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Capillary-driven desalination in a synthetic mangrove

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Science Advances  21 Feb 2020:
Vol. 6, no. 8, eaax5253
DOI: 10.1126/sciadv.aax5253
  • Fig. 1 Design elements and water flow in the synthetic mangrove.

    The schematic diagrams show the mangrove tree (left), the synthetic mangrove device (right), and their water transport mechanisms (center insets). In natural mangroves, capillary pressure created by evaporation into substomatal cavities (top left inset) brings about upward water transport through xylem channels (middle left inset) and desalination by aquaporin water channels in root cell membranes, which exclude salt from saline water in the soil (bottom left inset). The highly negative pressure in the root, Proot, overcomes the osmotic pressure of the saline water (πo), which enables water uptake through the root filtration system (i.e., Proot − πroot < Po − πo). The synthetic mangrove as shown comprises a nanoporous AAO membrane, a silica frit, and an RO membrane, mimicking the leaf, stem, and root of natural mangrove trees, respectively. Negative capillary pressure in the nanopores of the AAO membrane (top right inset) is the driving force for water transport through the silica frit (middle right inset) and desalination of saline water by a semipermeable RO membrane (bottom right inset). Hydrogel membranes are also used as leaves. Membranes were sealed using rubber O-rings, shown in black. Magnetic stirring is used to enhance mixing of the saline feed water and minimize concentration polarization at the membrane surface. Pleaf and Proot, xylem sap pressure at the leaf and root, respectively; Po, pressure in the soil or feed water; πroot, osmotic pressure of xylem sap at the root; πo, osmotic pressure of soil or feed water (~25 bar for seawater); γ, water surface tension; θ, water contact angle on nanopore wall; dp, nanopore diameter.

  • Fig. 2 Evaporation rate of water from the leaves of synthetic mangrove.

    An AAO membrane comprising nanopores with a diameter of 84 nm was used as the synthetic leaf. Deionized water was used as the feed solution. (A) Evaporation rates of pure water from the AAO membrane leaf under different relative humidities (RHs) and temperatures. The evaporation rates measured at each temperature were normalized to the corresponding vapor pressures. Solid line is the normalized evaporation rate, estimated by the convective mass transfer model at each RH and temperature. AAO membrane surface porosity of 40% and air velocity of 0.61 m/s, tangential to the AAO surface, were used in the model (note S2 and table S1). For clarity, only one solid line is plotted as the normalized lines for 20°, 30°, and 40°C were nearly overlaid. (B) Evaporation rates of pure water with and without root RO membrane. The evaporation rates were determined at 30°C and 40% relative humidity. Error bars represent ±SD for three different measurements.

  • Fig. 3 Rejection of dye molecules by the root membrane under negative pressure.

    Allura Red dye (1 mM) with Stokes diameter of ~1 nm was used as the feed solution. A commercial RO membrane was used as the root and an AAO membrane with a mean pore diameter of 84 nm was used as the leaf. The dye rejection experiments were performed at 30°C and 40% relative humidity. (A) UV-vis spectra of the feed (red dotted), retentate (red), permeate (blue), and deionized (DI) water as a blank (dashed green). The inset shows retentate and permeate solutions after dye rejection experiments. The volume of each vial shown is 2 ml. (B) Dye rejection with and without the root membrane. The upper images show the silica frit from the rejection experiments without (left) and with root membrane (right) before dissolution of dye for UV-vis quantification. Error bars represent ±SD for three different measurements. Photo credit: Yunkun Wang, Shandong University, China.

  • Fig. 4 Water uptake and desalination in the synthetic mangrove.

    (A and B) Pore size distributions in two different AAO membranes used as leaves, obtained from image analysis of SEM micrographs (insets). The mean pore diameters from Gaussian fits (dashed red line) for each AAO membrane were identified as 84 ± 1 nm (A) and 215 ± 43 nm (B). Error bars represent ±SD from at least five different SEM micrographs. (C and D) Water fluxes and salt rejections for AAO membranes with mean pore diameters of 84 nm (C) and 215 nm (D). Here, 0, 0.02, 0.05, 0.1, 0.25, 0.5 (0.6 and 0.7 M NaCl solutions for smaller-diameter AAO membrane experiments), and 1 M NaCl solutions were used as the feed, exerting osmotic pressures ranging from 0 to 49.0 bar. The effective pore diameter, dp,eff, for capillarity was estimated from the zero flux points, using the Young-Laplace equation (Eq. 3), to be 91 and 267 nm, respectively. Temperature and relative humidity were set as 30°C and 40%, respectively. (E) Mechanistic description of flux behavior, showing the evolution of the feed solution osmotic pressure (πo, purple line), root pressure difference (ΔP = PoProot, red line), water contact angle on the nanopore wall (θ, green line) (Fig. 4D), and water flux across the root membrane (blue line) as NaCl concentration increases in the feed solution. The sign convention of water flux is set as positive if water is withdrawn from the feed solution. (F) Anticipated configurations of water level in the synthetic mangrove. The curvature of the water meniscus increases (i.e., θ decreases) as the salt (NaCl) concentration increases to generate sufficient capillary pressure to overcome the osmotic pressure and drive selective water uptake through RO (stages 1 and 2). When the maximum curvature is reached, the osmotic pressure reaches the maximum capillary pressure that is achievable by nanopores of the given diameter (stage 3). Further increase of salt concentration results in negative water flux (i.e., osmosis), with the water level fully retracted from the nanopores (stage 4). Water fluxes in (C) and (D) are calculated from water (liquid) flow rates normalized to the area of RO membrane in the root (1.3 × 10−4 m2). Error bars represent ±SD for three different measurements.

  • Fig. 5 Desalination of hypersaline solutions and stable water uptake in a synthetic mangrove with a hydrogel leaf.

    (A) Schematic illustration of fabrication of a hydrogel-based synthetic leaf. A robust poly(HEMA) hydrogel film was formed by filling the pores of a macroporous PVDF membrane with monomer solution, polymerizing HEMA with cross-linking by ethylene glycol dimethacrylate (EGDMA) under UV irradiation, and immersing the membrane in water for >24 hours. (B) Water fluxes and salt rejections by RO at the root driven by negative pressure in a synthetic mangrove with poly(HEMA) leaves. NaCl solutions with concentrations up to 5 M were used as feed, with corresponding osmotic pressures of up to 406 bar. (C) Long-term performance experiment using 0.1 and 5 M NaCl as feed solutions. Temperature and relative humidity were set as 30°C and 40%, respectively. Water fluxes are calculated from water (liquid) flow rates normalized to the area of RO membrane in the root (1.3 × 10−4 m2). Error bars represent ±SD for three different measurements.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/8/eaax5253/DC1

    Note S1. Pressure drop across silica frit.

    Note S2. Water evaporation rate measurements and calculations.

    Note S3. Hydrogel and RO membrane characterization.

    Note S4. Dependency of water flux on hydraulic pressure in AAO nanopores.

    Note S5. Derivation of water flux through the hydrogel leaf.

    Fig. S1. Photograph of the synthetic mangrove.

    Fig. S2. SEM images of silica frit.

    Fig. S3. On-line water level monitoring system.

    Fig. S4. Vapor concentration boundary layer on the AAO membrane.

    Fig. S5. Initially stable flux followed by total loss of flux for AAO membranes with mean pore diameters of 84 nm as leaves.

    Fig. S6. Correlation between water flux and NaCl rejection.

    Fig. S7. Water flux and NaCl rejection without RO membrane root as a function of feed solution osmotic pressure.

    Fig. S8. Schematic of AAO pores, water meniscus, and vapor concentration boundary layer.

    Fig. S9. SEM images and energy dispersive x-ray spectroscopy (EDS) line scan profiles on the cross sections of PVDF and PVDF-poly(HEMA) membranes.

    Fig. S10. Infrared spectroscopy of PVDF microporous membrane and poly(HEMA) hydrogel in PVDF substrate.

    Fig. S11. X-ray photoelectron spectroscopy (XPS) characterization of hydrogel-based leaf.

    Fig. S12. Impact of feed concentration on evaporative water flux from poly(HEMA) hydrogel leaf.

    Fig. S13. Modeled profiles for water flux through the hydrogel-based leaf membrane.

    Table S1. Properties of anodic aluminum oxide (AAO) isotropic membrane filters.

    Table S2. Properties of poly(HEMA)-based hydrogel.

    References (4952)

  • Supplementary Materials

    This PDF file includes:

    • Note S1. Pressure drop across silica frit.
    • Note S2. Water evaporation rate measurements and calculations.
    • Note S3. Hydrogel and RO membrane characterization.
    • Note S4. Dependency of water flux on hydraulic pressure in AAO nanopores.
    • Note S5. Derivation of water flux through the hydrogel leaf.
    • Fig. S1. Photograph of the synthetic mangrove.
    • Fig. S2. SEM images of silica frit.
    • Fig. S3. On-line water level monitoring system.
    • Fig. S4. Vapor concentration boundary layer on the AAO membrane.
    • Fig. S5. Initially stable flux followed by total loss of flux for AAO membranes with mean pore diameters of 84 nm as leaves.
    • Fig. S6. Correlation between water flux and NaCl rejection.
    • Fig. S7. Water flux and NaCl rejection without RO membrane root as a function of feed solution osmotic pressure.
    • Fig. S8. Schematic of AAO pores, water meniscus, and vapor concentration boundary layer.
    • Fig. S9. SEM images and energy dispersive x-ray spectroscopy (EDS) line scan profiles on the cross sections of PVDF and PVDF-poly(HEMA) membranes.
    • Fig. S10. Infrared spectroscopy of PVDF microporous membrane and poly(HEMA) hydrogel in PVDF substrate.
    • Fig. S11. X-ray photoelectron spectroscopy (XPS) characterization of hydrogel-based leaf.
    • Fig. S12. Impact of feed concentration on evaporative water flux from poly(HEMA) hydrogel leaf.
    • Fig. S13. Modeled profiles for water flux through the hydrogel-based leaf membrane.
    • Table S1. Properties of anodic aluminum oxide (AAO) isotropic membrane filters.
    • Table S2. Properties of poly(HEMA)-based hydrogel.
    • References (4952)

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