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Reiko Matsushita/Shinta Watanabe
Gold and certain other precious metals are important components of computer chips, including those used in consumer electronics such as smartphones. However, these metals can be difficult to recover and recycle from e-waste. Japanese researchers have found that a pigment widely used by artists called Prussian blue can extract gold and platinum-group metals from e-waste much more efficiently than traditional bio-based absorbents, according to a recent article in Scientific Reports journal.
“The amount of gold contained in one tonne of cell phones is 300 to 400 grams, which is 10 to 80 times that in one tonne of natural ore,” the authors write. “The other elements have a similar situation. Consequently, recovering these valuable elements from e-waste is much more effective and efficient compared to collecting them from natural ore.”
Prussian blue is the first modern synthetic pigment. Admittedly, there was once a pigment known as Egyptian blue that had been used in ancient Egypt for millennia; The Romans called it caeruleum. But after the collapse of the Roman Empire, the pigment wasn’t used much, and eventually the secret of how it was made was lost. (Scientists have since figured out how to replicate the process.) So before Prussian blue was discovered, painters had to use indigo dye, smalt, or the expensive ultramarine found in lapis lazuli for deep blue hues.
It is believed that Prussian blue was first synthesized by accident around 1706 by a Berlin paint maker named Johann Jacob Diesbach. Diesbach attempted to create a red pigment by mixing potash, iron sulfate, and dried cochineal. But the potash he used appeared to be contaminated with blood – it’s thought to be a cut finger or some similar minor injury. The subsequent reaction produced a distinctive blue colored iron ferrocyanide and was eventually named Prussian blue (or Berlin blue).
The earliest known painting to use Prussian blue is currently that of Pieter van den Werff Entombment of Christ (1709), but the recipe was published in 1734, and Prussian blue was soon in widespread use among artists. Hokusai’s famous artwork, The Great Wave off Kanagawais one of the most famous works in which the pigment was used, along with Vincent van Gogh The starry night and many of the paintings from Pablo Picasso’s ‘Blue Period’.
The pigment has other uses. It is often used to treat heavy metal poisoning from thallium or radioactive cesium because its lattice-like network structure — similar to a jungle gym — can trap metal ions from those metals and prevent them from being absorbed by the body. Prussian blue helped remove cesium from the soil around the Fukushima power plant after the 2011 tsunami. Prussian blue nanoparticles are used in some cosmetics and used by pathologists as a dye to detect iron in, for example, bone marrow biopsy samples.
So, it is a very useful substance, which is why the Japanese authors of this latest publication decided to explore other potential practical uses. Using X-ray and UV spectroscopy, they analyzed how Prussian blue absorbs multivalent metals such as platinum, ruthenium, rhodium, molybdenum, osmium and palladium. They were surprised at how well the pigment retained its scaffold structure while replacing iron ions in the scaffold – the secret of its impressive uptake efficiency compared to bio-based absorbents. This is great news for e-waste recycling.
According to the authors, Prussian blue could also solve one of the challenges in the disposal of nuclear waste. Current practice involves converting radioactive liquid waste to a glass-like state at a reprocessing plant prior to disposal. However, platinum group metals can accumulate on the walls of the furnace and eventually lead to uneven heat distribution. Therefore, the melters must be flushed after each use, which in turn increases costs. Prussian Blue was able to remove these deposits without having to rinse the melters after each use.
DOI: Scientific Reports, 2022. 10.1038/s41598-022-08838-1 (About DOIs).
