Primary Energy Minimum Research Articles

Soil organic matter facilitates the transport of microplastic by reducing surface hydrophobicity

Groundwater contamination by microplastic (MP) causes increasing concern about the critical factors that determine MP transport mechanisms in the vadose zone. Strong water repellency of native microplastic surfaces and respective attractive hydrophobic interactions with components of the soil matrix limits its mobility. However, adsorption of dissolved organic matter (DOM) onto microplastic surfaces can potentially make them more hydrophilic, enhancing their mobility. Consequently, the main objective of this study was to systematically determine the effects of DOM on both the surface properties and the mobility of polystyrene microplastic (PSP), and to use the extended DLVO theory (XDLVO) to predict PSP transport behavior with natural DOM for the first time. Breakthrough experiments were conducted in columns filled with quartz sand, through which PSP (1 µm) were percolated at either 0 or 5 mg/L dissolved organic carbon (DOC) derived from beech (Fagus sylvatica) litter at ionic strengths ranging from 0.5 to 2.5 mM CaCl2, since the cation background also affects wettability and mobility. Using sessile drop contact angle and zeta potential measurements, XDLVO interaction energy components including Lewis acid-base energies were approximated. Breakthrough curve analysis confirmed that DOM coming from natural sources strongly enhanced the mobility of PSP in the quartz sand matrix, in particular at higher ionic strength. At 2.5 mM CaCl2, the presence of DOM increased PSP breakthrough from 1 % to 60 %. Consequently, in the absence of DOM, laser microscope images showed much more pronounced formation of microplastic clusters on quartz grain surfaces than at co-percolation of PSP with DOM. This behavior could be clearly predicted with the XLDVO approach, which suggests at 0 mg/L DOC a deep primary energy minimum between individual PSP and between PSP and quartz surfaces. In contrast, repulsive hydrophilic interactions between the surfaces were predicted at 5 mg/L DOC. We conclude that even low concentrations of DOM, as usually found in subsoil, can significantly enhance the mobility of microplastic in soils via rendering the surfaces more hydrophilic. This implies potentially serious repercussions for the spread of this contaminant in the environment.

Significant Mobility of Novel Heteroaggregates of Montmorillonite Microparticles with Nanoscale Zerovalent Irons in Saturated Porous Media

This study conducted laboratory column experiments to systematically examine the transport of novel heteroaggregates of montmorillonite (Mt) microparticles with nanoscale zerovalent irons (nZVIs) in saturated sand at solution ionic strengths (ISs) ranging from 0.001 to 0.2 M. Spherical nZVIs were synthesized using the liquid phase reduction method and were attached on the plate-shaped Mt surfaces in monolayer. While complete deposition occurred for nZVIs in sand, significant transport was observed for Mt-nZVI heteroaggregates at IS ≤ 0.01 M despite the transport decrease with an increasing loading concentration of nZVIs on Mt. The increased mobility of Mt-nZVI heteroaggregates was because the attractions between nZVIs and sand collectors were reduced by the electrostatic repulsions between the Mt and the collector surfaces, which led to a decreased deposition in the sand columns. Complete deposition occurred for the Mt-nZVI heteroaggregates at IS ≥ 0.1 M due to a favorable deposition at Derjaguin–Landau–Verwey–Overbeek (DLVO) primary energy minima. Interestingly, a large fraction of the deposited heteroaggregates was released by reducing IS because of a monotonic decrease of interaction energy with separation distance for the heteroaggregates at low ISs (resulting in repulsive forces), in contrast to the irreversible deposition of nZVIs. Therefore, the fabricated heteroaggregates could also have high mobility in subsurfaces with saline pore water through continuous capture and release using multiple injections of water with low ISs. Our study was the first to examine the transport of heteroaggregates of a plate-like particle with spherical nanoparticles in porous media; the results have important implications in the use of nanoscale zerovalent iron for in situ soil and groundwater remediation.

Lip balm drying promotes virus attachment: Characterization of lip balm coatings and XDLVO modeling

HypothesisDrying-induced decrease in lip balm surface energy enhances virus adhesion due to the emergence of strong hydrophobic colloid-surface interactions. ExperimentsA protocol was developed for preparing lip balm coatings to enable physicochemical characterization and adhesion studies. Surface charge and hydrophobicity of four brands of lip balm (dry and hydrated) and human adenovirus 5 (HAdV5) were measured and used to calculate the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) energy of interactions between lip balm coatings and HAdV5 as well as four other colloids: HAdV40, MS2 and P22 bacteriophages, and SiO2. Quartz crystal microbalance with dissipation monitoring (QCM-D) tests employed SiO2 colloids, HAdV5 and hydrated lip balms. FindingsDrying of lip balms results in a dramatic decrease of their surface energy (δΔGsws≥ 83.0 mJ/m2) making the surfaces highly hydrophobic. For dry lip balms, the interaction of the balm surface with all five colloids is attractive. For lip balms hydrated in 150 mM NaCl (ionic strength of human saliva), XDLVO calculations predict that hydrophilic colloids (MS2, P22, SiO2) may attach into shallow secondary minima. Due to the relative hydrophobicity of human adenoviruses, primary maxima in XDLVO profiles are low or non-existent making irreversible deposition into primary energy minima possible. Preliminary QCM-D tests with SiO2 colloids and HAdV5 confirm deposition on hydrated lip balms.

Differences in transport behavior of natural soil colloids of contrasting sizes from nanometer to micron and the environmental implications

Transport behaviors of nanoparticles (<100nm) and clay fractions (clay particles, <2μm; coarse clay particles, 1–2μm and fine clay particles, 0.1–1μm) extracted from two natural soils (Inceptisol from Jilin and Oxisol from Hainan, China) were investigated in saturated sand columns at 1–30mM NaCl and pH5–9. Increasing NaCl concentrations decreased the mobility, while increasing pH increased the mobility of soil particles of various sizes. At pH5 and 30mM NaCl, nanoparticles and clay fractions exhibited the different transport behaviors, and ripening was observed for Inceptisol nanoparticles while blocking for Oxisol nanoparticles in breakthrough curves (BTCs). The effluent mass recoveries (MRs) of nanoparticles were much more than that of clay particles for both two soils (>1.9-fold) at all tested conditions, except for Inceptisol at pH5 and 30mM NaCl (with comparable MR). According to Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations and particle-collector size ratios, both secondary energy minimum and physical straining led to the retention of clay fractions at pH5 and 30mM NaCl, whereas primary energy minimum and straining induced by simultaneous aggregation caused the retention of nanoparticles. The experimental attachment efficiency between soil particles of various sizes and sand collector for both two soils was in the order nanoparticles<fine clay particles<clay particles<coarse clay particles, indicating that soil nanoparticles had greater mobility than clay fractions. Consequently, nanoparticles are suggested to have greater risks to transport contaminants to surface and groundwater, once they are released from soils.

Quantifying bacterial attachment and detachment using leaching solutions of various ionic strengths after bacterial pulse

In this study, we quantified the attachment and detachment of bacteria during transport in order to elucidate the contributions of reversible attachment on bacterial breakthrough curves. The first set of breakthrough experiment was performed for a laboratory sand column using leaching solutions of deionized water and mineral salt medium (MSM) of 200 mM with reference to KCl solution by employing Pseudomonas putida as a model bacterium. In the second set of experiment, the ionic strengths of leaching solutions immediately after bacterial pulse were lowered to tenfold and 100-fold diluted system (2 and 20 mM MSM) to focus on the influence of physicochemical factor. Results have shown that bacterial retention occurred in the sand column due to the physical deposition and physicochemical attachment. The physicochemical attachment was attributed to the high ionic strength (200 mM MSM) of leaching solution and the formation of primary energy minimum. Replacing the 200 mM leaching solution with the lower ionic strengths after pulse resulted in the increased tailing of breakthrough curve due to the detachment from the attached bacteria. The detachment could be well explained by DLVO theory, which showed the formation of energy barrier and disappearance of the secondary minimum as the ionic strength gradually decreased. Analysis of mass recovery revealed that 12–20% of the attachment was due to physical and physicochemical attachment, respectively, where the latter consisted of 25–75% of irreversible and reversible attachment respectively.

Spontaneous Detachment of Colloids from Primary Energy Minima by Brownian Diffusion.

The Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy profile has been frequently used to interpret the mechanisms controlling colloid attachment/detachment and aggregation/disaggregation behavior. This study highlighted a type of energy profile that is characterized by a shallow primary energy well (i.e., comparable to the average kinetic energy of a colloid) at a small separation distance and a monotonic decrease of interaction energy with separation distance beyond the primary energy well. This energy profile is present due to variations of height, curvature, and density of discrete physical heterogeneities on collector surfaces. The energy profile indicates that colloids can be spontaneously detached from the shallow primary energy well by Brownian diffusion. The spontaneous detachment from primary minima was unambiguously confirmed by conducting laboratory column transport experiments involving flow interruptions for two model colloids (polystyrene latex microspheres) and engineered nanoparticles (fullerene C60 aggregates). Whereas the spontaneous detachment has been frequently attributed to attachment in secondary minima in the literature, our study indicates that the detached colloids could be initially attached at primary minima. Our study further suggests that the spontaneous disaggregation from primary minima is more significant than spontaneous detachment because the primary minimum depth between colloid themselves is lower than that between a colloid and a collector surface.

Plant Root Exudates Decrease Mobility of Smectite Colloids in Porous Media in Contrast to Humic Acid

Plant root exudates are primarily composed of carbohydrates (CHs), amino acids (AAs), and organic acids (OAs). Little is known about how plant root exudates influence the stability and mobility of clay colloids in the soil profile. In this study, transport behaviors of K+–saturated smectite colloids dispersed in maize (Zea mays L.) artificial root exudate (ARE) solution, humic acid (HA) solution, and deionized water through water-saturated sand columns were investigated with solution ionic strengths of 0.1 and 10 mmol L−1 KCl and pH of 5, 7, and 9. Results showed that smectite colloids were more aggregated and transported less in ARE solution, followed by water and HA solution. This trend became less apparent with increasing pH but more pronounced with increasing ionic strength, suggesting that enhanced stability and mobility of HA-dispersed smectite colloids probably resulted from increased electrostatic and/or steric repulsions. The results for CH-, AA-, or OA-dispersed smectite colloids revealed that the AA fraction was primarily responsible for enhanced colloid retention because positively charged amine groups in AAs (especially lysine) might neutralize the negative surface charge of colloids and promote intersurface bridging. The residual colloids, after flushing with deionized water (thus eliminating secondary energy minima), decreased in the order of ARE-, water-, and HA-dispersed colloids, suggesting greater retention by primary energy minima and pore straining for the ARE-dispersed smectite colloids. Overall, in contrast to HA, plant root exudates decreased the stability and mobility of smectite colloids, thus facilitating the retention of clay colloids in root zones during water percolation events.

Determination of parameters of a hybrid car powertrain by modeling of the energy consumption process

During operation of a hybrid powertrain with intermediate buffer energy storage, an inadmissible decrease in the amount of of stored energy may occur. The parameters of hybrid powertrain elements should be chosen so as to minimize the probability of such cases. This article describes a method for calculating powertrain parameters based on representation of energy consumption as a random function. The method is described by the example of a passenger car powertrain. The dependence of the minimum required power reserve of the primary energy source and minimum required buffer energy storage capacity is shown. The described method can be used to calculate the primary energy source power and buffer energy storage capacity of different hybrid vehicles using a random function specifying their energy consumption. Application of the method requires further experimental investigation of these parameters of random functions.

Release Kinetics of Multiwalled Carbon Nanotubes Deposited on Silica Surfaces: Quartz Crystal Microbalance with Dissipation (QCM-D) Measurements and Modeling

Understanding the kinetics of the release of carbon nanotubes (CNTs) from naturally occurring surfaces is crucial for the prediction of the environmental fate and transport of CNTs. In this study, the release kinetics of multiwalled CNTs (MWNTs) from silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were first deposited on silica surfaces under favorable deposition conditions (1.50 mM CaCl2 and pH 7.1) and the deposited MWNTs were then rinsed at different electrolyte solutions to induce the release of MWNTs from the primary energy minimum. The kinetics of MWNT release was shown to be first order with respect to the deposited MWNTs when complete release took place. A model that accounts for the releasable and unreleasable components of MWNTs was used to fit the experimental data in order to derive the release rate coefficients. When the CaCl2 concentration in the eluent was decreased, a larger fraction of deposited MWNTs was released and the release rate coefficient of the releasable MWNTs also increased. The rise in the surface charges of both MWNTs and silica surfaces with the drop in CaCl2 concentration likely resulted in the decrease in the height of the energy barrier, thus facilitating the release of the nanotubes. Moreover, when the initial surface concentrations of deposited MWNTs were over 1000 ng/cm(2), the release rate coefficient was lower than expected. The reduced release kinetics was likely due to the formation of large surface-bound MWNT clusters which had considerably lower diffusion coefficients than dispersed MWNTs or MWNT aggregates.

Particle deposition and mobilisation during deep bed filtration in oilfields

Deposition of latex microspheres on a borosilicate filter was carried out at gradually decreasing suspension velocities at conditions favourable for particle attachment and excluding particle retention due to size exclusion. Particle critical retention concentration followed quadratic function of suspension velocity agreeing with the modified particle attachment/detachment model based on mechanical equilibrium of a particle located on the porous matrix surface within experimental uncertainty in the entire range of studied velocities. Electrostatic force is the dominant factor in a strong particle-matrix attraction. Significant part of latex particles were able to overcome a low energy barrier and be trapped irreversibly in the primary energy minimum. The remaining deposited particles were attached in the secondary energy minimum. Due to a strong particle attraction to the surface, their detachment cannot be achieved by increasing fluid velocity only. Almost a fifth of the attached particles were removed after the reduction of salinity and increase in pH of solution.

Influence of Solution Chemistry on the Release of Multiwalled Carbon Nanotubes from Silica Surfaces

The release of multiwalled carbon nanotubes (MWNTs) that were deposited on silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were deposited on silica surfaces at elevated NaCl and CaCl2 concentrations before being rinsed with eluents of different solution chemistries to induce their remobilization. Energetically speaking, the MWNTs were released from the primary energy minimum when the background NaCl or CaCl2 concentrations were decreased at pH 7.1. The increase in electrostatic repulsion between MWNTs and silica likely caused a reduction in the energy barrier, which enabled the release of MWNTs. The degree of release increased in a stepwise fashion when the nanotubes were sequentially exposed to eluents of decreasing electrolyte concentrations, possibly due to the heterogeneity in nanotube surface charge densities. The degree of release via a successive reduction in NaCl concentration was lower at pH 4.0 than at 7.1 due to MWNTs and silica surfaces exhibiting a less negative surface charge at pH 4.0. Most of the deposited MWNTs were released when the pH was decreased from 7.1 to 4.0 at 1.5 mM CaCl2. This was attributed to the elimination of calcium bridging between the carboxyl groups on MWNTs and silanol groups on silica surfaces.

Adhesion of a Cylindrical Shell in the Presence of DLVO Surface Potential

A theoretical model is built for a micrometer size cylindrical shell adhering to a rigid surface in the presence of an electrolyte. In the presence of surface electrostatic double layers and van der Waals attraction according to the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, the shell deforms and settles in either the primary (1min) or secondary (2min) energy minimum depending on whether it has sufficient energy to overcome the repulsive energy barrier. The adhesion-detachment mechanics are constructed and solved computationally, yielding the relations between applied load, deformed profile, and mechanical stress distribution in the shell. The critical compressive load needed for transition from 2min to 1min is found for several repulsive barrier heights. At a critical pull-off tensile force, shell in the 1min detaches spontaneously at a nonzero contact area, but the one in the 2min detaches smoothly with the contact shrinking to a line contact. The model is relevant to bacterial adhesion in environmental engineering and microelectromechanical systems for microfluidics applications.

Fecal Indicator Bacteria Transport and Deposition in Saturated and Unsaturated Porous Media

Beach sediment and sand are recognized as nonpoint fecal indicator bacteria (FIB) sources capable of causing water quality and health risks for beach-goers. A comprehensive understanding of the key factors and mechanisms governing the migration and exchange of FIB between beach water column and sediment is desired to better predict FIB concentration variations and assess the associated risk. The transport and retention behavior of two model FIB Enterococcus faecalis (E. faecalis) and Escherichia coli (E. coli) was examined using packed-bed columns in both saturated and unsaturated porous media to evaluate FIB migration potentials at conditions simulating the coastal aquatic environment. Additionally, complementary cell characterization techniques were conducted to better understand the migration behaviors of both FIB strains observed in the column experiments. The mobility of the gram-positive species E. faecalis was much more sensitive to solution chemistry and column saturation level than that of the gram-negative species E. coli. Interaction energy calculations suggest that E. faecalis retention was largely governed by the combination of DLVO (Derjaguin-Landau-Verwey-Overbeek) and non-DLVO (most likely hydrophobic and/or polymer bridging) interactions in saturated porous media, while the combination of DLVO and steric interactions controlled the deposition of E. coli cells. The measured surface properties of the two FIB strains supported the distinct bacteria transport behaviors and the differences of the identified mechanisms for each strain. As a result, E. faecalis showed the least affinity to sand in freshwater and appeared to be irreversibly attached in primary energy minima at elevated salt conditions; whereas the retained E. coli cells were reversibly attached and mostly associated with the secondary energy minima at both freshwater and seawater conditions. In unsaturated porous media, E. faecalis cells seemed to prefer to attachment at air/water interface rather than sand surface, while E. coli showed a similar affinity to the two interfaces. It was proposed that the different surface characteristics of the two FIB strains resulted in the distinct transport and retention behavior in porous media. These results highlight the need for FIB management to consider variations in transport behavior between model FIB when assessing water quality and associated risks.

Role of Surface Roughness in Chemical Detachment of Colloids Deposited at Primary Energy Minima

This study theoretically and experimentally examined effects of surface roughness on detachment of colloids deposited under favorable chemical conditions on reduction of solution ionic strength. A superposition approach based on elemental geometric models was developed to estimate variation of Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction energies between a colloid and a rough surface under different solution chemistries. Theoretical analysis showed that most colloids attached at rough surfaces via primary‐minimum association are irreversible on reduction of solution ionic strength because primary minima are deeper and the detachment energy barriers are greater at lower ionic strength. A fraction of colloids initially attached at tips of nanoscale protruding asperities, however, will detach from a rough surface at low ionic strength because the net force acting on the colloids can become repulsive (i.e., calculated DLVO interaction energy curves show monotonic decreases of interaction energies with separation distance at low ionic strength). Column experiments were conducted with 1156‐nm polystyrene latex particles and rough sand (300–355‐μm diameter) to examine the detachment of colloids initially deposited at primary minima. Experimental results confirmed that a fraction of colloids are released at low ionic strengths. Our theoretical and experimental results are consistent with literature observations, adding convincing evidence to challenge the usual belief that colloids attached at primary minima are irreversible on reduction of solution ionic strength. Although the importance of surface heterogeneity on colloid deposition has been widely recognized, our study implies that surface heterogeneity also plays a critical role in colloid detachment under both favorable and unfavorable conditions.

Equilibrium Orientation of Nonspherical Janus Particles at Fluid–Fluid Interfaces

We study the equilibrium orientation of nonspherical Janus particles at an oil-water interface. Two types of nonspherical Janus particles are considered: Janus ellipsoids and Janus dumbbells. To find their equilibrium orientation, we calculate and minimize the attachment energy of each Janus particle as a function of its orientation angle with respect to the oil-water interface. We find that the equilibrium orientation of the interface trapped Janus particles strongly depends on the particle characteristics, such as their size, aspect ratio, and surface properties. In general, nonspherical Janus particles adopt the upright orientation (i.e., the long axis of ellipsoids or dumbbells is perpendicular to the interface) if the difference in the wettability of the two sides is large or if the particle aspect ratio is close to 1. In contrast, Janus particles with a large aspect ratio or a small difference in the wettability of the two regions tend to have a tilted orientation at equilibrium. Moreover, we find that Janus ellipsoids, under appropriate conditions, can be kinetically trapped in a metastable state due to the presence of a secondary energy minimum. In contrast, Janus dumbbells possess only a primary energy minimum, indicating that these particles prefer to be in a single orientation. The absence of a secondary minimum is potentially advantageous for obtaining particle layers at fluid-fluid interfaces with uniform orientation. Our calculation provides a detailed guidance for synthesizing nonspherical Janus particles that can be used as effective solid surfactants for the stabilization of multiphasic fluid mixtures and the modification of the rheological properties of fluid interfaces.

Modeling the effects of Ca 2+ and clay-associated organic carbon on the stability of colloids from topsoils

The goals of the study were to investigate the effects of the soil–water phase ionic strength, mainly monitored by the calcium ion (Ca 2+) concentration, on the stability behavior of easily dispersed topsoil colloidal clay-sized particles (<2 μm). The aggregation kinetics as a function of the Ca 2+ concentration was monitored by measuring the increase of the particle size over time with photon correlation spectroscopy. The critical coagulation concentrations ( CCC) of Ca 2+ were measured. The Hamaker constants ( A) characterizing the attractive chemical properties of the topsoil colloid surface were thus scaled according to the Derjaguin, Landau, Verwey, Overbeek (DLVO) theory by taking into account the electrokinetic behavior of the particles, measured by the ζ-potential. Effective values for the Hamaker constants of topsoil clay-sized colloids, clay minerals, and metal oxides were calculated by referring to reported values for crystalline silica or sand (quartz) particles. Potential-energy diagrams of interacting topsoil clay-sized colloids were calculated. The primary energy maximum and secondary energy minimum were used for modeling the aggregation kinetics along the Ca 2+ concentration by employing Marmur’s model. Coagulation in the secondary energy minimum can only explain the aggregation efficiency of topsoil colloids at low Ca 2+ concentrations (<2 mM Ca 2+) under unfavorable electrostatic conditions. The effect of surface-associated organic matter on the colloidal electrosteric stability was also investigated by comparing the topsoil colloid stability after the removal of organic matter.

Assessing bacterial adhesion using DLVO and XDLVO theories and the jet impingement technique

In this study, the adhesion of two bacterial strains ( Pseudomonas stutzeri PS, and Staphylococcus epidermidis, SE) to the glass and the indium tin oxide (ITO)-coated glass surfaces was examined qualitatively and quantitatively using the theoretical approaches and the jet impingement technique. A comparison between the DLVO and the extended DLVO (XDLVO) theories showed that the XDLVO predictions of bacterial adhesion and its reversibility are more accurate than DLVO predictions. The adhesion tests revealed that PS bacteria has much better adhesion rate than SE bacteria to both material surfaces, as predicted by XDLVO approach. Also both bacterial strains adhered better to the hydrophobic ITO-coated glass than to the hydrophilic glass surface, as predicted theoretically. Moreover, the microjet impingement technique was used not only to assess the bacterial adhesion strength on both materials, but also to verify the adhesion reversibility. The detachment stress values demonstrated that PS bacterial cells adhered strongly and irreversibly in the primary energy minimum, while SE bacterial cells adhered weakly and reversibly in the secondary energy minimum on both substrata surfaces. Also, the adhesion of both bacterial strains was found better and stronger on the hydrophobic ITO-coated glass surface than on the hydrophilic glass surface.

Influence of electrolyte species and concentration on the aggregation and transport of fullerene nanoparticles in quartz sands

The potential toxicity of nanoscale particles has received considerable attention, but the fate of engineered nanomaterials in the environment has been studied only under a limited set of conditions. In the present study, batch and column experiments were performed to assess the aggregation and transport of nanoscale fullerene (nC60) particles in water-saturated quartz sands as a function of electrolyte concentration and species. As the electrolyte concentration increased from 1 to 100 mM, the change in nC60 particle diameter was minimal in the presence of NaCl but increased by more than sevenfold in the presence of CaCl2. The latter effect was attributed to the agglomeration of individual nC60 particles, consistent with a net attractive force between particles and suppression of the electrical double layer. At low ionic strength (3.05 mM), nC60 particles were readily transported through 40- to 50-mesh quartz sand, appearing in the column effluent after introducing less than 1.5 pore volumes of nC60 suspension, with approximately 30% and less than 10% of the injected mass retained in the presence of CaCl2 or NaCl, respectively. At higher ionic strength (30.05 mM) and in finer Ottawa sand (100-140 mesh), greater than 95% of the introduced nC60 particles were retained in the column regardless of the electrolyte species. Approximately 50% of the deposited nC60 particles were recovered from 100- to 140-mesh Ottawa sand after sequential introduction of deionized water adjusted the pH to 10 and 12. These findings demonstrate that nC60 transport and retention in water-saturated sand is strongly dependent on electrolyte conditions and that release of deposited nC60 requires substantial changes in surface charge, consistent with retention in a primary energy minimum.

Effect of particle size and natural organic matter on the migration of nano- and microscale latex particles in saturated porous media

In the interest of fully assessing the potential environmental risks linked to “nanolitter,” we need to be able to predict the persistence, toxicity, and mobility of engineered nanomaterials in the natural subsurface environment. To examine the effects of particle size and natural organic matter on nanoparticle mobility, laboratory-scale filtration experiments were performed using different sized model nanomaterials (i.e., latex colloids having diameters of 50, 110, and 1500 nm) in the presence and absence of 5.0 mg/L Suwannee River humic acid (SRHA). At low ionic strengths (1–10 mM KCl), an increase in attachment efficiency ( α) with increasing particle size was observed. This result contrasts with predictions of particle filtration based on attachment in the primary energy minimum of the particle–grain interaction energy profile evaluated using Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The presence of SRHA generally resulted in a decrease in α over the range of experimental conditions investigated. Results of particle transport experiments combined with particle characterization measurements suggest that the decrease in colloid attachment in the presence of SRHA is related to the combined influence of the mechanisms of charge stabilization and steric stabilization.

Effect of Fouling Conditions and Cake Layer Structure on the Ultrasonic Cleaning of Ceramic Membranes

Homogeneous alumina membranes fouled by polystyrene latex particles at different pH values and ionic strengths were subjected to ultrasonic cleaning. Cleaning was more effective at high and low pH than at neutral pH. At low pH values, less repulsive particle‐particle interactions resulted in the removal of millimeter‐scale aggregates and highly effective cleaning. At near‐neutral pH, stronger repulsive particle‐particle interactions caused detachment to occur as individual particles from the cake layer rather than as flocs, which was a slightly less effective cleaning mechanism. Ultrasonic cleaning of cake layers formed at high ionic strength (>0.3 M KCl) was less effective than cleaning at lower ionic strength (<0.3 M KCl). High ionic strength caused particles to coagulate in solution and deposit as flocs on the membrane surface forming a highly permeable fouling layer. This fouling layer was resistant to ultrasound at the sub‐optimal cleaning conditions used in this study, perhaps due to particle attachment occurring within a primary energy minimum. Membrane cleaning experiments performed with particles of varying size showed that particle size was less important than the surface potential of the particles. For a given mass, particles that possessed the largest surface potential formed the thickest fouling layer, irrespective of particle size, and showed the greatest improvement in flux with ultrasonic cleaning. These results demonstrate that solution conditions influence ultrasonic cleaning of membranes primarily by modifying particle‐particle and particle‐membrane interactions as well as cake layer structure, rather than by impacting the extent or magnitude of cavitation events.