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Removing Coal Tar With Tiny Plastic Foam Orbs

Removing Coal Tar With Tiny Plastic Foam Orbs

Scientists develop a new "green" method for cleaning up a decades-long environmental mess.

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Thursday, July 14, 2016 - 15:45

Marcus Woo, Contributor

(Inside Science) -- Plastic foam isn't exactly good for the environment. A plastic foam cup, for instance, can sit in a landfill for up to a million years, according to some estimates. 

But scientists have now found that the material called polystyrene, which is also used to make Styrofoam, can help clean up a decades-long environmental mess: Soil contaminated with a thick, carcinogenic goo known as coal tar.

"It's a witch's brew, a black, viscous, evil-smelling chemical conglomerate," said Allen Hatheway, a geological engineer who has studied the issues surrounding coal tar for decades.

Polystyrene pellets, researchers found, can sop up the toxic gunk, part of a new process that can be one-third faster and almost 30 percent cheaper than conventional methods.

Some river sediments and soil throughout the country have been seeped in coal tar since the first half of the century. The muck is a byproduct of burning coal or oil to produce gas, which, from the 1800's to about 1950, provided lighting and heat for households across the world. Today, coal tar is sometimes used as a sealant for pavement on roads, but production dropped significantly once electricity replaced manufactured gas.   

When the coal tar spilled, it leaked into the environment, contaminating the soil, river sediments, and water. All tar is different, but in general, it's a mix of thousands of chemicals, many of which are harmful to human health. For example, about 10 percent of the tar's mass consists of carcinogenic molecules called polycyclic aromatic hydrocarbons, said co-author Nick Wilton, a graduate student at Tufts University in Medford, Massachusetts. 

"This material is very toxic to humans and [sediment-dwelling] organisms in the environment," he said. "It doesn't degrade. It really just lingers in the environment."

The EPA estimates up to 5,000 sites throughout the country are contaminated with coal tar. But some experts such as Hatheway, who was not involved in the research, say the number is as high as 50,000. The EPA's estimate didn't include every type of gas plant and facility that produced coal tar, he said. The cost of cleaning up in New York State alone, which has hundreds of sites, is estimated to be $3 billion. And the cleanup can take a long time. 

"They'll be cleaning coal tar sites longer than you and I will be alive," said co-author Albert Robbat, a chemist at Tufts. 

The main method used now involves digging up the contaminated sediment, which is often squishy mud, and using chemical stabilizers and cement to solidify it. Then, trucks haul the dirt to landfills as hazardous waste. 

While this is the most cost-effective method so far, transporting contaminated soil -- and replacing it with clean dirt -- isn't cheap. The noise and odors released from excavating the coal tar also disrupt the neighborhood. And cement is porous, so eventually, the coal tar could leach out and contaminate soil again. 

"You're just transporting the problem from one location to another," Robbat said. 

But Robbat and Wilton's research team may have found a better way. First, you scoop the contaminated sediment into an on-site tumbler like a concrete mixer. Then you pour in a chemical called CT1, which acts as a mobilizing agent that separates the tar from the soil. After adding recycled polystyrene pellets, you shake it for up to two hours. 

Because polystyrene has similar chemical properties as coal tar, the nasty gunk sticks to the pellets. And because polystyrene is light, all the pellets float to the top of the mixture, where you can easily collect them. The dirty pellets can then be recycled and used as an industrial fuel.

The researchers, who recently published their results in the journal Soil and Sediment Contamination, tested the method on about 55 pounds of contaminated sediment from the Grand Calumet River in Indiana and Illinois -- among the most toxic ever measured. Their approach removed more than 80 percent of the polycyclic aromatic hydrocarbons and 73 percent of the tar by mass. Repeating the process removed 94 percent of the carcinogenic molecules. 

The team is now scaling up their experiments to clean 30 cubic yards of sediment, enough to fill roughly 60 bathtubs. From there, Robbat said, it's straightforward to expand and clean 100 cubic yards, which would show that the technique could be used commercially. 

He ultimately wants to reach 300 cubic yards. Today, workers typically haul away about 125 cubic yards per day. 

Other researchers have explored similar methods, using a chemical called a surfactant that dissolves the molecules in the tar, which then sticks onto plastic pellets that can be removed. But it can take a couple days for the molecules to dissolve, and sometimes longer for the bigger ones. 

Instead of dissolving the tar's molecules, CT1 separates the tar from soil -- a much faster process that takes only a couple hours, Wilton explained.  

A recycled substance made from plants like corn and hemp, CT1 is a proprietary chemical from a New York-based company called GreenStract. Robbat and another member of the team, Christian Zeigler of Tufts, are minority shareholders of the company. 

Being able to recycle the plastic foam is another bonus. In 2009, about 2.5 million tons of polystyrene were classified as solid waste, according to the researchers, but less than one percent was recycled. 

Although Hatheway is skeptical that this process -- or any method for that matter -- can remove all of the coal tar in the most contaminated sites, he says this new approach can still make a significant impact on the stubborn stuff.

"It has a life that can be measured in geologic time," he said. "If nothing is done about it now, then nothing will."

 

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Marcus Woo is a freelance science writer based in the San Francisco Bay Area who has written for Wired, BBC Earth, BBC Future, National Geographic, New Scientist, Slate, Discover, and other outlets.