Researchers around the world are working flat out to find ways to actively remove carbon dioxide (CO₂) from the atmosphere. Because even if humanity were to manage to massively reduce its CO₂ emissions by 2050, there would still be emissions from agriculture, heavy industry, aviation and shipping that would be difficult to avoid and would have to be recovered from the atmosphere over many decades. The Intergovernmental Panel on Climate Change (IPCC) has been pointing this out in its synthesis and special reports for years. A recent report also shows how technologies and policies are lagging behind the need to actively remove CO₂ from the atmosphere.
While most developers have so far concentrated on removing the greenhouse gas directly from the air, attempts to filter it out of seawater have recently come to the fore. That seems even more rewarding, since the concentration of carbon dioxide in the sea is a hundred times higher than in the air.
Meritorious CO₂ sink: ocean
The ability of the oceans to absorb huge amounts of CO₂ has so far saved the earth from climate collapse. Between 30 and 40 percent of the CO₂ released by humans from fossil sources is dissolved in the world‘s oceans. In theory, they could probably absorb 80 percent of it – but that takes at least a thousand years. After all, the higher the CO₂ concentration in the sea, the more sluggish this process becomes over time. In addition, the already measurable slowing down of the ocean circulation is hindering the absorption of the greenhouse gas.
The chemical process in which the CO₂ in seawater turns into carbonic acid (H2CO3), which as a bicarbonate ion (HCO3-) is dissolved and the oceans acidified. Organisms whose shells and shells are made of lime are particularly at risk because the acid dissolves the lime. A reduction in the CO₂ concentration and thus the acidity also helps marine organisms.
The oceans themselves are what capture large amounts of CO₂ from the air. Air-sucking technologies can only accomplish this step with a high expenditure of energy. If the CO₂ content in seawater were reduced, the sea would then restore the concentration balance between air and water and absorb CO₂ again.
How the CO₂ comes out of the sea
This is exactly where researchers at the Massachusetts Institute of Technology (MIT) are now starting. Apparently, they found a particularly efficient and, above all, inexpensive method, which they recently reported on in the journal Energy and Environmental Science.
The authors calculated that the cost of collecting one tonne of CO₂ would be just 56 euros using their method. With all methods that filter CO₂ from the ambient air, the costs amount to several hundred euros per ton.
To achieve this, the researchers reversed the chemical reaction in their process, in which the carbon in the CO₂ absorbed from the sea in the water is partially converted into bicarbonate ions (HCO3-) is almost caught.
To do this, they direct the seawater into an electrochemical cell, in which it is first made even more acidic by the protons of an electrode. This dissolves the bicarbonate and releases the carbon as CO₂, which is then sucked off and collected by vacuum.
Before the water, which has been freed from CO₂ and is now much too acidic, can be pumped back into the sea, it has to be neutralized so that it does not harm marine life. To do this, it passes through a second chamber with reversed electrical voltage, which allows the protons from the first pass to be recovered. The water, which is now slightly alkaline again, then has the capacity to absorb new CO₂.
The two cells swap roles at regular intervals, so that the electrode that lost protons during the first run can regenerate when used in the second run, when the protons are collected.
Where the collected CO₂ can be stored
“At least locally, acidification could be reversed in this way,” emphasizes Kripa Varanasi from MIT’s mechanical engineering department. Nevertheless, the reintroduction of the alkaline water from the second step of the process must be distributed or far offshore to avoid a local increase in acid-binding capacity. In some cases, treated water could be reinjected into fish farms, for example, which generally have a tendency to acidify the water.
Just like with processes that extract CO₂ from the ambient air, the collected CO₂ must then be stored somewhere in such a way that it is removed from the atmosphere for a very long time. “You can certainly consider using the captured CO₂ as a feedstock to make chemicals or materials, but you won’t be able to use all of it as a feedstock,” says Alan Hatton of MIT’s Chemical Engineering Department. “There are no sales markets for many products that could be made from it, so that a significant part of the captured CO₂ has to be buried underground in any case.”
The system interfaces well with seawater desalination plants where the suction and discharge installations are already in place. Because the MIT technology works without chemicals that could affect the process water. It would also be possible for ships to at least compensate for their emissions by treating the water during the journey.
However, before the researchers can think of a first demonstration plant, it will probably be another two years. Because they still have to work on some improvements, such as the extraction of CO₂ or to prevent the precipitation of the minerals that are also present in seawater and pollute the electrochemical chambers and the electrodes.
(jl)