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.
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.