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The transition away from fossil fuels has led to a rush for new sources of minerals needed to build green technologies like batteries and solar panels. That has meant an expansion of mining, whether for lithium on the salt flats of Bolivia or for nickel and manganese contained within potato-sized nodules on the seafloor.
This push to mine so-called “critical minerals” comes with serious environmental and social downsides. But there could be a greener source for some of the rarest of these elements: seaweed.
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Some species of seaweed are known to accumulate high levels of certain minerals that exist in extremely low concentrations in seawater. Known as rare earth elements, they are key ingredients for the magnets in wind turbines and electronic motors of all sorts.
Several groups of researchers are now exploring ways to harvest these rare earth elements along with the seaweed, a new and still untested idea in the growing field of “biomining”.
Mining with macroalgae
Bokan Mountain juts above the thick coastal forest of Prince of Wales Island in south-eastern Alaska. Along its ridge, the mountain contains what may be one of the richest deposits of rare earth elements (REEs) in North America. These include elements like neodymium and dysprosium used to make the powerful magnets in wind turbines and other clean energy technologies.
While as a mining site for years, potential environmental impacts have made the project controversial. Now, at the University of Alaska Fairbanks and her colleagues have a plan to collect the mountain’s valuable minerals without mining at all. Instead, they will harvest seaweed.
“We know that seaweed can accumulate [some rare earth elements]. We want to know if they can hyperaccumulate,” she says, meaning they would store the minerals in their tissues at high enough concentrations to make it viable to harvest them.
The idea, backed by a nearly $2 million grant from the US Department of Energy’s research arm ARPA-E, is to grow species of seaweed – technically, macroalgae – that concentrate certain REEs in the waters around the mountain. These could then be extracted, purified further and used like REEs from any other source.
Runoff from the heavy rain in the region should carry higher-than-usual concentrations of REEs eroded from the deposit into the surrounding bays and inlets, says Umanzor, who recently returned from a field trip to sample water there. “That site is so metallic that navigation doesn’t work” because of the magnetic disruption, she says. The question is whether seaweed can be used to collect those magnetic minerals.
Parts per trillion
Umanzor’s project is one of several supported by an ARPA-E on “algal mining”, itself a part of efforts to support new “biomining” technologies that use plants or microbes to extract valuable minerals. The aim of all these is to develop new supplies of REEs and other so-called critical minerals needed for much of the technology required to switch from fossil fuels.
This is meant to avoid the environmental impacts of mining. The US government also wants to secure domestic sources of minerals to reduce dependence on a few countries that dominate mineral mining and refining.
According to a this month from the International Energy Agency, demand for REEs alone is set to nearly double by 2040, largely driven by clean technologies. In 2030, China is set to control more than half of REE mining and 77 per cent of refining, creating a risk of geopolitical disruptions to supply. Maybe seaweed could help.
“I like to think of it as environmentally positive mining,” says at Pacific Northwest National Laboratory (PNNL) in Washington state, who is also working on an ARPA-E backed algal mining project. He says the government’s support for the idea came after , in still unpublished work, found that some macroalgae species growing in tanks could accumulate REEs from seawater at concentrations a million times greater than than the seawater they were grown in.
“The ocean has a vast reserve of these elements but at very low concentrations” of roughly one part per trillion, says , also at PNNL. “If you could concentrate it significantly, you would get far ahead.”
Umanzor says it is unclear whether the algae benefit from accumulating the REEs or other minerals. But the main mechanism seems to be that negatively charged carbohydrates produced by the seaweed attract specific positively charged REEs. In brown seaweeds, for instance, a sugar called alginate that plays a role in the algae’s flexibility has to concentrate heavy REEs such as yttrium.
Other researchers and companies are working on related biomining approaches that involve growing plants on land to accumulate key minerals leached from the soil. However, an important difference with macroalgae in the ocean is that the moving water provides a perpetual supply of new material that can accumulate across the entire organism, not only in the roots. “Seaweed is more of a three-dimensional absorption organism,” says Umanzor.
At Bokan Mountain, Umanzor aims to answer some of the many basic questions that remain about whether such hyperaccumulating seaweed has any hope of providing a worthwhile supply of REEs. This includes how much of the mountain’s minerals are leaching out, and which ones and where they travel after reaching the water. Then there are questions about which species of macroalgae accumulate the most, how well they grow and how best to extract the minerals.
There are many ways the approach could fail – Umanzor says success would be a “miracle”. But if all goes as planned, she envisions dense seaweed farms harvesting REEs from the coastal waters around the mountain and beyond. She plans to begin farming experiments next year. “We will be looking at scales far beyond what is produced for food,” she says. “If we succeed, it’s going to be complementary to traditional mining.”
Seaweed bacon
Even if accumulation and extraction prove to be viable, the supply of REEs from seaweed is likely to be fairly limited, says at the University of Idaho, who isn’t involved with any of the ARPA-E projects. “But it’s something that we can integrate with other technologies,” he says. For instance, hyperaccumulating seaweed could be used to recover valuable materials contained within mining waste as part of a clean-up.
“You won’t ever grow seaweed just for these minerals,” says Edmunson. But other benefits could still make the approach worthwhile. Growing seaweed can help remove pollution from seawater and remove carbon dioxide from the atmosphere. Edmunson and his colleagues are also researching ways to extract minerals from seaweed while maintaining the rest of its biomass to use for other things, like fertiliser, fuel or protein for food. (A company called that makes a bacon substitute using seaweed protein is another ARPA-E grantee.)
Mining with macroalgae may even have potential beyond REEs. “The biological diversity of seaweed is just phenomenal,” says Edmunson. Pick a mineral found in seawater, he says, and “you’re likely to find an organism that is hyperaccumulating that for some reason”.
