Surfactant-enhanced Remediation Research Articles

Enhancing hexachlorocyclohexane solubility with surfactants and ionic liquids

Surfactant-enhanced remediation of soil can increase the solubility of hydrophobic organic contaminants in water and improve their removal. In this study, the effect of 24 traditional surfactants and surface-active ionic liquids (SAILs) on the solubility of hexachlorocyclohexane isomers (α-, β-, γ-, δ-HCH) was evaluated in batch-type experiments. The combination of two different proportions of a cationic SAIL and an anionic surfactant was also assessed. The solubilisation capacity of the surfactant varied depending on its concentration and structure, but also the HCH isomer considered. In general, the anionic surfactants yielded the best results, increasing solubilisation of HCH up to 2.4 times compared to the control (pure water). α-HCH and γ-HCH isomers were more easily solubilised than β-HCH and δ-HCH regardless of the surfactant’s type. The main advantage of ionic liquids being their tunability, some clues are given for formulating surfactants to enhance the solubilisation of HCH. The combination of a traditional surfactant and a SAIL influenced the critical micelle concentration, but not the solubilisation of the contaminant. Further studies on the use of blends for this application are required. The increase of solubility achieved with the studied surfactants encourages their application in washing HCH-contaminated soils through solubility enhancement.

Effect of cations on monochlorobenzene adsorption onto bentonite at the coexistence of Tween 80

The effect of several prevalent cations (including Na+, K+, Mg2+, Ca2+, Al3+, and Fe3+) on the adsorption of monochlorobenzene (MCB) onto bentonite was investigated at the coexistence of nonionic surfactant Tween 80 (T80) in surfactant-enhanced remediation (SER). They are all favorable for MCB and T80 adsorption, especially Mg2+ and Ca2+. Adsorption of MCB is strongly depended on T80 micelles. When its concentration exceeds the solubility, MCB is easier to bind with T80 micelles and be adsorbed by bentonite. Acidic environment can facilitate MCB and T80 adsorption, but the effect of cations on the adsorption is most significant under alkaline conditions. Adsorption capacity of MCB increases first followed by a slight decrease with increasing cations concentrations. The maximum adsorption rate of MCB determined is about 68.4% in a solution containing Mg2+ in the isothermal adsorption of MCB, while it is only 6.8% in a cation-free solution. Various characterizations showed that cations mainly changed the repulsion between bentonite particles and T80 micelles and the agglomeration and structure of bentonite, thus affecting the adsorption of MCB and T80 micelles. Our research demonstrated the nonnegligible promotion of MCB adsorption on bentonite by cations and acidic environment, which will adversely affect SER efficiency.

Effect of Solute on Interfacial Properties and Micelle Structure of Dodecylbenzenesulfonate (DBS): Experimental and Molecular Dynamics Studies.

A combined experimental and molecular dynamic simulation approach was used to examine the structure and interfacial properties of solute-saturated micelles. The properties of dodecylbenzenesulfonate (DBS) micelles were examined in dodecane and benzene hydrocarbon systems. Pyrene fluorescence was used to determine the aggregation number of surfactant monomers in the micelle systems. Molecular dynamic (MD) simulations using energy minimization applying the CHARMm force field with the TIP3P model for water. Comparison of the DBS/benzene and DBS/Dodecane micelles equilibrium structures via radial distribution function (RDF) and probability distribution function (PDF) analysis indicates that the area per head group for the DBS/Benzene micelle interface is significantly larger than that of the DBS/Dodecane at the interface. It was also determined that benzene molecules can move freely within the micelle while dodecane is strictly confined in the core of the micelle. The increased interfacial area per monomer caused by the insertion of benzene also reduces the effectiveness of the surfactant, which has implications for use in various environmental applications. However, the DBS/benzene micelle can solubilize many more hydrocarbon molecules in one micelle with less surfactant monomer (i.e., lower aggregation number) per micelle due to the increased available packing positions within the micelle. This, in turn, increases the efficiency of the surfactant in real-world applications which is consistent with previous laboratory results. Understanding the differing solubilization characteristics of surfactants against various classes of hydrocarbons in single solute systems is a necessary step to beginning to understand their solubilization properties in the mixed waste systems prevalent in most surfactant enhanced remediation (SEAR) strategies.

Solubilization and remediation of polycyclic aromatic hydrocarbons in groundwater by cationic surfactants coupled nanobubbles: Synergistic mechanism and application

Polycyclic aromatic hydrocarbons pose a severe threat to groundwater and the soil environment. Improving the efficiency of the surfactant-enhanced remediation (SER) technique is a research hotspot. Nanobubbles (NBs) were introduced to enhance the solubilization effect of cationic surfactant dodecyltrimethylammonium bromide (DTAB) on phenanthrene (PHE). The interaction between NBs and DTAB was favorable in reducing the critical micelle concentration of DTAB. The equilibrium solubilization of DTAB to PHE was upgraded by up to 30.4% with the synergy of NBs. The effect of DTAB concentration on the stability of NBs was an essential reason for the different upgrade rates. The kinetic study showed that the process of equilibrium solubilization was accelerated, while NBs were “consumed”. Based on the classic DLVO framework, OH– and inorganic ions in groundwater can participate in the formation of micelles and change the double electric layer thickness of bubbles, which are the key factors affecting the stability of NBs. The results revealed the colloidal interface characteristics and the particle behavior of micelles with NBs, providing new insight into the combined application of micelles or vesicles with NBs.

Cyclic solubilization and release of polycyclic aromatic hydrocarbons (PAHs) using gemini photosensitive surfactant combined with micro-nano bubbles: a promising enhancement technology for groundwater remediation

Polycyclic aromatic hydrocarbons (PAHs) have posed serious risks to the groundwater environment. Surfactant-enhanced remediation (SER) is a promising strategy, as it can increase the solubilization efficiency of PAHs contaminants. However, this technology is still hampered due to low efficiency and high cost for the separation of pollutants after solubilization. Through the combination of micro-nano bubbles (MNBs) with a gemini photosensitive surfactant (N1, N2-bis[4-[4-[(4-butylphenyl) azo] phenoxy] butyl]-N1, N2-tetramethylethane-1,2-diammonium bromide, AzoPBT), herein we present a significantly enhanced efficiency of phenanthrene (Phe) solubilization and surfactant cycling. Notably, the presence of MNBs was conducive to the dissolution and release of Phe by reducing the CMC and γ of trans-AzoPBT (from 0.52 mmol/L and 28.94 mN·m−1 to 0.39 mmol/L and 27.88 mN·m−1) and increasing the difference of interfacial properties between trans and cis-AzoPBT, which was owing to the electrostatic interaction. Groundwater chemical parameters, i.e., ions, pH and organic matter would affect the solubilization ability of trans-AzoPBT. Mg2+ and Ca2+ inhibited the solubilization while SO42− promoted the solubilization, depending on their radius, electronic quantity and solution pH. A change in the HA level up to 8 mg/L had a negatively effect on the solubilization ability. High release rate (69.39 %) of cis-AzoPBT for Phe and high recovery rate (89.37 %) for tans-AzoPBT were obtained in the AzoPBT/MNBs mixed aqueous solution. Compared to MNBs combined with conventional cationic surfactants, AzoPBT/MNBs mixed aqueous solution exhibited better synergistic solubilization performance, and the apparent solubility of Phe increased from 26.74 % to 31.61 %. Overall, this study demonstrates the enhanced solubilization of PAHs by photosensitive surfactants combined with MNBs, which provides valuable insights of the effective utilization of surfactants in the remediation of PAHs-contaminated groundwater with SER technology.

Comparison of the efficiency and microbial mechanisms of chemical- and bio-surfactants in remediation of petroleum hydrocarbon

Surfactant-enhanced remediation (SER) is one of the most effective methods for petroleum hydrocarbon-contaminated sites compared to single physical and chemical methods. However, biosurfactants are not as commonly used as chemical surfactants, and the actual remediation effects and related mechanisms remain undefined. Therefore, to comprehensively compare the remediation effects and biological mechanisms of biosurfactants and chemical surfactants, soil column leaching experiments including two biosurfactants (rhamnolipids and lipopeptide) and three commercially used chemical surfactants (Tween 80, Triton X-100, and Berol 226SA) were conducted. After seven days of leaching, rhamnolipids exhibited the highest petroleum hydrocarbon removal rate of 61.01%, which was superior to that of chemical surfactants (11.73–18.75%) in n-alkanes C10–C30. Meanwhile, rhamnolipids exhibited a great degradation advantage of n-alkanes C13–C28, which was 1.22–30.55 times that of chemical surfactants. Compared to chemical surfactants, biosurfactants significantly upregulated the soil's biological functions, including soil conductivity (80.90–155.56%), and soil enzyme activities of lipase (90.31–497.10%), dehydrogenase (325.00–655.56%), core enzyme activities of petroleum hydrocarbon degradation, and quorum sensing between species. Biosurfactants significantly changed the composition of Pseudomonas, Citrobacter, Acidobacteriota, and Enterobacter at the genus level. Meanwhile, chemical surfactants had less influence on the bacterial community and interactions between species. Moreover, the biosurfactants enhanced the microbial interactions and centrality of petroleum hydrocarbon degraders in the community based on the network. Overall, this work provides a systematic comparison and understanding of the chemical- and bio-surfactants used in bioremediation. In the future, we intend to apply biosurfactants to practical petroleum hydrocarbon-contaminated fields to observe realistic remediation effects and compare their functional mechanisms.

Dissolution and remobilization of NAPL in surfactant-enhanced aquifer remediation from microscopic scale simulations

In this paper, the dissolution and mobilization of non-aqueous phase liquid (NAPL) blobs in the Surfactant-Enhanced Aquifer Remediation (SEAR) process were upscaled using dynamic pore network modeling (PNM) of three-dimensional and unstructured networks. We considered corner flow and micro-flow mechanisms including snap-off and piston-like movement for two-phase flow. Moreover, NAPL entrapment and remobilization were evaluated using force analysis to develop the capillary desaturation curve (CDC) and predict the onset of remobilization. The corner diffusion mechanism was also applied in the modeling of interphase mass transfer to represent NAPL dissolution as the dominant mass transfer process. In addition, the effect of pore-scale heterogeneity on mass transfer rate coefficient and recovered residual NAPL was considered in the simulations. Sodium dodecyl sulfate (SDS) and Triton X-100 were used as the surfactant for the SEAR process. The results indicate that although surfactants enhance NAPL recovery during two-phase flow, surfactant-enhanced remediation of residual NAPL through dissolution is highly dependent on surfactant type. When SDS ─as a surfactant with high critical micelle concentration (CMC) and low micelle partition coefficient (Km)─ was injected into a NAPL contaminated site, the mass transfer rate coefficient decreased (due to considerable changes in interface chemical potentials) which leads to a significant reduction in NAPL recovery after the end of two-phase flow. In contrast, Triton X-100 (with low CMC and high Km) improved NAPL recovery, by enhancing solubility at surfactant concentrations greater than CMC which overcompensates the interphase mass transfer reduction.

Surfactant-enhanced treatment of oil-contaminated Arctic tundra soil: Ecotoxicological assessment

During 2018–2019 summer expeditions to the Yamal peninsula, we collected tundra gley soil samples at the oil-contaminated sites. The contamination level was determined by fluorimetry and was ranged from 30 to 107 g/kg. The soil quality was evaluated by its enzymatic activity; the low catalase activity of the soils confirmed the presumed slow soil natural attenuation and the need for land remediation. The oil-contained and background samples were ex-situ treated with the aqueous solutions of non-ionic BOK-6 and anionic AddiMax PV01 synthetic surfactants. The treatment efficiency reached 71 ± 2% and 97 ± 3% in the experimental design with BOK-6 and AddiMax PV01, respectively. We derived the ecotoxicity endpoints for the surfactant solutions, using infusoria Paramecium caudatum Ehrenberg and higher plants Raphanus sativus L. and Avena sativa L. The protozoan mortality was studied and resulted in LC 50 − 24 values << 0.1 mg/L and 339 mg/L for BOK-6 and AddiMax PV01, respectively. Both surfactant solutions and surfactant residues in soil showed the inhibition potential towards higher plants. BOK-6, with a strong relation to its chemical and ecotoxic profile, completely inhibited seed germination in the treated soil. The less ecotoxic surfactant AddiMax PV01 inhibited plant growth by 38% compared to control. The treatment increased ecotoxicity and dehydrogenase activity and further lowered the catalase activity of the background soil, and, therefore, the surfactant residues migrating to the non-contaminated territories should be controlled. By this study, we show the efficiency of the surfactant-enhanced remediation of arctic soils, but the associated environmental risks must be addressed. • Ex-situ treatment of oil-contaminated tundra gley soil reached 69%–99% efficiency. • Enzymes activity indicated slow natural attenuation potential of arctic soil. • Residues of toxic surfactant could cause greater ecotoxic effect than contamination. • Surfactants residues inhibited biological activity of non-contaminated arctic soil.

Synergistic Solubilization of Phenanthrene by Mixed Micelles Composed of Biosurfactants and a Conventional Non-Ionic Surfactant

This study investigated the solubilization capabilities of rhamnolipids biosurfactant and synthetic surfactant mixtures for the application of a mixed surfactant in surfactant-enhanced remediation. The mass ratios between Triton X-100 and rhamnolipids were set at 1:0, 9:1, 3:1, 1:1, 1:3, and 0:1. The ideal critical micelle concentration values of the Triton X-100/rhamnolipids mixture system were higher than that of the theoretical predicted value suggesting the existence of interactions between the two surfactants. Solubilization capabilities were quantified in term of weight solubilization ratio and micellar-water partition coefficient. The highest value of the weight solubilization ratio was detected in the treatment where only Triton X-100 was used. This ratio decreased with the increase in the mass of rhamnolipids in the mixed surfactant systems. The parameters of the interaction between surfactants and the micellar mole fraction in the mixed system have been determined. The factors that influence phenanthrene solubilization, such as pH, ionic strength, and acetic acid concentration have been discussed in the paper. The aqueous solubility of phenanthrene increased linearly with the total surfactant concentration in all treatments. The mixed rhamnolipids and synthetic surfactants showed synergistic behavior and enhanced the solubilization capabilities of the mixture, which would extend the rhamnolipids application.

Electrochemically reversible foam enhanced flushing for PAHs-contaminated soil: Stability of surfactant foam, effects of soil factors, and surfactant reversible recovery

Although surfactant foams enhanced-remediation for PAHs-contaminated soil has been proved to be an effective method, lack of simple/economic surfactant recovery methods from the eluent solutions limit its further remediation application for organic contaminated soil. Here, we prepared a electrochemically reversible ferrocene surfactant FcCH2N+(CH3)C12H25 (Fc12), then investigated the foaming ability and foam stability of Fc12 under its reduced (active state) and oxidation (inactive state) states and explored the flushing efficiency of reduced Fc12 foam for PAHs-contaminated soil and the recovery efficiency of collected eluent solution. The results showed that the foaming ability and foam stability of reduced Fc12 are greatly higher than those of oxidized Fc12, which is indicative of a well reversibly switchable characteristic of Fc12. The contaminated soil flushing efficiencies of reduced Fc12 for phenanthrene and pyrene were 65.28% and 46.45%. The respective desorption efficiency of phenanthrene and pyrene from collected eluent solutions were calculated to be 74.94% and 72.75% by electrochemical oxidation control, which indicates that Fc12 can be well recovered by simple electrochemical control. This study provides a feasible method for the recovery of surfactants from surfactant-enhanced remediation processes by simply electrochemical control.

Sorption-desorption mechanisms and environmental friendliness of different surfactants in enhancing remediation of soil contaminated with polycyclic aromatic hydrocarbons

The ionic and nonionic surfactants have different adsorption-desorption models for polycyclic aromatic hydrocarbons (PAHs) in soil-water system, due to the difference in the composition and charge of the hydrophilic groups. Surfactant eluents retained in the soil may also have a secondary effect on the soil environment. Thus, the aim of the study was to investigate sorption-desorption mechanisms and environmental toxicity of different surfactants in enhancing remediation of soil contaminated with PAHs. The distribution of PAHs between different surfactants and soil is influenced by the surfactant bilayer formation. The average molecular density, Sips isotherm, critical micelle concentration (CMC), and critical washing concentration (CWC) models were used to explore the mechanisms involved in the adsorption-desorption of cationic (cetyltrialkylammonium bromide, CTAB), anionic (sodium dodecyl benzene sulfonate, SDBS), and nonionic (TritonX-100, TX100) surfactants. These models were associated with the surfactant-enhanced remediation (SER) of naphthalene-contaminated soil. The mean concentrations and activity of indigenous bacteria were used to detect the toxic effect of the above surfactants on soil environment. The results of SER experiments showed that there are critical washing points (SDBS = 2.4 CMC, TX100 = 4.6 CMC, CTAB = 3.3 CMC) for different surfactants washing naphthalene (Nap). The values of CWC corresponding to critical washing points were key variables driving the need for added surfactants to remove or immobilize Nap. The CWC of different surfactants could be conveniently predicted by a model based on the surfactant-derived organic carbon. In addition, the mean concentration of viable bacteria in biological culture experiment was highest for TX100, followed by CTAB and SDBS. This study revealed the effects of average molecular density and surfactant-derived organic carbon on the distribution of Nap, and suggested a model for calculating CWC of different surfactants, to optimize SER technology. Biological culture experiment indicated that the high concentration and ionic surfactants had a significantly toxic effect on indigenous bacteria.

Desorption process and morphological analysis of real polycyclic aromatic hydrocarbons contaminated soil by the heterogemini surfactant and its mixed systems

Surfactant-enhanced remediation (SER) is an efficient and low-cost technology for polycyclic aromatic hydrocarbons (PAHs) contaminated sites. This study assessed the desorption processes and effects of Heterogemini surfactant (Dodecyldimethylammonium bromide/tetradecyldimethylammonium bromide, DBTB), two traditional surfactants (Hexadecyl trimethyl ammonium bromide, CTAB; Sorbitan monolaurate, Span 20) and their mixed systems on the real PAHs-contaminated soil from an abandoned coking plant, as well they were analyzed micro morphologically. DBTB had greater desorption capability for PAHs and favorable interaction with the traditional surfactants confirmed by reaction parameters βm and Gibbs. Whether for total PAHs (TPAHs) or different molecular weight PAHs, the mixed system Span 20/DBTB had larger molar solubilization ratio (MSR) and partition coefficient (Km) than CTAB/DBTB, the highest desorption rate for TPAHs reaching 68.83%. Additionally, microscopic morphology showed micelles of Span 20/DBTB were more dispersed and formed strings easily, explaining its good desorption capability. What resulted demonstrated the feasibility of DBTB, a novel Heterogemini surfactant, and its mixed systems remediating PAHs-contaminated soil of abandoned industrial site.

Selective removal of phenanthrene from SDBS or TX100 solution by sorption of resin SP850

Surfactant enhanced remediation (SER) is a promising technology for the removal of hydrophobic organic compounds (HOCs) from contaminated soils. Recycling of soil washing effluent by selective removing of HOCs from surfactant solutions, using techniques such as sorption, is a keyway to reduce the operation costs of SER. In this study, a potential and economical method to recycle washing effluent and lower operation costs of SER, i.e., selective sorption of phenanthrene from an anionic surfactant (sodium dodecylbenzene sulfonate, SDBS) or a nonionic surfactant (Triton X-100, TX100) solution by a resin, SP850, was developed. A modified selectivity parameter (S*), having a parabolic equation with a maximum S* value, was developed to evaluate the efficiency of the selective sorption process, the optimal surfactant concentration and SP850 dose for SER. For example, at the given SP850 dose of 1.0 g/L, the optimal concentrations of SDBS and TX100 for SER are about 15000 mg/L and 8000 mg/L, respectively. Moreover, the optimal added SP850 dose for recycling washing effluents is 1.5 g/L, which is independent of surfactant concentration. At the relatively high concentrations of surfactant used in SER, the selective sorption is depended on the sorption of phenanthrene by SP850 and the solubility enhancement of phenanthrene into surfactant micelles in solutions. The observed recycling soil washing effluent method by selective sorption using resins (e.g., SP850) as the adsorbents would be helpful for the application of SER in remediating contaminated soils.

Co-solubilization of polycyclic aromatic hydrocarbon mixtures in aqueous micellar systems and its correlation with FRET for enhanced remediation processes.

Surfactant enhanced remediation (SER) is an effective approach for decontaminating the PAH polluted soils. Solubilization and Cosolubilization of Phenanthrene (Ph), Pyrene (Py) and Perylene (Pe) as single, binary and ternary mixtures have been studied employing cationic (CTAB), anionic (SDS), non-ionic surfactant (Brij 30) and block copolymer (P123) micelles. In the single solute solubilization studies, solubility of Pe follows the order Brij 30 > CTAB > SDS whereas Ph or Py followed the order of CTAB > Brij 30 > SDS. In the cosolubilization studies, an increase, decrease or no change in the mutual solubility of PAHs was observed. Synergism in solubilization was observed most in P123 in both binary and ternary PAH mixture where more PAHs could get solubilized in the dense micellar shell region, thereby enhancing the micellar core volume leading to enhanced solubilization of PAHs. The solubilizates as pairs (Ph-Pe and Py-Pe) were further tested for any possible energy transfer in presence of surfactant based restricted host environments using spectrofluorometry and spectrophotometry. Based on the solubilization and cosolubilization an efficient non-radiative energy transfer (FRET) was observed between Ph/Py (donor) and Pe (acceptor) in the non-ionic surfactant system as well as in CTAB-Brij 58 mixed system. The results of this work may improve the effective utilization of surfactants in their correct evaluation for the removal of PAHs from contaminated soils or aquifers treated with SER technology.

Biodegradation of anionic surfactants in the bioremediation of oil-polluted soil

Surfactants are applied as emulsifiers or solubilizers by treatment of polluted soil. Theproblem of secondary pollution has arisen as result of the surfactant-enhanced remediation ofa polluted soil contaminated with hydrophobic organic compounds. Several studies haveshown that the surfactants are biodegradable in aerobic conditions and the biodegradabilitydepended on the chemical properties and concentration of surfactant. A study of the leachingof surfactants from the soil is important, as it is difficult to identify the reason for thereduction of concentration of pollutants in the soil: is it degradation or leaching? Theexperiments were carried out with a fine sandy soil in column tests and CaCO3 was added toincrease soil pH. The soil was treated twice with the bioremediation agent SR-100. The soilpH, the concentrations of anionic surfactant and petroleum hydrocarbons at the differentdepths of soil were determined. The microbial activity of soil fractions was evaluated byrespirometer. The concentration of surfactants was determined colorimetrically as MethyleneBlue active substances (MBAS). The concentration of anionic surfactants decreased in theupper layer of the columns, but it increased in the lower layers. It indicated the leaching ofthe anionic surfactants from soil during experiments of 60 days. The amounts of residualsurfactants were lower in the samples of polluted soil in comparison with unpolluted soil. Thesamples of lower soil fractions had higher microbial activity in comparison with upperfractions. Soil pH was measured as pHH20, pHKcl and pHcac12 instead of the pH of soilsolution, because soil was too dry. The pH of fine sandy soil was 5.8 and during theexperiment the value of pH increased in the lower layer of soil till pHH20 = 7.5.

Organo-layered double hydroxides for the removal of polycyclic aromatic hydrocarbons from soil washing effluents containing high concentrations of surfactants.

Disposal of soil washing effluent (SWE) resulting from the surfactant-enhanced remediation of soil containing hydrophobic organic contaminants (HOCs)is complicated because of the presence of high levels of surfactants. The synthesized layered double hydroxides (LDHs), modified with sodium dodecyl sulfonate (SDS) in different loading amounts (organo-LDHs),were evaluated in this study as sorbents for the removal of two typical HOCs, phenanthrene (PHE) and pyrene (PYR),from a simulative SWE. The results showed that the organo-LDHs can effectively sorb PHE and PYR from the SWE within an equilibrium time of 2 h. All isotherms were linear and the sorption capabilities of the organo-LDHs increased almost linearly with the increase in the amount of SDS loaded on the LDHs. Besides, the surface areas of the organo-LDHs decreased sharply with the increase in SDS loading owing to the hindrance of the exposed surface of the LDHs by the incorporated SDS. These findings indicated that partitioning dominated the sorption process rather than adsorption, and the strong affinity of HOCs towards the organic phase in LDHs assisted in the effective removal of polycyclic aromatic hydrocarbons (PAHs) from the SWE. Furthermore, the sorption capabilities of organo-LDHs towards PHE and PYR at the higher loading amounts of SDS were much greater than that of commercial activated carbon at the higher concentration ranges of PAHs.

Evaluation of ethoxylated nonionic surfactants for solubilization of chlorinated organic phases: Effects of partitioning loss and macroemulsion formation

Selection of proper surfactants is critical for applying surfactant-enhanced remediation (SER) to sites contaminated with nonaqueous phase liquids (NAPLs). Here, ethoxylated nonionic surfactants (Tween 20, Tween 40, Tween 80, and Triton X-100) were evaluated for their applicability to remedy chlorinated organic phases, chloroform (CF), trichloroethylene (TCE), and tetrachloroethylene (PCE), on the basis of solubilization capacity, partitioning behavior, and macroemulsion formation. The most hydrophilic CF was not relevant for SER applications since excessive surfactant partitioning into CF rendered only few of them available for its solubilization. In contrast, the more hydrophobic TCE and PCE, having moderate surfactant partitioning, were effectively solubilized. Among Tween surfactants, a more hydrophobic surfactant showed a larger solubilization potential for both chloroethylenes, but it suffered from a greater partitioning loss. Depending on the type and extent of NAPL contaminations, thus, a prior consideration should be given to either solubilization capacity or partitioning loss when selecting the optimal Tween surfactant. Compared to Tween surfactants, the more hydrophobic Triton X-100 showed greater partitioning losses into all three NAPLs. Of particular, its partitioning into CF and TCE was nearly complete, making impractical its application to the remediation of both organic liquids. The formation of macroemulsions, characterized by a high turbidity, may significantly deteriorate SER applicability by producing undesirable flows in aquifers. Their formation became more problematic with the increasing surfactant hydrophilicity and the increasing NAPL hydrophobicity. When these combinations are applied, it is critical to keep such surfactant concentrations as to exploit the solubilization potential but not to cause the macroemulsion formation.

A pore-scale investigation of heavy crude oil trapping and removal during surfactant-enhanced remediation

The presence of nonaqueous phase liquid (NAPL) in the subsurface presents significant challenges for soil and groundwater remediation. In particular, heavy crude oil, coal tar and/or bitumen present unique difficulties for removal and cleanup due to associated high viscosities, low aqueous solubilities, and limited mobility extraction potential. Although surfactant-enhanced aquifer remediation (SEAR) techniques have shown some promise for source removal, overall remediation (mobilization) performance will depend significantly on interfacial effects between the fluid and solid phases. A pore-scale study, implementing synchrotron X-ray microtomography (SXM), was conducted to understand and quantify the trapping and mobilization mechanisms and in-situ emulsification processes of heavy crude oil distributed within increasing complexity (i.e. physical heterogeneity) unconsolidated sands during surfactant flushing events. Pore-scale imaging analyses were conducted to quantify the changes in oil blob morphology before and after surfactant flushing events to assess the primary factors controlling the recovery. Results showed relatively low (10%) net recovery from the homogeneous sand after 5 pore volumes (PVs) of surfactant flushing and may be, in part, due to the more connected ganglia (i.e. single continuous) oil-phase. Such a condition may have limited the surfactant/oil contact resulting in relatively low interfacial activity and correspondingly inefficient oil mobilization and recovery. Negligible net oil recovery was achieved from the mildly-heterogeneous-sand and is likely due to the lower associated permeability of this particular porous medium. Furthermore, the oil-phase distribution within this medium primarily consisted of small disconnected blobs more readily exposed (in contact with) the surfactant solution. For the highly-heterogeneous-sand experiments, an average of 20% heavy-oil recovery resulted after each flushing event (total of ~37% after 5 PVs) and was attributed to more efficient reduction of interfacial tension associated with the increased surfactant-oil contact. The associated higher pH sand/fine‑carbonate system may have aided in maintaining a water-wet porous medium, a condition more conducive to higher oil recovery and displacement efficiency.

Reversible Solubilization of Pyrene by a Gas Switchable Surfactant Investigated by Molecular Dynamics Simulation.

The reversible solubilization behavior of pyrene by a CO2/N2 switchable surfactant (named N'-dodecyl- N, N-dimethylacetamidinium bicarbonate (DDAB)) was investigated with molecular dynamics (MD) simulations. We first individually simulated the aggregation of the inactive surfactant N'-dodecyl- N, N-dimethylacetamidines (DDA) and effective surfactant DDAB in water. Detailed structural properties analysis showed that DDAB molecules aggregated into a micelle, while the aggregation of DDA molecules was considered to be an oil droplet that was separated from the water phase. MD simulations revealed that pyrene molecule was solubilized in the interior hydrophobic region of the micelle as expected. Pyrene was adsorbed on the surface of the oil droplet which is due to the dense packing of DDA molecules inside the oil droplet. The simulated release process showed that the solubilized pyrene in the interior was squeezed out when the micelle was changed to an oil droplet. Reduced density gradient (RDG) function was used to study the weak interactions and explore the molecular driving force behind the reversible solubilization. The results demonstrated that repulsion effects of water molecules on the DDA headgroups play an important role on the pyrene release. Because of the persistent molecular motion of DDA molecules into the droplet center, pyrene was finally repelled out of the oil droplet. Our study provided a molecular mechanism into the reversible solubilization of a gas-controlled switchable surfactant. This is expected to be useful for surfactant-enhanced remediation (SER) experiments.

Solubilization of Nitrogen Heterocyclic Compounds Using Different Surfactants

In order to develop surfactant-enhanced remediation for nitrogen heterocyclic compounds (NHCs) (aniline, indole, and quinolone), the solubilization properties of micellar solutions of five surfactants, namely sodium dodecyl sulfate (SDS), rhamnolipid (RL), polysorbate (Tween 80), sorbitan monolaurate (Span 20), and iso-octyl phenoxy polyethoxy ethanol (TX-100) were investigated in this work. The solubilization capacities were quantified using critical micelle concentration (CMC) as well as thermodynamic and kinetic experiments. Besides, nuclear magnetic resonance (1H NMR) spectra were used to infer the locus of NHCs solubilized by SDS and TX-100. The results from the properties of five surfactants indicated that CMC was affected by temperature, while the micellization was spontaneous and could be both endothermic and exothermic based on the type of surfactant and temperature. Furthermore, the difference in compensation temperature was caused by different solubilization mechanism for various surfactants. The solubilization results showed that the solubilization of NHCs in the surfactant solutions followed a pseudo-first-order kinetic model. Meanwhile, the change in proton’s chemical shift depended on the structure of NHCs and the solubilization ability of surfactants. Finally, the orthogonal experiment (L16(43)) was elementarily designed to optimize the solubilization conditions of indole and the results showed that RL could be a better choice for solubilizing NHCs.