Preventing Over-Electrodialysis for Efficient CO2 Capture from Seawater

Most CO2 extraction techniques involve thermal cycles where CO2 is captured from a gas stream by a nucleophilic agent in a solvent and is extracted by heating the solvent. Such processes are energy-intensive and require a large solvent makeup at the end of each cycle. Electrodialysis (ED) of seawater is an efficient method to capture CO2 as the seawater is already equilibrated with CO2 in the air, which avoids processing large volumes of air that is otherwise necessary for other direct air capture techniques. The efficient capture of CO2 via ED requires selective removal of HCO3– and CO32– from seawater. However, the ED membranes generally are not selective to specific anions. Consequently, during the ED of seawater, HCO3–/CO32– are removed along with other anions such as Cl–, SO42–, and so on, causing over-dialysis of seawater. ED systems can be energy-intensive when over-electrodialysis causes preferential rejection of salt anions over CO2-capturing anions (i.e., HCO3– and CO32–) leading to excessive energy penalties. Here we evaluate the effect of dominant anions such as Cl– and SO42–on CO2 capture during ED. We observe this influence by monitoring the transference number of HCO3–, Cl–, and SO42– in the seawater at various concentrations. Interestingly, at lower concentrations of HCO3–, the CO2 capture rate reaches a maximum of ∼1.22 mmol/m2/s and then ultimately diminishes indicating complete exhaustion of HCO3–. Beyond this point, the electrodialysis system only works toward rejecting other salt anions. Using the optimal residence time, CO2 is preferentially captured at 0.71 mmol/m2/s continuously using simulated seawater. This work shows a systematic study to prevent over-electrodialysis and efficiently capture CO2. The insights from the influence of anions on CO2 capture can be a stepping stone to the large-scale implementation of such a process.