Exploring Salty Water Structure to Understand Carbon Sequestration in Deep Aquifers

Salty water can act as a large carbon sink, sequestering carbon dioxide in deep saline aquifers at high pressures. However, the actual fate of the gas depends on the structure of the salty water and how the two substances interact.

Polidori et al. studied NaCl solutions under high pressure and temperature conditions, which are models for the brine found in deep aquifers or the ocean water found in subduction zones on the seabed.

The researchers used neutron diffraction to image the structure of the liquid. They conducted two experiments with different chlorine isotopes, measuring the diffraction pattern for each solution and then subtracting one from the other. The result was information on the chloride ions and their surroundings.

“You’re asking the question: If I stand on a chloride ion and look around, what is it that I see?” said author Philip Salmon. “With this neutron diffraction, with isotope substitution, we can answer that question.”

By using two different types of pressure cells, one for easier temperature control and the other for better pressure manipulation, the scientists were able to examine the saline solutions at higher pressures than previous studies. However, to emulate hydrothermal conditions, the temperature will need to rise even further in future experiments.

The team conducted molecular dynamics simulations and found good agreement with their experimental results. The findings build a complete picture of how the ions and water molecules interact.

Looking toward the future, they hope to dissolve carbon dioxide into the NaCl solutions. Measuring the resultant structure will inform models on carbon sequestration mechanisms.

Source: “Structure and dynamics of aqueous NaCl solutions at high temperatures and pressures,” by Annalisa Polidori, Ruth F. Rowlands, Anita Zeidler, Mathieu Salanne, Henry E. Fischer, Burkhard Annighöfer, Stefan Klotz, and Philip S. Salmon, Journal of Chemical Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0067166 .