Influence of the injection of densified polymer suspension on the efficiency of DNAPL displacement in contaminated saturated soils
Graphical Abstract
Introduction
Dense non-aqueous phase liquids (DNAPLs) such as chlorinated organic compounds have been used for several years for various purposes, including but not limited to the cleaning of fabrics and metal degreasing. The vast consumption of these solvents during the last decades, as well as their inappropriate disposal have caused a plethora of environmental issues. Due to the density of these solvents, they can penetrate through the soil and the groundwater and, consequently, shape discontinuous trapped dense non-aqueous phase liquid (DNAPL) zones (Langwaldt and Puhakka, 2000, Zhang and Smith, 2002). The dissolution of some components of these organic compounds can result in serious groundwater pollution (Roy et al., 2004). Nowadays there are numerous contaminated sites around the world contending with the problems related to the contamination of the groundwater. Displacement and removal of these DNAPLs from their source zones are very challenging and costly (McCarty, 2010). In other words, their high density and interfacial tension as well as their low solubility make the performance of most extraction technologies inefficient (e.g. pump-and-treat-method). The trend in global soil remediation technologies shows that the approach has switched from the pump-and-treat method to more advanced methods, including thermal and chemical enhancement (Colombano et al., 2021, Colombano et al., 2020, Stroo et al., 2012).
Among the advanced remediation technologies, invading agents including surfactant, foam and polymer are widely used to displace the residing contaminant (DNAPL as pure phase) from the soil. In the case of the surfactant solution injection, using a high concentration of surfactant (i.e. higher than the critical micelle concentration) can cause the dissolution of the contaminant in the aqueous phase (Johnson et al., 1999). In the event of foam, the foam’s stability is one of the challenging issues in design and performance of foam injection (Ardakani et al., 2020, Kilbane et al., 1997, Wang and Mulligan, 2004).
Polymers have been widely used in the petroleum industry as one of the most effective enhanced oil recovery (EOR) technologies for different reservoir types (Alamooti and Malekabadi, 2018, Littmann, 1988, Liu, 2008, Sandiford, 1964). Although the process of DNAPL remediation of contaminated soils is similar to the EOR methods, there are some main differences. Firstly, the porous media in soils are several orders of magnitudes more permeable than reservoir rocks. In addition, the density of DNAPL is higher than the density of water and the location of these contaminants is close to groundwater and due to the high standards of remediation processes, the amount of residual DNAPLs should not exceed several ppm (Kilbane et al., 1997).
To mobilize the trapped ganglia of DNAPL in pore spaces, several forces including the capillary, viscous, gravity, and buoyancy forces are working simultaneously (Dejam et al., 2014, Mashayekhizadeh et al., 2011, Alamooti et al., 2020). The high surface tension of the DNAPLs makes the mobilization process more difficult but in the case of high permeable porous media, the capillary forces are not dominant. To mobilize ganglia of a non-aqueous phase liquid (here DNAPL) the summation of the viscous and gravity forces should be higher than the capillary forces present in polluted soil (Duffield et al., 2003, Jeong, 2005, Li et al., 2007, Pennell et al., 1996). In general, polymers are used to increase the viscous forces to improve the displacement of DNAPL by reducing the instabilities in the invading phase in porous media. Martel et al. (2004) used a polymer solution before and after a micellar solution (composed of a surfactant (12% Hostapur SAS from Clariant), an alcohol (12% n-butanol) and two solvents (19% d-limonene; 5% toluene)) to enhance the displacement efficiency of DNAPL. They injected polymer as a preflush slug to limit the mobility of the washing solution and avoid the adsorption of surfactant on the solids, and they injected it as postflush slug to push out the washing solution of porous media. They found that on the one hand using reduced velocity improved the dissolution of the micellar solution by increasing the contact time and on the other hand, it caused less mobilization of DNAPL due to a decrease in capillary number. They also showed that polymer solution (xanthan gum) can positively improve the front stability. Martel et al. (1998) used the xanthan polymer solution for mobility control during the soil remediation of NAPL in a multilayer system, and they found that the injection of a polymer solution after the surfactant increases the mobility of the surfactant in low permeable zones and decreases it in high permeable zones. Silva et al. (2013) used modeling to simulate the flow of the polymer-improved aquifer remediation. They showed that by using a biopolymer the sweep efficiency was improved more than 70%. Smith et al. (2008) illustrated that the polymer-improved remediation technique (xanthan gum and potassium permanganate) can be used as a robust technology for the displacement of contaminants including PCE.
Miller et al. (2000) introduced a density-motivated mobilization approach for the remediation of DNAPL contaminated soils. They elucidated the idea of modifying the balances between the capillary and buoyancy forces. By means of bench-scale columns and a two-dimensional tank, they performed several displacement experiments where they showed that the efficiency of densified brine solution (using NaI) on displacement of DNAPL can reach up to 70%. They also proved that the densified brine solution can be used as a barrier below the DNAPL pool to prevent the downward movement of the DNPAL when a surfactant solution was used as displacement agent.
From the rheological point of view, polymer solutions are considered as non-Newtonian fluids i.e. their viscosity is a function of shear rate. For the study of fluid flow in porous media, to link the shear rate at bulk scale and fluid velocity in porous media (Darcy velocity) the physical parameters of porous media including permeability, porosity, and tortuosity should be considered (Darby et al., 2017, Omirbekov et al., 2020). For the case of biopolymers such as xanthan and carboxymethyl cellulose (CMC), a shear thinning behavior is observed (Benchabane and Bekkour, 2008, Zhong et al., 2013).
In the case of chlorinated solvents, as they are much denser than water and mainly isolated near the groundwater resources, if a lighter fluid is injected to remediate the contaminated soil, buoyancy forces work against the displacement. Although polymer solutions can provide higher viscous forces, they are lighter than DNAPL and the gravity forces can influence the displacement efficiency of DNAPLs.
Despite numerous studies carried out to evaluate the performance of polymer solutions injection on the displacement of the DNAPLs, the question of how to handle the gravity forces in an open system (without no flow boundaries) of the contaminated soil using a polymer solution is not well addressed in the literature. Furthermore, although the performance of the viscous (e.g. polymer) and dense solutions on the displacement of DNAPL have been individually investigated, the literature is bereft of a study in which a single densified high viscosity mixture has been used to overcome the gravity and capillary forces at the same time. Although using only a polymer solution with higher injection rates can improve the viscous forces, but it can result in viscous fingering and soil push up. Therefore, the novelty of this work is the introduction of a new remediation technology in which a densified polymer suspension has been injected to overcome both the gravity and capillary forces. The injection of dense polymers will therefore allow forcing the polymer to remain at the bottom of the aquifer to better displace the DNAPL. To achieve this goal, we analyzed the performance of a densified polymer suspension on the displacement of DNAPL from contaminated soils where gravity forces are working against the sweeping process. Barite (BaSO4) particles were added to the CMC biopolymer to increase the density (Bern et al., 1996, Hanson et al., 1990) of the polymer suspension and to evaluate the permeability reduction in the displaced zone. Barite has very low solubility in water and is essentially considered nontoxic (Schulz et al., 2018). Also, it is normally ingested by patients who are going to do some X-ray tests on their digestive system (Merian et al., 2004). The main achievement of this study is to introduce densified polymer suspension for the improvement of the displacement efficiency of DNAPL. In addition, it gives insight on not only the application of densified polymer suspension on the displacement of DNAPL but also the role of boundaries on its performance. In this work, the experimental results on rheological behavior of the densified polymer suspension are provided and then the performance of the densification of the polymer suspension on displacement efficiency of DNAPL is discussed. The role of clogging of suspended particles on the transport of polymer suspension is analyzed. To have better understanding of the displacement process of DNAPL, the two-phase flow was simulated by means of the continuity and Darcy equations (Bear, 2013). The general advection-dispersion-reaction equation was used for the transport of barite and polymer in porous media (O’Carroll et al., 2013).
Section snippets
Material and methods (experimental and numerical)
In this section, first the 1D column and 2D tank experimental setups and procedures are explained. Then the method of image interpretation is presented. Finally, the governing equations are given and the numerical simulation method is described. The properties of experimental materials including the contaminant (DNAPL), barite-polymer suspension, soil physics as well as the suspension preparation are discussed in supplementary material part A.1.
Results
Firstly, the rheological behavior of polymer mixture with and without the barite particles was evaluated. Then, the experimental results of the barite particles clogging in the sand is discussed and finally, the results on the efficiency of the barite-polymer suspension on the displacement of the DNAPL is analyzed.
Characterization of the clogging of barite particles by finding attachment/detachment parameters
The set of experiments which are explained in Section 3.2 have been carried out to find the dispersivity of barite particles and parameters related to attachment and detachment of barite particles. As it is mentioned in material and methods section, the barite-CMC suspensions were injected at fixed injection rate corresponding to 1 m/day to a fully water saturated column of sand. The effluents of the column were monitored during injection until steady state conditions were reached. The
Conclusions
In this study, using two-dimensional tank experiments and corresponding numerical modeling of the displacement process in porous media, the efficiency of the injection of densified polymer suspensions on the displacement of DNAPL was investigated. It was shown that adding barite particles to densify the polymer solution could improve the displacement efficiency of DNAPL more than 4 times. Using one-dimensional column experiments, it was demonstrated that the barite-CMC suspensions can cause a
CRediT authorship contribution statement
Amir Alamooti: Methodology, Conceptualization, Visualization, Writing – original draft. Stéfan Colombano: Conceptualization, Supervision, Funding acquisition, Project administration, Writing − review & editing. Sagyn Omirbekov: Methodology, Conceptualization, Validation. Azita Ahmadi: Supervision, Validation, Writing – review & editing. Fabien lion: Resources, Visualization. Hossein Davarzani: Supervision, Writing – review & editing, Visualization.
Environmental Implication
Chlorinated DNAPL spills are frequent and due to their high toxicity they can negatively influence the soil and groundwater quality by penetrating through the soil and forming insoluble lenses. This study discusses a novel in situ remediation technique by introducing the densified polymer suspension for displacement of DNAPL from the contaminated zone. The experiments have been done on an unconfined system instead of an idealistic one to represent the real polluted site. This environmentally
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was performed as part of the PAPIRUS project. The authors would like to thank ADEME (French Environment and Energy Management Agency) for co-funding the project under the “GESIPOL” program and BRGM/DEPA and ADEME for providing the PhD grant for Amir Alamooti. The authors also gratefully acknowledge the financial support provided to the PIVOTS project by the “Région Centre – Val de Loire” and the European Regional Development Fund. We thank INOVYN for the assistance provided during
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