A biogeochemical framework for bioremediation of plutonium(V) in the subsurface environment

Abstract

Accidental release of plutonium (Pu) from storage facilities in the subsurface environment is a concern for the safety of human beings and the environment. Given the complexity of the subsurface environment and multivalent state of Pu, we developed a quantitative biogeochemical framework for bioremediation of Pu(V)O(2) (+) in the subsurface environment. We implemented the framework in the biogeochemical model CCBATCH by expanding its chemical equilibrium for aqueous complexation of Pu and its biological sub-models for including Pu's toxicity and reduction reactions. The quantified framework reveals that most of the Pu(V) is speciated as free Pu(V)O(2) (+) ((aq)), which is a problem if the concentration of free Pu(V)O(2) (+) is ≥28 μM (the half-maximum toxicity value for bacteria able to reduce Pu(V) to Pu(III)PO(4(am))) or ≥250 μM (the full-toxicity value that takes the bioreduction rate to zero). The framework includes bioreduction of Fe(3+) to Fe(2+), which abiotically reduces Pu(V)O(2) (+) to Pu(IV) and then to Pu(III). Biotic (enzymatic) reduction of Pu(V)O(2) (+) directly to Pu(III) by Shewanella alga (S. alga) is also included in the framework. Modeling results also reveal that for formation of Pu(III)PO(4(am)), the desired immobile product, the concentration of coexisting model strong ligand-nitrilotriacetic acid (NTA)-should be less than or equal to the concentration of total Pu(III).

In situ and ex situ bioremediation of radionuclide-contaminated soils at nuclear and norm sites

“…Due to high ionic charge, it can undergo hydrolysis and convert to a polymeric form at pH > 2. Information is limited on the influence of microbes on the solubility of Pu (Deo & Rittmann, 2012;Deo et al, 2011). The chemical speciation of Pu can be influenced by the soil's pH, redox conditions, organic content, mineralogy, and microbial activities (Francis, 2007).…”

Phosphate-Mediated Remediation of Metals and Radionuclides

“…Limitations to microbial reduction of metals and radionuclide also arise from low pH environments that must be neutralized to support growth of sulfate-and metalreducing communities [163,169,170]. Additionally, microbial reduction in mixed waste environments must be carefully considered so as to minimize contaminant migration of elements such as As and Pu that demonstrate greater mobility in their reduced valence states [171,172].…”

“…Furthermore, contaminated sites that are characterized by acidic to circumneutral porewater pH represent environments that can support stable mineral formation (Figures 2(a) and 2(b)), provided that carbonates are not present in significant concentrations (i.e.,P CO 2 < 10 −3.5 atm) [176,177]. Interestingly, investigations of microbial reduction of Cr, Np, Pu, and U have been shown to support subsequent phosphate precipitation reactions via thermodynamic modeling, chromatographic separation of actinides based on valence state, and X-ray analytical methods [154,155,172,178,179]. Unlike U, that is capable of forming phosphate minerals in both hexavalent and tetravalent states [50,179], the reduction of Cr, Np, and Pu is initially required for these contaminants to participate in phosphate precipitation reactions [154,155,172,178].…”

“…Interestingly, investigations of microbial reduction of Cr, Np, Pu, and U have been shown to support subsequent phosphate precipitation reactions via thermodynamic modeling, chromatographic separation of actinides based on valence state, and X-ray analytical methods [154,155,172,178,179]. Unlike U, that is capable of forming phosphate minerals in both hexavalent and tetravalent states [50,179], the reduction of Cr, Np, and Pu is initially required for these contaminants to participate in phosphate precipitation reactions [154,155,172,178]. To date, only pure culture or coculture studies have identified such coupled microbial interactions that offer an additional approach to control contaminant toxicity and mobility that are perpetuated by valence state cycling.…”