5:15 PM - 7:15 PM
[MZZ42-P10] Studies on changes of seawater carbonate chemistry during the ocean alkalinity
enhancement
Keywords:Carbon Neutral, Marine Carbon Dioxide Removal (mCDR), Ocean Alkalinity Enhancement (OAE)
Introduction
Ocean Alkalinity Enhancement (OAE), which focuses on the absorption of carbon dioxide (CO2) from the atmosphere by the oceans, is currently proposed as one of marine carbon dioxide removal (mCDR) methods. When CO2 dissolves in seawater, it reacts with H2O to produce carbonic acid (H2CO3), which subsequently dissociates into bicarbonate ions (HCO3-) and carbonate ions (CO32-), and the hydrogen ions (H+) released in this process cause ocean acidification. In OAE, the addition of alkaline substances to seawater can buffer the released H+ and reduce ocean acidification. In addition, as the pH increases, carbonate components in seawater tend to exist in the form of HCO3- and CO32-, which reduces the partial pressure of carbon dioxide in seawater (pCO2). When this value falls below atmospheric pCO2, the CO2 equilibrium between the atmosphere and ocean changes and allows additional CO2 absorption from the atmosphere.
Several previous studies have demonstrated the potential of this approach to significantly enhance the absorption of atmospheric CO2 and increase carbon sequestration by the ocean. However, limited fundamental research has been conducted on marine chemistry and geochemistry. In this study, experiments were conducted to confirm the potential for additional CO2 absorption by adding sodium carbonate (Na2CO3) to seawater. The pH, CO2, and total alkalinity (TA) of the seawater were continuously measured throughout the experiments.
Experimental methods
In the experiments, Na2CO3 was used to increase the total alkalinity by 250, 500, 750, 1000 and 2000 umol/kg respectively in approximately 600 mL of natural seawater from the Ogasawara Islands. 3 or 24 hours after the addition of Na2CO3, the changes in pH and pCO2 in the seawater were measured simultaneously. The seawater used in the experiment was aerated with outside air for approximately 40 minutes before the measurements. During the experiment, the seawater temperature was maintained at approximately 25 degrees Celsius using a thermostatic chamber. The salinity and total alkalinity of the seawater after the experiment were measured using a salt meter and an automatic titrator at the National Institute of Advanced Industrial Science and Technology (AIST), while the Dissolved Inorganic Carbon (DIC) was calculated from the titration curve during the alkalinity measurement.
Results/Discussion
Continuous measurements of pH and pCO2 over a three-hour period showed that the addition and subsequent dissolution of Na2CO3 resulted in a sharp increase in pH across all experiments, which then stabilized over the following three hours. Similarly, pCO2 in seawater decreased immediately after the addition of Na2CO3, dropping to levels below typical atmospheric pCO2 of approximately 400 ppm when total alkalinity increased to over 500 umol/kg. However, no significant changes in the final pCO2 values were observed when the alkalinity addition exceeded 1000 umol/kg. It is suggested that an increase in total alkalinity of more than 500 umol/kg enhances the potential for seawater to absorb CO2 from the atmosphere. However, an increase in TA exceeding 1000 umol/kg does not seem to improve the efficiency of CO2 absorption. Furthermore, an addition of 2000 umol/kg TA results in a significant loss of added alkalinity. Therefore, it is important for OAE efforts to accurately estimate the appropriate amount of alkaline material to be added to maximize CO2 absorption efficiency and minimize the risk of TA loss.
Ocean Alkalinity Enhancement (OAE), which focuses on the absorption of carbon dioxide (CO2) from the atmosphere by the oceans, is currently proposed as one of marine carbon dioxide removal (mCDR) methods. When CO2 dissolves in seawater, it reacts with H2O to produce carbonic acid (H2CO3), which subsequently dissociates into bicarbonate ions (HCO3-) and carbonate ions (CO32-), and the hydrogen ions (H+) released in this process cause ocean acidification. In OAE, the addition of alkaline substances to seawater can buffer the released H+ and reduce ocean acidification. In addition, as the pH increases, carbonate components in seawater tend to exist in the form of HCO3- and CO32-, which reduces the partial pressure of carbon dioxide in seawater (pCO2). When this value falls below atmospheric pCO2, the CO2 equilibrium between the atmosphere and ocean changes and allows additional CO2 absorption from the atmosphere.
Several previous studies have demonstrated the potential of this approach to significantly enhance the absorption of atmospheric CO2 and increase carbon sequestration by the ocean. However, limited fundamental research has been conducted on marine chemistry and geochemistry. In this study, experiments were conducted to confirm the potential for additional CO2 absorption by adding sodium carbonate (Na2CO3) to seawater. The pH, CO2, and total alkalinity (TA) of the seawater were continuously measured throughout the experiments.
Experimental methods
In the experiments, Na2CO3 was used to increase the total alkalinity by 250, 500, 750, 1000 and 2000 umol/kg respectively in approximately 600 mL of natural seawater from the Ogasawara Islands. 3 or 24 hours after the addition of Na2CO3, the changes in pH and pCO2 in the seawater were measured simultaneously. The seawater used in the experiment was aerated with outside air for approximately 40 minutes before the measurements. During the experiment, the seawater temperature was maintained at approximately 25 degrees Celsius using a thermostatic chamber. The salinity and total alkalinity of the seawater after the experiment were measured using a salt meter and an automatic titrator at the National Institute of Advanced Industrial Science and Technology (AIST), while the Dissolved Inorganic Carbon (DIC) was calculated from the titration curve during the alkalinity measurement.
Results/Discussion
Continuous measurements of pH and pCO2 over a three-hour period showed that the addition and subsequent dissolution of Na2CO3 resulted in a sharp increase in pH across all experiments, which then stabilized over the following three hours. Similarly, pCO2 in seawater decreased immediately after the addition of Na2CO3, dropping to levels below typical atmospheric pCO2 of approximately 400 ppm when total alkalinity increased to over 500 umol/kg. However, no significant changes in the final pCO2 values were observed when the alkalinity addition exceeded 1000 umol/kg. It is suggested that an increase in total alkalinity of more than 500 umol/kg enhances the potential for seawater to absorb CO2 from the atmosphere. However, an increase in TA exceeding 1000 umol/kg does not seem to improve the efficiency of CO2 absorption. Furthermore, an addition of 2000 umol/kg TA results in a significant loss of added alkalinity. Therefore, it is important for OAE efforts to accurately estimate the appropriate amount of alkaline material to be added to maximize CO2 absorption efficiency and minimize the risk of TA loss.