A fraction of world's seawater could meet energy needs for generations

“Under the sea, Darling it’s better, Down where it’s wetter, Take it from me!” sang a certain Jamaican crab in The Little Mermaid. Turns out he may have been right after all. Scientists say 0.1% of the world’s seawater contains enough critical elements to power humanity’s needs for 50,000 years, and are now exploring ways to extract them.Researchers at the US Department of Energy’s Pacific Northwest National Laboratory (PNNL) are developing three separate technologies and processes that either make this “liquid ore” more practical to mine or a useful “tool” in conventional mining. The first process pulls magnesium hydroxide directly from seawater, eliminating several complex steps necessary in conventional techniques. The second takes the concentrated brine left by desalination, turns it into acids and bases, and uses them as leaching agents for extracting other metals from ground-based ores. Finally, the third recruits seaweed to gather minerals that are otherwise too dilute to chase.Through these processes, PNNL says it has recovered magnesium, lithium, nickel, platinum-group metals and rare-earth metals, using either seawater or chemicals generated from it. Chemist Chinmayee Subban holds a PNNL device that can extract magnesium oxide from seawaterEdward Pablo | Pacific Northwest National Laboratory Seawater contains enormous quantities of critical minerals, such as magnesium, nickel, rare-earth metals, and even lithium. While there’s a probability that you are hearing about this for the first time, it’s actually been known for quite some time. In fact, the United States mined magnesium from seawater for 50 years before switching to imports in the 90s. Also, PNNL scientists managed to extract 5 grams of uranium from seawater in earlier research eight years ago.The catch, as there very often is, is that the critical elements mostly exist in concentrations that are too low to extract economically. According to the researchers, 600,000 gallons (2,271,247 liters) of water, enough to fill an Olympic-sized pool, contains only 0.42 kg (0.926 lb) of lithium and 0.00095 kg (0.0021 lb) of nickel, less than the weight of a standard paper clip. Magnesium, on the other hand, shows much more potential, with the same quantity of water containing 2,980 kg (6,570 lb) of the element, roughly the weight of a 2024 Ford F-250 Super Duty Cargo Truck … so, a lot.Now, techniques for mining magnesium from seawater do exist. However, these techniques are complex and involve numerous processes. They are also very customized to magnesium. What the PNNL researchers are trying to do is create techniques that can simplify the magnesium mining process while also providing means to extract other minerals using the same stream of water.“Our goal is really to maximize the dollars per cubic meter of seawater that we’re pumping and to use that water efficiently and responsibly,” explains Chinmayee Subban, a chemist and the project’s leader. “From seawater, we can extract these critical materials. The challenge will be to scale up these technologies so they can be economically feasible.”The researchers’ solution involves three distinct but related and integrable projects, with the first being extracting the magnesium. Traditional magnesium extraction involves the Dow process, a multi-stage, lengthy chemical process that uses lime (calcium oxide) to precipitate magnesium hydroxide, followed by hydrochloric acid treatment to form magnesium hydrochloride, and then electrolysis to obtain magnesium metal. That’s a summary, by the way.The researchers have created a different approach. Rather than mixing in lime to obtain magnesium hydroxide, PNNL flows seawater alongside (side-by-side) a base, such as sodium hydroxide, in a co-flow reactor. Magnesium ions react with hydroxide ions at the narrow boundary between the two streams, producing solid magnesium hydroxide. As that solid forms, it creates a thin barrier that limits further mixing and prevents calcium in the seawater from joining the precipitate. Fresh liquid continuously passes the reaction boundary before the system can settle into chemical equilibrium, allowing the reactor to produce high-purity magnesium hydroxide without the additional calcium-separation steps required after the bulk mixing that’s characteristic of the Dow process. A close-up of the system that extracts magnesium from seawater – a stream of sodium hydroxide and seawater flow next to each other, causing magnesium oxide to precipitate outChinmayee Subban | Pacific Northwest National Laboratory The team stops at magnesium hydroxide, as it is already a commercially valuable material that the US currently primarily imports. PNNL says its co-flow process, which it has patented, cuts at least four steps from the historical route.To save the cost and effort of pumping millions of liters of seawater solely to obtain minerals, the researchers propose bolting modular reactors onto coastal desalination plants, which already move enormous quantities of the stuff. PNNL environmental engineer Brooke Marten modeled the concept using California’s Carlsbad desalination plant.“The Carlsbad plant processes 108 million gallons of seawater per day,” Marten said. “With a 100 percent magnesium recovery rate, that would provide 524,000 kilograms [1,155,222 lb] of magnesium hydroxide every day, which is more than triple the United States’ current rate of magnesium hydroxide use.”After magnesium recovery, the water could proceed through reverse-osmosis desalination as usual, producing fresh water and leaving behind highly concentrated brine. In the conventional desalination process, the leftover brine mostly poses a disposal problem, as it is highly concentrated and must be carefully managed. However, this brings us to another part of the PNNL solution.The leftover brine is treated with the bipolar membrane electrodialysis (BPMED) process that uses electricity and specialized membranes to split the ultra-salty solution into acidic and alkaline streams. The alkaline stream can be returned to the sea, leaving behind the acidic byproduct, which the PNNL researchers have harnessed for another mining process.In conventional mining, acids “leach” metals by dissolving them out of crushed rock. The researchers used the acid as a leaching agent for olivine, a common greenish mineral containing small amounts of nickel. When they treated olivine containing just 0.27% nickel with waste acid from a demonstration-scale BPMED system, it leached 37% more nickel at room temperature than commercial hydrochloric acid of the same strength. Nickel can be extracted from olivine using a waste acid produced by separating seawater into a base and an acidChinmayee Subban | Pacific Northwest National Laboratory This brings us to the final technology that PNNL researchers are working on: extracting metals from sea plants. The researchers discovered that seaweeds accumulate critical minerals from the surrounding seawater into their tissues, sometimes in exponential concentrations.“Some critical materials show up in seaweed at concentrations a million times higher than the surrounding seawater,” said Scott Edmundson, a research botanist at PNNL’s Marine Research Laboratory.To extract the minerals, they grind harvested seaweed into a wet paste, add an acidic leaching liquid, and apply heat to break the chemical bonds holding the elements inside its tissues. Their initial target is to recover at least 50% of the mineral content, but PNNL says reaching that baseline efficiently remains difficult.At the same time, the researchers are also studying how to turn the leftover seaweed biomass into chemicals, fuel, and fertilizer. Perhaps Korea Institute of Geoscience and Mineral Resources’s technology that turns wet coffee grounds into biofuel comparable to anthracite charcoal can help in the fuel aspect. A diagram of the bipolar membrane electrodialysis processSara Levine | Pacific Northwest National Laboratory Now, remember the acidic “waste” obtained from the earlier BPMED process that found purpose as a leaching agent? In a separate experiment, researchers at PNNL fed the acidic BPMED stream to cultures of the fast-growing microalgae. The acid made carbon dioxide already dissolved in the seawater more available for photosynthesis, and the microalgae grew about three times faster. To be clear, this doesn’t really have anything to do with mineral mining, as the microalgae in question are not a mineral-accumulating type. The experiment just demonstrates another potential customer for the waste acid.So, in summary, groups of researchers, all at PNNL, developed separate but related projects for mining metals from seawater. The coflow process generates commercially valuable magnesium hydroxide, after which the leftover seawater, or regular seawater, is desalinated and split into acidic and alkaline byproducts. The acidic byproduct can then be used as a leaching agent to extract nickel from olivine (a rock, not sea-related) or as a growth supplement for algae. At the same time, researchers are trying to figure out how to extract critical minerals from seaweed.Now for some caveats. PNNL has recovered the named materials at research scale, but it has not demonstrated an economically viable commercial ocean-mining plant. Also, the environmental questions, including chemical discharge, brine management, intake effects, and ecosystem disruption, are acknowledged but not fully resolved here. Lastly, the seaweed work appears especially early-stage and would require cultivation, harvesting, and downstream extraction at enormous scale.Regardless, the interdisciplinary projects hold exciting potential and are currently being developed towards commercial viability.Source: PNNL