Modeling the Micro Scale
Scientists build realistic simulations for studying subsurface pollutants
In nature, certain microbes naturally feed upon pollutants, turning toxic chemicals and radionuclides into harmless or immobile products. At old landfills and other sites where these pollutants have reached the groundwater, these microorganisms could clean up the waste. To efficiently use these microscopic organisms, scientists need to understand their growth and metabolism in the subsurface.
Recently, scientists working to understand the microbes received a boost from two studies done with a new integrated microfluidics capability at the Department of Energy’s EMSL. These studies demonstrated how liquids mix and microbes grow in the gaps between clay, sand, and other soil particles; i.e., the pore scale. Reactions at the pore-scale can have a significant impact on the larger macro-scales (meters to kilometers) we can readily observe.
"Microfluidic structures are at the small scale, but they have implications for the larger scale," said Dr. Changyong Zhang, a geochemist who worked on both studies.
Growing at the edge of the mixing zone. When using bio-remediation, nutrient solutions that stimulate microbial growth are pumped into the ground. Scientists wanted to know how these solutions mix with contaminated groundwater already in the subsurface, and stimulate microbial growth and that results in pollutant degradation. The team used a micromodel, with channels spread out in a honeycomb pattern, and saw mixing between model nutrient solutions and groundwater. They found that microbial growth and pollutant degradation occur only at the mixing interface between nutrient solutions and groundwater, and that the pore structure affects the time scales of these processes. The experimental results from the study closely agreed with theoretical simulations. This data aids in the design and validation of remediation strategies.
This work was done by scientists from the University of Illinois at Urbana-Champaign and Pacific Northwest National Laboratory (PNNL).
Growing around soil aggregates: In a related study, scientists examined how Delftia acidovorans, a model bacteria isolated from an industrial waste site, grew in heterogeneous environments. The team created a micromodel with narrower and wider channels, mimicking the smaller pathways inside soil aggregates and the larger channels around them. The researchers showed that the microbes preferred to grow between soil aggregates. This information would influence how bioremediation is undertaken in different soil structures.
This study was done by scientists from PNNL, Los Alamos National Laboratory, University of Illinois at Urbana-Champaign, and the Helmholtz Center for Environmental Research in Germany.
Microfluidics resources available: At EMSL, researchers build simulations of real-world subsurface structures using micromodels. The models, etched onto silicon wafers or other surfaces, contain clear channels or pores that are between a few tens and a few hundreds of microns in diameter. Fluids are injected into the model using low-pulsation, high-precision syringe pumps.
Images of the reactions inside the channels can be obtained with a host of different microscopes available at EMSL. These and other capabilities are available to scientists through EMSL’s user process.
Scientific impact: When studying subsurface contamination, scientists need to bridge the gap between data about reactions measured in microns and reactions measured in kilometers. These microfluidic studies are bridging that gap by providing accurate data for large-scale computational simulations of interactions at the pore scale, the micrometer spaces around grains of clay, sand, and other soil matter. This work is part of EMSL’s ongoing efforts to bridge different scales.
Societal impact: Sand, clay, and other matter create complex mazes through the subsurface. These tiny pathways can significantly influence how liquids, gases, and microbes behave underground. Conducting studies using microfluidics and other EMSL capabilities provides needed data for herbicide remediation, nuclear waste cleanup, and carbon sequestration.
References: Willingham T, C Zhang, CJ Werth, AJ Valocchi, M Oostrom, and TW Wietsma. 2010. "Using Dispersivity Values to Quantify the Effects of Pore-Scale Flow Focusing on Enhanced Reaction Along a Transverse Mixing Zone." Advances in Water Resources 33(4):525-535. DOI:10.1016/j.advwatres.2010.02.004
Zhang CY, QJ Kang, X Wang, JL Zilles, RH Muller, and CJ Werth. 2010. "Effects of Pore-Scale Heterogeneity and Transverse Mixing on Bacterial Growth in Porous Media." Environmental Science & Technology 44:3085-3092.
Acknowledgments: The Willingham et al. research was funded by the National Science Foundation and an International Programs in Engineering Fellowship. The Zhang et al. work was funded by the National Research Initiative Grant from the U.S. Department of Agriculture National Institute of Food and Agriculture and EMSL.
Released: July 21, 2010