Influence of metal ion sorption on colloidal surface forces measured by atomic force microscopy

Atomic force microscopy (AFM) is employed to directly measure colloidal surface forces between a silica particle and a smooth glass plate in an aqueous solution with or without the presence of copper ions. Without the presence of copper ions, results show that the force between these two surfaces is repulsive and that its magnitude decreases with increasing ionic strength and decreasing pH. The surface forces are calculated based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for constant surface charge and are then compared with AFM force measurements. A good agreement between theory and experimental data is reported except at very small separation distances (<3 nm) between the silica particle and the glass plate. This behavior may be attributed to non-DLVO forces, such as the hydration effect that results from the bounded water molecules on the surface of the silica particle, or to surface roughness. When copper ions are present in acidic aqueous solutions, the magnitude of the force is found to be the same as that without the presence of copper ions, which indicates that no sorption of copper ions by the silica particle occurs under these conditions. Near neutral pH, sorption of copper ions causes charge reversal for the silica particle from negative to positive. Therefore, the force between the silica particle and the glass plate changes from repulsive to attractive. The transient zeta-potential of the silica particle during sorption of copper ions is determined by representing the experimental data with the DLVO theory. In alkaline solutions, where removal of copper ions is known to occur mainly by bulk precipitation, the measured force is similar to that without the presence of copper ions, which suggests that sorption does not occur under such conditions.     

Experimentally derived sticking efficiencies of microparticles using atomic force microscopy

The sticking efficiencies (alpha) of colloidal particles have been derived from the intersurface potential energy between 2 microm carboxylated polystyrene microspheres and a silica glass plate using the interaction force boundary layer model. The intersurface potential energies were calculated from force-distance data measured using atomic force microscopy (AFM) and from calculations based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. AFM forces were measured in aqueous solutions over a range of pH and ionic strength conditions, and DLVO calculations were performed on identical systems. In most conditions, sticking efficiencies that were calculated from AFM data are considerably largerthan values calculated from DLVO predictions. Sticking efficiencies vary between 0 and 1 and are strongly dependent upon solution chemistry. AFM-derived sticking efficiencies are consistent with measured microsphere and collector zeta-potentials; sticking efficiencies are lower for more negatively charged surfaces. These results provide the first alpha estimates of a microparticle-collector system that are calculated directly from physically measured interfacial nanoforces. This study clearly demonstrates that significant differences exist between DLVO- and AFM-derived sticking efficiencies.     

Interactions between Rotavirus and Suwannee River Organic Matter: Aggregation, Deposition, and Adhesion Force Measurement

Interactions between rotavirus and Suwannee River natural organic matter (NOM) were studied by time-resolved dynamic light scattering, quartz crystal microbalance, and atomic force microscopy. In NOM-containing NaCl solutions of up to 600 mM, rotavirus suspension remained stable for over 4 h. Atomic force microscopy (AFM) measurement for interaction force decay length at different ionic strengths showed that nonelectrostatic repulsive forces were mainly responsible for eliminating aggregation in NaCl solutions. Aggregation rates of rotavirus in solutions containing 20 mg C/L increased with divalent cation concentration until reaching a critical coagulation concentration of 30 mM CaCl(2) or 70 mM MgCl(2). Deposition kinetics of rotavirus on NOM-coated silica surface was studied using quartz crystal microbalance. Experimental attachment efficiencies for rotavirus adsorption to NOM-coated surface in MgCl(2) solution were lower than in CaCl(2) solution at a given divalent cation concentration. Stronger adhesion force was measured for virus-virus and virus-NOM interactions in CaCl(2) solution compared to those in MgCl(2) or NaCl solutions at the same ionic strength. This study suggested that divalent cation complexation with carboxylate groups in NOM and on virus surface was an important mechanism in the deposition and aggregation kinetics of rotavirus.     

Aquasols: on the role of secondary minima

Experiments are presented that test the hypothesis of deposition into and reentrainment from secondary minima during flow through porous media. The release of deposited particles following a decrease in ionic strength is inconsistent with deposition in the primary minimum of either simple DLVO interaction energy curves (which suggest that deposition is irreversible) or Born-DLVO interaction energy curves (which create a finite primary minimum that deepens with decreasing ionic strength). The observed release of particles is, on the other hand, consistent with deposition in the secondary minimum because this energy minimum decreases and can disappear with decreasing ionic strength. The implications for colloid transport of a reversible deposition process in the secondary minimum are very different from those of a process involving irreversible deposition in the primary minimum. First, particles that are continually captured and released will travel much farther in the subsurface than might be expected if the classic irreversible filtration model is applied. Second, and perhaps more significantly, deposition in the secondary well can increase with increasing particle size. Although particle transport by convective diffusion increases as particle size decreases, particle "attachment" in secondary minima decreases with decreasing particle size. Thus, smaller particles (those with diameters in the order of a few tens of nanometers) would be more effective in the facilitated transport of highly sorbing contaminants such as hydrophobic organic molecules, metals, and radionuclides. Other contaminants are themselves particles, such as viruses (tens of nanometers in diameter) and bacteria (near 1 microm in diameter). Due to this difference in size, viruses could be transported over much larger distances than bacteria. Third, the transport of colloids and, hence, the transport of contaminants associated with them, depends on the Hamaker constant of the particle-water-aquifer media system. Colloids of lower Hamaker constant are likely to be transported farther than colloids of higher Hamaker constant. The extent of adsorption of specific contaminants and the Hamaker constant for the particle-aquifer system are both characteristics of the particles and contribute to the effectiveness of colloid-facilitated transport. Finally, the solution chemistry of the pore waters (through pH, ionic strength, types of solutes, and the valence of the ions) ultimately controls the deposition and release of colloidal particles in porous media. The pH determines the charge density and surface potential of the surfaces. When the surfaces are similarly charged, their interaction can be unfavorable, with an energy barrier and secondary minimum. The ionic strength and valence of the ions determines the shape of the interaction energy curve, including the presence and height of the energy barrier and the presence and depth of the secondary well. Since the subsequent release of a particle depends on the mode in which the particle is deposited (primary or secondary), these factors are particularly important in determining the extent of colloid transport in the subsurface.     

Interaction of silica nanoparticles with a flat silica surface through neutron reflectometry

Neutron reflectometry (NR) was employed to study the interaction of nanosized silica particles with a flat silica surface in aqueous solutions. Unlike other experimental tools that are used to study surface interactions, NR can provide information on the particle density profile in the solution near the interface. Two types of silica particles (25 and 100 nm) were suspended in aqueous solutions of varying ionic strength. Theoretical calculations of the surface interaction potential between a particle and a flat silica surface using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were compared to the experimental data. The theory predicts that the potential energy is highly dependent on the ionic strength. In high ionic strength solutions, NR reveals a high concentration of particles near the flat silica surface. Under the same conditions, theoretical calculations show an attractive force between a particle and a flat surface. For low ionic strength solutions, the particle concentration near the surface obtained from NR is the same as the bulk concentration, while depletion of particles near the surface is expected because of the repulsion predicted by the DLVO theory.     

Investigation of interparticle forces in natural waters: effects of adsorbed humic acids on iron oxide and alumina surface properties

The nature of interparticle forces acting on colloid particle surfaces with adsorbed surface films of the internationally used humic acid standard material, Suwannee River Humic Acid (SHA), has been investigated using an atomic force microscope (AFM). Two particle surfaces were used, alumina and a hydrous iron oxide film coated onto silica particles. Adsorbed SHA dominated the interactive forces for both surface types when present. At low ionic strength and pH > 4, the force curves were dominated by electrostatic repulsion of the electrical double layers, with the extent of repulsion decreasing as electrolyte (NaCl) concentration increased, scaling with the Debye length (kappa(-1)) of the electrolyte according to classical theory. At pH approximately 4, electrostatic forces were largely absent, indicating almost complete protonation of carboxylic acid (-COOH) functional groups on the adsorbed SHA. Under these conditions and also at high electrolyte concentration ([NaCl] > 0.1 M), the absence of electrostatic forces allowed observation of repulsion forces arising from steric interaction of adsorbed SHA as the oxide surfaces approached closely to each other (separation < 10 nm). This steric barrier shrank as electrolyte concentration increased, implying tighter coiling of the adsorbed SHA molecules. In addition, adhesive bridging between surfaces was observed only in the presence of SHA films, implying a strong energy barrier to spontaneous detachment of the surfaces from each other once joined. This adhesion was especially strong in the presence of Ca2+ which appears to bridge SHA layers on each surface. Overall, our results show that SHA is a good model for the NOM adsorbed on colloids.     

Forces between colloid particles in natural waters

The origin and nature of interparticle forces acting on colloid surfaces in natural waters has been examined using an atomic force microscope. Natural colloids were represented by a surface film of iron oxide precipitated onto spherical SiO2 particles, and the effects of adsorbed natural organic matter (NOM), solution pH, and ionic composition on the force-separation curves were investigated. NOM from both riverine and marine environments was strongly adsorbed to the iron oxide surface. Under conditions of low ionic strength, the interparticle forces were dominated by electrostatic repulsion arising from negative functional groups on the NOM, except at very small separations (<10 nm) where repulsive forces arising from steric interference of the NOM molecules were also present. At high ionic strength (e.g., seawater) or low pH, the electrostatic forces were largely absent, allowing steric repulsion forces to dominate. In addition, adhesive bridging between surfaces by adsorbed NOM was observed, creating a strong energy barrier to spontaneous disaggregation of colloid aggregates. Our results demonstrate that adsorbed NOM dominates the surface forces and thus stability with respect to aggregation of natural water colloids.     

Localized attraction correlates with bacterial adhesion to glass and metal oxide substrata

Bacterial adhesion to surfaces does not always proceed according to theoretical expectations. Discrepancies are often attributed to surface heterogeneities that provide localized, favorable sites for bacterial attachment. The presence of these favorable deposition sites for bacteria, however, has never been directly measured. Atomic force microscopy (AFM) was used to quantify the distribution of attractive sites on clean substrata. Surfaces of silica and three different metal oxides mapped by adhesion force with regular or colloidal AFM tips showed a heterogeneous distribution of adhesion forces. Adhesion forces were normally distributed based on a colloid probe, but regular tips revealed a proportionately larger number of relatively more adhesive sites. No correlation was found between the average adhesion force (tip or colloid) and macroscopic adhesion tests using five strains of bacteria. However, when AFM tip results were compared to bacterial adhesion data on the basis of only the stickiest sites (the 5% of sites with the largest adhesion force), there was a good correlation of AFM data with adhesion data. These results demonstrate for the first time how overall bacterial adhesion to a surface effectively correlates with a relatively small fraction of highly adhesive sites rather than averaged adhesion force as detected using AFM.     

Interactions between natural organic matter and gold nanoparticles stabilized with different organic capping agents

The adsorption of natural organic matter (NOM) to the surfaces of natural colloids and engineered nanoparticles is known to strongly influence, and in some cases control, their surface properties and aggregation behavior. As a result, the understanding of nanoparticle fate, transport, and toxicity in natural systems must include a fundamental framework for predicting such behavior. Using a suite of gold nanoparticles (AuNPs) with different capping agents, the impact of surface functionality, presence of natural organic matter, and aqueous chemical composition (pH, ionic strength, and background electrolytes) on the surface charge and colloidal stability of each AuNP type was investigated. Capping agents used in this study were as follows: anionic (citrate and tannic acid), neutral (2,2,2-[mercaptoethoxy(ethoxy)]ethanol and polyvinylpyrrolidone), and cationic (mercaptopentyl(trimethylammonium)). Each AuNP type appeared to adsorb Suwannee River Humic Acid (SRHA) as evidenced by measurable decreases in zeta potential in the presence of 5 mg C L(-1) SRHA. It was found that 5 mg C L(-1) SRHA provided a stabilizing effect at low ionic strength and in the presence of only monovalent ions while elevated concentrations of divalent cations lead to enhanced aggregation. The colloidal stability of the NPs in the absence of NOM is a function of capping agent, pH, ionic strength, and electrolyte valence. In the presence of NOM at the conditions examined in this study, the capping agent is a less important determinant of stability, and the adsorption of NOM is a controlling factor.     

Relating organic fouling of reverse osmosis membranes to intermolecular adhesion forces

Organic fouling of reverse osmosis (RO) membranes and its relation to foulant--foulant intermolecular adhesion forces has been investigated. Alginate and Suwannee River natural organic matter were used as model organic foulants. Atomic force microscopy was utilized to determine the adhesion force between bulk organic foulants and foulants deposited on the membrane surface under various solution chemistries. The measured adhesion force was related to the RO fouling rate determined from fouling experiments under solution chemistries similar to those used in the AFM measurements. A remarkable correlation was obtained between the measured adhesion force and the fouling rate under the solution chemistries investigated. Fouling was more severe at solution chemistries that resulted in larger adhesion forces, namely, lower pH, higher ionic strength, presence of calcium ions (but not magnesium ions), and higher mass ratio of alginate to Suwannee River natural organic matter. The significant adhesion force measured with alginate in the presence of calcium ions indicated the formation of a crossed-linked alginate gel layer during fouling through intermolecular bridging among alginate molecules.     

Commercial titanium dioxide nanoparticles in both natural and synthetic water: comprehensive multidimensional testing and prediction of aggregation behavior

Engineered nanoparticles (ENPs) from industrial applications and consumer products are already being released into the environment. Their distribution within the environment is, among other factors, determined by the dispersion state and aggregation behavior of the nanoparticles and, in turn, directly affects the exposure of aquatic organisms to EPNs. The aggregation behavior (or colloidal stability) of these particles is controlled by the water chemistry and, to a large extent, by the surface chemistry of the particles. This paper presents results from extensive colloidal stability tests on commercially relevant titanium dioxide nanoparticles (Evonik P25) in well-controlled synthetic waters covering a wide range of pH values and water chemistries, and also in standard synthetic (EPA) waters and natural waters. The results demonstrate in detail the dependency of TiO(2) aggregation on the ionic strength of the solution, the presence of relevant monovalent and divalent ions, the presence and copresence of natural organic matter (NOM), and of course the pH of the solution. Specific interactions of both NOM and divalent ions with the TiO(2) surfaces modify the chemistry of these surfaces resulting in unexpected behavior. Results from matrix testing in well-controlled batch systems allow predictions to be made on the behavior in the broader natural environment. Our study provides the basis for a testing scheme and data treatment technique to extrapolate and eventually predict nanoparticle behavior in a wide variety of natural waters.     

Spatial variation in deposition rate coefficients of an adhesion-deficient bacterial strain in quartz sand

The transport of bacterial strain DA001 was examined in packed quartz sand under a variety of environmentally relevant ionic strength and flow conditions. Under all conditions, the retained bacterial concentrations decreased with distance from the column inlet at a rate that was faster than loglinear, indicating that the deposition rate coefficient decreased with increasing transport distance. The hyperexponential retained profile contrasted againstthe nonmonotonic retained profiles that had been previously observed for this same bacterial strain in glass bead porous media, demonstrating that the form of deviation from log-linear behavior is highly sensitive to system conditions. The deposition rate constants in quartz sand were orders of magnitude below those expected from filtration theory, even in the absence of electrostatic energy barriers. The degree of hyperexponential deviation of the retained profiles from loglinear behavior did not decrease with increasing ionic strength in quartz sand. These observations demonstrate thatthe observed low adhesion and deviation from log-linear behavior was not driven by electrostatic repulsion. Measurements of the interaction forces between DA001 cells and the silicon nitride tip of an atomic force microscope (AFM) showed that the bacterium possesses surface polymers with an average equilibrium length of 59.8 nm. AFM adhesion force measurements revealed low adhesion affinities between silicon nitride and DA001 polymers with approximately 95% of adhesion forces having magnitudes < 0.8 nN. Steric repulsion due to surface polymers was apparently responsible for the low adhesion to silicon nitride, indicating that steric interactions from extracellular polymers controlled DA001 adhesion deficiency and deviation from log-linear behavior on quartz sand.     

Influence of collector surface composition and water chemistry on the deposition of cerium dioxide nanoparticles: QCM-D and column experiment approaches

The deposition behavior of cerium dioxide (CeO(2)) nanoparticles (NPs) in dilute NaCl solutions was investigated as a function of collector surface composition, pH, ionic strength, and organic matter (OM). Sensors coated separately with silica, iron oxide, and alumina were applied in quartz crystal microbalance with dissipation (QCM-D) to examine the effect of these mineral phases on CeO(2) deposition in NaCl solution (1-200 mM). Frequency and dissipation shift followed the order: silica > iron oxide > alumina in 10 mM NaCl at pH 4.0. No significant deposition was observed at pH 6.0 and 8.5 on any of the tested sensors. However, ≥ 94.3% of CeO(2) NPs deposited onto Ottawa sand in columns in 10 mM NaCl at pH 6.0 and 8.5. The inconsistency in the different experimental approaches can be mainly attributed to NP aggregation, surface heterogeneity of Ottawa sand, and flow geometry. In QCM-D experiments, the deposition kinetics was found to be qualitatively consistent with the predictions based on the classical colloidal stability theory. The presence of low levels (1-6 mg/L) of Suwannee River humic acid, fulvic acid, alginate, citric acid, and carboxymethyl cellulose greatly enhanced the stability and mobility of CeO(2) NPs in 1 mM NaCl at pH 6.5. The poor correlation between the transport behavior and electrophoretic mobility of CeO(2) NPs implies that the electrosteric effect of OM was involved.     

原子力显微镜研究L-半胱氨酸对鲢肌球蛋白溶液热聚集行为的影响

利用原子力显微镜技术分析L-半胱氨酸（L-cysteine,L-Cys）对鲢肌球蛋白热聚集行为的影响。在肌球蛋白溶液中添加5?mmol/L（pH?7.0）的L-Cys溶液,分别进行未加热（25?℃、30?min）、一段式加热（90?℃、30?min）、二段式加热（40?℃、60?min＋90?℃、30?min）处理,分别测定溶解度、表面疏水性、聚集行为表面形貌的变化。结果表明：3?种加热方式低盐条件下L-Cys均显著提高肌球蛋白的溶解度（P＜0.05）;一段式加热时L-Cys显著提高高/低盐条件下肌球蛋白的表面疏水性（P＜0.05）,二段式加热时高盐条件下表面疏水性显著提高（P＜0.05）,其他条件下表面疏水性无明显变化。高/低盐条件下添加L-Cys均能显著改变肌球蛋白聚集行为的表面形貌,使聚集体更加分散,抑制了肌球蛋白的聚集。L-Cys对肌球蛋白溶解度、表面疏水性的影响进一步影响了其聚集行为,改变其聚集形貌。     

Interaction forces between colloids and protein-coated surfaces measured using an atomic force microscope

Bacterial surfaces contain proteins, polysaccharides, and other biopolymers that can affect their adhesion to another surface. To better understand the role of proteins in bacterial adhesion, the interactions between two different model colloids (glass beads and carboxylated latex microspheres) and four proteins covalently bonded to glass surfaces were examined using colloid probes and an atomic force microscope (AFM). Adhesion forces between an uncoated glass colloid probe and protein-coated surfaces, measured in retraction force curves, decreased in the order poly-D-lysine > lysozyme > protein A > BSA. This ordering was consistent with the relative calculated charges of the proteins at neutral pH and the zeta-potentials measured for glass beads and latex microspheres coated with these proteins. When the glass bead was coated with a protein (BSA), overall adhesion forces between the protein-coated colloid and the protein-coated surfaces were reduced, and the adhesion force for each protein decreased in the same order observed in experiments with the uncoated glass bead. When latex colloid probes were coated with BSA, adhesion forces were significantly larger than those measured with BSA-coated glass colloid probes under the same conditions, demonstrating that the nature of the underlying colloid can affect the measured interaction forces. In addition, the adhesion forces measured with the BSA-coated latex colloid increased in a different order (BSA < lysozyme < protein A < poly-D-lysine) than that observed using the BSA-coated glass colloid. It was also found that increasing the solution ionic strength consistently decreased adhesion forces. This result is contrary to the general observation that bacterial adhesion increases with ionic strength. It was speculated that conformational changes of the protein produced this decrease in adhesion with increased ionic strength. These results suggest the need to measure nanoscale adhesion forces in order to understand better molecular scale interactions between colloids and surfaces.     

Effects of natural organic matter and solution chemistry on the deposition and reentrainment of colloids in porous media

The role of humic acid in the transport of negatively charged colloids through porous media was examined. Adsorption of humic acid on latex colloids and silica collectors reduced the deposition of suspended particles and enhanced the reentrainment of deposited particles in porous media. These effects are considered to arise from additional electrostatic and steric contributions to the repulsive interaction energy due to the adsorption of negatively charged humic acid on both the suspended particles and the media collectors. At low ionic strength reversible deposition in shallow secondary minima is hypothesized to be the principal attachment mechanism, independent of the presence of humic acid. It is proposed that under these solution conditions, particle deposition and reentrainment are the result of a dynamic process, in which particles are continuously captured and released from secondary minima. At higher ionic strengths, deposition may be regarded as a combination of two mechanisms: capture in the primary well and capture in the secondary minimum. Theoretical calculations of the attachment efficiency were conducted using two existing mathematical models. The first model is based on deposition in the primary well (interaction force boundary layer, IFBL), and the second model is based on the Maxwell kinetic theory and deposition in the secondary minimum (Maxwell model). Simulations conducted with the Maxwell model provide significantly better fits of the experimental results than those conducted with the IFBL model.     

Thermal aggregation and gelation of kidney bean (Phaseolus vulgaris L.) protein isolate at pH 2.0: Influence of ionic strength

Thermal aggregation and gelation of kidney bean protein isolate (KPI) at pH 2.0 and varying ionic strengths (0–300 mM) were investigated using dynamic light scattering (DLS), atomic force microscopy (AFM), and turbidity and dynamic oscillatory measurements. DLS and AFM analyses showed that the extent of thermal aggregation at pH 2.0, or contour length of the worm-like and fine-stranded aggregates, progressively increased with increasing ionic strength. Turbidity and dynamic rheological analyses indicated that, the turbidity and mechanical moduli of the formed gels also increased with the increase in both ionic strength and protein concentration (c). The c dependence of the elastic modulus G′ could be well described using both fractal and percolation models, though in the case of fractal model applied, two distinct scaling regimes were observed. These results suggest that at pH 2.0, the thermal aggregation and gelation behaviors of the proteins in KPI could be remarkably affected by a change in electrostatic repulsion, and homogenous fine-stranded gels formed at ionic strengths in the 0–300 mM range.     

Effect of molecular scale roughness of glass beads on colloidal and bacterial deposition

Molecular-scale surface roughness and charge heterogeneity have been hypothesized as factors that can affect the deposition rates of colloids during their transport in porous media. To test their relative importance, a single batch of cleaned glass beads was divided in half and chemically treated with acid or base to alter surface roughness. Analysis of the topography of 20 glass beads with an atomic force microscope (AFM) indicated that the chromic acid-treated (rough) beads had a root-mean-square roughness of 38.1 +/- 3.9 nm, while the sodium hydroxide-treated (smooth) beads had root-mean-square surface roughness of 15.0 +/- 1.9 nm. AFM force volume imaging of glass bead surfaces did not reveal surface charge heterogeneity. Filtration experiments with inorganic colloids (latex microspheres, 1 microm diameter) consistently demonstrated that there was a greater retention of latex microspheres on rough than smooth glass beads suspended in either low (10(-5) M) or higher (10(-1) M) ionic strength (IS) solutions. Collision efficiencies for rough beads were 30-50% larger than for smooth beads. Collision efficiencies of bacteria using rough glass beads were also equal to or greater than those measured for smooth beads. In experiments with the perchlorate-reducing bacterial isolate KJ, collision efficiencies were significantly greater on rough rather than smooth beads for two different ionic strength solutions (IS = 0.05 or 1 M). In another case (IS = 0.1 M) for KJ, and in filtration experiments with E coli, collision efficiencies were not significantly different between the rough and smooth beads. We hypothesize that the consistently greater deposition rates of microspheres, but not bacteria, on rough rather than smooth beads are due in part to the presence of polymers on the surfaces of bacteria.     

Mobility of capped silver nanoparticles under environmentally relevant conditions

The mobility and deposition of capped silver (Ag) nanoparticles (NPs) on silica surfaces were characterized over a wide range of pH and ionic strength (IS) conditions, including seawater and freshwater. Two common organic capping agents (citrate and PVP) were evaluated. Both the capped Ag NPs and the silica surfaces were negatively charged under these environmentally relevant conditions, resulting in net repulsive electrostatics under most conditions. The steric repulsion introduced by the capping agents significantly reduced aggregation and deposition. In addition, the presence of natural organic matter in solution further decreased the deposition of either Ag NP on silica. Ag NPs were found to be highly mobile under these environmentally relevant conditions, with little or no deposition.     

Hierarchical approach to model multilayer colloidal deposition in porous media

Particle deposition is important in many environmental systems such as water and wastewater filtration, air pollution control, subsurface transport, biofilm formation and fouling, and thin film synthesis for use in remediation technologies. While continuum-level models have been developed to predict deposition dynamics in these systems, these models fail to explain transient dynamics of multilayer deposition from a mechanistic viewpoint. In this work, a multiscale approach has been developed to predict multiple layer irreversible colloidal deposition in the presence of interparticle electrostatic and van der Waals interactions in porous media. The approach combines the kinetic information obtained from the mesoscopic stochastic simulations of particle deposition with the macroscopic conservation equation describing colloidal transport. Sequential Brownian dynamics simulations are first performed by accounting for particle-particle (P-P) and particle-surface (P-S) interactions, and multilayered particle deposits are obtained. The available surface function quantifying the deposition kinetics is then obtained from the deposit microstructure. Deposition dynamics are studied at different ionic strengths and particle potentials that control the range and magnitude of interparticle interactions. Simulation results showed that the microstructure of the particle deposits formed under the influence of P-P and P-S electrostatic interactions exhibited significant variations with respect to ionic strength and could be qualitatively explained bythe interplay between the repulsive and attractive P-P and P-S interaction forces. The available surface function also varied significantly as a function of ionic strength. This basic understanding of the deposition dynamics at the mesoscale was then combined with the continuum-level transport equations to predict particle breakthrough curves in porous media. The approach is capable of capturing transient features of deposition dynamics, as demonstrated by the good agreement between the model predictions and the experimental observations.