Cleanup of Contaminated Ground Water with Nano Iron

05/23/13 Portland, Ore.

The “next big thing” for cleanup of contaminated soil and groundwater is to use nano-sized iron metal to degrade the pollutants. This technology passed from laboratory research to field-scale deployment in less than a decade.

The technology emerged so quickly that there are still outstanding questions regarding its performance in the field. A detailed pilot-scale study to address these questions was recently reported by researchers at the Oregon Health & Science University (OHSU).

The study, published in the February 2013 edition of Environmental Science & Technology, examined the fate of nano zerovalent iron (nZVI) during injection into a large three-dimensional model aquifer. The model aquifer is 10 x 10 meters in area, 2.4-meter deep, and filled with six layers of sand and clay. The facility for this was started 20 years ago by Professor Richard Johnson, and was one of only a few in the world with capacity for such large experiments.

“Previous large scale studies of nZVI injection have not included sufficient characterization to really know how effective the processes was,” said Johnson, the lead investigator on the project and professor in OHSU’s Division of Environmental and Biomolecular Systems (EBS). “We believe that the combination of monitoring methods, and modeling, that we used provides the first definitive evidence regarding what can actually be achieved in the field.”

Johnson led a team of eight OHSU researchers to monitor and track the nZVI injected into the aquifer. Each research member was responsible for one or more characterization methods. The resulting data, combined with numerical modeling of the results, provided the most detailed characterization to date of nZVI injection into the subsurface.

One important finding from the study is that simply monitoring the color in groundwater sampling wells can provide valuable information about the transport and fate of nZVI. Color changes not only are a sensitive indicator of nZVI breakthrough, but they also show when the nZVI has been oxidized by the aquifer. The resulting yellow color reflects the “reductant demand” of the aquifer.

In addition to the nZVI, the injected fluid contained several species that served as conservative tracers of fluid movement in the aquifer. These data, together with mathematical modeling of the results, showed that formation of bubbles of hydrogen gas (from reactions of the nZVI) influenced the flow of groundwater.

“This study clarifies both the pros and cons of this technology,” said Paul Tratnyek, co-author and professor with the OHSU Division of Environmental & Biomolecular Systems and co-Principal Investigator on the project. “Through this work, we hope to help ensure that when the technology gets used, it gets used correctly, so the environmental cleanup goals are reached with a minimum of surprises or failures.”

This study was funded by the Strategic Environmental Research and Development Program (SERDP) as a supplement to ER-1485 (Fundamental Study of the Delivery of Nanorion to DNAPL Source Zones in Naturally Heterogenous Field Systems), which was led by colleagues at Carnegie Mellon University. The full report form this project, including results from the pilot test, is available here.

nZVI injection well diagram

Conceptual model for treatment of contaminated groundwater by injection of nano-sized iron into the subsurface. Further explanation of this figure is available here.

Johnson science team at model aquifer 

Students and postdocs performing characterization methods used in this study. Additional photos can be viewed here.

water samples from monitoring well  

Photographs of water from a monitoring well 0.5 meters down-gradient from the nZVI injection well. Yellow indicates arrival of oxidized nZVI and black marks breakthrough of the injected material. Further data and explanation can be found in the journal article here.

 nZVI transport illustration

In the subsurface, nZVI (black dots) becomes deposited on aquifer grains (yellow circles), which limits its movement. Numerical modeling based on experimental data indicates that transport distances >1.5 meters will be difficult to achieve, even under aggressive injection conditions. Extended injection times did not significantly increase transport distances.

OHSU nZVI science team

Johnson, R. L., J. T. Nurmi, G. O’Brien Johnson, D. Fan, R. O’Brien Johnson, Z. Shi, J. Salter-Blanc Alexandra, P. G. Tratnyek, and G. V. Lowry. 2013. Field-scale transport and transformation of carboxymethylcellulose-stabilized nano zero-valent iron. Environ. Sci. Technol. 47(3): 1573-1580.