Colloid Transport Research Articles

Rapid Characterization and Modeling of Natural and Undefined Charge-Regulated Surfaces in Aqueous Systems.

The surfaces of most materials in aqueous systems are charged due to the ionization of surface functional groups. When these surfaces interact, the surface charge, electrostatic potential, and pH will vary as a function of separation distance, and this process is termed the charge-regulation effect. Charge regulation is a controlling factor in the adhesion and transport of colloids and microorganisms in aqueous systems, and its modeling requires representation of the pH-charge response of the surfaces, typically provided as the equilibrium constants (K) and site densities (N) of the dominant surface functional groups. Existing methods for obtaining these parameters demonstrate shortcomings when applied to many natural and man-made materials, such as weathered materials, materials with undefined or complex surface structures, and permeable materials, and for materials that do not provide the requisite high surface area in suspension due to small sample sizes. This hinders inclusion of the charge-regulation effect in colloid and microbial transport studies, and most studies of colloidal and microbial surface interactions use simplifying assumptions; a key example is the routine use of the constant potential assumption in DLVO modeling. Here we present a robust method that overcomes these issues and provides a rapid means to characterize charge-regulated surfaces using zeta potential data, without requiring a priori knowledge of the material composition. Applying a combined charge-regulation and Gouy-Chapman model, K and N values are obtained that accurately represent the electrostatic response of a charge-regulated surface. This method is demonstrated using activated carbon, aluminum oxide, iron (hydr)oxide, feldspar, and silica sand. The resulting K and N values are then used to show the variations in surface charge, electrostatic potential, and pH that can occur as these charge-regulated surfaces interact. This method provides a readily applied experimental approach for characterizing charge-regulated surfaces, with the overall goal to promote the inclusion of charge-regulated interactions into adhesion and transport studies with natural and undefined materials.

The Role of Stabilizer Concentration in the Mobility of Carbon‐Supported Nanozerovalent Iron (nZVI) in Fractured Media

AbstractThe mobility of nanozerovalent iron (nZVI) particles is of major importance when assessing their effectiveness for use in contaminated sites. Here, the influence of the stabilizer loading on nZVI mobility was investigated in fractured media. nZVI in the form of Carbo‐Iron Colloids (CIC), a composite material of activated carbon as a carrier for nZVI, was used for the tests. CIC was supplemented with carboxymethyl cellulose stabilizer, at three different loadings: 5% wt., 20% wt., and 80% wt., in a moderately saline solution of 250 mM. Transport experiments were conducted in a fractured chalk core, and the characteristics of the solutions and particles were analyzed. The effect of ionic strength was investigated by application of the 80% loading, also in a lower salinity solution of ~3 mM. Transport experiments were modeled, and all results were compared with especially high 1,600% wt. stabilizer loading, from a previous study. Among the different carboxymethyl cellulose loadings, significant differences in mobility and a correlation between mobility and stability were observed. Though, differences in stability were found insignificant. Higher stabilizer concentration and, to a greater extent, a higher number of free stabilizer molecules in solution enhance particle stability and, hence, mobility. A good fit was obtained between the transport experiment results and a model of solute and colloid transport in a single fracture. The model attachment parameters showed a good correlation with the decay rate constants of the stability tests, indicating yet again the significant role played by stability in CIC mobility and the possibility for mobility prediction.

The impact of nanoparticle aggregation on their size exclusion during transport in porous media: One- and three-dimensional modelling investigations

Greater particle mobility in subsurface environments due to larger size, known as size exclusion, has been responsible for colloid-facilitated transport of groundwater contaminants. Although size exclusion is not expected for primary engineered nanoparticles (NP), they can grow in size due to aggregation, thereby undergoing size exclusion. To investigate this hypothesis, an accurate population balance modelling approach and other colloid transport theories, have been incorporated into a three-dimensional transport model, MT3D-USGS. Results show that incorporating aggregation into the transport model improves the predictivity of current theoretical and empirical approaches to NP deposition in porous media. Considering an artificial size-variable acceleration factor in the model, NP breakthrough curves display an earlier arrival when aggregation is included than without. Disregarding the acceleration factor, aggregation enhances NP mobility at regions close to the injection point at a field scale and causes their retention at greater distances through alteration of their diffusivities, secondary interaction-energy minima, and settling behaviour. This results in a change of residual concentration profiles from exponential for non-aggregating dispersions to non-monotonic for aggregating dispersions. Overall, aggregation, hitherto believed to hinder the migration of NP in subsurface porous media, may under certain physicochemical conditions enhance their mobilities and deliver them to further distances.

Mycelial Effects on Phage Retention during Transport in a Microfluidic Platform.

Phages (i.e., viruses that infect bacteria) have been considered as good tracers for the hydrological transport of colloids and (pathogenic) viruses. However, little is known about interactions of phages with (fungal) mycelia as the prevalent soil microbial biomass. Forming extensive and dense networks, mycelia provide significant surfaces for phage-hyphal interactions. Here, for the first time, we quantified the mycelial retention of phages in a microfluidic platform that allowed for defined fluid exchange around hyphae. Two common lytic tracer phages (Escherichia coli phage T4 and marine phage PSA-HS2) and two mycelia of differing surface properties (Coprinopsis cinerea and Pythium ultimum) were employed. Phage-hyphal interaction energies were approximated by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) approach of colloidal interaction. Our data show initial hyphal retention of phages of up to ≈4 × 107 plaque-forming unit (PFU) mm-2 (≈2550 PFU mm-2 s-1) with a retention efficiency depending on the hyphal and, to a lesser extent, the phage surface properties. Experimental data were supported by XDLVO calculations, which revealed the highest attractive forces for the interaction between hydrophobic T4 phages and hydrophobic C. cinerea surfaces. Our data suggest that mycelia may be relevant for the retention of phages in the subsurface and need to be considered in subsurface phage tracer studies. Mycelia-phage interactions may further be exploited for the development of novel strategies to reduce or hinder the transport of undesirable (bio) colloidal entities in environmental filter systems.

Colloidal transport mechanisms and sequestration of U, Ni, and As in meromictic mine pit lakes

This study investigated the biogeochemistry of uranium (U), and the co-occurring elements nickel (Ni) and arsenic (As), in colloids and sediments from two meromictic mine pit lakes that have considerably different depths and geochemistry. In order to characterize the processes controlling metal speciation and cycling in the pits, the distribution and speciation of the elements in colloidal size fractions were analyzed using micro- and ultrafiltration in combination with transmission electron microscopy. Sediment traps collected fresh sediments over the course of one year below the chemocline of the pit lakes and were subsequently analyzed by scanning electron microscopy (SEM) for morphology and chemical digestions. The most common particles found in the shallower pit consisted of Ca-O and Fe-O colloids, while the particles in the deeper pit were composed of Ca-S-O. Filtration results showed a higher abundance of metals in larger colloidal fractions in aged samples, suggesting that colloids can act as metal accumulators. Sediment traps showed the formation of Fe-O, Fe-S, Al-Si, and Ce-P phases, which were observed to sorb U and Ni. The overall U removal was calculated to be 0.9 g/m2/year in both pits, despite considerably different geochemical conditions between the two, and the maximum removal rates for As (shallow pit) and Ni (deeper pit) were 4.7 g/m2/year and 0.6 g/m2/year, respectively. Bottom sediments were also collected from both pits, and characterized using sequential extractions, SEM, synchrotron-based X-ray absorption spectroscopy and Laue diffraction techniques. These techniques showed that the stability of metals in the sediment follows the order Ni < U < As. Nickel, found in the exchangeable and acid soluble fractions, could be easily mobilized by changes in aqueous ionic strength and pH. Similarly, pH and redox changes in the water may strongly affect U, due to its dominance in the acid soluble and reducible sediment fractions. U(VI) mineral phases confirmed by spectroscopy and Laue diffraction, such as vandendriesscheite and monazite, which were likely associated with the oxidizable sediment fraction, may increase overall U stability. Arsenic was more strongly bound to the sediments and mostly present in the oxidizable and residual (highly recalcitrant) fractions. The collective results demonstrate the importance of colloidal particles for metal sequestration processes and their potential importance on microbial communities in pit lakes, highlighting the need for improved characterization of colloids in environmental studies.

Colloidal transport of carbon and metals by western Siberian rivers during different seasons across a permafrost gradient

In contrast to fairly good knowledge of dissolved (<0.45 µm) and particulate (>0.45 µm) fluxes of carbon, nutrients and metals from the land to the ocean, colloidal (1 kDa−0.45 µm) forms of solutes are rarely quantified. This is especially true for Siberian rivers draining into the Arctic Ocean: because of organic-rich soils, colloidal fractions of elements are high and may sizably impact coastal biological processes. However, the main environmental parameters such as seasons, river size, climate, permafrost distribution and landscape parameters of the watershed controlling colloidal distribution of organic carbon (OC) and metals remain totally unknown. Here we used on-site centrifugation combined with ultrafiltration via 3, 30 and 100 kDa pore size membranes and 1 kDa dialysis to characterize colloidal size fractionation of OC and metals in 32 western Siberian rivers, ranging in size from 10 to 150,000 km2 watershed area, across a climate and permafrost gradient (from absent to continuous permafrost). The dominant forms of OC and metals was low molecular weight LMW< 3 kDa fraction and medium molecular weight (MMW3 kDa-30 kDa) colloids. The LMW< 3 kDa fraction of OC increased in the order spring < autumn < summer, following progressive replacement of allochthonous medium and high molecular weight colloids in spring by autochthonous LMW organic ligands in summer; the latter possibly occurred due to exometabolites of plankton and periphyton. The LMW OC fraction became low again in autumn, presumably due to appearance of allochthonous DOM washed out by autumn rains from peat soil at maximum thawing depth. The size of the watershed had subordinate influence on colloidal distribution compared to seasons. The effect of landscape parameters of the watershed on the colloidal status of solutes was subordinate to seasonal and permafrost-zone controls. Overall, the ongoing environmental changes in WSL will likely decrease the proportion of mineral (Fe, Al – bearing) colloids of C and trace metals exported by rivers to the Kara Sea. As a result, the flux of dissolved C, micro-nutrients and metal toxicants may become increasingly bioavailable to coastal aquatic biota.

Stochastic model for filtration by porous materials

Transport and filtration of colloids through permeable materials is controlled by the underlying heterogeneous structure of the host medium. A stochastic model is proposed that takes into account pore-scale heterogeneity, which controls the macroscopic deposition.

Lead was mobilized in acid silty clay loam paddy soil with potassium dihydrogen phosphate (KDP) amendment

The immobilization effectiveness between Pb and phosphorus in soil varies with soil types. To clarify the effect of phosphate on the availability of Pb in agricultural soil, a culture experiment with three types of paddy soil was performed with potassium dihydrogen phosphate (KDP) added. EDTA, DGT and in-situ solution extraction methods were used to represent different available Pb content. Results showed that the concentration of EDTA-Pb in HN soil was slightly elevated after exogenous KDP added. The supplement of 300 mg/kg KDP significantly increased the content of soluble Pb in both acid silty clay loam soil and neutral silty loam soil (increased by 104.65% and 65.12%, respectively). However, there was no significant influence of KDP on the concentration of DGT extracted Pb. XANES results showed that Pb(OH)2, PbHPO4, humic acid-Pb and GSH-Pb were the major speciation of Pb in soil colloids. The proportion of Pb(OH)2 and humic acid-bounded Pb in soil colloids were elevated after exogenous KDP added. Our results indicated that there was a mobilization effect of KDP on Pb by increasing the amount of colloidal Pb in soil solution, especially in acid silty clay loam paddy soil. Such colloid-facilitated transport might promote the uptake of Pb in rice and pose a potential threat to human health.

Improved understanding of particle transport in karst groundwater using natural sediments as tracers

Colloids and particles act as vectors for contaminant transport. In karst aquifers, particle transport is particularly efficient and plays critical roles in soil erosion and in the process of karstification. However, available techniques for particle tracing are either expensive or not representative for the transport of natural colloids and particles. We developed a new method for particle tracing, using natural sediments as artificial tracers, and first applied this method at a karst experimental site in the Alps. Suspended particles were injected into a swallow hole together with a conservative solute tracer for comparison. Breakthrough curves for 32 different particle size classes between 0.8 and 450 μm were recorded at a karst spring 230 m away using a mobile particle counter that allows quantitative detection at high temporal resolution. Results show that (i) sediments can be used as efficient particle tracers in karst groundwater; (ii) recoveries are similar for particles and solutes; (iii) mean velocity increases with increasing particle size; (iv) dispersion decreases with increasing particle size; (v) these observations point to exclusion processes. As a conclusion, this new experimental technique allows new insights into the transport and fate of colloids and particles in groundwater at affordable costs.

Modeling the transport of nitrate and natural multi-sized colloids in natural soil and soil amended with vermicompost

This study examines the impact of saturated-unsaturated flow conditions, different pore-water velocities, and vermicompost application on nitrate and colloid transport in soil. Twenty-four columns were filled with a sandy loam soil or the soil amended with vermicompost. Three sets of miscible-displacement experiments were conducted, varying the saturation and mean pore-water velocity. A non-reactive tracer test using nitrate was first conducted for each column, after which the colloid transport test was performed under the same conditions. The one-dimensional advection-dispersion equation was employed to model nitrate transport, while the coupled advection-dispersion equation with two-site kinetic model was applied to simulate colloid transport. Dispersivities determined from the nitrate tracer tests were very similar for all conditions for the two media, which is consistent with the experiment conditions (homogeneously packed columns and small changes in water saturation). The addition of the vermicompost caused a slight increase in dispersivity compared to the unamended soil. Employing nitrate dispersivities to model the colloid transport slightly reduced the accuracy of estimated breakthrough curves, which resulted in small changes in the fitted colloid transport coefficients. The experimental results revealed that colloid retention was greater under unsaturated conditions. Conversely, adding vermicompost to the soil reduced colloid retention. The findings of this study provide some recommendations for agricultural researchers and policy-makers to manage soil quality and control water resources pollution by understanding the critical role of hydraulic conditions (flow rate, water saturation) on transport of nitrate and colloids in soil.

Chemical Aging Changed Aggregation Kinetics and Transport of Biochar Colloids.

Little is known about aggregation and transport behaviors of aged biochar colloids in the terrestrial environment. This study investigated aggregation kinetics and transport of biochar colloids from aged (HNO3 treatment) and pristine pinewood biochars pyrolyzed at 300 and 600 °C (PB300 and PB600) in NaCl and CaCl2 solutions. In NaCl solutions, critical coagulation concentrations (CCCs) of aged PB300 and PB600 colloids (540 mM and 327 mM) were much greater than the CCCs of pristine biochar colloids (300 mM and 182 mM). This is likely due to substantial increase of negatively charged oxygen-containing functional groups (primarily carboxyl) on aged biochar surfaces. Intriguingly, in CaCl2 solutions the CCCs of the aged PB300 and PB600 colloids decreased to 25.2 mM and 32.1 mM from 58.6 mM and 41.7 mM for the pristine colloids, respectively. This probably resulted from greater surface charge neutralization and Ca2+ bridging for the aged biochar colloids. In salt solutions (e.g., 10 and 50 mM NaCl and 1 and 10 mM CaCl2), the aged biochar colloids showed higher mobility in porous media than the pristine biochar colloids. This study demonstrated that pristine and aged biochar colloids were stable in the solutions with environmentally relevant ionic strength, and the aging process might substantially increase their mobility in the subsurface.

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.

Effects of Early Structural Changes of Engineered Soils on Green Roof and Bioretention Performance

Engineered soils play an important role in urban hydrology e.g. in the functioning of green roofs and storm water bioretention beds. Water infiltration, colloid transport and heat transport are affected by changes in pore system geometry particularly due to development of macropores and clogging by particles. The rate of pedogenesis is often faster than in natural soils due to higher loads of particles as well as by extreme water regimes. In the presented project we assess the temporal changes of hydraulic properties of engineered soils in typical bioretention beds and green roofs by conducting field scale experiments. The aim is to elucidate changes in hydraulic properties by studying the structural changes of soils at the microscale by invasive and noninvasive methods. The outcomes of the research will lead to improved design and management procedures for green roofs and bioretention beds.

Two-site colloid transport with reversible and irreversible attachment: Analytical solutions

Analytical solutions for colloid transport in water saturated, one-dimensional, homogeneous porous media, under fully developed uniform flow are presented. The colloids can be either suspended in the aqueous phase or attached reversibly and/or irreversibly onto the solid matrix. Both attachment rates are assumed linear, and any possible blocking or ripening effects are neglected. The colloids can be either of neutral density or denser than the water. For dense colloids, gravity effects are accounted for. Gravity may enhance or hinder the migration of dense colloids, depending on the flow direction. Upstream boundary conditions for both instantaneous and broad-pulse colloid injections are accounted for. The new analytical solutions were used to successfully fit available experimental data.

Co-transport of U(VI), humic acid and colloidal gibbsite in water-saturated porous media

The release of uranyl from uranium tailing sites is a widely concerned environmental issue, with limited investigations on the effect of coexistence of various colloids. Gibbsite colloids extensively exist, together with ubiquitous humic substances, in uranium polluted waters at tailing sites, due to high concentration of dissolved Al in acid mine drainage. In this context, we investigated the co-transport of U(VI), gibbsite colloids and humic acid (HA) as a function of pH and ionic strength at a U(VI) concentration (5.0 × 10−5 M) relevant within mine tailings and related waste. It was found that, owing to electrostatic attraction, gibbsite colloids and HA associated with each other and transported simultaneously regardless of U(VI) presence. Besides the impact of pH and ionic strength, whether gibbsite colloids facilitated U(VI) transport depended on HA concentration. Gibbsite colloids impeded U(VI) transport at relatively low HA concentration (≤5 mg L−1), because associated colloids loaded with U(VI) were positively charged which favored colloid retention on negatively charged quartz sand in the column. U(VI) together with gibbsite colloids and low concentration HA was completely blocked at natural pH and/or high ionic strength. At relatively high HA concentration (20 mg L−1), however, the associated colloids showed negative zeta potential which facilitated U(VI) transport because of repulsion between negatively charged colloids and quartz sand. Meanwhile, high concentration of HA dramatically accelerated the transport of gibbsite colloids. These results implied that gibbsite colloids might imped U(VI) migration at uranium tailing sites unless the aquifers are enriched with abundant humic substances.

Quantifying colloid fate and transport through dense vegetation and soil systems using a particle-plugging tempered fractional-derivative model

Colloid contaminants are widely distributed in surface runoff from crop land and can be effectively removed by vegetative filter strips (VFS), whose quantification however proves difficult. Standard mechanism-based models contain many unknown parameters with intrinsic uncertainty, limiting their applicability and potential extension for other environmental conditions and colloid contaminant types. To remedy this limitation and capture the complex dynamics of colloids through the soil-vegetation system, this study proposes a parsimonious, particle-plugging tempered fractional advection-dispersion eq. (P-TFADE) with a few empirical parameters, which is built upon the promising fractional calculus engine. The P-TFADE model extends the promising tempered fractional derivative model by incorporating a plugging term, which is then proved to be able to capture both the plugging dynamics and tailing behavior of colloids under various hydrologic and geochemical conditions. Applications also show that the two critical parameters in the P-TFADE model, the time index (α) and plugging coefficient (Kp), can efficiently characterize the impact of the flowrate and ionic condition on transport of different sized colloids observed in our laboratory. In addition, the vegetation type determines the overall structure of the soil-vegetation system, whose impact on the colloid removal efficiency can be quantified by adding a parameter λ in the physical model. Therefore, the novel P-TFADE model can reduce the model uncertainty and help us further understand the nature of colloid dynamics through dense vegetation and soil systems.

Effect of sodium concentration on mobilization and fate of trace metals in standard OECD soil

The effect of different Na concentrations on the fate of trace metals (Cd, Cu, Ni, Zn) in standard OECD soil was evaluated by performing soil leaching column experiments. Five Na concentrations added in synthetic irrigation water (0, 1, 5, 10, 50 mM) were studied in order to evaluate the fate of the metals contained in both the irrigation water leachate and the soil layer. In all experiments, metals mostly accumulated on the top soil layer (0–0.5 cm), at variable concentrations according to the Na content in the artificial irrigation water. Nevertheless, concentration peaks of metal contamination occurred at different sampling time in the soil leachates depending on the metal and on influent water sodicity. Peaks of metals in the leachate appeared simultaneously with the release of organic matter and/or release of Al, suggesting significant involvement of colloids in metals transport. Sodium concentration (10–50 mM) was demonstrated to highly reduce colloidal mobilization leading to the accumulation of more than 95% of the influent metal in the top soil layer. Conversely, low Na concentrations (1–5 mM) favored colloidal transport leading to the recovery of metals in the soil leachates.

Characterizing surface electrochemical properties of simulated bulk soil in situ by streaming potential measurements

To evaluate the feasibility of using streaming potential to characterize the surface electrochemical properties of soil in situ, a quartz sand‐packed column was used to mimic the porous structure and architecture of bulk soil. A laboratory‐made apparatus was developed to measure in situ the streaming potential of quartz grains in the packed column under environmentally relevant physicochemical conditions (e.g. electrolyte pH, Fe oxide coating on sand grains, transport of goethite and amorphous Al hydroxide colloids, and the presence of humic acid). The results showed a significant positive relation between the measured streaming potentials and zeta potentials of the quartz grains examined. With decreasing electrolyte pH, the streaming potential of the quartz grains became less negative because of the greater protonation effect. Similarly, when Fe oxide was adsorbed on to the quartz surfaces, the streaming potential of Fe oxide‐coated quartz grains was less negative than that of uncoated quartz grains as a result of charge neutralization between positively charged Fe oxide and negatively charged sand grains. However, the streaming potential of Fe oxide‐coated quartz grains became more negative when humic acid was added to the electrolyte solution because of increased adsorption of negatively charged humic acid. Interestingly, an obvious change of streaming potential in the processes of colloidal goethite and amorphous Al hydroxide transport in the sand‐packed column grains occurred. Given that the overall positive charge of the colloidal amorphous Al was greater than that of goethite colloids, transport of amorphous Al colloids was found to exert a stronger effect on the variation in streaming potential of quartz sand. Taken altogether, the in situ streaming potential results showed that streaming potential can be used as a powerful index to characterize directly the surface electrochemical properties of quartz grains representing bulk soil.Highlights The variation in surface electrochemical properties of bulk soil needs investigation. We measured the streaming potential produced by simulated bulk soil using a laboratory‐made apparatus. The streaming potential of bulk soil varied with the change in environmentally relevant conditions. Streaming potential is a powerful index characterizing the surface electrochemical properties of bulk soil.

Rotation and Retention Dynamics of Rod-Shaped Colloids with Surface Charge Heterogeneity in Sphere-in-Cell Porous Media Model.

Colloid surface charge heterogeneity was incorporated into a three-dimensional trajectory model, which simulated particle translation and rotation via a force/torque analysis, to study the transport and retention dynamics of rod-shaped colloids over a wide size range in porous media under unfavorable conditions (energy barriers to deposition exist). Our previous study Li , K. ; Ma , H. Deposition Dynamics of Rod-Shaped Colloids during Transport in Porous Media under Favorable Conditions , Langmuir , 2018 , 34 , 9 , 2967 - 2980 , 10.1021/acs.langmuir.7b03983 for rod transport under favorable conditions (lacking energy barriers) demonstrated that particle rotation due to the coupled effect of flow hydrodynamics and Brownian rotation governed rod transport and retention. In this work, we showed that the shape of a colloid affected both transport process and colloid-collector interactions, but shape alone could not make rods to overcome energy barriers of over tens of kT for attachment under unfavorable conditions. The location of colloid surface heterogeneity did not affect transport but predominantly affected colloid-surface interactions by influencing the likelihood of heterogeneity patches facing the collector due to particle rotation. For surface heterogeneity located on the end(s) of a colloid, rods displayed enhanced retention compared with spheres; for surface heterogeneity located on the middle band, rods showed less retention compared with spheres. It was more effective to arrest a traveling rod when surface heterogeneity was located on the end relative to the side, because the tumbling motion greatly increased the likelihood of the end to intercept collector surfaces, and also because a rod would experience less repulsion with an end-on orientation relative to the collector surface compared to a side-on orientation due to the curvature effect. The influences of the particle aspect ratio on retention strongly depended upon the location of colloid surface heterogeneity. Our findings demonstrated that rods had distinct rotation and retention behaviors from spheres under conditions typically encountered in the environment; thus, particle rotation should be considered when studying the transport process of nonspherical colloids or spherical particles with inhomogeneous surface properties.

Colloid Transport in Porous Media: A Review of Classical Mechanisms and Emerging Topics

To celebrate the tenth anniversary of InterPore, we present an interdisciplinary review of colloid transport through porous media. This review aims to explore both classical colloid transport and topics that fall outside that purview and thus offer transformative insights into the physics governing transport behavior. First, we discuss the unique colloid characteristics relative to molecules and larger particles. Then, the classical advection–dispersion–filtration models (both conceptual and mathematical) of colloid transport are introduced as well as anomalous transport behaviors. Next, the forces of interaction between colloids and porous media surfaces are discussed. Fourth, applications that are interested in maximizing the transport of colloids through porous media are considered. Then the concept of motile, active biocolloids is introduced, and finally, colloid swarming as a newly recognized mode of transport is summarized.