Coprecipitation of Aluminum Hydroxides and Calcium Carbonates during Denitrification: A Stabilization Strategy for Oxidized Metals and Radionuclides
EMSL Project ID
25666
Abstract
Research conducted at the Field Research Center (FRC) at Oak Ridge National Laboratory and at the Old Rifle site has demonstrated that it is possible to stimulate indigenous microbial activity and create conditions favorable for the reductive precipitation of U and Tc. However, rapid reoxidation of biogenic U(IV) has been observed in field studies at the FRC following exposure to nitrate. Nitrate-dependent bio-oxidation of U(IV) is particularly important at sites where nitrate is present as a co-contaminant at very high concentrations (> 1 mM). Geochemical modeling and titration experiments with acidic groundwater show that increasing pH and bicarbonate concentrations will result in the precipitation of a wide range of solid phases (esp. Al, Si, and Fe(III) hydroxides and Mg and Mn carbonates) that can bind U(VI) and remove it from groundwater via several processes that we will refer to collectively as "coprecipitation". The overall objective of this project is to investigate biomineralization of oxidized metals and radionuclides through microbially-mediated "coprecipitation" in acidic groundwater/sediment systems. The specific objectives are:1. to evaluate the effect of microbially-mediated precipitation on pore- and field-scale hydraulic properties of acidic sediments.
2. to characterize the solubility and thermodynamic properties of the U(VI)-bearing solid phases and to evaluate their stability and reactivity under in situ conditions.
Laboratory experiments will be conducted using uncontaminated sediment and synthetic acidic groundwater designed to be representative of conditions from sites near the S-3 ponds at the FRC at ORNL and near the F-Area Seepage Basins at SRNL. Sediments will be packed into an intermediate-scale (~ 1 m) flow cell equipped with inlet, outlet, and sampling ports. Denitrifiying activity of native microorganisms will be stimulated by the addition of ethanol. Microbial activity will be monitored by analyzing pore water samples. Dual-energy gamma imaging will be used to monitor changes in sediment pack porosity resulting from mineral precipitation. Tracer tests will be conducted periodically as precipitates accumulate and the effects of precipitates on hydraulic properties (dispersivity, hydraulic conductivity, and mass transfer coefficients) will be evaluated. At the end of the experiment, the sediment pack will be destructively sampled to collect precipitate samples for detailed mineralogical characterization and for use in laboratory batch experiments at OSU.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2008-05-01
End Date
2009-05-03
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members