Looking for Answers Around Grains of Sand
Experiments reveal unexpected precipitation behavior, insights for cleanup and carbon sequestration
Small barricades of calcium carbonate form between the simulated soil particles in this study by Pacific Northwest National Laboratory and the University of Illinois at Urbana-Champaign. Enlarge Image
Results: Tiny cul-de-sacs and passages in the soil, that affect water flow and mixing, reverse the expected outcome of a common reaction that has environmental implications, according to scientists at Pacific Northwest National Laboratory and University of Illinois at Urbana-Champaign. The team studied the precipitate, solid calcium carbonate, CaCO3. They showed that higher concentrations of the precursors for CaCO3 result in less of the desired solid under dynamic flow and mixing conditions. This is because the solid forms in the passageways or pores and blocks further reactions.
"The heart and soul of what we learned from this study is counterintuitive to what people have thought using batch reactors," said Dr. Changyong Zhang, a geochemist at PNNL and the study's lead author.
Why It Matters: Cleaning up certain toxic materials in the groundwater relies on mixing of different solutions to halt the pollutant's migration. The rapid formation of CaCO3 could interfere with the mixing needed for groundwater remediation. While the study did not directly address geologic carbon storage, the results could be beneficial to sequestration studies.
Methods: The team began using clear, silicon micromodels at EMSL, a national scientific user facility at PNNL. The micromodels are thin, see-through structures, smaller than a trade paperback book. The models contain tiny passages that simulate the pores between and inside soil particles.
The team injected two solutions in parallel into the micromodel. The solutions mixed through the micromodel and formed CaCO3. They used nondestructive, real-time microscopic laser Raman techniques to analyze the CaCO3 that formed in the pores when the solutions met.
"The benefit of doing this research at the pore scale is being able to directly visualize what is happening at the flow interface," Zhang said.
In analyzing the materials that formed, the scientists showed results that are opposite to those from batch reactors, where solutions are well mixed. In a batch reactor, concentrated solutions produced higher levels of solid CaCO3 than weaker solutions. In the pore-scale flow experiments, concentrated solutions produced less CaCO3.
They found that less of the desired solid was formed because the solid CaCO3 quickly changed the porosity of the system, forming barriers in the pore spaces and blocking further reactions between the two solutions.
Further, they showed that the blockage of pores influenced the carbonate's ability to rearrange its internal structure. In a batch reactor, a less stable CaCO3 is formed first and transforms into a more stable mineral in a few hours. However, in the pore-scale flow experiments, once the less stable solid precipitates where the two liquids meet, the transformation was cut off and did not occur.
The results of this study were selected for quick release by Environmental Science and Technology.
What's Next: The team is conducting further studies into carbon sequestration to learn more about the conditions relevant to geologic reservoirs for carbon sequestration, such as high pressure and high temperature.
Acknowledgments: This work was supported by the Department of Energy's Office of Biological and Environmental Research, a Graduate Assistance in Areas of National Need fellowship from the U.S. Department of Education, PNNL's Laboratory Directed Research and Development Program under the Carbon Sequestration Initiative, and Los Alamos National Laboratory.
A portion of the experiments was done in DOE's EMSL, a national scientific user facility at PNNL.
This research was done by Changyong Zhang, Nancy Hess, Mart Oostrom, and Thomas Wietsma of PNNL, and Karl Dehoff, Albert Valocchi, Bruce Fouke, and Charles Werth of the University of Illinois at Urbana-Champaign.
Reference: Zhang C, K Dehoff, N Hess, M Oostrom, TW Wietsma, AI Valocchi, BW Fouke, and CI Werth. 2010. "Pore-Scale Study of Transverse Mixing Induced CaCO3 Precipitation and Permeability Reduction in a Model Surface Sedimentary System." Environmental Science and Technology 44:7833-7838. DOI: 10.1021/es1019788