Nonaqueous Phase Liquid Phase Research Articles

Novel phase transfer catalysis coupled with bifunctional oxidation for enhanced remediation of groundwater polluted with multiple NAPL: Performance and mechanisms

Structural differences among non-aqueous phase liquids (NAPLs) result in varying oxidation rates, limiting mass transfer between NAPLs and oxidants and seriously impairing the effectiveness of remediation via traditional in-situ chemical oxidation. To tackle this challenge, a novel approach is proposed for remediating multi-NAPL-polluted groundwater that leverages phase transfer catalysis (PTC) to enhance heterogeneous mass transfer by transferring oxidants from groundwater to NAPLs. Meanwhile, “oxidation-in-situ activation” is achieved through bifunctional oxidation using permanganate and peroxymonosulfate (PP). The proposed approach is referred to PTC-PP in this study. Herein, trichloroethene (TCE) and benzene serve as a representative multi-NAPL system. Experimental results indicated that PP significantly improved degradation efficiency of benzene in multi-NAPL system by at least 60.8% compared to single-oxidant systems, and further enhancement (17.6%) was achieved when PP was combined with PTC compared to PP alone. Dissolved Mn(II) and MnO2 generated by MnO4− reduction effectively activated peroxymonosulfate in PTC-PP system, with colloidal MnO2 being the most effective activator. Consequently, SO4•−, O2•− and 1O2 were formed in both NAPL and aqueous phases, while •OH was formed in aqueous phase, playing a crucial role in benzene oxidation. In phase transfer process of PTC-PP, the proportion of MnO4− transferred to benzene exceeded that to TCE. This finding illustrated that nondirectional phase transfer of oxidants posed a challenge for simultaneous promotion of TCE and benzene degradation. However, TCE and benzene removal efficiencies were both >75.7% by applying peroxymonosulfate after KMnO4 addition. These findings lay the theoretical groundwork for PTC-PP application in groundwater remediation.

Effect of Pumping Speeds on the Fate of Aniline in Different Soil Layer

Helan Mountain is an important ecological safety barrier in northwest China. In this study, a heterogeneous site polluted by aniline on Helan Mountain was the research object, and the TMVOC (A Simulator For Multiple Volatile Organic Chemicals) model of aniline restoration by pumping was optimized by employing a column experiment. Four typical layers of the soil medium were selected to explore the influence of soil settlement caused by different pumping speeds on the fate of aniline in different zones. The results show that the optimal pumping speed at the site is 3.24 × 106 m3/month and the latest remediation time is the 10th month after the start of the remediation. The larger the pumping speed is, the more obvious the sedimentation effect is. When the remediation is carried out at 5.18 × 106 m3/month, the NAPL (Non-Aqueous-Phase Liquid) phase removal rate decreases by 33.75% and the distribution of aniline to the NAPL phase increases, compared to that without considering the soil settlement. The fate of aniline in the source zone is the least affected by sedimentation, while that in the vadose zone is the most affected. The phase redistribution phenomenon is the most obvious in the water table fluctuation zone, and the NAPL phase aniline changes into gas and liquid phases. In addition, the NAPL phase concentration in the water table fluctuation zone is two orders of magnitude higher than that at 0.2 m below the water table. NAPL is the most sensitive to the relative settlement in the aquifer. The simulation results can provide a technical reference for the future application of P&T (Pump-and-Treat) technology in the remediation of organically contaminated sites to facilitate the sustainable use of soil. It is suggested that more attention should be paid to the water table fluctuation zone during the remediation of contaminated sites.

Three-Dimensional Model for Bioventing: Mathematical Solution, Calibration and Validation

Bioventing is an established technique extensively employed in the remediation of soil contaminated with petroleum hydrocarbons. In this study, the objective was to develop an improved foundational bioventing model that characterizes gas flow in vadose zones where aqueous and non-aqueous phase liquid (NAPL) are present and immobile, accounting for interphase mass transfer and first order biodegradation kinetics. By incorporating a correlation for the biodegradation rate constant, which is a function of soil properties including initial population of petroleum degrader microorganisms in soil, sand content, clay content, water content, and soil organic matter content, this model offers the ability to integrate a specific biodegradation rate constant tailored to the soil properties for each site. The governing equations were solved using the finite volume method in OpenFOAM employing the “porousMultiphaseFoam v2107” (PMF) toolbox. The equation describing gas flow in unsaturated soil was solved using a mixed pressure-saturation method, where calculated values were employed to solve the component transport equations. Calibration was done against a set of experimental data for a meso-scale reactor considering contaminant volatilization rate as the pre-calibration parameter and the mass transfer coefficient between aqueous and NAPL phase as the main calibration parameter. The calibrated model then was validated by simulating a large-scale reactor. The modelling results showed an error of 2.9% for calibrated case and 4.7% error for validation case which present the fitness to the experimental data, proving that the enhanced bioventing model holds the potential to improve predictions of bioventing and facilitate the development of efficient strategies to remediate soil contaminated with petroleum hydrocarbons.

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.

Modeling BTEX Multiphase Partitioning with Soil Vapor Extraction under Groundwater Table Fluctuation Using the TMVOC Model

The effects of groundwater table fluctuation (GTF) on the remediation of a petrochemically polluted riverside using soil vapor extraction (SVE) were investigated. The migration and transformation of benzene, toluene, ethylbenzene, and o-xylene (BTEX) in cases of natural attenuation, SVE without GTF, and SVE with GTF were simulated using the TMVOC model. The results showed that the optimized extraction well pressure and influencing radius of the target site were 0.90 atm and 8 m, respectively. The removal rates of BTEX in cases of natural attenuation, SVE without GTF, and SVE with GTF were 11.49%, 85.16%, and 97.33%, respectively. The removal rate of BTEX was maximized in the case of SVE with a GTF amplitude of 0.5 m to 1 m. The removal rates of benzene (99.99%), toluene (99.74%), ethylbenzene (96.37%), and o-xylene (94.72%) were maximized in the case of SVE with GTF. For the cases of SVE without GTF and SVE with GTF, mass losses of BTEX in gaseous (0.05 kg, 0.05 kg, respectively) and aqueous phases (5.46 kg, 5.87 kg, respectively) were consistent. However, the mass loss of BTEX in the non-aqueous phase liquid (NAPL) phase in the case of SVE with GTF (155.13 kg) exceeded that in the case of SVE without GTF (135.41 kg). This is because GTF positively affected both the solubility and volatility of BTEX in the NAPL phase. With the groundwater table decreasing, flows of gas and gaseous pollutants increased by 25% along the vertical section. At this stage, the removal rates of volatile organic compounds can be further improved by increasing the flow of the extraction well.

Thermal desorption mechanism of n-dodecane on unsaturated clay: Experimental study and molecular dynamics simulation

Thermal desorption technology can effectively remediate fuels-contaminated clayey soil. However, the microscopic mechanism for the contaminant desorption on clay is still unclear, especially with the existence of water on clay surfaces. In this study, a combination method including TGA experiments, multi-phase and multi-component kinetic models, and MD simulation was proposed to reveal the thermal desorption mechanism of n-dodecane in unsaturated clay. Results showed that the thermal desorption behavior of the free nonaqueous phase liquid (NAPL) and the adsorbed phase of n-dodecane or water could be identified by the multi-phase and multi-component kinetic models based on the desorption process rate obtained by thermogravimetric analysis (TGA) experiments. The activation energy of the NAPL phase (49–69.9 kJ/mol) was lower than the adsorbed phase (90 kJ/mol). The activation energy of the NAPL phase had the same linear relationship with its mass on kaolinite and montmorillonite, while adsorbed phase only existed on the kaolinite surface. MD simulation showed that water demonstrated competitive adsorption with n-dodecane on montmorillonite surfaces and prevented the formation of the adsorbed phase while having little influence on the n-dodecane adsorption on kaolinite surface, which agrees well with the kinetic analysis of the TGA experiments. With the combination of macroscopic experimental analysis and microscopic molecular simulation, it can be concluded that the mass of the NAPL phase limited the desorption behavior, while the interaction between the clay mineral surface and n-dodecane was the key factor that dominated the thermal desorption behavior of the adsorbed phase. The presented results provide new insight into the desorption mechanism of hydrocarbon on clay minerals, which is of significance for the design of thermal desorption remediation.

Modeling of soil gas radon as an in situ partitioning tracer for quantifying LNAPL contamination

In the last decades radon (Rn) has been widely proposed as a naturally occurring tracer for non-aqueous phase liquids (NAPL) in the soil. This work examines the feasibility of using soil gas data collected at some distance from the source zone for the application of the Rn deficit technique for the identification and quantification of NAPL contamination. To this end, we used a steady-state 1-D analytical solution that is based on a 3-layer model that allows to simulate the transport and distribution of Rn in the source zone, capillary fringe and overlying unsaturated soil. The analytical solution was first validated against a more detailed numerical model available in the literature. Then, a series of simulations were carried out to evaluate the vertical concentration profiles of Rn in soil gas above the source zone and in background location not impacted by NAPL. Simulation results showed that the parameters that most influence the migration and distribution of Rn in the subsurface are the distance of the soil gas probe from the source zone and, to a lower extent, the type of contamination (e.g. diesel or gasoline) and the soil type. On the basis of these results, we developed some easy-to-use nomographs to estimate the residual NAPL phase based on the observed radon deficit in soil gas and on the probe to source distance and soil and NAPL characteristics. According to the obtained results, the radon deficit technique results a feasible method for a qualitative identification of residual NAPL when radon in soil gas is measured at distances lower than 2 m from the contaminated zone. However, for an accurate quantitative estimation of the NAPL phase content, soil gas probes should be preferably located at distances lower than 1 m from the source zone.

Surfactant flooding makes a comeback: Results of a full-scale, field implementation to recover mobilized NAPL

Non-aqueous phase liquid (NAPL) remediation techniques using surfactants, such as enhanced pump and treat (also known as Surfactant–Enhanced Aquifer Remediation, “SEAR”) and micellar flooding provide a faster and more efficient way to recover NAPL from the subsurface. Micellar flooding is a recovery technique that relies on the ability of surfactants to mobilize the NAPL phase by reducing the interfacial tension between the aqueous phase and the NAPL. The application of micellar flooding for NAPL recovery has been limited to laboratory studies and some pilot–scale field applications primarily due to concerns that the technology might lead to uncontrolled movement of NAPL. This paper presents results from a full-scale field application of the micellar flood process designed to mobilize and recover an LNAPL (Jet fuel) from a surficial sandy aquifer located at a tank facility in western Jutland, Denmark. Phase behavior and flow experiments were conducted with field samples to identify suitable surfactant formulations. A field–scale simulation model was developed that indicated that a line–drive pattern with hydraulic control wells would be optimal for NAPL recovery. In addition to monitoring during the field implementation, monitoring was conducted immediately after and for a period of >1 year. The field implementation resulted in >90% recovery (approximately 36,000 Kg of LNAPL) based on the mass balance using laser–induced fluorescence (LIF) and chemical soil analysis (total petroleum hydrocarbon or TPH and BTEX) data. Post–surfactant flood site monitoring consisted of sampling water from multi–levels and from recovery wells. Groundwater samples were analyzed for total petroleum hydrocarbon (TPH) and benzene, toluene, ethylbenzene and xylene (BTEX). The pre–treatment and post–treatment mass discharges were also monitored, which led to a relationship between mass discharge with the mass reduction in the source zone. Also, the mass discharge Γ–model commonly used for DNAPL modeling was successfully implemented for LNAPL remediation. Studies of field applications of surfactant remediation processes are not readily available; it is especially rare to present a study of micellar flooding implementation for full-scale remediation processes.

Combining spectral induced polarization with X-ray tomography to investigate the importance of DNAPL geometry in sand samples

Although many studies have been performed to investigate the spectral induced polarization (SIP) response of nonaqueous phase liquid (NAPL)-contaminated soil samples, there are still many uncertainties in the interpretation of the data. A key issue is that altered pore space geometries due to the presence of a NAPL phase will change the measured IP spectra. However, without any information on the NAPL distribution in the pore space, assumptions are necessary for the SIP data interpretation. Therefore, experimental data of SIP signals directly associated with different NAPL distributions are needed. We used high-resolution X-ray tomography and 3D image processing to quantitatively assess NAPL distributions in samples of fine-grained sand containing different concentrations of tetrachloroethylene and link this to SIP measurements on the same samples. The total concentration of the sample constituents as well as the volumes of the individual NAPL blobs were calculated and used for the interpretation of the associated SIP responses. The X-ray tomography and image analysis showed that the real sample properties (porosity and NAPL distributions) differed from the targeted ones. Both contaminated samples contained less NAPL than expected from the manual sample preparation. The SIP results showed higher real conductivity and lower imaginary conductivity in the contaminated samples compared to a clean sample. This is interpreted as an effect of increased surface conductivity along interconnected NAPL blobs and decreased surface areas in the samples due to NAPL blobs larger than and enclosing grains. We conclude that the combination of SIP, X-ray tomography, and image analysis is a very promising approach to achieve a better understanding of the measured SIP responses of NAPL-contaminated samples.

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.

Application of nanotechnology in removal of NAPLs from contaminated aquifers: a source clean-up experimental study

This work investigates the removal of non-aqueous phase liquids (NAPLs) from groundwater resources using nanotechnology. We present results of a series of multiphase fluid displacement experiments conducted in a naturally occurring sandstone rock. These experiments involve injection of an aqueous suspension of silica nanoparticles to remove a trapped NAPL phase. Specifically, the effect of nanoparticle concentration on the efficiency of the NAPL removal is studied. Our results show that silica nanoparticles successfully remobilised the trapped NAPL phase and resulted in 13% increase in its removal efficiency. The optimal concentration for NAPL removal efficiency is found to be 0.3 wt%.

Impact of Soil Heterogeneity and NAPL Presence on Stable Carbon Isotope Signature Distribution During Reactive Transport

Multiphase flow and transport simulations were conducted to investigate the impact of soil heterogeneity and non-aqueous phase liquid (NAPL) presence on the distribution of stable carbon isotope signatures during contaminant transport with biodegradation. At a later time during the simulation of a homogeneous case with dense NAPL presence, significant carbon isotope signature (δ13C) values could only be observed in a narrow area at the bottom of the aquifer where NAPL accumulated. After this, the δ13C distribution remained relatively stable for a long time until all NAPL was dissolved into the groundwater and removed via biodegradation and groundwater flushing. These characteristics of δ13C distribution may only be captured when considering NAPL migration and dissolution. The simulation results demonstrated that δ13C values and their distribution significantly differed between the heterogeneous case and the homogeneous case, with respect to the maximum δ13C value and the shape of δ13C contours. When reaction rate constant varied for each soil type (each grid block) by relating it to soil permeability, the δ13C distribution demonstrated different patterns. In addition to geological heterogeneity, this indicates that the distribution of δ13C highly depends on the biological heterogeneity in the field. Therefore, this study suggests that, to avoid misinterpretation of isotope signature changes, geological and biological soil heterogeneities should be investigated. If a NAPL is present in the system, the NAPL phase transport and dissolution should be considered in addition to dissolved phase transport.

Sherwood correlation for dissolution of pooled NAPL in porous media

The rate of interphase mass transfer from non-aqueous phase liquids (NAPLs) entrapped in the subsurface into the surrounding mobile aqueous phase is commonly expressed in terms of Sherwood (Sh) correlations that are expressed as a function of flow and porous media properties. Because of the lack of precise methods for the estimation of the interfacial area separating the NAPL and aqueous phases, most studies have opted to use modified Sherwood expressions that lump the interfacial area into the interphase mass transfer coefficient. To date, there are only two studies in the literature that have developed non-lumped Sherwood correlations; however, these correlations have undergone limited validation. In this paper controlled dissolution experiments from pooled NAPL were conducted. The immobile NAPL mass is placed at the bottom of a flow cell filled with porous media with water flowing horizontally on top. Effluent aqueous phase concentrations were measured for a wide range of aqueous phase velocities and for two different porous media. To interpret the experimental results, a two-dimensional pore network model of the NAPL dissolution kinetics and aqueous phase transport was developed. The observed effluent concentrations were then used to compute best-fit mass transfer coefficients. Comparison of the effluent concentrations computed with the two-dimensional pore network model to those estimated with one-dimensional analytical solutions indicates that the analytical model which ignores the transport in the lateral direction can lead to under-estimation of the mass transfer coefficient. Based on system parameters and the estimated mass transfer coefficients, non-lumped Sherwood correlations were developed and compared to previously published data. The developed correlations, which are a significant improvement over currently available correlations that are associated with large uncertainties, can be incorporated into future modeling studies requiring non-lumped Sh expressions.

Effect of NAPL Source Morphology on Mass Transfer in the Vadose Zone.

The generation of vapor-phase contaminant plumes within the vadose zone is of interest for contaminated site management. Therefore, it is important to understand vapor sources such as non-aqueous-phase liquids (NAPLs) and processes that govern their volatilization. The distribution of NAPL, gas, and water phases within a source zone is expected to influence the rate of volatilization. However, the effect of this distribution morphology on volatilization has not been thoroughly quantified. Because field quantification of NAPL volatilization is often infeasible, a controlled laboratory experiment was conducted in a two-dimensional tank (28 cm × 15.5 cm × 2.5 cm) with water-wet sandy media and an emplaced trichloroethylene (TCE) source. The source was emplaced in two configurations to represent morphologies encountered in field settings: (1) NAPL pools directly exposed to the air phase and (2) NAPLs trapped in water-saturated zones that were occluded from the air phase. Airflow was passed through the tank and effluent concentrations of TCE were quantified. Models were used to analyze results, which indicated that mass transfer from directly exposed NAPL was fast and controlled by advective-dispersive-diffusive transport in the gas phase. However, sources occluded by pore water showed strong rate limitations and slower effective mass transfer. This difference is explained by diffusional resistance within the aqueous phase. Results demonstrate that vapor generation rates from a NAPL source will be influenced by the soil water content distribution within the source. The implications of the NAPL morphology on volatilization in the context of a dynamic water table or climate are discussed.

Steady State DNAPL Dissolution in Three-Dimensional Fractured Sandstone Network Experiments

The distribution of residual dense nonaqueous phase liquid (DNAPL) in the subsurface plays a critical role in the DNAPL dissolution kinetics. However, measuring residual DNAPL at the field scale in fractured bedrock settings is generally impractical. This research uses a three-dimensional (3D), bench-scale, fractured-rock network comprised of low-porosity sandstone to evaluate the dissolution kinetics of tetrachloroethylene (PCE) DNAPL at residual saturation during ambient groundwater conditions. To our knowledge, this work presents the first experiments to investigate DNAPL dissolution in 3D bench-scale fractured systems. DNAPL dissolution in the relatively uniform fracture network was evaluated and described using an effective parameter, the bulk mass transfer coefficient (KL). Results from dissolution experiments revealed a positive, statistically significant correlation between KL and DNAPL-water interfacial area, and between KL and DNAPL saturation, analogous to porous media experiments. While aperture size and uniformity influenced DNAPL trapping and interfacial area in the fracture network experiments, DNAPL-altered aperture size did not have a statistically significant influence on dissolution rates. The mass transfer behavior for our 3D fracture network was more similar to one-dimensional (1D) porous media experiments than to two-dimensional (2D) and 3D porous media experiments, which primarily used nonaqueous phase liquid (NAPL) pools. This result is likely due to a combination of the coexistence of the nonwetting NAPL phase and the flowing-aqueous phase in the larger fractures or at fracture intersections, as well as better mixing due to dynamic flow pathways in the network. Finally, residual NAPL saturations could not be replicated in the networks, possibly due to the unpredictable flow-switching behavior of multiphase fluids in fractured rock.

A review on the aging phenomena of organic components and their mass transfer through the NAPL interfacial phase

It occurs worldwide that the organic components of non-aqueous phase liquid (NAPL) enter the porous medium and become the source of contaminants in the subsurface. The transport of the organic components through NAPL interphase into the aqueous phase and the subsurface determines the extent of contamination, the persistence of residual NAPL phases and the techniques of remediation. During the transport process the NAPL interphase may experience “aging”, a physical and chemical change when NAPL is exposed to aqueous and or gaseous phases. This aging process alters vice versa the mass transfer behaviour of the organic contaminants in the porous medium.

介质非均质性对蒸汽强化SVE法修复TCE污染影响的二维土箱模拟研究<br>2D-Box Study on Effects of Media Heterogeneity on Remediation of TCE by Steam Enhanced Extraction

蒸汽强化SVE法(Steam Enhanced Extraction)是一个新近发展起来的并具有较好应用前景的土壤包气带非水相液体(NAPLs)污染修复方法，是把蒸汽和空气的混合气体注入污染土壤促进污染物的挥发，利用气流将土壤中污染物带出的技术。其实质是在人为施加的工程条件下非饱和多孔介质中NAPLs的运移转化过程。介质非均匀性影响着NAPLs去除过程，对NAPLs去除效率存在一定的影响。本研究选取三氯乙烯(Trichloroethylene, TCE)为典型污染物，采用不同质地的石英砂(粗砂、细砂和粉砂)模拟介质非均匀性，通过二维土箱气体模拟实验，分析介质非均质性对蒸汽强化气体抽提修复中TCE去除效率和去除过程的影响。结果表明：蒸汽强化SVE法较好的改进了SVE法去除过程中的“拖尾”现象。粗砂实验、细砂实验、粉砂实验的去除率分别为89.5%、88.2%、85.2%，受介质非均质的影响，细砂、粉砂区域的气体通透性降低，气体流经该区域出现“绕流”现象，从而减少了TCE随空气的去除量。同时介质非均质性影响实验进程中热量的传递，使热在非均质砂土中前进不均匀，进而影响TCE的去除过程，降低其去除效率。在细砂实验和粉砂实验里，均出现污染羽积累与下渗的现象，进一步降低了TCE去除效率。 Steam Enhanced Extraction is a newly developed and promising soil remediation technique for non-aqueous phase liquids (NAPLs) in vadose zone. Mixture of steam and air is injected into the contaminated soil to promote the volatilization of the pollutants. The remediation is a process of NAPLs transformation in unsaturated porous media un-der artificial engineering condition. Removal process and efficiency may be greatly impacted by medium heterogeneity. In this study, trichloroethylene (TCE) removal using Steam Enhanced Extraction was carried out in a 2-dimensional sandbox with three different layered sand structures under unsaturated conditions. Three kinds of sands (coarse, fine and silt) were used to simulate heterogeneous media. The results show that SEE perfectly improves “tailing” effect com-pared to soil vapor extraction (SVE). And the removal ratios from the three experiments in 100 minutes are 89.5%, 88.2% and 85.2%. Meanwhile, there were by-passing flows when gas through the coarse sand and fine sand area, which affected the total removal ratio and process. The layered structure with finer sand, in which the steam and air convection was faster, resulted in faster removal rate and larger removal ratio compared to that with clay sand. In the experiments packed with finer or clay sand, a small amount of NAPL phase TCE was cumulated, which also reduced the total re-move ratio.

Contaminant Mass Transfer from NAPLs to Water Studied in a Continuously Stirred Flow-Through Reactor

AbstractThe release of nonaqueous-phase liquids (NAPLs) from porous media to groundwater is a widespread environmental problem. The mass transfer of individual NAPL components controls both the extent of groundwater contamination and the persistence of the residual NAPL phase. In order to quantify this key process, small-scale experimental studies on NAPL-water mass transfer were performed in a dynamic system mimicking environmental conditions with “clean” water continuously flowing through the NAPL pool. To describe this process, a modified simulation method was developed and validated by the experimental data. The experimental system consisted of a custom-designed flow cell (with NAPL and water) connected to the peripheral equipment (e.g., pump, water source). This continuously stirred flow-through reactor was used to perform mass transfer experiments with simple and complex model NAPL–water systems. To simulate the experimental data (concentration versus time profiles of individual NAPL compounds), an ...

Determinability of NAPL mass by measuring concentrations downstream of a contaminant source

Organic contaminants in aquifers are often present as non-aqueous phase liquids (NAPL), which are long-lasting sources for groundwater contamination. The existing NAPL mass is an important parameter for the persistence of the source, but its determination is difficult. One possible detection method is based on the ideal multicomponent dissolution theory, using aqueous concentrations downstream of a fully mixed NAPL source to calculate its mass. In this publication, the applicability of this method is tested for a source size of about 5 m, using numerical methods. In contrast to fully mixed source zones, on this scale the NAPL sources are not in contact with each other, do not mix and develop independently over time. Highly soluble NAPL components can be depleted or the NAPL phase can be completely exhausted locally, while in other portions of the source zone NAPL is still present with all its components. Hence, the interpretation of the resulting aqueous concentrations downstream using the ideal dissolution theory leads to erroneous NAPL masses of several orders of magnitude in the investigated scenarios.

Effects of aging and mixed nonaqueous‐phase liquid sources in soil systems on earthworm bioaccumulation, microbial degradation, sequestration, and aqueous desorption of pyrene

The effects of loading and aging pyrene in soils in the presence of four environmentally common nonaqueous-phase liquids (NAPLs) (hexadecane, 2,2,4,4,6,8,8-heptamethylnonane [HMN], toluene, and dimethyl phthalate [DMP]) on its subsequent desorption from those soils, earthworm accumulation, biodegradation, and extractability were tested by using two dissimilar soils. The presence of each of the four NAPLs increased fractions and rates of pyrene desorption, and hexadecane slowed the effects of aging on these same parameters. Loading with hexadecane and HMN caused earthworm accumulation of pyrene to decrease. These results contrast with generally observed faster desorption rates resulting from NAPL addition, suggesting that additional factors (e.g., association of pyrene with NAPL phases and NAPL toxicities to earthworms) may impact bioaccumulation. The presence of HMN and toluene increased pyrene biodegradation, whereas hexadecane and DMP had the opposite effects. These results correlate with changes in the extractability of pyrene from the soils. After aging and biodegradation, hexadecane and DMP substantially increased pyrene residues extractable by methanol and decreased nonextractable fractions, whereas HMN and toluene had the opposite effects.