A little over a year ago, physicist Nabil Lawandy was stranded at the Charlotte, North Carolina airport, listening to endless news stories about sequestration and the threat of a government shutdown when scientific inspiration hit. Lawandy was struck by a mundane, but important way that the government could save hundreds of millions of dollars: by cleaning money and keeping it in circulation for longer.
The former Brown University professor had left academia in the mid-1990s to found Spectra Systems Corporation, a Rhode Island company that works on technologies that help central banks detect counterfeit currency. But he drew on an even more unlikely area of past experience when brainstorming about the best way to scrub dollar bills clean. He recalled that when he worked as a young scientist at NASA, pressurized gases would be used to clean the optics for satellites. What if the same technique were used on currency?
“Soiling, where the [bank]notes are just discolored and dark ... accounts for a lot of the rejected [bank]notes,” Lawandy said. “You don’t throw away your T-shirts and underwear; if you’d had to buy new underwear everyday, you’d be broke.”
In a study published last month in the journal Industrial and Engineering Chemistry Research, Lawandy described just how big the problem is. Each year, the world shreds and replaces around 150 billion banknotes, creating 150,000 tons of waste and spending somewhere in the ballpark of $10 billion simply replacing old bills with new ones. A study by a Dutch bank found that over half of the banknotes end up in the “shred” pile because they are too dirty.
So Lawandy and a colleague from Brown, Andrei Smuk, decided to try a money laundering experiment. They chose as their cleanser carbon dioxide gas at just the right temperature and pressure to become a “supercritical fluid”—a gas that acts like a liquid, able to dissolve substances, but that can also permeate porous solids.
In order to test the technique, the researchers had to get money properly dirty. Human skin secretes waxy sebum onto dollar bills, which turns from clear to brown when it oxidizes. To emulate it, they used motor oil and something called “Bey sebum” (made up mostly of beef tallow and other fatty compounds). They put the dirty banknotes in a chamber with high-pressure carbon dioxide, which resulted in a marked brightening of the banknotes, without the loss of any security features. The money also was no weaker or worn than it had been going in—unlike when a bill left in a pants pocket accidentally goes through the laundry.
Researchers also tested whether the cleaning technique only worked on single bills, or whether it could also work on piles of money—100-bill stacks called “straps.” It did. It also worked on “bundles”—10 straps together, meaning that it would be practical.
Lawandy said Spectra Systems is currently doing testing of the technology for two country’s central banks. Since they published their work in December, researchers have found ways to erase Sharpie or gel pens from banknotes—a technique that expands the technology’s utility by allowing graffitied bills to be salvaged. They also hope to see if they can clean the currency with an additive that would actually repair and strengthen the bills.
Lawandy said that although he can’t estimate how much a cleaning machine would cost, it would be far less costly to use one for a year than to replace the equivalent number of banknotes. Money-washing machines made up of 200-liter vessels could hold roughly 200,000 banknotes, and the cleaning procedure takes about an hour, meaning that a single machine could process more than a million banknotes a day.
The cleaning agent, carbon dioxide, would be recycled, and the sludge removed from the money could be discarded — or perhaps, he noted wryly, be put to new use.
“It’s pretty amazing. We extracted 4 grams out of a 100-gram strap,” Lawandy said. “From some dirty notes, that’s a significant amount of material, but that stuff is perfectly usable as a biodiesel. ... Maybe we can make candles out of it.”