Nonaqueous Phase Research Articles

Computational Insights into the Esterification of Palmitic Acid with Methanol in Water.

This work provides quantified explanations for the thermodynamic and kinetic characteristics of esterification in aqueous phase, and how phase transfer catalysts improve water phase esterification of fatty acids in a computational-experimental way. Self-catalyzed reaction mode with or without solvation effects, water participated reaction mode, and catalytic reaction mode (catalyzed by p-dodecyl benzene sulfonic acid, DBSA) are discussed. Our results show that the initial self-catalytic reaction mode undergoes the energy barrier of 100.1 kJ/mol, and rises to 148.9 kJ/mol when water molecule is involved, which hinders the esterification reaction. With the DBSA catalyst, this energy barrier will drop to 97.5 kJ/mol and the water phase esterification is successfully promoted with the yield of 81%. The key kinetic factor of binding energy is discussed as that water molecule has a strong reactant binding competitiveness (with the binding energy of -57.9 kJ/mol, and the value for the non-aqueous phase mode is 3.0 kJ/mol) and DBSA has the binding energy with the value of -45.3 kJ/mol, so it can compete with water to form reactant complexes. This work is a successful practice of a computation-experiment combined scheme, and provides a quantitative basis for the improvement of phase transfer catalysts on water phase esterification reactions. The calculation mode and method of aqueous esterification make it possible to convert bio-based fatty acids into fatty acid esters in fermentation broth.

Analysis of Dense Non-aqueous Phase Liquid Contaminants Effects on Soil Permeability by Response Surface Methodology

Soil contamination is considered a controversial issue in most countries. Nowadays, it is important to clearly understand how pollutants influence the soil from different sources. Today, hydrocarbons are one of the most important sources of soil contaminants, which is considered as a fundamental issue at the global level. The current study aims to analyze and model the effect of simultaneous parameters (time and concentration) of phenols and naphthalene with different percentages (10, 15, 20 and 25%) together with the amount of bentonite in fine-grained sandy soil. The designed experiments made use of response surface methodology (RSM) and Design-Expert software to carry out a computer-based simulation. According to the proposed model, the amount of bentonite is most affected by the permeability of the soil. The obtained results also showed that the permeability significantly decreases in the light of increasing the percentage of phenol and naphthalene coupled with the amount of bentonite and the age of contamination. On average, an 80% reduction of permeability was observed in contaminated soil, which was found in the soil contaminated with naphthalene. According to the results of the synergistic effects of time, the considerable impacts of both the percentage of hydrocarbon pollutants and the amount of bentonite on the reduction of permeability are quite evident.

Risk assessment and remediation of NAPL contaminated soil and groundwater

The presence of water-immiscible organic liquids—commonly called non-aqueous phase liquids or NAPLs—in soils and groundwater, is a worldwide environmental problem. Typical examples of NAPLs include: petroleum products, organic solvents and organic liquid waste from laboratories and industry. The molecular components of NAPLs present in soils, rocks and groundwater are readily transferred to the vapour and aqueous phases. The extent to which they do this is determined by their solubility (which is quite limited) and vapour pressure (which can be quite high). These molecular components, once dispersed in the vapour phase or dissolved in the aqueous phase, can provide a long-term source of harm to biotic receptors. The object of this lecture text is to examine how we can assess the degree of harm using quantitative risk assessment and how NAPL contaminated environments can be restored through the use of chemical, biological and physical remediation technologies.Graphical abstract

Synthesis, properties, and applications of an anionic–nonionic Gemini surfactant for highly efficient remediation of perchloroethylene-contaminated aquifers

Monomeric surfactants are currently used for the remediation of dense non-aqueous phase liquid-contaminated groundwater. However, they result in low remediation efficiency because of their high adsorption loss and low temperature and salt resistance. In this study, a novel and green Gemini surfactant, SNE(10)SID (sodium nonylphenol ethoxylate (10) sulfoitaconate diester), was synthesized for subsurface environment characteristics through esterification and sulfonation. This surfactant consisted of two long and straight hydrophobic chains, two selfsame non-ionic portions, and two anionic moieties covalently connected to each other by an itaconic acid spacer. The critical micelle concentration was ∼10−4 M. The interfacial tension between SNE(10)SID and perchloroethylene (PCE) was significantly low, and the value was found to be 1.0 mN/m. SNE(10)SID exhibited excellent activity under conditions of low temperature and in high salt systems. The solubility of PCE was up to 8863 mg/L in a 4 wt% SNE(10)SID solution, which was two orders of magnitude higher than the solubility recorded in pure water. SNE(10)SID exhibited a low adsorption loss (only 0.8 mg/g at a concentration of 2 g/L in medium sand). The PCE desorption ability of the surfactant was far significantly higher than the PCE desorption abilities of other monomeric surfactants in various aquifer media. Under conditions of dynamic flushing, the low-viscosity SNE(10)SID solution exhibited excellent remediation ability when PCE-contaminated simulated aquifers were used for the studies. The total PCE removal rate reached was 87.5% after the flushing of 10 pore volumes. These results revealed that SNE(10)SID was an excellent surfactant that could be used to realize efficient aquifer remediation.

Application of Solid Phase Extraction (SPE) Media Rods to Assess Degree of NAPL Encapsulation in In Situ Deposited NAPL Sediments

ABSTRACT Oil-particle aggregates (OPAs) develop from solid particles attaching to and/or penetrating into the surface of a non-aqueous phase liquid (NAPL) bead suspended in the water column. In situ deposited NAPL (IDN) sediments result from the deposition of OPAs on the sediment bed at the bottom of the water body. The extent of solid particle coverage on the surface of the oil bead defines the degree of encapsulation of the OPA (i.e., fully or partially encapsulated). Since important properties such as NAPL mobility and contaminant flux are controlled by the degree of OPA encapsulation, the ability to measure this physical property is critical in characterizing NAPL fate and transport in IDN sediments. This paper describes a semi-quantitative method to measure the degree of encapsulation within IDN sediment using the fluorescence response from rods coated with solid phase extraction (SPE) media. Specifically, Direct Analysis in Real Time (DART®) technology, which was developed and is distributed by Dakota Technologies, was applied to measure the amount of contact between the SPE material coated on a vertical rod and the PAHs contained in the NAPL found in the surrounding IDN sediment. Based on laboratory results in this paper, the magnitude of the fluorescence response correlates with the degree of encapsulation. For the single NAPL and various sediment combinations prepared in the laboratory for this study, fully (or nearly fully) encapsulated IDN sediments produced a DART® response below 15% reference emitter (%RE), a defined internal standard, whereas partially encapsulated IDN sediments produced a DART® response that was generally above 30%RE and as high as several thousand %RE. This difference in fluorescence response enables DART® technology to be applied as a line of evidence to determine the degree of NAPL encapsulation in an IDN sediment in field screening programs.

Three-Phase Equilibrium Calculations of Water/Hydrocarbon/Nonhydrocarbon Systems Based on the Equation of State (EOS) in Thermal Processes.

A simple and novel approach is proposed to represent the mutual solubility of water and hydrocarbon components based on equations of state at high temperatures in thermal recovery processes. Sϕreide and Whitson modifications are applied to the Peng–Robinson (PR) equation of state (EOS) so that all components, including the water component, can exist in all phases, reasonably representing gas solubility in water and water solubility in hydrocarbon phases. We propose an algorithm to assign binary interaction parameters (BIPs) for aqueous and nonaqueous phases. The water vapor pressure helps select initial K-values for stability analysis so that the aqueous phase can be split out first if present. The algorithm is tested by a wide range of variations in pressure, temperature, and composition. The results show the robustness of the algorithm and the effects of temperature and overall water mole fraction on phase behaviors in steam flooding processes.

Temperature influence on the NAPL-water interfacial area between 10 °C and 60 °C for trichloroethylene

Underground thermal energy storage (UTES) can contribute to renewable energy usability, especially in urban areas with the most demand and available infrastructure. But UTES may interact in those areas with non-aqueous phase liquids (NAPL) by increasing the temperature in storage formations. To determine temperature effects on NAPL dissolution rates into groundwater, the effective specific interfacial area (anw) between trichloroethylene (TCE) and water, as a function of temperature and TCE pore saturations, was calculated. The interfacial tension between the flushing solution and the NAPL, the adsorption coefficient and the retardation of a reactive tracer were determined by the drop weight method and interfacial tracer tests at 10 °C, 30 °C and 60 °C. From 10 to 60 °C anw increased by a factor of six to eight. Based on the results, a function to describe the anw between TCE and water was developed, which could improve numerical models on NAPL dissolution rates. The main mechanisms for the increase in anw are suggested to be NAPL blob migration on pore scale, and thermal-induced changes in wettability and in blob shape correlating with a temperature-induced decline in effective porosity of up to 32%. These results contribute to the understanding and predictability of UTES in contaminated aquifers, the general response of NAPL behavior on artificial increasing aquifer temperatures and the improvement of thermal and other groundwater remediation techniques.

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.

The implementation of genetic algorithm for the identification of DNAPL sources

Contamination caused by dense non-aqueous phase liquids (DNAPLs) is a serious peril to the groundwater supply quality. The DNAPLs are largely undetected and yet are likely to be a significant factor limiting site remediation. It is necessary to identify the sources of pollution and control pollution in the early stage. The primary objective of this research was to investigate the pollution source and intensity of migrating DNAPLs. This objective was accomplished through the combination of genetic algorithm (GA) and T2VOC numerical experimental simulation. This study considers two synthetic cases of numerical modeling based on the T2VOC from the TOUGH2 code family to assess and evaluate the efficacity of the algorithm. The first case identified concurrently the intensity as well as the location of the one unknown contaminated water source. The second case distinguished various contaminated water sources. For the single source pollution identification, the estimation of intensity error was 0.08%, and the error of GA at second and third true value calculation was 0.05% and 0.03%, respectively, achieving the goal of pollution identification accurately. While for the multiple sources, the deviation of intensity was ranging between 1.02 and 5.65%, the results suggested that source No.2 was mainly responsible for the leakage, and source No.3 adopted the secondary responsibility among others. These two cases suggested that GA established by optimal search strategy could be adopted for effective identification of DNAPL pollution sources. This study provides information that could facilitate the development and implementation of innovative remediation strategies.

Towards a digital twin for characterising natural source zone depletion: A feasibility study based on the Bemidji site

Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) may be a valid long-term management option at petroleum impacted sites. However, its future long-term reliability needs to be established. NSZD includes partitioning, biotic and abiotic degradation of LNAPL components plus multiphase fluid dynamics in the subsurface. Over time, LNAPL components are depleted and those partitioning to various phases change, as do those available for biodegradation. To accommodate these processes and predict trends and NSZD over decades to centuries, for the first time, we incorporated a multi-phase multi-component multi-microbe non-isothermal approach to representatively simulate NSZD at field scale. To validate the approach we successfully mimic data from the LNAPL release at the Bemidji site. We simulate the entire depth of saturated and unsaturated zones over the 27 years of post-release measurements. The study progresses the idea of creating a generic digital twin of NSZD processes and future trends. Outcomes show the feasibility and affordability of such detailed computational approaches to improve decision-making for site management and restoration strategies. The study provided a basis to progress a computational digital twin for complex subsurface systems.

Enhanced elimination of gaseous toluene and methanol emissions in a two-liquid phase trickling bioreactor: Performance evaluation, dynamic modeling, and microbial community shift

Negative interactions of the components usually restrict the application of trickling bioreactors (TBRs) for the simultaneous biodegradation of multi volatile organic compounds (VOCs). Modifications like the addition of non-aqueous phase (NAP) to TBRs can lead to design higher performance biological air cleaner systems for industries with mixed VOCs emissions. In this study, removal of toluene and methanol as hydrophobic and hydrophilic VOCs, respectively, was assessed in an up-flow two-liquid phase TBR (TLP-TBR) in the presence of silicone oil (5% v/v) as the NAP. The toluene and methanol inlet loading rates (ILRT and ILRM) were 18–36 and 0–187.2 g m−3 h−1, respectively, at a constant empty bed residence time of 60 s. The system could withstand against increasing ILRs so that removal efficiency (RE) of toluene >80% was obtained at various ILRs, while RE of methanol was almost stable at >90%. After optimization of a developed dynamic mathematical model, including mass transfer and kinetic mechanisms, the overall mass transfer coefficients of toluene and methanol were obtained as 1.9 × 10−4 and 2.1 × 10−4 m s−1, respectively. About 50% of biofilm depth was active for toluene biodegradation, indicating the prevalence of diffusion-limited regime for toluene removal. The microbial diversity shifted from the original inoculum to dominant toluene-degrading Gordonia and Burkholderia close to the gas inlet with relative abundance of 30% and 20%, respectively. While, methanol-degrading Hyphomicrobium sp. had higher contribution (19%) at top of the column. Therefore, the presence of NAP could facilitate the sustainable application of TBR through a suitable microbial composition shift.

Biotic and abiotic reductive dechlorination of chloroethenes in aquitards

Chlorinated solvents occur as dense nonaqueous phase liquid (DNAPL) or as solutes when dissolved in water. They are present in many pollution sites in urban and industrial areas. They are toxic, carcinogenic, and highly recalcitrant in aquifers and aquitards. In the latter case, they migrate by molecular diffusion into the matrix. When aquitards are fractured, chlorinated solvents also penetrate as a free phase through the fractures. The main objective of this study was to analyze the biogeochemical processes occurring inside the matrix surrounding fractures and in the joint-points zones. The broader implications of this objective derive from the fact that, incomplete natural degradation of contaminants in aquitards generates accumulation of daughter products. This causes steep concentration gradients and back-diffusion fluxes between aquitards and high hydraulic conductivity layers. This offers opportunities to develop remediation strategies based, for example, on the coupling of biotic and reactive abiotic processes. The main results showed: 1) Degradation occurred especially in the matrix adjacent to the orthogonal network of fractures and textural heterogeneities, where texture contrasts favored microbial development because these zones constituted ecotones. 2) A dechlorinating bacterium not belonging to the Dehalococcoides genus, namely Propionibacterium acnes, survived under the high concentrations of dissolved perchloroethene (PCE) in contact with the PCE-DNAPL and was able to degrade it to trichloroethene (TCE). Dehalococcoides genus was able to conduct PCE reductive dechlorination at least up to cis-1,2-dichloroethene (cDCE), which shows again the potential of the medium to degrade chloroethenes in aquitards. 3) Degradation of PCE in the matrix resulted from the coupling of reactive abiotic and biotic processes—in the first case, promoted by Fe2+ sorbed to iron oxides, and in the latter case, related to dechlorinating microorganisms. The dechlorination resulting from these coupling processes is slow and limited by the need for an adequate supply of electron donors.

Performance evaluation of a trickling bioreactor treating methanol vapor under one- and two-liquid phase conditions

This study compared the performance of a trickling bioreactor (TBR) for methanol removal with and without the addition of silicone oil as a non-aqueous phase. Due to an adaptation process based on activated sludge, the acclimation period was as short as 16 days. First, TBR was run under one-liquid phase (OLP) condition at empty bed gas residence time (EBRT) of 120 s, while methanol inlet concentration varied from 0.25 to 15.1 g m −3 . The results showed that OLP-TBR could be a promising approach when facing with methanol shock loads with average removal efficiencies (REs) over 80%. For OLP operation, the maximum elimination capacity (EC) was 411 g m −3 h −1 at a methanol inlet loading rate (ILRs) of 453 g m −3 h −1 . Then, silicone oil (10% v/v ) was added to TBR to provide two-liquid phase (TLP) condition, under methanol inlet concentrations of 0.55-9.2 g m −3 and two different EBRTs of 120 and 60 s. For the TLP operation, a maximum EC of 416 g m −3 h −1 was obtained for an ILR of 474 g m −3 h −1 under EBRT of 60 s. The addition of silicone oil negatively affected the TBR performance, particularly for methanol inlet concentrations higher than 5 g m −3 . Based on 16S rDNA analysis, Citrobacter koseri LMG 5519(T), Leifsonia shinshuensis JCM 10591(T), and Pandoraea pnomenusa DSM 16536(T) were identified as the methanol degrading species. • Methanol removal in one and two-liquid phase O/TLP-TBRs was studied. • Maximum elimination capacity (EC) was 411 g m -3 h -1 in OLP-TBR. • TLP operation slightly decreased TBR performance at inlet concentrations> 5 g m -3 . • Dominant bacteria in TBR was identified by 16S rDNA analysis. • Methanol mineralization to CO 2 decreased in TLP-TBR with more biomass formation.

Bringing Sustainability to Macroporous Polystyrene: Cellulose Nanocrystals as Cosurfactant and Surface Modifier in Deep Eutectic Solvent-Based Emulsion Templating

Macroporous polymers were produced from nonaqueous high internal phase emulsions (HIPEs) containing a deep eutectic solvent (DES) and stabilized by mixtures of unmodified cellulose nanocrystals (CNCs) and a nonionic surfactant. The effect of the CNC/surfactant ratio on HIPEs morphology and rheological properties was thoroughly investigated. Upon free-radical polymerization of styrene/divinylbenzene in the continuous phase, highly interconnected macroporous structures with cellulose-functionalized surfaces and robust mechanical properties were obtained. Additionally, the generation of nanocellulose dispersions from biomass in situ in the acidic DES internal phase of HIPEs and subsequent polymerization was proposed to synthesize interconnected macroporous polymers. Biomass incorporation into macroporous polymers further emphasizes the sustainable character of the DES-based emulsion templating introduced in this work.

A study of the influencing factors of DNAPLs transport in stochastic network fractures

The migration and distribution of dense non-aqueous phase liquid (DNAPLs) in fractured media are affected by many factors, including the physical and chemical properties of DNAPLs, geometric and chemical properties of fractures and leakage conditions. Previous studies were mostly conducted in a single fracture, while the migration of DNAPLs in stochastic network fracture were seldom examined. In this paper, stochastic network fractures are generated using the Monte Carlo method. The coordinates and apertures of fractures are identified and output by pixel scanning. The migration of Perchloroethylene (PCE) in the stochastic network fractures is simulated by PetraSim. The effects of the geometric properties of the fractures and the leakage conditions (including the leakage rate and the leakage location) on the migration of DNAPLs are discussed. The numerical simulation results show that the spatial variability of fractures also affects the migration path and spatial distribution of DNAPLs. With the increase of spatial variability of fracture width in the network fractures, dominant channels appear, the migration rate of DNAPLs accelerates, and the location of DNAPLs centroid and the spatial distribution of saturation change significantly. The leakage rate affects the migration range and spatial distribution of DNAPLs. The higher the leakage rate is, the faster the migration rate of DNAPLs will be. The more saturated the accumulated DNAPLs at the bottom of the model, the greater the spatial distribution of DNAPLs is. In the same network fracture, different leakage locations will also lead to different migration paths and distribution ranges of DNAPLs. The spatial variability of fractures in the gravity direction at different leakage locations is different, leading to different migration paths and spatial distribution of DNAPLs.

Influences of Dead‐End Pores in Porous Media on Viscous Fingering Instabilities and Cleanup of NAPLs in Miscible Displacements

AbstractImproving the understanding of mechanisms involved in a low miscible displacement efficiency is significant for a wide spectrum of applications in subsurface from environment such as groundwater remediation and sequestration to energy extraction such as enhanced oil recovery and geothermal recovery. Two key limiting factors to the efficiency are viscous fingering (VF) instability and dead‐end pores in porous media. Previous research on VF simply assumes all pores are well connected and fluids can be mobilized by convection. However, fluids trapped in dead‐end pores, such as nonaqueous phase liquids (NAPLs) in groundwater remediation, are inaccessible to convection, resulting in even less efficient displacements. Instead of the classic convection‐diffusion/dispersion equation, in this work, we use a fundamentally different capacitance model to incorporate the mass transfer between two pore types in miscible displacements. The hybrid pseudospectral and high‐order finite difference methods are employed to solve the governing equations in a fixed reference frame for simulating the flow dynamics. We found that the viscous fingering instabilities in well‐connected pore network led to an unstable, nonuniform distribution of trapped NAPLs in dead‐end pores network. Different with a traditional view that NAPLs are easily cleaned up, our study indicates the existence of dead‐end pores results in an inefficient cleanup of NAPLs in swept area because of the slow mass transfer of trapped NAPLs between two pore types. A simple model is developed to accurately predict the NAPL concentration behind finger trailing front, which has not been previously examined. Six flow regimes, four of which are new, are then identified.

Remobilization of LNAPL in unsaturated porous media subject to freeze-thaw cycles using modified light transmission visualization technique

The frequent use of light non-aqueous phase liquids (LNAPLs) in cold regions creates serious risks of soil and groundwater contamination. However, the subsurface remobilization behaviors of LNAPL in freezing and thawing unsaturated porous media are not well understood. The light transmission visualization (LTV) technique, which be previously used in two-phase system (water–air or water-NAPL), is modified to quantify LNAPL in three-(water–air-LNAPL) or four-phase (water–ice-air-LNAPL) systems during freezing and thawing. The pattern of LNAPL remobilization was observed in a two-dimensional flow chamber filled with unsaturated porous media subject to freeze–thaw cycles (FTCs) using the modified LTV technique. It is shown that LNAPL remobilization mainly occurs in the direction of the freezing front and horizontal with the number of FTCs. Meanwhile, LNAPL in the area below the freezing front is promoted to remigrate downward during freezing and the opposite direction during thawing. The analysis suggests that LNAPL remobilization is caused by a combination of freezing-induced pressure and changes in capillary pressure controlled by water migration, rather than through microcracks in the frozen porous media. The proposed modified LTV technique shows great promise for investigating LNAPL transport and distribution in freezing (unfreezing) unsaturated porous media that is non-destructive. The study offers a scientific basis for the prediction of the subsurface behavior of LNAPL at contaminated sites in widely distributed cold regions.

Field Study of Vertical Screening Distance Criteria for Vapor Intrusion of Ethylene Dibromide

AbstractSeveral regulatory agencies recommend screening petroleum vapor intrusion (PVI) sites based on vertical screening distance between a petroleum hydrocarbon source in soil or groundwater and a building foundation. U.S. Environmental Protection Agency (U.S. EPA) indicate the risk of PVI is minimal at buildings that are separated by more than 6 feet (1.8 m) from a dissolved‐phase source and 15 feet (4.6 m) from a light nonaqueous phase liquid (LNAPL) source. This vertical screening distance method is not, however, recommended at sites with leaded gasoline sources containing ethylene dibromide (EDB) because of a lack of field data to document EDB attenuation in the vadose zone. To help address this gap, depth‐discrete soil‐gas samples were collected at a leaded gasoline release site in Sobieski, Minnesota (USA). The maximum concentration of EDB in groundwater (175 μg/L) at the site was high relative to those observed at other leaded gasoline release sites. Soil gas was analyzed for EDB using a modification of U.S. EPA Method TO‐14A that achieved analytical detection limits below the U.S. EPA Vapor Intrusion Screening Level (VISL) for EDB based on a 10−6 cancer risk (<0.16 μg/m3). Concentrations of EDB in soil gas above LNAPL reached as high as 960 μg/m3 and decreased below the VISL within a source‐separation distance of 7 feet. This result coupled with BioVapor model predictions of EDB concentrations indicate that vertical screening distances recommended by regulatory agencies at PVI sites are generally applicable for EDB over the range of anticipated source concentrations and soil types at most sites.

Transport of chemotactic bacteria in granular media with randomly distributed chemoattractant-containing NAPL ganglia: Modeling and simulation

Chemotaxis is the biased movement of bacteria upon sensing chemical gradients, which can facilitate bioremediation of nonaqueous phase liquids (NAPLs) by transporting oil-degrading bacteria more efficiently to contaminants in groundwater aquifers. Transport phenomena of chemotactic bacteria in porous media can be described within the context of a convection-dispersion equation (CDE) that includes an additional convection-like chemotactic velocity. In this paper, we present a novel modification of a previously reported CDE by including a single first-order kinetic term which can differentiate bacterial sorption on porous matrix and chemoattractant-containing NAPL ganglia. Our model was solved numerically using a finite-element scheme to compute bacterial transport in granular media at a continuum level and also captured chemotaxis to discrete, randomly distributed NAPL ganglia of chemoattractant sources. Simulation results revealed the influence of oil ganglia on bacterial transport. It was found that chemotactic bacteria exhibited localized hotspots near NAPL ganglia because of chemotaxis toward and sorption on oil surfaces. The presence of oil ganglia reduced recovery of chemotactic bacteria by 71% and nonchemotactic bacteria by 44%, which accompanied enhanced retention inside the column. Model results suggested that chemotactic bacteria can locate NAPL and thereby sorb on surfaces more efficiently, which may overcome the limited bioavailability of contaminants dissolved in NAPLs. Simulated outputs demonstrated good agreement with experimental data in literature. Our model provides insight into the impact of chemotaxis and interaction with oil-phase chemoattractant sources on bacterial transport in porous media.

Density-regulated remediation of dense non-aqueous phase liquids using colloidal biliquid aphrons (CBLA): Force model of transport and distribution

Using colloidal biliquid aphrons (CBLAs) for density control has been proved to a promising technology in dense non-aqueous phase liquids (DNAPLs) contaminated aquifer remediation. However, the transport and distribution of CBLAs in aquifer is an urgent issue for actual application in groundwater. Especially considering the fact that CBLAs have a lower density than water. In this work, the role of buoyancy force on CBLA transport in water-saturated sandbox was investigated, and the force model of CBLA in pore space was developed. Furthermore, the density regulation of trichloroethylene (TCE) in sandbox was studied using CBLA. We found that buoyancy plays a significant role compared with other interaction forces in the transport of CBLA, and the sine of the rising angle of CBLA has a significant correlation with the force on CBLA. CBLA at 5 times the volume of TCE displaced the TCE at the bottom of the tank by upward mobility and the maximum concentration dramatically decreased to 31.23 mg/L. These results can be used for predicting the transport of CBLA (as well as other remediation reagents that are less dense than water) in aquifer and are beneficial to the subsequent remediation application of CBLA in actual contaminated sites.