CaCO3 forms slower in saline water than previously estimated

According to new research from Washington University in St. Louis’ McKelvey School of Engineering, the speed at which calcium carbonate (CaCO3) forms in saline water may be slower than previously estimated. The overestimation may be due to a lack of consideration of the effects of kinetics, the research found.

An article on the school’s website titled “When kinetics and thermodynamics should play together” by Brandie Jefferson explains how Young-Shin Jun, professor in the Department of Energy, Environment & Chemical Engineering at the school, came to this conclusion. These results were recently published in the Journal of Physical Chemistry C

At the core of Jun’s research was nucleation. “People often casually say ‘formation’ when they refer to the ‘growth’ of solids, but formation actually starts earlier, at the nucleation stage,” Jun said in the article. “Nucleation begins at the moment when all of the precursor parts have fallen into place, reaching a critical mass that creates a nucleus that is big enough and stable enough to continue to grow as calcium carbonate solids.”

Jun studied nucleation to find out when the CaCO3 actually forms at the northern Illinois Advanced Photon Source in the Argonne National Laboratory. Because nucleation happens at the nanoscale, the process has previously been difficult to study. However, using a grazing incidence small angle X-ray scattering (GISAXS), she was able to create unique environmental reaction cells and observe nucleation in aqueous environments, Jefferson reports. Nucleation was observed in waters of different saline concentrations.

Making her research unique, Jun focused on the balance of thermodynamics and kinetics for determining likelihood of nucleation, and more pointedly, how the kinetic factor influences the process.

“People have thought that kinetics is not important because it should be the same, no matter what,” June said in the article. However, Jun and her former doctoral student Qingyun Li quantitatively described the relationship between kinetics and thermodynamics of CaCO3 nucleation in saline water, using quartz as a substrate.

Their finding: Nucleation can more easily happen in high-saline water where interfacial energy is lower than pure water, but the kinetic factor is slow.

“If we account only for thermodynamics when we predict the system, we’re overestimating the rate of nucleation. The impact of kinetic factors should be included,” Jun explained in the article.

Jun also noted in the article that gaining this improved understanding of CaCO3 nucleation can help in designing sustainable water and energy production systems.

“It is an exciting finding. By changing the kinetics and thermodynamics, we can design a surface to prevent nucleation. By knowing when and where the nucleation happens, we can prevent or reduce it, extending the lifetime of pipelines or water purification membranes,” she explained. “Conversely, we can also increase nucleation where we need it, such as in geologic CO2 storage. This basic understanding gives us power and control.”