2020托福听力练习：微型机器人清洁废水（下载）

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　　Water laced with heavy metals—like the drinking water in Flint, Michigan—can be stripped of contaminants using chemicals, filters, membranes, even electric current.

　　But now researchers say they've created what could be a cheaper, more effective solution:

　　a fleet of microscopic, self-propelled, aquatic robots.

　　Each one the size of just 10 lined-up bacteria—so tiny that a billion will fit in a syringe.

　　Each tube-shaped microbot is a sandwich of three materials.

　　A graphene outer layer, which binds to heavy metals.

　　A middle layer of nickel, which gives the bots magnetic polarity, so they can be pulled through wastewater with magnets.

　　And platinum inside—for propulsion.

　　Just add a bit of peroxide to the wastewater, and it'll react with the platinum to form water and oxygen bubbles, which propel the tubes along.

　　In an hour, a swarm of 200,000 bots scavenged 80 percent of the lead from three millimeters of tainted water.

　　And the researchers estimate that it costs only about five cents a liter to do so.

　　The findings are in the journal Nano Letters.

　　The researchers envision the bots as a portable solution for small companies—they'd treat their water onsite instead of carting it to a treatment plant.

　　And after the robots do the rounds, the heavy metals can be stripped away.

　　Meaning companies can reuse the heavy metals, and, ultimately, keep them out of our waterways.