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HypothesesThe 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.FindingsOur 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.

This work investigates the role that pore structure plays in colloid retention across scales with a novel methodology based on image analysis. Experiments were designed to quantify–with robust statistics–the contribution from commonly proposed retention sites toward colloid immobilization. Specific retention sites include solid-water interface, air-water interface, air-water-solid triple point, grain-to-grain contacts, and thin films. Variable conditions for pore-water content, velocity, and chemistry were tested in a model glass bead porous medium with silver microspheres. Concentration signals from effluent breakthrough and spatial profiles of retained particles from micro X-ray Computed Tomography were used to compute mass balances and enumerate pore-scale regions of interest in three dimensions. At the Darcy-scale, retained colloids follow non-monotonic deposition profiles, which implicates effects from flow-stagnation zones. The spatial distribution of immobilized colloids along the porous medium depth was analyzed by retention site, revealing depth-independent partitioning of colloids. At the pore-scale, dominance and overall saturation of all retention sites considered indicated that the solid-water interface and wedge-shaped regions associated with flow-stagnation (grain-to-grain contacts in saturated and air-water-solid triple points in unsaturated conditions) are the greatest contributors toward retention under the tested conditions. At the interface-scale, xDLVO energy profiles were in agreement with pore-scale observations. Our calculations suggest favorable interactions for colloids and solid-water interfaces and for weak flocculation (e.g., at flow-stagnation zones), but unfavorable interactions between colloids and air-water interfaces. Overall, we demonstrate that pore-structure plays a critical role in colloid immobilization and that Darcy-, pore- and interface-scales are consistent when the pore structure is taken into account.

Deposition of engineered nanoparticles onto porous media from flowing suspensions is important for soil and groundwater quality. The deposition mechanism is controlled by interaction forces between particles and collectors. We investigated the origin and magnitude of opposing forces between silver and mica surfaces (representing nanosilver and sand grains) in solutions relevant to agricultural soils with direct measurements using a surface force apparatus. Solutions of variable NaNO3, Ca(NO3)2, and humic acid (HA) concentrations were used to differentiate individual contributing forces and quantify surface properties. The measured Hamaker constant for silver-water-mica was consistent with Lifshitz theory. Our results indicate that HA forms an adsorbed surface layer, but its charge, thicknesses, compressibility, and mass are significantly larger on mica than silver. Ca2+ primarily reduced the differences between the initially adsorbed HA layer properties on each surface, making them more similar. Force-distance profiles indicate that, when silver-mica systems were exposed to HA, osmotic-steric, electrostatic, and van der Waals forces dominate. Soft particle theory was deemed inappropriate for this system. Derjaguin's approximation was utilized to translate force measurements into interaction energy between nanosilver particles and mica collectors. We propose attachment efficiency estimates from measured surface properties, which suggest high particle mobility when nanosilver is applied to HA-rich agricultural soils with modest ionic strength.

Colloid deposition in granular media is relevant to numerous environmental problems. Classic filtration models assume a homogeneous pore space and largely ignore colloid aggregation. However, substantial evidence exists on the ubiquity of aggregation within porous media, suggesting that deposition is enhanced by it. This work studies the deposition process in relation to aggregate size and structure. We demonstrate that aggregation is induced at typical groundwater velocities by comparing the repulsive DLVO force between particle pairs to the hydrodynamic shear force opposing it. Column experiments imaged with high-resolution X-ray computed tomography are used to measure aggregate structure and describe their morphology probability distribution and spatial distribution. Aggregate volume and surface area are found to be power-law distributed, while Feret diameter is exponentially distributed with some flow rate dependencies caused by erosion and restructuring by the fluid shear. Furthermore, size and shape of aggregates are heterogeneous in depth, where a small number of large aggregates control the concentration versus depth profile shape. The range of aggregate fractal dimensions found (2.22-2.42) implies a high potential for restructuring or breaking during transport. Shear-induced aggregation is not currently considered in macroscopic models for particle filtration, yet is critical to consider in the processes that control deposition.

High degrees of spatial heterogeneity in hydrologic systems pose a major barrier for their protection and remediation. Dissolved and particulate contaminants are mixed and retained over timescales ranging from seconds to years due to their interactions with these structural heterogeneities. Over the last two decades, a new class of models has demonstrated its capacity to describe observed ‘anomalous transport’ behavior that is ubiquitous to nearly all flowing waters. The promise of these models lies in their potential for predicting transport using minimal parameters, while remaining faithful to the underlying complexity of the system. In this review, we highlight recent experimental studies that have improved our understanding of the structural controls of anomalous transport, as well as modeling studies that use these new insights to better predict contaminant fate.

This study investigates the functional correspondence between porescale hydrodynamics, mass transfer, pore structure and biofilm morphology during progressive biofilm colonization of a porous medium. Hydrodynamics and the structure of both the porous medium and the biofilm are experimentally measured with 3D particle tracking velocimetry and micro X-ray Computed Tomography, respectively. The analysis focuses on data obtained in a clean porous medium after 36 h of biofilm growth. Registration of the particle tracking and X-ray data sets allows to delineate the interplay between porous medium geometry, hydrodynamic and mass transfer processes on the morphology of the developing biofilm. A local analysis revealed wide distributions of wall shear stresses and concentration boundary layer thicknesses. The spatial distribution of the biofilm patches uncovered that the wall shear stresses controlled the biofilm development. Neither external nor internal mass transfer limitations were noticeable in the considered system, consistent with the excess supply of nutrient and electron acceptors. The wall shear stress remained constant in the vicinity of the biofilm but increased substantially elsewhere.

[This corrects the article DOI: 10.1371/journal.pone.0180374.].

X-ray tomography is a powerful tool giving access to the morphology of biofilms, in 3D porous media, at the mesoscale. Due to the high water content of biofilms, the attenuation coefficient of biofilms and water are very close, hindering the distinction between biofilms and water without the use of contrast agents. Until now, the use of contrast agents such as barium sulfate, silver-coated micro-particles or 1-chloronaphtalene added to the liquid phase allowed imaging the biofilm 3D morphology. However, these contrast agents are not passive and potentially interact with the biofilm when injected into the sample. Here, we use a natural inorganic compound, namely iron sulfate, as a contrast agent progressively bounded in dilute or colloidal form into the EPS matrix during biofilm growth. By combining a very long source-to-detector distance on a X-ray laboratory source with a Lorentzian filter implemented prior to tomographic reconstruction, we substantially increase the contrast between the biofilm and the surrounding liquid, which allows revealing the 3D biofilm morphology. A comparison of this new method with the method proposed by Davit et al (Davit et al., 2011), which uses barium sulfate as a contrast agent to mark the liquid phase was performed. Quantitative evaluations between the methods revealed substantial differences for the volumetric fractions obtained from both methods. Namely, contrast agent-biofilm interactions (e.g. biofilm detachment) occurring during barium sulfate injection caused a reduction of the biofilm volumetric fraction of more than 50% and displacement of biofilm patches elsewhere in the column. Two key advantages of the newly proposed method are that passive addition of iron sulfate maintains the integrity of the biofilm prior to imaging, and that the biofilm itself is marked by the contrast agent, rather than the liquid phase as in other available methods. The iron sulfate method presented can be applied to understand biofilm development and bioclogging mechanisms in porous materials and the obtained biofilm morphology could be an ideal basis for 3D numerical calculations of hydrodynamic conditions to investigate biofilm-flow coupling.

Ash plays an important role in controlling runoff and erosion processes after wildfire and has frequently been hypothesised to clog soil pores and reduce infiltration. Yet evidence for clogging is incomplete, as research has focussed on identifying the presence of ash in soil; the actual flow processes remain unknown. We conducted laboratory infiltration experiments coupled with microscope observations in pure sands, saturated hydraulic conductivity analysis, and interaction energy calculations, to test whether ash can clog pores (i.e. block pores such that infiltration is hampered and ponding occurs). Although results confirmed previous observations of ash washing into pores, clogging was not observed in the pure sands tested, nor were conditions found for which this does occur. Clogging by means of strong attachment of ash to sand was deemed unlikely given the negative surface charge of the two materials. Ponding due to washing in of ash was also considered improbable given the high saturated conductivity of pure ash and ash-sand mixtures. This first mechanistic step towards analysing ash transport and attachment processes in field soils therefore suggests that pore clogging by ash is unlikely to occur in sands. Discussion is provided on other mechanisms by which ash can affect post-fire hydrology. Journal compilation

This study underpins quantitative relationships that account for the combined effects that starting biomass and peak pyrolysis temperature have on physico-chemical properties of biochar. Meta-data was assembled from published data of diverse biochar samples (n=102) to (i) obtain networks of intercorrelated properties and (ii) derive models that predict biochar properties. Assembled correlation networks provide a qualitative overview of the combinations of biochar properties likely to occur in a sample. Generalized Linear Models are constructed to account for situations of varying complexity, including: dependence of biochar properties on single or multiple predictor variables, where dependence on multiple variables can have additive and/or interactive effects; non-linear relation between the response and predictors; and non-Gaussian data distributions. The web-tool Biochar Engineering implements the derived models to maximize their utility and distribution. Provided examples illustrate the practical use of the networks, models and web-tool to engineer biochars with prescribed properties desirable for hypothetical scenarios.

A fixed-bed sand column with graphene oxide (GO) layer was used to remove heavy metals (Cu(II) and Pb(II)) from an aqueous solution injected under steady flow. Due to the time constrained kinetic process of heavy metal sorption to GO, removal efficiency was affected by the injection flow rate. When injection flow rate changed from 1 to 5mLmin-1, the removal efficiency of the two metals decreased from 15.3% to 10.3% and from 26.7% to 19.0% for Cu(II) and Pb(II), respectively. Provided a fixed concentration of heavy metals in the injected flow, an increase in GO in column from 10 to 30mg resulted in an sharp increase in the removal efficiency of Pb(II) from 26.7% to 40.5%. When Cu(II) and Pb(II) were applied simultaneously, the removal efficiency of the two metals was lower than when applied by individually. GO-sand column performance was much better for the removal of Pb(II) than for Cu(II) in each corresponding treatment. When breakthrough curve (BTC) data were simulated by the convection-dispersion-reaction (CDER) model, the fittings for Cu in every treatment were better than that of Pb in corresponding treatment. Considering the small amount of GO used to enable the sand columns that resulted in a great increase in k value, compared to the GO-free sand columns, the authors propose GO as an effective adsorption media in filters and reactive barriers to remove Pb(II) from flowing water.

Hydraulic fracturing is expanding rapidly in the US to meet increasing energy demand and requires high volumes of hydrofracking fluid to displace natural gas from shale. Accidental spills and deliberate land application of hydrofracking fluids, which return to the surface during hydrofracking, are common causes of environmental contamination. Since the chemistry of hydrofracking fluids favors transport of colloids and mineral particles through rock cracks, it may also facilitate transport of in situ colloids and associated pollutants in unsaturated soils. We investigated this by subsequently injecting deionized water and flowback fluid at increasing flow rates into unsaturated sand columns containing colloids. Colloid retention and mobilization was measured in the column effluent and visualized in situ with bright field microscopy. While

The prediction of colloid transport in unsaturated porous media in the presence of large energy barrier is hampered by scant information of the proportional retention by straining and attractive interactions at surface energy minima. This study aims to fill this gap by performing saturated and unsaturated column experiments in which colloid pulses were added at various ionic strengths (ISs) from 0.1 to 50 mM. Subsequent flushing with deionized water released colloids held at the secondary minimum. Next, destruction of the column freed colloids held by straining. Colloids not recovered at the end of the experiment were quantified as retained at the primary minimum. Results showed that net colloid retention increased with IS and was independent of saturation degree under identical IS and Darcian velocity. Attachment rates were greater in unsaturated columns, despite an over 3-fold increase in pore water velocity relative to saturated columns, because additional retention at the readily available air-associated interfaces (e.g., the air-water-solid [AWS] interfaces) is highly efficient. Complementary visual data showed heavy retention at the AWS interfaces. Retention by secondary minima ranged between 8% and 46% as IS increased, and was greater for saturated conditions. Straining accounted for an average of 57% of the retained colloids with insignificant differences among the treatments. Finally, retention by primary minima ranged between 14% and 35% with increasing IS, and was greater for unsaturated conditions due to capillary pinning.

In this work, sand and bubble column experiments were conducted to explore the deposition mechanisms of graphene oxide (GO) particles in porous media with various combinations of moisture content and ionic strength. Sand column experimental results indicated that retention and transport of GO in porous media were strongly dependent on solution ionic strength. Particularly, GO showed high mobility under low ionic strength conditions in both saturated and unsaturated porous media. Increasing ionic strength dramatically increased the retention of GO particles in porous media, mainly through secondary-minimum deposition as indicated in the XDLVO interaction energy profiles. Recovery rates of GO in unsaturated sand columns were lower than that in saturated columns under the same ionic strength conditions, suggesting moisture content also played an important role in the retention of GO in porous media. Findings from the bubble column experiments showed that the GO did not attach to the air-water interface, which is consistent with the XDLVO predictions. Additional retention mechanisms, such as film straining, thus could be responsible to the reduced mobility of GO in unsaturated porous media. The experimental data of GO transport through saturated and unsaturated porous media could be accurately simulated by an advection-dispersion-reaction model. © 2013 Elsevier B.V..

The prediction of colloid transport in unsaturated porous media in the presence of large energy barrier is hampered by scant information of the proportional retention by straining and attractive interactions at surface energy minima. This study aims to fill this gap by performing saturated and unsaturated column experiments in which colloid pulses were added at various ionic strengths (ISs) from 0.1 to 50 mM. Subsequent flushing with deionized water released colloids held at the secondary minimum. Next, destruction of the column freed colloids held by straining. Colloids not recovered at the end of the experiment were quantified as retained at the primary minimum. Results showed that net colloid retention increased with IS and was independent of saturation degree under identical IS and Darcian velocity. Attachment rates were greater in unsaturated columns, despite an over 3-fold increase in pore water velocity relative to saturated columns, because additional retention at the readily available air-associated interfaces (e.g., the air–water–solid [AWS] interfaces) is highly efficient. Complementary visual data showed heavy retention at the AWS interfaces. Retention by secondary minima ranged between 8% and 46% as IS increased, and was greater for saturated conditions. Straining accounted for an average of 57% of the retained colloids with insignificant differences among the treatments. Finally, retention by primary minima ranged between 14% and 35% with increasing IS, and was greater for unsaturated conditions due to capillary pinning. [ABSTRACT FROM AUTHOR]

Naturally occurring polymers such as organic matter have been known to inhibit aggregation and promote mobility of suspensions in soil environments by imparting steric stability. This increase in mobility can significantly reduce the water filtering capacity of soils, thus jeopardizing a primary function of the vadose zone. Improvements to classic filtration theory have been made to account for the known decrease in attachment efficiency of electrostatically stabilized particles, and more recently, of sterically stabilized particles traveling through simple and saturated porous media. In the absence of an established unsaturated transport expression, and in the absence of applicable theoretical approaches for suspensions with asymmetric and nonindifferent electrolytes, this study presents an empirical correlation to predict attachment efficiency (a) for electrosterically stabilized suspensions in unsaturated systems in the presence of nonideal electrolytes. We show that existing models fall short in estimating polymer-coated colloid deposition in unsaturated media. This deficiency is expected given that the models were developed for saturated conditions where the mechanisms controlling colloid deposition are significantly different. A new correlation is derived from unsaturated transport data and direct characterization of microspheres coated with natural organic matter over a range of pH and CaCl2 concentrations. The improvements to existing transport models include the following: adjustment for a restricted liquid-phase in the medium, development of a quantitative term to account for unsaturated transport phenomena, and adjustments in the relative contribution of steric stability parameters based on direct measurements of the adsorbed polymer layer characteristics. Differences in model formulation for correlations designed for saturated systems and the newly proposed correlation for unsaturated systems are discussed, and the performance of the new model against a comprehensive set of experimental observations is evaluated. [ABSTRACT FROM AUTHOR]

Colloids play an important role in facilitating transport of adsorbed contaminants in soils. Recent studies showed that under saturated conditions colloid retention was a function of its concentration. It is unknown if this is the case under unsaturated conditions. In this study, the effect of colloid concentration on colloid retention was investigated in unsaturated columns by increasing concentrations of colloid influents with varying ionic strength. Colloid retention was observed in situ by bright field microscopy and quantified by measuring colloid breakthrough curves. In our unsaturated experiments, greater input concentrations resulted in increased colloid retention at ionic strength above 0.1 mM, but not in deionized water (i.e., 0 mM ionic strength). Bright field microscope images showed that colloid retention mainly occurred at the solid—water interface and wedge—shaped air—water—solid interfaces, whereas the retention at the grain—grain contacts was minor. Some colloids at the air—water—solid interfaces were rotating and oscillating and thus trapped. Computational hydrodynamic simulation confirmed that the wedge-shaped air—water—solid interface could form a "hydrodynamic trap" by retaining colloids in its low velocity vortices. Direct visualization also revealed that colloids once retained acted as new retention sites for other suspended colloids at ionic strength greater than 0.1 mM and thereby could explain the greater retention with increased input concentrations. Derjaguin—Landau—Verwey—Overbeek (DLVO) energy calculations support this concept. Finally, the results of unsaturated experiments were in agreement with limited saturated experiments under otherwise the same conditions. [ABSTRACT FROM AUTHOR]

The present study investigates the quality of four three-dimensional (3D) printing technologies to accurately reproduce the complex pore structure of a real undisturbed soil sample for laboratory experiments of transport in porous media at a 1:1 scale. Four state-of-the-art 3D printing technologies were evaluated (digital light synthesis, PolyJet with gel support material, low-force stereolithography, and PolyJet with water-soluble support material) using a combination of 3D image analysis from microtomopraphy and flow simulations of the pore structure produced with each 3D printing technique. Accuracy, as determined by matching solid and void volumes, permeability, connected porosity, specific surface area, and pore size distribution of the print against the original digital soil structure, was found to be substantially better for digital light synthesis, as compared to the other tested technologies. Repeatability, as determined by the same metrics but compared between identical prints, was found to be comparable across all printing technologies and did not significantly improve for prints at greater magnification (1.5 ×). Wettability of the samples was improved by plasma treatment of the prints. The thorough analysis herein presented demonstrates that advanced, yet relatively inexpensive 3D printing approaches can be used to generate real-scale high quality analogs of soils/rocks that are much needed for experimental laboratory work. Such a method can open countless opportunities for studying the coupling of pore-structure and hydrodynamics on reactive mass transport in environmental science and engineering, soil science, and other subsurface related fields. • Four 3D printed soil analogues were produced from an undisturbed soil image. • The digital light synthesis technology recreated the most accurate pore structure. • All four printing technologies were highly and similarly reproducible. • The wettability of the printed analogue can be tuned by plasma treatment. • We present an approach that produces accurate and identical soil/rock analogues. [ABSTRACT FROM AUTHOR]