The shortlist of 15 innovative ideas includes two British entries, plus a “Great Bubble Barrier” to catch plastics before they reach the ocean.
The environmental competition awards five winners with £1m each to develop projects designed to find solutions to repair and regenerate the earth.
Notpla Hard Material – a start-up run by Pierre Paslier and Rodrigo Garcia Gonzalez in London – makes packaging from seaweed and plants as an alternative to single-use plastic.
It has already created more than one million biodegradable takeaway food boxes for the firm Just Eat.
The other UK finalist – Low Carbon Materials (LCM), based in County Durham – uses unrecyclable plastic waste to make traditional concrete blocks carbon-zero.
Dr Natasha Boulding, one of the co-founders of LCM, said: “Until now, construction has been one of the hardest industries to decarbonise.
“With LCM, that could all change. We've turned concrete net-zero and now we need the world to start using it.”
Other finalists include The Great Bubble Barrier, from the Netherlands, which uses air pumped through a perforated tube to create a curtain of bubbles to catch plastic and prevent it from ending up in the ocean.
The City of Amsterdam Circular Economy is also a potential winner with its city-wide initiative to establish a fully circular economy by 2050, wasting nothing and recycling everything.
Elsewhere, Mukuru Clean Stoves from Kenya provides cleaner-burning stoves to reduce unhealthy indoor pollution and provide a safer way to cook.
The initiative was started by Charlot Magayi, who grew up in one of Nairobi's largest slums, Mukuru, and who used to sell charcoal for fuel.
Charlot suffered from repeated respiratory infections due to the charcoal pollution, and then sought an alternative solution after her daughter was severely burnt by a charcoal stove in 2012.
Her eco-stoves use processed biomass made from charcoal, wood and sugar cane and cause 90 per cent less pollution than an open fire, and she plans to create an even cleaner version which burns ethanol.
Prince William described the finalists as “innovators, leaders, and visionaries” and said they proved there are “many reasons to be optimistic about the future of our planet”.
“They are directing their time, energy, and talent towards bold solutions with the power to not only solve our planet's greatest environmental challenges, but to create healthier, more prosperous, and more sustainable communities for generations to come,” he added.
There are five Earthshot categories: Protect and restore nature; Clean our air, Revive our oceans; Build a waste-free world; and Fix our climate.
In launching the ambitious 10-year £50m prize, Prince William took inspiration from former US president John F Kennedy’s Moonshot project, which aimed to advance humankind’s achievements.
The winners will be announced in Boston in December.
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