Colloid Filtration Theory Research Articles

Impact of manure-related DOM on sulfonamide transport in arable soils

Field application of livestock manure introduces colloids and veterinary antibiotics, e.g. sulfonamides (SAs), into farmland. The presence of manure colloids may potentially intensify the SAs-pollution to soils and groundwater by colloid-facilitated transport. Transport of three SAs, sulfadiazine (SDZ), sulfamethoxypyridazine (SMPD), and sulfamoxole (SMOX), was investigated in saturated soil columns with and without manure colloids from sows and farrows, weaners, and fattening pigs. Experimental results showed that colloid-facilitated transport of SMOX was significant in the presence of manure colloids from fattening pigs with low C/N ratio, high SUVA280nm and protein C, while manure colloids from sows and farrows and weaners had little effect on SMOX transport. In contrast, only retardation was observed for SDZ and SMPD when manure colloids were present. Breakthrough curves (BTCs) of colloids and SAs were replicated well by a newly developed numerical model that considers colloid-filtration theory, competitive kinetic sorption, and co-transport processes. Model results demonstrate that mobile colloids act as carriers for SMOX, while immobile colloids block SMOX from sorbing onto the soil. The low affinity of SMOX to sorb on immobile colloids prevents aggregation and also promotes SMOX's colloid-facilitated transport. Conversely, the high affinity of SDZ and SMPD to sorb on all types of immobile colloids retarded their transport. Thus, manure properties play a fundamental role in increasing the leaching risk of hydrophobic sulfonamides.

微生物在饱和多孔介质中的迁移

微生物在多孔介质中的迁移是水文与水环境研究的热点问题。对该问题的深入探讨有助于遏制水传播疾病，提高受污染地下水微生物修复的效率。过去十五年来，对微生物在饱和多孔介质中迁移问题的研究获得很大进展。然而，传统的DLVO理论和胶体过滤理论解释微生物在多孔介质中的迁移过程时存在明显的不足。沥滤、异质性、优先流、微生物特征及生长衰亡过程等对生物胶体迁移存在显著的影响，其实验机理研究和模型模拟研究需要进一步深入。实验室的研究成果往往与野外条件下的不一致，这是加强野外研究的客观要求。野外实验不可控因素多，相对于实验室实验更加复杂。加强野外条件下的研究有助于深入理解生物胶体迁移的复杂规律，促进机理研究和模型模拟的发展。考虑多种机理的模型有复杂化的趋势，如何识别主要机理、简化模型、提高效率是模型研究要注意的问题。 Microbial transport in porous media is a hot issue in fields of hydrology and water environment. Re-searches on the issue are helpful to restrain water-borne diseases and to improve the efficiency of polluted groundwater recovery. Over the past 15 years, the understanding about microbial transport in a saturated porous media has been improved substantially. However, the DLVO theory and the colloid filtration theory cannot support a sufficient explanation in microbial transport. Straining, heterogeneity, pore scale force/ torque balance, microbial features, growth and death significantly affect the microbial transport of which mechanisms and modeling need to be enhanced in future studies. In addition, results from laboratorial studies are mostly different with field results, which highlights the necessity to do more field researches. In field experiments, there are many uncontrollable factors and complex situations compared with labora-torial experiments. More field experiments will offer valuable chances to get a thorough comprehension of microbial transport promoting development of transport mechanisms and models. New models tend to become more complicated when trying to integrate more mechanisms. Therefore, how to find the main mechanisms simplifying models and improving efficiency is also a key question.

An analytical model for nanoparticles concentration resulting from infusion into poroelastic brain tissue

We consider the infusion of a diluted suspension of nanoparticles (NPs) into poroelastic brain tissue, in view of relevant biomedical applications such as intratumoral thermotherapy. Indeed, the high impact of the related pathologies motivates the development of advanced therapeutic approaches, whose design also benefits from theoretical models.This study provides an analytical expression for the time-dependent NPs concentration during the infusion into poroelastic brain tissue, which also accounts for particle binding onto cells (by recalling relevant results from the colloid filtration theory). Our model is computationally inexpensive and, compared to fully numerical approaches, permits to explicitly elucidate the role of the involved physical aspects (tissue poroelasticity, infusion parameters, NPs physico-chemical properties, NP-tissue interactions underlying binding). We also present illustrative results based on parameters taken from the literature, by considering clinically relevant ranges for the infusion parameters. Moreover, we thoroughly assess the model working assumptions besides discussing its limitations.While not laying any claims of generality, our model can be used to support the development of more ambitious numerical approaches, towards the preliminary design of novel therapies based on NPs infusion into brain tissue.

The impact of immobile zones on the transport and retention of nanoparticles in porous media

AbstractNanoparticle transport and retention within porous media is treated by conceptualizing the porous media as a series of independent collectors (e.g., Colloid Filtration Theory). This conceptualization assumes that flow phenomena near grain‐grain contacts, such as immobile zones (areas of low flow), exert a negligible influence on nanoparticle transport and assumes that retention and release of particles depends only on surface chemistry. This study investigated the impact of immobile zones on nanoparticle transport and retention by employing synchrotron X‐ray computed microtomography (SXCMT) to examine pore‐scale silver nanoparticle distributions during transport through three sand columns: uniform iron oxide, uniform quartz, and well‐graded quartz. Extended tailing was observed during the elution phase of all experiments suggesting that hydraulic retention in immobile zones, not detachment from grains, was the source of tailing. A numerical simulation of fluid flow through an SXCMT data set predicted the presence of immobile zones near grain‐grain contacts. SXCMT‐determined silver nanoparticle concentrations observed that significantly lower nanoparticle concentrations existed near grain‐grain contacts throughout the duration of all experiments. In addition, the SXCMT‐determined pore‐scale concentration gradients were found to be independent of surface chemistry and grain size distribution, suggesting that immobile zones limit the diffusive transport of nanoparticles toward the collectors. These results suggest that the well‐known overprediction of nanoparticle retention by traditional CFT may be due to ignoring the influences of grain‐grain contacts and immobile zones. As such, accurate prediction of nanoparticle transport requires consideration of immobile zones and their influence on both hydraulic and surface retention.

Impact of biofilm on bacterial transport and deposition in porous media

Laboratory scale experiments were conducted to obtain insights into factors that influence bacterial transport and deposition in porous media. According to colloidal filtration theory, the removal efficiency of a filter medium is characterized by two main factors: collision efficiency and sticking efficiency. In the case of bacterial transport in porous media, bacteria attached to a solid surface can establish a thin layer of biofilm by excreting extracellular polymeric substances which can significantly influence both of these factors in a porous medium, and thus, affect the overall removal efficiency of the filter medium. However, such polymeric interactions in bacterial adhesion are not well understood and a method to calculate polymeric interactions is not yet available. Here, to determine how the migration of bacteria flowing within a porous medium is affected by the presence of surface-associated extracellular polymeric substances previously produced and deposited by the same bacterial species, a commonly used colloidal filtration model was applied to study transport and deposition of Pseudomonas fluorescens in small-scale columns packed with clean and biofilm coated glass beads. Bacterial recoveries were monitored in column effluents and used to quantify biofilm interactions and sticking efficiencies of the biofilm coated packed-beds. The results indicated that, under identical hydraulic conditions, the sticking efficiencies in packed-beds were improved consistently by 36% when covered by biofilm.

Correlation equations for average deposition rate coefficients of nanoparticles in a cylindrical pore

AbstractNanoparticle deposition behavior observed at the Darcy scale represents an average of the processes occurring at the pore scale. Hence, the effect of various pore‐scale parameters on nanoparticle deposition can be understood by studying nanoparticle transport at pore scale and upscaling the results to the Darcy scale. In this work, correlation equations for the deposition rate coefficients of nanoparticles in a cylindrical pore are developed as a function of nine pore‐scale parameters: the pore radius, nanoparticle radius, mean flow velocity, solution ionic strength, viscosity, temperature, solution dielectric constant, and nanoparticle and collector surface potentials. Based on dominant processes, the pore space is divided into three different regions, namely, bulk, diffusion, and potential regions. Advection‐diffusion equations for nanoparticle transport are prescribed for the bulk and diffusion regions, while the interaction between the diffusion and potential regions is included as a boundary condition. This interaction is modeled as a first‐order reversible kinetic adsorption. The expressions for the mass transfer rate coefficients between the diffusion and the potential regions are derived in terms of the interaction energy profile. Among other effects, we account for nanoparticle‐collector interaction forces on nanoparticle deposition. The resulting equations are solved numerically for a range of values of pore‐scale parameters. The nanoparticle concentration profile obtained for the cylindrical pore is averaged over a moving averaging volume within the pore in order to get the 1‐D concentration field. The latter is fitted to the 1‐D advection‐dispersion equation with an equilibrium or kinetic adsorption model to determine the values of the average deposition rate coefficients. In this study, pore‐scale simulations are performed for three values of Péclet number, Pe = 0.05, 5, and 50. We find that under unfavorable conditions, the nanoparticle deposition at pore scale is best described by an equilibrium model at low Péclet numbers (Pe = 0.05) and by a kinetic model at high Péclet numbers (Pe = 50). But, at an intermediate Pe (e.g., near Pe = 5), both equilibrium and kinetic models fit the 1‐D concentration field. Correlation equations for the pore‐averaged nanoparticle deposition rate coefficients under unfavorable conditions are derived by performing a multiple‐linear regression analysis between the estimated deposition rate coefficients for a single pore and various pore‐scale parameters. The correlation equations, which follow a power law relation with nine pore‐scale parameters, are found to be consistent with the column‐scale and pore‐scale experimental results, and qualitatively agree with the colloid filtration theory. These equations can be incorporated into pore network models to study the effect of pore‐scale parameters on nanoparticle deposition at larger length scales such as Darcy scale.

Predicting colloid transport through saturated porous media: A critical review

Understanding and predicting colloid transport and retention in water-saturated porous media is important for the protection of human and ecological health. Early applications of colloid transport research before the 1990s included the removal of pathogens in granular drinking water filters. Since then, interest has expanded significantly to include such areas as source zone protection of drinking water systems and injection of nanometals for contaminated site remediation. This review summarizes predictive tools for colloid transport from the pore to field scales. First, we review experimental breakthrough and retention of colloids under favorable and unfavorable colloid/collector interactions (i.e., no significant and significant colloid-surface repulsion, respectively). Second, we review the continuum-scale modeling strategies used to describe observed transport behavior. Third, we review the following two components of colloid filtration theory: (i) mechanistic force/torque balance models of pore-scale colloid trajectories and (ii) approximating correlation equations used to predict colloid retention. The successes and limitations of these approaches for favorable conditions are summarized, as are recent developments to predict colloid retention under the unfavorable conditions particularly relevant to environmental applications. Fourth, we summarize the influences of physical and chemical heterogeneities on colloid transport and avenues for their prediction. Fifth, we review the upscaling of mechanistic model results to rate constants for use in continuum models of colloid behavior at the column and field scales. Overall, this paper clarifies the foundation for existing knowledge of colloid transport and retention, features recent advances in the field, critically assesses where existing approaches are successful and the limits of their application, and highlights outstanding challenges and future research opportunities. These challenges and opportunities include improving mechanistic descriptions, and subsequent correlation equations, for nanoparticle (i.e., Brownian particle) transport through soil, developing mechanistic descriptions of colloid retention in so-called “unfavorable” conditions via methods such as the “discrete heterogeneity” approach, and employing imaging techniques such as X-ray tomography to develop realistic expressions for grain topology and mineral distribution that can aid the development of these mechanistic approaches.

Surface Roughness Impacts on Granular Media Filtration at Favorable Deposition Conditions: Experiments and Modeling.

Column tests were conducted to investigate media roughness impacts on particle deposition in absence of an energy barrier (i.e., high ionic strength). Media/collector surface roughness consistently influenced colloid deposition in a nonlinear, nonmonotonic manner such that a critical roughness size associated with minimum particle deposition could be identified; this was confirmed using a convection-diffusion model. The results demonstrate that media surface roughness size alone is inadequate for predicting media roughness impacts on particle deposition; rather, the relative size relationship between the particles and media/collectors must also be considered. A model that quantitatively considers media surface roughness was developed that described experimental outcomes well and consistently with classic colloid filtration theory (CFT) for smooth surfaces. Dimensionless-scaling factors froughness and fPCIF were introduced and used to develop a model describing particle deposition rate (kd) and colloid attachment efficiency (α). The model includes fitting parameters that reflect the impact of critical system characteristics such as ionic strength, loading rate, hydrophobicity. Excellent agreement was found not only between the modeled outcomes for colloid attachment efficiency (α) and experimental results from the column tests, but also with experimental outcomes reported elsewhere. The model developed herein provides a framework for describing media surface roughness impacts on colloid deposition.

Transport of carboxyl-functionalized carbon black nanoparticles in saturated porous media: Column experiments and model analyses

The aim of this study was to investigate the transport behavior of carboxyl-functionalized carbon black nanoparticles (CBNPs) in porous media including quartz sand, iron oxide-coated sand (IOCS), and aluminum oxide-coated sand (AOCS). Two sets of column experiments were performed under saturated flow conditions for potassium chloride (KCl), a conservative tracer, and CBNPs. Breakthrough curves were analyzed to obtain mass recovery and one-dimensional transport model parameters. The first set of experiments was conducted to examine the effects of metal (Fe, Al) oxides and flow rate (0.25 and 0.5mLmin−1) on the transport of CBNPs suspended in deionized water. The results showed that the mass recovery of CBNPs in quartz sand (flow rate=0.5mLmin−1) was 83.1%, whereas no breakthrough of CBNPs (mass recovery=0%) was observed in IOCS and AOCS at the same flow rate, indicating that metal (Fe, Al) oxides can play a significant role in the attachment of CBNPs to porous media. In addition, the mass recovery of CBNPs in quartz sand decreased to 76.1% as the flow rate decreased to 0.25mLmin−1. Interaction energy profiles for CBNP–porous media were calculated using DLVO theory for sphere–plate geometry, demonstrating that the interaction energy for CBNP–quartz sand was repulsive, whereas the interaction energies for CBNP–IOCS and CBNP–AOCS were attractive with no energy barriers. The second set of experiments was conducted in quartz sand to observe the effect of ionic strength (NaCl=0.1 and 1.0mM; CaCl2=0.01 and 0.1mM) and pH (pH=4.5 and 5.4) on the transport of CBNPs suspended in electrolyte. The results showed that the mass recoveries of CBNPs in NaCl=0.1 and 1.0mM were 65.3 and 6.4%, respectively. The mass recoveries of CBNPs in CaCl2=0.01 and 0.1mM were 81.6 and 6.3%, respectively. These results demonstrated that CBNP attachment to quartz sand can be enhanced by increasing the electrolyte concentration. Interaction energy profiles demonstrated that the interaction energy profile for CBNP–quartz sand was compressed and that the energy barrier decreased as the electrolyte concentration increased. Furthermore, the mass recovery of CBNPs in the presence of divalent ions (CaCl2=0.1mM) was far lower than that in the presence of monovalent ions (NaCl=0.1mM), demonstrating a much stronger effect of Ca2+ than Na+ on CBNP transport. Mass recovery of CBNPs at pH4.5 was 55.6%, which was lower than that (83.1%) at pH5.4, indicating that CBNP attachment to quartz sand can be enhanced by decreasing the pH. The sticking efficiencies (α) calculated from the mass recovery by colloid filtration theory were in the range from 2.1×10−2 to 4.5×10−1, which were far greater than the values (2.56×10−6–3.33×10−2) of theoretical sticking efficiencies (αtheory) calculated from the DLVO energy by the Maxwell model.

Retention and transport of an anaerobic trichloroethene dechlorinating microbial culture in anaerobic porous media

The influence of solution chemistry on microbial transport was examined using the strictly anaerobic trichloroethene (TCE) bioaugmentation culture KB-1®. A column was employed to determine transport behaviors and deposition kinetics of three distinct functional species in KB-1®, Dehalococcoides, Geobacter, and Methanomethylovorans, over a range of ionic strengths under a well-controlled anaerobic condition. A quantitative polymerase chain reaction (qPCR) was utilized to enumerate cell concentration and complementary techniques were implemented to evaluate cell surface electrokinetic potentials. Solution chemistry was found to positively affect the deposition rates, which was consistent with calculated Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction energies. Retained microbial profiles showed spatially constant colloid deposition rate coefficients, in agreement with classical colloid filtration theory (CFT). It was interesting to note that the three KB-1® species displayed similar transport and retention behaviors under the defined experimental conditions despite their different cell electrokinetic properties. A deeper analysis of cell characteristics showed that factors, such as cell size and shape, concentration, and motility were involved in determining adhesion behavior.

Transport of Human Adenoviruses in Water Saturated Laboratory Columns.

Groundwater may be contaminated with infective human enteric viruses from various wastewater discharges, sanitary landfills, septic tanks, agricultural practices, and artificial groundwater recharge. Coliphages have been widely used as surrogates of enteric viruses, because they share many fundamental properties and features. Although a large number of studies focusing on various factors (i.e. pore water solution chemistry, fluid velocity, moisture content, temperature, and grain size) that affect biocolloid (bacteria, viruses) transport have been published over the past two decades, little attention has been given toward human adenoviruses (hAdVs). The main objective of this study was to evaluate the effect of pore water velocity on hAdV transport in water saturated laboratory-scale columns packed with glass beads. The effects of pore water velocity on virus transport and retention in porous media was examined at three pore water velocities (0.39, 0.75, and 1.22cm/min). The results indicated that all estimated average mass recovery values for hAdV were lower than those of coliphages, which were previously reported in the literature by others for experiments conducted under similar experimental conditions. However, no obvious relationship between hAdV mass recovery and water velocity could be established from the experimental results. The collision efficiencies were quantified using the classical colloid filtration theory. Average collision efficiency, α, values decreased with decreasing flow rate, Q, and pore water velocity, U, but no significant effect of U on α was observed. Furthermore, the surface properties of viruses and glass beads were used to construct classical DLVO potential energy profiles. The results revealed that the experimental conditions of this study were unfavorable to deposition and that no aggregation between virus particles is expected to occur. A thorough understanding of the key processes governing virus transport is pivotal for public health protection.

Experimental investigation of virus and clay particles cotransport in partially saturated columns packed with glass beads

Suspended clay particles in groundwater can play a significant role as carriers of viruses, because, depending on the physicochemical conditions, clay particles may facilitate or hinder the mobility of viruses. This experimental study examines the effects of clay colloids on the transport of viruses in variably saturated porous media. All cotransport experiments were conducted in both saturated and partially saturated columns packed with glass beads, using bacteriophages MS2 and ΦX174 as model viruses, and kaolinite (KGa-1b) and montmorillonite (STx-1b) as model clay colloids. The various experimental collision efficiencies were determined using the classical colloid filtration theory. The experimental data indicated that the mass recovery of viruses and clay colloids decreased as the water saturation decreased. Temporal moments of the various breakthrough concentrations collected, suggested that the presence of clays significantly influenced virus transport and irreversible deposition onto glass beads. The mass recovery of both viruses, based on total effluent virus concentrations, was shown to reduce in the presence of suspended clay particles. Furthermore, the transport of suspended virus and clay-virus particles was retarded, compared to the conservative tracer. Under unsaturated conditions both clay particles facilitated the transport of ΦX174, while hindered the transport of MS2. Moreover, the surface properties of viruses, clays and glass beads were employed for the construction of classical DLVO and capillary potential energy profiles, and the results suggested that capillary forces play a significant role on colloid retention. It was estimated that the capillary potential energy of MS2 is lower than that of ΦX174, and the capillary potential energy of KGa-1b is lower than that of STx-1b, assuming that the protrusion distance through the water film is the same for each pair of particles. Moreover, the capillary potential energy is several orders of magnitude greater than the DLVO potential energy.

Hyperexponential and nonmonotonic retention of polyvinylpyrrolidone-coated silver nanoparticles in an Ultisol

The increasing application of engineered nanoparticles (ENPs) has heightened the concern that these ENPs would eventually be released to the environment and may enter into life cycle of living beings. In this regard, it is essential to understand how these ENPs transport and retain in natural soils because they are considered to be a major repository for ENPs. Herein, transport and retention of polyvinylpyrrolidone (PVP)-coated silver nanoparticles (PVP-AgNPs) were investigated over a wide range of physicochemical factors in water-saturated columns packed with an Ultisol rich in clay-size particles. Higher mobility of PVP-AgNPs occurred at larger soil grain size, lower solution ionic strength and divalent cation concentration, higher flow rate, and greater PVP concentrations. Most breakthrough curves (BTCs) for PVP-AgNPs exhibited significant amounts of retardation in the soil due to its large surface area and quantity of retention sites. In contrast to colloid filtration theory, the shapes of retention profiles (RPs) for PVP-AgNPs were either hyperexponential or nonmonotonic (a peak in particle retention down-gradient from the column inlet). The BTCs and hyperexponential RPs were successfully described using a 1-species model that considered time- and depth-dependent retention. Conversely, a 2-species model that included reversibility of retained PVP-AgNPs had to be employed to better simulate the BTCs and nonmonotonic RPs. As the retained concentration of species 1 approached the maximum solid-phase concentration, a second mobile species (species 2, i.e., the same PVP-AgNPs that are reversibly retained) was released that could be retained at a different rate than species 1 and thus yielded the nonmonotonic RPs. Some retained PVP-AgNPs were likely to irreversibly deposit in the primary minimum associated with microscopic chemical heterogeneity (favorable sites). Transmission electron microscopy and energy-dispersive X-ray spectroscopy analysis suggested that these favorable sites were positively charged sites on montmorillonite edges and goethite surfaces in the soil. Overall, our study highlights that the transport and especially retention of PVP-AgNPs are highly sensitive to the physicochemical factors, but mathematical modeling can accurately predict the fate of these ENPs in porous media which is important for better understanding the fate of these ENPs in point of exit and in the environment.

Column study for the evaluation of the transport properties of polyphenol-coated nanoiron

Injection of a nano zero valent iron (nZVI) suspension in the subsurface is a remedial option for obtaining the in situ reduction and immobilization of hexavalent chromium in contaminated aquifers. Prerequisite for the successful implementation of this technology is that the nanoparticles form a stable colloidal suspension with good transport properties when delivered in the subsurface. In this study we produced stable suspensions of polyphenol-coated nZVI (GT-nZVI) and we evaluated their transport behavior through representative porous media. Two types of porous materials were tested: (a) silica sand as a typical inert medium and (b) a mixture of calcareous soil and sand. The transport of GT-nZVI through the sand column was effectively described using a classic 1-D convection-dispersion flow equation (CDE) in combination with the colloid filtration theory (CFT). The calculations indicate that nZVI travel distance will be limited in the range 2.5–25cm for low Darcy velocities (0.1–1m/d) and in the order of 2.5m at higher velocities (10m/d). The mobility of GT-nZVI suspension in the soil–sand column is lower and is directly related to the progress of the neutralization reactions between the acidic GT-nZVI suspension and soil calcite.

Transport and retention of zinc oxide nanoparticles in porous media: Effects of natural organic matter versus natural organic ligands at circumneutral pH

The potential toxicity of nanoparticles (NPs) has received considerable attention, but there is little knowledge relating to the fate and transport of engineered ZnO NPs in the environment. Column experiments were performed at pH 7.3–7.6 to generate effluent concentrations and retention profiles for assessing the fate and transport of ZnO NPs (PZC=9.3, nominal size 20nm) in saturated quartz sands (256μm) in the presence of low natural organic matter (NOM) concentrations (1mg/L humic and fulvic acids) and millimolar natural organic ligands (NOL) levels (formic, oxalic, and citric acids). At circumneutral pHs, ZnO NPs were positively charged and immobile in sand. The presence of NOM decreased the attachment efficiency facilitating ZnO transport through sand columns. Conversely, ZnO transport in the presence of formic and oxalic acids was only slightly improved when compared to ZnO in DI water; whereas, citric acid showed no improvement. The distinct difference between NOM and NOL may have important implications with regard to ZnO transport in the subsurface environment. Experimental results suggested the presence of both favorable and unfavorable nanoparticle interactions causes significant deviations from classical colloid filtration theory (CFT).

Porous Media-Induced Aggregation of Protein-Stabilized Gold Nanoparticles

Gold-nanoparticles (AuNPs) are employed for cancer treatment, drug delivery, chemical analyses, and many other uses. As AuNP manufacture increases, it is imperative that we understand the environmental fate of these nanomaterials. We investigated the transport and stability of AuNPs under simulated groundwater conditions. Batch experiments indicated that 16 nm AuNPs stabilized with bovine serum albumin (BSA-cit-AuNPs) was slightly more stable under high ionic strength conditions than citrate-functionalized AuNPs (cit-AuNPs) of the same core size. Both types of AuNPs were injected into glass bead-packed columns and subjected to transport with varying NaCl and CaCl2 concentrations. BSA-cit-AuNPs deposited less than cit-AuNPs in the presence of increasing concentrations of CaCl2, but the opposite trend was observed in the presence of increasing concentrations of NaCl. This finding differed from the results obtained in the batch studies. Calculated attachment efficiencies (α) failed to reflect the observed experimental column data, with α at maximum only approaching 0.1 even though a majority of the AuNPs were retained in the column. Colloid filtration theory fails to predict and explain this discrepancy. We conclude that media induced nanoparticle aggregation is responsible for the inconsistency.

Experimental Study and Numerical Solution of Poly Acrylic Acid Supported Magnetite Nanoparticles Transport in a One-Dimensional Porous Media

Recently, iron nanoparticles have attracted more attention for groundwater remediation due to its potential to reduce subsurface contaminants such as PCBs, chlorinated solvents, and heavy metals. The magnetic properties of iron nanoparticles cause to attach to each other and form bigger colloid particles of iron nanoparticles with more rapid sedimentation rate in aqueous environment. Using the surfactants such as poly acrylic acid (PAA) prevents iron nanoparticles from forming large flocs that may cause sedimentation and so increases transport distance of the nanoparticles. In this study, the transport of iron oxide nanoparticles (Fe3O4) stabilized with PAA in a one-dimensional porous media (column) was investigated. The slurries with concentrations of 20,100 and 500 (mg/L) were injected into the bottom of the column under hydraulic gradients of 0.125, 0.375, and 0.625. The results obtained from experiments were compared with the results obtained from numerical solution of advection-dispersion equation based on the classical colloid filtration theory (CFT). The experimental and simulated breakthrough curves showed that CFT is able to predict the transport and fate of iron oxide nanoparticles stabilized with PAA (up to concentration 500 ppm) in a porous media.

Upscaling colloid transport and retention under unfavorable conditions: Linking mass transfer to pore and grain topology

[1] We revisit the classic upscaling approach for predicting Darcy-scale colloid retention based on pore-scale processes, and explore the implicit assumption that retention is a Markov process. Whereas this assumption holds under favorable attachment conditions, it cannot be assumed to hold under unfavorable conditions due to accumulation of colloids in the near-surface fluid domain. We develop a novel link between two-layer mass transfer parameters and the topologies of the pore and collector domains, starting with an elegant outcome of classic colloid filtration theory: that the likelihood of mass transfer from the bulk- to the near-surface fluid domain depends strongly on colloid proximity to the forward flow stagnation axis of each collector (grain). Applying this concept to colloid mass transfer from the near-surface to the bulk fluid domain yields the conclusion that such mass transfer predominantly occurs at rear-flow stagnation zones on collector surfaces. We support this concept with experimental proof that the alignment of rear and forward flow stagnation zones influences colloid mass transfer to surfaces. Colloid accumulation in the near-surface fluid domain under unfavorable conditions may produce extended tailing of colloids during elution in Darcy-scale studies by: (1) long residence times of colloids in the near-surface fluid domain; (2) direct propagation of near-surface colloids from upstream to downstream collectors. The latter generates correlated motion that violates the assumed independence of colloid retention on history of transport. We suggest an approach to upscaling that accounts for the above-described influences of colloid-surface interactions and pore/collector domain topology.

Transport of colloids in unsaturated packed columns: Role of ionic strength and sand grain size

The main objective of this study was to better understand the combined effects of ionic strength, and sand grain size on colloid fate and transport in unsaturated porous media. Spherical fluorescent polymer microspheres with three different sizes (0.075, 0.30 and 2.1μm), and laboratory columns packed with two size fractions of clean quartz sand (0.513 and 0.181mm) were used. The saturation level of the packed columns was set to 83–95% with solutions having a wide range of ionic strength (0.1–1000mM). The electrophoretic mobility of colloids and sand grains were evaluated for various ionic strength conditions. The single collector removal and collision efficiencies were quantified using the classical colloid filtration theory. Furthermore, theoretical collision efficiencies were estimated with appropriate DLVO energies using a Maxwell model. The experimental results suggested that the retention of the bigger colloids (2.1μm) was slightly higher compared to the conservative tracer and smaller colloids (0.3 and 0.075μm) in deionized-distilled-water, indicating attachment at air–water interfaces or straining. Moreover, relatively smaller attachment was observed onto fine than medium quartz sand. The mass recovery of the 0.3μm microspheres in NaCl solution was shown to significantly decrease with increasing ionic strength. Both the experimental and theoretical collision efficiencies based on colloid interactions with solid–water interfaces, were increased with increasing ionic strength.

A Field-Validated Model for In Situ Transport of Polymer-Stabilized nZVI and Implications for Subsurface Injection

Nanoscale zerovalent iron (nZVI) particles have significant potential to remediate contaminated source zones. However, the transport of these particles through porous media is not well understood, especially at the field scale. This paper describes the simulation of a field injection of carboxylmethyl cellulose (CMC) stabilized nZVI using a 3D compositional simulator, modified to include colloidal filtration theory (CFT). The model includes composition dependent viscosity and spatially and temporally variable velocity, appropriate for the simulation of push-pull tests (PPTs) with CMC stabilized nZVI. Using only attachment efficiency as a fitting parameter, model results were in good agreement with field observations when spatially variable viscosity effects on collision efficiency were included in the transport modeling. This implies that CFT-modified transport equations can be used to simulate stabilized nZVI field transport. Model results show that an increase in solution viscosity, resulting from injection of CMC stabilized nZVI suspension, affects nZVI mobility by decreasing attachment as well as changing the hydraulics of the system. This effect is especially noticeable with intermittent pumping during PPTs. Results from this study suggest that careful consideration of nZVI suspension formulation is important for optimal delivery of nZVI which can be facilitated with the use of a compositional simulator.