There is a good match between the total amount of CO2 absorbed by the planet’s seas and that retained by the atmosphere. Research on its removal from ocean waters without the use of chemicals – together with the measures and technologies already in place to eliminate its emissions – contributes to lowering the environmental impact generated by this greenhouse gas.
The reduction of CO2 in ocean waters through extraction processes represents, today – as we write – an opportunity heralding great hopes for the future, with a view to the global mitigation of greenhouse gas emissions and their definitive zeroing. But let’s take a step back. Only since the last few years the ocean has been recognized as a “global sink” for atmospheric carbon dioxide.
«The ocean is the largest carbon sink on the planet, a natural system that absorbs excess carbon dioxide from the atmosphere and stores it. About 40% of the CO2 put into the atmosphere by burning fossil fuels since the dawn of the industrial age has been absorbed by the ocean»
reads the paper “External Forcing Explains Recent Decadal Variability of the Ocean Carbon Sink”, edited by the Lamont-Doherty Earth Observatory, within Columbia University. And – its authors recall – the continuous process of acidification of ocean waters (direct consequence of the reaction of sea water in contact with the absorbed CO2) has already led to the destruction of coral reefs and harmful impacts on other forms of life in the seas. What has been done so far to stem this phenomenon?
CO2 reduction in the oceans: the search for a methodology free of the use of chemicals
In terms of CO2 reduction in the oceans, two approaches have been proposed to date, namely electrodeionization and electrodialysis, as explained by a team made up of some researchers from the Department of Chemical Engineering and the Department of Mechanical Engineering of the Massachusetts Institute of Technology (MIT), authors of the study described in “Asymmetric chloride-mediated electrochemical process for CO2 removal from oceanwater” :
«In the electrodeionization process, water splitting reactions occur, which in turn generate hydrogen and oxygen gases to modulate the pH, thus resulting in the removal of CO2. However, this approach brings with it a high overall energy consumption. Electrodialysis, on the other hand, provides a more energy-efficient medium, but is more costly and – crucially – presents risks of toxic substances leaking into the ocean»
It is necessary – underlines the study group – identify approaches that do not require the addition of chemicals, nor lead to reactions that lead to the formation of undesirable compounds in water.
CO2 from the air dissolves in seawater as carbonic acid, «which can be converted back to molecular CO2 simply by lowering the pH of ocean water and then removed as pure gas by vacuum stripping» the researchers speculate.
At that point, the pH of the treated water cansubsequently, be raised before it is returned to the ocean, with the dual advantage of countering the acidification of the seas and promoting the further absorption of CO2 from the atmosphere. But let’s take a closer look at what it is.
The electrochemical modulation of the pH of ocean waters
In terms of CO2 reduction in the oceans, the technique developed by the MIT team is based exclusively on electrochemical modulation of seawater pH, designed to initially release CO2 and then alkalize the treated water before it is returned to the ocean.
This approach – the authors specify – «it does not require expensive procedures or the addition of chemicals. It is easy to implement and does not lead to the formation of by-products or side streams».
What is the electrochemical modulation of the pH of the seas? The process consists of electrochemical cells without membranewhose electrodes (one in bismuth and the other in silverbothnon-polluting metals) release protons into the sea, thus driving the release of carbon dioxide, which dissolves into the water.
«It is a cyclic process, in which water is first acidified to convert dissolved inorganic bicarbonates into molecular carbon dioxide, which is then collected as a vacuum gas. And then seawater is fed to a second set of cells with a reverse voltage, to recover the protons and turn the acidic water to alkaline before releasing it back into the sea»
explains the study group. The removal of CO2 from marine waters and the subsequent reintroduction of alkaline water into the sea could – in the intentions of the authors – slowly begin to reverse the acidification process of the waterscaused by the accumulation of carbon dioxide.
Once carbon dioxide is removed from the water, it must then be disposed of, just as with all carbon removal processes. And it can be done, for example, by burying it in deep geological formations, under the seabed.
Or the removed CO2 can be chemically converted into a compound such as ethanol, which can be used as a transportation fuel.
Ocean CO2 reduction: applications and future research direction
«With this approach we won’t be able to treat emissions from the whole planet, but the reinjection of alkaline water into the sea could be done locally, for example in places such as fish farms, which tend to acidify seawater»
says the study team on CO2 reduction in the oceans. The CO2 removal modules can also be installed on fixed platforms in the seas or on merchant ships that ply the oceans or integrated with land-based desalination processesto take advantage of the water treatment plants already installed.
And the captured carbon dioxide can be injected directly from platforms into underground geological structures for long-term sequestration or used as feedstock for fuels or for the production of raw materials and specialty chemicals.
The system could also be implemented by vessels that treat water en route, in order to help mitigate the significant contribution of vessel traffic to overall greenhouse gas emissions.
There are already international mandates to reduce emissions from shipping, and this could help shipping companies offset some of their emissions and turn ships into ocean scrubbers.
Future research on the reduction of CO2 in the oceans foresees – as regards the structure of the electrodes – the replacement of silver with another material, easier to dispose of and dissolve.
Furthermore – the authors anticipate – work will have to be done on methods to overcome the fouling of the electrodes due to local conditions of high surface pH, which promote the formation of fouling:
«Several engineering and electrochemical approaches are already being studied to alleviate these problems. For example, in one of these, the degassed water was mixed with fresh ocean water before being fed to the regeneration cell, to reduce the overall pH increase in the flow channel». And the search continues.