Understanding CO2 solubility in aqueous salt solutions for effective carbon capture and storage
If you've ever left a soda bottle open and noticed it going flat, you've witnessed a fundamental scientific principle: gases, like carbon dioxide (CO2), can dissolve in and out of liquids. While this is a nuisance for your beverage, scientists believe this exact same process, on a colossal scale, could be one of our planet's best defenses against climate change.
The idea is to capture CO2 emissions from power plants and industries, then inject them deep underground into vast, porous rock formations filled with salty water, known as saline aquifers. Once there, the CO2 can be trapped for thousands of years, partly by dissolving into the brine—a process called solubility trapping.
But how much CO2 can the brine hold? The answer is not simple, as it depends on a complex cocktail of pressure, temperature, and the precise recipe of salts in the water. This article delves into the fascinating world of CO2 solubility in aqueous salt solutions, exploring the cutting-edge experiments and sophisticated models that scientists are developing to turn this promising concept into a precise and reliable climate solution 3 5 .
At its heart, the dissolution of CO2 in water is a balance between the gas above the liquid and the molecules dispersed within it. Increase the pressure, and you force more gas in, which is how soda cans are manufactured. However, when that water is not pure but instead a complex brine loaded with dissolved salts, the game changes completely.
The presence of ions like sodium (Na⁺), calcium (Ca²⁺), and chloride (Cl⁻) profoundly disrupts CO2's ability to dissolve. This is known as the "salting-out effect," where the dissolved ions effectively "crowd out" the CO2 molecules.
Accurately predicting CO2 solubility in these complex brines is essential for optimizing carbon capture, utilization, and storage (CCUS) processes 1 . It allows scientists to identify the best storage sites, calculate their true capacity, and predict the long-term fate of the injected CO2.
A 2024 study by Mousavi, Chapoy, and Burgass provides an excellent example of a comprehensive experimental approach designed to fill critical data gaps, particularly at high temperatures and for salts beyond common sodium chloride 6 .
The researchers set out to systematically measure the solubility of CO2 in pure water and in aqueous solutions of four different salts—sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), and magnesium chloride (MgCl2)—across a range of concentrations and temperatures 6 .
Creating precise aqueous solutions of different salts at specific concentrations
Placing brine samples in high-pressure cells and adjusting temperature and pressure
Using techniques like Raman spectroscopy to measure dissolved CO2 5
Using data to validate thermodynamic models like PC-SAFT 6
The data from experiments reveal clear and crucial trends about how different factors affect CO2 solubility in brine solutions.
The following table illustrates how CO2 solubility decreases as the salt concentration increases, a clear demonstration of the "salting-out" effect in different brine types at a constant temperature 6 .
| Salt Solution | Concentration (wt%) | CO2 Solubility (mole fraction) |
|---|---|---|
| Pure Water | 0% | 0.012 |
| Sodium Chloride (NaCl) | 10% | 0.008 |
| 20% | 0.005 | |
| Calcium Chloride (CaCl2) | 10% | 0.006 |
| 23.4% | 0.003 |
The type of salt also plays a major role. Divalent ions like Mg²⁺ and Ca²⁺ have a more pronounced salting-out effect than monovalent ions like Na⁺ and K⁺ 3 6 .
| Salt Type | Example Ions | Relative CO2 Solubility |
|---|---|---|
| Chloride Salts | Na⁺, Cl⁻ | Lower solubility than pure water |
| Divalent Chlorides | Ca²⁺, Mg²⁺, Cl⁻ | Lowest solubility among common salts |
| Potassium Carbonate | K⁺, CO₃²⁻ | Notably high CO2 affinity, especially at higher temperatures 1 |
The universal physical laws still hold true: CO2 solubility increases with pressure and decreases with temperature 1 3 .
| Pressure (MPa) | Temperature = 308 K | Temperature = 373 K |
|---|---|---|
| 5 MPa | 0.08 | 0.03 |
| 15 MPa | 0.14 | 0.09 |
Moving from laboratory experiments to predicting solubility in real-world formations requires a powerful set of computational tools. Researchers have developed a multi-faceted toolkit to tackle this challenge.
Physics-based models that use equations of state and theories of molecular interactions to calculate solubility.
A revolutionary new addition to the toolkit, ML uses algorithms trained on vast experimental datasets.
| Reagent/Solution | Role in Research | Example Use Case |
|---|---|---|
| Single-Salt Brines (e.g., NaCl, CaCl2) | Simplifies system to understand individual ion effects 3 | Foundational experiments to build basic models |
| Mixed-Salt & Real Formation Brines | Replicates the complex chemistry of actual aquifers 3 | Critical for validating model accuracy for real-world application |
| Amino Solutions (e.g., MEA, AMP) | Chemically absorbs CO2 in capture processes 4 9 | Used at the surface in carbon capture plants, not in geological storage |
The journey of understanding CO2 solubility in brine—from the fundamental "salting-out" effect observed in a lab beaker to the complex thermodynamic and machine learning models predicting behavior miles underground—is a testament to scientific ingenuity.
This research is far from an academic exercise; it is the bedrock of a safe and effective strategy to mitigate climate change. By accurately quantifying how much CO2 can be locked away in deep saline aquifers, scientists and engineers can de-risk CCUS projects, ensuring that the CO2 we work so hard to capture remains permanently stored.
It allows us to transform our understanding of a simple phenomenon, like a fizzy drink going flat, into a powerful tool for protecting our planet's future.