Collector Grain Research Articles

The effects of pH, ionic strength, and natural organics on the transport properties of carbon nanotubes in saturated porous medium

Carbon Nanotubes (CNTs) are widely utilized, and their inadvertent release into the soil and ground water systems may have detrimental environmental implications. Therefore, it is critical to comprehend the effect of several environmental conditions on the fate and transport of CNTs in porous media. In previous studies, a narrow range of pH, ionic strength, and collector grain size have only been considered while evaluating the transport and retention of functionalized multiwalled carbon nanotubes (f-MWCNTs). None have highlighted the effect of environmentally relevant concentrations of natural organic matter and particle concentrations on the mobility of f-MWCNTs in porous medium. The current study aims to fill the knowledge gap by evaluating the fate and transport of f-MWCNTs at different, environmentally relevant physicochemical conditions of pH (5, 7, and 9), ionic strengths (10, 50, and 100 mM), humic acid as representative of natural organic matter (0.1, 1, and 10 mg/L) and particle concentrations (10, 25, and 50 mg/L) through saturated porous medium (quartz sand). It was noted that a wide range of pH and humic acid imparted an overall negative charge on the particles, enhancing their mobility (C/C0 > 0.8). However, transport of the f-MWCNTs was hindered in the presence of high ionic strength and particle concentrations (C/C0 < 0.1). The transport profile was mathematically simulated using a one-dimensional advection-dispersion equation (ADE) and colloidal filtration theory (CFT). Additionally, interaction energy profiles based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory were estimated to gain a better understanding of the mechanism of f-MWCNT deposition. In light of the potential environmental consequences, the impact of real-world water systems (tap, river, lake, and ground water) on the transport behaviour of f-MWCNTs was also investigated. The study considerably advances our understanding of the various physicochemical and environmental factors that can critically impact the transport of f-MWCNT in porous media, which could prove beneficial for environmental applications and risk management.

A review on mobility of engineered carbon-based nanoparticles in porous media

Engineered nanoparticles have generated significant public and scientific excitement due to their unique physical, chemical and electrical properties, which have led to their application in a wide variety of industries. Among all these, carbon nanoparticles (CNPs) are widely manufactured nanoparticles which are utilised in a significant quantity of consumer products, such as reinforced concrete, plastics, sporting goods, electronics and biomedical applications. Due to their fast-track use, CNPs constitute a potential risk if they are released to soil and groundwater systems. Toxic effects of CNPs have been observed on the human body as well as the environment; therefore, their release and distribution into the environment has become an important topic of concern. Hence, it is essential to improve the current understanding of CNP transportation and retention into porous media. Several studies have investigated CNP mobility in packed sand columns under water-saturated conditions. This study reviews a significant number of studies which have found that CNP mobility is sensitive to a diversity of experimental conditions, including physical conditions (collector grain size, pore water velocity) and solution chemistry (ionic strength, pH). Further work should be done to understand the pattern of CNP mobility into subsurface environments considering realistic scenarios at field scale.

Modification, characterization and investigations of key factors controlling the transport of modified nano zero-valent iron (nZVI) in porous media

ABSTRACTEnhancement of nano zero-valent iron (nZVI) stability and transport in the subsurface environment is important for in situ degradation of contaminants. Various biodegradable dispersants (poly (acrylic acid) (PAA), Tween 20 and Reetha Extracts) have been tested to evaluate their effectiveness in this regard. Application of dispersants during the synthesis of nZVI have positively affected the reduction in particle size. The transport capacity in terms of fraction elution at different pore water velocities and collector grain size (filter media) was analyzed using correlation equation for the filtration model by Rajagopalan and Tien (RT model). At a surfactant concentration of 5% for PAA, Tween 20 and Reetha (Sapindus trifoliata) extracts, the lowest particle size and the highest zeta potential achieved are 8.67 nm and −55.29 mV, 75.24 nm and −62.68 mV, 61.6 nm and −37.82 mV, respectively. The trend of colloidal stability by The Derjaguin—Landau—Verwey—Overbeek (DLVO) Theory model for PAA and Reetha applied concentration was 3% > 4% > 5% > 2% > 1% > 0%. For Tween 20, modified nZVI particle shows a higher repulsive force with increasing Tween 20 concentration. Results indicated that some mechanisms such as aggregation, ripening and surface modification with different carrier pore water velocities had a considerable impact on nZVI retention in porous media. The results indicate that natural surfactant like Reetha extracts exhibits an alternative potential capacity for nZVI modification in comparison with synthetic surfactants (PAA and Tween 20).

Modelling clogging dynamism within dual-media pre-treatment rapid filters in seawater desalination

The dual-media rapid filtration is a popular pre-treatment technology in seawater desalination industry. However, it is still difficult for operators to optimize both the filtration and backwashing processes, particularly during the rapid filtering of moderate and high turbid (MHT) influents, due to the lack of a good understanding of the clogging dynamism incurred during operations. In this study, we attempt to model the incurred clogging dynamism via both experimental and computational means. For the former, experimental filtering of MHT influents was carried out within a lab-scale dual-media rapid pressure filter under varying experimental conditions. Concurrently, dimensionless analysis of the particle removal constant (RC) parameter was performed by considering the initial influent conditions and media characteristics. Good agreement was achieved between the predicted values from the dimensionless formulation of RC and the respective experimental values, which renders the likelihood of employing the formulation to improve the design of pre-treatment rapid filters for filtering MHT influents. Lastly, we attempt to model the clogging dynamism with the homogenization upscaling approach, by computationally resolving both the macroscale and microscale hydraulic gradient under clean and increasing clogging filter condition respectively to approximate the total clogging hydraulic gradient (CHG). Good agreement between the computational and semi-empirical values was achieved for the clean filter condition. Developing the solver in an open-source Open Field Operation and Manipulation (OpenFOAM) CFD software to investigate the particles’ interactions with a singular collector grain for approximating the microscale CHG is currently in progress.

Transport and retention of carbon dots (CDs) in saturated and unsaturated porous media: Role of ionic strength, pH, and collector grain size

Carbon-based engineered nanoparticles (ENPs) are widely used in consumer products due to their small size and unique physicochemical properties. Therefore, their release and distribution into the surface and subsurface environment is a subject of concern. Several studies have evaluated the transport and retention of carbon nanotubes and fullerenes, but none investigated the transport and retention of carbon dots (CDs). The aim of this research is to fill this knowledge gap by evaluating the transport and retention of CDs in saturated and unsaturated porous medium. Here, we investigate the effects of solution ionic strength (IS, 1–700 mM NaCl) and pH (4–9), the initial concentration of CDs (50–200 mg L−1), and porous media grain size (0.20–0.50 mm, 0.50–1 mm, 1–1.5 mm and 1.5–2 mm grain diameters) on the transport and retention of CDs in saturated (upward flow) and unsaturated (downward flow) quartz porous media. A mathematical model based on the advection-dispersion equation coupled with the second-order kinetics was used to fit the breakthrough curves and to calculate the attachment and straining rates under the different experimental conditions. These analyses were underpinned by characterization of CD surface functional groups, surface charge and aggregation under the different experimental conditions, calculation of CD-CD and CD-quartz sand interaction potential according to DLVO theory.Transport and retention of CDs in quartz porous media are consistent with those observed for other types of carbon-based ENPs such as fullerenes and carbon nanotubes. Mobility of CDs in both saturated and unsaturated porous media increases with the decrease in ionic strength, increase in pH, and increase in collector grain size. Retention of CDs increases with the increase in IS, decrease in pH and decrease in grain size. Generally, CDs mobility was higher under saturated than under unsaturated flow conditions, for the same experimental conditions. Overall, CDs tend to be highly mobile and could travel for long distances at a wide range of environmental conditions.

Deposition and mobilization of functionalized multiwall carbon nanotubes in saturated porous media: effect of grain size, flow velocity and solution chemistry

Carbon nanotubes (CNTs) are widely manufactured nanoparticles which are utilized in a number of consumer products, such as sporting goods, electronics and biomedical applications. Due to their accelerating production and use, CNTs constitute a potential environmental risk if they are released to soil and groundwater systems. It is, therefore, essential to improve the current understanding of environmental fate and transport of CNTs. The current study systematically investigated the effect of solution chemistry (pH and ionic strength) and physical conditions (collector grain size and flow rate) on the deposition and mobilization of functionalized multiwall carbon nanotubes (MWCNTs) using a series of column experiments under fully saturated conditions. A one-dimensional convection–dispersion model including collector efficiency for cylindrical nanoparticles was used to simulate the transport of MWCNTs in porous media. It was observed that an increase in pH resulted in increased mobility of MWCNTs. However, the transport of MWCNTs was strongly dependent on ionic strength of the background solution and a critical deposition concentration was observed between 3 and 4 mM NaCl concentration, with more than 99 % filtration of MWCNTs at 4 mM. The finer sand grains were able to filter a significant amount of MWCNTs (15 % more than coarse sand) from the inflow solution; this was likely caused by grain-to-grain straining mechanisms in the finer sand. A decrease in pore water velocity also led to more deposition of MWCNTs due to lowering of the kinetic energy of the particles. The results from this study indicated that a weak secondary minimum existed under unfavorable conditions for deposition, but the particles were trapped at both primary and secondary minimum.

Impact of Media Aging on the Removal of Cryptosporidium in Granular Media Filters

A study was conducted to assess the impact of media aging on Cryptosporidium parvum transport and retention in granular filters. Column experiments were conducted with sand, anthracite, and granular activated carbon (GAC) filters. All columns possessed an identical filter design and grain size distribution. Oocyst removal was evaluated with both new and aged media. The variation in performance among new media was attributed to the shape of the tested granular media and to the concentration of extractable metals on the surface of collector grains. Media aging significantly increased the treatment performance of sand (26%) and GAC (31%) but had no influence on the perfor- mance of anthracite. This enhanced performance of aged media correlated well with the extent of biofilm development. The best removal performance was observed with aged GAC (71 � 2%), which supported the highest amount of biomass according to polysaccharide and protein measurements. The results of this investigation are of particular interest because they suggest that media aging increases the removal of protozoan parasites in full-scale granular filters. DOI: 10.1061/(ASCE)EE.1943-7870.0000672. © 2013 American Society of Civil Engineers. CE Database subject headings: Water treatment; Filtration; Filters; Biofilm; Abatement and removal. Author keywords: Water treatment; Granular filtration; Media aging; Cryptosporidium; Biofilm.

Microhydrodynamics Simulation of Single-collector Granular Filtration

Abstract Granular filtration phenomenon is fundamentally investigated using dissipative hydrodynamics with a large collector grain and a non-Brownian sphere. Effects of interparticle forces balanced with hydrodynamic interactions are studied for particle trajectory analysis. In addition to conventional deposition of fine particles on collector surfaces (mostly near front hemispheres), the hydrodynamic trapping of rotating fine particles on the back hemisphere surface is fundamentally investigated using the exact solution of two-body hydrodynamics.

Representation of Heterogeneity in "Single Collector Efficiency" Equation for Multi Walled Carbon Nanotubes

Carbon nanotubes (type of engineered nanoparticles) are identified as a group of new and emerging contaminants. Because of their unique characteristics, novel models need to be developed in order to forecast their transport and fate in the saturated porous media. The movement of nanoparticles through saturated porous media has been previously modelled by clean-bed filtration theory. In this theory single collector efficiency (SCE) evaluates the fraction of transported particles that come into contact with the collector grain and can be removed from the aqueous phase. This equation assumes spherical geometry for nanoparticles. This assumption was identified as a short coming concerning carbon nanotubes (CNTs). To address this limitation, single collector efficiency equation was modified to accommodate the cylindrical shape of Multi-Walled Carbon Nanotubes (MWCNT), however, the underlying assumption for this empirical model is uniformity and homogeneity of the porous media. In this paper, further modification of the abovementioned equation is suggested in order to represent the heterogeneity of a natural porous media through replacing the “collector diameter”. This was achieved through utilising field measured properties such as hydraulic conductivity, porosity, and grain-size distribution. The resulting equation overcomes the limitations of current approaches and shows remarkable agreement with exact theoretical predictions of the single collector efficiency over a range of conditions commonly encountered in natural groundwater systems. Furthermore, the theoretical effect of natural heterogeneity on the movement of CNTs in saturated porous media is assessed. It was established that the use of an average grain size can greatly over-estimate the movement of CNTs while representing heterogeneity through the modified equation reduces the modelled mobility of CNTs. In addition, increasing heterogeneity (smaller uniformity coefficient) resulted in mobility reduction for CNTs.

Impact of Porous Media Grain Size on the Transport of Multi-walled Carbon Nanotubes

Nanoparticles possess unique physical, electrical, and chemical properties which make them attractive for use in a wide range of consumer products. Through their manufacturing, usage, and eventual disposal, nanoparticles are expected to ultimately be released to the environment after which point they may pose environmental and human health risks. One critical component of understanding and modeling those potential risks is their transport in the subsurface environment. This study investigates the mobility of one important nanoparticle (multi-walled carbon nanotubes or MWCNTs) through porous media, and makes the first measurements on the impact of mean collector grain size (d(50)) on MWCNT retention. Results from one-dimensional column experiments conducted under various physical and chemical conditions coupled with results of numerical modeling assessed the suitability of traditional transport models to predict MWCNT mobility. Findings suggest that a dual deposition model coupled with site blocking greatly improves model fits compared to traditional colloid filtration theory. Of particular note is that the MWCNTs traveled through porous media ranging in size from fine sand to silt resulting in normalized concentrations of MWCNTs in the effluent in excess of 60% of the influent concentration.

Anionic Homopolymers Efficiently Target Zerovalent Iron Particles to Hydrophobic Contaminants in Sand Columns

The transport of microscale carbonyl iron powder suspensions modified with anionic homopolymers was studied in water-saturated sand columns containing well-dispersed hydrophobic sand grains. Sand grains functionalized with hexadecyltrimethoxysilane were coated with a eutectic mixture of dichlorobenzenes that was solid at -10 °C and was mixed by grinding with unmodified sand grains. The dichlorobenzene coating liquefied at the temperature of the transport experiments, and the coated grains were thus mimetic of uniform droplets of dense nonaqueous phase liquid (DNAPL) contaminants. By comparing iron particle transport in uncontaminated columns with those that contained a small fraction of DNAPL-coated sand grains, sticking coefficients for both types of grains could be estimated. The anionic polyelectrolytes tested (polyacrylate, carboxymethylcellulose, alginate, and metasilicate) all gave low particle sticking coefficients (0.004-0.05) to unmodified sand, as expected from earlier studies. However, iron particles modified with the two moderately hydrophobic polymers (carboxymethylcellulose and polyacrylate) had 30-fold higher sticking coefficients (0.40 and 0.13, respectively) to the model DNAPL surface than they did to the sand surface. In contrast, no significant difference between the two kinds of collector grains was found with the more polar polymers (metasilicate and alginate). The trend in sticking coefficients was correlated with the surface energy of the polymer-modified iron surface as measured by the static contact angle method. From these data one can conclude that the hydrophobicity of the polymer dispersant is a key factor in targeting zerovalent iron to DNAPL source zones in soil and groundwater.

Injectivity Decline From Produced-Water Reinjection: New Insights on In-Depth Particle-Deposition Mechanisms

Summary Injectivity decline during produced-water reinjection (PWRI) originates not only from filter-cake buildup but also from in-depth deposition of oil droplets or solid particles. Physical modeling of particle-deposition mechanisms in porous media is thus of key interest for optimizing PWRI operations. The present work brings new insights on oil-droplet and solid-particle-deposition mechanisms in porous media. The experimental conditions were selected such that the ratio between pores and particle sizes is sufficiently large to ensure in-depth propagation. The parameters are the nature of the particles injected and a Peclet number calculated on the size of the collector grains (Peg) that encompasses in a nondimensional form the impact of both the flow rate and the particle size. The results are analyzed within the framework of the "colloidal approach." For oil droplets and solid particles, the collection efficiency (η) shows a transition from a behavior in which η varies as a power law of Peg, with exponent values −⅔ [(diffusion-limited deposition (DLD)] to −1 [reaction-limited deposition (RLD)] that are typical of the convection/diffusion regime, to a behavior characterized by an increase of η vs. Peg, typical of the hydrodynamic deposition regime. In the case of oil droplets (slightly charged), the transition occurs at a critical Peg value, PegC≈PegCgeom/10, corresponding to a diffusion-layer thickness around the collector grain of the same order of magnitude as the droplet diameter. In the case of electrosterically stabilized solid particles, the transition takes place at PegC≪PegCgeom for small particles and at PegC&amp;gt;PegCgeom for larger particles.

Transport and fate of bacteria in porous media: Coupled effects of chemical conditions and pore space geometry

Experimental and theoretical studies were undertaken to explore the coupled effects of chemical conditions and pore space geometry on bacteria transport in porous media. The retention of Escherichia coli D21g was investigated in a series of batch and column experiments with solutions of different ionic strength (IS) and ultrapure quartz sand. Derjaguin–Landau–Verwey–Overbeek (DLVO) calculations and results from batch experiments suggested that bacteria attachment to the sand surface was negligible when the IS was less than or equal to 50 mM. Breakthrough data from column experiments showed significant cell retention and was strongly dependent on the IS. This finding indicates that cell retention was dependent on the depth of the secondary energy minimum which increases with IS. When the IS of the influent bacteria‐free solution was decreased to 1 mM, only a small fraction of the retained bacteria was released from the column. The remaining retained bacteria, however, were recovered from the sand, which was excavated from the column and suspended in a cell‐free electrolyte having the original IS. These observations suggest that the solution chemistry is not the only parameter controlling bacteria retention in the porous media. Computational simulations of flow around several collector grains revealed the retention mechanism, which is dependent on both the solution chemistry and the pore space geometry. Simulations demonstrate that the pore space geometry creates hydrodynamically disconnected regions. The number of bacterial cells that may be transported to these relatively “immobile” regions will theoretically be dependent on the depth of the secondary energy minimum (i.e., the IS). Once bacteria are trapped in these immobile regions, reduction of the secondary energy minimum does not necessarily release the cells owing to hydrodynamic constraints.

Coupling of physical and chemical mechanisms of colloid straining in saturated porous media

Filtration theory does not include the potential influence of pore structure on colloid removal by straining. Conversely, previous research on straining has not considered the possible influence of chemical interactions. Experimental and theoretical studies were therefore undertaken to explore the coupling of physical and chemical mechanisms of colloid straining under unfavorable attachment conditions (pH=10). Negatively charged latex microspheres (1.1 and 3 μm) and quartz sands (360, 240, and 150 μm) were used in packed column studies that encompassed a range in suspension ionic strengths (6–106 mM) and Darcy water velocities (0.1–0.45 cm min −1). Derjaguin–Landau–Verwey–Overbeek (DLVO) calculations and torque analysis suggests that attachment of colloids to the solid–water interface was not a significant mechanism of deposition for the selected experimental conditions. Effluent concentration curves and hyperexponential deposition profiles were strongly dependent on the solution chemistry, the system hydrodynamics, and the colloid and collector grain size, with greater deposition occurring for increasing ionic strength, lower flow rates, and larger ratios of the colloid to the median grain diameter. Increasing the solution ionic strength is believed to increase the force and number of colloids in the secondary minimum of the DLVO interaction energy profile. These weakly associated colloids can be funneled to small regions of the pore space formed adjacent to grain–grain junctions. For select systems, the ionic strength of the eluant solution was decreased to 6 mM following the recovery of the effluent concentration curve. In this case, only a small portion of the deposited colloids was recovered in the effluent and the majority was still retained in the sand. These observations suggest that the extent of colloid removal by straining is strongly coupled to solution chemistry.

Surface Modifications Enhance Nanoiron Transport and NAPL Targeting in Saturated Porous Media

Rapid in situ degradation of chlorinated solvents present as nonaqueous phase liquids (NAPL) can be accomplished using reactive zerovalent nanoiron particles. Prior studies have shown that nanoiron transport in the subsurface is limited, and successful delivery of the nanoiron is essential for effective remediation. Here, the physical properties of bare and modified nanoiron are measured, and laboratory column reactors are used to compare the transport of three types of surface-modified nanoiron; triblock polymer-modified, surfactant-modified, and a commercially available polymer-modified nanoiron. The effect of particle concentration and solution ionic strength on the transport of each modified nanoiron is evaluated, and the filtration mechanisms for bare and modified particles are determined in microfluidic flow cells and quartz crystal microbalance (QCM) experiments that probe the particle–collector grain interaction. The effect of surface modification on nanoiron reactivity is evaluated in batch experiments. Transport of modified nanoiron does not directly correlate with ζ-potential or colloidal stability, but rather correlates to particle– grain interactions. Filtration of bare nanoiron is caused by straining and subsequent clogging rather than by deposition to clean sand grains, suggesting that filter ripening models rather than clean bed filtration models should be used to describe nanoiron transport at high particle concentrations. Surface modification decreased nanoiron reactivity by two to four times, but as high as a factor of nine depending on the modifier used. Amphiphilic triblock copolymer modified nanoiron with a high hydrophobe/hydrophile ratio shows promise for in situ targeting of NAPL, but requires further optimization.

Transport and Adhesion of Escherichia coli JM109 in Soil Aquifer Treatment (SAT): One-Dimensional Column Study

Bacteria transport and adhesion experiments under water-saturated and partially saturated conditions were examined over a wide range of ionic strength, from 1 to 100 mM KCl, CaCl(2), and MgCl(2), and at water contents of 0.15 and 0.22 in sand columns packed with three different sands, baked, sterilized, and raw sands in order to investigate the effects of ionic strength, water content, and porous media type on the microbial adhesion in soil aquifer treatment (SAT). Well-characterized Escherichia coli JM109 were used as model bacterial cells in this study. Column study results showed that bacterial deposition rates increased with increasing ionic strength and decreasing water content, and were higher in raw sand columns than those in other sand columns. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was applied to experimental results in order to consider the interaction energies between the bacterial cells and collector grains; results revealed that a considerable amount of bacterial cells was weakly deposited onto the solid surfaces in secondary minimum.

Virus transport in physically and geochemically heterogeneous subsurface porous media

A two-dimensional model for virus transport in physically and geochemically heterogeneous subsurface porous media is presented. The model involves solution of the advection–dispersion equation, which additionally considers virus inactivation in the solution, as well as virus removal at the solid matrix surface due to attachment (deposition), release, and inactivation. Two surface inactivation models for the fate of attached inactive viruses and their subsequent role on virus attachment and release were considered. Geochemical heterogeneity, portrayed as patches of positively charged metal oxyhydroxide coatings on collector grain surfaces, and physical heterogeneity, portrayed as spatial variability of hydraulic conductivity, were incorporated in the model. Both layered and randomly (log-normally) distributed physical and geochemical heterogeneities were considered. The upstream weighted multiple cell balance method was employed to numerically solve the governing equations of groundwater flow and virus transport. Model predictions show that the presence of subsurface layered geochemical and physical heterogeneity results in preferential flow paths and thus significantly affect virus mobility. Random distributions of physical and geochemical heterogeneity have also notable influence on the virus transport behavior. While the solution inactivation rate was found to significantly influence the virus transport behavior, surface inactivation under realistic field conditions has probably a negligible influence on the overall virus transport. It was further demonstrated that large virus release rates result in extended periods of virus breakthrough over significant distances downstream from the injection sites. This behavior suggests that simpler models that account for virus adsorption through a retardation factor may yield a misleading assessment of virus transport in “hydrogeologically sensitive” subsurface environments.

Analysis of an observed relationship between colloid collision efficiency and mean collector grain size

Previous investigators have attempted to use filtration theory to deconvolute the physical and chemical factors that influence colloid filtration in porous media. In the classic single-collector approach, bulk physical properties of the colloids and the medium are assumed to control the rate of colloid–collector collisions, while solution and surface chemical properties are assumed to control the probability of attachment. However, this assumed separation of physical and chemical variables will not apply when collector surface properties also vary with grain size. In recent work, we conducted column experiments with identical colloids and solution conditions, but with sand fractions of different mean grain size. The results indicated that both physical and chemical parameters varied with the collector grain size. However, this analysis was dependent on the particular model used to calculate the rate of colloid–collector collisions (the collector efficiency). To demonstrate that the variation of chemical parameters with grain size is not dependent on a particular physical transport model, in this work we apply and compare three different models for colloid filtration. The assumption of a constant attachment probability (collision efficiency) was found to be inconsistent with the experimental results. Instead, good results were obtained by considering that the collision efficiency varies linearly with the natural logarithm of the hydraulic conductivity of the collector medium. A linear relationship of this type was found for all three colloid filtration models. This analysis confirms that our data show a relationship between collector grain size, hydraulic conductivity, and attachment probability, and that the existence of such a relationship is not dependent on any particular model for colloid filtration.

Correlation of colloid collision efficiency with hydraulic conductivity of silica sands

Column experiments were conducted in the laboratory to study the relationship between local‐scale colloid filtration parameters and physical heterogeneity as represented by soil or rock hydraulic conductivity. Natural (polydisperse) sand was sieved into distinct classes to produce relatively homogeneous porous media for the experiments. Silica particles 0.45 μm in diameter were used as the transported colloids. The column experiments were carried out under carefully controlled chemical conditions. The hydraulic conductivity of each packed column was determined using a constant‐head permeameter arrangement. Conservative transport parameters (dispersivity and effective porosity) were obtained using a salt tracer. Comparison of solute and colloid breakthrough curves indicated that particle deposition rates were unsteady in the column. This behavior was analyzed using a blocking function which describes the alteration of the sand surface due to the accumulation of deposited colloids. By this method, clean‐bed collision efficiency factors were obtained for colloid filtration in each of the sand sizes. An inverse relationship was observed between the collision efficiency factor and the grain size of the collector medium. This relationship was attributed to a variation of surface heterogeneity with sand particle size. This theory is supported by the direct correlation of collision efficiency with surface heterogeneity parameters of the blocking function. The colloid filtration data were used to parameterize a linear correlation between the collision efficiency factor and the natural logarithm of the medium hydraulic conductivity. The collector efficiency, normally calculated from theoretical models for colloid transport in the vicinity of collector grains, was also shown to be a linear function of the natural logarithm of hydraulic conductivity. Correlations of this type can be used effectively in stochastic modeling of colloid transport through heterogeneous porous media.

The “Shadow Effect” in Colloid Transport and Deposition Dynamics in Granular Porous Media: Measurements and Mechanisms

The role of hydrodynamic and colloidal interactions in the transport and deposition dynamics of colloidal particles in granular porous media is systematically investigated. Colloid transport experiments were conducted with three suspensions of positively charged colloidal latex particles (133, 288, and 899 nm in diameter) and negatively charged packed quartz grains. The column experiments were carried out under a wide range of solution ionic strengths (10-5.5−10-2.0 M) and approach velocities (10-4.5−10-2.5 m/s) until a complete breakthrough was attained, thus allowing unambiguous determination of the maximum attainable surface coverage for each deposition run. Results show that the rate of blocking and the maximum attainable surface coverage are determined by a unique interplay between flow intensity, particle size, and solution ionic strength. It is suggested that the tangential or shear component of the fluid flow around collector grains creates a “shadow zone” on the collector surface down gradient of deposited particles where the probability of subsequent deposition is substantially reduced. The shadow zone is determined by the combined effect of hydrodynamic interaction and electrostatic double layer repulsion. Increasing the approach velocity and particle size and decreasing the solution ionic strength result in a larger area of the shadow zone and hence reduced maximum attainable surface coverages. It is also proposed that sand grain surface roughness influences the dynamics of particle deposition by creating shadow zones down gradient of surface protrusions where particle deposition is significantly hindered.