The dissolution of minerals in seawater takes time. It can require as long as several months for alkalinized seawater to exert its chemical effect and absorb additional carbon dioxide from the atmosphere. Some experts are developing electrochemical procedures to accelerate this process. They employ an electrochemical cell through which seawater is passed. The cell contains two electrodes. When an electric current is applied to the cell, the electrodes become a positively charged anode and a negatively charged cathode. The anode attracts bases, while the cathode attracts acids, result-ing in an “acid current” and a “base current”. Both of these currents can be used to influence the carbon dioxide concentration in seawater. Depending on the approach, the objective is either to increase the alkalinity of the seawater or to remove carbon dioxide directly from the seawater.

Two examples – how electrochemical methods can be applied

Scientists at the University of California have developed an electrochemical cell method by which the dissolved carbon dioxide in seawater reacts with and ultimately mineralizes the calcium and magnesium cations present in the water. The result is that carbon dioxide is chemically fixed through the formation of new rock material. The total carbon content of the water is thereby reduced, allowing it to absorb additional carbon dioxide from the atmosphere when it is reintroduced into the sea.
To achieve this chemical bonding, the seawater is passed through a machine called a flow reactor. In the machine the water flows across a network of electrodes that electrically charge it and electrochemically increase its alkalinity. In this state, the dissolved carbon dioxide and mineral components in the water can react instantly with one another. One result of this reaction is the formation of various solid materials such as calcium carbonate – CaCO₃ –, magnesium carbonate – MgCO₃ – and magnesium hydroxide – Mg(OH)₂ –, which can be further processed as mineral raw materials. Another result is water depleted in carbon dioxide, which is reintroduced into the sea. Hydrogen, which is in demand as a renewable fuel, is also produced in the process.
The researchers have calculated the scale at which this technology would have to be applied if the goal were to remove ten billion tonnes of carbon dioxide per year from the ocean and thus indirectly from the atmosphere. Worldwide, it would require the installation of around 1800 systems. The costs for the construction and operation of these flow reactors would reach several trillion US dollars. The necessary electricity would also have to come from renewable energy sources.

Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts have discovered a much less expensive solution. They channel seawater into an electrochemical cell where it is strongly acidified by protons from a bismuth electrode. This acidification causes the breakdown of the carbonates and hydrogen carbonates present in the water and frees up the carbon dioxide bound to them. This is then drawn off and collected. However, the acidified water has to be neutralized before it can be pumped back into the sea. This is achieved by passing it through a second cell with a reversed electrical charge, allowing the protons from the first pass to be recovered. This slightly basic water can then take up new carbon dioxide from the atmosphere. Here, the cost per tonne of carbon dioxide removed is 56 US dollars.
In contrast to the former method, however, the carbon dioxide removed is not bound in solid rock, but is gaseous and thus highly volatile. So it still must be further processed or stored in such a way that it does not escape back into the atmosphere. The second process can be conveniently integrated into existing seawater desalination plants where the necessary water intake and outlet installations are already in place. But before the MIT experts can build their first demonstration system there are several issues that need to be addressed. One of these is mineral precipitation that occurs during the process and contaminates the electrical terminals and electrodes of the cells.

fig. 7.7 > A flow generator forms the core of an electrochemical approach that has been developed by scientists in the US state of California. In the generator, carbon dioxide dissolved in the seawater reacts with and mineralizes calcium and magnesium cations. This decreases the total carbon content of the water so that it can absorb new carbon dioxide from the atmosphere when it is released back into the sea.