Opportunities for Particles and Particle Suspensions to Experience Enhanced Transport in Porous Media: A Review

Multiphase displacement manipulated by micro/nanoparticle suspensions in porous media via microfluidic experiments: From interface science to multiphase flow patterns

Multiphase displacement in porous media can be adjusted by micro/nanoparticle suspensions, which is widespread in many scientific and industrial contexts. Direct visualization of suspension flow dynamics and corresponding multiphase patterns is crucial to understanding displacement mechanisms and eventually optimizing these processes in geological, biological, chemical, and other engineering systems. However, suspension flow inside the opaque realistic porous media makes direct observation challenging. The advances in microfluidic experiments have provided us with alternative methods to observe suspension influence on the interface and multiphase flow behaviors at high temporal and spatial resolutions. Macroscale processes are controlled by microscale interfacial behaviors, which are affected by multiple physical factors, such as particle adsorption, capillarity, and hydrodynamics. These properties exerted on the suspension flow in porous media may lead to interesting interfacial phenomena and new displacement consequences. As an underpinning science, understanding and controlling the suspension transport process from interface to flow patterns in porous media is critical for a lower operating cost to improve resource production while reducing harmful emissions and other environmental impacts. This review summarizes the basic properties of different micro/nanoparticle suspensions and the state-of-the-art microfluidic techniques for displacement research activities in porous media. Various suspension transport behaviors and displacement mechanisms explored by microfluidic experiments are comprehensively reviewed. This review is expected to boost both experimental and theoretical understanding of suspension transport and interfacial interaction processes in porous media. It also brings forward the challenges and opportunities for future research in controlling complex fluid flow in porous media for diverse applications.

Co-transport of uranyl carbonate and silica colloids in saturated quartz sand under different hydrochemical conditions

Uranyl carbonate (UC) and silica colloids (cSiO₂) are widely distributed in carbonate-rich subsurface environments associated with uranium pollution. Mobile colloids such as cSiO₂ can affect uranium's transport efficiency in the groundwater environment. Therefore, elucidating the mechanism of UC and cSiO₂ co-transport in a saturated porous medium with different ionic strength (IS), pH, and UC concentration is essential for the prevention and control of groundwater radioactive pollution. At low UC concentrations (<2.1 × 10^-5 M), cSiO₂ is more prone to be deposited on the surfaces of quartz sand (QS) than UC, resulting in cSiO₂ preventing UC transport. Compared to pH 7 and 9, at pH 5 the adsorption of uranium [in the form of 81.5% UO₂CO₃(aq), 8.6% UO₂²⁺, and 5.2% UO₂OH⁺] on cSiO₂ renders cSiO₂ more prone to aggregate, causing smaller amounts of cSiO₂ (86.6%) and UC (55.8%) to be recovered. Mechanisms responsible for the evolution of the pH and zeta potential in effluents have been proposed. Chemical reactions (ligand-exchange reactions and deprotonation) that occur in the QS column between UC and cSiO₂/QS cause the pH of the suspension to varying, which in turn causes changes in the zeta potential and particle size of cSiO₂. Eventually, the recovery rates of cSiO₂ and UC are changed, depending upon the colloid particle size. Changes in ionic strength can seriously affect the stability of cSiO₂ particles, and that effect is more significant when UC is present. Moreover, colloidal filtration theory, a non-equilibrium two-site model, and the Derjaguin-Landau-Verwey-Overbeek theory successfully describe the individual-transport and co-transport of cSiO₂ and UC in the column. This study provides a strong basis for investigating UC pollution control in porous media.

Impacts of stratigraphic heterogeneity and release pathway on the transport of bacterial cells in porous media

In order to manage and control the pathogen release from waste streams of various municipal, industrial, and agricultural pollution sources, it is crucial to investigate the impact of release pathways of such contaminants on their fate and transport in groundwater, especially in respect to natural heterogeneities encountered in aquifers. In this laboratory scale study, we investigate the impacts of different release scenarios of Escherichia coli bacteria, including spatially distributed surface recharge and single-point deep injection, as well as mono-pulse and continuous injection on the transport of Escherichia coli within both single-layered and multilayer aquifers. The results demonstrate earlier arrival of bacteria breakthrough curve (BTC) than conservative solute within a single-layer system with textural and continuum scale heterogeneities, attributed to size exclusion mechanism and preferential flow paths. Size exclusion may be responsible for multiple peaked BTCs observed in all cases of mono-pulse injection of bacteria through both single layer and multi-layer systems. The higher breakthrough of bacteria suspension introduced through a distributed source compared to the point source injection at the same flow rate (19% and 53% in middle and top layers, respectively) suggests that natural hydrologic events such as storm may be more influential in the transport of pathogens in soils than point injections of bacteria in engineering applications such as bioremediation. Moreover, our results reveal that the concentration of the semi-steady state breakthrough formed under distributed and continuous injection condition increases significantly with an increase in the recharge flow rate. This would suggest that a variation in hydrologic conditions can significantly mobilize pathogens which are already deposited in soils.