The world’s oceans contain enormous amounts of magnesium, and scientists have developed a new method that could make it cheaper and more sustainable to recover this valuable mineral directly from seawater.
Magnesium salts are used in everything from dietary supplements and medicines to construction and industrial products.
Today, much of the world’s magnesium is obtained from minerals in rocks, which must be mined, crushed and heated.
These processes can consume large amounts of energy.
Researchers reporting in ACS Energy Letters have now developed an electrochemical system that uses specially designed bismuth electrodes to selectively remove magnesium ions from seawater.
The oceans represent a potentially enormous source. Magnesium is the second most abundant positively charged ion in seawater, after sodium. However, separating the two is difficult because seawater contains much more sodium than magnesium.
Traditional methods for extracting magnesium from seawater often require chemicals that cause the magnesium to form a solid material that can then be removed. Researchers have also experimented with electricity to separate magnesium, but these approaches can depend on expensive membranes.
The new method aims to make the process simpler and less costly.
The researchers built an electrochemical cell containing thin sheets of bismuth. The device had two channels separated by a membrane. Real seawater flowed through one channel while an electrolyte solution flowed through the other.
When an electric field was applied, the bismuth electrodes changed the local acidity of the seawater. This allowed the researchers to collect magnesium in the form of magnesium hydroxide without relying on large amounts of added chemicals.
They then reversed the direction of the electric field and switched the liquids flowing through the channels. This converted the magnesium hydroxide into magnesium chloride, a useful form of magnesium that has a wide range of applications.
By repeating the process, the researchers were able to concentrate the magnesium significantly. Its concentration increased eightfold, eventually producing a magnesium-to-sodium ratio of about 20 to 1.
According to the team, this level of selectivity is better than previously reported electrochemical membrane systems. Being able to separate magnesium from sodium efficiently is important if seawater extraction is to become commercially practical.
The researchers also estimated the economics of their method. They calculated that producing magnesium chloride could cost around $107 per ton, substantially below current market prices.
However, that estimate comes with an important limitation. It does not include some additional processing costs, such as drying the final magnesium salt. More research and larger-scale testing will therefore be needed to determine the true commercial cost.
The potential importance of the technology extends beyond magnesium supplements. Magnesium compounds are used in health care, manufacturing, construction and other industries, so developing new domestic sources could help strengthen supply chains.
The research also highlights a broader possibility: instead of viewing seawater mainly as something that needs to be desalinated, scientists could increasingly treat it as a vast source of useful materials.
If the technology can be successfully scaled up, the ocean could provide a plentiful new source of magnesium while reducing reliance on more energy-intensive mining and processing methods.
