Migration of radionuclides in porous rock in the presence of colloids: effects of kinetic interactions

This work investigates the colloid-facilitated migration of radionuclides with radioactive decay in porous media. The sorption processes for radionuclides with both the solid matrix and colloids are treated as equilibrium or nonequilibrium. An analytical solution is obtained from a simplified linear equilibrium interaction mechanism. In addition, the adsorption processes for radionuclides with colloids and porous rock can be assumed as nonequilibrium and modeled by the linear kinetic adsorption. The numerical method is employed to solve the coupled colloid and radionuclide transport equations under nonequilibrium sorption assumption. Moreover, the reaction rates of the adsorption processes for radionuclides with the solid matrix and colloids affect the transport characteristics of radionuclides. The fast reaction rate of radionuclides with colloids causes a higher concentration of radionuclides adsorbed on colloids in a dispersed phase and enlarges acceleration caused by colloids. However, the fast reaction rate for radionuclides with solid matrix increases the retardation effect caused by the solid matrix. This work developed a predictive model for the transport of colloid-facilitated radionuclides in porous medium and to assess the importance of various phenomenological coefficients, particularly parameters for the adsorption interactions.     

Colloids in the mortar backfill of a cementitious repository for radioactive waste

Colloids are present in groundwater aquifers and water-permeable engineered barrier systems and may facilitate the migration of radionuclides. A highly permeable mortar is foreseen to be used as backfill for the engineered barrier of the Swiss repository for low- and intermediate-level waste. The backfill is considered to be a chemical environment with some potential for colloid generation and, due to its high porosity, for colloid mobility. Colloid concentration measurements were carried out using an in-situ liquid particle counting system. The in-house developed counting system with three commercially available sensors allowed the detection of single particles and colloids at low concentrations in the size range 50-5000 nm. The counting system was tested using suspensions prepared from certified size standards. The concentrations of colloids with size range 50-1000 nm were measured in cement pore water, which was collected from a column filled with a highly permeable backfill mortar. The chemical composition of the pore water corresponded to a Ca(OH)2-controlled cement system. Colloid concentrations in the backfill pore water were found to be typically lower than approximately 0.1 ppm. The specific (geometric) surface areas of the colloid populations were in the range 240 m2 g(-1) to 770 m2 g(-1). The low colloid inventories observed in this study can be explained by the high ionic strength and Ca concentrations of the cement pore water. These conditions are favourable for colloid-colloid and colloid-backfill interactions and unfavourable for colloid-enhanced nuclide transport.     

Multi-Scale Characterization of Fractured Rocks Used as a Means for the Realistic Simulation of Pollutant Migration Pathways in Contaminated Sites: A Case Study

An integrated methodology is developed to quantify the geostatistical and transport properties of fractured media at multiple scales. Such information is helpful in developing numerical models and estimating the up-scaled transport coefficients of fractured formations. An oil-contaminated fractured site, overlying granite rock and situated in northern Spain, is investigated, and a macroscopic geological model that quantifies the regional distribution of faults and fractures over the entire area is established. The methodology is based on the measurement of fractured outcrops in the field (scale ～1-100 m), the collection of representative fractured samples and measurement of the fracture aperture (scale ～0.01-1 mm), and the analysis of macroscopic characteristics (scale ～1-5 km) of fracture/faults. The multi-scale fracture properties are utilized to construct a discrete fracture/fault network model which provides input data to a macroscopic simulator of contaminant transport in fractured porous media. The transient NAPL migration pathways are predicted for one scenario of pollution. Such information is helpful in the risk assessment of fractured contaminated sites.     

Experimental evidence of colloids and nanoparticles presence from 25 waste leachates

The potential colloids release from a large panel of 25 solid industrial and municipal waste leachates, contaminated soil, contaminated sediments and landfill leachates was studied. Standardized leaching, cascade filtrations and measurement of element concentrations in the microfiltrate (MF) and ultrafiltrate (UF) fraction were used to easily detect colloids potentially released by waste. Precautions against CO₂ capture by alkaline leachates, or bacterial re-growth in leachates from wastes containing organic matter should be taken. Most of the colloidal particles were visible by transmission electron microscopy with energy dispersion spectrometry (TEM–EDS) if their elemental MF concentration is greater than 200 μg l^?1. If the samples are dried during the preparation for microscopy, neoformation of particles can occur from the soluble part of the element. Size distribution analysis measured by photon correlation spectroscopy (PCS) were frequently unvalid, particularly due to polydispersity and/or too low concentrations in the leachates. A low sensitivity device is required, and further improvement is desirable in that field. For some waste leachates, particles had a zeta potential strong enough to remain in suspension. Mn, As, Co, Pb, Sn, Zn had always a colloidal form (MF concentration/UF concentration > 1.5) and total organic carbon (TOC), Fe, P, Ba, Cr, Cu, Ni are partly colloidal for more than half of the samples). Nearly all the micro-pollutants (As, Ba, Co, Cr, Cu, Mo, Ni, Pb, Sb, Sn, V and Zn) were found at least once in colloidal form greater than 100 μg l^?1. In particular, the colloidal forms of Zn were always by far more concentrated than its dissolved form. The TEM–EDS method showed various particles, including manufactured nanoparticles (organic polymer, TiO₂, particles with Sr, La, Ce, Nd). All the waste had at least one element detected as colloidal. The solid waste leachates contained significant amount of colloids different in elemental composition from natural ones. The majority of the elements were in colloidal form for wastes of packaging (3), a steel slag, a sludge from hydrometallurgy, composts (2), a dredged sediment (#18), an As contaminated soil and two active landfill leachates.These results showed that cascade filtration and ICP elemental analysis seems valid methods in this field, and that electronic microscopy with elemental detection allows to identify particles. Particles can be formed from dissolved elements during TEM sample preparation and cross-checking with MF and UF composition by ICP is useful. The colloidal fraction of leachate of waste seems to be a significant source term, and should be taken into account in studies of emission and transfer of contaminants in the environment. Standardized cross-filtration method could be amended for the presence of colloids in waste leachates.     

Speciation of Solutes in Drainage and Pore Waters of Two Former Lignite Mines (Germany)

An experimental speciation scheme consisting of ultra-filtration(1 kD), cation and anion exchange has been developed. Parallel calculations of the ion balances are performed using the computerroutine MINTEQA2. The experimental and computed ion balances suffer from non-equilibrium conditions, incalculability of metal-binding colloids or sub-colloids, inaccurate Eh measurements and analytical errors. In most samples colloidal or sub-colloidal structures of Fe oxy-hydroxides and/or gypsum influence the behavior of many trace elements. The comparison of theory and experiment has to be restricted to acid waters (pH < 5) where colloids (1–450 nm) and non-filterable colloids or sub-colloids (<1 nm) play a minor role. Yet, only a few transition metals such as Mn, Fe, Co, Ni, Cu, Zn, or Cd remain more or less free of colloidal influence in low pH waters. The effect of DOC on Al may be estimated by model substances like citrate.     

Laboratory waste minimization by recovery of silver as nano-silver colloidal dispersion from waste silver chloride

The use of cyclic experiments, where the product of one reaction becomes the starting material for the next experiment, was proposed as an effective protocol for waste minimization in an educational lab. A simple, cheap and pollution-free method was developed for recovering silver as nano-silver colloidal dispersion from waste silver chloride in the laboratories of the Faculty of Health, Safety and Environment. Silver nanoparticles of the size 5–18 nm were recovered in the presence of sodium borohydride as a reducing agent and polyvinylpyrrolidone as a stabilizer agent. The nano-silver particles were studied for their formation, structure, stability and size using UV–Vis spectroscopy, transmission electron microscopy and dynamic light scattering techniques. The antibacterial assays of nanoparticles showed satisfactory results for Escherichia coli ATCC25922, Staphylococcus aureus ATCC 29213, and Acinetobacter baumanii (Clinical isolate). A laboratory experiment was designed in which students synthesize yellow colloidal silver solution from chemical waste silver chloride and estimate particle size using visible spectroscopy.     

Migration of VOC Plume in the Subsurface Domain at the Y‐12 National Security Site

During the production of thermonuclear fusion weapons at the Y‐12 National Security Complex (Y‐12 NSC) in Oak Ridge, Tennessee, between 1950 and 1963, the regional environment was extensively contaminated by volatile organic compounds (VOCs). Old Salvage Yard (OSY) on the western side of the site has been characterized as the major source of VOCs. In order to analyze the long‐term fate and transport of chlorinated VOC sources, an integrated surface and subsurface flow and transport model was developed for the Y‐12 NSC using the hydrodynamic and transport numerical package MIKE‐SHE. The model was developed considering the recent hydrogeological investigations on preferential flow and transport pathways at the site. The model was calibrated using the recorded groundwater flow and water‐quality data. The modeling simulated migration of the VOC plume for the next 100 years. Considering a range of hydrogeological and transport parameters, uncertainty of the results is discussed. The modeling predicted that tetrachloroethene, trichloroethene, and 1,2‐dichloroethene may exceed human health–related risk levels for the next 10 to 20 years. However, the contamination is unlikely to migrate to surface water under the current hydrogeological conditions and will decay below acceptable risk levels within approximately 20 years. © 2013 Wiley Periodicals, Inc.     

Solidification and recycling of incinerator bottom ash through the addition of colloidal silica (SiO2) solution

The possibility of using incinerator bottom ash as a substitute for natural aggregates was investigated. Rough, porous surface of bottom ash, which diminishes the strength of solidified products, was improved by colloidal silica solution. As a result, a significant increase of mechanical strength was accomplished by a slight amount of silica (<1 wt% to total). Moreover, pozzolanic reaction was induced in initial cement hydration due to the nano-particle size of about 20 nm in colloidal silica solution. Cylindrical specimens and bricks were prepared from bottom ash added to a colloidal silica (SiO2) solution and cement, and then their compressive strengths were evaluated. Cylindrical specimens showed an increase of approximately 60% in compressive strength when colloidal solution containing 4 wt% silica particles was sprayed onto the bottom ash. The strength of bricks containing colloidal silica was in excess of 20 MPa, which meets the requirement of construction materials. Results of leaching tests based on Toxicity Characteristic Leaching Procedure (TCLP) proved that the solidified bottom ash possessed good chemical stability.     

Combined MODFLOW‐FRACTRAN Application to Assess Chlorinated Solvent Transport and Remediation in Fractured Sedimentary Rock

Detailed field investigations and numerical modeling were conducted to evaluate transport and fate of chlorinated solvent contamination in a fractured sedimentary bedrock aquifer (sandstone/siltstone/mudstone) at a Superfund site in central New Jersey. Field investigations provided information on the fractured rock system hydrogeology, including hydraulic gradients, bulk hydraulic conductivity, fracture network, and rock matrix, and on depth discrete contaminant distribution in fractures (via groundwater sampling) and matrix (via detailed subsampling of continuous cores). The numerical modeling endeavor involved application of both an equivalent porous media (EPM) model for flow and a discrete fracture network (DFN) model for transport. This combination of complementary models, informed by appropriate field data, allowed a quantitative representation of the conceptual site model (CSM) to assess relative importance of various processes, and to examine efficacy of remedial alternatives. Modeling progressed in two stages: first a large‐scale (20 km x 25 km domain) 3‐D EPM flow model (MODFLOW) was used to evaluate the bulk groundwater flow system and contaminant transport pathways under historic and current aquifer stress conditions and current stresses. Then, results of the flow model informed a 2‐D DFN transport model (FRACTRAN) to evaluate transport along a 1,000‐m flowpath from the source represented as a 2‐D vertical cross‐section. The combined model results were used to interpret and estimate the current and potential future extent of rock matrix and aqueous‐phase contaminant conditions and evaluate remedial strategies. Results of this study show strong effects of matrix diffusion and other processes on attenuating the plume such that future impacts on downgradient well fields under the hydraulic stresses modeled should be negligible. Results also showed futility of source remediation efforts in the fractured rock, and supported a technical impracticability (TI) waiver for the site. © 2013 Wiley Periodicals, Inc.     

Removal of dissolved organic matter and phthalic acid esters from landfill leachate through a complexation-flocculation process

Dissolved organic macromolecules (DOM) and micropollutants, such as phthalic acid esters (PAEs), are often enriched in landfill leachate. PAEs were detected in 10 leachate samples from three landfills in Hubei province, central China. In particular, di-(2-ethyhexyl) phthalate (DEHP) concentration was higher than 70microgL(-1) in most samples, and the highest DEHP concentration was in the colloid fraction (1000Da-0.45microm). Use of flocculants may result in flocculation and precipitation of colloids and associated contaminants, such as DEHP. Biodegradation is not effective for long chain PAEs or the landfill leachate, given the low ratio of BOD5/COD (less than 0.2) and deficient phosphorus content. Complexation-flocculation is therefore proposed as an alternative process for removing DOM and PAEs. Jar-test experiments were employed to investigate the mechanism and feasibility of the process. Among tested coagulants (ferric chloride, aluminum sulfate and poly aluminum chloride (PAC)), PAC was the most efficient. Hydrophobic contaminants with log KOC greater than 4 and DOM in wastewater can be removed simultaneously through the complexation-flocculation process.     

超滤膜系统污染物的形貌及组成分析

通过SEM、能谱、FTIR以及电感耦合等离子体质谱的分析方法对被污染超滤膜面的无机污染物、有机污染物和微生物进行了识别和分析。研究结果表明:超滤膜面无机污染物主要为Fe(OH)3、Cu(OH)2、Al(OH)3和Si等胶体物和少量CaSO4;有机污染物主要为多菌霉素类化合物;微生物可能富含铁类化合物。同时污染膜的清洗液分析结果说明该超滤膜污染物主要为无机胶体和有机物。     

Prediction of the Dispersal of Oil Transport in the Caspian Sea Resulting from a Continuous Release

A 3-D hybrid flow/transport model has been developed to predict the dispersal of oil pollution in coastal waters. The transport module of the model takes predetermined current and turbulent diffusivities and uses Lagrangian tracking to predict the motion of individual particles (droplets), the sum of which constitute a hypothetical oil spill. Currents and turbulent diffusivities used in the model have been generated by a numerical ocean circulation model (Princeton ocean model) implemented for the Caspian Sea. The basic processes affecting the fate of the oil spill are taken into account and parameterized in the transport model.The hybrid model is implemented for a simulated continuous release in the coastal waters of the Caspian Sea. The potential of the model for the prediction of the advective and turbulent transport and dispersal of oil spills is demonstrated.     

Evaluation of Cyto- and Genotoxicity of Poly(lactide-co-glycolide) Nanoparticles

This work reports on an analysis of the cyto- and genotoxicity of poly(lactide-co-glycolide) polymer nanoparticles, in an attempt to evaluate their mutagenic effects. Fibroblast (3T3) and human lymphocyte cell cultures were exposed to solutions containing three different concentrations of nanoparticles (5.4, 54 and 540 μg/mL, polymer mass/volume of solution). The nanoparticles were characterized in terms of their hydrodynamic diameters, zeta potentials and polydispersity indices. The morphology of the particles was determined by atomic force microscopy. The PLGA nanospheres presented a size of 95 nm, a zeta potential of −20 mV and a spherical morphology. Cellular viability assays using fibroblast cells showed no significant alterations compared with the negative control. A cytogenetic analysis of human lymphocyte cells showed no significant changes in the mitotic index in relation to the control, indicating that in the concentration range tested, the particles used in the experimental models did not present cyto- or genotoxicity. For the tests conducted in this work we can conclude that biodegradable and biocompatible PLGA nanospheres are not toxic in the cell cultures tested (fibroblast and lymphocyte cells) and in the range of concentrations employed. The results provide new information concerning the toxic effects of particles produced using PLGA.     

Cement-based solidification/stabilization of lead-contaminated soil at a utah highway construction site

This article describes portland cement-based solidification/stabilization (S/S) treatment of heavy metal-contaminated soil. The soil was discovered during highway construction in West Jordan, Utah. Environmental Chemical Corporation (ECC) performed an emergency response to remediate the soil under contract with the EPA and the United States Bureau of Reclamation (USBR). The soil was treated by S/S. Treatment of the soil, contaminated with lead and arsenic, involved: (1) excavation, (2) size segregation, (3) reduction of oversized particles, (4) addition and mixture of portland cement and cement kiln dust, and (5) beneficial reuse of the treated soil as a subbase. S/S treatment successfully reduced Toxicity Characteristic Leaching Procedure (TCLP) concentrations of the contaminants to below regulatory levels.     

System performance assessment of final repository for radioactive wastes using first-order reliability method

When a final option for radioactive waste is determined, it is necessary to demonstrate compliance of disposal system chosen with relevant regulations. Considering the large number of physical and chemical factors involved, the complexity of their interrelationships, and long time periods, a system approach is required. A stochastic analysis is also needed to ensure that these regulatory criteria can be met with confidence. Among a variety of models developed to treat the effect of uncertainty on system performance, the first-order reliability method is suggested as an attractive tool to stochastic problems incorporating any amount of probabilistic information. Based on the first-order approximation, the method can give a probability of failure against a preselected target value, and information concerning the sensitivity of the outcome to variations in the input random variables and their statistical moments. In this paper, for reliable prediction of the performance of repository for radioactive wastes, first-order reliability method is applied in treating the parameter uncertainties of predictive models. First, a thickness of canister corrosion and breach time of canister is calculated using a uniform corrosion model, combined with the diffusional transport modeling of radionuclides in the backfill material. Second, a fractional release rate for each radionuclide is derived from the evaluation of the performance of a waste package which consists of canister and backfill material. Third, a cumulative release rate at the accessible environment is obtained by geosphere transport modeling with the source term given as a fractional release rate. The proposed first-order reliability method can be applicable to a full range of problems occurring in radioactive waste management and beyond.     

Interactions of Natural Colloidal Material and Phenanthrene in the Aquatic Environment

Resuspension of contaminated aquatic sediments by natural and anthropogenic activities (i.e., dredging, boat activities, fish, wildlife, storms, runoff) increases the flux of natural colloidal material and colloidally bound contaminants into the overlying water column. Colloidal material extracted from lower Fox River sediments was analyzed for various physical and chemical characteristics and subjected to batch aggregation studies under controlled conditions of pH (～3–8) and colloid concentrations (5 and 9 mg L^?1 as TOC equivalents) in the presence of dissolved phenanthrene in solution. Under water chemistry conditions where pH and K⁺ concentration are typical of most natural waters (10^?2 M K⁺ and pH～8), the presence of phenanthrene in solution (average [phen] = 0.2–0.4 mg/L) prevents particle aggregation and decreases the settling rate of these particles. Ultimately, this increases the total concentration of colloidally bound contaminants in the water column. Dredging is the most popular remediation technique for removing contaminated sediments from the aquatic environment. However, this laboratory study suggests that for typical waters, dredging may potentially elevate the concentrations of contaminants found in the water column. © 2001 John Wiley & Sons, Inc.     

The Input and Transmission of Fallout Radionuclides Through Redó, a High Mountain Lake in the Spanish Pyrenees

Fallout radionuclides have increasing value as tracers of pathways for pollutant transport through catchment/lake systems, in addition to their more traditional role in dating sediment records. The objectivesof this study, carried out within the EU MOLAR project, were tomeasure atmospheric fluxes of fallout ²¹⁰Pb, ¹³⁷Cs and ⁷Be at Redó, to establish mass balances for theseradionuclides, and test and validate models of pollutant transport through the lake and its catchment. This was achieved by comparing measured fluxes and concentrations in the water column with theoretical estimates using simple compartment models. Several interesting points emerged. Differences betweensoil core and rainwater measurements suggest that Saharan dust may be an important source of fallout ²¹⁰Pb. Fluxes throughthe water column had a clear seasonal trend reflecting winter icecover. Significant concentrations of ¹³⁷Cs are still presentin the water column, due to continued inputs from the catchment and/or remobilisation from the bottom sediments.     

Fracture Characterization Using Borehole Radar: Numerical Modeling

Forward modeling of borehole radar data for a series of synthetic discrete-fracture network (DFN) models provides a conceptual framework for interpreting experimental field data at fractured rock sites. A finite-difference time-domain (FDTD) radar wave propagation model was developed for this purpose. Synthetic examples demonstrate the utility of single-hole reflection-mode and cross-hole transmission-mode borehole radar for (1) identification of fracture location and orientation, and (2) identification of fracture pore-fluid properties, which might change as a result of tracer tests or flooding exercises in support of resource development or site remediation. A two-dimensional, synthetic DFN was generated statistically based on hypothetical distributions of fracture length, orientation, aperture, permeability, and inter-connectivity. The DFN includes a zone of permeable fractures embedded within a network of lower-permeability fractures and a low-permeability rock matrix. We modeled the unconnected and non-permeable fractures as being filled with freshwater. To simulate tracer experiments, contaminant releases, or engineered-remediation processes, we considered alternately the inter-connected, permeable fractures to be filled with freshwater air, or saline water (tracer). Synthetic radar data sets for both single-hole reflection and cross-hole transmission modes were generated. The features in synthetic radargrams were then examined and compared to the DFN model to evaluate the likelihood of identifying fracture location, orientation, and pore fluid in field situations. This comparison demonstrates that (1) the replacement of freshwater with saline water in permeable fractures generally increases the amplitude of reflections from permeable, connected fractures; and (2) in general, radar reflection-mode data contains more information about fracture properties than transmission-mode data.     

Protein-Polymer Grafts via a Soy Protein Derived Macro-RAFT Chain Transfer Agent

Methodology to produce materials derived from renewable resources is of great importance in decreasing both environmental impact as well as dependence on fossil fuels. We report a straightforward method of polymer grafting to soy protein hydrolysates, which are available in surplus. Benzylthiocarbonate moieties were installed on the protein surface via amidation of free amino groups, creating a protein macro chain transfer agent (CTA) for reversible addition fragmentation transfer (RAFT) polymerization. We found that subjection of this soy protein macro-CTA (SP-CTA) to RAFT polymerization conditions with polar acrylate monomers resulted in protein-polymer nanometer-scale particles with solubility properties dictated by monomer polarity. Polymer grafting, particle size and polymerization were characterized by elemental analysis, transmission electron microscopy and gel permeation chromatography. We anticipate that this method of grafting will be of use in generation of new materials based on renewable resources.