Probing Solubility and pH of CO2 in aqueous solutions: Implications for CO2 injection into oceans

CO2 sequestration is among the most anticipated methods to mitigate the already detrimental concentrations of CO2 in the atmosphere. Among sequestration methods, CO2 injection into oceans is of great significance due to the oceans' large sequestration capacity. However, there are concerns about the changes in water pH as CO2 is injected into oceans. Previous studies in conditions representative of CCS in the ocean are scarce. In the current study, we experimentally measure the pH and solubility at pressures up to 400 atm, temperatures between 283 and 298 K, and different aqueous solutions in a high-pressure autoclave reactor. The results indicated that increasing pressure increases the solubility of CO2 in aqueous solutions, resulting in lower pH values. In contrast, increasing salinity and temperature lowers the solubility and, as a result, increases the system's pH. Among all the tested aqueous solutions, the synthetic seawater mimicked that of a potential injection point in the South China sea, exhibiting the highest salting-out effect and, therefore, the lowest solubility (i.e., the highest pH). The experimental dataset of this study was fed to a machine learning algorithm, Group modeling data handling (GMDH), to develop an explainable, white-box solubility model. The model could predict the pH as a function of solubility, temperature, pressure, and salinity with an accuracy of 0.87. The pH values from the model were compared to those from previous studies, and a good agreement among the values was found. Lastly, a parameter importance analysis was conducted to shed further light on the model's performance. Pressure and temperature were found to be the most and the least influential factors, respectively. As the implantation of the technology is currently being considered in China, the current study can pave the way to better understand the interactions and mechanisms involved in conditions representative of ocean sequestration before large-scale operations.