To manufacture the sponge, Mohammad Peydayesh, a senior scientist in Mezzenga’s group, and his colleagues denatured whey proteins under acidic conditions and high temperatures so they aggregated into protein nanofibrils in a gel. The scientists then dried the gel, creating a sponge out of these protein fibrils.
To recover gold in the lab experiment, the team salvaged the electronic motherboards from 20 old computers and extracted the metal parts. They dissolved these parts in an acid bath to ionize the metals.
When they placed the protein fibre sponge in the metal ion solution, the gold ions adhered to the protein fibres. Other metal ions can also adhere to the fibres, but gold ions do so very efficiently.
As the next step, the researchers heated the sponge. This reduced the gold ions into flakes, which the scientists subsequently melted down into a gold nugget. In this way, they obtained a nugget of around 450 milligrams out of the 20 computer motherboards. The nugget was 91% gold, which corresponds to 22 carats. The remainder was made out of copper.
Mezzenga’s calculations show that the technology is commercially viable, as procurement costs for the source materials added to the energy costs for the entire process are 50 times lower than the value of the gold that can be recovered.
Next, the researchers want to develop the technology to ready it for the market.
Although electronic waste is the most promising starting product from which they want to extract gold, there are other possible sources. These include industrial waste from microchip manufacturing or gold-plating processes. In addition, the scientists plan to investigate whether they can manufacture the protein fibril sponges out of other protein-rich byproducts or waste products from the food industry.
“The fact I love the most is that we’re using a food industry byproduct to obtain gold from electronic waste,” Mezzenga said in a media statement. “You can’t get much more sustainable than that!”