Plutonium (Pu) contamination of soils or groundwater can be a serious problem at DOE sites. Pu is an element with a complex chemistry, exhibiting a number of oxidation states in aquatic environments, predominantly Pu(IV) and Pu(V); although Pu(III) species have also been recently observed in microbially-active anaerobic systems. In surface and subsurface waters, the prevailing Pu oxidation state depends on the abundance of natural organic matter (NOM). NOM reducing moieties can significantly modify the mobility of Pu in the environment by producing colloidal organic carriers that strongly bind Pu, mostly as Pu(IV). As a consequence, NOM, as geopolymers (e.g., humic substances from decaying plant matter) and biopolymers (e.g., exopolymeric substances, EPS), can, at times, both retard and accelerate Pu transport through production of different types of negatively charged, colloidal organic carrier molecules. These molecules have different amphiphilic properties and molecular weights, and strong Pu binding affinities.
Objective 1: Build on our successful characterization of the chemical composition of organo-Pu species in water leachates from the remediated RFETS site by using a similar approach on other soils or aquifer sediments from different Pu-contaminated DOE sites. These leachates of ambient colloidal organic forms of Pu will then be compared to those that have formed over time when a Pu tracer was added to the same soils, and will be used for transport studies.
Objective 2: Determine the biogeochemical factors that give mobility and/or immobility to the colloidal organic carrier molecule. This requires a full molecular level characterization of the chemical composition of strongly Pu-binding biopolymers in NOM extracted from laboratory incubation, leaching and column studies with these soils.
Objective 3: Determine thermodynamic stability constants of Pu to the well-characterized Pu-carrying NOM compounds isolated from Pu contaminated soils.
Hypotheses: H1: Mobile organic Pu species from Pu contaminated soils contain biomarker compounds that can be used to predict the future spread of Pu contamination as well as Pu bioavailability. H2: Amphipilic EPS molecules of moderate molecular weight (e.g., 5-50 kDa) containing reducing moieties (e.g., hydroquinones, ferredoxins, or flavodoxins) with clustered ligand groups for Fe(III)-binding are most effective in immobilizing organo-Pu species that sorb to sediment particles, while colloids of ≤105 kDa molecular mass are potentially mobile in groundwater. H3: Binding constants of Pu to well characterized Pu-carrying NOM compounds isolated from Pu contaminated soils depend on applied Pu concentrations due to the well-known surface site heterogeneity effect.
Experimental design: The objectives of this proposal will be accomplished through controlled laboratory experiments at TAMUG, in collaboration with researchers from national laboratories and Pat Hatcher’s organic geochemistry group at ODU. At TAMUG, alpha counting for Pu, HPLC and IEF for separation, and GC-MS and ATR-FTIR will be used for NOM characterization and transport studies. These samples will be sent to ODU, for the applications of NMR techniques and EI-MS techniques will be applied to the isolates supplied from TAMUG. Additional collaborators are the SRNL Pu biogeochemistry group led by Dr. Dan Kaplan, the LLNL Actinide biogeochemistry group led by Annie Kersting, and the PNNL radiochemistry group led by Dr. Jon Schwantes.
The potential benefits of the project to DOE are a better understanding of the potential role and mobility of different organic bio- and geopolymeric Pu vectors that will greatly advance science in general, and in particular, environmental biogeochemistry of Pu at low, environmental levels. This is critical to the long-term DOE strategy to incorporate coupled physical, chemical and biological processes into decision making for environmental remediation and long-term stewardship. Present transport or risk models do not include any organic phases in their treatment of Pu transport, even though the effect(s) can be likely profound.