In situ bioreduction of technetium and uranium in a nitrate-contaminated aquifer

The potential to stimulate an indigenous microbial community to reduce a mixture of U(VI) and Tc(VII) in the presence of high (120 mM) initial NO3- co-contamination was evaluated in a shallow unconfined aquifer using a series of single-well, push-pull tests. In the absence of added electron donor, NO3-, Tc(VII), and U(VI) reduction was not detectable. However, in the presence of added ethanol, glucose, or acetate to serve as electron donor, rapid NO3- utilization was observed. The accumulation of NO2-, the absence of detectable NH4+ accumulation, and the production of N2O during in situ acetylene-block experiments suggest that NO3- was being consumed via denitrification. Tc(VII) reduction occurred concurrently with NO3- reduction, but U(VI) reduction was not observed until two or more donor additions resulted in iron-reducing conditions, as detected by the production of Fe(II). Reoxidation/remobilization of U(IV) was also observed in tests conducted with high (approximately 120 mM) but not low (approximately 1 mM) initial NO3- concentrations and not during acetylene-block experiments conducted with high initial NO3-. These results suggest that NO3(-)-dependent microbial U(IV) oxidation may inhibit or reverse U(VI) reduction and decrease the stability of U(IV) in this environment. Changes in viable biomass, community composition, metabolic status, and respiratory state of organisms harvested from down-well microbial samplers deployed during these tests were consistent with the conclusions that electron donor additions resulted in microbial growth, the creation of anaerobic conditions, and an increase in activity of metal-reducing organisms (e.g., Geobacter). The results demonstrate that it is possible to stimulate the simultaneous bioreduction of U(VI) and Tc(VII) mixtures commonly found with NO3- co-contamination at radioactive waste sites.     

Dissolution of nonuniformly distributed immiscible liquid: intermediate-scale experiments and mathematical modeling

The purpose of this work is to examine the effect of nonuniform distributions of immiscible organic liquid on dissolution behavior, with a specific focus on the condition dependency of dissolution (i.e., mass transfer) rate coefficients associated with applying mathematical models of differing complexities to measured data. Dissolution experiments were conducted using intermediate-scale flow cells packed with sand in which well-characterized zones of residual trichloroethene (TCE) and 1,2-dichloroethane (DCA) saturation were emplaced. A dual-energy gamma radiation system was used for in-situ measurement of NAPL saturation. Aqueous concentrations of TCE and DCA measured in the flow-cell effluent were significantly less than solubility, due primarily to dilution associated with the nonuniform immiscible-liquid distribution and bypass flow effects associated with physical heterogeneity. A quantitative analysis of flow and transport was conducted using a three-dimensional mathematical model wherein immiscible-liquid distribution, permeability variability, and sampling effects were explicitly considered. Independent values for the initial dissolution rate coefficients were obtained from dissolution experiments conducted using homogeneously packed columns. The independent predictions obtained from the model provided good representations of NAPL dissolution behavior and of total TCE/DCA mass removed, signifying model robustness. This indicates that for the complex three-dimensional model, explicit consideration of the larger scale factors that influenced immiscible-liquid dissolution in the flow cells allowed the use of a dissolution rate coefficient that represents only local-scale mass transfer processes. Conversely, the use of simpler models that did not explicitly consider the nonuniform immiscible-liquid distribution required the use of dissolution rate coefficients that are approximately 3 orders of magnitude smaller than the values obtained from the column experiments. The rate coefficients associated with the simpler models represent composite or lumped coefficients that incorporate the effects of the larger scale dissolution processes associated with the nonuniform immiscible-liquid distribution, which are not explicitly represented in the simpler models, as well as local-scale mass transfer. These results demonstrate that local-scale dissolution rate coefficients, such as those obtained from column experiments, can be used in models to successfully predict dissolution and transport of immiscible-liquid constituents at larger scales when the larger scale factors influencing dissolution behavior are explicitly accounted for in the model.     

Evolution of technetium speciation in reducing grout

Cementitious waste forms (CWFs) are an important component of the strategy to stabilize nuclear waste resulting from plutonium production by the U. S. Department of Energy. Technetium (99Tc) is an abundant fission product of particular concern in CWFs because of the high solubility and mobility of Tc(VII), pertechnetate (TcO4-), the stable form of technetium in aerobic environments. CWFs can more effectively stabilize 99Tc if they contain additives that chemically reduce mobile TcO4- to immobile Tc(IV) species. The 99Tc leach rate of reducing CWFs that contain Tc(IV) is much lower than that for CWFs that contain TcO4-. Previous X-ray absorption fine structure studies showed that Tc(IV) species were oxidized to TcO4- in reducing grout samples prepared on a laboratory scale. Whether the oxidizer was atmospheric O2 or NO3- in the waste simulant was not determined. In actual CWFs, rapid oxidation of Tc(IV) by NO3- would be of concern, whereas oxidation by atmospheric O2 would be of less concern due to the slow diffusion and reaction of O2 with the reducing CWF. To address this uncertainty, two series of reducing grouts were prepared using TcO4- containing waste simulants with and without NO3-. In the first series of samples, referred to as "permeable samples", the TcO4- was completely reduced using Na2S, and the samples were sealed in cuvettes made of polystyrene, which has a relatively large O2 diffusion coefficient. In these samples, all of the technetium was initially present as a Tc(IV) sulfide compound, TcSx, which was characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS data is consistent with a structure consisting of triangular clusters of Tc(IV) centers linked together through a combination of disulfide and sulfide bridges as in MoS3. From the EXAFS model, the stoichiometry of TcSx is TC3S10, which is presumably the compound generally referred to as "Tc2S7". The TcSX initially present in the permeable samples was steadily oxidized over 4 years. In the second series of samples, called "impermeable samples", the TcO4- was not initially completely reduced, and the groutsamples were sealed in cuvettes made of poly-(methyl methacrylate), which has a small O2 diffusion coefficient. In the impermeable samples, the remaining TcO4- continued to be reduced, presumably by blast furnace slag in the grout, as the samples aged. When the impermeable samples were opened and exposed to atmosphere, the lower-valent technetium species were rapidly oxidized to TcO4-.     

Development of an immobilization system for in situ micronutrients release

An immobilization system constituted by coated microcapsules was developed aiming at immobilizing probiotic bacteria capable of producing folate and release it in a sustained manner into the intestine. Despite no probiotic folate-producers have been immobilized so far, the system has been developed with this goal and this work reports its stability and ability to release folate under gastro-intestinal conditions.Microcapsules were made of alginate with three consecutive coatings of poly-l-lysine, sodium alginate and chitosan. Turbidity experiments showed a strong electrostatic interaction between these polymers. Fourier transform infrared spectroscopy (FTIR) and confocal analysis showed the stability of the coating materials when applied on the microcapsules, even after they were immersed in solutions simulating conditions in the stomach and small intestine (i.e. pH 2, 60 min and pH 7.2, 120 min, respectively). Coated microcapsules have an average diameter size ranged from 20 and 40 μm, and swelled upon exposure to a neutral medium, without dissolution as showed by microscopy analyses. Release experiments proved the ability of the coated microcapsules to release folic acid, at different rates, depending on the applied coating. Release experiments showed that the first coating (Ɛ-PLL) is characterized by Fickian diffusion as the main release mechanism of folic acid. Fickian rate constant (k_F) decreased with the number of consequent coatings, reflecting the decrease of predominance of Fick's behavior. Results showed that the developed coated microcapsules have suitable characteristics for encapsulation of folic acid aiming in situ release in the intestine.     

Reoxidation behavior of technetium, iron, and sulfur in estuarine sediments

Technetium is a redox active radionuclide, which is present as a contaminant at a number of sites where nuclear fuel cycle operations have been carried out. Recent studies suggest that Tc(VII), which is soluble under oxic conditions, will be retained in sediments as Fe(III)-reducing conditions develop, due to reductive scavenging as hydrous TcO2. However, the behavior of technetium during subsequent reoxidation of sediments remains poorly characterized. Here, we describe a microcosm-based approach to investigate the reoxidation behavior of reduced, technetium-contaminated sediments. In reoxidation experiments, the behavior of Tc was strongly dependent on the nature of the oxidant. With air, reoxidation of Fe(II) and, in sulfate-reducing sediments, sulfide occurred accompanied by approximately 50% remobilization of Tc to solution as TcO4-. With nitrate, reoxidation of Fe(II) and, in sulfate-reducing sediments, sulfide only occurred in microbially active experiments where Fe(II) and sulfide oxidation coupled to nitrate reduction was occurring. Here, Tc was recalcitrant to remobilization with <10% Tc remobilized to solution even when extensive Fe(II) and sulfide reoxidation had occurred. X-ray absorption spectroscopy on reoxidized sediments suggested that 15-50% of Tc bound to sediments was present as Tc(VII). Overall, these results suggest that Tc reoxidation behavior is not directly coupled to Fe or S oxidation and that the extent of Tc remobilization is dependent on the nature of the oxidant.     

Multicomponent reactive transport in an in situ zero-valent iron cell

Data collected from a field study of in situ zero-valent iron treatment for TCE were analyzed in the context of coupled transport and reaction processes. The focus of this analysis was to understand the behavior of chemical components, including contaminants, in groundwater transported through the iron cell of a pilot-scale funnel and gate treatment system. A multicomponent reactive transport simulator was used to simultaneously model mobile and nonmobile components undergoing equilibrium and kinetic reactions including TCE degradation, parallel iron dissolution reactions, precipitation of secondary minerals, and complexation reactions. The resulting mechanistic model of coupled processes reproduced solution chemistry behavior observed in the iron cell with a minimum of calibration. These observations included the destruction of TCE and cis-1,2-DCE; increases in pH and hydrocarbons; and decreases in EH, alkalinity, dissolved O2 and CO2, and major ions (i.e., Ca, Mg, Cl, sulfate, nitrate). Mineral precipitation in the iron zone was critical to correctly predicting these behaviors. The dominant precipitation products were ferrous hydroxide, siderite, aragonite, brucite, and iron sulfide. In the first few centimeters of the reactive iron cell, these precipitation products are predicted to account for a 3% increase in mineral volume per year, which could have implications for the longevity of favorable barrier hydraulics and reactivity. The inclusion of transport was key to understanding the interplay between rates of transport and rates of reaction in the field.     

烤烟叶片身份和结构与化学成分的关系及其近红外模型研究

通过相关分析和建立模型,研究了烤烟叶片身份和结构指标与化学成分的关系及其近红外速测的可行性。结果表明:烟草叶片身份和结构指标与单叶重以及烟草化学成分之间存在极密切的相关关系,特别是叶片厚度和叶面密度与钾含量的相关性最高,呈较好的幂指关系。烟草叶片厚度、叶面密度以及叶片疏松度均存在能建立较好预测模型的可能性。利用全国主产区441个烟叶样品建立的叶面密度近红外速测模型的参数为:决定系数（R2）90.28%,均方差（RMSECV）0.529,说明可以用近红外对烤烟叶面密度进行较准确的速测。      

Effects of progressive anoxia on the solubility of technetium in sediments

Technetium is a significant radioactive contaminant from nuclear fuel cycle operations. It is highly mobile in its oxic form (as Tc(VII)O4-) but is scavenged to sediments in its reduced forms (predominantly Tc(IV)). Here we examine the behavior of Tc at low concentrations and as microbial anoxia develops in sediment microcosms. A cascade of stable-element terminal-electron-accepting processes developed in microcosms due to indigenous microbial activity. TcO4- removal from solution occurred during active microbial Fe(III) reduction, which generated Fe(II) in the sediments and was complete before sulfate reduction began. Microbial community analysis revealed a similar and complex microbial population at all three sample sites. At the intermediate salinity site, PauII, a broad range of NO3-, Mn(IV), Fe(III), and SO4(2-) reducers were present in sediments including microbes with the potential to reduce Fe(III) to Fe(II), although no differences in the microbial population were discerned as anoxia developed. When sterilized sediments were incubated with pure cultures of NO3(-)-, Fe(III)-, and sulfate-reducing bacteria, TcO4- removal occurred during active Fe(III) reduction. X-ray absorption spectroscopy confirmed that TcO4- removal was due to reduction to hydrous Tc(IV)O2 in Fe(III)- and sulfate-reducing estuarine sediments.     

Evaluation of models to describe ruminal degradation kinetics from in situ ruminal incubation of whole soybeans

Different mathematical models were evaluated as candidates to describe ruminal dry matter (DM) and crude protein (CP) degradation kinetics of raw and roasted whole soybeans from data obtained using the in situ polyester bag technique. Three models were used: segmented with up to 3 straight lines (model I), negative exponential (model II), and rational function or inverse polynomial (linear over linear; model III). A fourth, a generalized sigmoidal model, was also considered but the data did not exhibit sigmoidicity, so it was dropped from the analysis. Lagged and nonlagged versions of each model were fitted to the DM and CP disappearance curves of 6 different feeds (2 cultivars of raw or differently heat-processed whole soybean). The comparison between lagged and nonlagged versions of each model, based on statistical and behavior characteristics, showed for all models that the discrete lag parameter did not significantly improve the fit to ruminal DM and CP disappearance curves. The comparison between models (using nonlagged equations) showed that models I and II gave better goodness-of-fit than model III. Based on biological characteristics, models II and III underestimated the undegradable DM and CP fractions, but there was no significant difference between models for extent of degradation.     

Mechanistic aspects of pyrite oxidation in an oxidizing gaseous environment: an in situ HATR-IR isotope study

The reaction of FeS2 (pyrite) with gaseous H2O, O2, and H2O/O2 was investigated using horizontal attenuated total reflection Fourier transform infrared spectroscopy (HATR-FTIR). Spectra were interpreted with the aid of hybrid molecular orbital/density functional theory calculations of sulfate-iron hydroxide clusters. Reaction of pyrite in gaseous H2O led primarily to the formation of iron hydroxide on pyrite. Exposure of the pyrite to gaseous O2 after exposure to H2O vapor led to the formation of sulfur oxyanions that included SO42-. Isotopic labeling experiments showed that after this exposure sequence the oxygen in the sulfate product was primarily derived from the H2O reactant. If, however, pyrite was exposed to gaseous O2 prior to pure H2O vapor, both SO42- and iron oxyhydroxide became significant products. Isotopic rabeling experiments using the O2-then-H2O sequence showed that the oxygen in the SO42- product was derived from both H2O and O2. The results indicate that H2O and O2 exhibit a competitive adsorption on pyrite, with H2O blocking surface sites for O2 adsorption. The extent of oxygen incorporation from either the H2O or the O2 component into the surface-bound sulfur oxyanion product appears to be a strong function of the relative concentration ratio of the reactant H2O and O2.     

Empirical models to correlate the basic physical and chemical indices of modified rice bran and mustard oil

Modified rice bran oil (RBO) and modified mustard oil (MO) were prepared by adding oleic acid and oxidized oil separately. The physical and chemical indices like dielectric constant, viscosity, peroxide value (PV), acid value (AV), free fatty acids (FFA), iodine value (IV), and saponification value (SV) of pure and modified RO and MO were determined and correlated. Dielectric constant and viscosity were found to decrease in a non-linear fashion with increase in temperature (30°C–75°C). The dependence of dielectric constant and viscosity with temperature was investigated using model empirical equations. The dielectric variation was studied using Akerlof and Oshry’s model equations, which exhibited high dependency of R² ranges from 0.99 to 0.997. The behaviour of viscosity with temperature was studied with Wright’s equation and the correlation coefficient (R²) was found to be 0.998. Model equations were developed, which relates dielectric constant with IV, SV, and PV with the regression coefficients R² = 0.955, 0.936, and 0.994, respectively. The developed equations can be used in processing, pipelining, and to predict the parameters at a desired temperature. On comparing the correlations between the physical and chemical properties of the oil samples, RBO and its modified forms exhibited more oxidative stability than modified MO.     

Effect of organic co-contaminants on technetium and rhenium speciation and solubility under reducing conditions

The chemical and biogeochemical reduction of pertechnetate (TcO4-) and perrhenate (ReO4-) have been compared alongside complexation of the reduced species by three anthropogenic ligands relevant to nuclear waste (ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and isosaccharinic acid (ISA)). An HPLC size-exclusion column coupled to ICP-MS was used to separate the species and quantify Tc and Re. During method development, ReO4- showed recalcitrance to direct chemical reduction by Sn(ll) under conditions that readily reduced TcO4- and resulted in Tc(IV)-organic complexes. In microcosm experiments of a silty loam soil containing Tc04-, ReO4-, and ISA (3.0 mM), EDTA (0.17 mM), or NTA (2.4 mM), anoxia developed to iron-reducing conditions during the 42 day experimental period. The majority of the TcO4- was reduced to particle-reactive Tc(IV) and removed from solution during nitrate reduction, but there was no chromatographic evidence of Tc(IV)-organic complexes in the porewater. Overall, the excess organic complexants added did not cause a measurable difference in the solubility of Tc(IV) over the control experiments in this organic-rich (12% organic carbon) soil. ReO4- did not undergo reduction, as shown by the constant porewater concentration and the chromatographic data, and thus Re does notfunction as an analogue forTc under environmental nitrate- and iron-reducing conditions.     

Adaptation of an osmotically pumped continuous in situ water sampler for application in riverine environments

We present the design of an osmotic water sampler that is adapted to and validated in freshwater. The sample is drawn into and stored in a continuous narrow bore tube. This geometry and slow pump rate (which is temperature dependent: 0.8 mL/d at 4 °C to 2.0 mL/d at 28 °C) minimizes sample dispersion. We have implemented in situ time-stamping which enables accurate study of pump rates and sample time defining procedures in field deployments and comparison with laboratory measurements. Temperature variations are common in rivers, and without an accurate time-stamping, or other defining procedure, time of sampling is ambiguous. The sampler was deployed for one month in a river, and its performance was evaluated by comparison with manually collected samples. Samples were analyzed for major ions using Ion Chromatography and collision reaction Inductively Couple Mass Spectrometry. Despite the differences of the two sampling methods (osmotic sampler averages, while manual samples provide snapshots), the two data sets show good agreement (average R(2) ≈ 0.7), indicating the reliability of the sampler and at the same time highlighting the advantages of high frequency sampling in dynamic environments.     

In situ lifetimes and kinetics of a reductive whey barrier and an oxidative ORC barrier in the subsurface

Permeable reactive barriers (PRB) are being used to engineer favorable field conditions for in-situ remediation efforts. Two redox adjustment barriers were installed to facilitate a 10-month research effort on the fate and transport of MTBE (methyl tert-butyl ether) at a site called the Michigan Integrated Remediation Technology Laboratory (MIRTL). Thirty kilograms of whey were injected as a slurry into an unconfined aquifer to establish an upgradient reductive zone to reduce O2 concentration in the vicinity of a contaminant injection source. To minimize the impact of contaminant release, 363 kg of oxygen release compound (ORC) were placed in the aquifer as a downgradient oxidative barrier. Dissolved oxygen and other chemical species were monitored in the field to evaluate the effectiveness of this technology. A transient one-dimensional advective-dispersive-reaction (ADR) model was proposed to simulate the dissolved oxygen transport. The equations were solved with commonly encountered PRB initial and constant/variable boundary conditions. No similar previous solution was found in the literature. The in-situ lifetimes, based on variable source loading, were estimated to be 1,661 and 514 days for the whey barrier and ORC barrier, respectively. Estimates based on either maximum O2 consumption/production or measured O2 curves were found to under- or overestimate the lifetime of the barriers. The pseudo-first-order rate constant of whey depletion was estimated to be 0.303/d with a dissolution rate of 0.04/d. The oxygen release rate constant in the ORC barrier was estimated to be 0.03/d. This paper provides a means to design and predict the performance of reactive redox barriers, especially when only limited field data are available.     

Calibration of an in situ membrane inlet mass spectrometer for measurements of dissolved gases and volatile organics in seawater

Use of membrane inlet mass spectrometers (MIMS) for quantitative measurements of dissolved gases and volatile organics over a wide range of ocean depths requires characterization of the influence of hydrostatic pressure on the permeability of MIMS inlet systems. To simulate measurement conditions in the field, a laboratory apparatus was constructed for control of sample flow rate, temperature, pressure, and the concentrations of a variety of dissolved gases and volatile organic compounds. MIMS data generated with this apparatus demonstrated thatthe permeability of polydimethylsiloxane (PDMS) membranes is strongly dependent on hydrostatic pressure. For the range of pressures encountered between the surface and 2000 m ocean depths, the pressure dependent behavior of PDMS membranes could not be satisfactorily described using previously published theoretical models of membrane behavior. The observed influence of hydrostatic pressure on signal intensity could, nonetheless, be quantitatively modeled using a relatively simple semiempirical relationship between permeability and hydrostatic pressure. The semiempirical MIMS calibration developed in this study was applied to in situ underwater mass spectrometer (UMS) data to generate high-resolution, vertical profiles of dissolved gases in the Gulf of Mexico. These measurements constitute the first quantitative observations of dissolved gas profiles in the oceans obtained by in situ membrane inlet mass spectrometry. Alternative techniques used to produce dissolved gas profiles were in good accord with UMS measurements.     

Uranium reoxidation in previously bioreduced sediment by dissolved oxygen and nitrate

Flow-through sediment column experiments examined the reoxidation of microbially reduced uranium with either oxygen or nitrate supplied as the oxidant. The uranium was reduced and immobilized via long-term (70 days) acetate biostimulation resulting in 62-92% removal efficiency of the 20 microM influent uranium concentration. Uranium reduction occurred simultaneously with iron reduction as the dominant electron accepting process. The columns were reoxidized by discontinuing the supply of acetate and either replacing the anaerobic gas used to purge the influent media with a gas mixture containing 20% oxygen (resulting in a dissolved oxygen concentration of 0.27 mM) or adding 1.6 mM nitrate to the influent media. Both oxygen and nitrate resolubilized the majority (88 and 97%, respectively) of the uranium precipitated during bioreduction within 54 days. Although oxygen is more thermodynamically favorable an oxidant than nitrate, nitrate-dependent uranium oxidation occurred significantly faster than oxygen-dependent uranium oxidation at the beginning of our experiment due, in part, to oxygen reacting more strongly with other reduced compounds. Nitrate breakthrough at the effluent of the column occurred within 12 h, which was significantly earlier than when oxygen was detected at the effluent (26 days). Although, over time, the majority of uranium was reoxidized by either oxidant, these results indicate that the type of oxidant and its reactivity with other reduced compounds will influence the fate of reduced uranium during a short-term oxidation event that may occur during a uranium bioremediation scenario.     

Mulching an ultisol in southern Nigeria: Effects on physical properties and maize and cowpea yields

A study carried out for two cropping seasons at Nsukka, southeastern Nigeria, to determine the minimum rate of straw mulch for optimising the physical conditions of the topsoil (0–20 cm depth) of a fragile ultisol and maize (Zea mays L) and cowpea (Vigna unguiculata L Walp) yields, found that soil water sorptivity, transmissivity, steady state infiltration rate, cumulative infiltration after 90 min and time to attain steady state infiltration were optimal at the 2.0 t ha^?1 mulch rate. Also, at this rate of mulching, water retention and per cent water-stable aggregates > 0.5 mm were maximal whereas soil compaction (measured by dry bulk density) was minimal. Between 2 and 10 days after saturation the surface soil had reductions of 11, 6, 6 and 6% in volumetric water content, respectively, on the control (0), 2.0, 4.0 and 8.0 t ha^?1 mulched plots, mainly due to evaporation. Supra-optimal soil temperatures (> 30°) were observed only on the bare plots whereas no significant differences in maximum soil temperature among the mulched plots were noticed. Maize and cowpea yields were optimal at the 4.0 t ha^?1 rate with respective increases over the bare plots of 80 and 67% at this rate.     

Multifunctional iron-carbon nanocomposites through an aerosol-based process for the in situ remediation of chlorinated hydrocarbons

Spherical iron-carbon nanocomposites were developed through a facile aerosol-based process with sucrose and iron chloride as starting materials. These composites exhibit multiple functionalities relevant to the in situ remediation of chlorinated hydrocarbons such as trichloroethylene (TCE). The distribution and immobilization of iron nanoparticles on the surface of carbon spheres prevents zerovalent nanoiron aggregation with maintenance of reactivity. The aerosol-based carbon microspheres allow adsorption of TCE, thus removing dissolved TCE rapidly and facilitating reaction by increasing the local concentration of TCE in the vicinity of iron nanoparticles. The strongly adsorptive property of the composites may also prevent release of any toxic chlorinated intermediate products. The composite particles are in the optimal range for transport through groundwater saturated sediments. Furthermore, those iron-carbon composites can be designed at low cost, the process is amenable to scale-up for in situ application, and the materials are intrinsically benign to the environment.