How Wetlands Naturally Clean Up Contaminants

The Science

Wetland environments are effective at mitigating migration of many groundwater contaminants because of their unique combination of geochemistry, microbiology, and hydrology. A recent study showed iron nanoparticles enriched near wetland plants roots bind natural organic matter to greatly immobilize uranium and possibly other contaminants.

The Impact

This study provides a new perspective on geochemical processes responsible for the observed enrichment of uranium in wetlands. Moreover, the findings shed light on a natural process of environmental remediation that potentially could be harnessed for strategies aimed at immobilizing a wide variety of groundwater contaminants.

Summary

Wetlands inhibit migration of groundwater contaminants through a series of biogeochemical processes that enhance the soil’s capacity to immobilize toxic metals. Past evidence has suggested that the root-impacted soil zone, known as the rhizosphere, might play an important role in contaminant immobilization in wetlands. Plants have adapted to grow in these waterlogged environments by transporting oxygen into the rhizosphere, thereby promoting oxidation of dissolved ferrous iron (Fe(II)) to form ferric iron (Fe(III)) oxyhydroxide and soluble uranium ( precipitates on the root surface, referred to as plaques). A recent study examined whether this type of Fe(II)/Fe(III) cycling in wetlands could participate in immobilizing U(VI)—a highly soluble form of uranium. To explore this possibility, researchers from Savannah River National Laboratory, Environmental Molecular Sciences Laboratory (EMSL), University of Georgia, U.S. Environmental Protection Agency, and Princeton University collected soil samples containing roots from the Tims Branch wetland on the Savannah River Site, a nuclear processing facility in South Carolina. The team characterized subsamples near and far from the roots using wet chemistry and various types of spectroscopy and microscopy. Mössbauer spectroscopy, X-ray computed tomography, transmission electron microscopy, helium ion microscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy were conducted at EMSL, a Department of Energy user facility. The analysis revealed wetland rhizosphere soil is enriched in Fe(III) nanoparticles. Moreover, U(VI) is concentrated in root plaques containing Fe(III)-oxyhydroxide precipitates. These results suggest dissolved Fe(II) from the wetland environment enters the rhizosphere and precipitates as Fe(III) nanoparticles capable of binding U(VI). Taken together, the findings suggest plant roots create biogeochemical conditions conducive to the formation of iron nanoparticles, which, in turn, play an important role in uranium enrichment and contaminant immobilization in wetlands.

Principal Investigator(s)

Daniel Kaplan
Savannah River National Laboratory
[email protected]

Ravi Kukkadapu
Environmental Molecular Sciences Laboratory
[email protected]

Funding

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Offices of Basic Energy Sciences and Biological and Environmental Research, including support of the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility, and Subsurface Biogeochemical Research program; and Princeton University.

References

Kaplan, D. I., R. Kukkadapu, J. C. Seaman, B. W. Arey, A. C. Dohnalkova, S. Buettner, D. Li, T. Varga, K. G. Scheckel, and P. R. Jaffé. 2016. “Iron Mineralogy and Uranium-Binding Environment in the Rhizosphere of a Wetland Soil,” Science of the Total Environment 569-570, 53-64. DOI: 10.1016/j.scitotenv.2016.06.120.