How electrochemistry could help recover magnesium from seawater
Seawater contains dissolved compounds that could provide valuable chemical resources. One of these resources is magnesium, an element used in lightweight alloys, medicines, agriculture and many industrial processes.
However, recovering magnesium from seawater is not straightforward. Magnesium ions are mixed with much larger quantities of sodium and chloride ions, as well as calcium, potassium and many other dissolved substances. Any useful method must therefore do more than collect magnesium. It must separate and concentrate it without bringing excessive amounts of the other ions along for the journey.
A recent study explored how electrodes containing bismuth could be used to recover magnesium from seawater. Although the researchers used specialised equipment, the process is based on ideas found in GCSE Chemistry, including electrolysis, oxidation, reduction, pH and precipitation.
Why recover magnesium from seawater?
Magnesium is used to produce strong, lightweight alloys for vehicles, electronic equipment and other products. Magnesium compounds also have important medical, agricultural and industrial uses.
The element can be obtained from rocks, underground salt deposits, brines and seawater. The oceans contain a vast supply of dissolved magnesium ions, but their relatively low concentration makes recovery difficult.
A successful method needs to:
separate magnesium from other dissolved ions;
transfer it from a large volume of seawater;
produce a smaller and more concentrated solution;
limit the energy and additional chemicals required.
The new research investigated whether electrochemistry could help achieve these aims.
Electrochemistry is the study and use of chemical reactions involving electricity. It includes familiar GCSE topics such as electrolysis and electrochemical cells.
The GCSE chemistry behind the research
My earlier guide, Electrolysis Made Easy, explains the basic theory of electrolysis. An electrolyte contains mobile ions, while oxidation and reduction reactions occur at the electrodes when an electric current passes through the system.
Reduction is the gain of electrons, while oxidation is the loss of electrons:
OIL RIG: Oxidation Is Loss, Reduction Is Gain
At GCSE, students are usually given inert electrodes. An inert electrode conducts electricity but does not take part in the chemical reaction.
Students then use two familiar rules to predict the products:
At the cathode, the reactivity series helps determine whether the metal or hydrogen is produced.
At an inert anode, a concentrated halide produces a halogen. Otherwise, oxygen is usually produced.
These rules work because the inert electrode provides a surface for the reaction without becoming one of the reactants.
The electrodes used in this research were different. Bismuth and bismuth oxychloride took part in reversible chemical reactions. The researchers could therefore oxidise or reduce the electrode material itself.
This created another possible electrode reaction. Instead of always discharging ions from the solution to produce a gas, the electrical current could change bismuth into bismuth oxychloride and back again.
That difference is central to understanding how the process worked.
Why was magnesium metal not produced?
It is important to distinguish between recovering magnesium ions and producing magnesium metal.
Magnesium is more reactive than hydrogen. During the electrolysis of an aqueous magnesium solution, hydrogen is normally produced at the cathode instead of magnesium metal. Magnesium cannot therefore be deposited easily from seawater using ordinary aqueous electrolysis.
Industrial magnesium production usually requires magnesium chloride to be separated, purified and dried before being electrolysed while molten.
The researchers did not attempt to produce magnesium metal directly. Their aim was to transfer magnesium ions from seawater into a much smaller volume of concentrated magnesium chloride solution.
Further processing would still be required to turn this into magnesium metal. No shiny strips of magnesium emerged from the machine ready for a science teacher to set alight.
Stage one: capturing magnesium from seawater
The researchers used an electrode containing bismuth oxychloride, BiOCl. During the magnesium-capture stage, bismuth oxychloride gained electrons and was reduced:
BiOCl + 2H⁺ + 3e⁻ → Bi + Cl⁻ + H₂O
The exact equation extends beyond the usual GCSE specification, but its effect can be understood using familiar principles.
The reaction uses hydrogen ions, H⁺, close to the electrode. This reduces the concentration of hydrogen ions in that small area, making the conditions around the electrode more alkaline.
Magnesium ions in the seawater can then react with hydroxide ions:
Mg²⁺ + 2OH⁻ → Mg(OH)₂
Magnesium hydroxide is insoluble in water, so it forms a solid precipitate.
A precipitate is an insoluble solid that forms from a solution.
The magnesium therefore began as dissolved Mg²⁺ ions in the seawater and became solid magnesium hydroxide close to the electrode.
Importantly, the researchers did not need to make the entire seawater sample strongly alkaline. The electrode created a small alkaline region around its surface, which was enough to make magnesium hydroxide form.
What happened at the other electrode?
Reduction cannot occur alone. If one substance gains electrons at the cathode, another substance must lose electrons at the anode.
In an ordinary GCSE experiment using inert electrodes, the chloride ions in seawater could be discharged at the anode:
2Cl⁻ → Cl₂ + 2e⁻
This would produce chlorine gas.
However, the researchers did not use an inert anode. They used bismuth, which could take part in the reaction. At this electrode, bismuth was oxidised to bismuth oxychloride:
Bi + Cl⁻ + H₂O → BiOCl + 2H⁺ + 3e⁻
In this equation, bismuth loses electrons, so bismuth is oxidised. The chloride ions become part of the solid bismuth oxychloride instead of losing electrons and forming chlorine gas.
This is not explained directly by the reactivity-series rule students use at the cathode. At the anode, several oxidation reactions may be possible:
chloride ions could be oxidised to chlorine;
hydroxide ions could be oxidised to oxygen;
the bismuth electrode could be oxidised to bismuth oxychloride.
Which reaction occurs depends on how easily each one takes place under the particular conditions. This includes the electrode material, the substances present, their concentrations and the voltage applied.
The researchers controlled the voltage so that the reversible bismuth reaction occurred instead of the unwanted reactions that would produce chlorine or oxygen.
A useful GCSE comparison is the electrolysis of copper sulfate using copper electrodes. With inert electrodes, oxygen is usually produced at the anode. With a copper anode, however, copper atoms lose electrons:
Cu → Cu²⁺ + 2e⁻
The copper anode dissolves instead of oxygen being produced. In both examples, a reactive electrode introduces another possible reaction.
Was the magnesium hydroxide attached to the electrode?
The magnesium hydroxide formed as a deposit on and within the rough, porous surface of the bismuth-containing electrode.
The supporting evidence showed that magnesium hydroxide had collected around the electrode. However, it did not show that the solid was attached by a particular chemical bond. It is therefore more accurate to describe it as a physical deposit formed on and within the porous coating.
After the capture stage, the researchers rinsed the cell gently with deionised water. Deionised water has had most of its dissolved ions removed.
Magnesium hydroxide is only slightly soluble in neutral water, so most of the solid remained on the electrode during the rinse.
The rinse removed seawater left inside the cell, including unwanted sodium ions. This reduced the amount of sodium that entered the final magnesium-rich solution.
The magnesium hydroxide was not filtered out and carried to another container. It remained deposited on the electrode while the liquid surrounding it was changed.
Stage two: collecting the magnesium
The researchers then replaced the seawater with a much smaller amount of liquid and reversed the polarity of the cell. Reversing the polarity swapped the roles of the electrodes.
The electrode that had reduced bismuth oxychloride during magnesium capture could now carry out the reverse reaction. Bismuth was oxidised back towards bismuth oxychloride:
Bi + Cl⁻ + H₂O → BiOCl + 2H⁺ + 3e⁻
This reaction produces hydrogen ions. The area around the electrode therefore becomes more acidic.
The hydrogen ions react with the deposited magnesium hydroxide:
Mg(OH)₂ + 2H⁺ → Mg²⁺ + 2H₂O
The solid magnesium hydroxide dissolves, releasing magnesium ions into the new liquid. Chloride ions are also present, so the collected product is a solution containing magnesium chloride.
In the scaled experiment, the researchers used one litre of seawater during the capture stage but only 10 millilitres of liquid during the collection stage.
This smaller amount of liquid is called the receiving solution because it receives the released magnesium ions.
Transferring magnesium from one litre of seawater into only 10 millilitres of receiving solution increased its concentration.
Why use a reversible electrode reaction?
The bismuth electrodes created small chemical environments that the researchers could control.
At the cathode, bismuth oxychloride was reduced. This used hydrogen ions and created alkaline conditions, causing magnesium hydroxide to form as a solid.
At the anode, bismuth was oxidised. This produced hydrogen ions and created acidic conditions, causing deposited magnesium hydroxide to dissolve.
Reversing the polarity swapped the roles of the two electrodes. Each electrode could therefore move between bismuth and bismuth oxychloride:
BiOCl ⇌ Bi
The overall process can be summarised as:
Capture: magnesium ions in seawater became solid magnesium hydroxide.
Release: the magnesium hydroxide dissolved into a smaller receiving solution containing chloride ions.
This use of local pH changes is important. It shows that electrochemistry can do more than deposit metals or produce gases. Electrode reactions can also control whether a substance forms as a solid or dissolves back into a solution.
How did the researchers measure the magnesium?
The researchers needed to determine how much magnesium had been removed from the seawater and how much had entered the receiving solution. They also needed to check whether unwanted ions, particularly sodium, had been transferred.
One of the main measuring techniques used was ICP-OES, which stands for inductively coupled plasma optical emission spectrometry.
The name is rather less friendly than the basic idea.
When atoms and ions receive energy, they can release particular wavelengths of light. This connects to the flame tests studied at GCSE. Different metal ions produce different flame colours because they release different wavelengths of visible light.
ICP-OES uses extremely hot plasma and sensitive equipment to identify elements from the light they produce. It can also measure how much of each element is present.
The researchers used it to measure magnesium and other elements in the seawater, rinse water and receiving solution. It provided evidence that the electrochemical separation had worked.
The team also used powerful microscopes and other methods to study the solid formed on the electrode. The results supported the conclusion that the deposit contained magnesium hydroxide.
What did the researchers achieve?
The researchers reused the receiving solution over several cycles. Each cycle collected more magnesium in the same small volume of liquid.
This increased the magnesium concentration from approximately 50 millimoles per litre to more than 500 millimoles per litre.
A mole is a unit chemists use to measure an amount of substance. A millimole is one-thousandth of a mole.
The scaled process produced:
approximately ninefold magnesium enrichment;
a final magnesium concentration of about 8.5 grams per litre;
a magnesium-to-sodium ratio of about 20 to 1;
approximately 10% recovery of the magnesium available in the seawater.
These results show that the method could separate and concentrate magnesium from both artificial seawater made in the laboratory and real seawater.
The high magnesium-to-sodium ratio is particularly important. Seawater contains far more sodium than magnesium, so separating the two is one of the main challenges.
Did the method work?
The experiment showed that magnesium could be separated from seawater and collected in a much more concentrated solution.
However, the researchers recovered only about 10% of the magnesium available in the seawater.
This is the method’s most important limitation. If it were used on a larger scale, approximately 90% of the magnesium would still be left behind. The process would therefore need to recover a much greater percentage before it could become an efficient way of producing magnesium compounds.
This does not mean that the experiment failed. It successfully demonstrated that the idea can work and that magnesium can be separated from the much larger quantity of sodium in seawater.
It should be viewed as an early demonstration rather than a finished industrial process.
Researchers would also need to investigate the total energy use, cost and performance of the method on a much larger scale.
Producing magnesium metal would require further stages. The magnesium chloride solution would need to be purified and completely dried before the molten magnesium chloride could be electrolysed. These later stages were not included in the experiment.
From GCSE electrolysis to real-world research
This research combines several ideas that students encounter during GCSE Chemistry:
electrolytes contain mobile ions;
reduction occurs when a substance gains electrons;
oxidation occurs when a substance loses electrons;
reactive electrodes can take part in electrolysis;
different possible electrode reactions compete;
electrode reactions can change the concentration of H⁺ ions;
metal hydroxides can form insoluble precipitates;
acids can dissolve metal hydroxides;
aqueous electrolysis does not normally produce highly reactive metals;
scientific measurements are needed to evaluate whether a process has worked.
At GCSE, these ideas are often taught through separate examples. In this research, they work together as parts of one separation process.
The study also demonstrates why GCSE product-prediction rules must be applied carefully. The familiar halide rule assumes an inert anode. If the electrode itself can react, oxidising the electrode may occur instead of discharging chloride or hydroxide ions.
The underlying definitions do not change:
reduction still occurs at the cathode;
oxidation still occurs at the anode;
electrons are still transferred.
What changes is the substance that undergoes the reaction.
Can we mine the sea?
Seawater contains valuable dissolved resources, including magnesium. This study shows that reversible bismuth electrochemistry can help capture magnesium as solid magnesium hydroxide and then release it into a smaller, more concentrated magnesium chloride solution.
The method does not produce magnesium metal directly. It also recovered only 10% of the available magnesium, so significant improvement would be needed before it could be used efficiently on an industrial scale.
Nevertheless, it provides a clear example of how familiar chemical principles can be applied to current scientific research.
GCSE electrolysis is therefore more than a set of rules for predicting products at electrodes. It provides a foundation for understanding how electricity can control reactions, separate substances and potentially recover valuable resources from complex mixtures.
Perhaps “mining the sea” will one day become an important source of useful materials. For now, this research offers a promising example of how familiar chemical principles can open up new ways of recovering the resources hidden in seawater.
Sources
D. J. Kim and colleagues, “Direct Magnesium Recovery from Ocean Waters Using Bismuth Electrochemistry”, ACS Energy Letters (2026), DOI: 10.1021/acsenergylett.6c01659.
American Chemical Society: “Extracting magnesium salt from seawater with electricity”.
Supporting Information for “Direct Magnesium Recovery from Ocean Waters Using Bismuth Electrochemistry”, including the experimental methods, measurements and scaled seawater experiments.
