Listen and read 18: Out of thin air - Thấm Tâm Vy

Nội dung text: Listen and read 18: Out of thin air - Thấm Tâm Vy

1. LISTEN AND READ 18 Both groups hope to get around these complications, but in different ways. The Watering Deserts researchers at Berkeley, led by Omar Yaghi, announced their results last year. They OUT OF THIN AIR propose to replace the zirconium-based metal-organic framework with one based on aluminium. Not only is aluminium cheaper than zirconium, it is better at binding to and New techniques may help provide water in arid areas then releasing water, making the trap’s operation smoother. On top of this, Dr Yaghi has found that by mixing graphite into the powder from which the framework is IF YOU LIVE in a desert, maintaining a supply of fresh water is a challenge. One compounded, in order to make it black, and therefore heat-absorbing, he can eliminate answer is desalination, but that needs a source of brine from which to remove the salt— the need for copper foam. which in turn requires that your desert be near the sea. Even in inland deserts, though, In Connecticut, meanwhile, Steven Suib and his colleagues, who have just reported moisture is often present in the air as water vapour. The problem is extracting this their result in Environmental Science and Technology Letters, propose getting rid of vapour effectively and cheaply. And that is what two groups of researchers—one at the both framework and foam, and replacing them with birnessite, a type of manganese University of Connecticut, the other at the University of California, Berkeley—hope dioxide. Birnessite is abundant in nature. It is also easy to make artificially. It is they have managed to do. therefore cheap. The ease with which water can be won from air depends on that air’s relative humidity. Like metal-organic frameworks, birnessite is riddled with tiny holes that allow air to This is a measure of its current vapour content as a percentage of its maximum possible move around inside it, to maximise adsorption. And it, too, is black and therefore heat- vapour content at its current temperature. A relative humidity of 100% means the air in absorbing. question is holding as much water vapour as it possibly can. A good way to get air to Both proposals work. Tested in desertlike conditions in a laboratory—and in Dr give up some of its moisture is therefore to cool it to the point where its relative Yaghi’s case in an actual desert, too—they absorb and regurgitate reasonable fractions humidity exceeds100%. of their weight of water every day. They are nothing like as productive as desalination Sometimes this happens naturally at night, causing mist and dew to form. These can be plants, and so would have to be built at large scale to generate water in commercially collected in special traps in areas where liquid water is otherwise rare. But if nocturnal useful quantities. But one thing deserts do have is lots of cheap land. If either or both of cooling does not bring air all the way up to 100% relative humidity, building water traps these inventions can be manufactured at scale, then the deserts may bloom—if not with out of special materials might give nature a helping hand. plants, at least with water-collection farms. [The Economist, US , January 11, 2019] Surface features Adsorption is a process which plucks water molecules from air that has less than 100% Notes: relative humidity by attaching them to the surface of a solid material. The molecules are held there by electrostatic connections called Van der Waals forces that link them with - desalination: khử muối the molecules of the pertinent surface. To collect a lot of water this way therefore - adsorption: sự hút bám (hóa học) - electrostatic: tĩnh điện requires a material that has two properties. One is a large surface area. The other is an - pertinent: thích đáng, đúng chỗ appropriate Van der Waals response. - to get around = to circulate: lưu hành; phổ biến Experimental traps that employ this principle have been made using substances called - birnessite: a phyllomanganate, which is a type of hydrated metal oxide that contains a high proportion of metal-organic frameworks. These are porous molecular networks through which air can manganese. - zirconium: a lustrous grey strong metallic element resembling titanium; it is used in nuclear reactors as circulate. Their porosity gives them a huge surface area. And by picking the right a neutron absorber; it occurs in baddeleyite but is obtained chiefly from zircon ingredients, such as zirconium, they can be given the necessary Van der Waals - to riddle: sàng lọc properties. Zirconium is, however, costly. - to regurgitate: phun ngược trở lại Moreover, once adsorbed, the water must then be released. This means warming the . adsorptive material—the warmth being provided by the sun, once it has risen. Here, metal-organic frameworks present a problem. They tend to reflect sunlight rather than absorbing it, and so do not heat up well. To overcome this, engineers build solid “foams” made of copper into the system. These heat up in the sun and transfer some of their heat to parts of the adjacent metal-organic framework where adsorbed water needs to be released. This works, but adding such foams makes an already costly technology even dearer. Thẩm Tâm Vy, Jan 11, 2020 LISTEN AND READ 18