James E. Banaszak

Subsurface interactions of actinide species and microorganisms: Implications for the bioremediation of actinide-organic mixtures

By reviewing how microorganisms interact with actinides in subsurface environments, we assess how bioremediation controls the fate of actinides. Actinides often are co-contaminants with strong organic chelators, chlorinated solvents, and fuel hydrocarbons. Bioremediation can immobilize the actinides, biodegrade the co-contaminants, or both. Actinides at the IV oxidation state are the least soluble, and microorganisms accelerate precipitation by altering the actinides oxidation state or its speciation. We describe how microorganisms directly oxidize or reduce actinides and how microbiological reactions that biodegrade strong organic chelators, alter the pH, and consume or produce precipitating anions strongly affect actinide speciation and, therefore, mobility. We explain why inhibition caused by chemical or radiolytic toxicities uniquely affects microbial reactions. Due to the complex interactions of the microbiological and chemical phenomema, mathematical modeling is an essential tool for research on and application of bioremedation involving co-contamination with actinides. We describe the development of mathematical models that link microbiological and geochemical reactions. Throughout, we identify the key research needs.

Full-scale application of focused-pulsed pre-treatment for improving biosolids digestion and conversion to methane

We tested at full-scale the innovative Focused Pulsed (FP) technology for pre-treating waste sludge in order to improve methane gas production and biosolids reduction in sludge digestion, but without incurring problems of odors, toxicity, and high costs for chemical or energy consumption. FP pre-treatment of a mixture of primary and secondary sludge increased the soluble COD by 160% and DOC 120% over the control. FP pre-treatment of 63% of the input waste sludge increased biogas production by over 40% and reduced biosolids requiring disposal by 30% when compared to the plant baseline. FP pre-treatment also correlated with a shift of the bacterial and archaeal communities. The most significant change was that the acetate-cleaving Methanosaeta became the dominant methanogen. Full FP pre-treatment should increase biogas production and biosolids removal by 60% and 40%, respectively. Full FP pre-treatment should generate energy benefits of at least 2.7 times and as high as 18 times the FP energy input, depending on heat recovery from FP treatment. For a plant treating 76,000 m3/d of wastewater (380 m3-sludge/d), FP treatment should generate an annual economic benefit of approximately

Reduction of Np(V) and precipitation of Np(IV) by an anaerobic microbial consortium

540,000 net of electricity and other operating and maintenance costs. This represents a payback period of three years or less.

Mathematical Modelling of the Effects of Aerobic and Anaerobic Chelate Biodegradation on Actinide Speciation

A combination of experimental, analytical, and modeling investigations shows that an anaerobic, sulfate-reducing consortium reduced Np(V) to Np(IV), with subsequent precipitation of a Np(IV) solid. Precipitation of Np(IV) during growth on pyruvate occurred before sulfate reduction began. H2 stimulated precipitation of Np(IV) when added alone to growing cells, but it slowed precipitation when added along with pyruvate. Increasing concentrations of pyruvate also retarded precipitation. Accumulation of an intermediate pyruvate-fermentation product – probably succinate – played a key role in retarding Np(IV) precipitation by complexing the Np(IV). Hydrogen appears to have two roles in controlling Np precipitation: donating electrons for Np(V) reduction and modulating intermediate levels. That Np(V) is microbially reduced and subsequently precipitated under anaerobic conditions has likely beneficial implications for the containment of Np on lands contaminated by radionuclides, but complexation by fermentation intermediates can prevent immobilization by precipitation.

Subsurface interactions of actinide species and microorganisms : implications for the bioremediation of actinide-organic mixtures.

Biodegradation of natural and anthropogenic chelating agents directly and indirectly affects the speciation, and hence, the mobility of actinides in subsurface environments. We combined mathematical modelling with laboratory experimentation to investigate the effects of aerobic and anaerobic chelate biodegradation on actinide [Np(IV/V), Pu(IV)] speciation. Under aerobic conditions, nitrilotriacetic acid (NTA) biodegradation rates were strongly influenced by the actinide concentration. Actinide-chelate complexation reduced the relative abundance of available growth substrate in solution and actinide species present or released during chelate degradation were toxic to the organisms. Aerobic bioutilization of the chelates as electron-donor substrates directly affected actinide speciation by releasing the radionuclides from complexed form into solution, where their fate was controlled by inorganic ligands in the system. Actinide speciation was also indirectly affected by pH changes caused by organic biodegradation. The two concurrent processes of organic biodegradation and actinide aqueous chemistry were accurately linked and described using CCB ATCH, a computer model developed at Northwestern University to investigate the dynamics of coupled biological and chemical reactions in mixed waste subsurface environments. CCBATCH was then used to simulate the fate of Np during anaerobic citrate biodegradation. The modelling studies suggested that, under some conditions, chelate degradation can increase Np(IV) solubility due to carbonate complexation in closed aqueous systems.

Fate of neptunium in an anaerobic, methanogenic microcosm

By reviewing how microorganisms interact with actinides in subsurface environments, we assess how bioremediation controls the fate of actinides. Actinides often are co-contaminants with strong organic chelators, chlorinated solvents, and fuel hydrocarbons. Bioremediation can immobilize the actinides, biodegrade the co-contaminants, or both. Actinides at the IV oxidation state are the least soluble, and microorganisms accelerate precipitation by altering the actinides oxidation state or its speciation. We describe how microorganisms directly oxidize or reduce actinides and how microbiological reactions that biodegrade strong organic chelators, alter the pH, and consume or produce precipitating anions strongly affect actinide speciation and, therefore, mobility. We explain why inhibition caused by chemical or radiolytic toxicities uniquely affects microbial reactions. Due to the complex interactions of the microbiological and chemical phenomena, mathematical modeling is an essential tool for research on and application of bioremediation involving co-contamination with actinides. We describe the development of mathematical models that link microbiological and geochemical reactions. Throughout, we identify the key research needs.

Radiotoxicity of neptunium(V) and neptunium(V)-nitrilotriacetic acid (NTA) complexes towards Chelatobacter heintzii

Neptunium is found predominantly as Np(IV) in reducing environments, but Np(V) in aerobic environments. However, currently it is not known how the interplay between biotic and abiotic processes affects Np redox speciation in the environment. In order to evaluate the effect of anaerobic microbial activity on the fate of Np in natural systems, Np(V) was added to a microcosminoculated with anaerobic sediments from a metal-contaminated fresh water lake. The consortium included metal-reducing, sulfate-reducing, and methanogenic microorganisms, and acetate was supplied as the only exogenous substrate. Addition of more than 10{sup {minus}5} M Np did not inhibit methane production. Total Np volubility in the active microcosm, as well as in sterilized control samples, decreased by nearly two orders of magnitude. A combination of analytical techniques, including VIS-NIR absorption spectroscopy and XANES, identified Np(IV) as the oxidation state associated with the sediments. The similar results from the active microcosm and the abiotic controls suggest that microbian y produced Mn(II/HI) and Fe(II) may serve as electron donors for Np reduction.

Focused-Pulsed sludge pre-treatment increases the bacterial diversity and relative abundance of acetoclastic methanogens in a full-scale anaerobic digester

The objective of this work was to investigate the toxicity mechanisms of neptunium and the neptunium-NTA complex towards Chelatobacter heintzii. The results show that metal toxicity of aquo NpO{sub 2}{sup +} may significantly limit growth of Cl heintzii at free metal ion concentrations greater than {approx} 10{sup {minus}5} M. However, neptunium concentrations {ge} 10{sup {minus}4} M do not cause measurable radiotoxicity effects in C. heintzii when present in the form of a neptunium-NTA complex or colloidal/precipitated neptunium-phosphate. The neptunium-NTA complex, which is stable under aerobic conditions, is destabilized by microbial degradation of NTA. When phosphate was present, degradation of NTA led to the precipitation of a neptunium-phosphate phase.