¿ìè¶ÌÊÓƵ

Strings of plastic balls dangled in the ocean could harvest enough cobalt for hundreds of thousands of electric car batteries. The heavy metal is a key battery ingredient, but onshore reserves are running low. So engineers in the US want to mine it from brine.

Maha Haji and Alexander Slocum at the Massachusetts Institute of Technology say the system could catch enough dissolved cobalt from seawater each year to make a battery for every Tesla Model 3 that has rolled off the production line so far. In total, repurposing 76 unused oil rigs in the Gulf of Mexico could produce enough cobalt for half a million electric vehicle batteries.

Growth in sales of electric cars mean global demand for cobalt could outstrip supply for the first time next year, according to Europe’s Joint Research Centre. However, seawater swims with dissolved minerals and the world’s oceans carry about 500 million tonnes of cobalt, dwarfing the 7 million tonnes in known reserves on land.

The proposal would be to fill plastic spheres, each about the size of a beach ball and riddled with holes, with absorbent materials and strap them to long ropes immersed in the ocean. The absorbent materials, such as algae or lemon peel, would bind with the dissolved cobalt more than other minerals and pull it from solution. Every few weeks the chains of balls would be dragged back in to collect the cobalt they soak up.

The technique has already been used in lab tests to harvest uranium. Cobalt is a stiffer challenge because its concentration in seawater is about eight times lower.

The study does not tackle the economics and whether the process could be made cheap enough to be carried out on a large scale. However, one way to reduce costs could be to use waste materials, such as recycled plastic bottles to make the balls. The team says further studies would be needed to assess the environmental impact.