Abdesselam Abdelouas

Reduction of U(VI) to U(IV) by indigenous bacteria in contaminated ground water

Abstract We report on bio-catalyzed reduction and immobilization of U(VI) species (0.25 mg/l to 235 mg/l) in ground water in the presence of high concentrations of nitrate, sulfate and carbonate. We studied ground water from the uranium mill tailings site near Tuba City, Arizona (USA). Experiments with the ground water were conducted in the presence of the Navajo sandstone host rock. Uranium in solution is complexed by carbonate. Two indigenous denitrifying bacteria were identified Pseudomonas aeruginosa and P. stutzeri , and one sulfate reducing bacterium, Shewanella putrefaciens , also known as Fe(III)-reducer. S. putrefaciens can use U(VI) as an electron acceptor, instead of Fe(III). Ethanol was used as the organic carbon source. Microbially mediated reactions are sequential in the order of decreasing redox intensity. Metabolic reduction of nitrate to gaseous species (N 2 , N 2 O) was complete within 1 week at 16°C. The sulfate concentration remained constant. Some of the U(VI) coprecipitated with aragonite/calcite or was adsorbed on biomass during denitrification. Subsequently, the enzymatically catalyzed reduction of U(VI) to U(IV) was complete within 3 weeks but was accompanied by reduction of sulfate to sulfide. U(IV) precipitated as a uraninite solid solution (U, Ca)O 2 , adhering to the bacteria. The final concentration in solution was ≤1 μg/l. U(VI) was not reduced by sulfide. Complexation of U(VI) by carbonate made its reduction by sulfide even slower than in pure water. The bio-catalyzed reaction is the faster process under the conditions given by the composition of the ground water.

Biological reduction of uranium in groundwater and subsurface soil

Biological reduction of uranium is one of the techniques currently studied for in situ remediation of groundwater and subsurface soil. We investigated U(VI) reduction in groundwaters and soils of different origin to verify the presence of bacteria capable of U(VI) reduction. The groundwaters originated from mill tailings sites with U concentrations as high as 50 mg/l, and from other sites where uranium is not a contaminant, but was added in the laboratory to reach concentrations up to 11 mg/l. All waters contained nitrate and sulfate. After oxygen and nitrate reduction, U(VI) was reduced by sulfate-reducing bacteria, whose growth was stimulated by ethanol and trimetaphosphate. Uranium precipitated as hydrated uraninite (UO2 x xH2O). In the course of reduction of U(VI), Mn(IV) and Fe(III) from the soil were reduced as well. During uraninite precipitation a comparatively large mass of iron sulfides formed and served as a redox buffer. If the excess of iron sulfide is large enough, uraninite will not be oxidized by oxygenated groundwater. We show that bacteria capable of reducing U(VI) to U(IV) are ubiquitous in nature. The uranium reducers are primarily sulfate reducers and are stimulated by adding nutrients to the groundwater.

Chemical reactions of uranium in ground water at a mill tailings site

We studied soil and ground water samples from the tailings disposal site near Tuba City, AZ, located on Navajo sandstone, in terms of uranium adsorption and precipitation. The uranium concentration is up to 1 mg/l, 20 times the maximum concentration for ground water protection in the United States. The concentration of bicarbonate (HCO3−) in the ground water increased from ≤7×10−4 M, the background concentration, to 7×10−3 M. Negatively charged uranium carbonate complexes prevail at high carbonate concentrations and uranium is not adsorbed on the negatively charged mineral surfaces. Leaching experiments using contaminated and uncontaminated sandstone and 1 N HCl show that adsorption of uranium from the ground water is negligible. Batch adsorption experiments with the sandstone and ground water at 16°C, the in situ ground water temperature, show that uranium is not adsorbed, in agreement with the results of the leaching experiments. Adsorption of uranium at 16°C is observed when the contaminated ground water is diluted with carbonate-free water. The observed increase in pH from 6.7 to 7.3 after dilution is too small to affect adsorption of uranium on the sandstone. Storage of undiluted ground water to 24°C, the temperature in the laboratory, causes coprecipitation of uranium with aragonite and calcite. Our study provides knowledge of the on-site uranium chemistry that can be used to select the optimum ground water remediation strategy. We discuss our results in terms of ground water remediation strategies such as pump and treat, in situ bioremediation, steam injection, and natural flushing.

Oxidative dissolution of uraninite precipitated on Navajo sandstone

Abstract Column and batch experiments were conducted with sandstone and ground water samples to investigate oxidation of uraninite precipitated by microbially mediated reduction of U(VI), a contaminant in ground water beneath a uranium mill tailings site near Tuba City, AZ, USA. Uraninite precipitated together with mackinawite (FeS 0.9 ) because Fe(III) from the sandstone and sulfate, another contaminant in the water were reduced together with U(VI). After completion of U(VI) reduction, experiments were conducted to find out whether uraninite is protected by mackinawite against reoxidation. Uncontaminated ground water from the same site, containing 7 mg/l of dissolved oxygen, was passed through the columns or mixed with sandstone in batch experiments. The results showed that small masses of uraninite, 0.1 μg/g of sandstone, are protected by mackinawite from reoxidation. Uraninite masses on the order of 0.1 μg/g correspond to U(VI) concentrations of 0.5 mg/l, typically encountered in uranium contaminated ground waters. Mackinawite is an effective buffer and is formed in sufficient quantity to provide long-term protection of uraninite. Uranium concentrations in ground water passed through the columns are too low (4 μg/l) to distinguish between dissolution and oxidative dissolution of uraninite. However, batch experiments showed that uraninite oxidation takes place.

Using Cytochrome c{sub 3} to Make Selenium Nanowires

We report on a new method to make nanostructures, in this case selenium nanowires, in aqueous solution at room temperature. We used the protein cytochrome c{sub 3} to reduce selenate (SeO{sub 4}{sup 2{minus}}) to selenium (Se{sup 0}). Cytochrome c{sub 3} is known for its ability to catalyze reduction of metals including U{sup VI} {yields} U{sup IV}, Cr{sup VI} {yields} Cr{sup III}, Mo{sup VI} {yields} Mo{sup IV}, Cu{sup II} {yields} Cu{sup 0}, Pb{sup II} {yields} Pb{sup 0}, Hg{sup II} {yields} Hg{sup 0}. Nanoparticles of Se{sup 0} precipitated from an aqueous solution at room temperature, followed by spontaneous self-assembling into nanowires. Cytochrome c{sub 3} was extracted from the sulfate-reducing bacteria Desulfovibrio vulgaris (strain Holdenborough) and isolated by the procedure of DerVartanian and Legall.

Surface layers on a borosilicate nuclear waste glass corroded in MgCl2 solution

Surface layers on the French borosilicate nuclear waste glass, R7T7, corroded in MgCl2 solution were studied to determine the composition, structure and stability of crystalline phases. The characteristics of the phases constituting the surface layer varied with the parameter SV × t, the glass surface area (S) to solution volume (V) ratio, times time (t). At low SV × t values ( 98% of the neodymium released from the glass were precipitated in the surface layer. In the 463 day experiment, 86% of the neodymium in the surface layer was in solid solution with powellite, the rest with saponite. Uranium was contained exclusively in saponite. High SV ratios, typical of disposal conditions for vitrified high-level radioactive waste, favor retention of actinides in fairly insoluble corrosion products. Observation of similar corrosion products on natural glasses as on nuclear waste glasses lend support to the hypothesis that the host phases for actinides observed in the laboratory are stable over geological periods of time.

FORMATION OF HYDROTALCITE-LIKE COMPOUNDS DURING R7T7 NUCLEAR WASTE GLASS AND BASALTIC GLASS ALTERATION

Alteration experiments have been performed using RTT7 and synthetic basaltic glasses in MgCl2−CaCl2 salt solution at 190°C. The duration of experiments ranged from 0.25 to 463 days. The alteration products were studied by Scanning Electron Microscope (SEM), Scanning Transmission Electron Microscope (STEM), X-ray diffraction (XRD) and Electron Spectrometry for Chemical Analysis (ESCA). For both glasses, the early alteration product is a hydrotalcite-like compound [Mg6Al2CO3(OH)16·4H2O] in which HPO42−, SO42− and Cl− substitutes for CO32−. The measured basal spacing is 7.68 Å for the hydrotalcite formed from R7T7 glass and 7.62 Å for the hydrotalcite formed from basaltic glass which reflect the high Al/Al + Mg ratios x (0.34 ≤ x ≤ 0.46). The chemical microanalyses show that the hydrotalcite is subsequently covered by a silica-rich gel which evolves into saponite after a few months. These results support the use of basaltic glasses alteration patterns in Mg-rich solution, to understand the long-term behavior of R7T7 nuclear waste glass.

Chloride and Organic Chlorine in Forest Soils: Storage, Residence Times, And Influence of Ecological Conditions

Recent studies have shown that extensive chlorination of natural organic matter significantly affects chlorine (Cl) residence time in soils. This natural biogeochemical process must be considered when developing the conceptual models used as the basis for safety assessments regarding the potential health impacts of 36-chlorine released from present and planned radioactive waste disposal facilities. In this study, we surveyed 51 French forested areas to determine the variability in chlorine speciation and storage in soils. Concentrations of total chlorine (Cl(tot)) and organic chlorine (Cl(org)) were determined in litterfall, forest floor and mineral soil samples. Cl(org) constituted 11-100% of Cl(tot), with the highest concentrations being found in the humus layer (34-689 mg Cl(org) kg(-1)). In terms of areal storage (53 - 400 kg Cl(org) ha(-1)) the mineral soil dominated due to its greater thickness (40 cm). Cl(org) concentrations and estimated retention of organochlorine in the humus layer were correlated with Cl input, total Cl concentration, organic carbon content, soil pH and the dominant tree species. Cl(org) concentration in mineral soil was not significantly influenced by the studied environmental factors, however increasing Cl:C ratios with depth could indicate selective preservation of chlorinated organic molecules. Litterfall contributions of Cl were significant but generally minor compared to other fluxes and stocks. Assuming steady-state conditions, known annual wet deposition and measured inventories in soil, the theoretical average residence time calculated for total chlorine (inorganic (Cl(in)) and organic) was 5-fold higher than that estimated for Cl(in) alone. Consideration of the Cl(org) pool is therefore clearly important in studies of overall Cl cycling in terrestrial ecosystems.

The effect of high power ultrasound on an aqueous suspension of graphite

Ultrasound treatment was used to study the decrease of the granulometry of graphite, due to the cavitation, which allows the erosion by separating grains. At a smaller scale, cavitation bubble implosion tears apart graphite sheets as shown by HRTEM, while HO(*) and H(*) radicals produced from water sonolysis, generate oxidative and reductive reactions on these sheet fragments. Such reactions form smaller species, e.g. dissolved organic matter. The methodology proposed is very sensitive to unambiguously identifying the in situ composition of organic compounds in water. The use of the atmospheric pressure chemical ionization (APCI) Fourier transform mass spectrometry (FTMS) technique minimizes the perturbation of the organic composition and does not require chemical treatment for analysis. The structural features observed in the narrow range (m/z<300) were mainly aromatic compounds (phenol, benzene, toluene, xylene, benzenediazonium, etc.), C(4)-C(6) alkenes and C(2)-C(10) carboxylic acids. Synthesis of small compounds from graphite sonication has never been reported and will probably be helpful to understand the mechanisms involved in high energy radical reactions.

Precipitation of technetium by subsurface sulfate-reducing bacteria

Summary To study the interaction between Tc and subsurface bacteria, we conducted batch experiments with soil and groundwater or sterilized deionized water. The system water/soil was amended with lactate and phosphate for bacterial growth. Nitrate and sulfate were added to stimulate the growth of indigenous denitrifying and sulfate-reducing bacteria. During denitrification Tc-concentration did not change with time. In the presence of sulfate-reducing bacteria, Tc-concentrations decreased in reacted waters which could be attributed to Tc(VII) reduction and precipitation of TcO2 and/or TcS2. Coprecipitation with newly formed iron sulfide is expected to contribute to Tc removal. Additional experiments with U and Tc showed that these elements were simultaneously reduced by sulfate-reducing bacteria. This work shows that 1) subsurface mixed cultures of denitrifying bacteria do not remove Tc from solution, this is different from uranium and 2) sulfate-reducing bacteria reduce and remove Tc from aqueous solutions and thus in situ bioremediation of subsurface waters and soils may be possible with such ubiquitous bacteria.