Enhanced Soil Vapor Extraction Research Articles

Transient Simulation of 1, 4-Dioxane Fate and Transport During Enhanced Soil Vapor Extraction Treatment

1, 4-Dioxane is an emerging contaminant a that is fully miscible in water and resistant to biodegradation. The presence of 1, 4-Dioxane in vadose zone creates prolonged environmental issues and risk human health. To date, treatment technologies for 1, 4-Dioxane vadose zone remediation are still being developed. In this work, a numeric simulation tool was developed to investigate the fate and transport of 1, 4-Dioxane in the treatment zone of enhanced soil vapor extraction (XSVE) which utilizes heated air injection to improve contaminant removal efficacy. The results suggested identify source zone is critical for XSVE treatment, inaccurate heat injection point would prolong treatment duration.

A soil-column gas chromatography (SCGC) approach to explore the thermal desorption behavior of hydrocarbons from soils.

A soil-column gas chromatography approach was developed to simulate the mass transfer process of hydrocarbons between gas and soil during thermally enhanced soil vapor extraction (T-SVE). Four kinds of hydrocarbons-methylbenzene, n-hexane, n-decane, and n-tetradecane-were flowed by nitrogen gas. The retention factor k' and the tailing factor T f were calculated to reflect the desorption velocities of fast and slow desorption fractions, respectively. The results clearly indicated two different mechanisms on the thermal desorption behaviors of fast and slow desorption fractions. The desorption velocity of fast desorption fraction was an exponential function of the reciprocal of soil absolute temperature and inversely correlated with hydrocarbon's boiling point, whereas the desorption velocity of slow desorption fraction was an inverse proportional function of soil absolute temperature, and inversely proportional to the log K OW value of the hydrocarbons. The higher activation energy of adsorption was found on loamy soil with higher organic content. The increase of carrier gas flow rate led to a reduction in the apparent activation energy of adsorption of slow desorption fraction, and thus desorption efficiency was significantly enhanced. The obtained results are of practical interest for the design of high-efficiency T-SVE system and may be used to predict the remediation time.

Electrode Design for Soil Decontamination with Radio‐Frequency Heating

AbstractRadio‐frequency heating to enhance soil decontamination requires adjusted solutions for the electrode design depending on scale and remediation technique. Parallel plate electrodes provide widely homogeneous field and temperature distributions and are, therefore, most suitable for supporting biodegradation processes. For thermally enhanced soil vapor extraction, certain temperature gradients can be accepted and, therefore, the less‐demanding geometry of rod‐shaped electrodes is usually applied. For electrode lengths of some meters, a design with an air gap has to be used in order to focus heating to the desired depth. Perforated rod electrodes may be simultaneously employed as extraction wells. Placing an oxidation catalyst in situ within the electrodes is an option for handling of highly loaded air flows. Coaxial antenna may be utilized to selectively heat soil compartments far from the surface of the soil.

Influence of in Situ Steam Formation by Radio Frequency Heating on Thermodesorption of Hydrocarbons from Contaminated Soil

Thermal desorption of a wide spectrum of organic contaminants, initiated by radio frequency (RF) heating, was studied at laboratory and pilot-plant scales for an artificially contaminated soil and for an originally contaminated soil from an industrial site. Up to 100 °C, moderate desorption rates were observed for light aromatics such as toluene, chlorobenzene, and ethylbenzene. Desorption of the less volatile contaminants was greatly enhanced above 100 °C, when fast evaporation of soil-water produced steam for hydrocarbon stripping (steam-distillation, desorption rates increased by more than 1 order of magnitude). For hydrocarbons with low water solubility (e.g., aliphatic hydrocarbons), the temperature increase above 100 °C after desiccation of soil again led to a significant increase of the removal rates, thus showing the impact of hydrocarbon partial pressure. RF heating was shown to be an appropriate option for thermally enhanced soil vapor extraction, leading to efficient cleaning of contaminated soils.

Risk-based economic decision analysis of remediation options at a PCE-contaminated site

Remediation methods for contaminated sites cover a wide range of technical solutions with different remedial efficiencies and costs. Additionally, they may vary in their secondary impacts on the environment i.e. the potential impacts generated due to emissions and resource use caused by the remediation activities. More attention is increasingly being given to these secondary environmental impacts when evaluating remediation options. This paper presents a methodology for an integrated economic decision analysis which combines assessments of remediation costs, health risk costs and potential environmental costs. The health risks costs are associated with the residual contamination left at the site and its migration to groundwater used for drinking water. A probabilistic exposure model using first- and second-order reliability methods (FORM/SORM) is used to estimate the contaminant concentrations at a downstream groundwater well. Potential environmental impacts on the local, regional and global scales due to the site remediation activities are evaluated using life cycle assessments (LCA). The potential impacts on health and environment are converted to monetary units using a simplified cost model. A case study based upon the developed methodology is presented in which the following remediation scenarios are analyzed and compared: (a) no action, (b) excavation and off-site treatment of soil, (c) soil vapor extraction and (d) thermally enhanced soil vapor extraction by electrical heating of the soil. Ultimately, the developed methodology facilitates societal cost estimations of remediation scenarios which can be used for internal ranking of the analyzed options. Despite the inherent uncertainties of placing a value on health and environmental impacts, the presented methodology is believed to be valuable in supporting decisions on remedial interventions.

A Laboratory and Pilot Study of Thermally Enhanced Soil Vapor Extraction Method for the Removal of Semi-Volatile Organic Contaminants

Laboratory and field pilot study was conducted to evaluate the effectiveness of thermally enhanced soil vapor extraction (SVE) system for the removal of semi-volatile organic contaminants (SVOCs) from soils. Several parametric studies were conducted to evaluate the effect of temperature and air flow rate on the removal of SVOCs. Column studies near ambient temperature show very little efficiency for SVOCs removal. The removal rates of SVOCs are highly dependent on temperature. Moving fronts in the effluent concentrations of contaminants are observed, according to the order of volatility of contaminant. This result is attributed to continuous changes in the liquid composition toward less volatile fractions, thus increasing the mole fraction of the less volatile fractions remaining in the liquid mixture. It is also found that increased air flow rate results in high removal rate of contaminants. However, too high air flow rate brings about the mass transfer limitation on the volatilization of the contaminants. Results from steady-state column studies show that mass transfer limitation is reduced at high temperature, possibly due to the increase in the diffusivity of contaminant in the liquid and gas phase. A field pilot study of a hot air injection method for the remediation of diesel-contaminated soil has shown advantages over the conventional SVE. Within 30 days of test operation, TPH concentrations at unsaturated soils were dramatically reduced, corresponding to more than 95% removal. However, temperature profiles and the removal rates of SVOCs near groundwater level indicate that moisture content would be a limiting factor in the real application of hot air injection.

Application of Nonisothermal Multiphase Modeling to In Situ Soil Remediation in Söderkulla

Nonisothermal multiphase modeling with the T2VOC numerical simulator was used as an integration tool in an in situ remediation project in which trichloroethylene (TCE)‐contaminated soil was treated with thermally (steam) enhanced soil vapor extraction. Numerical simulation studies were employed in the iterative conceptualization of the migration of the contaminants and the effects of different treatment operations, in the planning of optimal system design and control of the remediation processes, as well as for integration of information obtained from field investigations during different phases of the project. Modeling proved to be a valuable tool in planning and decision making. It was, however, difficult to forecast the costs or the duration of the project, since the actual TCE mass and distribution was poorly known. The power of modeling lies in its ability to compare the effect of different strategies or measures on the cleanup efficiency, and in studying the associated effects during the treatments.

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.

Predicting hydrocarbon removal from thermally enhanced soil vapor extraction systems: 1. Laboratory studies

Conventional soil vapor extraction (SVE) systems have a limited effectiveness at removing semi-volatile chemicals from soil. Raising chemical vapor pressures by heating soil in-situ can decrease remediation time and help remove semi-volatile chemicals that otherwise would not be removed by conventional SVE. The increased compound removal rate that results from use of thermally enhanced SVE was investigated in laboratory studies. Increased soil temperatures (50–150°C) increased both the rate of removal of the compounds studied and the range of compounds that were removed in column studies. The column studies indicated that if soil temperatures are raised enough to elevate the vapor pressure of a compound above 70 Pa, SVE will remove most of the compound from the soil. Thermally enhanced column study hydrocarbon removal rate constants were shown to have a definable relationship with vapor pressure. The relative removal rate constants also demonstrated an Arrhenius relationship with temperature. Laboratory studies can be used to develop these relationships and the results can be extrapolated within certain temperature ranges and compound types for a given soil.

Predicting hydrocarbon removal from thermally enhanced soil vapor extraction systems: 2. Field study

The effectiveness of a thermally enhanced soil vapor extraction (SVE) system to remove semi-volatile organic chemicals (SVOCs) was investigated in a field study. The data allowed the calculations of SVOC removal rates at several temperatures. A previous laboratory study using the same field soils had developed a relationship between SVOC removal rate constants and inverse temperature. The laboratory and the field SVOC removal rate constants were compared and a linear log–log relationship between the laboratory and the field SVOC removal rate constants resulted. Subsequent analyses indicated that it was possible to use laboratory determined SVOC removal rate relationships to estimate SVOC removal from in situ field soil. The time dependence of SVOC concentration reduction using in situ thermally enhanced SVE systems was also shown.

Vadose Zone Characterization at a Contaminated Field Site Using Partitioning Interwell Tracer Technology

The partitioning interwell tracer test (PITT) has been demonstrated to be effective in determining volume and spatial distribution of residual nonaqueous phase liquids (NAPLs) in the saturated subsurface. Use of PITT technology in the vadose zone has been limited to this point. Two PITTs were completed in the vadose zone of a NAPL contaminated site at Kirtland Air Force Base in Albuquerque, NM. The PITTs were completed before and after a thermally enhanced soil vapor extraction remediation effort, to provide performance assessment. Methane was used as a nonpartitioning tracer. Four perfluorocarbons (perfluoromethylcyclohexane, perfluoro-1,3-dimethylcyclohexane, perfluoro-1,3,5-trimethylcyclohexane, and perfluorodecalin) were utilized as air−NAPL partitioning tracers. Difluoromethane was used as an air−water partitioning tracer to estimate water saturation. The complex light NAPL (LNAPL) consisted of hydrocarbon fuels ranging from approximately 12−70 carbons in size. Laboratory experimental data and thermodynamic modeling were used to characterize compositional changes in the NAPL due to the remediation effort. About 260 kg of NAPL were removed from the subsurface by the remediation effort. The PITT assessment of the remediation was compared favorably to the results from soil borings taken at the site. The PITT has been shown to be a useful technology for both NAPL characterization and remedial performance assessment in the vadose zone.

In‐Line Measurement of Trichloroethylene Vapors Using Tin Dioxide Sensors

AbstractDuring thermally enhanced in situ remediation of soils and ground water, gas streams are generated with varying temperatures, moisture content, and organic compound concentrations. In this study, we evaluated the performance of tin dioxide sensors for measuring trichloroethylene (TCE) concentrations in gas streams from a thermally enhanced soil vapor extraction system. Temperature, pressure, moisture content, and vapor flow rates affected the resistivity of the sensors, and thus the signal. When fluctuations in these parameters were eliminated by condensing excess water and healing to a constant temperature prior to measurement, the sensors provided reliable in‐line measurement of TCE concentrations. Gas tracers such as methane were easily monitored in‐line, providing quick and inexpensive data on subsurface vapor flow velocities and direction.

Examining the economics of remediation by fluid injection with vacuum extraction

AbstractEnhanced methods of in‐situ remediation based on patented technology involving fluid injection with vacuum extraction have been used successfully at the Sand Creek Superfund Site in Commerce City, Colorado. Approximately 177,000 pounds of volatile organic compounds (VOCs) were removed from the subsurface in six months, two months ahead of schedule. Remediation goals were achieved on this thermally enhanced soil vapor extraction project by using vertical and horizontal wells interchangeably in vacuum or pressure service for vapor extraction, dual vacuum extraction, heated vapor reinjection, and air sparging. Although VOCs consisted of mixed chlorinated and petroleum hydrocarbons, the petroleum hydrocarbons, some in the form of nonaqueous phase liquids, had not been fully characterized. This article examines the evolution of the remedial design from that conceptualized in the Record of Decision (ROD) of the U.S. EPA, presents the rationale for the selection of alternative system components, and provides a cost analysis of the selected remedial technology, with comparisons to that of alternatives considered for use at Sand Creek.