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Homes, restaurants, and the coffee industry collectively produce about 6 million tons of spent coffee grounds every year. Researchers have now come up with a practical way to use some of this waste. They have made a rubbery foam from used coffee powder and silicone that can pull lead and mercury ions from water (ACS Sustainable Chem. Eng. 2016, DOI:10.1021/acssuschemeng.6b01098). The spongelike material could be used to clean heavy-metal-contaminated water on a large scale, its creators say.
Spent coffee grounds are already used as fertilizer and converted into biodiesel. Scientists have also studied the ability of coffee grounds to remove heavy metals from wastewater, since the grounds contain charged amine, carboxylic, and phenolic groups that adsorb heavy metal ions. But filtering grounds out of the water postcleanup is difficult.
So Despina Fragouli and her colleagues at the Italian Institute of Technology decided to fix coffee into a rubbery, porous material—a silicone foam that is cheap and easy to make.
The researchers made the foam by mixing finely ground used coffee powder and a small amount of sugar into a solution of the elastomer acetoxy polysiloxane and a polydimethylsiloxane surfactant, and allowing the mixture to dry and polymerize overnight. Then the researchers dipped the material into warm water to dissolve the sugar crystals, leaving behind pores and yielding a spongy foam that contains 60 to 70% coffee by weight.
To test its ability to clean wastewater, the team immersed the foam in aqueous solutions of varying concentrations of lead and mercury ions for 30 hours. Each gram of the foam could adsorb a maximum of about 13 mg of lead ions and up to 17 mg of mercury. The foam adsorbed more than five times as much lead by weight as spent coffee powder. “This makes it realistic to use spent coffee in a large scale application,” Fragouli says. By using a sufficient amount of foam, it should be possible to remove enough metal ions to meet drinking water standards, she adds.
“We are exploring ways to remove metal ions from the foams without altering their functionality so they can be reused,” Fragouli says. The researchers are also planning to make fully biodegradable foams to make disposal simpler and more cost effective. The team has also begun making the foam from elastomers that spontaneously become porous during polymerization by forming gas bubbles, eliminating the need for sugar in the process.
No one has attempted to make composite coffee foams for water remediation before, says Constantine M. Megaridis, a mechanical engineer at the University of Illinois, Chicago. “Millions of tons of spent coffee wind up in landfills every year, so the proposed method not only reduces the solid waste stream but removes dangerous heavy metal pollutants from water,” he says. But collecting enough coffee grounds for large-scale application of this technology could be difficult, he adds, and doing it cost effectively would require a system to be set up with commercial users like hotels and restaurants.
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Chemical & Engineering News
ISSN 0009-2347
Copyright © 2016 American Chemical Society
Microscopic chinks in a material can spread and grow into larger fissures—ones that can split apart the plastics and composites used in airplanes, spacecraft, electronics, sports equipment, and pipes. A new, simple technique uses embedded microcapsules to reveal tiny, invisible cracks in a wide variety of plastics by making the cracks glow (ACS Cent. Sci. 2016, DOI: 10.1021/acscentsci.6b00198). Such an early warning strategy to detect these fractures could allow engineers to replace or repair critical components and prevent catastrophes.
Microscopic capsules loaded with various patching materials have been mixed into paints, plastics, and electronics to give the materials self-healing properties. Some researchers have tried to extend this technique to monitoring structural materials for damage. Typically, they have filled capsules with dyes or fluorescent molecules that change color or glow when they react with certain functional groups within the polymer or with triggering compounds. But such early warning systems are complex or have to be redesigned for each polymer or composite.
Nancy R. Sottos, Jeffrey S. Moore, and colleagues at the University of Illinois, Urbana-Champaign, came up with a simple, sensitive system that’s independent of the polymer. They embed plastics with microcapsules filled with molecules that glow on their own after being released. The slightest crack in the plastic ruptures these capsules, triggering a bright blue fluorescent signal that can be detected under ultraviolet light for days. It’s as if the material “bruises,” letting you quickly identify a damaged part before it fails, says Michael Keller, a mechanical engineer at the University of Tulsa, who was not involved in the work.
The system uses polyurethane capsules about 110 µm wide filled with a dilute solution of 1,1,2,2-tetraphenylethylene (TPE) in an organic solvent. The TPE molecules fluoresce brightly only when they aggregate. When damage ruptures the capsules, the solvent evaporates, and the molecules form crystalline deposits on the capsule shell that shine under UV light.
To test the system, the researchers made coatings of epoxy, polyurethane, polydimethylsiloxane, polyurethane, and polyacryclic acid, each containing 10% by weight of the capsules. Scratches made with a blade were undetectable under visible light but shone bright blue when inspected with a handheld UV lamp. The researchers could detect cracks smaller than 2 µm in size and as long as 40 days after the damage occurred.
“We really are interested in incorporating this technology with self-healing technology,” saysMaxwell J. Robb, a postdoctoral researcher in Moore’s laboratory. They also plan to explore the use of the capsules inside composites and other materials.
A lot of time and money is spent today inspecting structural composite parts or overdesigning them so they do not fail, Keller says. He adds that the approach might not work for cracks deeper within a material because the solvent might not evaporate and the light might be harder to spot.
Christoph Weder, a polymer chemist who studies self-healing materials at the University of Fribourg, adds that long-term stability might be a challenge since the solvents could evaporate prematurely, and the need for UV light adds a step. Nevertheless, its simplicity and versatility for various polymers makes the strategy promising, he says. “When I first saw this advance, I said, ‘Four-letter word,’ we should have thought of this.”