Salmon Sperm Used to Recycle Rare Earth Elements
The supply of several rare earth elements (REEs), such as neodymium, are limited. REEs are only found in a few places on Earth and current extraction methods are costly and damage the environment. For this reason, the recycling of these rare earth elements is important.
Japanese researchers have found an environmentally friendly alternative for recovering and separating REEs from ore and advanced materials such as neodymium magnets, using salmon sperm, also known as milt.
The team, led by Yoshio Takahashi, a professor of environmental chemistry at Hiroshima University found that several REEs bound strongly to phosphate-containing molecules on the surface of bacterial cells. This sparked an interest in DNA, which contains a lot of phosphate. However, since most DNA samples are soluble in water and would require a solid substrate attached to them the scientists were driven toward salmon milt, which is largely made up of DNA, but is solid and insoluble.
In the experiment, the team fragmented the salmon milt into small pieces, creating a powder. They then prepared a mixed standard solution containing all the REEs, including the three main metals used in neodymium magnets – neodymium, dysprosium and trivalent iron. On addition of milt to the prepared solution, the milt was found to absorb the rare earth elements equally as well as bacteria. The study showed that the phosphate site in the milt allows for it to be used as an adsorbent for REEs due to the metal ions having a high affinity for the phosphate in the powder.
The REEs were recovered from the milt powder by adding acid to the solution and separating the various substances using a centrifuge.
Jean-Claude Bunzil, a scientist who investigates REEs at the Swiss Federal Institute of Technology, found the use of cheap milt interesting, commenting that “although the proposed protocol does not suppress dissolving the magnets in strong acid, it deserves attention”. Bunzil also suggested that “milt might be particularly useful for recycling the metals within the alloys found in electronic circuits, mobile phones and hard disk drives.” However, he understands the difficulty in scaling up this process, feeling this deserves more attention.
Milt is cheap and readily available, with the Japanese fishing industry disposing of thousands of tons of salmon sperm each year. The results of this study showed that the milt has a sufficiently high affinity to adsorb REEs and is therefore an environmentally friendly alternative to the current methods of recycling rare earth metals.
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