The potential for uranium transport in the subsurface is decreased in anaerobic conditions through the formation of sparingly soluble UO2(s), a process often facilitated by dissimilatory metal and sulfate reducing bacteria.  Additionally, uranium may adsorb on a variety of mineral surfaces decreasing its solution concentration.  Within my dissertation research, I explored the profound impact of uranyl speciation, and specifically the presence of dissolved calcium on uranium redox cycling. I demonstrated a novel remediation strategy of sequestering uranium via incorporation into iron oxide minerals (Stewart et. al, 2009) and demonstrated this in a natural setting at a redox interface in Rifle, CO (Stewart et. al, 2015).


Selenium can be extremely toxic, causing gastrointestinal and endocrine damage to both humans and animals at higher concentrations. During mining operations, Se is often oxidized, increasing its mobility in the environment, as a byproduct of mining for other metals; however, native microbial populations have the metabolic capacity to re-reduce Se back to a more stable form when oxygen concentrations decrease to micro-aerophilic levels.  I measured soil respiration rates in mine waste rock samples to quantify how rapidly microbial populations in oxidized waste rock materials consume oxygen. We combined these observations with X-ray spectroscopy to identify different solid phase products from the selenium-bearing mine waste materials to understand native microbial populations’ influence on selenium biogeochemistry in these settings.


Treatment processes for metals in industrial stormwater can be cost prohibitive. Therefore, passive remediation strategies using natural materials and microbial processes have the potential to provide affordable solutions for removing metals from aqueous waste streams, which is important from regulatory and environmental quality perspectives. My current research explores key chemical, biological, and physical processes promoting removal of hexavalent chromium from an industrial stormwater treatment system by association with granular organic peat media.


A Multi-Modal Approach to Unpacking Iron Biogeochemical Processes in Buoyant Hydrothermal Plumes. Stewart, B.D., J.V. Sorensen, K. Wendt, J.B. Sylvan, C. R. German, K.   Anantharaman, G.J. Dick, J.A Breier,  and B. M. Toner.  Chemical Geology. Submitted.

Large Nickel Isotope Fractionation Caused by Surface Complexation Reactions with Hexagonal Birnessite.  Sorensen, J.V., B. Gueguen, B.D. Stewart, J. Peña, O. Rouxel, and B. M. Toner. Chemical Geology 2020, 537, 119481.

Reactivity of Uranium and Ferrous Iron with Natural Iron Oxyhydroxides.  Stewart, B.D., A.C. Cismasu, K.H. Williams, B.M. Peyton, and P.S. Nico.   Environmental Science and Technology 2015, 49(17), 10357-10365.

Influence of Chelating Agents on Biogenic Uraninite Reoxidation by Fe(III) (hydr)oxides

Stewart, B.D., C. Girardot, N. Spycher, R.K. Sani, and B.M. Peyton. Environmental Science and Technology 2013, 47(1), 364-371.

Reoxidation of Biogenic Reduced Uranium – A Challenge Toward Bioremediation

Singh, G., S. Sengor, A. Bhalla, S. Kumar, J. De, B.D. Stewart, N. Spycher, T.M. Ginn, B.M. Peyton, and R.K. Sani. Critial Reviews in Environmental Science and Technology 2013, 43(17).

Detection of Biological Uranium Reduction using Magnetic Resonance

Vogt, S.J., B.D. Stewart, J.D. Seymour, B.M. Peyton, and S.L. Codd. Biotechnology and Bioengineering 2012, 109, 877-883.

On Modeling Biogenic Uraninite Precipitation and Reoxidation by Iron(III)(Hydr)oxides: Thermodynamic and Kinetic Considerations

Spycher, N., M. Issarangkun, B.D. Stewart, S. Sengor, E. Belding, T.M. Ginn, B.M. Peyton, and R.K. Sani. Geochimica Cosmochimica Acta 2011, 75, 4426-4440.

Effect of Uranium(VI) Speciation on Simultaneous Microbial Reduction of Uranium(VI) and Iron(III)

Stewart, B.D., R.T. Amos, and S. Fendorf. Journal of Environmental Quality 2011, 40, 90-97.

Influence of Uranyl Speciation and Iron Oxides on Uranium Biogeochemical Redox Reactions

Stewart, B.D., R.T. Amos, P.S. Nico, and S. Fendorf. Geomicrobiology Journal 2011, 28, 444-456.

Impact of Uranyl-Calcium-Carbonato Complexes on Uranium(VI) Adsorption to Synthetic and Natural Sediments

Stewart, B.D., M.A. Mayes, and S. Fendorf. Environmental Science and Technology 2010, 44(3), 928-934.

Kinetic and Mechanistic Constraints on the Oxidation of Biogenic Uraninite by Ferrihydrite

Ginder-Vogel, M.G., B.D. Stewart, and S. Fendorf. Environmental Science and Technology 2009, 44(1), 163-169.

The Influence of Calcium on the Biogeochemical Fate of Uranium

Stewart, B.D., and S. Fendorf. VDM Publishing House 2009.

Incorporation of Uranium(VI) into Fe (hydr)oxides during Fe(II) Catalyzed Remineralization

Nico, P.S., B.D. Stewart, and S. Fendorf. Environmental Science and Technology 2009, 43 (19), 7391-7396.

Stability of Uranium Incorporated into Fe (hydr)oxides Under Fluctuating Redox Conditions

Stewart, B.D., P.S. Nico, and S. Fendorf. Environmental Science and Technology 2009, 43, 4922-4927.

Speciation-Dependent Microbial Reduction of Uranium within Iron-Coated Sands

Neiss, J., B.D. Stewart, P.S. Nico, and S. Fendorf. Environmental Science and Technology 2007, 41, 7343-7348.

Quantifying Constraints Imposed by Calcium and Iron on Bacterial Reduction of Uranium(VI)

Stewart, B.D., J. Neiss, and S. Fendorf. Journal of Environmental Quality 2007, 36, 363-372.