Colloid Deposit Morphology Controls Permeability in Porous Media

Summary

Processes occurring in soils and aquifers play a crucial role in contaminant remediation and carbon cycling. The flow of water through porous media like soils and aquifers is essential for contaminant remediation and carbon cycling and depends on the permeability, which determines how much water flows for a given hydraulic driving force. Widely recognized is that colloids (fine particles including soils, chemical precipitates, and bacteria) often control permeability and that colloid deposit morphology (the structure of deposited colloids) is a fundamental aspect of permeability. Until recently, however, no experimental techniques were available to measure colloid deposit morphology within porous media. A recent study, led by the University of Colorado Denver in collaboration with Lawrence Berkeley National Laboratory, used a custom-designed experimental apparatus to perform a series of experiments using static light scattering (SLS) to characterize colloid deposit morphology within refractive index matched (RIM) porous media during flow through a column. Real-time measurements of permeability, specific deposit, and deposit morphology were conducted with initially clean porous media at various ionic strengths and water velocities. Decreased permeability (i.e., increased clogging) correlated with colloid deposit morphology, specifically with lower fractal dimension and smaller radius of gyration.

These observations suggest a deposition scenario in which large and uniform aggregates become deposits, reducing porosity, and lead to higher fluid shear forces, which then decompose the deposits, filling the pore space with small and dendritic fragments of aggregate. Accordingly, for the first time, observations are available to quantify the relationship between the macroscopic variables of ionic strength and water velocity and the pore-scale variables of colloid deposit morphology, which can be conceptualized as an emergent property of the system. This research paves the way for future studies to quantify the complex feedback process between flow, chemistry, and biology in soils and aquifers.

Principal Investigator(s)

Eric Roth
University of Colorado, Denver

Funding

This research was supported by the Subsurface Biogeochemistry Research Program (SBR; award DE-SC0006962) of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science. B.G. was supported as part of the Subsurface Science Scientific Focus Area funded by BER within the DOE Office of Science (award DE-AC02-05CH11231).

References

Roth, E.J.; Gilbert, B.; and Mays, D.C. “Colloid deposit morphology and clogging in porous media: Fundamental insights through investigation of deposit fractal dimension.” Environmental Science & Technology 49, 12263–70 (2015). DOI:10.1021/acs.est.5b03212.