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Out of Sight, Out of Mind?

Within-Grain Pore Connectivity Determines Rate of Contaminant Diffusion

Using EMSL’s computational capabilities, researchers found the slowing rate of desorption and diffusion from the granular medium to the surrounding liquid phase is governed by pore connectivity. The less connectivity, the greater the probability that contaminant molecules will take a very long time to leave the grain.

There was a time when disposal of contaminants followed one guiding principal: “Out of sight, out of mind.” But with growing concern for ecosystem and human health, the environmental and scientific communities need to better understand the mechanisms and rates at which subsurface contaminants desorb from hosting media – granular soil, sand and sediments – and diffuse into the environment.

Researchers at Iowa State University and the University of Texas at Arlington collaborated with Pacific Northwest National Laboratory (PNNL) scientists to develop models of the microscopic transfer of uranium into and out of porous material. They investigated diffusion in microenvironments and developed approaches to scale microscopic mass transfer to larger systems.

Because existing models that predict the rates of desorption and diffusion from hosting media did not account for real-world grain-scale properties of hosting media, the research team focused on the desorption, diffusion, and migration of uranium from inside the individual grains of the hosting media, consistent with conditions at nuclear disposal sites.

EMSL computational capabilities were used for numerical calculations and evaluations of diffusion and mass transfer in complex granular porous media, and the results were surprising.

The fundamental process of desorption and diffusion from the granular medium to the surrounding liquid phase was found to be much slower than projected by previous models. Researchers also found the rate of desorption dramatically decreased over time.

This phenomenon is governed by connectivity – the interconnected architecture of microscopic pores inside individual grains (for example, sand grains).  The less pore connectivity, the greater the probability that contaminant molecules will take a very long time to leave the grain. Researchers developed probability equations for migration based on the connectivity of micropores. According to lead PNNL scientist Chongxuan Liu, various factors can influence diffusion inside grains and contaminant release into environment, but the most important factors are the grain-scale pore architecture and the contaminant’s history in the hosting medium.     

What’s next? Both laboratory and field research is currently underway to experimentally characterize grain-scale pore architecture and to investigate the larger-scale manifestation of the grain-scale contaminant diffusion. The research is supported by the Subsurface Science Focus Area (SFA) and Integrated Field Research Challenge (IFRC) programs at PNNL, funded by the U.S. Department of Energy's Office of Biological and Environmental Research (BER).

Scientific impact: Greater insight into pore connectivity’s impact on desorption rates from solid media to the surrounding liquid phase will allow scientists to more accurately predict contaminant migration and project the progress and effectiveness of remediation programs.

Societal impact: The study, which applies to all manner of contaminants, from uranium to common volatile organic compounds such as trichloroethene and tetrachloroethylene, gives environmental scientists a better understanding of how contaminants will behave in the future. And based on this, makers of public policy will have better information with which to make better long-term decisions.

Reference: Ewing, R. P., Q. Hu, and C. Liu (2010), Scale dependence of intragranular porosity, tortuosity, and diffusivity, Water Resour. Res., 46, W06513, doi:10.1029/2009WR008183.

Acknowledgments: This work was funded by the U.S. Department of Energy's Office of Biological and Environmental Research (BER).

 

Released: September 07, 2010