Polymer Coatings Affect Transport and Remobilization of Colloidal Activated Carbon in Saturated Sand Columns: Implications for In Situ Groundwater Remediation.

Colloidal activated carbon (CAC) is an emerging technology for the in situ remediation of groundwater impacted by per- and polyfluoroalkyl substances (PFAS). In assessing the long-term effectiveness of a CAC barrier, it is crucial to evaluate the potential of emplaced CAC particles to be remobilized and migrate away from the sorptive barrier. We examine the effect of two polymer stabilizers, carboxymethyl cellulose (CMC) and polydiallyldimethylammonium chloride (PolyDM), on CAC deposition and remobilization in saturated sand columns. CMC-modified CAC showed high mobility in a wide ionic strength (IS) range from 0.1 to 100 mM, which is favorable for CAC delivery at a sufficient scale. Interestingly, the mobility of PolyDM-modified CAC was high at low IS (0.1 mM) but greatly reduced at high IS (100 mM). Notably, significant remobilization (release) of deposited CMC-CAC particles occurred upon the introduction of solution with low IS following deposition at high IS. In contrast, PolyDM-CAC did not undergo any remobilization following deposition due to its favorable interactions with the quartz sand. We further elucidated the CAC deposition and remobilization behaviors by analyzing colloid-collector interactions through the application of Derjaguin-Landau-Verwey-Overbeek theory, and the inclusion of a discrete representation of charge heterogeneity on the quartz sand surface. The classical colloid filtration theory was also employed to estimate the travel distance of CAC in saturated columns. Our results underscore the roles of polymer coatings and solution chemistry in CAC transport, providing valuable guidelines for the design of in situ CAC remediation with maximized delivery efficiency and barrier longevity.

Important Role of Concave Surfaces in Deposition of Colloids under Favorable Conditions as Revealed by Microscale Visualization.

This study conducted saturated column experiments to systematically investigate deposition of 1 µm positively charged polystyrene latex micro-colloids (representing microplastic particles) on negatively charged rough sand, glass beads, and soil with pore water velocities (PWV) from 4.9 × 10-5 to 8.8 × 10-4 m/s. A critical value of PWV was found below which colloidal attachment efficiency (AE) increased with increasing PWV. The increase in AE with PWV was attributed to enhanced delivery of the colloids and subsequent attachment at concave locations of rough collector surfaces. The AE decreased with further increasing PWV beyond the threshold because the convex sites became unavailable for colloid attachment. By simulating the rough surfaces using the Weierstrass-Mandelbrot equation, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) interaction energy calculations and torque analysis revealed that the adhesive torques could be reduced to be comparable or smaller than hydrodynamic torques even under the favorable conditions. Interestingly, scanning electron microscopic experiments showed that blocking occurred at convex sites at all ionic strengths (ISs) (e.g., even when the colloid-colloid interaction was attractive), whereas at concave sites, blocking and ripening (i.e., attached colloids favor subsequent attachment) occurred at low and high ISs, respectively. To our knowledge, our work was the first to show coexistence of blocking and ripening at high ISs due to variation of the collector surface morphology.

Quantitative Linking of Nanoscale Interactions to Continuum-Scale Nanoparticle and Microplastic Transport in Environmental Granular Media.

Quantitative linkage of fundamental physicochemical characteristics to rate coefficients used in simulations of experimentally observed transport behaviors of nanoparticles and microplastics (colloids) in environmental granular media is an active area of research. Quantitative linkage is herein demonstrated for (i) colloids ranging from nano- to microscale; in two field-based granular media of contrasting grain size, (ii) natural fine sand at the column scale; and (ii) streambed-equilibrated commercial pea gravel at the field scale. Continuum-scale rate coefficients were linked to nanoscale interactions via mechanistic pore-scale colloid trajectory simulations that predicted and defined fast- and slow-attaching subpopulations, as well as nonattaching subpopulations that either remained in the near-surface pore water or re-entrained to bulk pore water. These subfractions of the classic collector efficiency were upscaled to continuum-scale rate coefficients that produced experimentally observed colloid breakthrough-elution concentration histories and nonexponential colloid distributions from the source. The simulations explained transition from hyperexponential to nonmonotonic colloid distributions from the source as driven accumulation of mobile near-surface colloids due to relatively strong secondary minimum interaction and weak diffusion for microscale colloids. The assumption of depletion of the fast-attaching colloid subpopulation by attachment to grain surfaces produced the experimentally observed contrasting distances across which nonexponential colloid distribution from the source occurred in the fine sand versus pea gravel. Rate coefficients were quantitatively calculated from physicochemical parameters and the following three fit parameters: (i) fractional coverage by nanoscale heterogeneity; (ii) efficiency of return to the near-surface domain; and (iii) in explicit simulations, characteristic velocity for scaling transfer to near-surface pore water.

Pathogen Prevalence in Fractured versus Granular Aquifers and the Role of Forward Flow Stagnation Zones on Pore-Scale Delivery to Surfaces.

Lesser pathogen prevalence is well recognized in granular versus fractured aquifers; however, the impact of residence time (inactivation/death) versus removal (pore-scale delivery to surfaces) on pathogen prevalence remains unaddressed. The objective of this study was to examine the specific role of pore-scale delivery to surfaces (removal) as an explanation of contrasting pathogen prevalence in granular versus fractured media from Wisconsin. Inactivation/death was obviated by the use of nonbiological colloids in column transport experiments conducted in representative media from the two Wisconsin sites. Trends in retention as a function of colloid size were examined using nano- to microsized (0.1-4.2 µm) carboxylate-modified polystyrene latex microspheres that represented virus- to protozoa-sized pathogens. Several orders of magnitude greater removal of all colloid sizes were observed in granular relative to those in fractured media, whereas the size corresponding to minimum retention contrasted between the two media. Particle trajectory simulations in collectors (flow fields with surfaces) representing granular versus fractured media captured the observed contrasting retention and trends with colloid size. These results demonstrate that flow impingement on surfaces at forward flow stagnation zones drives contrasting pore-scale delivery to surfaces in granular versus fractured media and potentially the observed contrasting pathogen prevalence in granular versus fractured aquifers.

A Unified Force and Torque Balance for Colloid Transport: Predicting Attachment and Mobilization under Favorable and Unfavorable Conditions.

Colloid attachment and detachment behaviors concern a wide range of environmental contexts but have typically been mechanistically predicted exclusive of one another despite their obvious coupling. Furthermore, previous mechanistic prediction often addressed packed column contexts, wherein specific forces and torques on the colloid could not be well-constrained, preventing robust predictions. These weaknesses were addressed through direct observation experiments under conditions where perfect sink assumptions fail and allow calibration of the contact between the colloid and collector. Attachment and flow perturbation experiments in the presence of colloid-collector attraction (favorable conditions) permitted calibration of contact parameters without the complexity that comes with colloid-collector repulsion (unfavorable conditions). Combining calibrated contact parameters with discrete representative nanoscale heterogeneity, developed to predict unfavorable attachment, provided an independent means to predict unfavorable detachment. The result was mechanistic prediction of colloid attachment and detachment that quantitatively agreed with experimental observation for both ionic strength and flow perturbation results, improving significantly upon previous qualitative prediction.

Hydrodynamic versus Surface Interaction Impacts of Roughness in Closing the Gap between Favorable and Unfavorable Colloid Transport Conditions.

Recent experiments revealed that roughness decreases the gap in colloid attachment between favorable (repulsion absent) and unfavorable (repulsion present) conditions through a combination of hydrodynamic slip and surface interactions with asperities. Hydrodynamic slip was calibrated to experimentally observed tangential colloid velocities, demonstrating that slip length was equal to maximum asperity relief, thereby providing a functional relationship between slip and roughness metrics. Incorporation of the slip length in mechanistic particle trajectory simulations yielded the observed modest decrease in attachment over rough surfaces under favorable conditions, with the observed decreased attachment being due to reduced colloid delivery rather than decreased attraction. Cumulative interactions with multiple asperities acting within the zone of colloid-surface interaction were unable to produce the observed dramatic increased attachment and decreased reversibility with increased roughness under unfavorable conditions, necessitating inclusion of nanoscale attractive heterogeneity that was inferred to have codeveloped with roughness. Simulated attachment matched experimental observations when the spatial frequency of larger heterodomains (nanoscale zones of attraction) increased disproportionately relative to smaller heterodomains as roughness increased, whereas attachment was insensitive to asperity properties, including the number of interactions per asperity and asperity height; colloid detachment simulations were highly sensitive to these parameters. These cumulative findings reveal that hydrodynamic slip moderately decreases colloid bulk delivery, nanoscale heterogeneity dramatically enhances colloid attachment, and multiple interactions among asperities decrease detachment from rough surfaces.

A binomial modeling approach for upscaling colloid transport under unfavorable attachment conditions: Emergent prediction of non-monotonic retention profiles.

We used a recently developed simple mathematical network model to upscale pore-scale colloid transport information determined under unfavorable attachment conditions. Classical log-linear and non-monotonic retention profiles, both well-reported under favorable and unfavorable attachment conditions, respectively, emerged from our upscaling. The primary attribute of the network is colloid transfer between bulk pore fluid, the near surface fluid domain (NSFD), and attachment (treated as irreversible). The network model accounts for colloid transfer to the NSFD of down-gradient grains and for reentrainment to bulk pore fluid via diffusion or via expulsion at rear flow stagnation zones (RFSZs). The model describes colloid transport by a sequence of random trials in a 1D network of Happel cells, which contain a grain and a pore. Using combinatorial analysis that capitalizes on the binomial coefficient, we derived from the pore-scale information the theoretical residence time distribution of colloids in the network. The transition from log-linear to non-monotonic retention profiles occurs when the conditions underlying classical filtration theory are not fulfilled, i.e., when a NSFD colloid population is maintained. Then, nonmonotonic retention profiles result, potentially both for attached and NSFD colloids. The concentration maxima shift downgradient depending on specific parameter choice. The concentration maxima were also shown to shift downgradient temporally (with continued elution) under conditions where attachment is negligible, explaining experimentally-observed down-gradient transport of retained concentration maxima of adhesion-deficient bacteria. For the case of zero reentrainment, we develop closed form, analytical expressions for the shape and the maximum of the colloid retention profile.

Contribution of Nano- to Microscale Roughness to Heterogeneity: Closing the Gap between Unfavorable and Favorable Colloid Attachment Conditions.

Surface roughness has been reported to both increase as well as decrease colloid retention. In order to better understand the boundaries within which roughness operates, attachment of a range of colloid sizes to glass with three levels of roughness was examined under both favorable (energy barrier absent) and unfavorable (energy barrier present) conditions in an impinging jet system. Smooth glass was found to provide the upper and lower bounds for attachment under favorable and unfavorable conditions, respectively. Surface roughness decreased, or even eliminated, the gap between favorable and unfavorable attachment and did so by two mechanisms: (1) under favorable conditions attachment decreased via increased hydrodynamic slip length and reduced attraction and (2) under unfavorable conditions attachment increased via reduced colloid-collector repulsion (reduced radius of curvature) and increased attraction (multiple points of contact, and possibly increased surface charge heterogeneity). Absence of a gap where these forces most strongly operate for smaller (<200 nm) and larger (>2 µm) colloids was observed and discussed. These observations elucidate the role of roughness in colloid attachment under both favorable and unfavorable conditions.

Total Mercury and Methylmercury Response in Water, Sediment, and Biota to Destratification of the Great Salt Lake, Utah, United States.

Measurements of chemical and physical parameters made before and after sealing of culverts in the railroad causeway spanning the Great Salt Lake in late 2013 documented dramatic alterations in the system in response to the elimination of flow between the Great Salt Lake's north and south arms. The flow of denser, more-saline water through the culverts from the north arm (Gunnison Bay) to the south arm (Gilbert Bay) previously drove the perennial stratification of the south arm and the existence of oxic shallow brine and anoxic deep brine layers. Closure of the causeway culverts occurred concurrently with a multiyear drought that resulted in a decrease in the lake elevation and a concomitant increase in top-down erosion of the upper surface of the deep brine layer by wind-forced mixing. The combination of these events resulted in the replacement of the formerly stratified water column in the south arm with one that was vertically homogeneous and oxic. Total mercury concentrations in the deep waters of the south arm decreased by approximately 81% and methylmercury concentrations in deep waters decreased by roughly 86% due to destratification. Methylmercury concentrations decreased by 77% in underlying surficial sediment, whereas there was no change observed in total mercury. The dramatic mercury loss from deep waters and methylmercury loss from underlying sediment in response to causeway sealing provides new understanding of the potential role of the deep brine layer in the accumulation and persistence of methylmercury in the Great Salt Lake. Additional mercury measurements in biota appear to contradict the previously implied connection between elevated methylmercury concentrations in the deep brine layer and elevated mercury in avian species reported prior to causeway sealing.

Prediction of Nanoparticle and Colloid Attachment on Unfavorable Mineral Surfaces Using Representative Discrete Heterogeneity.

Despite several decades of research there currently exists no mechanistic theory to predict colloid attachment in porous media under environmental conditions where colloid-collector repulsion exists (unfavorable conditions for attachment). It has long been inferred that nano- to microscale surface heterogeneity (herein called discrete heterogeneity) drives colloid attachment under unfavorable conditions. Incorporating discrete heterogeneity into colloid-collector interaction calculations in particle trajectory simulations predicts colloid attachment under unfavorable conditions. As yet, discrete heterogeneity cannot be independently measured by spectroscopic or other approaches in ways directly relevant to colloid-surface interaction. This, combined with the fact that a given discrete heterogeneity representation will interact differently with differently sized colloids as well as different ionic strengths for a given sized colloid, suggests a strategy to back out representative discrete heterogeneity by a comparison of simulations to experiments performed across a range of colloid size, solution IS, and fluid velocity. This has recently been performed for interaction of carboxylate-modified polystyrene latex (CML) microsphere attachment to soda lime glass at pH 6.7 with NaCl electrolyte. However, extension to other surfaces, pH values, and electrolytes is needed. For this reason, the attachment of CML (0.25, 1.1, and 2.0 µm diameters) from aqueous suspension onto a variety of unfavorable mineral surfaces (soda lime glass, muscovite, and albite) was examined for pH values of 6.7 and 8.0), fluid velocities (1.71 × 10(-3) and 5.94 × 10(-3) m s(-1)), IS (6.0 and 20 mM), and electrolytes (NaCl, CaSO4, and multivalent mixtures). The resulting representative heterogeneities (heterodomain size and surface coverage, where heterodomain refers to nano- to microscale attractive domains) yielded colloid attachment predictions that were compared to predictions from existing applicable semiempirical expressions in order to examine the strengths and weaknesses of the discrete heterogeneity approach and opportunities for improvement.

Power law size-distributed heterogeneity explains colloid retention on soda lime glass in the presence of energy barriers.

This article concerns reading the nanoscale heterogeneity thought responsible for colloid retention on surfaces in the presence of energy barriers (unfavorable attachment conditions). We back out this heterogeneity on glass surfaces by comparing mechanistic simulations incorporating discrete heterogeneity with colloid deposition experiments performed across a comprehensive set of experimental conditions. Original data is presented for attachment to soda lime glass for three colloid sizes (0.25, 1.1, and 1.95 µm microspheres) under a variety of ionic strengths and fluid velocities in an impinging jet system. A comparison of mechanistic particle trajectory simulations incorporating discrete surface heterogeneity represented by nanoscale zones of positive charge (heterodomains) indicates that a power law size distribution of heterodomains ranging in size from 120 to 60 nm in radius was able to explain the observed retention for all conditions examined. In contrast, uniform and random placement of single-sized heterodomains failed to capture experimentally observed colloid retention across the range of conditions examined.

Release of colloids from primary minimum contact under unfavorable conditions by perturbations in ionic strength and flow rate.

Colloid release from surfaces in response to ionic strength and flow perturbations has been mechanistically simulated. However, these models do not address the mechanism by which colloid attachment occurs, at least in the presence of bulk colloid-collector repulsion (unfavorable conditions), which is a prevalent environmental condition. We test whether a mechanistic model that predicts colloid attachment under unfavorable conditions also predicts colloid release in response to reduced ionic strength (IS) and increased fluid velocity (conditions thought prevalent for mobilization of environmental colloids). The model trades in mean-field colloid-collector interaction for discrete representation of surface heterogeneity, which accounts for a combination of attractive and repulsive interactions simultaneously, and results in an attached colloid population (in primary minimum contact with the surface) having a distribution of strengths of attraction. The model moderates equilibrium separation distance by inclusion of steric interactions. By using the same model parameters to quantitatively predict attachment under unfavorable conditions, simulated release of colloids (for all three sizes) from primary minimum attachment in response to perturbations qualitatively matched experimental results, demonstrating that both attachment and detachment were mechanistically simulated.

A review of exosome separation techniques and characterization of B16-F10 mouse melanoma exosomes with AF4-UV-MALS-DLS-TEM.

Exosomes participate in cancer metastasis, but studying them presents unique challenges as a result of their small size and purification difficulties. Asymmetrical field flow fractionation with in-line ultraviolet absorbance, dynamic light scattering, and multi-angle light scattering was applied to the size separation and characterization of non-labeled B16-F10 exosomes from an aggressive mouse melanoma cell culture line. Fractions were collected and further analyzed using batch mode dynamic light scattering, transmission electron microscopy and compared with known size standards. Fractogram peak positions and computed radii show good agreement between samples and across fractions. Ultraviolet absorbance fractograms in combination with transmission electron micrographs were able to resolve subtle heterogeneity of vesicle retention times between separate batches of B16-F10 exosomes collected several weeks apart. Further, asymmetrical field flow fractionation also effectively separated B16-F10 exosomes into vesicle subpopulations by size. Overall, the flow field flow fractionation instrument combined with multiple detectors was able to rapidly characterize and separate exosomes to a degree not previously demonstrated. These approaches have the potential to facilitate a greater understanding of exosome function by subtype, as well as ultimately allow for "label-free" isolation of large scale clinical exosomes for the purpose of developing future exosome-based diagnostics and therapeutics.

Biased cyclical electrical field flow fractionation for separation of sub 50 nm particles.

Cyclical electrical field flow fractionation (CyElFFF) is a technique for characterizing and separating nanoparticles based on their size and charge using cyclical electric fields. The high diffusion rate of nanoparticles has prevented CyElFFF from being applicable to particles smaller than 100 nm. In this work, the diffusion challenges associated with nanoparticles was resolved using biased cyclical electric fields. This new method, biased cyclical electrical field flow fractionation (BCyElFFF), achieves baseline separation of 15 and 40 nm gold nanoparticles. Theoretical considerations show that the optimal resolution is achieved when the applied bias yields electrical transport that counteracts the diffusive transport of nanoparticles. BCyElFFF greatly extends separation capabilities of the cyclical electrical field flow fractionation to sub 50 nm nanoparticles and provides a powerful alternative to other separation and characterization techniques capable of separating nanoparticles smaller than 50 nm.

Extending applicability of correlation equations to predict colloidal retention in porous media at low fluid velocity.

In this work, we analyzed causes for a recently noted shortcoming of filtration models, which is to predict collector efficiencies greater than unity under low fluid velocity conditions. For Eulerian flux approaches, both the underlying mechanistic model and the correlation equation used to export model results may contribute to this error. For particle trajectory approaches, the error results solely from the correlation equation, not from the underlying mechanistic model, making correction a relatively simple endeavor. Whereas a fitted saturation factor was recently used in a correlation equation to try to force collector efficiencies to remain below unity, we herein develop a different saturation factor based on classic mass transfer relationships to extend the applicability of our correlation equation to low fluid velocities.

Relating mechanistic fate with spatial positioning for colloid transport in surface heterogeneous porous media.

HYPOTHESES: The transport behavior of colloids in subsurface porous media is altered by surface chemical and physical heterogeneities. Understanding the mechanisms involved and distribution outcomes is crucial to assess and control groundwater contamination. The multi-scale processes that broaden residence time distribution for particles in the medium are here succinctly described with an upscaling model. Experiments/model: The spatial distribution of silver particles along glass bead-packed columns obtained from X-ray micro-computed tomography and a mechanistic upscaling model were used to study colloid retention across interface-, collector-, pore-, and Darcy-scales. Simulated energy profiles considering variable colloid-grain interactions were used to determine collector efficiencies from particle trajectories via full force-torque balance. Rate coefficients were determined from collector efficiencies to parameterize the advective-dispersive-reactive model that reports breakthrough curves and depth profiles. FINDINGS: Our results indicate that: (i) with surface heterogeneity, individual colloid-grain interactions are non-unique and span from repulsive to attractive extremes; (ii) experimentally observed spatial positioning of retention at grain-water interfaces and grain-to-grain contacts is governed respectively by mechanistic attachment to the grain surface and retention without contact at rear-flow stagnation zones, and (iii) experimentally observed non-monotonic retention profiles and heavy-tailed breakthrough curves can be modeled with explicit implementation of heterogeneity at smaller scales.

Nanospecific inhibition of pyoverdine siderophore production in Pseudomonas chlororaphis O6 by CuO nanoparticles.

CuO nanoparticles (NPs) exhibit dose-dependent toxicity to bacteria, whereas sublethal concentrations of these NPs change bacterial metabolism. Siderophores are model metabolites to study the impact of sublethal levels of metallic NPs on bacteria because they are involved in survival and interaction with other organisms and with metals. We report that a sublethal level of CuO NPs modify the production of the fluorescent siderophore pyoverdine (PVD) in a soil beneficial bacterium, Pseudomonas chlororaphis O6. The production of PVD was inhibited by CuO NPs but not by bulk CuO nor Cu ions at concentrations equivalent to those released from the NPs. The cell responses occurred despite the NPs forming near micrometer-sized aggregates. The CuO NPs reduced levels of periplasmic and secreted PVD and impaired expression from genes encoding proteins involved in PVD maturation in the periplasm and export through cell membranes. EDTA restored the fluorescence of PVD quenched by Cu ions but did not generate fluorescence with cultures of NP-challenged cells, confirming the absence of PVD. Consequently, depending on the bacterium, this nanoparticle-specific phenomenon mediating cellular reprogramming through effects on secondary metabolism could have an impact on critical environmental processes including bacterial pathogenicity.

Colloidal transport and deposition through dense vegetation.

The effectiveness of submerged synthetic aquatic vegetation on removal of colloids from flowing water was investigated to explore retention of particulate nonpoint source pollutants in aquatic systems. In colloid transport experiments, the deposition rate coefficient of colloids in dense vegetation is often taken as spatially constant. This assumption was tested by experiments and modeling aimed at quantifying changes in colloid retention with travel distance in submerged synthetic aquatic vegetation. Experiments were performed in a 10-m long, 0.6-m wide flume with a 5-cm water depth under different fluid velocities, initial colloid concentrations, and solution pH values. A model accounting for advection, dispersion and first-order kinetic deposition described the experimental data. The colloid deposition rate coefficient showed a power-law decrease with travel distance, and reached a steady state value before the end of the flume. Measured changes in colloid properties with transport distance (ζ potential and size) could not explain the observed decrease. While gravity was shown to contribute to the decrease, its impact was too weak to explain the decreasing power law trend, suggesting that processes operating in granular media to produce similar outcomes may also apply to submerged vegetation.

Surface heterogeneity on hemispheres-in-cell model yields all experimentally-observed non-straining colloid retention mechanisms in porous media in the presence of energy barriers.

Many mechanisms of colloid retention in porous media under unfavorable conditions have been identified from experiments or theory, such as attachment at surface heterogeneities, wedging at grain to grain contacts, retention via secondary energy minimum association in zones of low flow drag, and straining in pore throats too small to pass. However, no previously published model is capable of representing all of these mechanisms of colloid retention. In this work, we demonstrate that incorporation of surface heterogeneity into our hemispheres-in-cell model yields all experimentally observed non-straining retention mechanisms in porous media under unfavorable conditions. We also demonstrate that the predominance of any given retention mechanism depends on the coupled colloid-collector-flow interactions that are governed by parameters such as the size and spatial frequency of heterogeneous attractive domains, colloid size, and solution ionic strength. The force/torque balance-simulated retention is shown to decrease gradually with decreasing solution ionic strength, in agreement with experimental observations. This gradual decrease stands in sharp contrast to predictions from mean field theory that does not account for discrete surface heterogeneity.