Soil Vapor Extraction Process Research Articles

NON-ISOTHERMAL MODELING OF SOIL VAPOR EXTRACTION SYSTEM INCLUDING SOIL TEMPERATURE EFFECT

Soil vapor extraction (SVE) is a proven effective in-situ technology for the removal of volatileorganic compounds (VOCs) from the subsurface. SVE process is highly sensitive to temperature.Studying annual soil temperature variation with depth declares that there is a considerable temperaturevariation in the upper few meters that may affect the overall efficiency of SVE process.A numerical model was developed to aid in investigation of field-scale soil vapor extractionprocess. The model is three-dimensional, time dependent that simulates nonisothermal vapor flow andtransport of multicomponent mixtures in soil and keeps track of the distribution of each compound inthe other three immobile phases (NAPL, aqueous, and sorbed). Rate limited interphase mass transferwith linear driving force expressions were used to model volatilization of oil into gas phase. A localequilibrium partitioning was assumed between gas, water, and solid phase. The model equations werediscretized using a standard Galerkin finite element method and solved using set iterative solutionalgorithm.Simulation of hypothetical field-scale problems was done. The physical domain described a threedimensionalsystem with flow to a single extraction well. A hypothetical soil temperature variationwith depth was incorporated with the model. The result of these simulations showed that thistemperature variation has a considerable effect on system efficiency and may play a role in optimumsystem configuration.

Predicting soil concentrations and remediation target values of BTEX by an off-gas based mass transfer model

Soil vapor extraction (SVE) is a widely used technology for the remediation of volatile organic compounds (VOCs) contaminated soils. Residual concentrations of VOCs are crucial for assessing the SVE process and planning when to stop this process, however, the measurement of their residual concentrations in the soil is complicated. Herein, a pseudo-first-order sequential reaction model was established to predict the mass transfer of the BTEX (benzene, toluene, ethylbenzene, xylene) between the soil and off-gas during the SVE process. Based on this mass transfer model, the residual concentrations of BTEX in the soil during the trailing stage could be accurately estimated (R2 > 0.89) by their off-gas concentrations that were directly monitored in real time. Considering the removal efficiency and operating costs, a concept of the remediation target values (RTV) was proposed for the SVE technology, and its relevant model (R2 > 0.92, NRMSE = 6.4–16.8 %) was established based on the experimental data. The remediation endpoint can be further estimated based on the RTV with an overall accuracy of 84–100 %. These findings provide a simple and fast way to predict VOC concentrations in soil with easy-to-know factors and online monitoring of off-gas concentrations and will guide and optimize the SVE process toward more economical and efficient techniques for soil remediation.

Combining Soil Vapor Extraction and Electrokinetics for the Removal of Hexachlorocyclohexanes from Soil.

This paper focuses on the evaluation of the mobility of four hexachlorocyclohexane (HCH) isomers by soil vapor extraction (SVE) coupled with direct electrokinetic (EK) treatment without adding flushing fluids. SVE was found to be very efficient and remove nearly 70 % of the four HCH in the 15-days of the tests. The application of electrokinetics produced the transport of HCH to the cathode by different electrochemical processes, which were satisfactorily modelled with a 1-D transport equation. The increase in the electric field led to an increase in the transport of pollutants, although 15 days was found to be a very short time for an efficient transportation of the pollutants to the nearness of the cathode. Loss of water content in the vicinity of the cathode warns about the necessity of using electrokinetic flushing technologies instead of simple direct electrokinetics. Thus, results point out that direct electrokinetic treatment without adding flushing fluids produced low current intensities and ohmic heating that contributes negatively to the performance of the SVE process. No relevant differences were found among the removal of the four isomers, neither in SVE nor in EK processes.

Removal of Aliphatic Hydrocarbons from Gas Oil Contaminated Clay Soil via Soil Vapor Extraction

In this research, the performance of soil vapor extraction (SVE) for the removal of aliphatic hydrocarbons from gas oil contaminated soil and the effect of soil type on this method is studied. To explore the effect of soil type in the removal of hydrocarbons, SVE tests were conducted on 3 types of soil: 1) fine sand, 2) fine sand with 20% of kaolinite clay and 3) fine sand with 40% of kaolinite clay. Three extraction periods of 8, 16 and 24 hours were used. The results have shown that the efficiency of this method exceeds 78% in the removal of all hydrocarbons from fine sand, while an increase in clay significantly decreases the efficiency. Accordingly, the efficiency of this method decreases by 53.1% and 54.6% in fine sand with 20% of clay and fine sand with 40% of clay respectively. Furthermore, the results show that the higher concentration of some heavy and medium hydrocarbon leads to the more increase in early hours of SVE process, which indicates the alteration of hydrocarbons into each other.

Degradation of off-gas toluene in continuous pyrite Fenton system

Degradation of off-gas toluene from a toluene reservoir and a soil vapor extraction (SVE) process was investigated in a continuous pyrite Fenton system. The removal of off-gas toluene from the toluene reservoir was >95% by 8h in the pyrite Fenton system, while it was ∼97% by 3h in classic Fenton system and then rapidly decreased to initial level by 8h. Continuous consumption of low Fe(II) concentration dissolved from pyrite surface (0.05–0.11mM) was observed in the pyrite Fenton system, which can lead to the effective and successful removal of the gas-phase toluene due to stable production of OH radical (OH). Inhibitor and spectroscopic test results showed that OH was a dominant radical that degraded gas-phase toluene during the reaction. Off-gas toluene from the SVE process was removed by 96% in the pyrite Fenton system, and remnant toluene from rebounding effect was treated by 99%. Main transformation products from toluene oxidation were benzoic acid (31.4%) and CO2 (38.8%) at 4h, while traces of benzyl alcohol (1.3%) and benzaldehyde (0.7%) were observed. Maximum operation time of continuous pyrite Fenton system was estimated to be 56–61d and its optimal operation time achieving emission standard was 28.9d.

Carbon Tetrachloride Flow and Transport in the Subsurface of the 216‐Z‐9 Trench at the Hanford Site

As a result of past practices, up to 580 m3 carbon tetrachloride (CT) was discharged to waste sites at the 200 West Area of the USDOE's Hanford Site near Richland, WA. Three‐dimensional modeling was conducted to enhance the current conceptual model of CT distribution beneath the major disposal site (216‐Z‐9). The simulations, using the STOMP code, focused on migration of dense nonaqueous phase liquid (DNAPL) consisting of CT and codisposed organics under scenarios with differing sediment properties, sediment distribution, waste properties, and waste disposal history. Simulation results support a conceptual model for CT distribution where CT in the DNAPL phase migrated primarily in a vertical direction below the disposal site and where some CT DNAPL likely migrated across the water table into the regional aquifer. Results also show that the lower permeability Cold Creek unit retained more CT DNAPL within the vadose zone than other hydrologic units during the infiltration and redistribution process. Due to the relatively high vapor pressure of the CT, the resulting vapor plumes are extensive and influenced by density‐driven advection. Any continued migration of CT from the vadose zone to the groundwater is likely through interaction of vapor phase CT with the groundwater and not through continued DNAPL migration. Additional simulations assessed the impacts of soil vapor extraction (SVE) as a remediation method. These simulations showed rapid CT removal associated with the assumed local equilibrium of CT between the phases. Additional efforts are needed to enhance the understanding of rate‐limited volatilization to improve simulation of the SVE process and to provide a basis for refining the design and operation of SVE systems.

Characterization of SnO2 ceramic gas sensor for exhaust gas monitoring of SVE process.

A Figaro-type gas sensor system was investigated for the monitoring of volatile organic contaminants (VOC) in the exhaust gas from a soil vapor extraction (SVE) process. Benzene, toluene, ethyl benzene and xylene (BTEX), and their mixtures, were tested as representative contaminants. Reasonably good correlation factors >0.98 were obtained between the GC analyses and the sensor responses for each component, and for the total gas concentrations. Although the composition of the exhaust gas from SVE process, as well as the amount of each component, change with time, the sensor can be used to estimate the residual amount of contaminants by measuring the total concentrations in the exhaust gas. The sensor can be utilized as a valuable tool for the monitoring of SVE process by indicating when the operation to remediate a contaminated site should be stopped. The proposed ceramic gas sensor system may be a good alternative to existing methods, because it can satisfy the essential monitoring necessities of SVE processes, and has many advantages over other fully equipped instrumentation, as a cost-effective device, with long-term monitoring stability.

Solar‐powered soil vapor extraction for removal of dense nonaqueous phase organics from soil

Both surface and groundwater are threatened by buried waste at a number of sites because of subsurface plumes containing dense non‐aqueous phase liquids (DNAPLs), hazardous metals, and radionuclides. In this work, a novel process is explored which represents an enhancement to thermal extraction methods as well as a mobile system that is suitable for remediation of remote sites. Solar radiation is collected by modular concentrators which transfer the concentrated solar radiation to an optical waveguide (OW) transmission line consisting of low‐loss optical fibers. The OW line transmits the high intensity solar radiation to the contaminated site where the solar flux is injected into the soil for thermally enhanced soil vapor extraction (SVE). The organic compounds driven out of the soil go through off‐gas treatment. A small prototype was constructed and used to heat soil contaminated with trichloroethylene (TCE). The thermal efficiency of the solar SVE process conducted using the small scale experiment was approximately 50%. The moisture loss in the soil sample correlated to the soil heating time. Short heating times were sufficient to remove TCE from soil samples below detection limits.

Gas Permeability and Diffusivity in Undisturbed Soil: SVE Implications

Accurate prediction of air transport parameters in variably saturated soil is a prerequisite for realistic modeling of soil vapor extraction (SVE) systems for cleanup of contaminated soil sites. Improved models for estimating gas diffusivity and gas permeability in undisturbed sandy and loamy soils from soil total porosity, air-filled porosity, and soil-water retention properties were developed based on gas diffusivity and permeability data from the literature representing more than 3,500 undisturbed soil core samples. The new models gave significant improvements in prediction accuracy compared with existing gas diffusivity and permeability models. The new models provide good predictions of gas diffusivity and permeability from other soil parameters that are easier to measure. The models therefore seem promising for reducing expenses associated with measurements of field parameters during preliminary remedial investigations at contaminated sites. The new predictive models were used in a two-dimensional numerical transport code for simulating SVE in trichloroethylene contaminated soil to evaluate the impact of air-filled porosity on SVE efficiency. Results imply that stochastic variations in air-filled porosity cause increased contaminant removal rates in the early stages of the SVE process.