Concentrating Alkalinity for Direct Air Capture of Carbon Dioxide: Using Osmotic Pressure for Concentration and Separation

Abstract

Climate change has resulted in the pressing need to globally manage carbon sources and sinks. Reducing emissions as much and as fast as possible must be prioritized to avoid the worst harms from global warming, which will be felt most by vulnerable populations. However, if warming is to be halted this century, it has now become apparent that removing atmospheric CO2, on the scale of gigatonnes of CO2 per year, will also be necessary to offset the hardest-to-avoid emissions. Direct air capture technologies, which are industrial processes for chemically removing atmospheric CO2, have been introduced and are being deployed as one such approach for carbon removal. This work specifically contributes to the understanding of aqueous dissolved inorganic carbon-based direct air capture technologies in the following ways: First, we propose a new approach for removing atmospheric CO2, the alkalinity concentration swing (ACS), driven by concentrating and diluting aqueous alkaline solution, which increases and decreases the partial pressure of CO2 of the solution, respectively. Second, we improve on the ACS process by introducing a selectivity step, which separates bicarbonate ions from carbonate ions. A theoretical investigation reveals that bicarbonate-enrichment allows for reaching higher cycle capacity, higher CO2 partial pressure, and improved absorption rates. Third, nanofiltration is experimentally studied, and confirmed as a mechanism to enrich bicarbonate ions, reaching bicarbonate-carbonate selectivity factors above 30. Fourth, we experimentally demonstrate the ability to use reverse osmosis, a membrane-based separation process driven by applied pressure, as a method for concentrating alkalinity. We find that our proposed approach has significant limitations, but it has certain promising pathways for improvement and feasible deployment, if absorption rate shortcomings are able to be sufficiently addressed through promoters and if energy recovery steps are implemented. Fifth, a generalized theory for acid-base concentration swings is developed, introducing a new conceptual framework for controlling pH through osmotic drive. Sixth, absorption and outgassing rates are experimentally studied for alkaline and weak acid solutions, with and without CO2 promoters. And finally, seventh, a broader process perspective on dissolved inorganic carbon-based DAC is introduced to examine the trade-off between the three key figures of merit: volumetric cycle capacity, absorption flux, and cycle energy.