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Cactus needles give clues on how to battle smog

Cactus needles give clues on how to battle smog

Researchers looking for new methods to fight the mainland's chronic air pollution find inspiration in the needles of a cactus

When Ju Jie and her colleagues published the results of their research into the fog-collecting abilities of cactus in the science journal in December, she hoped it might one day help save the life of a thirsty desert explorer.

The PhD student from the Institute of Chemistry at the Chinese Academy of Sciences had a picture in her mind of a device small enough for a backpack and cheap enough for adventurous youngsters.

She never considered finding an application in a city of 20 million people that spans nearly 17,000 square kilometres.

The smog in Beijing last month, however, changed her mind. Far exceeding internationally accepted danger levels, it choked her, blinded her and made her head spin.

"If smog is closely related to fog, we can reduce the smog by collecting fog," she said. "If our device can remove fog in a lab, many such devices can produce a similar effect in the city. If the pollutants are removed with fog, we can breathe again.

"The more I think about it, the more I am obsessed with the idea."

Ju's idea is just one of many proposals that surfaced on the mainland after the record-breaking smog. It may be the most radical and science-fiction-like, but it is hardly the most expensive or least feasible.

Fog plays an important role in the creation of smog. The tiny water droplets, suspended in the air, are not harmful themselves, but they adhere to particles of pollutants from coal-fired power plants, factories and car exhausts, forming smog.

The fog also creates a static weather system that traps pollutants in a small area. As the water condenses, it draws heat from near ground level, creating a zone in which the air hardly moves. Smog often follows fog.

Scientists have long been intrigued by the water collecting capabilities of the cactus. Many studies have examined the plant's lengthy roots, its wax-like membrane that keeps water from escaping, and other properties that help the cactus thrive in parched deserts.

The plant's needle-like spines, however, had been considered a self-defence mechanism, guarding against grazing animals. Ju's team, under the instruction of Professor Jiang Lei, put the needles under an electron microscope and discovered that they are in fact a crucial water-collecting device.

Video footage captured by Ju's team showed that in a foggy environment, a large water droplet formed on a needle every few seconds and was quickly absorbed by the cactus.

Other forms of life, spiders for instance, sometimes collect and use fog water. Spider sometimes sip water droplets that form on their webs, a phenomenon that inspired humans to install large nylon nets in Peru - where fog is abundant but water scarce.

But the unique capability of the cactus is that no matter which direction its needles are facing, even if held upside down, they can still keep water droplets moving from the tip of the needle to the root. Ju said the secret lay in the needle's micro-surface structure, which gave the water droplet an uneven shape. An unevenly shaped water ball tends to move along a designated course due to unequal internal pressure.

"We have studied many fog collection systems in nature and the cactus is probably the most efficient for the job," Ju said. "With a device similar to a cactus we can pretty much avoid evaporation, as all water droplets will automatically fall in your cup."

Ju's fog collector, for the time being, exists largely on paper. Her team is still calculating how much water it could draw from the air in a given period of time. The team is also experimenting with some coating chemicals that could make water droplets move even faster than on a cactus needle. Their biggest headache is turning a laboratory prototype into manufactured reality.

Ju said she was optimistic that her fog collector would soon be on the market. The team has filed two patents for the device and is working with a company in Beijing to commercialise the design.

"We can make it to almost any size, any shape and any colour, if it is to be planted in large numbers all over the city," she said. "I hope our fog collector can make our city clean, beautiful and futuristic."

New ideas tend to draw criticism and Ju's is no exception. Professor Zhai Jianping, a specialist in pollution removal technology at Nanjing University's school of environment, said that the absence of a any form of power was an advantage of Ju's device, but could also become a fatal shortcoming.

"Without forced air flow, the device could only absorb water droplets near it, while the city's smog can reach up to a kilometre high," Zhai said. "But with sufficient air flow the device would become unnecessary because the wind would disperse the smog quickly."

Other scientists have proposed other ideas. Wang Yuesi, a researcher with the academy's Institute of Atmospheric Physics, said some had proposed artificially inducing rain to reduce the smog and improve visibility, but that was only possible when humidity reached a very high level, and more problems would arise after the polluted water reached the ground.

Many scientists have urged the government to mandate the installation of dust-removal devices at power plants and factories, but the costs of such devices were so high that most plants turned them on only during inspections by officials, Wang said.

Xue Huiwen, an associate professor of environmental science at Peking University, said that smog was a difficult problem to solve.

"Until we move factories out of China, we will have no choice but to live with pollution."


This article appeared in the South China Morning Post print edition as: Scientists look to nature in battle against smog