Colloid Filtration Theory Research Articles

Insights into the controlling factors of the transport of tire wear particles in saturated porous media: The facilitative role of aging and fulvic acid

The widespread distribution and potential adverse effects of tire wear particles (TWPs) on soil and groundwater quality pose a growing environmental concern. This study investigated the transport behavior of TWPs in saturated porous media and elucidated the underlying mechanisms influenced by environmental factors. Additionally, the effects of key environmental factors, such as aging, ionic strength, cation species, medium type, and natural organic matter (NOM), on the transport of TWPs were evaluated. The results showed that aging processes simulated through O3 and UV irradiation altered the physicochemical properties of TWPs, increased the mobility of TWPs at low ionic strengths. However, the high ionic strengths and the presence of Ca2+ significantly inhibited the mobility of TWPs due to enhanced aggregation. The transport mechanism of the original and aged TWPs shifted from blocking to ripening under favorable retention conditions (i.e., high ionic strengths, divalent cations, and fine sands). Interestingly, the presence of fulvic acid (FA) inhibited the ripening of the three TWPs, significantly promoting their transport through a spatial site resistance mechanism. The two-site kinetic attachment model (TSKAM), extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, and colloid filtration theory (CFT) were applied to describe the transport behavior of the TWPs. The study provided a comprehensive understanding of the transport behavior of TWPs in groundwater environments, highlighting the environmental risks associated with their widespread distribution.

Comparative study of polystyrene microplastic transport behavior in three different filter media: Quartz sand, zeolite, and anthracite

Microplastics (MPs) are emerging contaminants that are attracting increasing interest from researchers, and the safety of drinking water is greatly affected by their transportation during filtration. Polystyrene (PS) was selected as a representative MPs, and three filter media (quartz sand, zeolite, and anthracite) commonly found in water plants were used. The retention patterns of PS-MPs by various filter media under various background water quality conditions were methodically investigated with the aid of DLVO theory and colloidal filtration theory. The results show that the different structures and elemental compositions of the three filter media cause them to exhibit different surface roughnesses and surface potentials. A greater surface roughness of the filter media can provide more deposition sites for PS-MPs, and the greater surface roughness of zeolite and anthracite significantly enhances their ability to inhibit the migration of PS-MPs compared with that of quartz sand. However, surface roughness is not the only factor affecting the migration of MPs. The lower absolute value of the surface potential of anthracite causes the DLVO energy between it and PS-MPs to be significantly lower than that between zeolite and PS-MPs, which results in stronger retention of PS-MPs by anthracite, which has a lower surface roughness, than zeolite, which has a higher surface roughness. The transport of PS-MPs in the medium is affected by the combination of the surface roughness of the filter media and the DLVO energy. Under the same operating conditions, the retention efficiencies of the three filter materials for PS-MPs followed the order of quartz sand < zeolite < anthracite. Additionally, the conditions of the solution markedly influenced the transport ability of PS-MPs within the simulated filter column. The transport PS-MPs in the simulated filter column decreased with increasing solution ionic strength and cation valence. Naturally, dissolved organic matter promoted the transfer of PS-MPs in the filter layer, and humic acid had a much stronger facilitating impact than fulvic acid. The study findings might offer helpful insight for improving the ability of filter units ability to retain MPs.

Natural mineral colloids facilitated transport of EE2 in saturated porous media: Effects of humic acid and conjugate form

Steroid estrogens have posed significant ecological risks to aquatic organisms due to their potent endocrine-disrupting effects. The role of natural mineral colloids in facilitating the transport of hydrophobic organic pollutants in the environment has been confirmed, but the control mechanisms of colloids on 17α-Ethinylestradiol (EE2) migration in the subsurface environment are often still not well understood. This study combined the batch sorption equilibrium experiments and dynamic transport simulations to reveal the interface interactions and co-transport characteristics between illite colloids and EE2 at both macroscopic and microscopic levels. The existing form changes of EE2 and the influence of coexisting humic acid (HA) during transport in porous media were also specifically investigated. The batch experiments demonstrated that the primary mechanisms governing EE2 sorption onto illite colloids involved surface sorption and hydrogen bonding. The coexistence of HA could load onto the surface of illite colloids, thereby enhancing the colloidal sorption capacity for EE2. Transport experiments demonstrated that the migratory ability of EE2 in silty clay was limited, but illite colloids could significantly promote its penetration, with the peak penetration content (Ct/C0) increasing from 0.64 to 0.77. In the absence of HA, EE2 primarily transported in a dissolved form, accounting for 62.86% of the total concentrations. When HA concentrations were increased to 10 mg/L and 20 mg/L, the proportion of colloidal conjugate EE2 in the effluents reached 52.13% and 54.49%, respectively. The enhanced transport of EE2 by HA was primarily attributed to the improved migration ability of illite colloids and the increased proportion of illite-EE2 conjugate, resulting in a maximum Ct/C0 value of 0.94. The validity of these results was further confirmed by employing calculations based on the Derjaguin–Landau–Verwey–Overbeek and Colloidal Filtration Theory. This study provides new insights of understanding the transport of EE2 in subsurface environment.

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.

Comparing the Fate and Transport of MS2 Bacteriophage and Sodium Fluorescein in a Karstic Chalk Aquifer.

Groundwater flow and contaminant migration tracing is a vital method of identifying and characterising pollutant source-pathway-receptor linkages in karst aquifers. Bacteriophages are an attractive alternative tracer to non-reactive fluorescent dye tracers, as high titres (>1012 pfu mL-1) can be safely released into the aquifer, offering improved tracer detectability. However, the interpretation of bacteriophage tracer breakthrough curves is complicated as their fate and transport are impacted by aquifer physicochemical conditions. A comparative tracer migration experiment was conducted in a peri-urban catchment in southeast England to characterise the behaviour of MS2 bacteriophage relative to sodium fluorescein dye in a karstic chalk aquifer. Tracers were released into a stream sink and detected at two abstraction boreholes located 3 km and 10 km away. At both sites, the loss of MS2 phage greatly exceeded that of the solute tracer. In contrast, the qualitative shape of the dye and phage breakthrough curves were visually very similar, suggesting that the bacteriophage arriving at each site was governed by comparable transport parameters to the non-reactive dye tracer. The colloid filtration theory was applied to explain the apparent contradiction of comparable tracer breakthrough patterns despite massive phage losses in the subsurface. One-dimensional transport models were also fitted to each breakthrough curve to facilitate a quantitative comparison of the transport parameter values. The model results suggest that the bacteriophage migrates through the conduit system slightly faster than the fluorescent dye, but that the former is significantly less dispersed. These results suggest that whilst the bacteriophage tracer cannot be used to predict receptor concentrations from transport via karstic flow paths, it can provide estimates for groundwater flow and solute contaminant transit times. This study also provides insight into the attenuation and transport of pathogenic viruses in karstic chalk aquifers.

Exploring the Impact of Surface Functionalization on the Reaction, Magnetophoretic, and Collective Transport Behavior of Nanoscale Zerovalent Iron.

This study provides a systematic analysis of the transport and magnetophoretic behavior of nanoscale zerovalent iron (nZVI) particles, both bare and surface functionalized by poly(ethylene glycol) (PEG) and carboxymethyl cellulose (CMC), after undergoing a chemical reaction. Here, a simple and well-investigated chemical reaction of methyl orange (MO) degradation by nZVI was used as a model reaction system, and the sand column transport and low-gradient magnetophoretic profiles of the nanoparticles were measured before and after the reaction. The results were compared over time and analyzed in the context of extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to understand the particle interactions involved. The colloidal stability of both bare and functionalized nZVI particles was enhanced after the reaction due to the consumption of metallic Fe content, resulting in a significant drop in their magnetic properties. As a result, they exhibited improved mobility across the sand column and a slower magnetophoretic collection rate compared to the unreacted particles. Here, the colloidal filtration theory (CFT) was employed to analyze the transport behavior of nZVI particles across the packed sand column. It has been observed that the surface properties of the reacted functionalized particles changed, possibly due to the entrapment of degraded products within the polymer adlayer. Moreover, quartz crystal microbalance with dissipation (QCM-D) measurements were performed to reveal the viscoelastic contribution of the adlayer formed by both bare and functionalized nZVI particles after the reaction on influencing their transport behavior across the sand column. Finally, we proposed the implementation of a high-gradient magnetic trap (HGMT) to reduce the transport distance of the colloidally stable CMC-nZVI, both before and after the reaction. This study sheds light on the behavioral changes of iron nanoparticles after the reaction and highlights environmental concerns regarding the presence of reacted nanoparticles.

An extended colloid filtration theory for modeling Escherichia coli transport in 3-D fracture networks

Microbial transport in fractured carbonate rock using enhanced solutions is a significant and neglected research topic in the literature. We propose an extended colloid filtration theory (CFT) combined with a particle-tracking following streamlines (PTFS) model for the rapid prediction of breakthrough curves (BTCs) and plumes of pathogens in three-dimensional (3-D) discrete fracture networks (DFNs). We adapted CFT in porous media to pathogen transport in fractures containing Terra Rossa (soil) deposits. As an example of the model capability, a simulation was used to predict the 3-D motion field and Escherichia coli count in groundwater originating from the Forcatella managed aquifer recharge (MAR) Facility (Brindisi, Italy) using a DFN composed of 3,900 fractures. In arid regions, MAR facilities are significant for sustaining basic human needs, such as freshwater supply for drinking and crop production. The Markov chain Monte Carlo (MCMC) technique was applied to E. coli counts in the collected water samples to increase data representativeness. The pathogen transport coefficients were further supported by batch filtration tests carried out in the CNR/IRSA Laboratory (Bari, Italy). The mean E. coli attachment rate coefficient of 0.15 × 10−8 m2 d–1 (sticking efficiency = 1.1 × 10−8 m) resulted in a 2.1 log10 removal in 600 m of reclaimed water filtration. The simulation output visualized the E. coli 3-D pathways in groundwater and the positions of contaminated groundwater spring outflows on Forcatella Beach. The simulation results agreed with the mean MCMC output of E. coli concentrations in bathing water under unperturbed geochemical and environmental flow and transport conditions. However, results indicate that concentrations of pathogenic strains, parasites, and enteric viruses may enter the marine environment of MAR sites during flood periods.

Co-transport of Microplastics and a surrogate for Human Enteric Viruses in a saturated column packed with Quartz Sand

Groundwater can be contaminated with infective human enteric viruses from various sources, such as wastewater treatment plant discharge, landfills, septic tanks, agricultural practices, and artificial groundwater recharge. Anthropogenic pollutants, such as microplastics, may exhibit an affinity to transport biocolloids (bacteria, viruses) further and reduce their degradation rates in the natural environment. Human enteric viruses (poliovirus, hepatitis A, rotavirus, and adenovirus) can adsorb to the abiotic surface of microplastics and are simultaneously present in wastewater discharge. These newly formed clumps of pathogens and microplastics could penetrate deeper into soils as vectors for preferential flow and threaten groundwater systems, triggering a higher risk for drinking water and possibly followed by a disease outbreak. The mechanisms behind the adsorption of human enteric viruses on microplastic surfaces and their potential role in prolonging virus survival and promoting environmental transport remain unclear. This study aims to explore the possibility of co-transport of microplastics and human enteric viruses in saturated porous media, using PRD1 bacteriophage as a surrogate. PRD1 bacteriophages have been widely used as surrogates of rotavirus because they share many fundamental properties and features. Column experiments were performed using quartz sand (soil grain size: 0.60 - 1.30 mm) as a porous media in a 30 cm long and 7 cm diameter column. The column experiments were conducted by maintaining Darcy velocity of 2.65 m/day. Three different influent solution scenarios were considered in the experiments: PRD1 mixed with microplastics, PRD1 alone, and microplastics alone. The enumeration of PRD1 in the effluent solution was performed using quantitative polymerase chain reaction (qPCR) as well as the culture method, in order to differentiate between infective and inactive virus transport. Microplastics were quantified using Solid-Phase Cytometry (SPC). Results were analyzed by calculating the collision and sticking efficiencies of the microplastics and PRD1 using the classical colloid filtration theory and Hydrus 1D modeling tool. There was no evidence of interference or inhibition of microplastics on the performance of qPCR and DNA extraction in the methodological setup. Additionally, the efficacy of qPCR and DNA extraction methods did not yield significantly different results across any of the influent solution conditions. Preliminary results suggest that the presence of microplastics enhanced the transport of PRD1, which led to reduced attachment of PRD1 in the porous media. The concentration of infective phages showed a delayed sharp increase, indicating that there may be a sorption mechanism that delays their breakthrough. It is possible that a portion of the active phages possess a higher sticking efficiency and that population heterogeneity contributes to this phenomenon. A comprehensive understanding of the processes that govern virus transport with globally distributed microplastics is crucial for protecting public health.

In-depth understanding of transport behavior of sulfided nano zerovalent iron/reduced graphene oxide@guar gum slurry: Stability and mobility

Nanoscale zero-valent iron (NZVI), which has the advantages of small particle size, large specific surface area, and high reactivity, is often injected into contaminated aquifers in the form of slurry. However, the prone to passivation and agglomeration as well as poor stability and mobility of NZVI limit the further application of this technology in fields. Therefore, sulfided NZVI loaded on reduced graphene oxide (S-NZVI/rGO) and guar gum (GG) with shear-thinning properties as stabilizers were used to synthesize S-NZVI/rGO@GG slurries. SEM, TEM, and FT-IR confirmed that the dispersion and anti-passivation of NZVI were optimized in the coupled system. The stability and mobility of the slurry were improved by increasing the GG concentration, enhancing the pH, and decreasing the ionic strength and the presence of Ca2+ ions, respectively. A modified advection-dispersion equation (ADE) was used to simulate the transport experiments considering the strain and physicochemical deposition/release. Meanwhile, colloidal filtration theory (CFT) demonstrated that Brownian motion plays a dominant role in the migration of S-NZVI/rGO@GG slurry, and the maximum migration distance can be increased by appropriately increasing the injection rate. Extended-Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory showed that the excellent stability and migration of S-NZVI/rGO@GG slurry mainly came from the GG spatial forces. This study has important implications for the field injection of S-NZVI/rGO@GG slurry. According to the injection parameters, the injection range of S-NZVI/rGO@GG slurry is effectively controlled, which lays the foundation for the promotion of application in actual fields.

Clean water production from plastic and heavy metal contaminated waters using redox-sensitive iron nanoparticle-loaded biochar

The unceasing release of tiny plastics (microplastics and nanoplastics) and their additives, like metal ions, into the aquatic systems from industries and other sources is a globally escalating problem. Their combined toxic effects and human health hazard are already proven; hence, their remediation is requisite. This study utilised the nano-zerovalent iron-loaded sugarcane bagasse-derived biochar (nZVI-SBC) for simultaneous removal of Nanoplastics (NPs) of different functionality and size along with metal ions (Ni2+, Cd2+, AsO43−, and CrO42−). Batch and column experiments were conducted, and the results showed an efficient removal of contaminants with maximum sorption of carboxylate-modified NPs of size 500 nm (qmax = 90.3 mg/g) among all three NPs types. Significant removal was observed in Cd2+ in case of cations and CrO42− in case of anions with qmax = 44.0 and 87.8 mg/g, respectively. Kinetics and the isotherm modelling better fitted the pseudo-second-order kinetic model and Sips isotherm model, respectively for both NPs and metal ions. The designed material worked well in pH range of 4–8, ionic strength 1–20 mM and in complex aqueous matrices, with >90% removal. FTIR, zeta potential and the imaging analysis of the reaction precipitates confirmed the electrostatic attraction, pore retention and complexation as the potential mechanisms for removing NPs, whereas, XPS studies confirmed the reduction co-precipitation and surface complexation as the possible mechanism for removing metal ions. High values of attachment efficiency factor calculated from colloidal filtration theory (CFT) validated the experimental results and justified the high sorption of carboxylate modified 500 nm NPs particles. The synthesized material successfully removed both NPs of varying size and functionality and metal ions simultaneously with significant efficacy in complex environmental samples proving the broad applicability of material in realistic environmental conditions and different types of water treatment processes.

Mathematical modeling the transport of few-layer graphene in saturated porous media

This mathematical modeling study incorporates an existing set of effluent data for FLG-NPs transport through water-saturated quartz sand to estimate kinetic particle mobility parameters for a range of experimental ionic strengths (IS) and organic macromolecules (OMs) concentrations. Parameter estimation was done by implementing several NP filtration models in inverse analyses: (i) the clean-bed colloid filtration theory model (CFT, ii) maximum retention capacity model (MRC), and (iii) two subclasses of a two-attachment-site model (2S). It was found that as solution IS (1-100 mM NaCl) increased, the kinetic conditions for particle-collector attachment transitioned from unfavorable (i.e. fractional attachment) to favorable (i.e. diffusion-limited), and the unbounded subclass of 2S lost the better numerical performance compared to CFT and MRC; as the difference between the normalized sum of squared residuals (NSSR) of the unbounded subclass of 2S and CFT decreased from 20% at 1 mM to less than 1% at 100 mM NaCl suggesting the adequacy of CFT to estimate mobility parameters at high ionic strength. As the OMs concentrations increased from 0.8 to 50 total organic carbon (mg (TOC)/L) in the system, the unbounded subclass of 2S showed the best outcomes. Simulation results from all models predicted that attachment efficiency decreases with increasing OM concentration consistent with the observed enhancement of FLG mobility at higher OM levels. The FLG detachment rate coefficient slightly increased with an increase of OM concentration in the influent. The unbounded four-parameter subclass of 2S exhibited a numerical advantage in comparison with CFT and MRC models for estimating FLG mobility parameters at low IS (1 mM NaCl) and high OMs concentration (50 mg (TOC)/L). However, the CFT model yielded similar goodness of fit values very similar to 2S and MRC models at high IS (100 mM NaCl) and low OMs concentration (0.8 mg (TOC)/L).

Capture of colloidal fine suspended particle by aquatic vegetation under rainfall

The capture of colloidal fine suspended particles by vegetation plays an important role in water quality of the shallow aquatic system under rainfall. Quantifying impact of rainfall intensity and vegetation condition on this process remains poorly characterized. In this study, the colloidal particle capture rates under three rainfall intensities, four vegetation densities and with submerged or emergent vegetation were investigated in different travel distance in a laboratory flume. Considering vegetation as porous media, non-Darcy's law with rainfall as a source term, was coupled with colloid first-order deposition model, to simulate the particle concentration changes with time, determining the particle deposition rate coefficient (kd), representing capture rate. We found that the kd increased linearly with rainfall intensity; but increased and then decreased with vegetation density, suggesting the existence of optimum vegetation density. The kd of submerged vegetation is slightly higher than emergent vegetation. The single collector efficiency (η) showed the same trend as kd, suggesting colloid filtration theory well explained the impact of rainfall intensity and vegetation condition. Flow hydrodynamic enhanced the kd trend, e.g., the theoretical strongest flow eddy structure represented in the optimum vegetation density. This study is helpful for the design of wetland under rainfall, to remove colloidal suspended particles and the hazardous material, for the protection of the downstream water quality.

Comparative reductions of norovirus, echovirus, adenovirus, Campylobacter jejuni and process indicator organisms during water filtration in alluvial sand

Sand filtration is a cost-effective means of reducing microbial pathogens in drinking-water treatment. Our understanding of pathogen removal by sand filtration relies largely on studies of process microbial indicators, and comparative data from pathogens are sparse. In this study, we examined the reductions of norovirus, echovirus, adenovirus, bacteriophage MS2 and PRD1, Campylobacter jejuni, and Escherichia coli during water filtration through alluvial sand. Duplicate experiments were conducted using 2 sand columns (50 cm long, 10 cm diameter) and municipal tap water sourced from chlorine-free untreated groundwater (pH 8.0, 1.47 mM) at filtration rates of 1.1–1.3 m/day. The results were analysed using colloid filtration theory and the HYDRUS-1D 2-site attachment-detachment model. The average log10 reduction values (LRVs) of the normalised dimensionless peak concentrations (Cmax/C0) over 0.5 m were: MS2: 0.28; E. coli: 0.76; C. jejuni: 0.78; PRD1: 2.00; echovirus: 2.20; norovirus: 2.35; and adenovirus: 2.79. The relative reductions largely corresponded to the organisms' isoelectric points rather than their particle sizes or hydrophobicities. MS2 underestimated virus reductions by 1.7–2.5 log, and the LRVs, mass recoveries relative to bromide, collision efficiencies, and attachment and detachment rates differed mostly by ∼1 order of magnitude. Conversely, PRD1 reductions were comparable with those of all 3 viruses tested, and its parameter values were mostly within the same orders of magnitude. E. coli seemed an adequate process indicator for C. jejuni with similar reductions. Comparative data describing pathogen and indicator reductions in alluvial sand have important implications for sand filter design, risk assessments of drinking-water supplies from riverbank filtration and the determination of safe setback distances for drinking-water supply wells.

Pore-scale morphology effects on colloid deposition by trajectory tracking simulations

The migration and deposition of colloids in natural porous media is a phenomenon of high importance in hydrocarbon recovery, wastewater treatment and contaminant hydrogeology. Commonly described by colloid filtration theory (CFT), and using a single representative collector element, the theoretical framework is not accounting for morphological effects and heterogeneity of realistic porous media, including the local variations of balance of forces or presence of hydraulically stagnant regions in the vicinity of grain-to-grain contacts. So far developed numerical models of colloidal transport in porous media accounting for all major interactions and pore structure were applied to 2D systems. Known 3D simulations are limited to particulate flow with a reduced set of interactions.We propose a coupled colloid transport model combining computational fluid dynamics (CFD) and discrete element model (DEM) approaches, where the former provides the velocity field at a given time step, while the second calculates the updated colloid positions accounting for a set of forces acting on each colloid. Relationships between colloid retention and morphological characteristics, such as the pore coordination number and surface to volume ratio of a random sphere pack are investigated by pore partitioning of a digitized representation. The role of stagnation zones is quantified by expressing the capture probability of colloids as a function of distance to the nearest grain-to-grain contact measured along the curved surface.The analysis of pore space morphology of the collector and the simulated filtration process reveals a strong relation between colloid retention and pore connectivity. The simulations successfully mimic the expected effects of ionic strength of the carrier fluid, such as the increasing role of grain-to-grain contacts in colloid retention at low ionic strength.

Review of Colloid (Nano‐ and Micro‐Particle) Transport and Surface Interaction in Groundwater. William P. Johnson and Eddy F. Pazmino. The Groundwater Project. 111 pp. ISBN: 978‐1‐77470‐070‐9. DOI: https://doi.org/10.21083/978‐1‐77470‐070‐9

Review of <i>Colloid (Nano‐ and Micro‐Particle) Transport and Surface Interaction in Groundwater</i>. William P. Johnson and Eddy F. Pazmino. The Groundwater Project. 111 pp. ISBN: 978‐1‐77470‐070‐9. DOI: https://doi.org/10.21083/978‐1‐77470‐070‐9

Predicting colloid transport and deposition in an array of collectors

Colloid transport and deposition are influenced by pore structures and transport history. In contrast to the homogeneous single-collector efficiency assumption in the classical colloid filtration theory (CFT), it is realized that for accurate prediction of deposition the pore geometry and variation of single-collector efficiency along the transport direction need to be carefully considered. The axisymmetric Happel sphere-in-cylinder array models are developed. Comprehensive sets of numerical simulations are performed covering a wide range of parameters. We propose an upscaling equation of deposition rate coefficient by adopting the average collector efficiency and the array geometric model. The predictions based on the proposed model are compared with those from the classical CFT and previous experiments. We find that the collector efficiencies decrease substantially along the array in most natural conditions, especially within the first few collectors, which is attributed to the gravitational sedimentation and interception mechanisms. Detailed comparison with existing experimental data demonstrates that the upscaled model more accurately predicts the deposition rate than the equations based on single-collector efficiencies, especially for small and large colloids. Our work suggests the significance of considering array packing structures in CFT and can have implications for predicting colloid transport in industrial and field applications.

Evaluating Single‐Collector Efficiencies of Colloid Deposition From Lagrangian Simulations: Geometric Models and Particle Release Scenarios

AbstractSingle‐collector efficiency is of paramount importance in colloid filtration theory and widely used to represent the average filtration efficiency of a specific porous medium. In this work we present new formulations (unifying the stochastic and limiting trajectory cases) for efficient evaluation of the single‐collector efficiency with Lagrangian simulations in axisymmetric flow field for two commonly adopted particle‐releasing scenarios: from the envelope boundary (Happel sphere‐in‐cell) and from the upstream projected area (sphere‐in‐cylinder). We introduce an equivalent captured flux and define a capture indicator variable for each particle, to avoid calculation of local capture probability in previous studies. The single‐collector efficiencies obtained with the proposed formulations using particle trajectory simulations in axisymmetric flow field are in excellent agreement with full 3D simulation results, which validates our approach. The results are also compared with the Levich equation and published correlation equations. The differences between the two particle‐releasing scenarios are clarified and their impact on the single‐collector efficiency is quantified. The proposed axisymmetric simulation method can significantly reduce the computational time for obtaining single‐collector efficiency in comparison to 3D modeling approach, and can also be conveniently applied to studying particle transport in other practical scenarios involving axial symmetry, such as constricted tubes, impinging jet, stagnation‐point flow.

Removal of bacterial plant pathogens in columns filled with quartz and natural sediments under anoxic and oxygenated conditions

Irrigation with surface water carrying plant pathogens poses a risk for agriculture. Managed aquifer recharge enhances fresh water availability while simultaneously it may reduce the risk of plant diseases by removal of pathogens during aquifer passage. We compared the transport of three plant pathogenic bacteria with Escherichia coli WR1 as reference strain in saturated laboratory column experiments filled with quartz sand, or sandy aquifer sediments. E. coli showed the highest removal, followed by Pectobacterium carotovorum, Dickeya solani and Ralstonia solanacearum. Bacterial and non-reactive tracer breakthrough curves were fitted with Hydrus-1D and compared with colloid filtration theory (CFT). Bacterial attachment to fine and medium aquifer sand under anoxic conditions was highest with attachment rates of max. katt1 = 765 day-1 and 355 day-1, respectively. Attachment was the least to quartz sand under oxic conditions (katt1 = 61 day-1). In CFT, sticking efficiencies were higher in aquifer than in quartz sand but there was no differentiation between fine and medium aquifer sand. Overall removal ranged between < 6.8 log10 m−1 in quartz and up to 40 log10 m−1 in fine aquifer sand. Oxygenation of the anoxic aquifer sediments for two weeks with oxic influent water decreased the removal. The results highlight the potential of natural sand filtration to sufficiently remove plant pathogenic bacteria during aquifer storage.

Colloid‐Pollutant Migration through Saturated Vegetative Filter Strips with Various Hydrodynamics Condition under Rainfall

AbstractFlow hydrodynamic condition impacts the deposition process on vegetation and removal of colloid and colloid‐associated contaminants in saturated vegetative filter strips (VFSs) under rainfall. A 9.6‐m‐long flume was constructed to investigate the retention of colloids through VFSs, with saturated soil and simulated rainfall. Colloidal filtration theory was applied to calculate the colloid deposition rate coefficient (kd) on vegetation surface. The result showed that removal efficiencies (η) of the single‐stem decreased, but colloid kd on the flume scale vegetation increased with increased Reynolds number (Re). Flume experiment found that the colloid retention rate in VFSs decreased as the Re increased. Since deposition on vegetation surface and diffusion into saturated soil are two main removal mechanisms, the discrepancy between kd and retention rate suggests that colloid diffusion into the soil was the dominant process of colloid removal in saturate VFSs. The research deepens the understanding of colloid transport mechanism in VFSs, and provides theoretical basis for the design of VFSs.

Nano-SiO2 transport and retention in saturated porous medium: Influence of pH, ionic strength, and natural organics

Nano silica (nSiO2), induces potential harmful effects on the living environment and human health. It is well established that SiO2 facilitates the co-transport of a variety of other contaminants, including heavy metals and pesticides. The current study focused on the systematic evaluation of the effects of multiple physicochemical parameters such as pH (5, 7, and 9), ionic strength (10, 50, and 100 mM), and humic acid (0.1, 1, and 10 mg/L) on the transport and retention of nSiO2 in saturated porous medium. Additionally, the influent concentration of nSiO2 (10, 50, and 100 mg/L) was also varied. Our experimental findings indicate that the size of nSiO2 aggregates was directly related to the pH, ionic strength, HA, and particle concentration had a significant impact on the breakthrough curves (BTCs). The stability provided by the varying concentrations of pH and humic acid had a significant effect on the size of nSiO2 aggregates and transport (C/C0 > 0.7). The presence of a greater magnitude of negative charge on the surface of both nSiO2 and quartz sand resulted in less aggregation and enhanced flow of nSiO2 through the sand column. The Electrostatic and steric repulsion forces were the primary governing mechanisms in relation to the size of nSiO2 aggregates, affecting the single-collector efficiency and attachment efficiency, which determined the maximal transport of nSiO2. Conversely, a probable increase in Van der Waals force of attraction exacerbated the particle deposition and reduced their mobility for high ionic strength, and particle concentrations (C/C0 < 0.1). The formation of large nSiO2 aggregates, in particular, was principally responsible for the enhancement of nSiO2 retention in sand columns over a broad range of IS and particle concentration. The interaction energy profiles based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory were determined to understand the mechanism of nSiO2 deposition. Aditionally, all the experimental BTCs were mathematically simulated and justified by the colloidal filtration theory (CFT). Considering the environmental ramifications, the transport behavior of nSiO2 was further evaluated in various natural matrices such as river, lake, ground, and tap water. The nSiO2 suspended in the river, lake, and tap water had significantly higher mobility (C/C0 > 0.7), whereas groundwater indicated higher retention (C/C0 < 0.3). The study advances our collective knowledge of physicochemical and environmental parameters that can affect particle mobility.