Foam for Environmental Remediation: Generation and Blocking Effect

Enhancing pollutant removal from contaminated soils using yield stress fluids as selective blocking agents

This work presents a novel technique consisting in the use of yield stress fluids as blocking agents in porous media presenting pore-scale heterogeneities. The key feature of this method is that yield stress fluids only flow through the pores having a minimum size that depends on the applied pressure gradient. These fluids remain immobile in more and more pores as the pressure gradient is decreased. Therefore, the dimension of the pores which are invaded by the yield stress fluid can be controlled by adjusting the applied pressure gradient. Moreover, yield stress fluids are highly suitable blocking agents given the extremely high viscosity values that they exhibit in the pores. This allows for the diversion of the flow from greater to smaller pores during subsequent waterflooding stages, thus enhancing pollutant removal from the flow paths of small hydraulic conductance. A series of multiphase flow experiments were conducted in this study using well-characterized cores of artificial A10 sintered silicate. In these experiments, semidilute aqueous solutions of xanthan gum biopolymer were used as yield stress fluids to block the greatest pores. By doing so, considerably more pollutant was recovered by waterflooding. Furthermore, it was shown that an increase in polymer concentration does not always lead to a decrease in the size of the pores invaded by the blocking agent. Indeed, concentrated polymer solutions generate higher pressure gradients throughout the porous medium, which facilitates the invasion of small pores. Nevertheless, depending on the value of the yield stress-pressure gradient ratio, they may also develop extremely high viscosities that slow down their flow through such small pores. This work also presents a method to measure the volume of blocked pores using the results of tracer tests. The reported results suggest that using a polymer solution developing a yield stress as a selective blocking agent is a promising technique for soil remediation.

Lab on a chip for a low-carbon future

Transitioning our society to a sustainable future, with low or net-zero carbon emissions to the atmosphere, will require a wide-spread transformation of energy and environmental technologies. In this perspective article, we describe how lab-on-a-chip (LoC) systems can help address this challenge by providing insight into the fundamental physical and geochemical processes underlying new technologies critical to this transition, and developing the new processes and materials required. We focus on six areas: (I) subsurface carbon sequestration, (II) subsurface hydrogen storage, (III) geothermal energy extraction, (IV) bioenergy, (V) recovering critical materials, and (VI) water filtration and remediation. We hope to engage the LoC community in the many opportunities within the transition ahead, and highlight the potential of LoC approaches to the broader community of researchers, industry experts, and policy makers working toward a low-carbon future.

The era of low-permeability sites remediation and corresponding technologies: A review

Rational utilization of soil resources and remediation of contaminated soils are imperative due to the rapidly growing demand for clean soils. Currently, many in-situ remediation technologies are less suitable at low-permeability sites due to the limitations of soil permeability. This work defines a low-permeability site as a site with hydraulic conductivity less than 10^-4 cm/s, and summarizes the migration characteristics of representative contaminants at low-permeability sites, and discusses the principles and practical applications of different technologies suitable for the remediation of low-permeability sites, including electrokinetic remediation technology, polymer flushing technology, fracturing technology, and in-situ thermal remediation technology. Enhanced and combined remediation technologies are further described because one remediation technology cannot remediate all contaminants. The prospects for the application of remediation technologies to low-permeability sites are also proposed. This work highlights the necessity of low-permeability sites remediation and the urgent need for new remediation technologies, with the hope to inspire future research on low-permeability sites.

Foam injection for enhanced recovery of diesel fuel in soils: Sand column tests monitored by CT scan imagery

The use of surfactant foam for the remediation of diesel fuel, a Light Non-Aqueous Phase Liquid (LNAPL), was investigated in sand column experiments using X-ray Computed Tomography (CT). A preliminary series of tests were carried out on six surfactant candidates in order to measure their physical properties, including critical micelle concentrations and interfacial tensions (IFT) with the LNAPL. Batch tests for foam stability were carried out with and without added LNAPL, in order to measure the half-life of foam columns produced with each surfactant candidate. Foam flow-rate co-injection tests were carried out for each surfactant candidate in 405 cm³ sand columns contaminated with LNAPL at residual saturation. These tests revealed that a 1:1 mixture of sodium dodecyl sulfate and cocamidopropyl betaine, injected at a total volumetric flow-rate (Q_foam) of 45 mL/min, resulted in successful generation and propagation of foam within the contaminated porous medium. Finally, two sand column tests, carried out respectively under high- and low-pressure conditions, were imaged with a CT-scanner in order to compare and contrast foam morphology evolution as well as the LNAPL desaturation dynamics involved in both scenarios. The saturation profiles extracted from CT images provided valuable new insights.

Localized Delivery of Liquid Fertilizer in Coarse-Textured Soils Using Foam as Carrier

Agrochemicals and fertilizers are central to modern agriculture and are credited with the large increase of crop yield as a part of the Green Revolution of the 1960’s. Timely and targeted fertilizer application to crop root zones enhances effectiveness and reduces unintended release to the environment. This is particularly important for highly mobile liquid fertilizers (e.g., nitrate) that can be mobilized with infiltrating water to bypass root-bearing soil volumes. We report a novel liquid fertilizer delivery method using foam as carrier. The high degree of control and mechanical stability of liquid fertilizer foam (defined dispersed gas bubbles in a continuous liquid phase) injection into coarse soils (most susceptible to preferential flows) is proposed a novel delivery method to targeted root zone volumes at concentrations and geometry that promote uptake and reduces losses. This note and preliminary communication meant to serve a proof of concept report comparing foam and conventional liquid fertilizer applications. The results indicate that foam-delivery reduced fertilizer leaching thus improving its retention in soil for similar flow conditions of liquid delivery. Theoretical estimates suggest that the effects of fertilizer retention could be enhanced in more localized (3-D) injection of foam fertilizers and other agrochemicals thus enhancing agronomic efficiency and reducing environmental risk of contamination.

Measuring and modeling nanoparticle transport by foam in porous media

In this paper, an experimental study of nanoparticle transport by foam is presented. Bubbles made of N₂-gas were stabilized with either a cationic surfactant (Cetyl Trimethyl Ammonium Bromide, CTAB), silica nanoparticles, or a combination of them. The concentrations of the surface active materials were selected upon foamability and stability tests. Column-flood tests were run until steady-state changing nanoparticle concentration, foam quality (f_g), and flow rate. A synergistic behaviour of surfactant and nanoparticles help the formation of a strong foam. The measurements were used to validate a mechanistic model, presented in our earlier work (Li and Prigiobbe, 2020), which couples foam and nanoparticles transport with agglomeration and extended-DLVO theory. The model agrees well with the measurements and results show that an high-quality (ca. 90% gas fraction) can be used to carry nanoparticles and the efficient increases with flow velocity. This opens the opportunity for the application of foam as a carrier of nanoparticles in subsurface applications such as the remediation of contaminated sites and makes the model a valuable tool to design and predict such operations.

Experimental study of foam propagation and stability in highly permeable porous media under lateral water flow: Diverting groundwater for application to soil remediation

Foam propagation and stability in highly permeable porous media, encountered in soil pollution applications, are still challenging. Here, we investigated the application of foam for blocking the aquifer to divert the flow from a contaminated zone and, therefore, ease the remediation treatments. The main aim was to better understand the critical parameters when the foam is injected into a highly permeable aquifer with high groundwater flow velocity (up to 10 m/day). A decimetric-scale 2D tank experimental setup filled with 1 mm glass beads was used. The front part of the 2D tank was made of transparent glass to photograph the foam flow using the light-reflected method. The water flow was generated horizontally through injection and pumping points on the sides of the tank. The pre-generated foam was injected at the bottom center of the tank. Water streamlines (using dye tracing) and water saturation were investigated using image interpretation. Results show that 100% of the water flow was diverted during the injection of the foam. Foam stability in porous media depends significantly on the horizontal water flow rate. Recirculating water containing the surfactant increases foam stability. The main mechanism of destruction was identified as the dilution of the surfactant in water. However, the head-loss measurements showed that despite foam destruction, the relative permeability of the water phase in the media remained quite low. Injection of foam increases the radius of gas propagation, thanks to foam's high viscosity, compared to a pure gas injection case. These results are new highlights on the efficiency of foam as a blocking agent, showing that it can also serve as a means for gas transport more efficiently in porous media, especially for soil remediation applications.

Use of saponin foam reinforced with colloidal particles as an application to soil remediation: Experiments in a 2D tank

Foam can be used to achieve environmental remediation in case of contamination caused by light non aqueous phase spills. However, when it comes in contact with oily pollutants, foam becomes weaker and its life time is greatly reduced. Such weakening can be dampened by using silica particles -together with saponin surfactant- which were shown to reinforce foam in bulk and 1D sandpack experiments. Here is addressed both foam propagation in a 2D porous media when buoyancy and gravity interfere, and foam behaviour when in contact with floating oil. Therefore, macroscopic foam displacement, and specific liquid and gas phases behaviours were studied in a 2D-tank. A piston-like displacement was observed during foam propagation in the absence of oil, while foam liquid phase was influenced by gravity and did not propagate homogeneously on entire tank height. In the presence of oil, foam was partly destroyed, which increased the local permeability of gas and created new preferential paths for gas flow. This effect was partially avoided via a surfactant concentration increase, but solid colloidal particles turned out to be a more efficient stabilizing agent, by significantly increasing foam strength and its oil-tolerance.

Foam trapping in a 3D porous medium: in situ observations by ultra-fast X-ray microtomography

One of the challenges in the study of foam transport in 3D porous media is to have an adequate spatial and temporal resolution to get a better understanding of the local phenomenon at the pore scale in a non-destructive way. We present an experimental study in which ultra-fast X-ray microtomography is used to investigate the foam trapping while the foam is flowing in a 3D porous medium. Preformed aqueous foam is injected into a rotating cell containing a 3D granular medium made of silica grains. The use of rotating seals allows the cell to rotate continuously at a rate of one revolution per second, compatible with the fast X-ray tomography at SOLEIL synchrotron. We visualize the foam flow and track the trapping of bubbles with an acquisition time of about one second and a spatial resolution of a few microns (pixel size of one micron). This allows us to extract the characteristics and reliable statistics about trapped bubbles inside the granular medium and to observe their local behavior. With this setup and technique we obtain access to the dynamics of foam trapping during the flow and the texture variations of the foam in the trapped zones. These local trapping events are well correlated with the macroscopical measurement of the pressure gradient over the cell.

Field demonstration of foam injection to confine a chlorinated solvent source zone

A novel approach using foam to manage hazardous waste was successfully demonstrated under active site conditions. The purpose of the foam was to divert groundwater flow, that would normally enter the source zone area, to reduce dissolved contaminant release to the aquifer. During the demonstration, foam was pre generated and directly injected surrounding the chlorinated solvent source zone. Despite the constraints related to the industrial activities and non-optimal position of the injection points, the applicability and effectiveness of the approach have been highlighted using multiple metrics. A combination of measurements and modelling allowed definition of the foam extent surrounding each injection point, and this appears to be the critical metric to define the success of the foam injection approach. Information on the transport of chlorinated solvents in groundwater showed a decrease of contaminant flux by a factor of 4.4 downstream of the confined area. The effective permeability reduction was maintained over a period of three months. The successful containment provides evidence for consideration of the use of foam to improve traditional flushing techniques, by increasing the targeting of contaminants by remedial agents.