Mary P. Neu

Enhanced Exopolymer Production and Chromium Stabilization in Pseudomonas putida Unsaturated Biofilms

ABSTRACT Chromium-contaminated soils threaten surface and groundwater quality at many industrial sites. In vadose zones, indigenous bacteria can reduce Cr(VI) to Cr(III), but the subsequent fate of Cr(III) and the roles of bacterial biofilms are relatively unknown. To investigate, we cultured Pseudomonas putida, a model organism for vadose zone bioremediation, as unsaturated biofilms on membranes overlaying iron-deficient solid media either containing molecular dichromate from potassium dichromate (Cr-only treatment) or with deposits of solid, dichromate-coated hematite (Fe+Cr treatment) to simulate vadose zone conditions. Controls included iron-deficient solid medium and an Fe-only treatment using solid hematite deposits. Under iron-deficient conditions, chromium exposure resulted in lower cell yield and lower amounts of cellular protein and carbohydrate, but providing iron in the form of hematite overcame these toxic effects of Cr. For the Cr and Fe+Cr treatments, Cr(VI) was completely reduced to Cr(III) that accumulated on biofilm cells and extracellular polymeric substances (EPSs). Chromium exposure resulted in elevated extracellular carbohydrates, protein, DNA, and EPS sugars that were relatively enriched in N-acetyl-glucosamine, rhamnose, glucose, and mannose. The proportions of EPS protein and carbohydrate relative to intracellular pools suggested Cr toxicity-mediated cell lysis as the origin. However, DNA accumulated extracellularly in amounts far greater than expected from cell lysis, and Cr was liberated when extracted EPS was treated with DNase. These results demonstrate that Cr accumulation in unsaturated biofilms occurs with enzymatic reduction of Cr(VI), cellular lysis, cellular association, and extracellular DNA binding of Cr(III), which altogether can facilitate localized biotic stabilization of Cr in contaminated vadose zones.

Plutonium(V/VI) Reduction by the Metal-Reducing Bacteria Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1.

ABSTRACT We examined the ability of the metal-reducing bacteria Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1 to reduce Pu(VI) and Pu(V). Cell suspensions of both bacteria reduced oxidized Pu [a mixture of Pu(VI) and Pu(V)] to Pu(IV). The rate of plutonium reduction was similar to the rate of U(VI) reduction obtained under similar conditions for each bacteria. The rates of Pu(VI) and U(VI) reduction by cell suspensions of S. oneidensis were slightly higher than the rates observed with G. metallireducens. The reduced form of Pu was characterized as aggregates of nanoparticulates of Pu(IV). Transmission electron microscopy images of the solids obtained from the cultures after the reduction of Pu(VI) and Pu(V) by S. oneidensis show that the Pu precipitates have a crystalline structure. The nanoparticulates of Pu(IV) were precipitated on the surface of or within the cell walls of the bacteria. The production of Pu(III) was not observed, which indicates that Pu(IV) was the stable form of reduced Pu under these experimental conditions. Experiments examining the ability of these bacteria to use Pu(VI) as a terminal electron acceptor for growth were inconclusive. A slight increase in cell density was observed for both G. metallireducens and S. oneidensis when Pu(VI) was provided as the sole electron acceptor; however, Pu(VI) concentrations decreased similarly in both the experimental and control cultures.

Uncovering f-element bonding differences and electronic structure in a series of 1 : 3 and 1 : 4 complexes with a diselenophosphinate ligand

Understanding the bonding trends within, and the differences between, the 4f and 5f element series with soft donor atom ligands will aid elucidation of the fundamental origins of actinide (An) versus lanthanide (Ln) selectivity that is integral to many advanced nuclear fuel cycle separation concepts. One of the principal obstacles to acquiring such knowledge is the dearth of well characterized transuranic molecules that prevents the necessary comparison of 4f versus 5f coordination chemistry, electronic structure, and bonding. Reported herein is new chemistry of selenium analogues of dithiophosphinate actinide extractants. LnIII and AnIII/IV complexes with the diselenophosphinate [Se2PPh2]− anion have been synthesized, structurally and spectroscopically characterized, and quantum chemical calculations performed on model compounds in which the phenyl rings have been replaced by methyl groups. The complexes [LnIII(Se2PPh2)3(THF)2] (Ln = La (1), Ce (2), Nd (3)), [LaIII(Se2PPh2)3(MeCN)2] (4), [PuIII(Se2PPh2)3(THF)2] (5), [Et4N][MIII(Se2PPh2)4] (M = Ce (6), Pu (7)), and [AnIV(Se2PPh2)4] (An = U (8), Np (9)), represent the first f-element diselenophosphinates. In conjunction with the calculated models, complexes 1–9 were utilized to examine two important factors: firstly, bonding trends/differences between trivalent 4f and 5f cations of near identical ionic radii; secondly, bonding trend differences across the 5f series within the AnIV oxidation state. Analysis of both experimental and computational data supports the conclusion of enhanced covalent bonding contributions in PuIII–Se versus CeIII–Se bonding, while differences between UIV–Se and NpIV–Se bonding is satisfactorily accounted for by changes in the strength of ionic interactions as a result of the increased positive charge density on NpIV compared to UIV ions. These findings improve understanding of soft donor ligand binding to the f-elements, and are of relevance to the design and manipulation of f-element extraction processes.

Plutonium speciation affected by environmental bacteria

Summary Plutonium has no known biological utility, yet it has the potential to interact with bacterial cellular and extracellular structures that contain metal-binding groups, to interfere with the uptake and utilization of essential elements, and to alter cell metabolism. These interactions can transform plutonium from its most common forms, solid, mineral-adsorbed, or colloidal Pu(IV), to a variety of biogeochemical species that have much different physico-chemical properties. Organic acids that are extruded products of cell metabolism can solubilize plutonium and then enhance its environmental mobility, or in some cases facilitate plutonium transfer into cells. Phosphate- and carboxylate-rich polymers associated with cell walls can bind plutonium to form mobile biocolloids or Pu-laden biofilm/mineral solids. Bacterial membranes, proteins or redox agents can produce strongly reducing electrochemical zones and generate molecular Pu(III/IV) species or oxide particles. Alternatively, they can oxidize plutonium to form soluble Pu(V) or Pu(VI) complexes. This paper reviews research on plutonium-bacteria interactions and closely related studies on the biotransformation of uranium and other metals.

OXIDATION STATE DETERMINATION OF PLUTONIUM AQUO IONS USING X-RAY ABSORPTION SPECTROSCOPY

Abstract Four oxidation states (III, IV, V, and VI) of Pu may coexist under environmentally relevant conditions. An efficient method to determine the states actually present in various matrices would enhance the ability to model the fate and transport of plutonium in process streams and in the environment. This communication establishes that the L3 X-ray absorption near-edge structure (XANES) spectra of Pu are primarily determined by the valence state and the presence or absence of the trans dioxo moiety, consistent with previous U and Np XANES studies. The edge energies were observed to shift progressively to higher energy with increasing valence, with an average 1.68 eV increase per formal oxidation state increase. In addition, the general spectral shape of the (III) and (IV) species is clearly different from the dioxo-containing (V) and (VI) species, with the first maximum much larger and sharper for the (III) and (IV) spectra than for the (V) and (VI) spectra.

Neptunium(V) hydrolysis and carbonate complexation: Experimental and predicted neptunyl solubility in concentrated NaCl using the Pitzer approach

Abstract The solid-liquid equilibrium of Np (V) has been studied in NaCl at 25°C and 0.01 atm CO 2 . The equilibrium solids were characterized using X-ray powder diffraction, and the Np (V) solution species were characterized using NIR absorption spectroscopy. The solid phases NaNPO 2 CO 3 · n H 2 O at [CO 3 ] −3 M and Na 3 NPO 2 (CO 3 ) 2 · n H 2 O at [CO 3 ] > 10 −3 M were found as solubility-limiting solid equilibrium phases. Pure Np (V) carbonato complexes were formed in solution; hydroxo, bicarbonato, or mixed hydroxocarbonato Np (V) complexes were determined not to be significant. The comparison of Np (V) solubility data in NaCl and NaClO 4 solutions indicated a stabilization of Np (V) in solution due to the interaction with chloride ions. Absorption spectra of NpO 2 + and NpO 2 CO 3 in 5 M NaCl were shifted slightly towards higher wavelengths, also suggesting an interaction with chloride ions. The Pitzer approach was applied to parameterize experimental data and to predict Np (V) solubility in brine solutions. The activity of water changes with electrolyte concentration; thus incorporation of the reported molecules of hydration water in NaNPO 2 CO 3 · n H 2 O ( n = 3.5) to determine the chemical potential of this solid improved the agreement between predicted solubility and experimental data. NaNPO 2 CO 3 · n H 2 O and NaAmO 2 CO 3 · n H 2 O showed the same solubility in 3 and 5 M NaCl.

Structural and spectroscopic characterization of plutonyl(VI) nitrate under acidic conditions.

The plutonyl(VI) dinitrate complex [PuO(2)(NO(3))(2)(H(2)O)(2)]·H(2)O (1) has been structurally characterized by single-crystal X-ray diffraction and spectroscopically characterized by solid-state vis-NIR and Raman spectroscopies. Aqueous solution spectroscopic studies indicate only weak plutonyl(VI) nitrate complexation, with the mononitrate complex dominating and negligible dinitrate formation, even in concentrated nitric acid.

Low-valent molecular plutonium halide complexes.

Treatment of plutonium metal with 1.5 equiv of bromine in tetrahydrofuran (thf) led to isolation of PuBr3(thf)4 (1), which is a new versatile synthon for exploration of non-aqueous Pu(III) chemistry. Adventitious water in the system resulted in structural characterization of the eight-coordinate complex [PuBr2(H2O)6][Br] (2). The crystal structure of PuI3(thf)4 (3) has been determined for the first time and is isostructural with UI3(thf)4. Attempts to form a bis(imido) plutonyl(VI) moiety ([Pu(NR)2](2+)) by oxidation of PuI3(py)4 with iodine and (t)BuNH2 resulted in crystallization of the Pu(III) complex [PuI2(thf)4(py)][I3] (4). Dissolution of a Pu(IV) carbonate with a HCl/Et2O solution in thf gave the mixed valent (III/IV) complex salt [PuCl2(thf)5][PuCl5(thf)] (5) as the only tractable product. Oxidation of Pu[N(SiMe3)2]3 with TeCl4 afforded the Pu(IV) complex Pu[N(SiMe3)2]3Cl (6), which may prove to be a useful entry route for investigation of organometallic/non-aqueous tetravalent plutonium chemistry.

SPECTROSCOPIC INVESTIGATION OF THE FORMATION OF PUO2CL+ AND PUO2CL2 IN NACL SOLUTIONS AND APPLICATION FOR NATURAL BRINE SOLUTIONS

The chloride complexation of the PuO22+ ion has been studied in acidic NaCl solutions with electrolyte concentrations as high as 5 mol kg−1 at 23°C by using conventional absorption spectrophotometry. Plutonyl and its complexes have ionic strength-dependent molar absorptivities that were determined in NaClO4, the first essential step in the quantitative analysis of chloride complexation. The distributions of species for the Pu complexes, PuO22+, PuO2Cl+, and PuO2Cl2o, formed under the conditions investigated, were determined by peak-fitting of optical absorption spectra. The apparent stability constants of the Pu(VI) chloro complexes were calculated at each NaCl concentration. Specific ion-interaction theory parameters were determined for the plutonyl chloro complexes and the electrolyte constituents, then compared with the literature data. The calculated values for log β° were determined to be 0.23 ± 0.03 and −1.7 ± 0.2 for the mono and bis chloro complexes, respectively. Spectra of Pu(VI) in brines representative of waters at the Waste Isolation Pilot Plant, the licensed nuclear waste repository in a salt formation at Carlsbad, NM, USA, were measured and modeled by using the thermodynamic data and ion interaction parameters were determined. In these brines, less than 10% of the total Pu(VI) concentration exists as the Pu(VI) aquo ion, whereas about 90% is present as Pu(VI) chloro complexes.

Reaction chemistry of plutonium with vanadium pentoxide in molten salts

Abstract Vanadium pentoxide, V2O5, has been proposed as an oxidant for the stabilization of reduced forms of plutonium within pyrochemical salt residues from plutonium pyrochemical processes at the Rocky Flats Environmental Technology Site because of its large reduction potential and its ability to react via normal solid-state reactions. However, when V2O5 was used to oxidize actual process residues results were highly variable. This paper discusses the reaction chemistry of PuCl3, Pu° and PuOCl with V2O5 under a variety of conditions, including in the presence and absence of a NaCl/KCl salt matrix. This work is the first systematic study of the solid-state oxidation of plutonium species by vanadium pentoxide. For PuCl3 a greater than or equal to 1:1 V2O5:PuCl3 ratio is needed for complete oxidation. Increasing the amount of V2O5 from 1 to 2 equivalents increases the contribution of side reactions, including reactions of V2O5 with the process equipment and/or salt matrix, and produces undesirable non-volatile ternary and quaternary vanadium compounds. The oxidation of PuOCl is similar to that of PuCl3. Greater than a 1:1 V2O5:PuOCl ratio is required to achieve adequate conversion of PuOCl to PuO2. Oxidation of Pu° by V2O5 is more complex than the PuCl3 or PuOCl oxidations. Complete oxidation of Pu° to Pu(IV) in the presence of the salt matrix does not occur with up to 6 equivalents of V2O5. However, conversion of Pu° to Pu(IV) occurs rapidly in the absence of the salt matrix.