University of Southern California Undergraduate Research Symposium for Creative and Scholarly Work, April 9-11, 2012 Removal of Radioactive Uranium from Groundwater using Nanoparticle Technology and Adsorption Mechanisms Student Researchers: Hannah Gray and Aditi Yokota-Joshi Faulty Advisor: Professor Massoud Pirbazari; Research Scientist: Dr. Varadarajan Ravindran Doctoral Student Advisors: Ryan Thacher and Kuo-Hsun Tsai Lab Coordinator: Erick Hernandez Sonny Astani Department of Civil and Environmental Engineering, Viterbi School of Engineering, University of Southern California Work sponsored by Women in Science and Engineering (WiSE) Grant

Introduction to Uranium  Uranium is a radioactive element that can be released into groundwater through anthropogenic and natural sources, including leaching from uranium mill tailing, hydraulic fracturing, and accidents in nuclear reactors. Radioactive uranium is toxic to humans and can increase risk of cancer, kidney disease, neurological damage, and birth defects.  Uranium exists in two valence states:  Soluble, mobile hexavalent U(VI)  Insoluble, immobile tetravalent U(IV).  This difference in solubility can be exploited to remove dissolved uranium from contaminated groundwater.

Figure 1. Uranium tailing ponds, Colorado Figure 2. Fukushima nuclear power plant accident Figure 3. Uranium-related health effects and birth defects.

Goals of Research  Conventional removal techniques are invasive and chemical-intensive. The primary goal of this research is to develop alternative means of uranium removal by:  Simulate U(VI) behavior in groundwater systems by investigating adsorption of U(VI) to common soils  Assess the use of nZVI for removal of U(VI) from groundwater  Investigating microbial remediation through bio- catalyzed reduction of U(VI)

Materials  The Beckman 6000 Liquid Scintillation Counter (LSC) was used to measure radionuclide activity. The liquid scintillation counting technique detects photons released during radioactive decay events, and uses photon counts to determine Disintegrations per Minute (DPM). Sample preparation for the LSC included addition of a scintillation cocktail and 0.2 N HNO 3 to reduce loss of counts from interferences.  Rate studies and adsorption tests were performed on a shaker unit to ensure uniform mixing within bottles, and pH was controlled through a phosphate buffer addition.

Methods  Sand adsorption experiments were performed with glass bottles of 20g sand and 0 to 1000 ug/L uranium. The samples were mixed continuously for 72 hrs and then analyzed for U(VI) by liquid scintillation counting.  Removal of U(VI) by nZVI was investigated under oxic and anoxic conditions in 100 mL batch reactors, containing 0.1 g/L nZVI and mg/L U(VI), resulting in a ratio of nZVI to U(VI) of between 20:1 and 1:1. Adsorption experiments were handled in a nitrogen glove box to prevent iron oxidation. For anaerobic testing, the uranium solutions were purged with nitrogen and placed in anaerobic glass jars to prevent any oxidation.

nZVI Adsorption Isotherm Determination  Zero-valent iron nanoparticles (nZVI) have a very high specific surface area, are highly mobile, and have been found effective in the removal of heavy metals and chlorinated compounds by an adsorption-reduction reaction. nZVI reduces soluble U(VI) to insoluble U(IV) via adsorption and precipitation of uranyl ions onto the nZVI particles.  Four key isotherm models were developed - oxic and anoxic adsorption models, and humic acid oxic and anoxic adsorption models. Humic acid, in 30 mg/L concentration, reflects the naturally present humic acid in groundwater.

Figure 4. TEM Micrograph and diagram to illustrate adsorption of uranium oxide nodules on sand. Fe 0  U(VI)  UO 2 (CH 3 COO) 2 ·2H 2 O Sorption Fe 0 U(IV)  U(VI) UO 2 (CH 3 COO) 2 ·2H 2 O Reduction Sorption Figure 5. TEM Micrograph and diagram to illustrate reduction and adsorption of uranium nodules on zero-valent iron nanoparticles.

Langmuir Isotherm for uranium and sand  Adsorption of uranium on sand was found to follow the Langmuir isotherm, indicating that the organic layer on the sand media was capable of uranium removal. This result was further supported by the humic acid adsorption isotherm.

Add nZVIFiltration Figure 5. The removal of Uranium contamination by nZVI reduction and filtration. The rate of this process was studied to develop the isotherm models for nZVI and U(VI) shown below.

The nZVI-U(VI) equilibrium data fit a Langmuir isotherm model, representing the behavior of nZVI in the presence of U(VI) aerobically and anaerobically. Below, further testing with humic acid verified these findings, while also reflecting groundwater’s natural characteristics. Langmuir Isotherm for uranium and nZVI

Summary of Results  nZVI successfully reduces U(VI) to U(IV), enabling adsorption and removal, up to 93% removal in some cases.  The data suggests the adsorption of uranium onto nZVI follows a Langmuir isotherm model.  Aerobic adsorption works more effectively for U 238 removal than anaerobic adsorption, with a adsorption capacity of  Humic acid positively impacts the aerobic adsorption of U 238 onto nZVI, by both an increase in the rate of adsorption and in the total quantity adsorbed.  Humic acid negatively impacts the anaerobic adsorption of U 238 onto nZVI, by reducing the total quantity of uranium adsorbed.

Conclusions  The Langmuir isotherm model describes a relation between the maximum adsorption capacity and the net enthalpy of adsorption. For nZVI, adsorption capacity is:  mg U(VI)/g nZVI for aerobic nZVI  mg U(VI)/g nZVI for aerobic nZVI with humic acid  *** (TBD) mg U(VI)/g nZVI for anaerobic nZVI  *** (TBD) mg U(VI)/g nZVI for anaerobic nZVI with humic acid  The increased adsorption with the aerobic addition of humic acid might suggest ***. Additionally, aerobic groundwater with naturally occurring humic acid would result in effective removal of uranium by nZVI.

Future Work  Future work will include  Developing isotherm models for U(VI)-nZVI adsorption with additional organic materials present.  Comparing the removal of uranium by sulfur-reducing bacteria (SRB) versus the nZVI technology to evaluated the best treatment option for uranium- contaminated groundwater.  Performing dynamic sand column isotherm tests, integrated with both SRB and nZVI, thus maximizing U(VI) reduction to U(IV).  Applying bioremediation columns as an in-situ or ex- situ treatment for contaminated sites.

Acknowledgements Special thanks to Professor Donal Manahan of the USC College Department of Biological Sciences and the USC Women in Science and Engineering for their assistance in this research project.

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