Technetium (Tc) reduction by chemically and biologically reduced smectite clay
EMSL Project ID
18005
Abstract
Oxidized form of technetium (Tc7+) is highly toxic and soluble when they are present in water systems(1). Large quantities of these radioactive heavy metals are released from storage tanks of radioactive wastes (Hanford and Oak Ridge for example). The physical (pump and treat), and biological (bio-stimulation and bio-augmentation) methods to remove these metals from groundwater systems have many disadvantages. For example,the pump and treat method is expensive and not environmentally sound. If metal-reducing bacteria are injected into contaminated sites, they may quickly die off, or attach to mineral surfaces, hence minimize their transport and decrease efficiency.
In this proposal, we intend to use reduced iron-rich smectite (nontronite) to reduce the oxidized Tc7+ to reduced form Tc4+. This activated clay possesses several advantages over classical methods of groundwater remediation. For example, large quantities of activated smectite can be injected into the contaminated site, or can be created in-situ from naturally present smectite by chemically or biologically reducing it. Therefore, this hypothesis makes the clay, a foe for current practice of Tc reduction, into a friend. We have performed several preliminary experiments demonstrating that biologically reduced smectite (nontronite) can efficiently and completely reduced the oxidized form of Tc, thus immobilizing it in groundwater systems. In this proposal, we intend to elaborate this research through following laboratory experiments:
Aqueous species of Tc(VII) will be prepared from ammonium pertechnetate. Reduced smectite will be produced by either microbial (Shewanella putrefaciens CN32) or chemical (dithionite-citrate-bicarbonate) reduction of Fe(III) in smectite. The rate and extent of Tc(VII) reduction by Fe(II) in the smectite structure will be measured through direct extraction and liquid scintillation counting of 99Tc. Structural changes in smectite, spatial associations of immobilized Tc (epitaxial growth or surface adsorption/complexation) before and after reaction with Tc will be analyzed by X-ray diffraction and transmission electron microscopy (TEM) methods. This specific proposal to EMSL user facility is to request TEM time to determine the fate of reduced Tc in relation to clay mineral smectite.
New rate laws will be defined based on the laboratory measured rates of Tc(VI) reduction from Fe(II) associated in three different environments in bioreduced smectite: structural, surface adsorbed and free in solution. Ions speciation models will be performed using MINTEQA2 and the rate of Tc reduction will be performed using STELLA software. These three rate laws will be combined by certain stoichiometric factor that accounts the role and rate of each process. The first order rate law of Fe(III) reduction in smectite as a function of surface site concentration of Fe(III) will be also be incorporated. In order to make the modeling realistic to existing processes, attempts to relate the rate to thermodynamic constraints based on free energy of half reaction pairs of Tc, surface adsorbed and soluble Fe(II) will also be added in kinetic term. Finally, the complete model from the formation of activated smectite to Tc immobilization will be purposed. The significance and role of each parameter will be discussed to predict their role in anoxic natural groundwater conditions.
Project Details
Project type
Exploratory Research
Start Date
2006-02-16
End Date
2007-03-22
Status
Closed
Released Data Link
Team
Principal Investigator