There’s more to gold than a luminous shine. The precious metal also conducts electricity easily and resists corrosion, properties that make it irreplaceable in the electronics industry. Experts estimate it takes at least 1,000 kg of ore to produce a gram of gold—the same amount that’s used in about 40 mobile phones. Chemists at the Swiss Federal Institute of Technology, Lausanne (EPFL) discovered a material that can extract gold from solution, which could make mining gold from electronic waste, sewage sludge, and even sea water practical. Pulling gold from such waste sources could offer an alternative to digging up ore from the ground.
The new gold-grabbing material is a composite made by exposing para-phenylenediamine (pPDA) to the metal organic framework, or MOF, known as iron-1,3,5-benzenetricarboxylate. The MOF catalyzes the polymerization of pPDA creating a high-surface-area polymer filled with pores (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b09555).
“We’ve opened the polymer up and given access to all this beautiful functionality,” explains Wendy L. Queen, who led the research effort with graduate student Daniel T. Sun. The polymer is redox active, Queen says, so not only does it grab gold ions, but it also reduces those ions to metallic gold. The composite can selectively harvest gold from solutions containing multiple metals because of this reduction chemistry. “Any other metal that this material grabs that doesn’t have a similar reduction potential to gold, we can release into solution by a chemical treatment process. But because the gold is reduced it stays in the material,” Queen says.
A gram of the composite can extract nearly a gram of gold—pulling out 90-99% of the gold ions present in solution in a matter of minutes. The composite works in wastewater, fresh water, ocean water, and solutions used to leach gold from electronic waste and sewage sludge ash. To harvest the gold, the researchers heat the composite to 900 °C to burn off the organic material, and treat the resulting material with acid to remove the MOF’s iron. The purity of the resulting gold is 23.9 karats—the highest value ever reported for gold extracted from these sources.
Huanting Wang, a chemical engineer at Monash University who studies MOFs, says the composite “will potentially make a significant impact in gold extraction from spent electronic devices and other sources.”
Queen notes that the MOF is inexpensive and estimates that the starting materials to make the composite cost $2.50/kg when purchased on the metric ton scale. The EPFL team is currently working on an apparatus that could pull gold from flowing water.