Impact of Ionic Strength on Colloid Retention in a Porous Media: A Micromodel Study

Release of deposited colloids in the soil porous media during two-phase flow poses potential health hazard due to the facilitated transport of contaminants towards groundwater reservoirs. Considerable uncertainties exist concerning the impact of ionic strength on pore-scale mechanisms of colloid mobilization during transient flow. This study aims to investigate the effect of ionic strength on colloid retention and mobilization using a glass micromodel. The behavior of Carboxylate modified Polystyrene latex particles of 5 ?m diameter in saline solution (i.e., 100 mM & 1 mM of NaCl at pH 10) was visualized with an optical microscope during saturated and twophase flow. We found that colloid aggregation and attachment on Solid-Water Interfaces (SWI) was increased with increase in ionic strength. CO2 injection into the saturated micromodel mobilized the previously attached colloids on SWI, retained at the Gas- Water Interfaces (GWI) due to capillary forces and thus were transported through the micromodel. Imbibition mobilize colloids from GWI and are transported or reattached on SWI depending on the ionic strength of pore water. The greater adhesive forces of colloids at higher ionic strength was resulted in thin film attachment during drainage and reattachment of colloids mobilized from GWI on SWI during imbibition. The acquired images showed the application of a micromodel for the visualization of colloid retention and re-mobilization through the porous media.This publication was made possible by partial funding from NPRP grant # NPRP8- 594-2-244 from the Qatar National Research Fund (a member of Qatar Foundation). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of funding agencies

TORT3D: A MATLAB code to compute geometric tortuosity from 3D images of unconsolidated porous media

Tortuosity is a parameter that plays a significant role in the characterization of complex porous media systems and it has a significant impact on many engineering and environmental processes and applications. Flow in porous media, diffusion of gases in complex pore structures and membrane flux in water desalination are examples of the application of this important micro-scale parameter. In this paper, an algorithm was developed and implemented as a MATLAB code to compute tortuosity from three-dimensional images. The code reads a segmented image and finds all possible tortuous paths required to compute tortuosity. The code is user-friendly, easy to use and computationally efficient, as it requires a relatively short time to identify all possible connected paths between two boundaries of large images. The main idea of the developed algorithm is that it conducts a guided search for connected paths in the void space of the image utilizing the medial surface of the void space. Once all connected paths are identified in a specific direction, the average of all connected paths in that direction is used to compute tortuosity. Three-dimensional images of sand systems acquired using X-ray computed tomography were used to validate the algorithm. Tortuosity values were computed from three-dimensional images of nine different natural sand systems using the developed algorithm and compared with predicted values by models available in the literature. Findings indicate that the code can successfully compute tortuosity for any unconsolidated porous system irrespective of the shape (i.e., geometry) of particles. 1 2017 Elsevier B.V.Scopu

Influence of Water Table Fluctuation on Natural Source Zone Depletion in Hydrocarbon Contaminated Subsurface Environments

Most of the prediction theories regarding dissolution of organic contaminants in the subsurface systems have been proposed based on the static water conditions and the influence of water fluctuations on mass removal requires further investigations. In this study, it was intended to investigate the effects of water table fluctuations on biogeochemical properties of the contaminated soil at the smear zone between the vadose zone and the groundwater table. An automated 60 cm soil column system was developed and connected to a hydrostatic equilibrium reservoir to impose the water regime by using a multi-channel pump. Four homogenized hydrocarbon contaminated soil columns were constructed and two of them were fully saturated and remained under static water conditions while another two columns were operated under water table fluctuations between the soil surface and 40 cm below it. The experiments were run for 150 days and relevant geochemical indicators as well as dissolved phase concentrations were analyzed at 30 and 50 cm below the soil surface in all columns. The results indicated significant difference in terms of biodegradation effectiveness between the smear zones exposed to static and water table fluctuation conditions. This presentation will provide an overview of the experimental approach, mass removal efficiency, and key findings.This publication was made possible by funding from NPRP grant # NPRP9-093-1-021 from the Qatar national research fund (a member of Qatar Foundation). We acknowledge that all the Gas Chromatography analyses were accomplished in the Central Laboratories unit, Qatar University

Statistical Analysis of the Effect of Water Table Fluctuation and Soil Layering on the Distribution of BTEX on Soil and Groundwater Under Anaerobic Condition

Crude oil, gasoline, and diesel fuel spills pollute groundwater in many coastal areas. BTEX is a hydrocarbon of concern due to its high-water solubility, which allows it to spread widely in the subsurface environment. The mobile phase of LNAPLs percolates through porous soil and accumulates above the water table. Subsurface geological, pollutant morphology, and hydrogeologic site features make natural attenuation difficult to understand. Texture and vertical spatial variability affect soil hydraulic properties and water and contaminant distribution in soil profiles. Changes in rainfall strength and frequency and increased water demand may increase groundwater level oscillations in the next century. Five sets of columns, including one soil column and one equilibrium column, were operated for 150 days. One of the columns was operated under a steady state condition (S), and four columns under transient water table condition. The stable column (S), and the Fluctuating column 1 (F1) contain homogenized soil, while the fluctuating columns 2, 3, and 4 contains heterogenous soil. ORP values at the middle of the columns varied cyclically with WTF. EC values affected greatly by fluctuation and temperature and the statistical test p-value 3.119e-10 0.05). Soil layering affects the attenuation of BTEX, as the peak concentrations for benzene occurred at second imbibition cycle for the homogeneous soil, while for the heterogeneous soil occurred between second and fourth imbibition cycles

The Dependent Clogging Dynamics and Its Impact on Porous Media Permeability Reduction

The dynamics of fine particle entrapment, transport, and deposition within pore systems, particularly the ability of mobile fines to impair permeability within porous media, are critical to a variety of natural and manmade phenomena, impacting oil and gas recovery, slope stability, filter capacity, and the efficiency of lab-on-chip diagnostics in medical disciplines. According to the research, clogging of pore throats in the porous media is not a random process; clogged throats, in particular, modify flow conditions and promote subsequent clogging nearby which is called dependent clogging. Over the last several decades, significant efforts have been made to identify and parameterize the role of dependent clogging in permeability reduction, with studies applying a combination of physical investigation and numerical simulation to this objective. In this work, we deploy a coupled computational fluid dynamics-discrete element method-based framework to investigate fines migration and consequent pore-throat clogging within a geologically realistic pore system extracted from an x-ray microtomographic image of a sand pack. The analysis of the simulation results revealed a spatial correlation between the clogged throats, implying that throats in close proximity became clogged dependently around the same time. Furthermore, dependent clogging was observed to be more frequent than independent clogging and it impacts system permeability more efficiently. This suggests that the distribution of clogged throats has a significant impact on the system's permeability reduction other than the total number of clogged throats.This publication was supported by Qatar University Grant (QUHI-CENG-22/23-517)

Release of colloids in saturated porous media under transient hydro-chemical conditions: A pore-scale study

The deposition and consequent release of colloids pose a significant challenge to the environment, groundwater quality and human health. Subsurface soil contains numerous types of colloids that exhibit a diverse range of interaction favorability with the soil grains and their impact on release behavior remains unclear. The objective of this study was to investigate, at the pore-scale, the impact of colloid interaction favorability on colloid deposition and subsequent release in response to perturbations of flow rate and solution chemistry in saturated porous media. Pore-scale experiments were conducted using a micromodel that is geometrically representative of a real sand-stone rock. Favorable, medium-favorable and unfavorable colloids (i.e., repulsion absent, repulsion at long-range of separation distances and attraction absent with the micromodel surface, respectively) were deposited in the micromodel and then a series of colloid release experiments were conducted at different conditions including increasing the flow rate, decreasing the ionic strength and increasing the solution pH. Favorable colloids exhibited extensive deposition on the collector center where the flow streamlines are parallel to the collector surface, as adhesion forces overcome hydrodynamic forces. However, at medium and high ionic strength, deposition in Forward Flow Stagnation Zone (FFSZ) was dominant for unfavorable colloids as the hydrodynamic forces are negligible. Pore-scale images showed that, upon perturbations in flow rate and solution chemistry, colloids that were initially deposited on collector centers were more susceptible to release as compared to colloids that were initially deposited in FFSZs. The negligible hydrodynamic drag forces in FFSZ and deep primary minimum interaction at short separation distances were the major factors that hindered the release of colloids in FFSZ under transient hydro-chemical conditions. The intensity of colloid deposition and release decreases as the favorability of colloids decreases and as the ionic strength decrease for unfavorable colloids. This study provides a clear insight to the pore-scale colloid deposition and release mechanisms during transient hydro-chemical conditions that help in the modeling of environmental and engineering applications including managed aquifer recharge, groundwater contamination and wastewater treatment processes. 2021 The Author(s)Open Access funding provided by the Qatar National Library. This publication was made possible by partial funding from NPRP grant #NPRP8-594-2-244 from the Qatar National Research Fund (a member of Qatar Foundation). Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of funding agencies. Safna Nishad was funded by the Graduate Assistantship program at Qatar University. The authors would like to thank the Center for Advanced Materials (CAM) at Qatar University for help in Zeta Potential Analysis.Scopu

Pore-scale simulation of fine particles migration in porous media using coupled CFD-DEM

Transport of fine particles in porous media has attracted a considerable interest over the past several decades given its importance to many industrial and natural processes. In order to control fine particles transport in porous media, the behavior itself needs to be predicted and analyzed. Which could be quite costly to do in a laboratory experiment, hence the modelling of such a behavior would be an optimum way to study. In this work, we present a coupled computational fluid dynamics-discrete element method (CFD-DEM) based modelling framework that is capable of simulating the migration of a large number of fine particles, on the colloids size through physically realistic porous media. The model was parallelized using Message Passing Interface (MPI) protocol to resolve particle-particle and particle-grain interactions for a large number of fine particles. The model's geometry was generated from computed tomography images of sand packs, the CFD solver were set to solve the fluid flow while the DEM were to solve the particles physical interactions. Immersed Boundary Method (IBM) were used to couple the two solvers in a resolved manner. The model's setup was first validated to capture single phase fluid flow behavior and then the coupling of the fluid flow and particle interaction was compared to experimental results. In order to validate the coupled CFD-DEM model, velocity profiles of fine particles, their diffusion in the pore space and the percentage of fine particles retained in the pore space obtained from micromodel experiment were compared to values obtained from simulations using the same pore space geometry and initial experimental conditions. The presented framework was shown to capture the bulk dynamics of fine particulate transport and deposition within geologically realistic and complex flow domains while minimizing execution time. The model was used to study the impact of flow velocity and size of fines on permeability reduction of porous media due to migration of fine particles. Simulations indicate that permeability reduction due to fine migration in porous media is directly proportional to flow velocity. The time required to develop bridging and subsequent clogging of the pore space that lead to permeability reduction decreases as the flow velocity increases. The size of the fine particles has a significant impact on permeability reduction where the reduction in permeability increases as the size of particles increases. A faster and larger reduction in permeability was observed when a suspension of polydisperse particles was injected as opposed to a monodisperse suspension. 2022 The AuthorsOpen Access funding provided by the Qatar National Library. This publication was made possible by funding from NPRP grant #NPRP8-594-2-244 from the Qatar National Research Fund (a member of Qatar Foundation). Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of funding agencies.Scopu

Sewage enhanced bioelectrochemical degradation of petroleum hydrocarbons in soil environment through bioelectro-stimulation

The impact of readily biodegradable substrates (sewage and acetate) in bioelectroremediation of hydrocarbons (PW) was evaluated in a bench-scale soil-based hybrid bioelectrochemical system. Addition of bioelectro-stimulants evidenced efficient degradation than control operation. Acetate and sewage were exhibited power density of 1126 mW/m2 and 1145 mW/m2, respectively, which is almost 15 % higher than control (without stimulant, 974 mW/m2). Increased electrochemical activity was correlated well with total petroleum hydrocarbons (TPH) degradation through addition of acetate (TPHR, 525 mg/L, 67.4 %) and sewage (TPHR, 560 mg/L,71.8 %) compared to the control operation (TPHR, 503 mg/L, 64.5 %). Similarly, chemical oxygen demand (COD) reduction was also enhanced from 69.0 % (control) to 72.1 % and 74.6 % with acetate and sewage, respectively. Sewage and acetate also showed a positive role in sulfates removal, which enhanced from 56.0 % (control) to 62.9 % (acetate) and 72.6 % (sewage). This study signifies the superior function of sewage as biostimulant compared to acetate for the bioelectroremediation of hydrocarbons in contaminated soils

Biodegradation Kinetics of Benzene and Naphthalene in the Vadose and Saturated Zones of a (Semi)-Arid Saline Coastal Soil Environment

Biodegradation is a key process for the remediation of sites contaminated by petroleum hydrocarbons (PHCs), but this process is not well known for the (semi)-Arid coastal environments where saline conditions and continuous water level fluctuations are common. This study differs from the limited previous studies on the biodegradation of PHCs in Qatari coastal soils mainly by its findings on the biodegradation kinetics of the selected PHCs of benzene and naphthalene by indigenous bacteria. Soil samples were collected above, across, and below the groundwater table at the eastern coast of Qatar within a depth of 0 to-40 cm. Environmental conditions combining low oxygen and high sulfate concentrations were considered in this study which could favor either or both aerobic and anaerobic bacteria including sulfate-reducing bacteria (SRB). The consideration of SRB was motivated by previously reported high sulfate concentrations in Qatari soil and groundwater. Low-and high-salinity conditions were applied in the experiments, and the results showed the sorption of the two PHCs on the soil samples. Sorption was dominant for naphthalene whereas the biodegradation process contributed the most for the removal of benzene from water. Losses of nitrate observed in the biodegradation experiments were attributed to the activity of nitrate-reducing bacteria (NRB). The results suggested that aerobic, NRB, and most likely SRB biodegraded the two PHCs, where the combined contribution of sorption and biodegradation in biotic microcosms led to considerable concentration losses of the two PHCs in the aqueous phase (31 to 58% after 21 to 35 days). Although benzene was degraded faster than naphthalene, the biodegradation of these two PHCs was in general very slow with rate coefficients in the order of 10-3 to 10-2 day-1 and the applied kinetic models fitted the experimental results very well. It is relevant to mention that these rate coefficients are the contribution from all the microbial groups in the soil and not from just one.Scopu