Sponge extracts gold from e-waste

This sponge captures tiny amounts of gold in e-waste

ben guarino

According to a recent United Nations report, if all the 62 million tons of e-waste produced each year were loaded into garbage trucks, they would circle the Earth bumper-to-bumper. And that terrifying traffic jam must be hiding an astonishing amount of precious metals, including gold, which is essential for electronics because it conducts electricity, stretches into wires, and doesn’t corrode easily. Modern iPhones use gold in the camera, circuit board, and USB-C connector. On a pound-for-pound basis, there is more gold in a cell phone than in ore from a typical gold mine.

But extracting precious metals from discarded electronic equipment is a grueling process. Recycling facilities use energy-intensive smelters to process e-waste under intense heat. Alternatively, they use corrosive agents to break down bulk electronics into liquids filled with metal ions. This approach requires complex electrochemical processes and toxic treatments to extract valuable elements in metallic form. The search for environmentally friendly methods that eliminate these additional steps has led materials scientists down some unusual alleys. For example, airgel made from whey protein, a byproduct of cheese, can capture gold ions from computer motherboards soaked in acid. For another experimental material this month, Proceedings of the National Academy of Sciencescombines graphene (a sheet of carbon one atom thick) with chitosan, a sugar found in shrimp shells. The chitosan spontaneously attaches to the carbon sheet, so the sponge essentially builds itself.

In the first experiment, the study authors used a sponge to filter water containing gold ions. The pale yellow liquid turned transparent, and gold particles piled up on the graphene surface. Once there, the ions reacted with chitosan, a natural reducing agent that helps transform the gold back into its metallic form. The researchers also tested the sponge on partially processed e-waste. When the scientists increased the acidity of the liquid to pH 3, the chitosan in the sponge captured the remaining gold, ignoring other metals. Although an acidic environment is required, the use of sponges could potentially eliminate the need for further processing that relies on poisons such as cyanide to obtain metallic gold from liquids. “Our method allows us to efficiently recover gold directly from waste mixtures,” say study co-authors Daria Andreeva Baumler and Konstantin Novoselov, materials scientists at the National University of Singapore. .

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This new material is one of the most powerful gold adsorbents ever created. (Although adsorption is similar to the well-known absorption, advertisementAdsorbed substances accumulate on the surface, absWhat is absorbed is taken into the body. It’s the difference between a lump of mustard on your chin and a freshly eaten hot dog. ) The sponge collected up to 99.5 percent gold by weight from liquids with gold concentrations as low as 3 ppm.

“As far as I know, this is the highest value ever,” says Raffaele Mezzenga, a physicist at the Swiss Federal Institute of Technology Zurich and author of the whey protein aerogel study. He was not involved in the study of the combination of graphene and chitosan. . He notes that while the sponge is efficient, the parts needed to make it aren’t cheap, and he questions whether it would be a viable option “under real-world operating conditions.” Adapting the technology for use on an industrial scale is “the very next step in our research,” said Novoselov and Andreeva Baumler.