Simulation of Diffusive Transport of Radionuclides in the Soil of a Radioactive Waste Disposal Site

The diffusive transport of ¹³⁷Cs, ⁹⁰Sr, and ⁶⁰Co in the clay of a radioactive waste disposal site at PINSTECH was studied to assess the safety of the underlying permeable zone against the release of these radionuclides from buried waste containers in the clay. Diffusion coefficients of these radionuclides were estimated by reservoir to sediment diffusion method via their stable counterparts in a laboratory experiment. A curve-fitting procedure was applied on the measured concentration-time profiles of the reservoir using the one-dimensional solute transport equation with a nonlinear least squares technique. Distribution coefficients were determined in laboratory batch experiments. Diffusive transport simulations were performed with the estimated values of diffusion coefficients and distribution coefficients using the one-dimensional solute transport equation describing Fickian diffusion, equilibrium adsorption, and radioactive decay. The transport simulation results showed that ¹³⁷Cs, ⁹⁰Sr, and ⁶⁰Co will transport distances of 4.33, 3.77, and 1.51 meters, respectively, in the clay before their activity concentrations will drop to clearance levels set by the International Atomic Energy Agency (IAEA), below which the waste is treated as non-radioactive. This showed that concentrations more than clearance levels will not be able to transport to the permeable zone at a minimum depth of seven meters from the ground surface if the waste containers are disposed in a trench below which a clay layer with a thickness of 4.33 meters or more exists.     

Analysis of copper corrosion in compacted bentonite clay as a function of clay density and growth conditions for sulfate‐reducing bacteria

Aims: To investigate the relationships between sulfate‐reducing bacteria (SRB), growth conditions, bentonite densities and copper sulfide generation under circumstances relevant to underground, high‐level radioactive waste repositories. Methods and Results: Experiments took place 450 m underground, connected under in situ pressure to groundwater containing SRB. The microbial reduction of sulfate to sulfide and subsequent corrosion of copper test plates buried in compacted bentonite were analysed using radioactive sulfur (³⁵SO₄^2?) as tracer. Mass distribution of copper sulfide on the plates indicated a diffusive process. The relationship between average diffusion coefficients (D_s) and tested density (ρ) was linear. D_s (m² s^?1) = ?0·004 × ρ (kg m^?3) + 8·2, decreasing by 0·2 D_s units per 50 kg m^?3 increase in density, from 1·2 × 10^?11 m² s^?1 at 1750 kg m^?3 to 0·2 × 10^?11 m² s^?1 at 2000 kg m^?3. Conclusions: It is possible that sulfide corrosion of waste canisters in future radioactive waste repositories depends mainly on sulfide concentration at the boundary between groundwater and the buffer, which in turn depends on SRB growth conditions (e.g., sulfate accessibility, carbon availability and electron donors) and geochemical parameters (e.g., presence of ferrous iron, which immobilizes sulfide). Maintaining high bentonite density is also important in mitigating canister corrosion. Significance and Impact of the Study: The sulfide diffusion coefficients can be used in safety calculations regarding waste canister corrosion. The work supports findings that microbial activity in compacted bentonite will be restricted. The study emphasizes the importance of growth conditions for sulfate reduction at the groundwater boundary of the bentonite buffer and linked sulfide production.     

Influence of Bentonite Proportion in Natural Clay on Pb2+ ions Sorption: Response Surface Methodology,Kinetics and Equilibrium Studies

The influence of natural clay's bentonite proportion on Pb²⁺ sorption capacity was investigated using response surface methodology (RSM), kinetics and equilibrium studies. Experiments were conducted at different initial pH (3–7), bentonite to clay ratio (0–100%), initial Pb²⁺ ions concentration (20–120 mg/L) and sorbent dosage (0.2–1 g). Under the RSM study, data obtained from 27 experiments undertaken were found to fitted second-order polynomial model (R² = 0.998 and R²-predicted = 0.994). Analysis of variance showed that the Pb²⁺ sorption capacity was influenced according to the order; initial concentration> mass of adsorbent > initial pH > bentonite proportion. Optimal operating conditions were obtained at initial pH 5, 0.2 g sorbent dosage, 30% bentonite to clay ratio and 100 mg/L Pb²⁺ ion concentration. Regardless of the bentonite proportion, Pb²⁺ sorption kinetics followed pseudo-second-order associated with intra-particle diffusion. The sorption isotherm for the clay which was described by Freundlich yielded higher adsorption capacity (25 mg/g) while that of the bentonite was described by Langmuir model with lower maximum sorption capacity of 15 mg/g. These results suggest that sorption of the Pb²⁺ ions was not likely to significantly impact on the removal of Pb²⁺ ions during electrokinetic remediation process of clay having different proportion of bentonite.     

Pore‐network modeling of biofilm evolution in porous media

The influence of bacterial biomass on hydraulic properties of porous media (bioclogging) has been explored as a viable means for optimizing subsurface bioremediation and microbial enhanced oil recovery. In this study, we present a pore network simulator for modeling biofilm evolution in porous media including hydrodynamics and nutrient transport based on coupling of advection transport with Fickian diffusion and a reaction term to account for nutrient consumption. Biofilm has non‐zero permeability permitting liquid flow and transport through the biofilm itself. To handle simultaneous mass transfer in both liquid and biofilm in a pore element, a dual‐diffusion mass transfer model is introduced. The influence of nutrient limitation on predicted results is explored. Nutrient concentration in the network is affected by diffusion coefficient for nutrient transfer across biofilm (compared to water/water diffusion coefficient) under advection dominated transport, represented by mass transport Péclet number >1. The model correctly predicts a dependence of rate of biomass accumulation on inlet concentration. Poor network connectivity shows a significantly large reduction of permeability, for a small biomass pore volume. Biotechnol. Bioeng. 2011;108: 2413–2423. © 2011 Wiley Periodicals, Inc.     

Radionuclide transport model for risk evaluation of high-level radioactive waste in Northwestern China

The proper disposal of high-level radioactive waste (HLRW) is highly challenging. Numerical simulations are helpful methods of evaluating the risks of radionuclide transport. This paper examines the hydrogeological conditions of a prospective HLRW repository in China. A regional radionuclide transport model is first constructed using the TOUGH2-MP/EOS7R module at a site in northwestern China to evaluate radionuclide transport behavior. A flow model calibration under a steady state shows that the simulated and observed hydraulic heads match well. Hypothetical radionuclides (⁹⁰Sr, ¹³⁷Cs, ²³⁵U and ²³⁹Pu) are assumed to be instantaneously released at three locations with large groundwater velocities. Transport modeling shows that ²³⁵U is the most sensitive element and has the plume size (2400 m) after 10,000 years. Sensitivity analyses of parameters, including the permeability, distribution coefficient and diffusion coefficient, are carried out. The results show that the distribution coefficient is the most critical parameter for nuclide transport. These findings will provide a preliminary reference for the radionuclide migration process at a prospective HLRW disposal site, which could be informative for safety analyses and further transport in and around the repository system.     

Alleviation of Both Water and Nutrient Limitations is Necessary to Accelerate Ecological Restoration of Waste Rock Tips

Hard rock quarries are commonly located close to national parks and special areas of conservation and are generally regarded as visually intrusive. Consequently, restoration strategies that effectively accelerate natural plant regeneration processes are required. Slate waste tips present extreme conditions for plant establishment with multiple potential limiting factors (e.g., lack of organic matter, nutrients, and poor water retention). In this study, we investigated ecological strategies to accelerate natural regeneration at the largest slate quarry in Europe. A field experiment was conducted to assess ecosystem restoration using a contrasting set of native woody species. Treatments included amendments of waste tips with: polyacrylamide gel to increase water‐holding capacity; mineral fertilizer to increase nutrient supply; and two treatments that increased both (organic waste or boulder clay addition). Ecosystem recovery was evaluated through above‐ and below‐ground productivity (plant and microbial, respectively) and soil analyses. Neither increasing nutrient supply (with mineral fertilizer) nor water‐holding capacity (with polyacrylamide gel) was sufficient, alone, to improve plant establishment. However, both boulder clay and organic waste amendment significantly enhanced plant growth. There was a marked positive interaction in the effects on tree growth of the amendment with organic waste and boulder clay. Large interactions occurred between tree species and substrate amendments. The growth of N₂‐fixing species was strongly favored over non‐fixers where there was no addition of material increasing soil nitrogen supply, whereas the growth advantage of pioneer species over non‐pioneers was greatest with fertilizer, organic waste, or clay additions. Organic waste addition had the greatest positive impact on soil processes.     

Molecular Weight Dependence of Exciton Diffusion in Poly(3‐hexylthiophene)

A joint experimental and theoretical study of singlet exciton diffusion in spin‐coated poly(3‐hexylthiophene) (P3HT) films and its dependence on molecular weight is presented. The results show that exciton diffusion is fast along the co‐facial π–π aggregates of polymer chromophores and about 100 times slower in the lateral direction between aggregates. Exciton hopping between aggregates is found to show a subtle dependence on interchain coupling, aggregate size, and Boltzmann statistics. Additionally, a clear correlation is observed between the effective exciton diffusion coefficient, the degree of aggregation of chromophores, and exciton delocalization along the polymer chain, which suggests that exciton diffusion length can be enhanced by tailored synthesis and processing conditions.     

Ageing of added copper in bentonite without and with humic acid

Abstract

The ageing of metals in soils plays an important role in assessing ecological risk and soil quality criteria, it therefore is necessary to understand the ageing processes. Using a sequential extraction procedure and a parabolic diffusion equation, the ageing of added copper(Cu) in bentonite without and with humic acid (HA) was investigated as a function of incubation time over one year. At the same time, the effect of pH on Cu ageing in bentonite with HA was also studied. The most labile Cu fractions (water-soluble Cu, Cu extracted by NH₄NO₃ and EDTA) gradually transformed into less extractable forms. The modelled diffusion equation suggested that the ageing processes were controlled by surface nucleation/precipitation and diffusion. In the presence of HA, the contribution from surface nuclea-tion/precipitation increased significantly, whereas that from diffusion decreased, this indicated the addition of HA significantly enhanced the short-term ageing by complexation with Cu ions, but strongly inhibited the further diffusion of Cu ions into interlayer of bentonite during long-term ageing owing to the more stable complexes formed on solid surface. With increasing pH in bentonite with HA, the apparent diffusion rate coefficient (D/r²) increased greatly, which showed that during long-term ageing pH played an important role in reducing the extractability as a result of the significant increase in residual Cu (RES-Cu), the most inert fraction, due to micropore diffusion.     

Valorization of Foundry Sand in Clay Bricks at Industrial Scale

In this article, foundry sand as waste material has been valorized in ceramic brick manufacturing at industrial scale. The employment of a waste coming from one industry as an input for another is one of the key concepts of industrial ecology. To study the environmental behavior of the ceramic bodies in different life cycle stages, three leaching tests have been developed. We used an EN 12457 equilibrium leaching test with distilled water and a Wastewater Technology Centre acid neutralization capacity (WTC‐ANC) leaching test with different acidic leachates to carry out the environmental evaluation under different granular scenarios to ascertain the possibilities of the reuse or disposal of this granular material at the end of its useful life (end‐of‐life stage). Finally, we used a NEN 7345 diffusion leaching test for construction materials, with the aim of studying the environmental assessment at the use stage. Regulated pollutants in both stages have been evaluated. Furthermore, other soluble salts have been analyzed because they are closely related to the efflorescence phenomenon in bricks. Results indicate that core and green sand from the foundry industry can be used to replace clay content in construction materials, and that these foundry‐sand‐based ceramics improve some soluble salt results. Despite this fact, at the end‐of‐life stage in an inert waste landfill, lead, arsenic and chromium can be an environmental problem, both for commercial bricks and for foundry‐sand‐based bricks. This work can contribute to the determination of viability of sustainable processes of brick manufacturing that use foundry wastes as raw materials.     

Transcription of the Campylobacter jejuni cell division gene ftsA

Abstract Long-lived radioactive waste will be buried several hundred meters below ground in metal canisters surrounded by a buffer of compacted bentonite. Sulfate-reducing bacteria present in the bentonite may induce canister corrosion by production of hydrogen sulfide. Here we show that survival of sulfate-reducing bacteria in bentonite depends on the availability of water and that compacting a high quality bentonite to a water activity ( a _w) of 0.96 was lethal for the species investigated.     

Computational Oral Absorption Simulation for Low‐Solubility Compounds

Bile micelles play an important role in oral absorption of low‐solubility compounds. Bile micelles can affect solubility, dissolution rate, and permeability. For the pH–solubility profile in bile micelles, the Henderson–Hasselbalch equation should be modified to take bile‐micelle partition into account. For the dissolution rate, in the Nernst–Brunner equation, the effective diffusion coefficient in bile‐micelle media should be used instead of the monomer diffusion coefficient. The diffusion coefficient of bile micelles is 8‐ to 18‐fold smaller than that of monomer molecules. For permeability, the effective diffusion coefficient in the unstirred water layer adjacent to the epithelial membrane, and the free fraction at the epithelial membrane surface should be taken into account. The importance of these aspects is demonstrated here using several in vivo and clinical oral‐absorption data of low‐solubility model compounds. Using the theoretical equations, the food effect on oral absorption is further discussed.     

Utilization of Illinois PCC Dry Bottom Ash for Compacted Landfill Barriers

Compacted soil barriers are one of the most important components of municipal waste landfills. The material used to construct a landfill liner and/or cap must prevent the flow of fluids through them. Soils with low values of permeability (such as compacted clays) are often used to construct landfill barriers. Natural sands and other cohesionless materials are used to construct hydraulic barriers by adding admixtures to modify their properties. Several studies have been conducted that dealt with determining geotechnical engineering properties of sand-bentonite mixtures. Pulverized coal combustion (PCC) dry bottom ash is a coal combustion by-product of burning coal to produce electricity. Because of the increasing costs associated with the disposal of bottom ash and the environmental regulations in place, there is a need to develop alternate methods for profitable and environmentally safe uses of this waste material. Most scientists and researchers have concluded that bottom ash has geotechnical characteristics similar to those of sands. However, information on the use of bottom ash, with or without admixtures, in the construction of landfill barriers is limited. Most of the available literature on the engineering properties of bottom ash deals with its use as a fill material. The physical and chemical characteristics of bottom ash depend on several factors including type of coal used and type of boiler and collection system. This paper presents the results of an experimental study conducted to determine the possible use of Illinois PCC dry bottom ash amended with bentonite to construct landfill barriers. Test results presented show that the average value of hydraulic conductivity of Illinois PCC dry bottom ash with 15% bentonite content is close to the acceptable value required for its use as hydraulic barrier. Therefore, it was concluded that Illinois PCC dry bottom ash, modified with 15% or higher bentonite content, is likely to provide adequate hydraulic conductivity for its use to construct landfill barriers.     

Design of Carbon Nanotube‐Based Gas‐Diffusion Cathode for O2 Reduction by Multicopper Oxidases

Multicopper oxidases, such as laccase or bilirubin oxidase, are known to reduce molecular oxygen at very high redox potentials, which makes them attractive biocatalysts for enzymatic cathodes in biological fuel cells. By designing an enzymatic gas‐diffusion electrode, molecular oxygen can be supplied through the gaseous phase, avoiding solubility and diffusion limitations typically associated with liquid electrolytes. In doing so, the current density of enzymatic cathodes can theoretically be enhanced. This publication presents a material study of carbon/Teflon composites that aim to optimize the functionality of the gas‐diffusion and catalytic layers for application in enzymatic systems. The modification of the catalytic layer with multiwalled carbon nanotubes, for example, creates the basis for stronger π–π stacking interactions through tethered enzymatic linkers, such as pyrenes or perylene derivates. Cyclic voltammograms show the effective direct electron contact of laccase with carbon nanotube‐modified electrodes via tethered crosslinking molecules as a model system. The polarization behavior of laccase‐modified gas‐diffusion electrodes reveals open‐circuit potentials of +550 mV (versus Ag/AgCl) and current densities approaching 0.5 mA cm² (at zero potential) in air‐breathing mode.     

Mixing and sulphate-reducing activity of bacteria in swelling, compacted bentonite clay under high-level radioactive waste repository conditions

AIM: The fate of micro-organisms in the bentonite clay surrounding high-level radioactive waste (HLW)-containing copper canisters in a future Swedish underground (500 m) repository were investigated. METHODS AND RESULTS: Laboratory experiments were designed in which the mixing of various bacterial species with swelling bentonite was studied. A clear trend of fewer cultivable bacteria at depth was seen in the clay. This trend was consistent as the incubation time was increased from 8 h to 28 weeks. Sulphate-reducing bacteria were found to be active, reducing sulphate at the lowest density studied, 1.5 g cm-3, but sulphate reduction activity ceased at higher densities. CONCLUSIONS: The number of viable micro-organisms in an HLW repository bentonite clay buffer will decrease rapidly during swelling and very few viable cells will be present at full compaction. SIGNIFICANCE AND IMPACT OF THE STUDY: Sulphate-reducing bacteria will most probably not be able to induce corrosion of HLW-containing copper canisters.     

In situ effective diffusion coefficient profiles in live biofilms using pulsed‐field gradient nuclear magnetic resonance

Diffusive mass transfer in biofilms is characterized by the effective diffusion coefficient. It is well documented that the effective diffusion coefficient can vary by location in a biofilm. The current literature is dominated by effective diffusion coefficient measurements for distinct cell clusters and stratified biofilms showing this spatial variation. Regardless of whether distinct cell clusters or surface‐averaging methods are used, position‐dependent measurements of the effective diffusion coefficient are currently: (1) invasive to the biofilm, (2) performed under unnatural conditions, (3) lethal to cells, and/or (4) spatially restricted to only certain regions of the biofilm. Invasive measurements can lead to inaccurate results and prohibit further (time‐dependent) measurements which are important for the mathematical modeling of biofilms. In this study our goals were to: (1) measure the effective diffusion coefficient for water in live biofilms, (2) monitor how the effective diffusion coefficient changes over time under growth conditions, and (3) correlate the effective diffusion coefficient with depth in the biofilm. We measured in situ two‐dimensional effective diffusion coefficient maps within Shewanella oneidensis MR‐1 biofilms using pulsed‐field gradient nuclear magnetic resonance methods, and used them to calculate surface‐averaged relative effective diffusion coefficient (D_rs) profiles. We found that (1) D_rs decreased from the top of the biofilm to the bottom, (2) D_rs profiles differed for biofilms of different ages, (3) D_rs profiles changed over time and generally decreased with time, (4) all the biofilms showed very similar D_rs profiles near the top of the biofilm, and (5) the D_rs profile near the bottom of the biofilm was different for each biofilm. Practically, our results demonstrate that advanced biofilm models should use a variable effective diffusivity which changes with time and location in the biofilm. Biotechnol. Bioeng. 2010;106: 928–937. © 2010 Wiley Periodicals, Inc.     

Effect of Bentonite on Compacted Clay Landfill Barriers

Compacted clay barriers (liner and cap) are one of the most important components of municipal waste landfills. On-site soils are generally used to construct the clay barriers as long as they can be compacted to standard specifications, including hydraulic conductivity. Wherever the available on-site soils are not suitable to be used for constructing clay barriers, soils amended with bentonite are commonly used. This article presents the results of an experimental study conducted on compacted clay-bentonite mixtures to develop data on the effects of bentonite on engineering properties of compacted clay-bentonite mixtures. Clay-bentonite mixtures with bentonite contents of up to 7% were tested to determine consistency using Atterberg limits, moisture-density relationships using standard Proctor compaction tests, shear strength using unconfined compression tests, hydraulic conductivity using triaxial flexible-wall hydraulic conductivity tests, and consolidation properties using one-dimensional consolidation tests. Unconfined compression tests were also performed with 11% bentonite content. The laboratory test results indicated that liquid limit, plastic limit, and plasticity index increased linearly with increased amount of bentonite. The addition of bentonite resulted in decreased maximum dry unit weight but the optimum moisture content increased slightly. Unconfined compression strength of compacted clay-bentonite mixtures increased linearly with an increase in the amount of bentonite. Hydraulic conductivity of compacted clay-bentonite mixtures decreased nonlinearly with increased amount of bentonite, but a linear relationship was observed between logarithm of hydraulic conductivity and bentonite content. The compression index increased slightly from 0 to 3% bentonite content but increased nonlinearly beyond 3% bentonite content, whereas the swelling index of clay-bentonite mixtures has been observed to increase approximately linearly with increase in the amount of bentonite.     

Permeability of lysozyme tetragonal crystals to water

Diffusion of water within cross-linked tetragonal crystals of hen egg-white lysozyme has been measured and simulated on a computer using the X-ray structure of water-filled channels within the crystal lattice. Relative to the self-diffusion coefficient of bulk water molecules, the experimental diffusion coefficient of water within the crystal was found to be 13 times reduced in the (001) crystallographic plane and 5 times reduced in the [001] direction. Comparison of the experimental and computer simulated diffusion coefficients shows that steric limitations for water diffusion are mostly responsible for this reduction of the water diffusion in the crystal, with the self-diffusion coefficient of intracrystalline water reduced by no more than 30–40% as compared to that of bulk water.     

Shear‐induced diffusion of red blood cells measured with dynamic light scattering‐optical coherence tomography

Quantitative measurements of intravascular microscopic dynamics, such as absolute blood flow velocity, shear stress and the diffusion coefficient of red blood cells (RBCs), are fundamental in understanding the blood flow behavior within the microcirculation, and for understanding why diffuse correlation spectroscopy (DCS) measurements of blood flow are dominantly sensitive to the diffusive motion of RBCs. Dynamic light scattering‐optical coherence tomography (DLS‐OCT) takes the advantages of using DLS to measure particle flow and diffusion within an OCT resolution‐constrained three‐dimensional volume, enabling the simultaneous measurements of absolute RBC velocity and diffusion coefficient with high spatial resolution. In this work, we applied DLS‐OCT to measure both RBC velocity and the shear‐induced diffusion coefficient within penetrating venules of the somatosensory cortex of anesthetized mice. Blood flow laminar profile measurements indicate a blunted laminar flow profile and the degree of blunting decreases with increasing vessel diameter. The measured shear‐induced diffusion coefficient was proportional to the flow shear rate with a magnitude of ~0.1 to 0.5 × 10^?6 mm². These results provide important experimental support for the recent theoretical explanation for why DCS is dominantly sensitive to RBC diffusive motion.      

Model‐Based Investigation on the Mass Transfer and Adsorption Mechanisms of Mono‐Pegylated Lysozyme in Ion‐Exchange Chromatography

Recent studies highlighted the potential of PEGylated proteins to improve stabilities and pharmacokinetics of protein drugs. Ion‐exchange chromatography (IEX) is among the most frequently used purification methods for PEGylated proteins. However, the underlying physical mechanisms allowing for a separation of different PEGamers (proteins with a varying number of attached PEG molecules) are not yet fully understood. In this work, mechanistic chromatography modeling is applied to gain a deeper understanding of the mass transfer and adsorption/desorption mechanisms of mono‐PEGylated proteins in IEX. Using a combination of the general rate model (GRM) and the steric mass action (SMA) isotherm, simulation results in good agreement with the experimental data are achieved. During linear gradient elution of proteins attached with PEG of different molecular weight, similar peak heights, and peak shapes at constant gradient length are observed. A superimposed effect of increased desorption rate and reduced diffusion rate as a function of the hydrodynamic radius of PEGylated proteins is identified to be the reason of this anomaly. That is why the concept of the diffusion‐desorption‐compensation effect is proposed. In addition to the altered elution orders, PEGylation results in a considerable decrease of maximum binding capacity. By using the SMA model in a kinetic formulation, the adsorption behavior of PEGylated proteins in the highly concentrated state is described mechanistically. An exponential increase in the steric hindrance effect with increasing PEG molecular weight is observed. This suggests the formation of multiple PEG layers in the interstitial space between bound proteins and an associated shielding of ligands on the adsorber surface to be the cause of the reduced maximum binding capacity. The presented in silico approach thus complements the hitherto proposed theories on the binding mechanisms of PEGylated proteins in IEX.     

Dispersion of protein solutions in short, open capillary tubes as a method for rapid molecular weight determination

A method has been developed by which the molecular weight of proteins and other freely diffusing species can be estimated on the basis of chromatographic peak shapes developed by injection of a sample into an open capillary tube in a liquid chromatography system. In chromatographic peaks obtained from such a system, there are contributions from both convection and diffusion. Thus, peak shape is dependent upon the diffusion coefficient of the molecular species, the flow rate, and the length of the capillary tube. In the work reported here it has been found that for samples of different proteins ranging from 2000 to 14,000 molecular weight, each injected at the same mobile phase flow rate, the ratio (R) of h1, the height of the peak primarily due to convection, to h2, the height of the "makeup" peak, primarily due to diffusion from the capillary wall, is a direct measure of protein molecular weight. Linear plots of R vs molecular weight are obtained under certain conditions.