salt will be permeated through the membrane. The amount of salt in the draw side would be
As an example for appropriate FO testing, Sapkota et al. recently reported an equation: (6)
Due to the low water flux (0.3 l m–2 h–1) reported by Zhuet al., ΔV can be considered small. Following the simplifications in ref (6), eq 2 simplifies to
In this case, the rejection rate shows no time dependence, as it takes both into account, the permeated water and salt. Hypothetically for a nonselective membrane, the rejection rate would be 0%.
In Figure 4g of the article, the authors provide a NaCl rejection rate for different thicknesses of the membranes. It is not further described whether these experiments were conducted with or without sucrose draw solution.
If it was conducted without a draw solution, the rejection rate has no meaning. The water permeates from the DI water side to the saltwater side, whereas the salt diffuses from the saltwater side to the DI water side. If it was conducted with sucrose draw solution, an estimate of the true rejection rate is possible. (5,6) The authors report a water permeance PW of 0.3 l m–2 h–1 with a Na+ permeation rate PNa of ∼0.02 mol m–2 h–1 in Figure 4g of the article. These values can be used to estimate the correct rejection rate by utilizing eq 4:
It may be noted that in the methods section of the article, the authors provide two starting NaCl concentrations, 0.1 and 0.5 M, but it is not clear which one was used for the experiments. We assumed that 0.5 M was used, which provides the 86.7% rejection rate calculated above. This may not be the correct rejection rate as for an appropriate calculation one needs to know the exact experimental parameters. In contrast, in the article, 99% rejection is claimed. In the FO experiments, no permeation rate for the ions is given. By that, an estimation of the correct rejection rate is not possible. However, the calculation of the rejection rate in Figure 4g indicates that the rejection rate calculated in the FO experiment of the article needs to be analyzed carefully, as well. Our calculated value for salt rejection of 86.7% is much closer to the value given in the article for reverse osmosis RO mode of 80%. The reason behind the inability to completely block NaCl in both modes might originate from stripping of the hydration shell, as mentioned by the authors or framework defects in the 2-D laminar structure. (7,8) However, the authors report a low permeation rate of Al3+ of 1 × 10–4 mol m–2 h–1 that suggests the general possibility of TM to block salt ions. If the permeation rate and water permeance are similar in FO experiments compared to the recorded values from the U-tube setup in the article, the rejection rate can be estimated >99% for Al3+ ions with eq 4. To validate that further, FO experiments are necessary.
It may also be noted that in the article the water flux is sometimes given in L m–2 h–1 and in L m–2 h–1 bar –1. However, in the FO experiments and in the ion permeation measurements, no pressure was applied.
Author Information
- Tobias Foller - School of Materials Science and Engineering, University of New South Wales Sydney, Kensington NSW 2052, Australia;
https://orcid.org/0000-0003-2636-4542;
- Rakesh Joshi - School of Materials Science and Engineering, University of New South Wales Sydney, Kensington NSW 2052, Australia;https://orcid.org/0000-0002-7497-9499;
References
This article references 8 other publications.
- 1Zhu, J.; Wang, L.; Wang, J.; Wang, F.; Tian, M.; Zheng, S.; Shao, N.; Wang, L.; He, M. Precisely Tunable Ion Sieving with an Al13–Ti3C2Tx Lamellar Membrane by Controlling Interlayer Spacing. ACS Nano 2020, 14, 15306, DOI: 10.1021/acsnano.0c05649Google ScholarThere is no corresponding record for this reference.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=&md5=9874b665cc7a056b8e2f928dd3112440
- 2Abraham, J.; Vasu, K. S.; Williams, C. D.; Gopinadhan, K.; Su, Y.; Cherian, C. T.; Dix, J.; Prestat, E.; Haigh, S. J.; Grigorieva, I. V.; Carbone, P.; Geim, A. K.; Nair, R. R. Tunable Sieving of Ions Using Graphene Oxide Membranes. Nat. Nanotechnol. 2017, 12 (6), 546– 550, DOI: 10.1038/nnano.2017.21Google Scholar2https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXls1Gru70%253D&md5=262d57c0d4658bacdfe7006439e85672Tunable sieving of ions using graphene oxide membranesAbraham, Jijo; Vasu, Kalangi S.; Williams, Christopher D.; Gopinadhan, Kalon; Su, Yang; Cherian, Christie T.; Dix, James; Prestat, Eric; Haigh, Sarah J.; Grigorieva, Irina V.; Carbone, Paola; Geim, Andre K.; Nair, Rahul R.Nature Nanotechnology (2017), 12 (6), 546-550CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Graphene oxide membranes show exceptional mol. permeation properties, with promise for many applications. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ∼9 Å (ref. 4), which is larger than the diams. of hydrated ions of common salts. The cutoff is detd. by the interlayer spacing (d) of ∼13.5 Å, typical for graphene oxide laminates that swell in H2O. Achieving smaller d for the laminates immersed in H2O proved to be a challenge. Here, the authors describe how to control d by phys. confinement and achieve accurate and tunable ion sieving. Membranes with d from ∼9.8 Å to 6.4 Å are demonstrated, providing a sieve size smaller than the diams. of hydrated ions. In this regime, ion permeation is thermally activated with energy barriers of ∼10-100 kJ mol-1 depending on d. Importantly, permeation rates decrease exponentially with decreasing sieve size but H2O transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for the entry of H2O mols. and large slip lengths inside graphene capillaries. Building on these findings, the authors demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.
- 3Shon, H. K.; Phuntsho, S.; Zhang, T. C.; Surampalli, R. Y. Forward Osmosis; American Society of Civil Engineers: Reston, VA, 2015.Google ScholarThere is no corresponding record for this reference.
- 4Song, X.; Liu, Z.; Sun, D. D. Nano Gives the Answer: Breaking the Bottleneck of Internal Concentration Polarization with a Nanofiber Composite Forward Osmosis Membrane for a High Water Production Rate. Adv. Mater. 2011, 23 (29), 3256– 3260, DOI: 10.1002/adma.201100510Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXntVGksrk%253D&md5=a414b0f1454ce6aa8049292b4e255f68Nano Gives the Answer: Breaking the Bottleneck of Internal Concentration Polarization with a Nanofiber Composite Forward Osmosis Membrane for a High Water Production RateSong, Xiaoxiao; Liu, Zhaoyang; Sun, Darren DelaiAdvanced Materials (Weinheim, Germany) (2011), 23 (29), 3256-3260CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)Internal concn. polarization was controlled with a nanofiber composite forward osmosis membrane for a high water prodn. rate.
- 5McCutcheon, J. R.; McGinnis, R. L.; Elimelech, M. A Novel Ammonia—Carbon Dioxide Forward (Direct) Osmosis Desalination Process. Desalination 2005, 174 (1), 1– 11, DOI: 10.1016/j.desal.2004.11.002Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXjvVKjtrw%253D&md5=e1551cf12c366e0b5c989de70bebac63A novel ammonia-carbon dioxide forward (direct) osmosis desalination processMcCutcheon, Jeffrey R.; McGinnis, Robert L.; Elimelech, MenachemDesalination (2005), 174 (1), 1-11CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)A forward (direct) osmosis (FO) desalination process is presented. The process uses an ammonium bicarbonate draw soln. to ext. water from a saline feed water across a semi-permeable polymeric membrane. Very large osmotic pressures generated by the highly sol. ammonium bicarbonate draw soln. yield high water fluxes and can result in very high feed water recoveries. Upon moderate heating, ammonium bicarbonate decomps. into ammonia and CO2 gases that can be sepd. and recycled as draw solutes, leaving the fresh product water. Expts. with a lab.-scale FO unit utilizing a flat sheet cellulose tri-acetate membrane demonstrated high product water flux and relatively high salt rejection. The results further revealed that reverse osmosis (RO) membranes are not suitable for the FO process because of relatively low product water fluxes attributed to severe internal concn. polarization in the porous support and fabric layers of the RO membrane.
- 6Sapkota, B.; Liang, W.; VahidMohammadi, A.; Karnik, R.; Noy, A.; Wanunu, M. High Permeability Sub-Nanometre Sieve Composite MoS2Membranes. Nat. Commun. 2020, 11 (1), 1– 9, DOI: 10.1038/s41467-020-16577-yGoogle ScholarThere is no corresponding record for this reference.
- 7Ritt, C. L.; Werber, J. R.; Deshmukh, A.; Elimelech, M. Monte Carlo Simulations of Framework Defects in Layered Two-Dimensional Nanomaterial Desalination Membranes: Implications for Permeability and Selectivity. Environ. Sci. Technol. 2019, 53 (11), 6214– 6224, DOI: 10.1021/acs.est.8b06880Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXptFygu7c%253D&md5=3c18fe1544b3725dde53c170ae7f1526Monte Carlo Simulations of Framework Defects in Layered Two-Dimensional Nanomaterial Desalination Membranes: Implications for Permeability and SelectivityRitt, Cody L.; Werber, Jay R.; Deshmukh, Akshay; Elimelech, MenachemEnvironmental Science & Technology (2019), 53 (11), 6214-6224CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)Two-dimensional nanomaterial (2-dimensional NM) frameworks, esp. those comprising graphene oxide, have received extensive research interest for membrane-based sepn. processes and desalination. However, the impact of horizontal defects in 2-dimensional NM frameworks, which stem from non-uniform deposition of 2-dimensional NM flakes during layer build-up, has been almost entirely overlooked. We apply Monte Carlo simulations, under idealized conditions in which the vertical interlayer spacing allows for H2O permeation while perfectly excluding salt, on both the formation of the laminate structure and mol. transport through the laminate. The simulations show that 2-dimensional NM frameworks are extremely tortuous (tortuosity ≈103), with H2O permeability decreasing from 20 to <1 L m-2 h-1 bar-1 as thickness increased from 8-167 nm. Addnl., we find that framework defects allow salt to percolate through the framework, hindering H2O-salt selectivity. 2-dimensional NM frameworks with a packing d. of 75%, representative of most 2-dimensional NM membranes, are projected to achieve <92% NaCl rejection at a H2O permeability of <1 L m-2 h-1 bar-1, even with ideal interlayer spacing. A high packing d. of 90%, which to the knowledge has yet to be achieved, could yield comparable performance to current desalination membranes. Maximizing packing d. is therefore a crit. tech. challenge, in addn. to the already daunting challenge of optimizing interlayer spacing, for the development of 2-dimensional NM membranes.
- 8Lu, X.; Gabinet, U. R.; Ritt, C. L.; Feng, X.; Deshmukh, A.; Kawabata, K.; Kaneda, M.; Hashmi, S. M.; Osuji, C. O.; Elimelech, M. Relating Selectivity and Separation Performance of Lamellar Two-Dimensional Molybdenum Disulfide (MoS2) Membranes to Nanosheet Stacking Behavior. Environ. Sci. Technol. 2020, 54 (15), 9640– 9651, DOI: 10.1021/acs.est.0c02364Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXht1Ohu7nF&md5=77dd77ef6f86ea45ec7ad37d4aa3077aRelating selectivity and separation performance of lamellar two-dimensional molybdenum disulfide (MoS2) membranes to nanosheet stacking behaviorLu, Xinglin; Gabinet, Uri R.; Ritt, Cody L.; Feng, Xunda; Deshmukh, Akshay; Kawabata, Kohsuke; Kaneda, Masashi; Hashmi, Sara M.; Osuji, Chinedum O.; Elimelech, MenachemEnvironmental Science & Technology (2020), 54 (15), 9640-9651CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)Increased demand for highly selective and energy-efficient sepns. processes has stimulated substantial interest in emerging two-dimensional (2D) nanomaterials as a potential platform for next-generation membranes. However, persistently poor sepn. performance continues to hinder the viability of many novel 2D-nanosheet membranes in desalination applications. In this study, we examine the role of the lamellar structure of 2D membranes on their performance. Using self-fabricated molybdenum disulfide (MoS2) membranes as a platform, we show that the sepn. layer of 2D nanosheet frameworks not only fails to demonstrate water-salt selectivity but also exhibits low rejection toward dye mols. Moreover, the MoS2 membranes possess a mol. wt. cutoff comparable to its underlying porous support, implying negligible selectivity of the MoS2 layer. By tuning the nanochannel size through intercalation with amphiphilic mols. and analyzing mass transport in the lamellar structure using Monte Carlo simulations, we reveal that small imperfections in the stacking of MoS2 nanosheets result in the formation of catastrophic microporous defects. These defects lead to a precipitous redn. in the selectivity of the lamellar structure by negating the interlayer sieving mechanism that prevents the passage of large penetrants. Notably, the imperfect stacking of nanosheets in the MoS2 membrane was further verified using 2D X-ray diffraction measurements. We conclude that developing a well-controlled fabrication process, in which the lamellar structure can be carefully tuned, is crit. to achieving defect-free and highly selective 2D desalination membranes.
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References
This article references 8 other publications.
- 1Zhu, J.; Wang, L.; Wang, J.; Wang, F.; Tian, M.; Zheng, S.; Shao, N.; Wang, L.; He, M. Precisely Tunable Ion Sieving with an Al13–Ti3C2Tx Lamellar Membrane by Controlling Interlayer Spacing. ACS Nano 2020, 14, 15306, DOI: 10.1021/acsnano.0c05649There is no corresponding record for this reference.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=&md5=9874b665cc7a056b8e2f928dd3112440
- 2Abraham, J.; Vasu, K. S.; Williams, C. D.; Gopinadhan, K.; Su, Y.; Cherian, C. T.; Dix, J.; Prestat, E.; Haigh, S. J.; Grigorieva, I. V.; Carbone, P.; Geim, A. K.; Nair, R. R. Tunable Sieving of Ions Using Graphene Oxide Membranes. Nat. Nanotechnol. 2017, 12 (6), 546– 550, DOI: 10.1038/nnano.2017.212https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXls1Gru70%253D&md5=262d57c0d4658bacdfe7006439e85672Tunable sieving of ions using graphene oxide membranesAbraham, Jijo; Vasu, Kalangi S.; Williams, Christopher D.; Gopinadhan, Kalon; Su, Yang; Cherian, Christie T.; Dix, James; Prestat, Eric; Haigh, Sarah J.; Grigorieva, Irina V.; Carbone, Paola; Geim, Andre K.; Nair, Rahul R.Nature Nanotechnology (2017), 12 (6), 546-550CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Graphene oxide membranes show exceptional mol. permeation properties, with promise for many applications. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ∼9 Å (ref. 4), which is larger than the diams. of hydrated ions of common salts. The cutoff is detd. by the interlayer spacing (d) of ∼13.5 Å, typical for graphene oxide laminates that swell in H2O. Achieving smaller d for the laminates immersed in H2O proved to be a challenge. Here, the authors describe how to control d by phys. confinement and achieve accurate and tunable ion sieving. Membranes with d from ∼9.8 Å to 6.4 Å are demonstrated, providing a sieve size smaller than the diams. of hydrated ions. In this regime, ion permeation is thermally activated with energy barriers of ∼10-100 kJ mol-1 depending on d. Importantly, permeation rates decrease exponentially with decreasing sieve size but H2O transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for the entry of H2O mols. and large slip lengths inside graphene capillaries. Building on these findings, the authors demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.
- 3Shon, H. K.; Phuntsho, S.; Zhang, T. C.; Surampalli, R. Y. Forward Osmosis; American Society of Civil Engineers: Reston, VA, 2015.There is no corresponding record for this reference.
- 4Song, X.; Liu, Z.; Sun, D. D. Nano Gives the Answer: Breaking the Bottleneck of Internal Concentration Polarization with a Nanofiber Composite Forward Osmosis Membrane for a High Water Production Rate. Adv. Mater. 2011, 23 (29), 3256– 3260, DOI: 10.1002/adma.2011005104https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXntVGksrk%253D&md5=a414b0f1454ce6aa8049292b4e255f68Nano Gives the Answer: Breaking the Bottleneck of Internal Concentration Polarization with a Nanofiber Composite Forward Osmosis Membrane for a High Water Production RateSong, Xiaoxiao; Liu, Zhaoyang; Sun, Darren DelaiAdvanced Materials (Weinheim, Germany) (2011), 23 (29), 3256-3260CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)Internal concn. polarization was controlled with a nanofiber composite forward osmosis membrane for a high water prodn. rate.
- 5McCutcheon, J. R.; McGinnis, R. L.; Elimelech, M. A Novel Ammonia—Carbon Dioxide Forward (Direct) Osmosis Desalination Process. Desalination 2005, 174 (1), 1– 11, DOI: 10.1016/j.desal.2004.11.0025https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXjvVKjtrw%253D&md5=e1551cf12c366e0b5c989de70bebac63A novel ammonia-carbon dioxide forward (direct) osmosis desalination processMcCutcheon, Jeffrey R.; McGinnis, Robert L.; Elimelech, MenachemDesalination (2005), 174 (1), 1-11CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)A forward (direct) osmosis (FO) desalination process is presented. The process uses an ammonium bicarbonate draw soln. to ext. water from a saline feed water across a semi-permeable polymeric membrane. Very large osmotic pressures generated by the highly sol. ammonium bicarbonate draw soln. yield high water fluxes and can result in very high feed water recoveries. Upon moderate heating, ammonium bicarbonate decomps. into ammonia and CO2 gases that can be sepd. and recycled as draw solutes, leaving the fresh product water. Expts. with a lab.-scale FO unit utilizing a flat sheet cellulose tri-acetate membrane demonstrated high product water flux and relatively high salt rejection. The results further revealed that reverse osmosis (RO) membranes are not suitable for the FO process because of relatively low product water fluxes attributed to severe internal concn. polarization in the porous support and fabric layers of the RO membrane.
- 6Sapkota, B.; Liang, W.; VahidMohammadi, A.; Karnik, R.; Noy, A.; Wanunu, M. High Permeability Sub-Nanometre Sieve Composite MoS2Membranes. Nat. Commun. 2020, 11 (1), 1– 9, DOI: 10.1038/s41467-020-16577-yThere is no corresponding record for this reference.
- 7Ritt, C. L.; Werber, J. R.; Deshmukh, A.; Elimelech, M. Monte Carlo Simulations of Framework Defects in Layered Two-Dimensional Nanomaterial Desalination Membranes: Implications for Permeability and Selectivity. Environ. Sci. Technol. 2019, 53 (11), 6214– 6224, DOI: 10.1021/acs.est.8b068807https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXptFygu7c%253D&md5=3c18fe1544b3725dde53c170ae7f1526Monte Carlo Simulations of Framework Defects in Layered Two-Dimensional Nanomaterial Desalination Membranes: Implications for Permeability and SelectivityRitt, Cody L.; Werber, Jay R.; Deshmukh, Akshay; Elimelech, MenachemEnvironmental Science & Technology (2019), 53 (11), 6214-6224CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)Two-dimensional nanomaterial (2-dimensional NM) frameworks, esp. those comprising graphene oxide, have received extensive research interest for membrane-based sepn. processes and desalination. However, the impact of horizontal defects in 2-dimensional NM frameworks, which stem from non-uniform deposition of 2-dimensional NM flakes during layer build-up, has been almost entirely overlooked. We apply Monte Carlo simulations, under idealized conditions in which the vertical interlayer spacing allows for H2O permeation while perfectly excluding salt, on both the formation of the laminate structure and mol. transport through the laminate. The simulations show that 2-dimensional NM frameworks are extremely tortuous (tortuosity ≈103), with H2O permeability decreasing from 20 to <1 L m-2 h-1 bar-1 as thickness increased from 8-167 nm. Addnl., we find that framework defects allow salt to percolate through the framework, hindering H2O-salt selectivity. 2-dimensional NM frameworks with a packing d. of 75%, representative of most 2-dimensional NM membranes, are projected to achieve <92% NaCl rejection at a H2O permeability of <1 L m-2 h-1 bar-1, even with ideal interlayer spacing. A high packing d. of 90%, which to the knowledge has yet to be achieved, could yield comparable performance to current desalination membranes. Maximizing packing d. is therefore a crit. tech. challenge, in addn. to the already daunting challenge of optimizing interlayer spacing, for the development of 2-dimensional NM membranes.
- 8Lu, X.; Gabinet, U. R.; Ritt, C. L.; Feng, X.; Deshmukh, A.; Kawabata, K.; Kaneda, M.; Hashmi, S. M.; Osuji, C. O.; Elimelech, M. Relating Selectivity and Separation Performance of Lamellar Two-Dimensional Molybdenum Disulfide (MoS2) Membranes to Nanosheet Stacking Behavior. Environ. Sci. Technol. 2020, 54 (15), 9640– 9651, DOI: 10.1021/acs.est.0c023648https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXht1Ohu7nF&md5=77dd77ef6f86ea45ec7ad37d4aa3077aRelating selectivity and separation performance of lamellar two-dimensional molybdenum disulfide (MoS2) membranes to nanosheet stacking behaviorLu, Xinglin; Gabinet, Uri R.; Ritt, Cody L.; Feng, Xunda; Deshmukh, Akshay; Kawabata, Kohsuke; Kaneda, Masashi; Hashmi, Sara M.; Osuji, Chinedum O.; Elimelech, MenachemEnvironmental Science & Technology (2020), 54 (15), 9640-9651CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)Increased demand for highly selective and energy-efficient sepns. processes has stimulated substantial interest in emerging two-dimensional (2D) nanomaterials as a potential platform for next-generation membranes. However, persistently poor sepn. performance continues to hinder the viability of many novel 2D-nanosheet membranes in desalination applications. In this study, we examine the role of the lamellar structure of 2D membranes on their performance. Using self-fabricated molybdenum disulfide (MoS2) membranes as a platform, we show that the sepn. layer of 2D nanosheet frameworks not only fails to demonstrate water-salt selectivity but also exhibits low rejection toward dye mols. Moreover, the MoS2 membranes possess a mol. wt. cutoff comparable to its underlying porous support, implying negligible selectivity of the MoS2 layer. By tuning the nanochannel size through intercalation with amphiphilic mols. and analyzing mass transport in the lamellar structure using Monte Carlo simulations, we reveal that small imperfections in the stacking of MoS2 nanosheets result in the formation of catastrophic microporous defects. These defects lead to a precipitous redn. in the selectivity of the lamellar structure by negating the interlayer sieving mechanism that prevents the passage of large penetrants. Notably, the imperfect stacking of nanosheets in the MoS2 membrane was further verified using 2D X-ray diffraction measurements. We conclude that developing a well-controlled fabrication process, in which the lamellar structure can be carefully tuned, is crit. to achieving defect-free and highly selective 2D desalination membranes.