Graphene oxide sponge soaks up gold from electronic waste
A self-assembled composite of graphene oxide and chitosan can capture gold from electronic waste many times more efficiently and selectively than existing materials, researchers in Singapore have shown. The material could potentially eliminate several purification steps and make industrial recycling more economically competitive.
Because of the inertness of gold, its mining has a significant environmental footprint, usually requiring toxic chemicals such as mercury or cyanide to extract it from other components of the ore. This stability, together with its high electrical conductivity and ductility, makes it useful in electronics. Recycling gold to reduce mining faces the same extraction problems. Unwanted components are decomposed using one of several possible processes such as immersion in aqua regia (a concentrated mixture of hydrochloric acid and nitric acid). This yields a mixture of gold(I) and gold(III) ions mixed with copper, nickel, zinc and many others. ‘Now electrolysis is used,’ says Daria Andreeva at the National University of Singapore; ‘Electrolysis is a very long process that can take days or even weeks, so it’s a very interesting approach to look at how to make [separation] more efficient from an energy or time point of view.’
In previous work, Andreeva and colleagues led by Kostya Novoselov – who shared the 2010 Nobel prize in physics with Andre Geim for his work on graphene – developed self-assembled membranes from graphene oxide and other materials for applications ranging from tunable water filtration to corrosion prevention. In the new work, the researchers combined a solution of chitosan with a dispersion of graphene oxide flakes. When freeze dried, it formed a sponge-like material with ion-binding sites that could selectively capture and reduce both gold(I) and gold(III) ions – which must usually be extracted separately. Their material demonstrated substantially higher capacity to adsorb both ions: previous adsorbents have captured around 0.3g gold(I) and 2g gold(III) per gram of adsorbent – theirs captured 6.2g gold(I) and 16.8g gold(III).
