# Stability and flow behavior of polymer-enhanced foams for improved in-situ remediation of hydrocarbons: Effect of polymer-surfactant interactions

Adil Baigadilov, Stéfan Colombano, Sagyn Omirbekov, Maxime Cochennec, Dorian Davarzani, Fabien Lion, Hugues Bodiguel, Laurent Oxarango

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## ► To cite this version:

Adil Baigadilov, Stéfan Colombano, Sagyn Omirbekov, Maxime Cochennec, Dorian Davarzani, et al. Stability and flow behavior of polymer-enhanced foams for improved in-situ remediation of hydrocarbons: Effect of polymer-surfactant interactions. Journal of Hazardous Materials, 2025, 486, pp.137004. 10.1016/j.jhazmat.2024.137004. hal-04870962

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Submitted on 31 Oct 2025

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# Stability and flow behavior of polymer-enhanced foams for improved in-situ remediation of hydrocarbons: Effect of polymer-surfactant interactions

Adil Baigadilov<sup>a,b,c,*</sup>, Stéfan Colombano<sup>a</sup>, Sagyn Omirbekov<sup>d</sup>, Maxime Cocheméc<sup>a</sup>, Dorian Davarzani<sup>a</sup>, Fabien Lion<sup>a</sup>, Hugues Bodiguel<sup>b</sup>, Laurent Oxarango<sup>c</sup>

## HIGHLIGHTS

* Polymer-surfactant interactions are crucial for foam stability and flow behavior.

* Xanthan Gum increases foam stability by 3.6–10.8 times against destructive hydrocarbons.

* Polymer-enhanced foams improve mobility reduction factor and pollutant sweeping.

* Recovery yields exceed 95 % with polymer-enhanced foams.

## GRAPHICAL ABSTRACT

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## ARTICLE INFO

**Keywords:**

Polymer-enhanced foams

Polymer-surfactant interactions

Xanthan Gum

Hydrocarbon contamination

Porous media

In-situ remediation

## ABSTRACT

Conventional in-situ hydrocarbon remediation technologies face challenges associated with high costs and low long-term efficacy. Aqueous foam injection presents a promising approach by enhancing volumetric sweeping efficiency. This study investigates the efficiency of polymer-enhanced foams (PEFs) for in-situ remediation of hydrocarbon-contaminated soil, focusing on the impact of Xanthan Gum (XG) biopolymer on foam stability against antifoaming diesel and the flow behavior in soil matrices. We examined two PEFs: Sodium Dodecyl Sulfate (SDS)-based and a blend of SDS and Cocamidopropyl Hydroxysultane (SDS-CAHS: SC)-based. Bulk foam tests pre-evaluated foam stability, while 1D sandpack experiments assessed PEFs' performance in porous media mimicking contaminated soil remediation. Stability tests showed that XG strengthens the foam by increasing liquid phase viscosity and improving overall foam stability. The findings emphasize the importance of the interactions inside polymer-surfactant complexes, where SDS was more impacted by XG than SC due to repulsive

<footnote><sup>a</sup> BRGM, Orléans F-45060, France</footnote>

<footnote><sup>b</sup> Univ. Grenoble Alpes, CNRS, Grenoble INP, LRP, Grenoble 38000, France</footnote>

<footnote><sup>c</sup> Univ. Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, Grenoble 38000, France</footnote>

<footnote><sup>d</sup> National Laboratory Astana, Nazarbayev University, Astana 010000, Kazakhstan</footnote>

<footnote>* Corresponding author at: BRGM, Orléans F-45060, France.</footnote>

<footnote>E-mail addresses: a.baigadilov@brgm.fr, adil.baigadilov@gmail.com (A. Baigadilov).</footnote>
forces and hydrophobic interactions. Foam flow experiments revealed PEFs' higher mobility reduction factors (MRF) and noticeable recovery improvement of the free-phase product (≥95%) compared to traditional surfactant-based foams. This research provides valuable insights into optimizing PEF compositions, potentially guiding future scale-up applications for hydrocarbon-contaminated sites.

# 1. Introduction

Soil contamination by petroleum hydrocarbons is a prevalent issue streaming from industrial activities, oil spills, and leakage from underground storage tanks [1]. Petroleum hydrocarbons are complex mixtures with a wide range of aliphatic and aromatic organic compounds known as hydrophobic organic contaminants (HOCs). When a significant quantity of HOCs is released into the subsurface, it infiltrates downwards through the unsaturated zone and depresses the capillary fringe. It forms a floating free-phase light non-aqueous phase liquid (LNAPL) lens that disperses laterally above the water table [2]. The persistence of such toxic compounds in soil matrices affects soil quality and productivity and threatens water resources and human health through bioaccumulation [3]. On many occasions, fast remediation can be achieved by ex-situ techniques based on polluted soil excavation; however, it proves inadequate for inaccessible urban areas. Consequently, developing effective and sustainable in-situ remediation technologies is a critical environmental priority.

Traditional pumping-skimming technologies are no longer considered the best course of action to treat groundwater due to long treatment duration and low recovery efficiency (≤60%) [4]. Other in-situ remediation methods, including bioaugmentation, thermal desorption, and chemical oxidation treatment, have limitations such as high costs and potential secondary pollution [5,6]. In this context, surfactant-enhanced remediation techniques have emerged as promising alternatives due to their effectiveness in HOCs' mobilization by lowering the interfacial tension between the water and LNAPL phases. However, unfavorable mobility ratio and density differences could generate displacement instabilities due to viscous fingerings and gravitational underrides [7-9]. Thus, surfactant foams have gained attention among these techniques for their potential to increase the contact area between the surfactant and the contaminant and to improve the volumetric sweep, thereby enhancing remediation efficiency [10]. It is mainly accomplished by the advantages of the pseudoplastic foam behavior associated with high viscosity and low density.

In porous media (e.g., soil matrix), foam is considered a gas dispersion in the liquid phase, where at least some part of it is discontinued by thin surfactant-based lamellae [11]. Foam can be categorized as weak or strong based on its texture, defined as the number of lamellae per unit volume [12,13]. The dynamic behavior of foam within the soil, such as continuous lamellae creation and destruction, controls the foam texture along the flow path. Thus, the successful application of foam is based on two main features: 1) the kick-off of foam generation due to the dominance of lamellae creation over destruction (i.e., the transition from coarse to fine textured foam) and 2) foam strength (i.e., degree of mobility reduction). Finely-textured strong foam can significantly reduce the mobility of the gas phase, thus improving the sweeping. However, to ensure good sweeping efficiency, the pressure gradient near an injection point should be relatively high to withstand the expanding radial flow, which causes the foam to slow down and weaken [14-16]. Moreover, many foam systems that reduce mobility would fail in the presence of oily organic compounds [17-20]. It is mainly due to oil-spreading conditions, where the surface forces favor oil entering the aqueous-gas interface and destabilize the foam [21,22].

Foam and/or lamellae stability against oil is a complex phenomenon highly dependent on the surfactant formulation, among other factors [19,23]. One of the ways to improve foam's tolerance to organic compounds is the appropriate selection of surfactant or surfactant blends. Recently, [24] have introduced a binary surfactant mixture of Sodium

Dodecyl Sulfate (SDS) and Cocamidopropyl Hydroxysultaine (CAHS), which demonstrated significantly enhanced foam stability against antifoaming diesel oil. It was attributed to the formation of a stable pseudo-emulsion film by the mixture, creating a higher oil entry barrier. However, lamellae disruption by the organic phase during foam flooding still occurred, limiting complete contaminant sweeping. This limitation motivates the exploration of additional stabilizing agents to further enhance foam structure and resistance. For the last decade, researchers have been interested in combining a surfactant system with colloidal nanoparticles to reinforce the stability of the foam [25-28]. Despite the perspective of this approach, there are still concerns about the potential adverse impacts of nanoparticles on the environment, aquatic life, and human health [29-31]. Thus, the use of hydrosoluble biopolymer additives offers a promising avenue to address the abovementioned challenges.

Polymer incorporation into the foaming solution has proven feasible to improve the foam's sweeping performance in the petroleum industry [32-34], but it is still less recognized in environmental remediation applications. The studies by [26] and [35] are the only works in which polymer-enhanced foam (PEF) was used for remediation purposes. [35] investigated CO₂ foam stabilization using Xanthan Gum (XG) biopolymer with dodecyl trimethylammonium chloride (DTAC) surfactant in combination with β-cyclodextrin. Biopolymer/β-cyclodextrin/surfactant (bP/βCD/S) system showed promising results for foam stability and mobility control in porous media. However, no contaminant was involved in their study; thereby, no discussion was available concerning the stability of PEF in the presence of HOCs. [26] also combined XG polymer with Saponin surfactant to enhance N₂ foam stability, but this time in the presence of oil contaminant. They observed higher RF for PEF compared to conventional surfactant foam; however, according to the authors, it did not provide the necessary structural reinforcement to the foam. The increased viscosity of foam's aqueous phase due to XG incorporation made foamability more challenging, resulting in weaker foam overall. Thus, both studies highlighted the need for further research to test and optimize different polymer-surfactant combinations and better understand their mechanisms in varying field conditions (i.e., different contaminant types and soils with distinct intrinsic properties).

Although PEFs hold significant potential, it is crucial to acknowledge their inherent limitations. Generally, they are non-trivial systems involving complex intermolecular physicochemical mechanisms. For instance, the efficiency of PEFs remains greatly affiliated with interactions in polymer-surfactant pairs, which might be challenging to predict and control [36,32,37,38]. Additionally, despite the benefits of reduced continuous phase mobility due to increased viscosity, polymers may hinder foam generation and reduce foamability, potentially resulting in weaker foams [26,32]. Nevertheless, previous studies suggest that the polymer presence enhances foam's ability to reduce gas mobility and improve sweep efficiency, leading to better remediation outcomes. Therefore, in this study, we focused on the role of Xanthan Gum as a foam enhancer, a biopolymer known for its high viscosity and stability under various environmental conditions [38-40]. XG is particularly interesting due to its low environmental impact and shear-thinning behavior [41,42]. Such features of the biopolymer offer a more environmentally friendly alternative to synthetic polymers, aligning with the goals of sustainable remediation.

The main objectives of the current paper were two-fold: 1) to assess the impact of XG on the stability of two surfactant foams from our previous studies [24] in the presence of diesel oil and 2) to evaluate the
dynamic performance of these PEFs to sweep LNAPL (free product) from a polluted highly permeable sandy soil to establish their capability for practical remediation applications. An additional research question of our work was to identify whether the behavior of bulk PEFs can forecast their flow performance in a porous medium. Through extensive laboratory experiments, including bulk and foam flow in porous media evaluations, this study aims to understand better how polymer-surfactant interactions influence foam properties and remediation efficacy.

## 2. Material and methods

### 2.1. Foam components

Two types of polymer-enhanced foams were used in our studies: 1) Anionic (Sodium Dodecyl Sulfate (SDS): ReagentPlus®, Sigma Aldrich) surfactant-based PEF and 2) Anionic-Zwitterionic, SDS-CAHS (Cocamidopropyl Hydroxysultaine: Cola®Teric CBS, Colonial Chemicals), surfactant mixture-based PEF. SDS-CAHS (SC) was blended at 50:50 based on [24]. SDS and SC surfactant solutions of 1 wt% concentration (active material) were prepared with Milli-Q water (18 MΩ/cm). All PEF solutions were made by introducing an anionic biobased polymer (Xanthan Gum (XG): Xanthomonas campestris-43708, Sigma Aldrich) at various concentrations into the surfactant-based foaming solutions. They were stirred for at least 12 h to ensure homogeneity. The stirring was done at a low speed (250 rpm) to avoid foaming before the experiments. Nitrogen gas (N₂: AZOTE U, Air Liquid) was used as the gas phase to generate foam in this work.

### 2.2. Soil and contaminant

The model soil used in this study was unconsolidated highly permeable packing of 0.4–2 mm grain size silica sand (coarse sand: BB, Sibelco). The soil porosity was φ=34.81 ± 0.5 % with an intrinsic permeability k=255 ± 10μm². Porosity was measured by weighing the soil samples before and after water saturation. The permeability was estimated based on Darcy’s law from the pressure gradient measured at various imposed injection flow rates of water in the porous media set-up (Fig. 1b). Marble sand (fine sand: SMB, Oviv SA) with φ=36.23 ± 0.06 % and k=30 ± 5μm² was used for foam generation before injection into the soil samples.

Hydrocarbons with TPH in the range of 10–28 carbon atoms (Diesel oil: Excellium Truck Diesel, TotalEnergies SE) were employed to mimic soil contamination. Manufacturer-developed antifoaming additives in diesel ensured the creation of maximum adverse conditions to investigate foam-based remediation techniques. The main characteristics of the contaminant are provided in Tab. 1.

### 2.3. Foam stability tests

#### 2.3.1. Set-up and procedure

Stability tests were conducted using Dynamic Foam Analyzer (DFA-100, Krüss), schematically shown in Fig. 1(a). The vertically positioned prismatic column of the device (hereinafter called DFA prism) was initially filled with 50 ± 1 mL of tested foaming solution and 1 % vol. of diesel oil, which was introduced on top of the solution. It helped to minimize the pre-emulsification of the diesel oil. N₂ gas was sparged through a porous glass filter at the bottom of the DFA prism at a constant volumetric flux of 200 mL/min for 60 s to generate foam. Optical sensors located along the prism and the light source were used to track the foam height (decay profile) and the height of the liquid drained (liquid drainage) from the foam during the precisely defined time after the gas sparging stopped (Fig. 1a). Measured parameters of the bulk scale foam were continuously monitored by KRUSS’s “Advance” software. All tests were replicated to guarantee the reliability of the results.

#### 2.3.2. Foam stability evaluation

We used foam volumetric stability (FVS) and foam liquid stability (FLS) proposed in the literature [43,44] to characterize foam stability. They can be calculated based on the heights of foam and liquid recorded by DFA-100 and are defined as:

$$FVS = \frac{V_{t(foam)}}{V_{0(foam)}}, \qquad (1)$$

<table><caption>**Table 1**

Physical properties of diesel oil measured at ambient temperature (20 °C).</caption><thead><tr><th>Organic phase</th><th>ρ (g/cm³)</th><th>μ (mPa·s)</th><th>σ (mN/m)</th><th>γ (mN/m)</th></tr></thead><tbody><tr><td>Diesel</td><td>0.823 ± 0.005</td><td>3.01 ± 0.09</td><td>26.11 ± 0.56</td><td>19.40 ± 0.37</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Experimental set-ups used to conduct: (a) Bulk foam tests (DFA-100) and (b) Foam flow experiments (1D sandpack columns: Foam Generator and Main).</figcaption></figure>
$$FLS = \frac{V_{t(lq)}}{V_{0(lq)}}, \qquad (2)$$

where $V_{0(foam)}$ and $V_{0(lq)}$ are the initial volumes of foam and liquid, i.e. right after the sparging has ceased. $V_{t(foam)}$ and $V_{t(lq)}$ are the volumes of foam and liquid at a given time t. One can define the half-life time of the foam $t_{1/2(foam)}$ when the FVS is reduced to half of the original volume. Similarly, $t_{1/2(lq)}$ corresponds to the time for drainage of 50 % of the liquid initially retained in foam.

## 2.4. Foam flow experiments

### 2.4.1. Set-up and procedure

Fig. 1(b) shows the schematic of the experimental set-up used. 1D glass column, later called the Main Column, was 0.3 m in length and 0.04 m in diameter. It was packed vertically by introducing coarse silica sand into the column under manual vibrations to have a uniformly compacted porous medium. In total, 12 sand column experiments were conducted, with a new porous medium prepared for each test. Therefore, the packaging procedure was always thoroughly followed to ensure similar properties between the media. The properties of each porous medium (pore volume (-), porosity (Φ), intrinsic permeability (k)) were determined after complete water saturation. This was achieved by placing the sandpack vertically and injecting water from the bottom to the top at a flow rate of $Q = 1$ mL/min. Prior to the water saturation, the porous media were rinsed with $CO_2$ gas to avoid trapped air bubbles. For hydrocarbon displacement experiments, the water-saturated soil sample was contaminated with diesel oil from the top to the bottom with a low flow rate ($Q = 1$ mL/min) to ensure gravity-stable displacement.

The diesel-contaminated porous medium was placed horizontally and flooded first with the tested foaming solution for 3 pore volumes (PV). This volume was sufficient to ensure that the diesel phase was set at residual saturation and to maximize surfactant adsorption. Thus, it guaranteed a good foam generation during the evaluation. After the solution preflush, the pre-generated foam was injected into the studied Main Column to displace the diesel. Two pressure sensors (33x, Keller) were connected upstream and downstream of the sandpack to record pressure drop evolution during the experiments. Graduated cylinders were placed under the outlet tube to sample the effluent during the experiment.

The foam was pre-generated by co-injection of $N_2$ gas and foaming solution into a polyvinyl chloride column packed with fine marble sand following the abovementioned procedure. The foam generator column was 3 times shorter than the Main one with the same internal diameter ($L = 0.1$ m, ID = 0.04 m). Co-injection was controlled by a mass flow controller (El-Flow Select F-201CV, Bronkhorst) and a piston metering pump (1HM, Eldex). It lasted until the fine-textured stable foam was found at the outlet before FG's connection to the studied sandpack.

All experiments were conducted at room temperature and atmospheric pressure conditions with the total flow rate of injected foam $Q_{foam} = 3$ mL/min ($Q_{foam} = Q_{gas} + Q_{liq}$). Foam quality, defined in Eq. (3), was fixed at $f_g = 0.85$ and selected according to previous works on oil displacement by pre-generated foam [45,46].

$$f_g = \frac{Q_{gas}}{Q_{gas} + Q_{liq}}. \qquad (3)$$

### 2.4.2. Assessment of foam displacement efficacy

The main advantages of foam in environmental remediation applications are flow diversion from high to low permeability zones and improved sweeping efficiency due to the foam's high apparent viscosity, which results from the greatly restricted mobility of gas by lamellae. Dimensionless parameters such as gas Mobility Reduction Factor (MRF) and foam Resistance Factor (RF) [47] provide the easiest way to compare the gas mobility change in different conditions (different gas types, formulations and concentrations of foaming solutions,

permeabilities of the media, etc.) without incorporating the more complex phenomena such as porous medium type and mineralogy, surfactant adsorption on solid surfaces, wettability effects on foam at the pore walls and so on [13].

Thus, to assess the performance of XG-enhanced SDS and SC foams in the unconsolidated porous media, we used Mobility Reduction Factor (MRF), here defined as the ratio of foam to no-foam apparent gas viscosities ($\mu_{gapp}$):

$$MRF = \frac{\mu_{gapp}^f}{\mu_{gapp}^0}. \qquad (4)$$

Since both are compared at the same injection conditions ($Q_{foam} = Q_{no-foam}$) and derived from Darcy's law, MRF can be simplified as

$$MRF = \frac{\nabla P_{foam}}{\nabla P_{no-foam}}, \qquad (5)$$

where $\nabla P_{foam}$ and $\nabla P_{no-foam}$ represent the pressure gradients measured along the studied porous medium. The $\nabla P_{no-foam}$ was obtained by performing gas injection ($Q_{gas} = 3$ mL/min) into representative porous media with similar physical properties ($\phi$, k) using the same sand.

We also used Breakthrough Time (BT), defined as the time required for foam to reach the outlet of the sandpack, to qualitatively and quantitatively examine the stability of the foam displacement front invading the highly permeable sandy soil after external pre-generation for both clean and contaminated media. BT was determined by visually observing the outlet and monitoring pressure changes within the sandpack, which provided consistent, observation-based measurements across all foam types and compositions. This standardized method ensures a fair comparison of BT across different foam formulations. Foam injection can sometimes exhibit a two-step breakthrough behavior — an initial breakthrough of weaker gas-dominated foam followed by a second breakthrough of stronger, stable foam. However, the analysis of the paper centers on this latter strong foam BT, as it represents the formation of a stable foam front and the onset of effective displacement. The low values of BT suggest the plug flow regime with good sweeping (i.e., less PV of foaming solution injection).

## 2.5. Other measurements

Rheological and interfacial properties of the SDS- and SC-based foaming solutions at various XG concentrations (0.01, 0.02, and 0.05 wt%) were obtained at ambient temperature (20 °C) following the previous studies of polymer solutions [41,42]. A rotational stress-controlled rheometer (Haake Mars 60, Thermofisher) equipped with cone-plate geometry (C60/1•/Ti and TMP60) was used to measure the dynamic viscosities of the studied solutions under the applied shear rates (imposed range: 0.1–100 s⁻¹). Gas-liquid-oil interfacial tensions were measured by the pendant drop method using a Drop Shape Analyzer (DSA-100, Krüss).

# 3. Results and discussion

## 3.1. Bulk polymer-enhanced foam (PEF) stability

Foam is a metastable two-phase system that tends to irreversibly evolve over time [48]. The bulk scale foam's stability is essentially governed by three main phenomena: gravitational drainage, diffusive coarsening, and bubble coalescence. The latter two occur mainly after sufficient thinning of the lamellae caused by the liquid content reduction in foam as a result of drainage. Once the foam is generated, however, before the so-called jamming transition [49], a certain amount of liquid holds-up despite the continuous phase redistribution inside the foam due to gravitational and capillary forces. Eventually, the liquid flows downward, concentrating on the lower part of the foam. The presence of
the oil phase tends to adversely impact foam stability. It forces the bubbles to coalesce at rather greater liquid content due to entering the gas-liquid interface and subsequent dewetting of the lamellae [50,51].

Xanthan Gum (XG) is a polymer with a strong viscosifying ability, even at rather low concentrations. Thus, this XG property is expected to primarily affect the drainage of the liquid in foam. To understand Xanthan's impact on foam drainage behavior in the presence of diesel oil, we plotted the drainage rate as a function of time for SDS and SC PEFs at different polymer concentrations in Fig. 2. The drainage rate was obtained by taking the numerical gradient of the height of the liquid fraction at the bottom of the DFA prism (see Fig. 1a) over time. This allowed us to determine the effect of polymer concentration and the role of surfactant used on foam drainage.

From Fig. 2, one can see that SDS foam shows the highest drainage rate in the absence of polymer immediately after the end of foaming. This indicates poor resistance of SDS foam to diesel oil. In contrast, the polymer-free SC foam was on hold-up before liquid accumulation at the bottom of the foam mentioned earlier. Such behavior suggests greater diesel tolerance resulting in the greater entry barrier of the surfactant mixture [24,52,53]. However, by adding XG to the foaming solution, one can observe that the hold-up time increased for both surfactant systems. Due to the increased dynamic viscosity of the foaming solution caused by the presence of polymer, the jamming transition was delayed. Upon reaching the jamming transition, regardless of the polymer concentration in both surfactant systems, we observed an immediate elevation of the drainage rate. Saint-Jalmes and Langevin [54] found that drainage velocity depends not only on physical parameters such as foam bubble size and liquid fraction but also on the bulk viscosity of the solution. Their findings align with the behavior revealed by the drainage profiles of SDS and SC PEFs in our work. Indeed, one can notice in Fig. 2 a difference in the amplitude of the drainage rate, which decreases with the XG concentration and, therefore, with the dynamic viscosity of the bulk solution.

The drainage rate profiles provide an insight into the effect of the polymer on the drainage itself as a distinct phenomenon occurring in PEF and do not demonstrate its contribution to overall stability. Thus, to determine the impact of XG on foam stability in the presence of diesel, we plotted the foam half-life as a function of the drainage half-life in Fig. 3. The impact of the XG concentration on foam stability was nonlinear and varied depending on the surfactant used in the foaming solution. For instance, for SC PEF, one can see increased stability by a factor of 1.4, 2.8, and 3.4 at 0.01, 0.02, and 0.05 wt% XG concentrations, respectively. On the other hand, for SDS PEF at the same

concentrations of XG, the stability increased by 3.6, 4.2, and 10.8 times compared to polymer-free foam (Fig. 3).

Studies of [37] and [55] reported two types of surfactant-polymer interactions (weakly and strongly interacting), where foam stability depends on the electrostatic forces between the two and the interfacial properties of each. The strong interaction is related to the system containing the surfactant and polymer with opposite charges. On the other hand, the weak interaction suggested by [37] occurs between equally charged surfactant-polymer pairs, such as the SDS-XG pair in this work. The strong electrostatic repulsive force between equally charged polymer and surfactant molecules promotes the rapid adsorption of surfactant molecules on the freshly created gas-liquid interfaces (facilitates foaming) and decelerates the film thinning [37].

Apart from the weak and strong interactions described above, the interaction between anionic surfactant and anionic polymer through hydrophobic areas of molecules in surfactant-rich conditions (well above critical micell concentration) was suggested in the literature [41, 56,57]. Such interaction should not adversely affect the stability of the foam; instead, it may enhance it by increasing the dynamic viscosity of the foaming solution [57]. This was confirmed by the dynamic viscosity curves of SDS and SC foaming solutions as a function of shear rate at various XG concentrations shown in Fig. 4.

Although all polymer-containing solutions exhibited shear-thinning behavior, the SDS-XG systems demonstrated higher viscosity values at XG concentration above 0.01 wt%. Hence, it could explain the superiority of drainage half-life time for SDS PEF compared to SC PEF, which increased with polymer concentration (Fig. 3). Additionally, it was supported by lower drainage rates and longer hold-ups for SDS PEFs observed in Fig. 2.

The SC-XG pair represents a strongly interacting system in our study. This is due to the positively charged part of the hydrophilic head of the bipolar CAHS surfactant. Electrostatic forces and the structural similarity of the hydrophobic tails of the CAHS and SDS molecules led to 1) XG molecules shrinkage due to disordered attachments of surfactant monomers caused by electrostatic and hydrophobic attractions [55,58]; 2) inhibition of the synergistic effect in SC surfactant mixture as a result of insufficient amount of free CAHS molecules at the interface [59]. The first could explain the lower viscosity contribution of XG in SC PEF than in SDS one observed in Fig. 4 and Fig. 2. The second could explain an insignificantly increased SC PEF stability at relatively high XG concentrations (Fig. 3). While at lower concentrations (0.01 wt% and 0.02 wt%), there were still enough free CAHS molecules to cover freshly generated gas-liquid-oil interfaces.

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Drainage rate for SDS (left) and SC (right) surfactant solutions at different polymer concentrations in the presence of diesel oil. The solid lines indicate the average value of the triplicates, while the colored area is delimited by the minimum and maximum values we have computed.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 3. Foam half-life as a function of drainage half-life time for SDS and SC surfactant solutions at different polymer concentrations, in the presence of 1 % vol. of diesel oil. The solid lines indicate the average value of the triplicates, while the colored area is delimited by the minimum and maximum values we have measured.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 4. Dynamic viscosity of SDS-based and SC-based foaming solutions as a function of shear rate at various polymer concentrations.</figcaption></figure>

## 3.2. Polymer-enhanced foam (PEF) flow performance in porous media

To ensure clarity and consistency in our experimental approach, we prioritized polymer concentrations that demonstrated significant stabilization effects in bulk foam tests. As shown in Section 3.1, the lowest concentrations of Xanthan Gum used in this study had minimal impact on foam stability, primarily due to insufficient increase in the viscosity of the continuous liquid phase. Given this limited effect, we focused on the 0.02 and 0.05 wt% of XG, as these concentrations showed the most promising stability and viscosity enhancements necessary for effective foam performance in porous media.

### 3.2.1. PEFs flow behavior in porous media

To examine the effect of PEF composition on the flow behavior, we first plotted the MRF as a function of cumulative injected pore volumes of foaming solution (PV) for SDS-based and SC-based foams at different

polymer concentrations in Fig. 5. Corresponding pressure gradients (∇P) of each foam injection experiment are displayed on the right.

From Fig. 5, one can observe a positive influence of XG polymer on the performance of both surfactant foam SDS and SC foams in contaminant-free porous media. The addition of 0.02 %wt. of XG resulted in a significant decrease in BT (–71 % for SDS- and –58 % for SC-based foams) and a noticeable increase in MRF (+12 % for SDS and +19 % for SC). The positive effect was also confirmed by the experiment at the lowest XG content (0.01 wt%) for SDS-based foam, showing rather high mobility reduction and earlier breakthroughs of strong foam. However, it is worth noting that the improvement was not obvious at the higher concentration of polymer (0.05 wt%), corresponding to the findings from the foam stability tests (see Section 3.1), where the half-life demonstrated a non-linear increase with XG concentration. The final values of MRF and strong foam BT for all tested formulations are also reported in Table 2.
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Mobility reduction factor (MRF) and corresponding pressure gradients ( ∇ P) as a function of cumulative injected pore volumes (PV) of foaming solution for SDS and SC PEFs in the absence of diesel oil. Vertical dashed lines indicate strong foam breakthrough time (BT)).</figcaption></figure>

<table><caption>**Table 2**

Strong foam breakthrough time (BT) and final mobility reductions (MRF) of SDS- and SC-based PEFs in the absence and presence of residual diesel.</caption><thead><tr><th rowspan="2">Foaming solution</th><th>BT(PV)</th><th>MRF(-)</th><th>BT(PV)</th><th>MRF(-)</th></tr><tr><th colspan="2">No diesel</th><th colspan="2">With diesel</th></tr></thead><tbody><tr><td>SDS</td><td>0.85</td><td>1305.1</td><td>1.80</td><td>570.3</td></tr><tr><td>SDS+XG(0.01 wt%)</td><td>0.48</td><td>1194.9</td><td>0.98</td><td>1074.6</td></tr><tr><td>SDS+XG(0.02 wt%)</td><td>0.25</td><td>1460.7</td><td>0.70</td><td>1352.4</td></tr><tr><td>SDS+XG(0.05 wt%)</td><td>0.24</td><td>1550.3</td><td>0.24</td><td>1481.2</td></tr><tr><td>SC</td><td>0.64</td><td>1335.9</td><td>1.39</td><td>1275.8</td></tr><tr><td>SC+XG(0.02 wt%)</td><td>0.27</td><td>1591.5</td><td>0.49</td><td>1520.0</td></tr><tr><td>SC+XG(0.05 wt%)</td><td>0.24</td><td>1640.0</td><td>0.40</td><td>1494.6</td></tr></tbody></table>

It's important to acknowledge that foam injection may exhibit a two-step breakthrough behavior during the foam's first invasion into the porous media. It was evidenced in Fig. 5 by the changes in slope for black curves below 0.5 pore volume of injection with lower magnitudes of MRF compared to the final ones. Weak foam breakthroughs first due to gravity segregation and preferential flow through high-permeability zones induced by the dominance of the rate of lamellae destruction over the rate of creation in the transition zone [60,24,61]. However, as the liquid phase drains and capillary pressure increases, foam generation is promoted, resulting in a second breakthrough of highly viscous finely

textured strong foam. Thus, the decreases in BT with monotonic evolution of MRF suggest a more piston-like displacement of the foam front when XG was present. This behavior is likely induced by the higher viscosity of the PEFs' flushing solutions, which tend to stabilize the front by preventing gas segregation and reducing lamellae destruction near the invasion front.

Regarding the measured ∇ P from Fig. 5, they were found to reach 0.25–0.3 MPa.m⁻¹ during the experiments. According to some previous works [62–64], the limit of ∇ P for using foam in an in-situ remediation context should be kept below 0.1 MPa.m⁻¹ to respect the challenges associated with soil uplifting. Our injection rates and column design generated pressure gradients above this limit. Since the present study focuses on the comparison between different foaming solutions, we considered it preferable to select injection conditions that guarantee a stable flow of foam (strong foam) along the entire length of the pack. Moreover, identical injection rates (Q<sub>foam</sub>), gas fractions (f<sub>g</sub>), and gas type between the experiments enable a direct comparison of PEF formulations.

### 3.2.2. Impact of diesel and sweeping efficiency of PEFs

Fig. 6 depicts the evolution of MRF and ∇ P during the injection of PEFs into sandpacks with diesel residual saturation (S<sub>dr</sub>: 13.1 – 20.7 %) after the foaming solution pre-flushing. The corresponding BT

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Mobility reduction factor (MRF) and pressure gradients ( ∇ P) as a function of cumulative injected pore volumes (PV) of foaming solution at residual diesel saturation (S<sub>dr</sub>: 13.1 – 20.7 %) for SDS- and SC-based PEFs.</figcaption></figure>
and MRF can also be found in Table 2.

In all studied cases, the diesel at residual saturation impacted both MRF and BT. A severe adverse effect was observed for SDS foam without polymer. The very low MRF (≤ 570) compared to all other foam compositions (≥ 1275) points out to an inability to form a strong foam, which is consistent with the results from the stability tests (see Section 3.1). In contrast, the incorporation of XG in SDS-based foam showed a contrary trend. From Fig. 6, at lower concentrations of XG (0.01 wt% and 0.02 wt%), one may observe some decrease of MRF with delayed BT due to diesel presence. Interestingly, at high concentration (0.05 wt%), the MRF and BT were rather similar to those obtained from diesel-free experiments. It suggests a poorly affected or an unaffected propagation front during the initial sweeping stage for two concentrations, respectively, indicating considerable foam improvement.

The SC-based PEFs, however, exhibited a less consistent trend with increasing XG concentration. One should remember that the SC surfactant mixture is intrinsically less sensitive to antifoaming diesel than SDS due to more stable pseudo-emulsion films [24]. This property was confirmed by the higher MRF obtained for the polymer-free SC foam. Nevertheless, diesel oil had some impact reflected by slightly elevated BT values. This limited impact was further suppressed by 0.02 wt% of Xanthan Gum (+19 %). Strikingly, in contrast to SDS-, the efficiency of SC-based PEF at 0.05 wt% found to be decreased evidenced by lower MRF and higher BT (see Fig. 6 and Tab. 2).

Although the strongest detrimental effect of diesel was on SDS foam compared to SC, the XG addition appeared more obviously efficient for the SDS-based PEFs since it preserved the hydraulic behavior of the foam flow. Considering the observations from Fig. 4, where the dynamic viscosities of the SDS-XG pair were higher than the SC-XG one at similar concentrations, it might be concluded that the strong interactions between surfactant and polymer might also influence the effect of polymer on foam flow behavior. These observations are in great agreement with other researchers, who argued that the surfactant-polymer interactions dictate the rate and efficiency of exchanges between the interface and continuous liquid phase during the stretching-contraction movement of lamellae in porous media [32,33]. It was particularly significant in SC-based PEFs in our study due to the opposite charges of the two.

The observed trends and findings were further strengthened by the recovery yields from diesel-contaminated sandy soil. Fig. 7 demonstrates recovery yields of diesel after pre-flushing with foaming solutions and foam injections for all studied cases. One can notice that PEFs consistently achieved higher recovery than conventional surfactant foams, aligning with previous works [65,66]. The increased diesel recovery was attributed to the improved hydraulic behavior of foam by

elevated solution viscosity and enhanced stability. Similarly, the improved tolerance to diesel in the SC mixture over SDS alone did not lead to a substantial increase in recovery yields, indicating that the addition of XG rather than the surfactant composition predominantly drives the observed phenomena. Residual diesel saturation decreased to as low as 2–3 % for all tested PEFs.

### 3.3. Discussion on the role Xanthan Gum in PEFs' improved performance in porous media

The mechanisms driving polymer-enhanced foams (PEFs) are complex, involving both physical and chemical interactions. Thus, it is not intuitive to explicitly point out the exact ones, responsible for the observed enhanced stability and efficiency in porous media in the current study. However, a few are discussed below that may explain the Xanthan Gum's (XG) impact on foam behavior.

Foam flow in porous media is a dynamic process. Lamellae stretch and contract as they pass through pore bodies and throats [67,68]. In PEFs, polymer molecules like Xanthan Gum (XG) can interact with dynamic lamellae in multiple ways. Some studies suggest that polymer molecules occupy the thin films between gas bubbles, increasing lamellae thickness, but potentially reducing the formability [26,69]. Others propose that the polymer primarily resides in the thicker liquid films surrounding the bubbles, particularly within the Plateau borders [70,71]. Regardless of their specific location, the polymer's presence generally increases foam mobility reduction and flow resistance, mainly due to the increased viscosity of the continuous liquid phase.

Generally, polymers are known to improve the so-called "plugging capacity" of the foam [72-74], the ability of foam to effectively divert the flow from highly permeable thief zones of a porous medium [75]. This effect is particularly beneficial when lamellae undergo partial destruction during the flow, as the viscous continuous liquid phase sustains the plugging capacity. Additionally, aqueous solutions prepared by Xanthan Gum behave as pseudoplastic fluids [76,77], which further aids in displacing the organic phase by allowing the foam to maintain its structural property under varying flow conditions. Hence, the principal advantage of PEFs over traditional surfactant foams observed in our study lies in their improved organic phase removal before or during PEF infection, rather than by stabilizing the foam against the destructive impact of oil [71,78]. The SDS-based PEF, despite SDS's relatively low tolerance to diesel, achieved notable mobility reduction and sweeping efficiency, an effect that was further amplified with increasing XG concentration. Similarly, SC-based PEFs maintained high MRF and recovery yields despite Xanthan Gum's weaker viscosifying effect on SC. Although the

<figure><img src="image_11.png" /><figcaption>Fig. 7. Recovery yields of diesel oil by SDS and SC PEFs and by corresponding foaming solutions as a function of XG concentration.</figcaption></figure>
presence of XG significantly increases the viscosity of the continuous phase, the role of surfactant-polymer interactions varies with surfactant type and charge.

The interaction is relatively weak in the SDS-XG system, where both components carry similar charges. It allows XG molecules to remain mainly in the continuous phase, promoting an even distribution of SDS across lamellae. Furthermore, the weak interaction in the SDS-XG pair enhances electrostatic repulsion within the lamellae, stabilizing the foam structure during flow through the Marangoni effect. This effect facilitates surfactant diffusion to stretched or contracted regions of the interface. The significantly increased MRF values for SDS-based PEFs in diesel-contaminated media demonstrate how this electrostatic repulsion aids in stabilizing the foam. Conversely, in the SC-XG system, interactions between CAHS (Cocamidopropyl Hydroxysultaine) and XG are rather strong due to the zwitterionic nature of the surfactant, which leads to the formation of disordered surfactant-polymer complexes. These complexes may involve electrostatic attraction between the negatively charged carboxyl groups of XG and the positively charged quaternary ammonium groups of CAHS, besides hydrogen bonding and hydrophobic interactions. This can reduce the concentration of free surfactant at the interface, impacting foam generation and stability. The effect is particularly pronounced in porous media, where foam stability and generation depend highly on surfactant concentration due to the extensive surface area of the generated bubbles [19,79]. Indeed, the limited MRF observed for SC-XG PEF compared to SDS-XG PEF in our study could be attributed to this depletion of available surfactant at higher XG concentrations. This was evident in the MRF curve instabilities at 0.05 wt% compared to 0.02 wt% XG for SC-based PEFs. This behavior aligns with observations by [33], who noted similar limitations in associative polymer foams.

Therefore, the results from our study offer a very promising perspective for PEFs' application in environmental science and engineering. Following the traditional pump-and-treat method, they could also be directly injected into the soil matrix through an injection well towards the extraction well without any pre-flushings. Studies, such as those conducted by [80], have demonstrated that PEFs remain effective even at relatively high organic phase saturations compared to regular surfactant foams. However, further laboratory tests are required to confirm the interest in such an approach, considering the injection of PEFs without pre-flushing and/or a porous media with higher diesel oil saturations.

### 3.4. Correlations between bulk foam and foam flowing in porous media

There is a definite separation in the physics governing the behavior

of bulk foam and foam flowing in porous medium [81,24,61,23]. However, forecasts of foam behavior in porous media based on bulk foam tests are of great interest due to their cost-effectiveness and capability to provide essential insight into the interactions between foam components (see Section 3.1). Numerous attempts have already been made to relate the properties of bulk foam and foam in porous media in the literature. Typically, it considers the correlations between the maximum lifetime of foam and pressure drop-based parameters (i.e., gas mobility reduction or apparent viscosity). Some research studies have found positive correlations in the absence of oil [82,19,83]. In the other works, the contrary, a failure in its presence was observed [84,19, 23]. Hence, no clear relationship has yet been established between the two states of foam.

In Fig. 8(a), we first plotted MRF (obtained with porous media experiments) as a function of foam half-life (obtained with bulk foam experiments) to anticipate PEF behavior in porous media based on bulk foam experiments. However, from Fig. 8(a), no obvious relation can be drawn between the two due to the rather good plug-flow behavior of all tested solutions at the residual diesel condition except XG-free SDS foam (see Section 3.2.2). We assume that the high plugging capacity of PEFs and the pre-flushing of porous media can lessen the full destructive impact of diesel oil on foam. Moreover, pressure drop-based parameters from foam injection experiments usually reach their maximum after the displacement of most of the oil from the medium [23]. Thus, other types of relationships are required for PEFs.

In Fig. 8(b), we correlate two other parameters of bulk and foam in porous media used in our study, such as drainage half-life and breakthrough time, respectively. BT was normalized with respect to the total injected pore volume of the solution. Fig. 8(b) depicts the exponential decay of breakthrough time with drainage time. It suggests a certain threshold above which the displacement efficiency by foam and mobility reduction in porous medium become almost independent of drainage and stability of bulk PEF, respectively. It is likely due to the confinements posed by the porous medium since foam survival is a complex function of not only lamellae stability but also petro-physical properties of the medium in which foam flows [48]. When the conditions are favorable due to lamellae stability or lower oil saturation, the foam will have a finer texture and reach maximum gas mobility reduction. As the gas phase's mobility reduces, the aqueous phase saturation will decrease, leading to higher capillary pressure between the two phases. The foam coalescence rate will increase once it approaches the so-called limiting capillary pressure ($P_c^*$) [85]. $P_c^*$, in turn, highly depends on the aqueous phase saturation and soil morphology [86].

Thus, numerous factors influence the complicated relationship between two states of foam, including the experimental conditions and

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** MRF of PEFs as a function of foam half-life (a) and normalized BT of PEFs as a function of drainage half-life (b) for the various concentrations of XG in the presence of diesel oil.</figcaption></figure>
physics governing foam behavior in two different states. It highly depends on foaming component interactions, particularly in the PEF case. Our work demonstrated that the anticipations can not be done by correlating solely the one single parameter obtained from bulk and flowing conditions as it is done in common practices [19,87]. Hence, a more holistic approach is required, as we have proposed in our work for PEFs. However, it remains an open path for future research.

## 4. Conclusion

This study investigated the performance of polymer-enhanced foams (PEFs) for the remediation of diesel-contaminated soil, focusing on the impact of Xanthan Gum (XG) on foam stability and flow behavior. Bulk foam tests revealed that XG improved the stability of both SDS- and SC-based PEFs, but its effect was less pronounced than the enhancement provided by the CAHS in the SC mixture compared to SDS alone. XG also improved the drainage half-time, particularly for the SDS-XG system due to better synergy between the surfactant and polymer. Foam flow experiments demonstrated that XG enhanced the plugging capacity of the foams due to the increased viscosity of the aqueous phase. Surprisingly, the superior diesel tolerance of SC-based PEFs observed in bulk experiments did not translate into a significant advantage in terms of flow performance and diesel removal in contaminated sandpacks.

Our findings demonstrate that the careful selection of both polymer and surfactant is crucial for developing effective PEFs. The concentration of XG should be optimized to balance foam stability, mobility control, and foamability. While XG improved the performance of both SDS and SC-based foams, the SDS-XG system showed better synergy and greater enhancement, particularly in the presence of diesel. This study provides valuable insights for the formulation of PEFs for soil remediation. It highlights the importance of considering polymer-surfactant interactions and the potential limitations of relying solely on bulk foam stability tests to predict flow behavior in porous media. By understanding these factors, more targeted and efficient PEF formulations can be developed for various contaminants and site-specific conditions. Future research should focus on understanding the molecular dynamics between polymers and different surfactants, exploring alternative bio-polymers, and conducting large-scale studies to evaluate the practical applications and economic viability of PEFs for soil remediation.

## Environmental Implication

Soil and aquifer contamination by petroleum hydrocarbons threatens both the environment and public health. Conventional remediation methods are often impractical for hard-to-reach, heterogeneous areas. Aqueous foam injections offer a novel solution, but their stability is compromised in the presence of hydrocarbons. Our study introduces polymer-enhanced foams (PEFs), incorporating Xanthan Gum to increase the stability against diesel oil. These biodegradable, non-toxic PEFs improve hydrocarbon mobilization and recovery, providing a sustainable remediation approach. This research contributes to the development of robust remediation technologies essential for environmental protection, effectively mitigating the adverse impacts of hydrocarbon spills on soil and groundwater.

## CRediT authorship contribution statement

**Adil Baigadilov:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Stéfan Colombano:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **Sagyn Omirbekov:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Conceptualization. **Hugues Bodiguel:** Writing – review & editing, Validation, Supervision, Methodology, Conceptualization. **Laurent Oxarango:** Writing – review & editing,

Validation, Supervision, Methodology, Conceptualization. **Maxime Coheennec:** Writing – review & editing, Validation, Software, Formal analysis, Data curation. **Dorian Davarzani:** Writing – review & editing, Supervision. **Fabien Lion:** Software, Resources.

## 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.

## Data Availability

Data will be made available on request.

## Acknowledgments

This study was conducted as part of the "MOBILMOUSSE" project. The authors would like to thank BRGM (French Geological Survey), COLAS ENVIRONNEMENT, and EDF (Électricité de France) for co-financing the project. We also greatly acknowledge the financial support of the PIVOTS project, funded by the "Regione Centrale-Val-de-Loire" and the European Regional Development Fund. Special thanks are extended to the Bolashak International Scholarship of the Government of Kazakhstan for providing a PhD grant to Adil Baigadilov. Additionally, we acknowledge research grant no. AP19679429 from the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan.

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