Hans W. Papenguth

Uranium association with halophilic and non-halophilic bacteria and archaea

Summary We determined the association of uranium with bacteria isolated from the Waste Isolation Pilot Plant (WIPP), Carlsbad, New Mexico, and compared this with known strains of halophilic and non-halophilic bacteria and archaea. Examination of the cultures by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) showed uranium accumulation extracellularly and/or intracellularly to a varying degree. In Pseudomonas fluorescens and Bacillus subtilis uranium was associated with the cell surface and in the latter it was present as irregularly shaped grains. In Halobacterium halobium, the only archeon studied here, uranium was present as dense deposits and with Haloanaerobium praevalens as spikey deposits. Halomonas sp. isolated from the WIPP site accumulated uranium both extracellularly on the cell surface and intracellularly as electron-dense discrete granules. Extended X-ray absorption fine structure (EXAFS) analysis of uranium with the halophilic and non-halophilic bacteria and archaea showed that the uranium present in whole cells was bonded to an average of 2.4±0.7 phosphoryl groups at a distance of 3.65±0.03 Å. Comparison of whole cells of Halomonas sp. with the cell wall fragments of lysed cells showed the presence of a uranium bidentate complex at 2.91±0.03 Å with the carboxylate group on the cell wall, and uranyl hydroxide with U-U interaction at 3.71±0.03 Å due to adsorption or precipitation reactions; no U-P interaction was observed. Addition of uranium to the cell lysate of Halomonas sp. resulted in the precipitation of uranium due to the inorganic phosphate produced by the cells. These results show that the phosphates released from bacteria bind a significant amount of uranium. However, the bacterially immobilized uranium was readily solubilized by bicarbonate with concurrent release of phosphate into solution.

Surface complexation clues to dolomite growth

Calcium and magnesium adsorb in near-stoichiometric proportions to dolomite over wide ranges in [Ca{sup 2+}]/[Mg{sup 2+}], ionic strength, and solution composition pointing to minimal mixing of metal cations between the CaCO{sub 3} and MgCO{sub 3} layer edges exposed at the dolomite surface. Near-neutral pH Mg and Ca adsorb as hydrated ions, or, in sulfate-rich solutions, as metal sulfate complexes. Near-stoichiometric adsorption of Ca and Mg points to dehydration and subsequent carbonation of adsorbed Mg as the likely rate-limiting step for dolomite growth at near-Earth surface conditions. We propose that one path for dolomite growth from low-temperature natural waters is through the initial adsorption of Mg-sulfate complexes onto either (1) growing dolomite crystals or (2) rate-limiting dolomite nucleii. Field relations, as well as homogeneous synthesis at low temperatures (25{degrees}C < T < 100{degrees}C) support this hypothesis and provide a mechanistic explanation for dolomite growth from sulfate-rich natural waters. 36 refs.

Metal sorption to dolomite surfaces

Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, P{sub CO{sub 2}}, and pH at 25 C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, P{sub CO{sub 2}}, and pH at 25 C. Metal adsorption is typically favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na{sup +} with the dolomite surface, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca{sup 2+} in solution will decrease Kds. At the same time, to the extent that high P{sub CO{sub 2}} increase Ca{sup 2+} levels, JK{sub d}s will decrease with CO{sub 2} levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Am-carbonate complexes appear to sorb; Pu-complexes might sorb as well; U-carbonate complexation leads to desorption). This indirect CO{sub 2} effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to affect to actinide K{sub d}s.

Kinetic modeling of microbially-driven redox chemistry of radionuclides in subsurface environments: Coupling transport, microbial metabolism and geochemistry

Microbial reactions play an important role in regulating pore water chemistry as well as secondary mineral distribution in many subsurface systems and, therefore, may directly impact radionuclide migration in those systems. This paper presents a general modeling approach to couple microbial metabolism, redox chemistry, and radionuclide transport in a subsurface environment. To account for the likely achievement of quasi-steady state biomass accumulations in subsurface environments, a modification to the traditional microbial growth kinetic equation is proposed. The conditions for using biogeochemical models with or without an explicit representation of biomass growth are clarified. Based on the general approach proposed in this paper, the couplings of uranium reactions with biogeochemical processes are incorporated into computer code BIORXNTRN Version 2.0. The code is then used to simulate a subsurface contaminant migration scenario, in which a water flow containing both uranium and a complexing organic ligand is recharged into an oxic carbonate aquifer. The model simulation shows that Mn and Fe oxyhydroxides may vary significantly along a flow path. The simulation also shows that uranium(VI) can be reduced and therefore immobilized in the anoxic zone created by microbial degradation.

Toxicity of Actinides to Bacterial Strains Isolated from the Waste Isolation Pilot Plant (WIPP) Environment

The possibility of toxic effects from several actinide elements to bacteria isolated from the Waste Isolation Pilot Plant (WIPP) site has been investigated. This study is part of an extensive ongoing research program that endeavors to validate the suitability and safety of the WIPP site as a transuranic (TRU) radioactive waste repository. The motivation for the toxicity studies was to determine the eventual fate of the actinides after their contact with microorganisms relevant to the WIPP site. The toxicity studies investigated possible adverse effects to the growth of the bacteria due to actinide toxicity. Actinide interactions with the bacteria will impact actinide transport or retardation by the bacterial species.

Contribution of Mineral-Fragment Type Pseudo-Colloids to the Mobile Actinide Source Term of the Waste Isolation Pilot Plant (WIPP)

The Waste Isolation Pilot Plant (WIPP) is being developed in southeastern New Mexico by the U.S. Department of Energy (DOE) as a repository for transuranic waste produced by defense programs. Regulations promulgated by the U.S. Environmental Protection Agency (EPA) place limits on the cumulative release of radionuclides to the accessible environment over 10,000 years and require performance assessments to demonstrate WIPP compliance with these regulatory standards.

The interaction of plutonium with bacteria in the repository environment

Microorganisms in the nuclear waste repository environment may interact with plutonium through (i) sorption, (ii) intracellular accumulation, and (iii) transformation of chemical speciation. These interactions may retard or enhance the mobility of Pu by precipitation reactions, biocolloid formation, or production of more soluble species. Current and planned radioactive waste repository environments, such as deep subsurface halite and granite formations, are considered extreme relative to life processes in the near-surface terrestrial environment. There is a paucity of information on the biotransformation of radionuclides by microorganisms present in such extreme environments. In order to gain a better understanding of the interaction of plutonium with microorganisms present in the waste repository sites we investigated a pure culture (Halomonas sp.) and a mixed culture of bacteria (Haloarcula sinaiiensis, Marinobacter hydrocarbonoclasticus, Altermonas sp., and a g-proteobacterium) isolated from the Waste ...

Retardation of Colloidal Actinides Through Filtration in Intrusion Borehole Backfill at the Waste Isolation Pilot Plant (WIPP)

A depth filtration model was used to evaluate filtration of four types of colloids (mineral fragments, humics, microbes, and mature actinide intrinsic colloids) and their agglomerates by borehole backfill material in the event of inadvertent human intrusion to the Waste Isolation Pilot Plant (WIPP). The WIPP is a proposed repository sited in bedded salt for transuranic wastes generated under our nation’s defense programs. Under a human intrusion scenario involving two or more boreholes, flow from an underlying brine reservoir could potentially result in the migration of colloids from the repository up a borehole and then outward to the overlying Culebra Dolomite aquifer. However, in the performance assessment of the WIPP it is assumed that any intrusive borehole would be backfilled immediately after the infringement. The borehole filler material is assumed to have the hydraulic properties of either degraded concrete or grout, or silty sand with a maximum permeability of 10−11 m2 (worse case scenario). Mechanisms of filtration were modeled by trajectory analysis with borehole particles regarded as collectors. The dominant filtration mechanisms were diffusion and interception. The collision efficiency (α) of the colloid particles to the collector grains presented the greatest uncertainty for the filtration modeling. Extensive review of the literature indicated that the colloidal particles displaying the lowest collision efficiencies are microbes and particles stabilized by humic substances (0.1 to 0.01 and approximately 0.001, respectively). These collision efficiency values are based upon low ionic strength water; collision efficiencies have been shown to increase, by orders of magnitude in some cases, upon increasing ionic strength. Conservative α values ranging from 10−2 to 10−3 were used in our model calculations. The model predicts that most of the entrained colloids and colloid agglomerates will be filtered out by the borehole filling within a few meters (or fractions of meters) of brine flow distance. Particles displaying the least efficient filtration (e.g., particles between 1 and 5 μm) would have their concentrations reduced by about an order of magnitude over the travel distance (395 m) between the repository and borehole interface with the Culebra.