Ever since humanity discovered the metal gold, it has almost permanently nestled everywhere around us. In our modern times, gold is not only used to make jewellery but due to its various unique properties, is used in making electronic components, medicines and even as an ingredient in cooking. Interestingly, one of the most common uses of gold today is in smartphones, laptops and digital cameras. Due to gold’s better conductivity and general resistance to oxidation and corrosion in varying environments, gold is used in circuit boards
and numerous electronic connectors. All this gold lasts nearly forever, but not the electronics it is buried in. Use ofelectronic devices such as, smartphone, laptops, computers, tablets, printers is increasing and thus the use of gold in their (devices design). One of the most common uses of gold today is in smartphones, but with a limited lifespan, these smartphones gets thrown in the waste along with the gold. In the electronic devices dominated generation/civilization, taking into consideration the growing heaps of electronic-waste all over the world in general and in India in particular and possibility of large quantity of gold extraction from this waste, many innovative, safe, economical technologies are always proposed by the researchers and technologists. No wonder in 2016, the executive summary of e-waste noted a massive 44.7 million tons of e-waste in just one year. By 2021, the world will likely be cluttered with a full 52.2 million tonnes of such waste, which today consists mainly of fridges, washing machines and other domestic appliances, but also increasingly mobile phones and computers.
Gold from waste
From circuit boards to discarded CPU’s, e-waste is a huge concern for recycling. To put that in perspective, a printed circuit board (PCBs) (used in cellular phones and personal computers) contains about 280 g/ton-waste of gold. With about 44.7 million tonnes of waste, it means that there are about 12,000 tonnes of gold in that waste, which is about 10,00,000 standard gold bars lying in trash heaps. In fact, Japan is even looking to recycle old technology, including smartphones, to create Olympic medals for Tokyo 2020 Games. But the problem lies in how the gold is extracted. Methods such as precipitation, ion exchange, solvent extraction, and flotation for gold recovery are available. But these methods have major disadvantages like the use of toxic chemicals, high reagent requirements and the generation of toxic secondary wastes that require proper disposal. Hence a better alternative for the extraction of gold is the need of the hour. And this alternative is what researchers from across India have developed using plant leaves.
Gold extraction using plant leaves
Recently, Indian scientists have proposed the use of plant leaves to extract Gold from e-waste. The plant Lagerstroemia speciosa can be used to extract gold from e-waste and wastewater due to its unique properties. Scientists from North Maharashtra University, Debajyoti Paul from Department of Earth Sciences, Indian Institute of Technology Kanpur and Dipak J. Garole from Directorate of Geology and Mining, Government of Maharashtra have created this method. The method of gold extraction from e-waste can be divided into following different parts:
- A bio-sorbent that adsorbs gold
- To remove gold from the e-waste
- To use the bio-sorbent to adsorb the gold
- To retrieve the gold from the bio-sorbent
For the first step, the leaves of Lagerstroemia speciosa, commonly known as “Pride of India”, are used. These can bond to specific metal ions, e.g., gold. But using only the leaves to adsorb gold is not very effective. So scientists made a bio-sorbent with the leaves and polyethyleneimine (PEI). Polyethyleneimine (PEI) has been widely used for the adsorption and recovery of various metal ions from aqueous media. First, Lagerstroemia speciosa plant leaves are collected, washed with ultrapure water, air dried at room temperature and powdered. Then PEI is used to prevent leaching and to increase adsorption. The leaves powder is suspended in a PEI solution and stirred for four hours at 25°C using a magnetic stirrer. This solution is developed into a PEI-LS bio-sorbent solid, separated from the suspension using vacuum filtration.
The above flow chart shows how the extraction of gold is done. This is done by treating the waste to various acidic treatments, obtaining an acid-leached solution of the gold. This acidic gold solution is then mixed with the PEI-LS bio-sorbent under experimental conditions and viewed under a scanning electron microscope and in an energy-dispersive X-ray spectroscope. The EDS spectrum of gold-sorbed PEI-LS reveals the presence of gold peaks, with an adsorption rate of 97.9%. The electron microscopic and spectroscopic images of the bio-sorbent before mixing in the acidic leached gold solution and after show the presence of gold in the solution. Then, the gold has to be recovered from the bio-sorbent. This is done using various eluting agents. Acidic thiourea has the most recovery, with about 96.9% of the gold retrieved from the bio-sorbent. This technique gives us a low-cost and eco-friendly bio-sorbent by a simple PEI modification of Lagerstroemia speciosa leaves powder that will extract gold from e-wastes. This bio-sorbent can even be used to recover gold from the wastewater of gold mines or jewellery polishing water and finally, the bio-sorbent has a good reusability rate after each adsorption process.