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Humans have many reasons to thank mangroves. These stilt-rooted marsh trees store massive amounts of carbon in tropical coasts, maintain nurseries for a wide variety of commercially important fish species, and protect coastal areas from storm damage. In addition to these benefits to people, mangroves are unique among trees because they thrive in shallow marine waters that are too salty for most living things to grow.
To better understand exactly how mangroves can grow in the ocean and pump fresh water into their leaves, engineers built what they are dubbing "synthetic mangroves," pressure-driven devices that can draw fresh water from the saltier seas. and they have the potential to help manage stormwater in cities.
The synthetic mangrove they built resembles a large French press rather than a tree, but has some version of the most important parts of a mangrove: "roots" that exclude salt, a strong "stem" and thirsty "leaves. In a real mangrove, the pressure difference between the leaves and the rest of the tree acts like suction on a straw, drawing water from the roots and through the stem. The natural membranes in the roots filter the salt. For the synthetic mangrove, the great challenge was to get the fresh water to lift the “stem” towards the leaves without creating bubbles. The team landed on a layer made of silica for the stem and hydrogel for the leaves, and found that their synthetic mangrove worked like a real one, even with super-salty water.
Although the design helps engineers better understand how mangroves absorb water while leaving salt behind, we shouldn't expect false mangrove forests to sprout up instead of desalination plants - treatment centers that convert saltwater into freshwater - anytime soon. The elegant pressure-driven desalination process of synthetic mangroves works on a small scale, but a large mangrove-inspired desalination plant would pay a hefty energy bill. Instead, the engineers who created the faux mangrove have another idea for using their aspirant trees: incorporate them into the design of cities to make the buildings more resistant to storm surge.
In one city, yes, "the buildings themselves would absorb excess groundwater and evaporate the water from their walls and roofs," they write in a recent Science Advances article explaining their invention. Like a mangrove, these buildings would depend on pressure differences to absorb water, making them self-sufficient buffers against the storms that flood city streets. At least 30 of the so-called sponge cities using related technologies already exist in China, and the design may gain traction as storms and storm surges increase in intensity with climate change.
The researchers built a “synthetic mangrove”, shown to the right, to successfully copy how real mangroves absorb fresh water from the ocean. Y. Wang, J. Lee, J. R. Werber, M. Elimelech, Capillary desalination in a synthetic mangrove. Sci. Adv. 6, eaax5253 (2020).
As people continue to cut down real and living mangrove forests in alarming numbers, it is worth remembering that synthetic mangroves do not reproduce the full benefits of real mangroves. Mangrove-inspired urban design shows promise in protecting people from the most extreme effects of storm surge and flooding, but in many places, the best way to do this is to leave old-fashioned, living mangrove forests intact to do the right thing. that they do best.