Colloids are fine particles (1 nm to ~ 10 μm) that stay suspended in water. Examples of colloids include soil particles or minerals, particulate organic carbon, synthetic nanoparticles, virus, bacteria, and protozoa. Their mobility in water and soil plays a critical role in the cycling of nutrients, elements, and trace metals, which are needed to sustain lives on Earth. Because colloids could carry several contaminants, colloid mobility is also linked to many negative environmental effects including facilitated transport of contaminants and increase in export of highly adsorbing contaminants to receiving water bodies. My research in this category examines the physical, geochemical, and biological processes that affect the transport of particles or colloids and particulate contaminants through soil and water. Most of the work on subsurface transport is done with unrealistic simplistic systems that do not embrace the complexity of the matrix or hydrology. My previous and current work in this category embraces those complexities while still emphasizing mechanistic science. I am interested to study how a change in weather patterns during climate change affects the net export or accumulation of colloids and colloid-associated contaminants in water resources.
These fundamental research studies help design natural and engineered stormwater treatment systems that are resilient to climate extremes. To know more about the applications of the study on colloid transport, see my research studies on stormwater treatment and asbestos remediation.
- Prof. Joseph N Ryan, University of Colorado (Ph.D. advisor)
- Prof. James E Saiers, Yale University
- Dr. Ronald W Harvey, United State Geological Survey (USGS)
- Prof. Alexandria Boehm, Stanford University.
- Prof. Jane Willenbring, University of Pennsylvania
Selected publications on this category are provided below.
Intact soil core setup in the laboratory to study colloid transport in heterogeneous soil.
Effect of dry-wet cycles on the transport of colloids in the subsurface.
During climate change, the occurrence of long drying cycles and high-intensity rainfall is expected to increase in some part of the earth. This study examines how drying duration affects the mobility of colloids in a fractured soil. Subjecting intact soil cores to multiple dry-wet cycles with varying drying duration, we found that increases in drying duration first increased the mobility of colloids until a critical drying duration (that depends on soil properties) and then any further increases in drying duration decreased the colloid mobilization. These results reveal that antecedent drying duration and soil hydraulic properties are two key factors that affect that mobility of colloids in the subsurface. This research is published at Environmental Science and Technology here. PDF.
Effect of freeze-thaw cycles on the transport of colloids and heavy metals in the subsurface.
During climate change, the occurrence of freeze-thaw cycles is expected to change in the northern hemisphere, but very little is known about the impact of the changing climate on the mobility colloids and colloid-associated contaminants in soil. This study examines how a frequent freeze-thaw cycle would change the export of colloids and colloid-associated contaminants to water resources. Subjecting pre-contaminated intact soil cores to multiple freeze-thaw cycles, we found that, compared to dry-wet cycles (control), freeze-thaw cycles create new preferential flow paths, generate soil colloids that are enriched with iron oxides and clays, and cause a rapid transport of colloids and colloid-associated heavy metals or radionuclides through the preferential flow paths. These results indicate that frequent freeze-thaw cycle could increase the mobility of sequestered contaminants in the subsurface and increase the risk of groundwater and surface water contamination. This research is published at Environmental Science and Technology here. PDF.
Effect of intermittent rainfall events on the net export of bacteria and pathogens to water resources.
During climate change, rainfall pattern on the earth surface is expected to change. Most areas are expected to get few but high-intensity rainfall events. This study examines the mechanism of pathogen transport in subsurface during intermittent rainfall events. Using microspheres of different sizes as surrogates to pathogens, we found that intermittent rainfall events increased the release of sequestered microspheres from a soil core pre-contaminated with microspheres. The microspheres mobility increased with increases in microsphere size and permeability of the soil. This result highlights the effect of intermittent high-intensity rainfall on water quality and health risk. This research is published in Vadose Zone Journal (here) and featured on the cover (here). PDF.
Colloid mobilization hysteresis: Role of pore water exchange between macropore and matrix.
An exchange of water between preferential flow paths and soil matrix contributes to a long-term release of dissolved contaminant, but whether and how this exchange can affect the release of colloids are unknown. Limited study in this area has been recognized as one of the major challenge to advance the understanding of colloid transport in the subsurface. Using an intact soil core in this study, we provided a direct evidence showing how the exchange of water between preferential flow paths and matrix affects the amount of colloid mobilized in soils. We showed that colloid mobilization during a rainfall event depends on the ionic strength of the previous rainfall (hence termed as colloid mobilization hysteresis). Different amounts of colloids were mobilized during two rainfalls of the same solution ionic strength following an exposure to a lower or higher ionic strength water. The result revealed that the interaction of infiltrating water (new water) with water in soil matrix (old water) either decreased or increased the ionic strength of pore water, which in turn affected the amount of colloids accumulated during flow pause and mobilized from the pores during next rainfall cycle. This research is published at Environmental Science and Technology here. PDF.
The studies were supported through funding from the U.S. Department of Energy Environmental Science and Management Program (Grant # DOE-FG02-08ER64639).