Reverse osmosis (RO) desalination generates large volumes of concentrated brine and is associated with significant CO2 emissions, posing environmental challenges for coastal regions. A potential mitigation strategy involves saturating desalination brine with CO2 and injecting it into deep saline coastal aquifers, where elevated pressure and geochemical interactions may promote long-term carbon sequestration while preventing brine discharge to the marine environment.
This geochemical modeling study investigates the geochemical evolution of desalination brine saturated with CO2 under in situ aquifer conditions, focusing on dissolved inorganic carbon (DIC), pH, alkalinity, and major cation concentrations. The analysis follows a sequential process consisting of four stages: (1) initial desalination brine, (2) CO2 dissolution into the brine at elevated pressure (up to 13 atm), (3) equilibration with aquifer host-rock calcium carbonate (CaCO3), and (4) cation exchange processes with the host rock, including mixing with ambient saline groundwater (SGW).
Saturating desalination brine with CO2 at high pressure leads to substantial CO2 dissolution, resulting in a marked decrease in pH and a pronounced increase in DIC concentrations. Subsequent equilibration with CaCO₃ buffers acidity, causing pH recovery and increases in alkalinity, Ca2+ concentration, and DIC, with an approximately 1:1 normality ratio between alkalinity and Ca2+, indicating carbonate mineral dissolution. Mixing with SGW moderates solute concentrations while preserving the geochemical trends established in earlier stages. Finally, cation exchange reactions release Ca2+ and Mg2+ to the brine and adsorb Na+ and K+ onto the solid matrix, exhibiting nonlinear behavior during mixing and inducing only minor changes in pH, alkalinity, and DIC.
The results demonstrate that high-pressure CO2 dissolution, followed by carbonate buffering and cation exchange, governs the geochemical stability of CO2-enriched desalination brine in saline aquifers. These findings support the feasibility of combined brine management and CO2 sequestration in deep coastal aquifers and provide a basis for future coupled hydrogeochemical modeling.