Researchers Explore ‘Biomining’ Seaweed for Critical Minerals

In a bright, open laboratory nestled along Washington State’s Sequim Bay, among rows of glassware filled with seawater and green and purple seaweed, researchers are investigating a new way to produce the critical minerals that are vital to everyday life.

Minerals known as rare earth elements, like neodymium, are some of the most important ingredients in our electronics, vehicles, and buildings and are traditionally mined underground. Now, researchers at the Department of Energy’s (DOE’s) Pacific Northwest National Laboratory (PNNL) are probing a new ore source, floating just off the nation’s coastlines: seaweed.

Researchers at PNNL’s Sequim campus have been growing several species of seaweed in seawater from Sequim Bay and investigating different methods of extracting minerals from their leaf-like tissues. 

“The ocean is the single largest source of a lot of critical minerals we need for high-tech applications,” said Michael Huesemann, principle investigator on the biomining work at PNNL’s Sequim laboratory in Washington State. “If we’re able to tap seawater and do it in a way that’s responsible, that potentially allows for a domestic source of critical materials.” 

An untapped source of critical minerals

Researchers have long theorized that the ocean could be a new frontier for producing valuable minerals, said Scott Edmundson, research botanist at PNNL’s Sequim laboratory.

As wind, rain, and running water erode rocks, and as humans fertilize the soil, critical minerals end up flushing into the ocean. Simultaneously, minerals travel from deep under Earth’s crust to the ocean floor through undersea volcanoes and hydrothermal vents. 

The problem is that the ocean is so vast—holding 300 million cubic miles of water—that it renders the mineral concentrations much too dilute to be extracted with conventional technologies. The only way to access those minerals is to concentrate them with new approaches. 

As it turns out, seaweeds are fantastic collectors of minerals, Edmundson said. Although researchers aren’t sure of the why or how, what they do know is that the very seaweed species you see scattered along the shoreline may hold a range of critical minerals. And occasionally, the concentration of those minerals in the seaweed exceeds that of ocean water by over a millionfold. 

Huesemann and his colleagues, funded by DOE’s Advanced Research Project Agency – Energy, have been investigating different species of seaweed to figure out which minerals they concentrate, and how much. For instance, they found that a leathery brown seaweed species called Fucus is particularly good at concentrating nickel in its tissues. A green, leafy seaweed called Ulva, meanwhile, seems better at bioaccumulating rare earth elements. Ultimately, the researchers found that Ulva (better known as sea lettuce) is the best at concentrating a number of critical minerals. 

Now that the researchers understand which species of seaweed best concentrates which mineral, the next challenge rears: How do you separate the minerals out? Cobalt or dysprosium aren’t sprinkled on the surface of the seaweed like powdered sugar. The minerals are incorporated into the seaweed’s tissues, Edmundson said.

Which means that the chemical bonds holding the minerals in place need to be broken.

Mining seaweed for minerals

The researchers have experimented with several methods for extracting minerals from seaweed, trying to find the least energy-intensive process. After many rounds of tests, the researchers found a process that’s produced good initial results: after enough seaweed has grown, the researchers first grind it down into a paste, then mix that paste with an acidic liquid that can break off whatever mineral they’re aiming for.

Known as a “lixiviant,” acidic liquids are used in standard ore mining to extract minerals from rock. When the seaweed paste is mixed with the lixiviant, the pH of the mixture decreases (or becomes more acidic), causing the target mineral to break off the seaweed. The mixture is also submitted to high temperatures, which can help break chemical bonds. 

The team’s initial baseline goal is to extract at least 50 percent of the critical mineral content from the seaweed biomass—a task that has not proven to be straightforward, Huesemann said. The researchers have experimented with different lixiviants, different temperatures, and processing the mixture multiple times to get out as much critical mineral as possible.

Economics of biomining

Perfecting the extraction method is just the beginning, Huesemann said. For biomining to be a viable process, the team is also analyzing the economic costs and benefits associated with the process. At one point the researchers used heat to dry the seaweed samples first, but now they skip that step to save energy. Lixiviants are another cost, so the researchers have been exploring using waste acids from other processes, such as ocean alkalinity enhancement, to help extract minerals.

On a positive note, seaweed grows fast and doesn’t need fresh water. What’s more, once the minerals are extracted, the remaining biomass can be repurposed for uses such as biofuel feedstock or for bioproducts like plastics, building material, or even adhesives.

Edmundson also pointed out that because of the significant diversity among the different seaweed species and what minerals they bioaccumulate, critical mineral extraction could be tailored to future technology needs. 

“The diversity is so high that you can pick and grow the organism you want for the specific critical mineral,” Edmundson said. “The critical mineral of today might not be the critical mineral of tomorrow. And the flexibility of seaweed’s bioaccumulation of minerals is such that it allows us to fine-tune the technology to meet the need of the moment.”