Filtration Of Colloids Research Articles

Sedimentation of heterogeneous particles in a porous material

Filtration of suspensions and colloids in porous materials occurs during the construction and operation of hydraulic structures, tunnels and underground storage facilities. Filtration models are used to calculate the penetration of grout into loose soil, when treating drinking water and industrial wastewater. During the filtration process, suspended particles pass through large pores and get stuck at the entrance of small-diameter pores. The trapped particles form a stationary deposit. A model of filtration of a polydisperse suspension in a porous material is considered. The purpose of the work is to study sediment profiles – the dependence of the concentration of deposited particles on the distance to the porous material inlet at a fixed time. An exact solution to the model was constructed using the method of characteristics. It has been shown that when filtering a polydisperse suspension, the distribution of sediment differs for different types of particles. The sediment profile of the largest particles always decreases monotonically, but the sediment profile of the smallest particles is not monotonic. It decreases at short times, then a maximum point appears on the graph, moving along the porous medium as time increases. After the maximum point reaches the porous material exit, the sediment profile becomes monotonically increasing. The sediment profiles of intermediate size particles and the total sediment profile are either monotonic or non-monotonic depending on the model parameters. The behavior of maximum points of non-monotonic profiles has been studied.

Concentration polarizations of PEG and silica colloids in ceramic nanofiltration

A large decrease in permeability is often observed during the filtration of nano-sized colloids, while fouling is widely regarded as the main explanation for this phenomenon. The osmotic pressure or concentration polarization (CP) of colloids can also contribute to the flux decline. However, the contribution of CP to flux loss cannot be determined by the traditional CP model. In this study, the effect of fouling and CP/osmotic pressure on flux was distinguished. The CP values of polyethylene glycol (PEG) and silica-colloids were determined by the osmotic pressures near the membrane surface and in the feed. The CP induced by colloids accounted for 43–95% of the flux loss in our experiments. Silica exhibited higher CP values (127–460), compared to 7–71 for PEG. This was attributed to the slower back diffusion caused by the larger colloids, as evidenced by the diffusion coefficients of 4.30 × 10−11 m2/s for silica (10 nm) and 1.45 × 10−10 m2/s for PEG (2.9 nm). Although the CP was mitigated by increasing the cross-flow velocity, CP values of 31 and 250 were observed for PEG and silica at high Reynolds number of 7317, respectively. The experimentally obtained CP values were also compared with those calculated by the film diffusion model.

Exact solution to non-linear filtration in heterogeneous porous media

Many technological processes of chemical and environmental engineering are associated with the filtration of fine particles in porous media, specifically in heterogeneous reservoirs. A one-dimensional model of deep bed filtration of suspensions and colloids in a heterogeneous porous medium is derived. The model includes the mass balance equation with variable porosity for the concentrations of suspended and retained particles and the deposit growth equation with the non-linear filtration function depending on the retained concentration and on the spatial coordinate. Using the method of characteristics, a formula for the curvilinear concentration front is obtained. The problem is reduced to one nonlinear equation in partial derivatives of the first order. An exact solution is obtained for the hyperbolic heterogeneity, and a Riemann invariant providing a relation between the solutions on the characteristics, is found. The properties of the solutions are studied. The cases of infinite and finite filtration time are considered.

Stochastic model for filtration by porous materials

Transport and filtration of colloids through permeable materials is controlled by the underlying heterogeneous structure of the host medium. A stochastic model is proposed that takes into account pore-scale heterogeneity, which controls the macroscopic deposition.

The impact of wetland on neutral mine drainage from mining wastes at Luanshya in the Zambian Copperbelt in the framework of climate change.

The impact of a natural wetland ("dambo" in Zambia) on neutral mine drainage at Luanshya in the Zambian Copperbelt has been investigated during an intermediate discharge period (July) using a multi-method characterization of solid phase samples, sequential extraction analysis, X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy combined with water analyses, isotopic analyses, and geochemical modeling. In the wetland, the principal identified solid phases in sediments were carbonates, gypsum, and ferric oxyhydroxides. A significant portion of the ochres was present as insoluble hematite. Mine drainage pH values decrease, and log [Formula: see text] values increase after inflow of water into the wetland; dissolved and suspended concentrations of Fe, Mn, Cu, and Co also decrease. Based on speciation calculations, there is no precipitation of secondary Cu and Co minerals in the period of sampling, but it can occur later in dry period when the flow rate is reduced. Concentrations of sulfate decrease, and values of δ34S(SO4) in the wetland increase in parallel, suggesting sulfate reduction is occurring. In more advanced dry period, the discharge in mine drainage stream is probably much lower and water can reach supersaturation with respect to minerals such as gypsum, which has been found in sediments. Wetlands have a positive impact on mine drainage water quality due to the removal of metals by adsorption, co-precipitation, and filtration of colloids. However, there can also be a rebound of contamination by seepage inflow downstream from the wetland. Ongoing climate change with extreme hydrologic events may enhance differences between dry and rainy seasons with resulting faster mobilization of contaminants.

Asymptotic model of size-exclusion grouting

In the construction of foundations of buildings and structures on fragile ground, various technologies of soil grouting are used. When pouring fine-grained concrete into the porous soil, the concrete grains are filtered in the pores of the soil. The filtration process depends on the ratio of the pore sizes of the soil and the solid particles of the injected concrete mortar.Injection of a carrier fluid with small solid particles in a porous medium forms a dynamic concentrations front of suspended and retained particles, separating the suspended particles and the hollow part of the porous frame. The purpose of the study is to construct and calculate an asymptotic model near the concentrations front for the filtration of monodisperse suspension in a porous medium with size-exclusion mechanism of particles retention.The classical mathematical model for one-dimensional filtration of suspensions and colloids in a porous medium is based on the geometric ratio of the particles and pores sizes: the particles freely pass through the large pores and get stuck in the pore throats with sizes smaller than the particles diameter. The model is determined by a system of two quasilinear first-order partial differential equations with the gap between boundary and initial conditions. To construct an asymptotic expansion in the vicinity of the concentrations front, a special small parameter is used that specifies the distance to the front. This parameter provides direct determination of the asymptotic terms from the recurrent system of ordinary linear differential equations of the first order.Near the concentrations front of the suspended and retained particles, a nonlinear high-order asymptotics is constructed for the filtration problem of solid particles transported by a carrier fluid in a porous medium. The obtained solution is zero before the front and nonzero behind the front. Approbation of the asymptotic expansion is carried out. It is shown that, for a linear blocking filtration coefficient, the asymptotics coincides with the exact solution.The asymptotic model of deep bed filtration makes it possible to obtain exact formulas for the high-order asymptotic expansion near the dynamic concentrations front of suspended and retained particles. The asymptotics improves the ability to fine-tune the filtration model depending on the properties of the porous soil and the grout.

Microfluidic Mimic for Colloid Membrane Filtration: A Review

This review provides an overview of the recent improvements of microfluidic membrane mimics. A special focus is given to the filtration of colloids in this device. Methods for on-chip membrane filtration have undergone significant development and improvement over the past two decades. Many efforts have been made to develop a single chip microfluidic platform that integrates the benefits of microfluidics and membrane science and technology. This review addresses the potential for microfluidic devices to serve as microfiltration membranes for separation purposes, as well as, micro-sized tools to study colloidal fouling phenomena at the pore scale.

CALCULATION OF LONG-TERM FILTRATION IN A POROUS MEDIUM

he filtration problem in a porous medium is an important part of underground hydromechanics. Filtration of suspensions and colloids determines the processes of strengthening the soil and creating waterproof walls in the ground while building the foundations of buildings and underground structures. It is assumed that the formation of a deposit is dominated by the size-exclusion mechanism of pore blocking: solid particles pass freely through large pores and get stuck at the inlet of pores smaller than the diameter of the particles. A one-dimensional mathematical model for the filtration of a monodisperse suspension includes the equation for the mass balance of suspended and retained particles and the kinetic equation for the growth of the deposit. For the blocking filtration coefficient with a double root, the exact solution is given implicitly. The asymptotics of the filtration problem is constructed for large time. The numerical calculation of the problem is carried out by the finite differences method. It is shown that asymptotic approximations rapidly converge to a solution with the increase of the expansion order.

Development and implementation of a pass/fail field-friendly method for detecting sildenafil in suspect pharmaceutical tablets using a handheld Raman spectrometer and silver colloids

A simple, fast, sensitive and effective pass/fail field-friendly method has been developed for detecting sildenafil in suspect Viagra and unapproved tablets using handheld Raman spectrometers and silver colloids. The method involves dissolving a portion of a tablet in water followed by filtration and addition of silver colloids, resulting in a solution that can be measured directly through a glass vial. Over one hundred counterfeit Viagra and unapproved tablets were examined on three different devices during the method development phase of the study. While the pass/fail approach was found to be 92.6% effective on average, the efficacy increased to 97.4% on average when coupled with the software’s “Discover Mode” feature that allows the user to compare a suspect spectrum to that of a stored sildenafil spectrum. The lowest concentration of sildenafil in a water/colloid solution that yielded a “Pass” was found to be 7.6μg/mL or 7.6 parts per million (ppm). For the analysis of suspect tablets, this value was found to be as low as 10μg/mL and as high as 625μg/mL. This variability was likely related to the tablet formulation, e.g., concentration of sildenafil, presence and concentration of water-soluble and/or water-insoluble ingredients. However, since most counterfeit Viagra and unapproved tablets contain >50mg sildenafil per tablet, such low concentrations will not be encountered often. Limited in-lab and in-field validation studies were conducted in which analysts/field agents followed the procedure outlined in this study for small sample sets. These individuals were provided with written instructions, a ∼20min demonstration regarding how to perform the procedure and use the instrument, and a kit with field-friendly supplies (purified bottled water from a local grocery store, disposable plastic pipettes, eye-dropper with a silver colloid solution, etc.). The method proved to be 98.3% and 91.7% effective for the in-lab and in-field validation studies, respectively, which demonstrated the ruggedness, simplicity and practicality of the method.

Highly permeable twinned alumina nanoparticles for the precoat filtration of fine colloids

Abstract Pretreatment is key to the success of any fine filtration process. Precoat filtration is a common method to reduce fine particulate matter in feed streams. In this study, precoat filters were formed by the deposition of the diatomaceous earth (DE) or twinned alumina nanosheets (TAN) particles on a substrate. The TAN particles were produced via metal salt hydrolysis. The performance of the precoat filters was investigated during the constant-pressure filtration of a bentonite solution. The results showed that the TAN precoat exhibited enhanced flow properties and reduced the turbidity of the filtrate more rapidly than either of the DE precoats. The TAN precoat reached the required turbidity level of ≤0.10 NTU at a flux that was up to 28 times higher than the fluxes obtained by the DE precoats. The superior performance of the TAN precoat was explained by (1) the ability of the TAN precoat to resist compaction during filtration due to the unique twinning of the alumina nanosheets forming the TAN particles, (2) the strong attractive forces between the bentonite and TAN particles based on their opposite surface charges, and (3) the isotropic permeability of the TAN aggregates.

Generic Mechanochemical Grafting Strategy toward Organophilic Carbon Nanotubes.

Although carbon nanotubes (CNTs) have been produced in industrial scale, their poor dispersibility in organic solvents still imposes a huge challenge for their practical applications. In the present work, we propose a generic mechanochemical grafting strategy to prepare the organo-soluable CNTs, which is facile, efficient, and scalable. Significantly, the solvent spectrum of the CNTs suspension can be simply extended by changing the chemical composition of the grafted elastomer chains. The prospect of the organo-solubale CNTs is demonstrated by the free-standing buckypapers by direct filtration of the CNT colloids. Such buckypapers exhibit great potential as robust and ultraflexible conductors due to the combination of high toughness and stable conductivity under cyclic bending and twisting. Furthermore, this facile surface modification strategy of CNTs also enables remarkable improvement in mechanical properties of CNT-based rubber composites. We envision that the present work offers a facile yet efficient strategy for scalable production of organosoluable CNTs and other nanoparticles, which is of great scientific and technological interest.

Compression and expansion properties of filter cake accompanied with step change in applied pressure in membrane filtration

Abstract In order to well elucidate the true nature of filter cake formed in membrane filtration of aqueous colloids, the transient flux decline behaviors accompanied with step-up and step-down changes in the applied filtration pressure during the course of constant pressure filtration were investigated by carrying out both microfiltration experiments of kaolin slurry and ultrafiltration experiments of nanosilica sol under various experimental conditions. When the applied pressure was increased stepwise, for each colloid the plot of the reciprocal filtration rate vs. the filtrate volume per unit membrane area referred to as the Ruth plot tends to approach a linear relationship obtained in constant pressure filtration conducted under the increased pressure condition from the beginning of filtration, after a transient period when the preformed cake under the initial pressure was compressed. In contrast, in the case of the step-down change in the applied pressure, when the volume of the preformed cake is adequate, the plot after a transient period tends to approach a higher straight line parallel to a linear relationship for constant pressure filtration conducted under the decreased pressure condition from the beginning. Such flux decay behaviors were well described by the expression having two constants: one is the constant representing the final filterability for the preformed filter cake, and the other is the rate constant describing the change rate in the structure of preformed cake resulting from a pressure jump. Moreover, the influence of experimental conditions on the cake reversibility and the rate constant was revealed both for kaolin slurry and for nanosilica sol.

Filtration of protein colloids by fibrous membranes: A mechanistic investigation using packed bed filtration approach

Abstract This research aimed to use packed bed filtration as a novel approach to investigate fibrous membrane filtration of aqueous particles, a novel environmental application of fibrous materials and a new frontier for engineering colloidal science. For this purpose, bovine serum albumin (BSA) was filtered through a pre-characterized glass bead column to determine attachment efficiencies (α) of BSA onto glass beads. Subsequently, BSA was filtered through a glass-fiber membrane possessing similar surface elemental compositions as the glass beads. By simulating the glass fiber membrane as a packing of spherical glass particles with a diameter equivalent to fiber diameter, α values pertinent to BSA-glass fiber attachment were calculated using the clean bed efficiency model. A side-by-side comparison of the α values show that membrane filtration exhibited greater α values than column filtration, the α values calculated for membrane filtration were always about 1.5 times of those determined under similar conditions in column filtration. However, α values for the two systems varied with solution chemistry in similar trends and the maximal α values for both systems were both found at the isoelectric point of BSA (pHIEP = 4.78). This finding implies that fibrous membrane filtration may be studied using classical packed bed filtration theories.

Migration of Contaminants in Fractured-Porous Media in the Presence of Colloids: Effects of Kinetic Interactions

The presence of colloids in groundwater can enhance contaminant migration by reducing retardation effects. The field experiments performed in a water-conducting feature of a shear zone at the Grimsel Test Site in Switzerland indicate that the sorption processes for contaminants on mobile and immobile colloids are kinetically controlled and that colloid filtration occurs. To enable the modelling of those experiments, an appropriate model of colloid-facilitated contaminant transport in fractured-porous media is developed. The physical system is modelled as a single planar fracture with adjacent fully saturated porous rock matrix. Contaminants can diffuse into the rock matrix but colloids cannot. In the mathematical model, the 1D advective contaminant transport along the fracture is coupled with contaminant diffusion into the rock matrix perpendicular to the fracture. Radioactive decay and sorption processes for contaminants in the rock matrix (linear equilibrium), on the fracture surface (linear equilibrium and linear kinetic reactions) and on mobile and filtered colloids (linear kinetic approach), are considered. The model for colloid transport includes filtration of colloids in the fracture and their remobilization. A useful approach is developed that can be applied to adequately describe a natural system (crystalline rock) with our double-porosity model (single fracture integrated into porous rock). Numerical solutions are obtained using an implicit finite difference scheme and realized in the transport code COFRAME. The transport code is validated by the comparison of calculated results with field experiment data. Some sets of simulations are performed to study the effect of kinetics for interaction processes.

Colloid Filtration in Surface Dense Vegetation: Experimental Results and Theoretical Predictions

Understanding colloid and colloid-facilitated contaminant transport in overland flow through dense vegetation is important to protect water quality in the environment, especially for water bodies receiving agricultural and urban runoff. In previous studies, a single-stem efficiency theory for rigid and clean stem systems was developed to predict colloid filtration by plant stems of vegetation in laminar overland flow. Hence, in order to improve the accuracy of the single-stem efficiency theory to real dense vegetation system, we incorporated the effect of natural organic matter (NOM) on the filtration of colloids by stems. Laboratory dense vegetation flow chamber experiments and model simulations were used to determine the kinetic deposition (filtration) rate of colloids under various conditions. The results show that, in addition to flow hydrodynamics and solution chemistry, steric repulsion afforded by NOM layer on the plants stem surface also plays a significant role in controlling colloid deposition on vegetation in overland flow. For the first time, a refined single-stem efficiency theory with considerations of the NOM effect is developed that describes the experimental data with good accuracy. This theory can be used to not only help construct and refine mathematical models of colloid transport in real vegetation systems in overland flow, but also inform the development of theories of colloid deposition on NOM-coated surfaces in natural, engineered, and biomedical systems.

A General Approach for Predicting the Filtration of Soft and Permeable Colloids: The Milk Example

Membrane filtration operations (ultra-, microfiltration) are now extensively used for concentrating or separating an ever-growing variety of colloidal dispersions. However, the phenomena that determine the efficiency of these operations are not yet fully understood. This is especially the case when dealing with colloids that are soft, deformable, and permeable. In this paper, we propose a methodology for building a model that is able to predict the performance (flux, concentration profiles) of the filtration of such objects in relation with the operating conditions. This is done by focusing on the case of milk filtration, all experiments being performed with dispersions of milk casein micelles, which are sort of ″natural″ colloidal microgels. Using this example, we develop the general idea that a filtration model can always be built for a given colloidal dispersion as long as this dispersion has been characterized in terms of osmotic pressure Π and hydraulic permeability k. For soft and permeable colloids, the major issue is that the permeability k cannot be assessed in a trivial way like in the case for hard-sphere colloids. To get around this difficulty, we follow two distinct approaches to actually measure k: a direct approach, involving osmotic stress experiments, and a reverse-calculation approach, that consists of estimating k through well-controlled filtration experiments. The resulting filtration model is then validated against experimental measurements obtained from combined milk filtration/SAXS experiments. We also give precise examples of how the model can be used, as well as a brief discussion on the possible universality of the approach presented here.

FUNMIG Integrated Project results and conclusions from a safety case perspective

Abstract The scope of the FUNMIG Integrated Project (IP) was to improve the knowledge base on biogeochemical processes in the geosphere which are relevant for the safety of radioactive waste repositories. An important part of this project involved the interaction between data producers (research) and data users (radioactive waste management organisations in Europe). The aim thereof was to foster the benefits of the research work for performance assessment (PA), and in a broader sense, for the safety case of radioactive waste repositories. For this purpose a specifically adapted procedure was elaborated. Thus, relevant features, events and processes (FEPs) for the three host rock types, clay, crystalline and salt, were taken from internationally accepted catalogues and mapped onto each of the 108 research tasks conducted during the FUNMIG project by a standardised procedure. The main outcome thereof was a host-rock specific tool (Task Evaluation Table) in which the relevance and benefits of the research results were evaluated both from the PA and research perspective. Virtually all generated data within FUNMIG are related to the safety-relevant FEP-groups “transport mechanisms” and “retardation”. Generally speaking, much of the work within FUNMIG helped to support and to increase confidence in the simplified PA transport and retardation models used for calculating radionuclide (RN) transport through the host rock. Some of the studies on retardation processes (e.g. coupled sorption-redox processes at the mineral–water interface) yielded valuable data for all three rock types dealt within the IP. However, most of the studies provided improved insight regarding host-rock specific features and processes, the majority of this work being dedicated to clay-rich and crystalline host rocks. For both of these host rock types, FUNMIG has significantly contributed to improving understanding on a conceptual level, both by providing new experimental data at different spatial scales and by developing improved modelling approaches. The disposal concept in salt host rocks differs from those in other host rock types in that the host rock is, at least in part, regarded as near-field from which under normal scenarios no release of radionuclides is possible. Corresponding investigations in FUNMIG concerned therefore not salt host rocks but an example of an overburden under the premise of radionuclide escape from the salt barrier. Selected highlights with regard to FUNMIG’s achievements include: For argillaceous host rocks, the systematic effort of investigating and comparing diffusion and sorption processes at different scales in different clay rocks using a variety of methods has substantially increased the knowledge basis for future safety cases. For crystalline host rocks, valuable data on the generation, transport and filtration of clay colloids from the near-field and their impact on RN transport under realistic conditions have been obtained. The results from studies on organic colloids and on biofilms, including their interaction with radionuclides, have been shown to be of interest for future safety cases of salt-host rocks. Among the main research issues from a PA perspective that need to be addressed in the future, the following are noted: (i) the question of irreversibility of RN sorption to colloids in fractures of crystalline rocks, (ii) a comprehensive model for cation and anion diffusion in clays for different scales and (iii) the applicability of mechanistic retardation models for strongly sorbing radionuclides in intact clay and crystalline host rocks. An important lesson learnt from the interaction between research and PA is that it would be desirable to apply a similar evaluation procedure for proposed research tasks before the start of the research work. In this regard, the procedures developed within FUNMIG are a useful tool for planning future Integrated Projects.

Slow colloidal aggregation and membrane fouling

We observed that the concept of critical flux, although established on physical bases, does not describe all typical fouling situations found in membrane filtration. We especially focus on the slow flux decline that is observed in many industrial membrane applications, and that has found several types of explanations that we briefly discuss. In order to get a better understanding of this situation, we have considered the orders of magnitude of the slow aggregation kinetics that are expected to happen within a boundary layer, on an ultrafiltration or microfiltration membrane in operation. The results help to understand that whereas the critical flux is limiting cross-flow filtration of stable colloids, it should be combined to kinetics aspects of slow aggregation in cases of colloids of intermediate stability (metastable). We discuss some consequences on the design and operation of processes using membrane filtration.

Role of bentonite colloids on europium and plutonium migration in a granite fracture

Abstract To establish when the presence of colloids can favour contaminant migration is a critical task for the performance assessment of a high level radioactive waste repository. In this study, the effects of the presence of bentonite colloids on the migration of highly sorbing elements, such as Eu and Pu, in a granite environment were investigated. Special efforts were made to quantify the filtration of bentonite colloids in this medium and to determine the experimental uncertainties that could bias the interpretation of the results. The migration of bentonite colloids in a granite fracture was studied in the laboratory by dynamic column experiments under low water flow rates (4–11 mL/h). The breakthrough curves of bentonite colloids always showed a peak in a similar position to conservative tracers, but the colloids recovery critically depended on their initial concentration and on the water flow rate. In the presence of colloids, the breakthrough curves of Eu and Pu always showed a peak in the same position as conservative tracers, thus indicating the migration of these radionuclides to be mostly colloid-driven. The recovery of Pu adsorbed onto the colloids was approximately that expected when the experimentally determined bentonite colloid filtration in the column was accounting for but the Eu recovery was always significantly lower.

Straining of Nonspherical Colloids in Saturated Porous Media

We explore the effects of colloid shape on straining kinetics by measuring the filtration of spherical and nonspherical colloids within saturated columns packed with quartz sand. Our observations demonstrate that the transport of peanut-shaped colloids matches the transport of spherical colloids with diameters equal to the minor-axis length of the peanut-shaped colloids. The straining rates of the spherical colloids vary linearly with the ratio of colloid diameter (d(p)) to sand-grain diameter (d(g)) for 0.0083 < d(p)/d(g) < 0.06. This linear relationship also quantifies the straining rates of the peanut-shaped particles provided that the particle's minor axis length is used for d(p). Results of pore-scale simulations reveal that a peanut-shaped particle adopts a preferred orientation as it approaches a pore-space constriction such that its major axis tends to align with the local flow direction. The extent of this reorientation increases with the particle's aspect ratio. Findings from this research suggest that straining is sensitive to changes in colloid shape and thatthe kinetics of this process can be approximated on the basis of measurable properties of the nonspherical colloids and porous media.