Recovering precious metals from e-waste

08 April 2016 | Research
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Assoc Prof Yew's 2 presents a greener way to recover precious metals such as gold from electronic waste

A team of NUS researchers has discovered that electronic waste is not just a load of garbage. Instead, precious metals such as gold, platinum and palladium can be safely and sustainably recovered from electronic waste.

Working together with international collaborators, NUS Synthetic Biology for Clinical and Technological Innovation (SynCTI) researchers found that synthetically engineered microbes can be used to recycle the precious metals. The process, the first such attempt in using synthetic biology for recycling of metals from electronic waste, was developed over a period of three years.

Electronic waste represents a growing portion of unwanted materials in modern cities, posing a major disposal challenge. This presents a pressing need to recycle in order to protect the environment and conserve natural resources.

“Ultimately, we can’t continue to mine precious metals such as gold from ores as we’ll run out of these natural resources, so all these circulating metals have to be recovered,” said Principal Investigator Associate Professor Yew Wen Shan. Supporting this call to recycle is the fact that electronic waste contains a higher concentration of gold compared to gold ores which typically contain other fillers.

Currently, harsh acids such as concentrated sulphuric and nitric acids are used in metal recycling, to oxidise the metal into its soluble state. Electrolysis then reduces the solution to obtain the solid precious metals.

Assoc Prof Yew’s research, funded by the National Research Foundation, aims to replace industrial standard practices. His method recovers precious metals, and removes toxic ones such as lead and mercury, in a greener and more sustainable way.

e waste 2

Assoc Prof Yew showing the electronic waste (right hand) and recycled gold with the bioreactor in the foreground

Within a bioreactor, the team synthetically engineered a soil bacterium known as Chromobacterium violaceum to produce a biochemical derived from the amino acid glycine. The biochemical oxidises the electronic waste in place of harsh acids used conventionally. Subsequently, another class of engineered enzymes is expressed for biological reduction, effectively substituting electrolysis.

“There are existing enzymes that convert glycine into biochemicals. What we have done is to look at this mechanism and improve its efficiency, basically making it even better, in order to oxidise and reduce gold and other precious metals which the naturally occurring enzymes could not,” explained Assoc Prof Yew. He added that the ideal enzymes and solutions would probably not even exist as nature has never been exposed to such large amounts of electronic waste before, rendering them unnecessary, until now. This is where synthetic biology plays a part.

The process takes longer than current methods — one week to obtain the solid precious metals compared to about one hour. However, it can be scaled up to allow for a greater volume of recycling since there are no concerns regarding accidental leakage of toxic chemicals into the environment. 

The novel technique is able to recover 85 per cent of gold from electronic waste and Assoc Prof Yew is confident the team can achieve its target of 99.9 per cent gold recovery for commercialisation. The team is currently working with leading metal recycling company Cimelia Resource Recovery to encourage the adoption of its research as the industry “Gold Standard” in sustainable electronic waste management.

See media coverage.