For
coffee lovers, the first cup of the morning is one of life’s best aromas. But
did you know that the leftover grounds could eliminate one of the worst smells
around—sewer gas?
In
research to develop a novel, eco-friendly filter to remove toxic gases from the
air, scientists at The City College of New York (CCNY) found that a material
made from used coffee grounds can sop up hydrogen sulfide gas, the chemical that
makes raw sewage stinky.
Teresa
Bandosz, PhD, CCNY professor of chemistry and chemical engineering, develops
and tests materials that scrub toxic gases like hydrogen sulfide from air in
industrial facilities and pollution control plants. Much like the grains of
charcoal packed into the filter of a tabletop water pitcher, her filters use a
form of charcoal called “activated carbon.”
Carbon
producers already use materials like coal, wood, peat, fruit pits, and coconut
shells to make filters. Bandosz realized that our modern coffee culture could
supply an abundant source of eco-friendly organic waste. But coffee grounds
also come equipped with a special ingredient that boosts their smell-fighting
power.
Caffeine,
the stimulant that gives coffee its energy jolt, contains nitrogen. This
element cranks up carbon’s ability to clean sulfur from the air, a process
called adsorption. “We should not neglect the natural biomass that is rich in
this element,” she and colleagues assert in the Journal of Hazardous Materials. The National Science Foundation
(NSF) and the Army Research Office funded the research.
Usually,
making carbon adsorbents more reactive to toxins requires treating the original
with a nitrogen-rich chemical such as ammonia, melamine, or urea, the main
nitrogen-containing substance in mammal urine. “All of these,” the researchers
note, “significantly increase the cost of adsorbents.”
To
make their new filter, Bandosz and her colleagues carbonized old coffee
grounds, essentially turning them into charcoal.
To
do so, they prepared a slurry of coffee grounds, water and zinc chloride, a
chemical “activator.” The team then dried and baked the mixture at temperatures
of up to 800 C. The process of activation fills the carbon with scores of
minute holes about 10 to 30 A in diameter, roughly equivalent to 10 to 30
hydrogen atom-widths across. These densely packed pores are blanketed with
nitrogen, perfect to capture hydrogen sulfide molecules passing through.
Hydrogen
sulfide gas isn’t just a smelly nuisance for sewage plant neighbors; it can be
deadly. Human noses are so sensitive to the rotten-egg scent of this toxin that
it can overwhelm the sense of smell, Bandosz explained. “When someone is exposed
to high concentrations of hydrogen sulfide, the nose will stop detecting it,”
she said. “There have been cases in which workers died of hydrogen sulfide
exposure in sewer systems.” Bandosz suspects that the coffee-based carbon could
also separate out other pollutants from the air and water.
With
the ubiquitous motto to “Reduce, Reuse, Recycle,” and coffee-ground carbon’s
special affinity for a toxic gas, Bandosz hopes coffee grounds can be
commercially developed into the next green waste filter. For now, however, she
recycles them on her own: “I put them outside under the plants in my garden,
especially those that like acidic soil,” she said. They are a great fertilizer,
of course, packed as they are with nitrogen-rich caffeine.
