According to the cohesion-tension theory, mangrove trees desalinate salty water using highly negative pressure (or tension) that is generated by evapo

Capillary-driven desalination in a synthetic mangrove

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2021-06-08 14:00:08

According to the cohesion-tension theory, mangrove trees desalinate salty water using highly negative pressure (or tension) that is generated by evaporative capillary forces in mangrove leaves. Here, we demonstrate a synthetic mangrove that mimics the main features of the natural mangrove: capillary pumping (leaves), stable water conduction in highly metastable states (stem), and membrane desalination (root). When using nanoporous membranes as leaves, the maximum osmotic pressures of saline feeds (10 to 30 bar) allowing pure water uptake precisely correspond to expected capillary pressures based on the Young-Laplace equation. Hydrogel-based leaves allow for stable operation and desalination of hypersaline solutions with osmotic pressures approaching 400 bar, fivefold greater than the pressure limits of conventional reverse osmosis. Our findings support the applicability of the cohesion-tension theory to desalination in mangroves, provide a new platform to study plant hydraulics, and create possibilities for engineered membrane separations using large, passively generated capillary pressures.

Mangroves are salt-tolerant trees that grow in tropical and subtropical coastal regions around the world. To survive in their saline or brackish environments, mangroves tightly control water and ion uptake at the roots, enabling xylem saps to be nearly salt free (1–3). Salt exclusion is partially achieved by physical blockage of the nonselective apoplastic route, in which water and ions circumvent the cell membrane by moving through the cell walls of root cells (1, 4). This blockage stems from the deposition of a waxy substance called suberin within the cell walls of endodermal cells, rendering the otherwise permeable cell walls essentially impermeable. Elimination of the apoplastic route results in uptake occurring primarily via the symplastic pathway, in which water and ions permeate the cell membrane and subsequently pass cell to cell through intracellular cytosol until entering the xylem for long-range transport. The cell membrane acts as the discriminating barrier. Ions are largely excluded from passive transport through the lipid bilayer, owing to the high solvation energy of ions in the hydrophobic core (5). Ions must therefore pass through selective pumps and channels, allowing the cell to control ion uptake. Water readily permeates the cell membrane, with the permeability of the membrane being enhanced by salt-excluding water channels called aquaporins (6). Owing to the selectivity of the cell membrane, salt exclusion in mangroves can reach up to 99% (1–3, 7).

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