Molecular simulations of NAPL removal from mineral surfaces using microemulsions and surfactants

Larger aggregate formed by self-assembly process of the mixture surfactants enhance the dissolution and oxidative removal of non-aqueous phase liquid contaminants in aquifer

Surfactants can transfer non-aqueous phase liquid (NAPL) contaminants to the aqueous phase, and enhance the removal of the latter in groundwater. However, the extensive use of surfactants causes secondary contamination and increases the non-target consumption of oxidants. It is pressing to develop a surfactant with high phase transfer efficiency and sound compatibility with oxidants to minimize the use of surfactants for groundwater remediation. The phase transfer capability of different surfactants and their binary mixtures, their enhanced KMnO4 oxidation performance for NAPL contaminants as well as influencing factors were investigated to solve the above-mentioned question. The results showed that Tween20, SDBS and BS-12 perform best in terms of phase transfer capability among nonionic, anionic and amphoteric surfactants respectively, and only SDBS and BS-12 produce a synergistic effect among the binary mixtures. The CMC of SDBS/BS-12 was lower than its ideal CMC value, and the self-assembly process of SDBS/BS-12 also formed larger aggregates, which improved the phase transfer performance. Compared to other single surfactants, the removal efficiency of petroleum hydrocarbons in the aquifer sediments was raised by 7.4-33.8 % using the mixed surfactant. The SDBS/BS-12 mixture was compatible with KMnO4 and boosted the reaction of NAPL contaminants with KMnO4 by transferring from the NAPL phase to the aqueous phase. As a result, the NAPL toluene and phenanthrene removal efficiency increased from 37 % and 29 % to 80 % and 86 % respectively. Natural organic matters inhibited the phase transfer efficiency of the SDBS/BS-12 mixture, whereas anions and monovalent cations enhanced the phase transfer capability of the mixture. High-valent cations led to precipitation in the SDBS/BS-12, which could be eliminated by adding Na2Si2O5. The SDBS/BS-12 mixture delivered the same phase transfer efficiency with the dosage of 1.73-23.07 % of other single surfactants, and its cost was equivalent to 0.25-41.7 % of the latter, thus embracing bright application prospects.

Field demonstration of in-situ microemulsion flushing for enhanced remediation of multiple chlorinated solvents contaminated aquifer

The remediation of in-situ microemulsion flushing for multiple chlorinated solvents contaminated groundwater is challenging, because different chlorinated solvent has major influence on microemulsion formation and solubilization behaviors. This work was conducted to evaluate the remediation effectiveness for various chlorinated solvents contaminated site and monitor the disturbance of groundwater during in-situ microemulsion flushing process. Groundwater at this site was contaminated with chlorobenzene (MCB), chloroaniline and nitrochlorobenzene. The medium layer was mainly composed of fine and silty sand, with average hydraulic conductivity of 4.97 m/d. Results of this field-scale test indicated in-situ microemulsion flushing successfully enhanced the apparent solubility of various chlorinated solvents. Post-flushing concentration of various chlorinated solvents were 1.33-71.6-fold the concentration of pre-flushing values at 10 sampling locations within the test zone. This field was flushed with 16.8 m3 microemulsion, removing approximately 18.49 kg chlorinated solvents. Besides, a trend in the desorption order of various chlorinated solvents was observed. The least hydrophobic pollutant was flushed first, followed by contaminants of increasing hydrophobicity. In addition, during remediation process, the indexes of groundwater fluctuated insignificantly, indicating the reagent had little disturbance to aquifer. This field work demonstrated the feasibility of in-situ microemulsion enhanced remediation via increasing apparent solubility of multiple chlorinated solvents.

Molecular Dynamics Simulation of Polyacrylamide Adsorption on Calcite

In poorly consolidated carbonate rock reservoirs, solids production risk, which can lead to increased environmental waste, can be mitigated by injecting formation-strengthening chemicals. Classical atomistic molecular dynamics (MD) simulation is employed to model the interaction of polyacrylamide-based polymer additives with a calcite structure, which is the main component of carbonate formations. Amongst the possible calcite crystal planes employed as surrogates of reservoir rocks, the (1 0 4) plane is shown to be the most suitable surrogate for assessing the interactions with chemicals due to its stability and more realistic representation of carbonate structure. The molecular conformation and binding energies of pure polyacrylamide (PAM), hydrolysed polyacrylamide in neutral form (HPAM), hydrolysed polyacrylamide with 33% charge density (HPAM 33%) and sulfonated polyacrylamide with 33% charge density (SPAM 33%) are assessed to determine the adsorption characteristics onto calcite surfaces. An adsorption-free energy analysis, using an enhanced umbrella sampling method, is applied to evaluate the chemical adsorption performance. The interaction energy analysis shows that the polyacrylamide-based polymers display favourable interactions with the calcite structure. This is attributed to the electrostatic attraction between the amide and carboxyl functional groups with the calcite. Simulations confirm that HPAM33% has a lower free energy than other polymers, presumably due to the presence of the acrylate monomer in ionised form. The superior chemical adsorption performance of HPAM33% agrees with Atomic Force Microscopy experiments reported herein.

Influencing of hydrogeochemical conditions and engineering parameters on phase behaviors and remediation performance of in-situ microemulsion for residual PCE in aquifers

In-situ microemulsion has great potential for remediation of chlorinated solvent contaminated aquifers due to its efficient solubilization, and the in-situ formation and phase behaviors of microemulsion is a key factor in determining the remediation performance. However, the role of aquifer properties and engineering parameters on microemulsion in-situ formation and phase transition has been rarely attended. In this work, the influences of hydrogeochemical conditions on in-situ microemulsion phase transition and solubilization ability for tetrachloroethylene (PCE) were explored, and the formation condition, phase transition and removal efficiency for in-situ microemulsion flushing under various flushing conditions were investigated. The results indicated that the cations (Na⁺, K⁺, Ca²⁺) all facilitated the microemulsion phase altering from Winsor I → III → II, whereas the anions (Cl^-, SO₄^2-, CO₃^2-) and pH variation (5-9) had no profound influence on phase transition. Besides, the solubilization capacity of microemulsion was enhanced by the pH variation and the cations, which was proportional to the cation concentration of groundwater. The column experiments demonstrated that PCE underwent the phase transition from emulsion to microemulsion and then to micellar solution during the flushing process. The formation and phase transition of microemulsion were mainly related to injection velocity and PCE residual saturation in aquifers. The slower injection velocity and higher residual saturation were profitable to the in-situ formation of microemulsion. In addition, the removal efficiency could achieve 99.29 % for residual PCE at 12 °C, enhancing with the finer porous medium, lower injection velocity and intermittent injection. Furthermore, the flushing system exhibited high biodegradability and weak reagent adsorption onto the aquifer media, presenting a low environmental risk. This study provides valuable information on the in-situ microemulsion phase behaviors and the optimal reagent parameters, facilitating the application of in-situ microemulsion flushing.

A pH-responsive phosphoprotein washing fluid for the removal of phenanthrene from contaminated peat moss in the cold region

Oil pollution is one of the major environmental concerns in the petroleum industry. In this study, a cheap food-grade sodium caseinate (NaCas) was used as a pH-responsive washing fluid in the remediation of phenanthrene (PHE) affected peat moss. The effects of environmental factors on the removal of PHE were systematically investigated. The results showed that increasing NaCas concentration and washing temperature improved the PHE mobilization, while high salinity and humic acid dosage displayed a negative effect. The factorial analysis revealed that three individual factors and two interactions exhibited significant effects on the washing performance. Due to the pH-responsive property of NaCas, the turbidity, total organic carbon (TOC), and chemical oxygen demand (COD) of the washing effluent were remarkably reduced by simply adjusting the solution acidity, improving the practical application of such a washing method. Significantly, the toxicity modeling proved that NaCas can reduce the binding energy between PHE and superoxide dismutase (SOD) of the selected marine organism, and thus relieve the toxicity of PHE to the organisms. Given these advantages, NaCas-assisted washing can be a viable option for the remediation of contaminated peat moss.

Mesoscale Aggregation of Sulfur-Rich Asphaltenes: In Situ Microscopy and Coarse-Grained Molecular Simulation

Asphaltene aggregation is critical to many natural and industrial processes, from groundwater contamination and remediation to petroleum utilization. Despite extensive research in the past few decades, the fundamental process of sulfur-rich asphaltene aggregation still remains not fully understood. In this work, we have investigated the particle-by-particle growth of aggregates formed with sulfur-rich asphaltene by a combined approach of in situ microscopy and molecular simulation. The experimental results show that aggregates assembled from sulfur-rich asphaltene have morphologies with time-dependent structural self-similarity, and their growth rates are aligned with a crossover behavior between classic reaction-limited aggregation and diffusion-limited aggregation. Although the particle size distribution predicted using the Smoluchowski equation deviates from the observations at the initial stage, it provides a reasonable prediction of aggregate size distribution at the later stage, even if the observed cluster coalescence has an important effect on the corresponding cluster size distribution. The simulation results show that aliphatic sulfur exerts nonmonotonic effects on asphaltene nanoaggregate formation depending on the asphaltene molecular structure. Specifically, aliphatic sulfur has a profound effect on the structure of rod-like nanoaggregates, especially when asphaltene molecules have small aromatic cores. Interactions between aliphatic sulfur and the side chain of neighboring molecules account for the repulsive forces that largely explain the polydispersity in the nanoaggregates and corresponding colloidal aggregates. These results can improve our current understanding of the complex process of sulfur-rich asphaltene aggregation and sheds light on designing efficient crude oil utilization and remediation technologies.

Microemulsion: a novel alternative technique for edible oil extraction_a mechanistic viewpoint

Microemulsions, as isotropic, transparent, nano size (<100 nm), and thermodynamically stable dispersions, are potentially capable of being used in food formulations, functional foods, pharmaceuticals, and in many other fields for various purposes, particularly for nano-encapsulation, extraction of bioactive compounds and oils, and as nano-reactors. However, their functionalities, and more importantly their oil extraction capability, strongly depend on, and are determined by, their formulation, molecular structures and the type, ratio and functionality of surfactants and co-surfactants. This review extensively describes microemulsions (definition, fabrication, thermodynamic aspects, and applications), and their various mechanisms of oil extraction (roll-up, snap-off, and solubilization including those by Winsor Types I, II, III, and IV systems). Applications of various food grade (natural or synthetic) and extended surfactants for edible oil extraction are then covered based on these concepts.

Wettability Reversal on Dolomite Surfaces by Divalent Ions and Surfactants: An Experimental and Molecular Dynamics Simulation Study

Due to the importance of the dolomite mineral in carbonate reservoirs, the wettability characteristics of dolomite surfaces were studied with both experiments and molecular dynamics simulations. Contact angle measurements confirm that the dolomite surface can be rendered oil-wet by carboxylates (acidic components of crude oil) and that the cationic surfactant can reverse the oil-wetness more effectively than the anionic surfactant used in this study. The oil-wetness of an aged dolomite chip was reduced when treated with MgSO₄ solution at 80 °C, while CaCl₂, MgCl₂, and Na₂SO₄ solutions did not produce any significant wettability alteration. The effects of surfactants and divalent ions, Ca²⁺, Mg²⁺, and SO₄^2- (also referred to as Smart Water ions), were simulated with two model dolomite surfaces containing point defects and step vacancies, respectively. The results indicate that the cationic surfactant can weaken the attraction between the oil phase and the carboxylates, while the anionic surfactant tends to maintain the oil-wetness of the dolomite surface by replacing the carboxylates through competitive adsorption. All Ca²⁺, Mg²⁺, and SO₄^2- ions can act as potential determining ions, and the detachment of carboxylates is due to the repulsion from SO₄^2- ions drawn close to the surface in the presence of adsorbed Mg²⁺.

Chemical computational approaches for optimization of effective surfactants in enhanced oil recovery

The surfactant flooding becomes an attractive method among several Enhanced Oil Recovery (EOR) processes to improve the recovery of residual oil left behind in the reservoir after secondary oil recovery process. The designing of a new effective surfactant is a comparatively complex and often time consuming process as well as cost-effective due to its dependency on the crude oil and reservoir properties. An alternative chemical computational approach is focused in this article to optimize the performance of effective surfactant system for EOR. The molecular dynamics (MD), dissipative particle dynamics (DPD) and density functional theory (DFT) simulations are mostly used chemical computational approaches to study the behaviour in multiple phase systems like surfactant/oil/brine. This article highlighted a review on the impact of surfactant head group structure on oil/water interfacial property like interfacial tensions, interface formation energy, interfacial thickness by MD simulation. The effect of entropy in micelle formation has also discussed through MD simulation. The polarity, dipole moment, charge distribution and molecular structure optimization have been illustrated by DFT. A relatively new coarse-grained method, DPD is also emphasized the phase behaviour of surfactant/oil/brine as well as polymer-surfactant complex system.

Surfactant-enhanced remediation of oil-contaminated soil and groundwater: A review

Oil leakage, which is inevitable in the process of extraction, processing, transportation and storage, seriously undermines the soil and groundwater environment. Surfactants can facilitate the migration and solution of oil contaminants from nonaqueous phase liquid (NAPL) or solid phase to water by reducing the (air/water) surface tension, (oil/water) interfacial tension and micellar solubilization. They can effectively enhance the hydrodynamic driven remediation technologies by improving the contact efficiency of contaminants and liquid remediation agents or microorganism, and have been widely used to enhance the remediation of oil-contaminated sites. This paper summarizes the characteristics of different types of surfactants such as nonionic, anionic, biological and mixed surfactants, their enhancements to the remediation of oil-contaminated soil and groundwater, and examines the factors influencing surfactant performance. The causes of tailing and rebound effects and the role of surfactants in suppressing them are also discussed. Laboratory researches and actual site remediation practices have shown that various types of surfactants offer diverse options. Biosurfactants and mixed surfactants are superior and worth attention among the surfactants. Using surfactant foams, adding shear-thinning polymers, and combining surfactants with in-situ chemical oxidation are effective ways to resolve tailing and rebound effects. The adsorption of surfactants on soils and aquifer sediments decreases remediation efficiency and may cause secondary pollution, Therefore the adsorption loss should be noticed and minimized.

Interfacial and molecular interactions between fractions of heavy oil and surfactants in porous media: Comprehensive review

The oil production by the natural energy in oil reservoirs is decreasing gradually. Only 25-30% of the world's reservoirs can be produced naturally, and different methods are employed to recover the remaining oil. The use of surfactants is one of the promising methods for unlocking the residual oil after natural depletion. In such a method, one of the main challenges is to study how surfactant, oil, and water interact and how porous media affect these interactions. Molecular dynamics (MD) simulation provides an opportunity to gain insights into this challenge. MD simulation can be used to study interactions between surfactant, oil, and water statically and dynamically in porous media. This paper presents a comprehensive review of interactions between surfactants and fractions of oil/heavy oil, including asphaltene, resin, aromatics, and saturates. Also, it explains the probable mechanisms of oil detachment from reservoir rock in the presence of surfactants. A thorough grasp of molecular interactions between surface-active agents and different fractions of oil helps us to develop successful surfactant-based oil recovery methods.

Precipitant Effects on Aggregates Structure of Asphaltene and Their Implications for Groundwater Remediation

Asphaltenes generally aggregate, then precipitate and deposit on the surfaces of environmental media (soil, sediment, aquifer, and aquitard). Previous studies have recognized the importance of asphaltene aggregates on the wettability of aquifer systems, which has long been regarded as a limiting factor that determines the feasibility and remediation efficiency of sites contaminated by heavy oils. However, the mechanisms/factors associated with precipitant effects on asphaltene aggregates structure, and how the precipitant effects influence the wettability of surfaces remain largely unknown. Here, we observe the particle-by-particle growth of asphaltene aggregates formed at different precipitant concentrations. Our results show that aggregates for all precipitant concentrations are highly polydisperse with self-similar structures. A higher precipitant concentration leads to a more compacted aggregates structure, while precipitant concentration near to onset point results in a less compact structure. The well-known Smoluchowski model is inadequate to describe the structural evolutions of asphaltene aggregates, even for aggregation scenarios induced by a precipitant concentration at the onset point where the Smoluchowski model is expected to explain the aggregate size distribution. It is suggested that aggregates with relative high fractal dimensions observed at high precipitant concentrations can be used to explain the relatively low Stokes settling velocities observed for large asphaltene aggregates. In addition, asphaltene aggregates with high fractal dimensions are likely to have high density of nanoscale roughness which could enhance the hydrophobicity of interfaces when they deposit on the sand surface. Findings obtained from this study advance our current understandings on the fate and transport of heavy oil contaminants in the subsurface environment, which will have important implications for designing and implementing more effective and efficient remediation technologies for contaminated sites.

A positively charged calcite surface model for molecular dynamics studies of wettability alteration

A new model for a positively charged calcite surface was developed to allow realistic molecular dynamics studies of wettability alteration on carbonate rocks. The surface charge was introduced in a manner consistent with the underlying calcite geochemistry and with the conclusions of recent quantum mechanical studies. The simulations using the new surface model demonstrate that the experimentally observed wettability behavior of calcite is represented correctly. In particular, the model surface became oil-wet due to the adsorption of the carboxylate species. Furthermore, the oil-wet conditions were reversed more effectively by a cationic surfactant than by an anionic one, in agreement with the majority of experimental observations. Finally, with simulated smart water, the well-documented wettability alteration abilities of Ca²⁺ and SO₄^2- could be explained by the formation of ion-pairs and competitive adsorption onto the surface, respectively. The simulation results with the new surface model conceptually agree with the electric double layer expansion being the predominant mechanism for the low salinity effect in oil recovery enhancement. The proposed calcite surface model will benefit future simulation studies on the wettability characteristics of carbonate rocks, and facilitate the design and optimizations of chemical agents and formulations to enhance the oil recovery from carbonate reservoirs.

Pore-scale dynamics of nanofluid-enhanced NAPL displacement in carbonate rock

This study presents a pore-scale investigation of two-phase flow dynamics during nanofluid flooding in subsurface formations containing non-aqueous phase liquids (NAPLs) such as crude oils. The goal was to gain fundamental understanding of the dominant displacement mechanisms of NAPL at different stages of nanofluid injection in a carbonate rock using x-ray microtomography integrated with a miniature core-flooding system. The nanofluid consisted of surfactant-based microemulsions with in-situ synthesized silica nanoparticles. After establishing its initial wettability state, the carbonate core sample was subjected to various pore volumes (PV) of nanofluid flooding (from 0.5 to 10) to examine the impact on NAPL flow dynamics. We found that most NAPL mobilization occurred within the first PV of injection, removing nearly 50% of NAPL from the rock. The nanofluid invaded into larger pores first due to a sharp decrease in NAPL/brine interfacial tension (from 14 to 0.5 mN/m) and contact angle (from 140 to 88°). With higher amount of nanofluid delivered into the pores through advection, over 90% of NAPL droplets were emulsified and their size decreased from 9 to 3 μm. Subsequent nanofluid injection could further remove NAPL from the smaller pores by altering the thickness of NAPL layers adsorbed on the rock. This dynamic solubilization process reached equilibrium after 5 PV of injection, leading to a reduced layer thickness (from 12 to 0.2 μm), a narrower in-situ contact angle distribution around 81°, and an additional 16% of NAPL removal.

Numerical modelling of the impact of surfactant partitioning on surfactant-enhanced aquifer remediation

The partitioning of surfactants into non-aqueous phase liquids (NAPLs) during Surfactant-Enhanced Aquifer Remediation (SEAR) is potentially an important and non-negligible phenomenon that can strongly impact remediation efficiency. This paper numerically investigates the impact of surfactant partitioning on the enhanced NAPL dissolution and mobilization mechanisms and the overall NAPL removal from the subsurface. For demonstration, a multiphase model is used to simulate a hypothetical SEAR consisting of Triton X100 surfactant solution for the removal of perchloroethylene (PCE) entrapped in contaminated porous medium at the core/column scale. The simulations are conducted for two-dimensional homogenous and three-dimensional heterogeneous systems. By simultaneously incorporating spatial heterogeneity of porous media, injection rate, and endpoint mobility ratio into the model, we delineate the interplay of surfactant partitioning with flow and transport dynamics. Our results show that surfactant partitioning from the aqueous phase across the interface to the NAPL phase can undermine both efficiency of the enhanced dissolution and mobilization of NAPL species. This undermining is more pronounced for when aqueous phase mobility is less than the mobility of the NAPL phase. For such conditions interfacial tension between the two phases is reduced less for partitioning than non-partitioning cases (due to loss of surfactant into NAPL phase) and a secondary water front is formed due to partitioning that makes aqueous phase breaks through earlier.