  We describe a new principle - the Alkalinity Concentration Swing (ACS) - for
direct air capture of carbon dioxide driven by concentrating an alkaline
solution that has been exposed to the atmosphere and loaded with dissolved
inorganic carbon. Upon concentration, the partial pressure of carbon dioxide
increases, allowing for extraction and compression. We find that higher
concentration factors result in proportionally higher outgassing pressure, and
higher initial alkalinity concentrations at the same concentration factor
outgas a higher concentration of CO2 relative to the feed solution. We examine
two desalination technologies, reverse osmosis and capacitive deionization, as
possible implementation for the ACS, and evaluate two simplified corresponding
energy models. We compare the ACS to incumbent technologies and make estimates
on water, land, and energy requirements for capturing one million tonnes of CO2
per year. We find that estimates for the lower end of the energy range for both
reverse osmosis and capacitive deionization approaches are lower than or
roughly equal to incumbent direct air capture approaches. For most conditions,
we find an inverse relationship between the required energy and water
processing volume per million tonnes of CO2. Realizing the ACS requires a
simple alkaline aqueous solvent (e.g. potassium alkalinity carrier) and does
not require heat as a driving mechanism. More generally, the ACS can be
implemented through industrial-scale desalination approaches, meaning current
technology could be leveraged for scale-up.
