Size dispersion and colloid mediated radionuclide transport in a synthetic porous media

Size dispersion effects during the migration of natural submicron bentonite colloids (<200 nm) through a ceramic column are observed for the first time by laser-induced breakdown detection (LIBD) at ppm (parts per million) mass concentration. Larger size fractions ( approximately 200 nm) arrive prior to smaller size fractions (<100 nm) at the column outlet in agreement with model predictions and earlier findings with carboxylated polystyrene spheres. By addition of trace amounts of americium(III) and plutonium(IV), colloid mediated transport of these radionuclides is studied. The peak arrival times of Pu-244 and Am-241, as measured by ICP-MS, match the bentonite colloid breakthrough and occur significantly prior to the conservative tracer (HTO) indicating the colloid-borne migration of tri- and tetravalent radionuclides.     

A triple continuum one-dimensional transport model for colloid facilitated contaminant migration in sets of parallel fractures with fracture skin

A triple continuum one-dimensional transport model is developed to analyse colloid facilitated contaminant transport in fractured geological formations. The model accounts for contaminant transport in the fracture, reversible deposition onto fracture surfaces and onto the colloids, diffusion into the rock formation and irreversible deposition of colloids onto the fracture surfaces. Sorption of the contaminant onto the fracture surfaces and onto suspended and deposited colloids are assumed to follow the linear equilibrium assumption (LEA); whereas the irreversible deposition of colloids onto the fracture skin surface is assumed to be governed by the linear kinetic sorption isotherms. The resulting coupled contaminant transport equations are solved using a numerical model employing fully implicit finite difference method based formulation. Results clearly demonstrate that the presence of the fracture skin significantly influences colloid facilitated contaminant migration in fractured formations. Fracture skin porosity and fracture skin diffusion coefficient are demonstrated to be the critical fracture skin properties that affect colloid facilitated contaminant migration in fractures. The impact of different colloid parameters on contaminant transport is investigated. The distribution coefficient for contaminant sorption onto the suspended colloids is found to be the most significant colloid related parameter influencing contaminant migration in fractured formation with fracture skin.     

A comparison of clay colloid and artificial microsphere transport in natural discrete fractures

Transport of monodispersed buoyant 1-mum latex microspheres, dense 1.34-microm montmorillonite particles, Li(+) and Br(-) was investigated in a naturally fractured chalk core with an average equivalent hydraulic aperture of 183 microm. Studied parameters were: tracer arrival time, C/C(0) values, mass recovery, size distribution and the impact of initial concentration. Breakthrough time of both colloidal tracers was faster than that of the soluble tracers. Significantly lower recovery and slightly slower breakthrough time were observed for the clay particles relative to the microspheres, apparently mainly due to the former's higher density, resulting in preferential gravitational settling of the clay particles. However, variable surface charge and nonuniform shape and size of the clay particles may also play a role in the observed differences. From the theoretical scale ratio, the time interval calculation seems to be a major factor in colloid recovery. Clay-particle size fractionation was observed (0.62 vs 1.34 microm at the outflow and inflow, respectively), and there was no significant influence of the initial concentration (100 and 500 mg/L) on transport properties. Our observations indicate that colloid density is a dominant property for their transport in fractures. This work emphasizes the need for caution when the results of studies in which buoyant colloids are used as tracers are extrapolated to natural systems in which clay colloids are present.     

Experimental investigation of acoustically enhanced colloid transport in water-saturated packed columns

The effects of acoustic wave propagation on the transport of colloids in saturated porous media were investigated by injecting Uranine (conservative tracer) as well as blue and red polystyrene microspheres (colloids of different diameters; 0.10 and 0.028 mum, respectively) into a column packed with glass beads. Experiments were conducted by maintaining the acoustic pressure at the influent at 23.0 kPa with acoustic frequencies ranging from 30 to 150 Hz. The experimental results suggested that colloid size did not affect the forward and reverse attachment rate coefficients. The acoustic pressure caused an increase in the effective interstitial velocity at all frequencies for the conservative tracer and colloids of both sizes, with maximum increase at 30 Hz. Furthermore, acoustics enhanced the dispersion process at all frequencies, with a maximum at 30 Hz.     

Application of simplified desorption method to study on sorption of americium(III) on bentonite

To elucidate the sorption behavior of americium(III) on bentonite, which is a mixture of montmorillonite clay, quartz and other minerals, simplified desorption experiments were applied to the solid phases collected after the sorption experiments. The sorption–desorption behavior was examined in the final pH range from 2 to 8. The desorption experiments revealed that most of the Am was sorbed on the montmorillonite moiety of the bentonite. The sorption of Am on montmorillonite was divided into two types: one was the “exchangeable” sorption, in which the sorbed Am was desorbed with a 1 M KCl aqueous solution, and the rest was the “unexchangeable” sorption. The exchangeable sorption was ion exchange of mostly Am³⁺. The unexchangeable sorption was the strong sorption of Am hydroxides. An accessory iron mineral, pyrite, might be involved in the Am sorption on bentonite at neutral pH.     

Sorption behavior of Np (IV), Np (V) and Am (III) in the disturbed zone between engineered and natural barriers

In order to be more confident of the performance assessment of high-level radioactive waste disposal, radionuclide transport must be investigated in more detail in the disturbed host rock region adjacent to the engineered barriers where disturbance has been introduced during the construction and waste-emplacement period. Geochemical, hydrological, and rock-mechanical properties should be quite different from those of undisturbed host rock. We have to elucidate the effect of bentonite intrusion into intersecting fractures from the standpoint of radionuclide confinement. In the present work, sorption distribution ratios (Kd's) of Np and Am are measured experimentally for various values or redox potential (Eh) in a simulated rock fracture filled with bentonite. The Kd of Am is approximately 6.5×10³ ml/g and found to be insensitive to the redox potential. Under anaerobic conditions, the Kd of Np is approximately 6×10⁴ ml/g. Under aerobic conditions, Kd is as small as 30 to 100 ml/g. This is the first report to measure the sorption behavior of Np and Am in a simulated rock fracture filled with bentonite (namely, in a disturbed zone) under pH, Eh and ionic strength control. We aan make use of these Kd data for numerically evaluating the mass transfer from bentonite filled fractures into the water-flowing fracture network¹.     

Thermal behavior and dehydroxylation kinetics of naturally occurring sepiolite and bentonite

Sepiolite and bentonite have a wide range of industrial applications based on their physicochemical properties such as surface area, thermal behavior, chemical composition, and mineralogic composition. The thermal behavior and kinetics of naturally occurring sepiolite and bentonite were determined in order to give an idea about the potential use of naturally occurring clay minerals in possible applications. Naturally occurring sepiolite and bentonite samples were heated to the temperature that was achieved at the end of the dehydroxylation process. Mineralogic and thermal characteristics of raw and heat treated samples were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, and nitrogen adsorption/desorption analyses. Changes in the structure following heat treatment were used for the evaluation of the dehydroxylation properties of the samples. The dehydroxylation properties of the minerals are strongly affected by the crystal structure. Kinetic analyses, which were related to the dehydroxylation of naturally occurring sepiolite and bentonite, were conducted using dynamic thermogravimetry/derivative thermogravimetry analysis under nitrogen atmosphere. Flynn–Wall–Ozawa, Kissenger–Akahira–Sunose, and Friedman isoconversional methods were used to determine the activation energies of the dehydroxylation reactions of the samples. The results indicate that the activation energy of naturally occurring sepiolite showed a little variation at a particular conversion rate (0.3–0.7), while the activation energy of naturally occurring bentonite showed a significant variation within the range of variation of the conversion rate. The present study shows that the dehydroxylation reactions of naturally occurring sepiolite and bentonite were single mechanism reaction and complex mechanism reaction, respectively.     

Effect of contact time,pH, humic acid and temperature on the sorption of radionuclide Am(III) onto attapulgite

Attapulgite has been applied in the sorption of metal and radionuclide ions since its discovery. Herein, radionuclide Am(III) sorption onto attapulgite was carried out at 25 °C in 0.01 mol/L NaNO₃ solutions. Effects of contact time, Am(III) initial concentration, pH, humic acid and temperature on Am(III) sorption onto attapulgite were investigated. The sorption of Am(III) increases with increasing contact time and reaches a maximum value within 24 h at different Am(III) initial concentration. The fast sorption velocity indicates that strong chemical sorption or strong surface complexation contributes to the sorption of Am(III) onto attapulgite under the experimental conditions. The experimental data can be described well by the pseudo-second-order rate model. The sorption of Am(III) onto attapulgite is strongly dependent on pH values and surface complexation is the main sorption mechanism. The presence of HA enhances the sorption of Am(III) onto attapulgite at pH < 8.5, whereas, at pH > 8.5, little effect of HA on Am(III) sorption is observed. The Langmuir, Freundlich and D-R models were used to simulate the sorption data at different pH values and the results indicated that Langmuir model simulates the experimental data better than Freundlich and D-R models. The thermodynamic parameters indicates that the sorption of Am(III) onto attapulgite is an endothermic and spontaneous process. The results suggest that the attapulgite is a suitable material as an adsorbent for preconcentration and immobilization of Am(III) from aqueous solutions.     

Active colloids on fluid interfaces

We review recent work on active colloids at interfaces, including self-propelled colloids that move by generating a propulsive force, and driven colloids that move under external fields. Features unique to fluid interfaces alter the flows generated at interfaces by active colloid motion, and hydrodynamic interactions with these layers. We emphasize recent observations of natural swimmers, like bacteria, and bio-mimetic colloids including self-propelled phoretic and Marangoni swimmers, and magnetically driven colloids. We discuss active colloid interaction with boundaries and with each other. We conclude with a discussion of open issues and opportunities to design active colloids as active surface agents that manipulate interfacial properties and the transport in the vicinity of interfaces.     

Coupled Influence of Colloidal and Hydrodynamic Interactions on the RSA Dynamic Blocking Function for Particle Deposition onto Packed Spherical Collectors

The influence of electrostatic double-layer and hydrodynamic interactions on random sequential adsorption (RSA) of colloidal particles onto packed spherical collectors was investigated using inverse analysis of colloid breakthrough data obtained from well-controlled particle deposition experiments. Deposition experiments were carried out using monodisperse aqueous suspensions of positively charged latex colloids and packed columns of negatively charged uniform glass beads for different combinations of ionic strength, particle size, and approach velocity. From the experimental particle breakthrough data, the initial particle deposition rates and the virial coefficients of the dynamic blocking function based on RSA mechanics were determined. The magnitudes of the virial coefficients were observed to increase from the hard sphere values with increasing flow rates and decreasing ionic strengths of the background electrolyte. Particle size also plays a significant role in governing the deposition dynamics. The deviation from the hard sphere RSA behavior becomes more prominent for larger particles. Copyright 2000 Academic Press.     

Colloid dispersion in a uniform-aperture fracture

This research investigates the dispersion of colloids through fracture systems by exploring experimentally and numerically the transport and dispersion of 1.0-, 0.11-, and 0.043-mum diameter fluorescent carboxylate-modified microspheres and chloride at various flow rates through variable-length, synthetic Plexiglas fractures (flow cells). A dimensionless number describing each experiment is varied by changing the colloid size, flow rate, and fracture length. Surface characteristics of the microspheres and Plexiglas favor repulsive interactions, thereby minimizing the chance of colloid filtration and remobilization. Full recovery of the colloids is typically observed, thereby supporting the assumption of negligible colloid filtration. In comparison to chloride transport, there is increased tailing for colloid plumes traveling through the flow cell. This increased tailing is attributed to Taylor dispersion phenomena (dispersion due to an advection gradient). In the synthetic fractures investigated here, colloid dispersion due to the velocity gradient is evident, but fully developed Taylor conditions are not realized. A particle-tracking algorithm is run inversely to estimate the effective dispersion rate for the colloid plume in each experiment as a function of the experimental parameters (flow rate, fracture length, and colloid size). Results suggest that the log of the effective dispersion rate of the colloid plume increases linearly with the log of the dimensionless number comprising experimental parameters.     

Influence of natural organic matter and ionic composition on the kinetics and structure of hematite colloid aggregation: implications to iron depletion in estuaries

The stability and aggregation behavior of iron oxide colloids in natural waters play an important role in controlling the fate, transport, and bioavailability of trace metals. Time-resolved dynamic light scattering experiments were carried out in a study of the aggregation kinetics and aggregate structure of natural organic matter (NOM) coated hematite colloids and bare hematite colloids. The aggregation behavior was examined over a range of solution chemistries, by adjusting the concentration of the supporting electrolyte-NaCl, CaCl2, or simulated seawater. With the solution pH adjusted so that NOM-coated and bare hematite colloids were at the same zeta potential, we observed a significant difference in colloid stability which results from the stability imparted to the colloids by the adsorbed NOM macromolecules. This enhanced stability of NOM-coated hematite colloids was not observed with CaCl2. Aggregate form expressed as fractal dimension was determined for both NOM-coated and bare hematite aggregates in both NaCl and CaCl2. The fractal dimensions of aggregates formed in the diffusion-limited regime indicate slightly more loosely packed aggregates for bare hematite than theory predicts. For NOM-coated hematite, a small decrease in fractal dimension was observed when the solution composition changed from NaCl to CaCl2. For systems in the reaction-limited regime, the measured fractal dimensions agreed with those in the literature. Colloid aggregation was also studied in synthetic seawater, a mixed cation system to simulate estuarine mixing. Those results describe the important phenomena of iron oxide aggregation and sedimentation in estuaries. When compared to field data from the Mullica Estuary, U.S.A., it is shown that collision efficiency is a good predictor of the iron removal in this natural system.     

Characterization of heavy-metal-containing seepage water colloids by flow FFF, ultrafiltration, ELISA and AAS

Heavy-metal-containing humic colloids from seepage water samples of three different municipal waste disposal plants were characterized in terms of molecular weight, hydrodynamic radius and heavy metal content. The size distribution of the colloids was determined with ultrafiltration (UF) and flow field-flow fractionation (flow FFF). The humic colloids in the seepage water samples were characterized using an off-line coupling of flow FFF with an enzyme-linked immunosorbent assay (ELISA) for humic substances. The heavy metals in the different size fractions obtained by UF and flow FFF were determined using atomic absorption spectroscopy (AAS). The colloid size distributions obtained with UF showed a maximum of the distribution in the range 1–10 nm. Seepage water samples with high colloid concentrations had a second maximum in the range 0.1–1 m. The determination of colloid size with flow FFF gave different colloid size distributions for the three waste disposal seepage waters, whereas water from the oldest disposal plant showed the smallest colloid size with a maximum at 0.9 nm and water from the most recent plant showed the largest colloid size with a maximum at 1.3 nm. The determination of particle classes with regard to the chemical composition using a scanning electron microscope with energy dispersive X-ray fluorescence detector (SEM/EDX) showed that the particles can be divided into five classes: silicates, insoluble salts, iron(hydr)oxides, carbonates and organic colloids (humic colloids). Flow FFF/ELISA off-line coupling showed that the most frequently occurring colloids of the seepage waters were humic colloids and investigation of the UF-size-fractions with AAS showed that up to 77% of the total mass of a heavy metal element can be bound to particles, especially to humic colloids. Additionally, the distributions of the heavy metals Fe, Cu and Zn were investigated with flow FFF/AAS off-line coupling. These results also showed that a substantial amount of these heavy metals (up to 46%) was bound to humic colloids.     

Aggregation of amphiphilic polymers in the presence of adhesive small colloidal particles

The interaction of amphiphilic polymers with small colloids, capable to reversibly stick onto the chains, is studied. Adhesive small colloids in solution are able to dynamically bind two polymer segments. This association leads to topological changes in the polymer network configurations, such as looping and cross-linking, although the reversible adhesion permits the colloid to slide along the chain backbone. Previous analyses only consider static topologies in the chain network. We show that the sliding degree of freedom ensures the dominance of small loops, over other structures, giving rise to a new perspective in the analysis of the problem. The results are applied to the analysis of the equilibrium between colloidal particles and star polymers, as well as to block copolymer micelles. The results are relevant for the reversible adsorption of silica particles onto hydrophilic polymers, used in the process of formation of mesoporous materials of the type SBA or MCM, cross-linked cyclodextrin molecules threading on the polymers and forming the structures known as polyrotaxanes. Adhesion of colloids on the corona of the latter induce micellization and growth of larger micelles as the number of colloids increase, in agreement with experimental data.     

Transport of kaolinite colloids through quartz sand: influence of humic acid, Ca(2+), and trace metals

To evaluate the risk of contaminant transport by mobile colloids, it seems essential to understand how colloids and associated pollutants behave during their migration through uncontaminated soil or groundwater. In this study, we investigated at pH 4 the influence of flow velocity, humic acid, solution Ca(2+) concentrations, and trace metals (Pb(2+), Cu(2+)) on the transport and deposition of kaolinite particles through a pure crystalline quartz sand as porous medium. A short-pulse chromatographic technique was used to measure colloid deposition. Adsorption of humic acid to the kaolinite increase its negative surface charge and then decrease colloid deposition. Experiments with different flow rates showed that humic-coated kaolinite colloid deposition followed a first-order kinetic rate law. The deposition rate coefficients of humic-coated kaolinite colloids increase with increasing Ca(2+) concentration in the suspension. The effect of trace metals on the mobility is studied by injecting two suspensions with different concentrations of Pb(2+) and Cu(2+). At very low cation concentration, the fraction of colloids retained is low and roughly independent of the nature of divalent cations. At high concentration, the deposition is higher and depends on the affinity of divalent cations toward humic-coated kaolinite colloids.     

Stable cluster formation in aqueous suspensions of iron oxyhydroxide nanoparticles

Metal oxide and oxyhydroxide nanoparticles are important components of natural aqueous systems and have application in photocatalysis. Uncoated (oxyhydr)oxide nanoparticles can form charge-stabilized colloids in water, but the precise regimes of dispersion and aggregation have been determined for very few nanomaterials. We studied the colloidal behavior of approximately 6 nm nanoparticles of iron oxyhydroxide (FeOOH), a common natural nanoscale colloid, and found that these nanoparticles formed stable suspended clusters under a range of aqueous conditions. Light and X-ray scattering methods show that suspended fractal nanoclusters are formed between pH 5 and 6.6 with well-defined maximum diameters that can be varied from 25 nm to approximately 1000 nm. The nanoclusters retain a very high surface area, and persist in suspension for at least 10 weeks in solution. The process is partially reversible because optically transparent suspensions are regained when nanoparticles that aggregated and settled at pH >7 are adjusted to pH 4 without stirring. However, completely redispersed nanoparticles are not obtained even after one month. Because nanocluster formation is controlled predominantly by surface charge, we anticipate that many metal oxide and other inorganic nanoparticles will exhibit equivalent cluster-forming behavior. Our results indicate that natural nanoparticles could form stable nanoclusters in groundwater that are likely to be highly mobile, with implications for the long-range transport of surface sorbed contaminants.     

Organoclay sorption of benzene as a function of total organic carbon content

The sorption of benzene to bentonite, activated carbon, and two organo-clays synthesized with the quaternary ammonium organic cations hexadecyltrimethylammonium (HDTMA) and benzyltriethylammonium (BTEA) was quantified as a function of total organic carbon content. The unmodified bentonite sorbed no benzene, while the activated carbon exhibited the strongest benzene uptake. For the organoclays, organic cations were exchanged onto Wyoming bentonite at four different percentages of the clay's cation exchange capacity. For HDTMA clay, in which partitioning is the dominant sorptive medium, it was determined that benzene sorption increased as the total organic carbon content increased (as the clay became more hydrophobic). In contrast, the sorption of benzene to BTEA clay, an adsorptive clay, decreased as the total organic carbon content of the clay was increased. It is believed that the sorptive capacity of BTEA clay decreases due to the formation of positively charged dimers on the clay's surface that block access to the sorptive sites. The organoclays sorbed more benzene than the unmodified bentonite, but less than the activated carbon.     

表面活性剂改性的螯合剂有机膨润土对水中有机污染物和重金属的协同吸附研究

采用季铵盐阳离子(CTMA+)和有机螯合剂(Am)复合改性膨润土(IMB), 制得一系列螯合剂柱撑膨润土IMB-CTMA-Am. 利用X射线衍射(XRD)分析、差热-热重(TG-DTA)分析、比表面积测定(N2-BET)以及元素分析等手段对吸附剂样品进行了表征. 结果表明, CTMA+和Am已柱撑进入膨润土层间. 吸附实验结果表明, IMB-CTMA-Am能同时有效地去除有机污染物对硝基苯酚(PNP)和重金属铅(Pb2+), 其对水中对硝基苯酚的强吸附能力来源于分配作用的增加和层间距的增大, 而与比表面积无关. 傅里叶变换红外光谱(FTIR)显示, IMB-CTMA-Am吸附Pb2+后的N—H吸收峰向低频方向移动, 表明Pb2+和膨润土层间的Am形成了稳定的配合物. 有机螯合剂Am和Pb2+所形成配合物的稳定性越大, IMB-CTMA-Am对Pb2+离子的吸附能力也就越强.     

Colloid filtration theory and the Happel sphere-in-cell model revisited with direct numerical simulation of colloids

The transport of colloids and bacterial cells through saturated porous media is a complex phenomenon involving many interrelated processes that are often treated via application of classical colloid filtration theory (CFT). This paper presents a numerical investigation of CFT from the Lagrangian perspective, to evaluate the role of some of the classical assumptions underlying the theory and to demonstrate a means to include processes relevant to bacterial transport that were inadequately characterized or neglected in the original formulation, including Brownian diffusion and potentially hysteretic potential functions. The methodology is based on conducting a Lagrangian trajectory analysis within Happel's sphere-in-cell porous media model to obtain the collection efficiency (eta), the frequency at which colloids or bacteria make contact with the solid phase of the porous medium. The Lagrangian framework of our model lends itself to mechanistic modeling of the biological processes that may be important in subsurface bacterial transport. The numerical study presented here focuses on the size range of bacterial colloids and smaller (down to 10 nm). Results of our model runs are in good agreement with the deterministic trajectory analysis of Rajagopalan and Tien (when diffusion is neglected) and in excellent agreement with the analytical solution to the Smoluchowski-Levich approximation of the convective-diffusion equation (when external forces and interception are neglected). Simple addition of our result for the deterministic eta to our result for the Smoluchowski-Levich eta matches the overall Rajagopalan and Tien eta to within 5% error or less for all cases studied. When we simulate diffusion and the deterministic forces together, our results diverge from the Rajagopalan and Tien eta as the particle size decreases, with discrepancies as large as 73%. These results suggest that accurate prediction of eta values for bacteria-sized (and all submicrometer) colloids requires simultaneous consideration of the primary transport mechanisms.