Discarded electronics, or e-waste, often contain large amounts of gold (Au) and other heavy metals. An environmentally sustainable route to the recovery of such resources combines e-waste with food waste. A low-density aerogel derived from whey protein, a byproduct of the cheesemaking industry, has been developed to selectively adsorb Au from the remains of discarded electronics.

Researchers from ETH Zurich, Switzerland, first placed whey protein into an acidic solution and heated it, which unraveled the proteins from tiny balls into strands. Freeze-drying the solution then forms a lightweight gel-based puck with high porosity.

The potential for the gel to adsorb Au from a solution also containing copper, lead and other metals at the same concentration was evaluated. The aerogel sucked up 93% of the Au while removing less than 10% of any of the other metals. Subsequent trials with real e-waste applied the new adsorbent to dissolved computer motherboards in aqua regia. Au ions from the mixture settled on the surface of the aerogel and were reduced, forming metallic Au.

Each gram of aerogel captured 190 mg of the desired element, and burning the material freed the Au, turning it into a tiny hunk of metal. This process represents an impressive improvement over the activated carbon typically used as an adsorbent, as each gram of the latter secured only about 60 mg of Au from an e-waste mixture.

A paper describing this precious metal recovery technology is published in Advanced Materials.