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From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
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Bureau Seminar, February 17, 2012

Experimental results showing carbon isotope fractionation during gas-phase transport of CO2. A useful process tracer for diffusion and adsorption?

Link to streaming video: available 02.17.2012 at 8:55am

Dr Toti Larson
Jackson School of Geosciences

Toti Larson

Toti Larson is a Research Scientist in the Department of Geological Sciences at UT Austin since January, 2012. He is managing the Stable Isotope laboratory led by Jaime Barnes and Dan Breecker. Prior to UT, Toti worked at Los Alamos National Laboratory as a Technical Staff Member in the Earth and Environmental Sciences Division. His focus of research has been in the application of stable isotopes to environmental, chemical and nuclear forensics and geological exploration.

Diffusion and adsorption processes during gas phase transport differentially affect the stable carbon isotope values of CO2. During simple diffusion the lighter 12CO2 is preferentially partitioned in the tails of a CO2 plume (chromatographic peak) and the heavier 13CO2 is partitioned in the center of the plume (chromatographic peak). In contrast, systems dominated by adsorption processes yield a stable isotope profile that more closely resembles a Rayleigh fractionation pattern. Using 1-dimensional column experiments with montmorillonite, quartz sand, and a blank column we demonstrate a +18‰ carbon isotope fractionation (enrichment) as a result of CO2 desorption from montmorillonite. Results from a quartz-sand packed column and a blank column are similar with a smaller 2-4‰ difference in δ13C across the peak. Mixing and/or minor adsorption in the quartz-sand column may obscure diffusion effects. In parallel to the 1-D column experiments, batch-style adsorption experiments yield consistent results; adsorption of CO2 to montmorillonite with a coupled 4‰ carbon isotope fractionation whereas little effect was observed with quartz sand. These results demonstrate the importance of understanding the isotopic effects of CO2 in different substrates, and potentially offers a tracer tool that can be used to quantify surface area and/or transport distance of CO2. With the analytical tools we have developed in the Stable Isotope Lab for Critical Zone Gases in the Department of Geological Sciences these experiments can readily be expanded to include mixed gases including alkanes, nitrogen, oxygen, and argon.

 

 
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