Surfactant foam technology for in situ removal of heavy chlorinated compounds-DNAPLs

Mobilization of DNAPL lenses in heterogeneous aquifers using shear-thinning PEO polymers: Experimental and numerical study

Polymer solution injection has emerged as a promising method for the remediation of NAPL (non-aqueous phase liquids)-contaminated aquifers. This technique enhances recovery efficiency by modifying viscous forces, stabilizing the displacement front, and minimizing channeling effects. However, there remains a significant gap in understanding the behavior of polymer solutions, particularly those with different molecular weights (MW), for mobilizing DNAPL (dense non-aqueous phase liquids) trapped in heterogeneous aquifers, especially within low-permeability layers. In this study, we address this gap by investigating the mobilization of DNAPL lenses confined by low-permeability layers through the injection of polyethylene oxide (PEO) polymers of varying MW. PEO solutions with MW of 5 M (million) and 8 Mg/mol displayed shear-thinning behavior for shear rates of 0.01 to 100 s-1, while the 1 Mg/mol solution showed shear-thinning below 10 s-1 and Newtonian behavior above. PEO solutions in porous media exhibit Newtonian behavior at low-to-moderate shear rates for all MWs, likely due to confinement-limited entanglement. Adsorption studies found non-significant PEO adsorption on soil surfaces, likely due to its large molecular size. Post-flushing of PEO-saturated columns with water led to notable permeability reductions attributed to viscous fingering. Column tests indicated a decrease of the residual DNAPL saturation with the capillary number (Ca), more sharply in low permeability soils. 2D cell tests identified three stages of DNAPL mobilization: initial stabilization, sharp recovery increase upon PEO arrival, and a final stabilization at residual saturation. The duration of each transition was found to be influenced by concentration. Numerical simulations accurately mirrored these stages and provided additional insights into PEO viscosity distribution and DNAPL mobilization patterns in heterogeneous media. The results highlighted that higher injection rates promote mobilization from the two low permeability layers surrounding the DNAPL bank from both sides and the upper zone, while lower rates mainly drive mobilization from the upper side. Using numerical simulations the performance of PEO injection on displacement of DNAPL in multiple lenses and various position of recovery points was evaluated.

The effectivity and applicability of a novel sugar-based anionic and nonionic Gemini surfactant synthetized for the perchloroethylene-contaminated groundwater remediation

Surfactant-enhanced aquifer remediation (SEAR) has effectively removed dense nonaqueous phase liquids (DNAPLs) from the contaminated aquifers. However, restricted by structural defects, typical monomeric surfactants undergo precipitation, high adsorption loss, and poor solubilization in aquifers, resulting in low remediation efficiency. In this study, a novel sugar-based anionic and non-ionic Gemini surfactant (SANG) was designed and synthesized for SEAR. Glucose was introduced into SANG as a non-ionic group to overcome the interference of low temperature and ions in groundwater. Sodium sulfonate was introduced as an anionic group to overcome aquifer adsorption loss. Two long-straight carbon chains were introduced as hydrophobic groups to provide high surface activity and solubilizing capacity. Even with low temperature or high salt content, its solution did not precipitate in aquifer conditions. The adsorption loss was as low as 0.54 and 0.90 mg/g in medium and fine sand, respectively. Compared with typical surfactants used for SEAR, SANG had the highest solubilization and desorption abilities for perchloroethylene (PCE) without emulsification, a crucial negative that Tween80 and other non-ionic surfactants exhibit. After flushing the contaminated aquifer using SANG, > 99 % of PCE was removed. Thus, with low potential environmental risk, SANG is effectively applicable in subsurface remediation, making it a better surfactant choice for SEAR.

Surfactant foam injection for remediation of diesel-contaminated soil: A comprehensive study on the role of co-surfactant in foaming formulation enhancement

Aqueous foam injection is a promising technique for in-situ remediation of soil and aquifers contaminated by petroleum products. However, the application efficiency is strongly hindered by foam's instability upon contact with hydrocarbons. Addressing this, we propose a new binary surfactant mixture of Sodium Dodecyl Sulfate (SDS) and Cocamidopropyl Hydroxysultaine (CAHS). This study investigates CAHS's role as a co-surfactant in enhancing foam stability against antifoaming diesel oil under static and dynamic conditions. Using a dynamic foam analyzer (DFA-100), we assessed static foam's stability by monitoring decay profiles and bubble growth over time. The results revealed that the highest stability can be reached at a CAHS to SDS ratio of 50:50, increasing the half-life of the foam by 7.7 times. Remarkably, our analyses at bulk and bubble scales also elucidated the mechanisms behind the enhanced foam stability of the proposed binary surfactant mixture in the absence and presence of diesel. Additionally, in a 1D sand column, the SDS-CAHS mixture demonstrated more than twofold improvement of the Resistance Factor, attributed to the better survival of the lamellae due to the reduced rate of their destruction. This formulation also yielded a recovery improvement of >10 % compared to SDS foam. The significant improvements in stability and performance of the SDS-CAHS (50:50) mixture were credited to a robust pseudo-emulsion film formation, creating a higher oil entry barrier. This reinforcement and the surfactant molecules' synergistic interactions at the gas-liquid-oil interface significantly contributed to the overall effectiveness.

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.

Applications of functional nanoparticle-stabilized surfactant foam in petroleum-contaminated soil remediation

Surfactant foam (SF) can be used to remediate petroleum-contaminated soil because of its easy transfer to inhomogeneous and low-permeability formations. Nanoparticles (NPs) not only stabilize SF under extreme conditions but also impart various functions, aiding the removal of petroleum contaminants. This review discusses the stabilization mechanisms of nanoparticle-stabilized SF (NP-SF) as well as the effects of NP size, chargeability, wettability, and NP-to-surfactant ratio on foam stability. SF stabilized by inert SiO₂ NPs is most commonly used to remediate soil contaminated with crude oil and diesel. Low dose of SF stabilized by nano zero-valent iron is cost-effective for treating soil contaminated with chlorinated organics and heavy metal ions. The efficiency and recyclability of Al₂O₃/Fe₃O₄ NPs in the remediation of diesel and crude oil contamination could be enhanced by applying a magnetic field. This review provides a theoretical basis and practical guidelines for developing functional NP-SF to improve the remediation of petroleum-contaminated soils. Future research should focus on the structural design of photocatalytic NPs and the application of catalytic NP-SF in soil remediation.

An Overview on the Treatment of Oil Pollutants in Soil Using Synthetic and Biological Surfactant Foam and Nanoparticles

Oil-contaminated soil is one of the most concerning problems due to its potential damage to human, animals, and the environment. Nanoparticles have effectively been used to degrade oil pollution in soil in the lab and in the field for a long time. In recent years, surfactant foam and nanoparticles have shown high removal of oil pollutants from contaminated soil. This review provides an overview on the remediation of oil pollutants in soil using nanoparticles, surfactant foams, and nanoparticle-stabilized surfactant foams. In particular, the fate and transport of oil compounds in the soil, the interaction of nanoparticles and surfactant foam, the removal mechanisms of nanoparticles and various surfactant foams, the effect of some factors (e.g., soil characteristics and amount, nanoparticle properties, surfactant concentration) on remediation efficiency, and some advantages and disadvantages of these methods are evaluated. Different nanoparticles and surfactant foam can be effectively utilized for treating oil compounds in contaminated soil. The treatment efficiency is dependent on many factors. Thus, optimizing these factors in each scenario is required to achieve a high remediation rate while not causing negative effects on humans, animals, and the environment. In the future, more research on the soil types, operating cost, posttreatment process, and recycling and reuse of surfactants and nanoparticles need to be conducted.

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.

Assessment of shear-thinning fluids and strategies for enhanced in situ removal of heavy chlorinated compounds-DNAPLs in an anisotropic aquifer

The removal of chlorinated organic hydrocarbons (COHs) -DNAPLs was studied in permeability-contrasted sandboxes with an egg-box shaped substratum. Aqueous solutions were compared to viscous shear-thinning fluids (xanthan solution and foam). Interfacial and viscous effects were compared by increasing the capillary number of injected fluids. Non-spatially targeted DNAPL recovery (NSTR) where the driving force was the injection pressure, was compared to spatially targeted DNAPL recovery (STR) where a pumping system allowed the controlled flow. A historical contamination made of a complex mixture of COHs and hexachlorobutadiene (HCBD) as a model were used. NSTR results showed that DNAPL recovery with non-viscous liquids did not exceed 40%. The best results were obtained for xanthan solutions with surfactant ~ 1.3 ×CMC for which pure phase recovery amounted to 88% and 93% for HCBD and for the historical DNAPL, respectively. The STR strategy showed similar recovery yields, whereas xanthan concentrations were 10-times lower. Mass balances on DNAPL showed that at most, 0.15% of COHs was dissolved in the aqueous effluents. NZVI (1 g.l^-1) were delivered in xanthan in view of the chemical degradation of residual COHs and showed a 65% transmission through the low permeability soil.

Interaction quantitative modeling of mixed surfactants for synergistic solubilization by resonance light scattering

In situ flushing through surfactant-enhanced aquifer remediation (SEAR) technology has long been recognized as a promising technique for NAPL removal from contaminated aquifers. However, there have been few studies on the choice of surfactants. In this work, the interaction quantitative model between resonance light scattering intensity and the concentration of binary surfactant mixtures NP-10+SDBS and NP-10+CTAB was established, and the mechanism of binary surfactant interaction was explored through the model by the resonance light scattering method. The relationship between the model constants and NAPL solubilization was also investigated to better address the application of surfactants in practical NAPL-contaminated site remediation. The critical micelle concentrations (CMCs) of nonylphenol ethoxylate (NP-10), dodecyl benzene sulfonate (SDBS), hexadecyl trimethyl ammonium bromide (CTAB), and the binary surfactant mixtures were measured by resonance light scattering (RLS), which were consistent with those obtained from surface tension measurements. In all cases, the RLS signals exhibited similar variations with surfactant concentration. A quantitative calculation model based on the RLS measurement data was established, and the binding constants K_NP-10+SDBS and K_NP-10+CTAB were calculated to be 0.66 and 1.51 L·mmol^-1, respectively, according to the equilibrium equations. The results showed that the binding constants have a significant positive correlation with NAPL solubilization.

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.

Remediation of soils contaminated by hydrophobic organic compounds: How to recover extracting agents from soil washing solutions?

A lot of soil (particularly, former industrial and military sites) has been contaminated by various highly toxic contaminants such as petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenyls (PCBs) or chlorinated solvents. Soil remediation is now required for their promotion into new industrial or real estate activities. Therefore, the soil washing (SW) process enhanced by the use of extracting agents (EAs) such as surfactants or cyclodextrins (CDs) has been developed for the removal of hydrophobic organic compounds (HOCs) from contaminated soils. The use of extracting agents allows improving the transfer of HOCs from the soil-sorbed fraction to the washing solution. However, using large amount of extracting agents is also a critical drawback for cost-effectiveness of the SW process. The aim of this review is to examine how extracting agents might be recovered from SW solutions for reuse. Various separation processes are able to recover large amounts of extracting agents according to the physicochemical characteristics of target pollutants and extracting agents. However, an additional treatment step is required for the degradation of recovered pollutants. SW solutions may also undergo degradation processes such as advanced oxidation processes (AOPs) with in situ production of oxidants. Partial recovery of extracting agents can be achieved according to operating conditions and reaction kinetics between organic compounds and oxidant species. The suitability of each process is discussed according to the various physicochemical characteristics of SW solutions. A particular attention is paid to the anodic oxidation process, which allows either a selective degradation of the target pollutants or a complete removal of the organic load depending on the operating conditions.

A new foam-based method for the (bio)degradation of hydrocarbons in contaminated vadose zone

An innovative foam-based method for Fenton reagents (FR) and bacteria delivery was assessed for the in situ remediation of a petroleum hydrocarbon-contaminated unsaturated zone. The surfactant foam was first injected, then reagent solutions were delivered and propagated through the network of foam lamellae with a piston-like effect. Bench-scale experiments demonstrated the feasibility of the various treatments with hydrocarbon (HC) removal efficiencies as high as 96 %. Compared to the direct injection of FR solutions, the foam-based method led to larger radii of influence and more isotropic reagents delivery, whereas it did not show any detrimental effect regarding HC oxidation. Despite 25 % of HCs were expelled from the treated zone because of high foam viscosity, average degradation rates were increased by 20 %. At field-scale, foam and reagent solutions injections in soil were tracked both using visual observation and differential electric resistivity tomography. The latter demonstrated the controlled delivery of the reactive solutions using the foam-based method. Even if the foam-based method duration is about 5-times longer than the direct injection of amendment solutions, it provides important benefits, such as the confinement of harmful volatile hydrocarbons during Fenton treatments, the enhanced reagents delivery and the 30 % lower consumption of the latter.

Controlled treatment of a high velocity anisotropic aquifer model contaminated by hexachlorocyclohexanes

Xanthan gels were assessed to control the reductive dechlorination of hexachlorocyclohexanes (HCHs) and trichlorobenzenes (TCBs) in a strong permeability contrast and high velocity sedimentary aquifer. An alkaline degradation was selected because of the low cost of NaOH and Ca(OH)₂. The rheology of alkaline xanthan gels and their ability to deliver alkalinity homogeneously, while maintaining the latter, were studied. Whereas the xanthan gels behaved like non-Newtonian shear-thinning fluids, alkalinity and Ca(OH)₂ microparticles had detrimental effects, yet, the latter decreased with the shear-rate. Breakthrough curves for the NaOH and Ca(OH)₂ in xanthan solutions, carried out in the lowest permeability soil (9.9 μm²), demonstrated the excellent transmission of alkalinity, while moderate pressure gradients were applied. Injection velocities ranging from 1.8 to 3.8 m h^-1 are anticipated in the field, given the permeability range from 9.9 to 848.7 μm². Despite a permeability contrast of 8.7 in an anisotropic aquifer model, the NaOH and the Ca(OH)₂ both in xanthan gels spread only 5- and 7-times faster in the higher permeability zone, demonstrating that the delivery was enhanced. Moreover, the alkaline gels which were injected into a high permeability layer under lateral water flow, showed a persistent blocking effect and longevity (timescale of weeks), in contrast to the alkaline solution in absence of xanthan. Kinetics of alkaline dechlorination carried out on the historically contaminated soil, using the Ca(OH)₂ suspension in xanthan solution, showed that HCHs were converted in TCBs by dehydrodechlorination, whereas the latter were then degraded by reductive hydrogenolysis. Degradation kinetics were achieved within 30 h for the major and most reactive fraction of HCHs.

Surfactant-enhanced aquifer remediation: Mechanisms, influences, limitations and the countermeasures

In recent years, surfactant-enhanced aquifer remediation (SEAR) has attracted increasing interest duo to the high efficiency of removing non-aqueous phase liquids (NAPLs) from aquifer. A thorough understanding of SEAR is necessary for its successful implementation in field remediation. This paper reviewed the SEAR technology in a comprehensive way based on the recent research advances. Firstly, an overview of the basic processes and mechanisms underlying the technology was presented. Secondly, applications of SEAR and the factors that influence the performance were summarized. Thirdly, the key limitations of SEAR, which are downward migration of dense-NAPLs, secondary pollution of surfactants, adsorptive, precipitative and partitioning loss of surfactants, and heterogeneity of the aquifer, were reviewed. Finally, the recent advances in modifying SEAR to overcome the limitations were discussed in detail. The review will promote our understanding of SEAR technology and provide some useful information to improve the performance of SEAR in applications.

Soil flushing pilot test in a landfill polluted with liquid organic wastes from lindane production

Sites contaminated by Dense Non-Aqueous Liquid Phases (DNAPLs) containing chlorinated compounds are a ubiquitous problem caused by spills or the dumping of wastes with no concern for the environment. Their migration by gravity through the subsurface and their accumulation far below ground level make in-situ treatments the most appropriate remediation technologies. In this work, an aqueous solution containing a non-ionic and biodegradable surfactant was injected in the Sardas alluvial layer contaminated at some points with DNAPL (formed by a mixture of more than 28 chlorinated compounds) from lindane production. A volume of 5.28 m³ of an aqueous surfactant emulsion (13 g L^-1) was injected at 14.5 m b g.l in the permeable layer (gravel-sand), at a flow rate of 0.6 m³ h^-1 and the groundwater was monitored within a test cell (3.5 m radius) built ad hoc. The flow of the injected fluids in the subsurface was also evaluated using a conservative tracer, bromide (130 mg L^-1), added to the surfactant solution. Concentration of contaminants, chloride, bromide and surfactant, surface tension and conductivity were measured at the injection point and at three monitoring points over time. High radial dispersion was noticed resulting in high dilution of the injected fluids. The surfactant was not adsorbed in the soil during the injection time, the adsorption of the surfactant took place in the meantime (15 h) between its injection and the groundwater (GW) extraction. The concentration of chlorinated compounds dissolved from the soil in the surfactant aqueous phase when equilibrium was reached (about 850 mg L^-1) is related to the moderate average contamination of the soil in the test cell (about 1230 mg kg^-1). In contrast, the extraction of the free DNAPL in the altered marls layer was highly enhanced due to the addition of the surfactant. Finally, it was found that the surfactant and the contamination did not migrate from the capture zone.

Fractional-Flow Theory for Non-Newtonian Surfactant-Alternating-Gas Foam Processes

Foam can improve sweep efficiency in gas-injection-enhanced oil recovery. Surfactant-alternating-gas (SAG) is a favored method of foam injection. Laboratory data indicate that foam can be non-Newtonian at low water fractional flow fw, and therefore during gas injection in a SAG process. We investigate the implications of this finding for mobility control and injectivity, by extending fractional-flow theory to gas injection in a non-Newtonian SAG process in radial flow. We make most of the standard assumptions of fractional-flow theory (incompressible phases, one-dimensional displacement through a homogeneous reservoir, instantaneous attainment of local equilibrium), excluding Newtonian mobilities. For this initial study, we ignore the effect of changing or non-uniform oil saturation on foam. Non-Newtonian behavior at low fw implies that the limiting water saturation for foam stability varies as superficial velocity decreases with radial distance from the well. We discretize the domain radially and perform Buckley–Leverett analysis on each narrow increment in radius. Solution characteristics move outward with fixed fw. We base the foam model parameters and non-Newtonian behavior on laboratory data in the absence of oil. We compare results to mobility and injectivity determined by conventional simulation, where grid resolution is usually limited. For shear-thinning foam, mobility control improves as the foam front propagates from the well, but injectivity declines somewhat with time. This change in mobility ratio is not that at steady state at fixed water fractional flow in the laboratory, however, because the shock front in a non-Newtonian SAG process does not propagate at fixed fractional flow (though individual characteristics do). Moreover, the shock front is not governed by the conventional condition of tangency to the fractional-flow curve, though it continually approaches this condition. Injectivity benefits from the increased mobility of shear-thinning foam near the well. The foam front, which maintains a constant dimensionless velocity for Newtonian foam, decelerates somewhat with time for shear-thinning foam. For shear-thickening foam, mobility control deteriorates as the foam front advances, though injectivity improves somewhat with time. Overall, however, injectivity suffers from reduced foam mobility at high superficial velocity near the well. The foam front accelerates somewhat with time. Conventional simulators cannot adequately represent these effects, or estimate injectivity accurately, in the absence of extraordinarily fine grid resolution near the injection well.

Comparative assessment of a foam-based oxidative treatment of hydrocarbon-contaminated unsaturated and anisotropic soils

In situ delivery of liquid reagents in vadose zone is limited by soil anisotropy and gravity. The enhanced delivery of persulfate (PS) as oxidant, using a new foam-based method (F-PS) was compared at bench-scale to traditional water-based (W-PS) and surfactant solution-based (S-PS) deliveries. The goal was to distribute PS uniformly in coal tar-contaminated unsaturated and anisotropic soils, both in terms of permeability and contamination. Water was the less efficiently delivered fluid because of the hydrophobicity of the contaminated soils. Surfactant enhanced PS-distribution into contaminated zones by reducing interfacial tension and inverting soil wettability. Regardless of coal tar contamination contrasts (0 vs. 5 and 1 vs. 10 g kg _soil^-1) or strong permeability contrasts, PS-solution injection after foam injection led to the most uniform reagents delivery. While PS-concentration varied more than 5-times between zones using W-PS and S-PS methods, it varied less than 1.6-times when the F-PS one was used. Finally, despite unfavorable conditions, the foam-based method did not show any detrimental effect regarding the oxidation of hydrocarbons compared to the W-PS and S-PS methods carried out in ideal conditions. Moreover, hydrocarbon degradation rates were slightly higher when using F-PS than S-PS due to a lower surfactant content in the targeted zone.

A review of sources, environmental occurrences and human exposure risks of hexachlorobutadiene and its association with some other chlorinated organics

Research on hexachlorobutadiene (HCBD) has increased since its listing in the Stockholm Convention on Persistent Organic Pollutants in 2011. However, thorough reports on recent data regarding this topic are lacking. Moreover, potential associations between HCBD and some chlorinated organics have usually been ignored in previous research. In this review, possible formation pathways and sources, current environmental occurrences and human exposure risks of HCBD are discussed, as well as the association with several organochlorine compounds. The results reveal that unintentional production and emission from industrial activities and waste treatments are the main sources of HCBD. Similar precursors are found for HCBD and chlorobenzenes, indicating the presence of common sources. Although recent data indicates that levels of HCBD in the environment are generally low, risks from human exposure to HCBD, together with other pollutants, may be high. More attention in the future needs to be paid to the mixed contamination of HCBD and other pollutants from common sources.

A review on the application of chemical surfactant and surfactant foam for remediation of petroleum oil contaminated soil

Soil, exposed to petroleum oil contaminants (in the form of petrol, diesel, gasoline, crude oil, used motor oil), may cause potential damage to the environment, animal and human health. In this review article, mechanisms of the petroleum oil contaminant removal from soil by chemical surfactant systems such as surfactant solution, surfactant foam and nanoparticle stabilized surfactant foams are explained. Laboratory based research works, reported within the last decade on the application of similar systems towards the removal of petroleum oil contaminant from the soil, have been discussed. It is an important fact that the commercial implementation of the chemical surfactant based technology depends on the environmental properties (biodegradability and toxicity) of the surfactants. In recent times, surfactant foam and nanoparticle stabilized surfactant foam are becoming more popular and considered advantageous over the use of surfactant solution alone. However, more research works have to be conducted on nanoparticle stabilized foam. The impact of physicochemical properties of the nanoparticles on soil remediation has to be explored in depth.

Comparison of zero-valent iron and iron oxide nanoparticle stabilized alkyl polyglucoside phosphate foams for remediation of diesel-contaminated soils

Stable surfactant foam might play a vital role in the effective remediation of diesel oil contaminated soil-a major environmental hazard. This paper, first of its kind, is reporting the remediation of diesel-contaminated desert soil, coastal soil and clay soil by aqueous alkylpolyglucoside phosphate (APG-Ph) surfactant foams stabilized by Fe⁰ and Fe₃O₄ nanoparticles. Zero-valent iron (Fe⁰, ∼28 nm) and iron oxide (Fe₃O₄, ∼20 nm) nanoparticles are synthesized by liquid-phase reduction and precipitation methods, respectively. The effect of these nanoparticles on foamability, foam stability, surface tension and remediation of diesel-contaminated soils are examined at various concentrations (volume %) of alkylpolyglucoside phosphate (APG-Ph) surfactant and nanoparticles (mg/l). The maximum values of foamability and foam stability recorded for 0.1 vol % APG-Ph foam stabilized by 3.5 mg/l Fe⁰ are 108.3 and 110.4 mL, respectively. At the same conditions, the Fe₃O₄ results in 99.4 and 87.5 mL, respectively, depicting the better performance of Fe⁰. Reduction in surface tension of 0.1 vol % APG-Ph solution (50.75 mN/m) with the addition of 3.5 mg/l Fe⁰ (9.51 mN/m) and Fe₃O₄ (19.45 mN/m) nanoparticle is observed. Both the nanoparticles enhance remediation. The foam formed with 0.1 vol % APG-Ph and stabilized by 3.5 mg/l Fe⁰ shows the maximum diesel removal efficiency of 95.3, 94.6, and 57.5% for coastal soil, desert soil and clay soil, respectively. On the other hand, Fe₃O₄ (3.5 mg/l) stabilized APG-Ph foam of the same concentration shows merely 76.0, 79.6 and 51.6% diesel removal efficiency for coastal soil, desert soil, and clay soil, respectively. The rate of diesel removal by zero-valent iron and iron oxide nanoparticle stabilized foams are found to be well described by the first order kinetic model. Higher foamability, foam stability, and reducing capacity accompanying lower surface tension, compared to those of the Fe₃O₄ nanoparticle stabilized foam, could explain higher diesel removal efficiency of the Fe⁰ nanoparticle stabilized foam.

Targeted delivery of hydrogen for the bioremediation of aquifers contaminated by dissolved chlorinated compounds

Dihydrogen (H₂) gas injection is a promising option to enhance the reductive biodehalogenation of contaminants in groundwater. However, it is challenging to ensure its targeted delivery at the right places in plumes, and for the long times required for bioremediation. In this paper, the ability of surfactant foam to retain H₂ in the saturated zone and to enhance its release in the dissolved form was compared to traditional biosparging. H₂ gas was injected, either alone, or as foam, in a 2D saturated cell packed with glass beads. This cell was continuously flushed with deoxygenated water to mimic aquifer circulation, and H₂ was studied both in terms of gas distribution in the cell and dissolved concentrations downstream the injection zone. Experimental results are discussed in conjunction with simulations obtained using modeling. Both show that the viscous behavior of foam allows to efficiently retain greater volumes of H₂ gas, 3.5 times higher than biosparging. Moreover, it is retained in a dense manner around the injection point, making possible the targeted delivery of this reagent. Besides, the gas dissolution in groundwater showed to be steadier and more persistent when gas was injected as foam, with dissolution rate constants observed to be 1.12 à 1.58 times lower. Finally, the retained foamed-gas persistently reduced water's relative permeability 1.7 to 5 times, diverting the groundwater flow from the treated zone despite the fast elution of the surfactant. Hence, when H₂-foam injection is targeted to plume's contaminant concentration hotspots, on top of enhancing bioremediation, it can reduce contaminant diffusion to groundwater.

Enhanced remedial reagents delivery in unsaturated anisotropic soils using surfactant foam

Homogeneous delivery of solution of oxidant in unsaturated soils is limited by soil anisotropy and gravity. An innovative injection strategy using foam was developed to improve in situ delivery. Primary foam injection before oxidant solution enhanced both the lateral and uniform delivery of reactant in isotropic and anisotropic (permeability, contamination) soils. The oxidant spread isotropically through the foam water network. This sequential injection heavily improved the delivery radius of influence (ROI), while limiting contact between surfactant and solution of oxidant in order to preserve the selective oxidation of petroleum hydrocarbons contaminant (TPH). Prior foam injection allowed uniform delivery of the solution of oxidant across the region occupied by the foam, regardless of the soil permeability contrast (1:18), whereas poor ROI were observed for the direct injection of oxidant. Experiments in contamination contrasted soils showed that foam was able to propagate in highly TPH contaminated soils (max 60% velocity reduction for 22 g.kg_{dry soil}^-1). As for permeability contrast, foam is expected to enhance reagents delivery in such contexts. This novel strategy was proven to be efficient, even for complex anisotropic conditions, and should allow to cut field costs and uncertainties associated to poor reagents delivery.

3D numerical modelling of a pulsed pumping process of a large DNAPL pool: In situ pilot-scale case study of hexachlorobutadiene in a keyed enclosure

Remediation of dense non-aqueous phase liquids (DNAPLs) represents a challenging issue because of their persistent behaviour in the environment. This pilot-scale study investigates, by means of in situ experiments and numerical modelling, the feasibility of the pulsed pumping process of a large amount of a DNAPL in an alluvial aquifer. The main compound of the DNAPL is hexachlorobutadiene (HCBD), added in 2015 to the persistent organic pollutants list (POP). A low-permeability keyed enclosure was built at the location of the DNAPL source zone in order to isolate a finite volume of soil and a 3-month pulsed pumping process was applied inside the enclosure to exclusively extract the DNAPL. The water/DNAPL interface elevation at both the pumping well and an observation well was recorded. The cumulated pumped volume of DNAPL was also monitored. A total volume of about 20 m³ of pure DNAPL was recovered since no water was extracted during the process. The three-dimensional and multiphase flow simulator TMVOC was used and a conceptual model was elaborated and generated with the pre/post-processing tool mView. Numerical simulations reproduce the pulsed pumping process and show an excellent match between simulated and field data of DNAPL cumulated pumped volume and a reasonable agreement between modelled and observed data for the evolution of the water/DNAPL interface elevations at the two wells. This study offers a new perspective in remediation since DNAPL pumping system optimisation may be performed where a large amount of DNAPL is encountered.

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.

Shear-thinning fluids for gravity and anisotropy mitigation during soil remediation in the vadose zone

Surfactant foam has been proposed as an effective treatment fluid for in situ environmental remediation of soils. In the vadose zone, it could improve treatment homogeneity, but its use remains challenging. To better understand and predict foam formation and propagation in vadose zone, we studied them in 24 soils with wide range of properties (including permeability: 2 10^-12 to 3.3 10^-9 m²). Foam rheology showed to be complex and mostly influenced by soil permeability and grading. Below 2 10^-11 m², foam propagation velocity was not influenced by permeability. Conversely, slight shear thinning to Newtonian behavior was observed for higher permeabilities. Benefits for remediation in anisotropic vadose zones and the injection strategies (mobility control agent or blocking agent) were discussed. Moreover, different methods of foam injection were compared over the range of soil permeability. It showed that "surfactant alternating gas" method was the most suitable for soil permeability lower than 5 10^-10 m² to avoid soil fracturing. Conversely, in higher permeability soils, pre-generated foam was required to get high viscosity foam. Foam and xanthan polymer solution behaviors were compared across the range of permeability studied. They show similarities, and the benefits of one among the other should be evaluated for each specific case.

Assessment of flushing methods for the removal of heavy chlorinated compounds DNAPL in an alluvial aquifer

Immiscible mobilization and foam flushing were assessed as low surfactant consuming technologies, for the enhanced recovery of dense non-aqueous phase liquid (DNAPL) residual at a site contaminated by heavy chlorinated compounds. Preliminary experiments in well-controlled conditions demonstrated the phenomena involved in these remediation technologies and their limitations. Furthermore, we characterized the technologies according to by their surfactant consumption (per kg of DNAPL recovered) and the final DNAPL saturation reached. Surfactant foam flushing (SFF) produced lower DNAPL saturation than immiscible mobilization, thanks to its higher viscosity. However, its efficiency is strongly correlated to the pressure gradient (▽P) used during injection, and that is limited by risks of soil fracturing. The two technologies were tested in field cells (10m×10m×10m) delimited by cement/bentonite walls anchored in the clayey substratum. The deepest soil layer was the most contaminated. It was composed of silt-sandy soil and had an average hydraulic conductivity of 10^-4ms^-1. Field results show that we should now model flushing fluid propagation to design efficient set-ups for recovering the displaced DNAPL.

In situ remediation of tetrachloroethylene and its intermediates in groundwater using an anaerobic/aerobic permeable reactive barrier

Tetrachloroethylene (PCE) is among the most ubiquitous chlorinated compounds found in groundwater contamination. Its chlorinated degradation by-products remain highly toxic. In this study, an anaerobic/aerobic permeable reactive barrier system consisting of four different functional layers was designed to remediate PCE-contaminated groundwater. The first (oxygen capture) layer maintained the dissolved oxygen (DO) concentration at < 1.35 mg/L in influent supplied to the second (anaerobic) layer. The third (oxygen-releasing) layer maintained DO concentration at > 11.3 mg/L within influent supplied to the fourth (aerobic) layer. The results show that 99% of PCE was removed, mostly within the second layer (anaerobic). Furthermore, the toxic by-products trichloroethylene (TCE), dichloroethylene (DCE), and vinyl chloride (VC) were further degraded by 98, 90, and 92%, respectively, in layer 4 (aerobic). Thus, the designed anaerobic/aerobic permeable reactive barrier system could control both PCE and its degradation by-products, showing great potential as an efficient remediation alternative for the in situ treatment of PCE-contaminated groundwater.

Phytoscreening as an efficient tool to delineate chlorinated solvent sources at a chlor-alkali facility

Chlorinated ethenes (CE) are among the most common volatile organic compounds (VOC) that contaminate groundwater, currently representing a major source of pollution worldwide. Phytoscreening has been developed and employed through different applications at numerous sites, where it was generally useful for detection of subsurface chlorinated solvents. We aimed at delineating subsurface CE contamination at a chlor-alkali facility using tree core data that we compared with soil data. For this investigation a total of 170 trees from experimental zones was sampled and analyzed for perchloroethene (PCE) and trichloroethene (TCE) concentrations, measured by solid phase microextraction gas chromatography coupled to mass spectrometry. Within the panel of tree genera sampled, Quercus and Ulmus appeared to be efficient biomonitors of subjacent TCE and PCE contamination, in addition to the well known and widely used Populus and Salix genera. Among the 28 trees located above the dense non-aqueous phase liquid (DNAPL) phase zone, 19 tree cores contained detectable amounts of CE, with concentrations ranging from 3 to 3000 μg L^-1. Our tree core dataset was found to be well related to soil gas sampling results, although the tree coring data were more informative. Our data further emphasized the need for choosing the relevant tree species and sampling periods, as well as taking into consideration the nature of the soil and its heterogeneity. Overall, this low-invasive screening method appeared useful to delineate contaminants at a small-scale site impacted by multiple sources of chlorinated solvents.

Surfactant foam flushing for in situ removal of DNAPLs in shallow soils

An innovative process combining surfactant foam and surfactant flushing (SF) for the remediation of a shallow saturated soil contaminated with a dense non-aqueous phase liquid (DNAPL) residual was investigated at bench-scale. First, foam was formed by injecting DHSS surfactant and nitrogen in alternation in soil (SAG process) at constant pressure. The effect of pressure gradients set point (≤90kPam^-1) on DNAPL recovery was investigated. Foam front mobilized DNAPL with a piston-like fast propagation (1.8md^-1) despite the low pressure gradients. 34-60% of DNAPL residual was extracted with low surfactant consumption (<0.4kgkg^-1 DNAPL removed). Then, a solubilizing agent (Tergitol) was injected into soil, previously treated with foam, to improve the DNAPL removal. It led a removal up to 95% whereas only 2 porous volume (PV) of solubilizing solution were injected. As a comparison, when the solubilizing agent was injected without foam pre-treatment, 40 PV would have been required to lead the removal up to 90%. Considering the overall treatment, the technology we developed requires 10-times less surfactant than the traditional SF technology. This technology is especially suitable for very shallow soils where injection pressure has to be kept at a low level to avoid soil heaving.

Static Adsorption of an Ethoxylated Nonionic Surfactant on Carbonate Minerals

The static adsorption of C_12-14E₂₂, which is a highly ethoxylated nonionic surfactant, was studied on different minerals using high-performance liquid chromatography (HPLC) combined with an evaporative light scattering detector (ELSD). Of particular interest is the surfactant adsorption in the presence of CO₂ because it can be used for foam flooding in enhanced oil recovery applications. The effects of the mineral type, impurities, salinity, and temperature were investigated. The adsorption of C_12-14E₂₂ on pure calcite was as low as 0.01 mg/m² but higher on dolomite depending on the silica and clay content in the mineral. The adsorption remained unchanged when the experiments were performed using a brine solution or 0.101 MPa (1 atm) CO₂, which indicates that electrostatic force is not the governing factor that drives the adsorption. The adsorption of C_12-14E₂₂ on silica may be due to hydrogen bonding between the oxygen in the ethoxy groups of the surfactant and the hydroxyl groups on the mineral surface. Additionally, thermal decomposition of the surfactant was severe at 80 °C but can be inhibited by operating in a reducing environment. Under reducing conditions, adsorption of C_12-14E₂₂ increased at higher temperatures.