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Pushing the limits of size selectivity in nanoscale solute separations

Abstract

Transport of a spherical solute through a cylindrical pore has been modelled for decades using well-established hindered transport theory, predicting solutes with a size smaller than the pore to be rejected nonetheless because of convective and diffusive hindrance; this rejection mechanism prevents extremely sharp solute separations by a membrane. Whereas the model has been historically verified, solute transport through near-perfect isoporous membranes may finally overcome this limitation. Here encouraging solute rejections are achieved using nanofabricated, defect-free silicon nitride isoporous membranes. The membrane is challenged by a recirculated feed to increase the opportunity for interactions between solutes and the pore array. Results show the membrane completely reject solutes with greater size than the pore size while effectively allowing smaller solutes to permeate through. With effectively increasing the number of interactions, we propose that a steeper size-selective rejection curve may be achieved. With this traditional hurdle overcome, there is new promise for unprecedented membrane separations through judicious process design and extremely tight pore-size distributions.

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Fig. 1: Solute rejection of ideal isoporous membranes and polydisperse membranes.
Fig. 2: Fabrication of silicon nitride isoporous membranes.
Fig. 3: Pore-size distribution of silicon nitride isoporous membranes.
Fig. 4: Water and dextran transport through silicon nitride membranes.
Fig. 5: A 3D-printed crossflow device and filtration system are assembled to characterize water and solute transport.

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Data availability

The data that support the findings of this study are available via Figshare at https://doi.org/10.6084/m9.figshare.24773811.v1 (ref. 44).

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Acknowledgements

This work was supported as part of the Advanced Materials for Energy-Water Systems (AMEWS) Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences at Argonne National Laboratory under contract DE-AC02-06CH11357. We would like to thank P. Griffin at the Soft Matter Characterization Facility of the University of Chicago for the helpful discussion of GPC analysis. We would also like to thank S. Yim and J. Dhanasekaran for their support of the nanofabrication process, dextran sample GPC measurements and solute transport discussion, respectively.

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S.B.D. and P.F.N. conceived the idea and designed the research. F.G., W.C., J.G.E. and R.Z.W. conducted the experiments and analysed the experimental results. R.D. performed GPC. N.J.Z. executed TEM. F.G., W.C. and J.G.E. contributed to drafting and revising the paper. S.B.D. and P.F.N contributed to proofreading and revising the paper. S.B.D. and P.F.N. supervised the project and contributed to the funding acquisition.

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Correspondence to Seth B. Darling.

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Supplementary discussion, calculation and Figs. 1–13.

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Gao, F., Chen, W., Eatman, J.G. et al. Pushing the limits of size selectivity in nanoscale solute separations. Nat Water 2, 521–530 (2024). https://doi.org/10.1038/s44221-024-00252-3

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