Inside an old gas tank, pink and white mold grows atop crispy cakes of spent espresso.
Todd Broockerd, vice president of Kitten Coffee, a roaster and espresso bar in Brooklyn, dumps the grounds into a big, black bag and chucks that into a trash hauling bin. A putrid aroma — rotting cheese, with a hint of citrus — permeates the air.
Each year, coffee manufacturers, restaurants, cafes and home brewers worldwide produce about six billion tons of coffee waste like the cakes in Mr. Broockerd’s bin. If not rotting in a dump or fertilizing a garden, the grounds end up in animal feed and biofuels.
But researchers in Italy have found a new home for the stinky old coffee bits — by infusing them into a porous foam that removes heavy metals from polluted water, according to a study published this month in ACS Sustainable Chemistry and Engineering.
“We use a lot of coffee here in Italy,” said Despina Fragouli, the author of the study and a materials scientist at the Istituto Italiano di Tecnologia. She and her team develop new compounds from agricultural waste — like turning cacao husks into a material for preserving and packaging food.
Naturally, they wondered “What about coffee?”
It turns out, coffee is made of chemical components that are really good at trapping heavy metals like mercury and lead, which are common water pollutants, and poisonous in high or sustained doses. Previously, other researchers used coffee as a powder to remove lead from drinking water, but getting the powder out afterward wasn’t easy.
By integrating coffee powder into a foam, Dr. Fragouli’s team is offering a possible solution, although the method has yet to be perfected for everyday, practical uses.
To manufacture the foam, they dried old espresso grounds from local bars into powder and mixed it with silicon and sugar. After the mixture solidified, they dipped it in water to dissolve the sugar, leaving behind holes to create the absorbent texture.
They dropped their little foam block in water containing lead and mercury and measured the metallic concentrations over time. For a water sample with metallic concentrations of 16 parts per million (which are 1,000 times greater than what the Environmental Protection Agency considers a threat in drinking water), a piece of foam the size of a postage stamp removed 99 percent of the metals from the water within 30 hours.
But the metals needed time to interact with the coffee, and the foam wasn’t as effective in moving water, removing only 67 percent of the metals. And at a certain point, absorption maxed out.
If this were to be used practically in the future, Dr. Fragouli said, the water would need to be moving very slowly through a super thick layer of foam, or contained in a large foam-lined tank for hours. She has been discussing the material both with Italian industries that produce metallic waste and those that generate coffee grounds.
Meanwhile, “Ours go into the trash unless the garden next door comes by and asks for them,” Mr. Broockerd said. “It’s interesting to me that something that smells so bad could act as a cleaning agent.”