A theoretical model of air and steam co-injection to prevent the downward migration of DNAPLs during steam-enhanced extraction

When steam is injected into soil containing a dense volatile non-aqueous phase liquid contaminant the DNAPL vaporized within the heated soil region condenses and accumulates ahead of the steam condensation front. If enough DNAPL accumulates, gravitational forces can overcome trapping forces allowing the liquid contaminant to flow downward. By injecting air with steam, a portion of the DNAPL vapor remains suspended in equilibrium with the air, decreasing liquid contaminant accumulation ahead of the steam condensation front, and thus reducing the possibility of downward migration. In this work, a one-dimensional theoretical model is developed to predict the injection ratio of air to steam that will prevent the accumulation of volatile DNAPLs. The contaminated region is modeled as a one-dimensional homogeneous porous medium with an initially uniform distribution of a single component contaminant. Mass and energy balances are combined to determine the injection ratio of air to steam that eliminates accumulation of the contaminant ahead of the steam condensation front, and hence reduces the possibility of downward migration. The minimum injection ratio that eliminates accumulation is defined as the optimum injection ratio. Example calculations are presented for three DNAPLs, carbon tetrachloride (CCl4), trichloroethylene (TCE), and perchloroethylene (PCE). The optimum injection ratio of air to steam is shown to depend on the initial saturation and the volatility of the liquid contaminant. Numerical simulation results are presented to validate the model, and to illustrate downward migration for ratios less than optimum. Optimum injection ratios determined from numerical simulations are shown to be in good agreement with the theoretical model.     

Co-injection of air and steam for the prevention of the downward migration of DNAPLs during steam enhanced extraction: an experimental evaluation of optimum injection ratio predictions

When steam is injected into soil containing a dense volatile non-aqueous phase liquid contaminant, the DNAPL vaporized within the heated soil region condenses and accumulates ahead of the steam condensation front. If enough DNAPL accumulates, gravitational forces can overcome trapping forces allowing the liquid contaminant to flow downward. By injecting air with steam, a portion of the DNAPL vapor remains suspended in equilibrium with the air, decreasing liquid contaminant accumulation ahead of the steam condensation front, and thus reducing the possibility of downward migration. In a previous work, a theoretical model was developed to predict the optimum injection ratio of air to steam that would eliminate accumulation of DNAPL ahead of the temperature front and thus minimize the potential for downward migration. In this work, the theoretical model is summarized, and an experiment is presented in order to evaluate the optimum injection ratio prediction. In the experiment, a two-dimensional water saturated sand pack is contaminated with a known mass of TCE (DNAPL). The system is then remediated by co-injecting air and steam at the predicted optimum injection ratio, calculated based on the average contaminant soil concentration in the sand pack. Results for the co-injection of air and steam are compared to results for the injection of pure steam or pure air. Injection at the predicted optimum injection ratio for a volumetric average NAPL saturation, reduced accumulation of the contaminant ahead of the condensation front by over 90%, as compared to steam injection alone. This indicates that the optimum injection ratio prediction is a valuable tool for limiting the spreading of DNAPL during steam-enhanced extraction. Injection at the optimum injection ratio resulted in earlier recovery of contaminant than for steam injection alone. Co-injection of steam and air is also shown to result in much higher recovery rates than air injection alone.     

Modeling of air sparging of VOC-contaminated soil columns

Air sparging is a remediation technology currently being applied for the restoration of sites contaminated with volatile organic compounds (VOCs). Attempts have been made by various researchers to model the fate of VOCs in the gas and liquid phase during air sparging. In this study, a radial diffusion model with an air–water mass transfer boundary condition was developed and applied for the prediction of VOC volatilization from air sparging of contaminated soil columns. The approach taken was to use various parameters such as mass transfer coefficients and tortuosity factors determined previously in separate experiments using a single air channel apparatus and applying these parameters to a complex system with many air channels. Incorporated in the model, is the concept of mass transfer zone (MTZ) where diffusion of VOCs in this zone was impacted by the volatilization of VOCs at the air–water interface but with negligible impact outside the zone. The model predicted fairly well the change in the VOC concentrations in the exhaust air, the final average aqueous VOC concentration, and the total mass removed. The predicted mass removal was within 1% to 20% of the actual experimental mass removed. The results of the model seemed to suggest that air-sparged soil columns may be modeled as a composite of individual air channels surrounded by a MTZ. For a given air flow rate and air saturation, the VOC removal was found to be inversely proportional to the radius of the air channel. The approach taken provided conceptual insights on mass transfer processes during air sparging operations.     

The Transfer of Trichloroethyene (TCE) from a Shower to Indoor Air: Experimental Measurements and Their Implications

Experiments were performed to measure the transfer of trlchloroethylene (TCE), a volatile organic compound (VOC), from tap water In showers to Indoor air. In these experiments, the loss of TCE from tap water in the shower is based on the difference between Influent and effluent concentrations. We have developed and previously published a three-compartment model, which we use to simulate the 24-h concentration history of VOCs in the shower, bathroom, and remaining household volumes resulting from the use of contaminated tap water. An important input to this model is the transfer efficiency of the VOC from water to air. The experiments reveal that the transfer efficiency of TCE from shower water to air has an arithmetic mean value of 61 percent and an arithmetic standard deviation of 9 percent. Analysis of the results shows that there Is no statistically significant difference between the transfer efficiency measured with hot (37°C) or cold (22°C) shower water and that there Is no statistically significant change In transfer efficiency with time during a 20-mln shower. The implications for exposure assessment are considered.     

The transfer of trichloroethylene (TCE) from a shower to indoor air: experimental measurements and their implications

Experiments were performed to measure the transfer of trichloroethylene (TCE), a volatile organic compound (VOC), from tap water in showers to indoor air. In these experiments, the loss of TCE from tap water in the shower is based on the difference between influent and effluent concentrations. We have developed and previously published a three-compartment model, which we use to simulate the 24-h concentration history of VOCs in the shower, bathroom, and remaining household volumes resulting from the use of contaminated tap water. An important input to this model is the transfer efficiency of the VOC from water to air. The experiments reveal that the transfer efficiency of TCE from shower water to air has an arithmetic mean value of 61 percent and an arithmetic standard deviation of 9 percent. Analysis of the results shows that there is no statistically significant difference between the transfer efficiency measured with hot (37 degrees C) or cold (22 degrees C) shower water and that there is no statistically significant change in transfer efficiency with time during a 20-min shower. The implications for exposure assessment are considered.     

The transfer of trichloroethylene (TCE) from a shower to indoor air: experimental measurements and their implications

Experiments were performed to measure the transfer of trichloroethylene (TCE), a volatile organic compound (VOC), from tap water in showers to indoor air. In these experiments, the loss of TCE from tap water in the shower is based on the difference between influent and effluent concentrations. We have developed and previously published a three-compartment model, which we use to simulate the 24-h concentration history of VOCs in the shower, bathroom, and remaining household volumes resulting from the use of contaminated tap water. An important input to this model is the transfer efficiency of the VOC from water to air. The experiments reveal that the transfer efficiency of TCE from shower water to air has an arithmetic mean value of 61 percent and an arithmetic standard deviation of 9 percent. Analysis of the results shows that there is no statistically significant difference between the transfer efficiency measured with hot (37 degrees C) or cold (22 degrees C) shower water and that there is no statistically significant change in transfer efficiency with time during a 20-min shower. The implications for exposure assessment are considered.     

Experimental investigation of pneumatic soil vapor extraction

Soil Vapor Extraction (SVE) is a common remediation technique for removing volatile organic compounds from unsaturated contaminated soils. Soil heterogeneities can however cause serious limitations to the applicability of SVE due to air bypassing low permeable areas of the soil, leading to diffusion limitation of the remediation. To enhance removal from areas subject to diffusion limitation a new remediation technique, pneumatic soil vapor extraction, is proposed. In contrast to traditional SVE, in which soil vapor is extracted continuously by a vacuum pump, pneumatic SVE is based on enforcing a sequence of large pressure drops on the system to enhance the recovery from the low-permeable areas. The pneumatic SVE technique was investigated in the laboratory using TCE as a model contaminant. 2D-laboratory tank experiments were performed on homogeneous and heterogeneous sand packs. The heterogeneous packs consisted of a fine sand lens surrounded by a coarser sand matrix. As expected when using traditional SVE, the removal of TCE from the low permeable lens was extremely slow and subject to diffusion limitation. In contrast when pneumatic venting was used removal rates increased by up to 77%. The enhanced removal was hypothesized to be attributed to mixing of the contaminated air inside the lens and generation of net advective transport out of the lens due to air expansion.     

Nonideal transport of reactive contaminants in heterogeneous porous media: 7. distributed-domain model incorporating immiscible-liquid dissolution and rate-limited sorption/desorption

The purpose of this work is to present a distributed-domain mathematical model incorporating the primary mass-transfer processes that mediate the transport of immiscible organic liquid constituents in water-saturated, locally heterogeneous porous media. Specifically, the impact of grain/pore-scale heterogeneity on immiscible-liquid dissolution and sorption/desorption is represented in the model by describing the system as comprising a continuous distribution of mass-transfer domains. With this conceptualization, the distributions of the initial dissolution rate coefficient and the sorption/desorption rate coefficient are represented as probability density functions. Several sets of numerical experiments are conducted to examine the effects of heterogeneous dissolution and sorption/desorption on contaminant transport and elution. Four scenarios with different combinations of uniform/heterogeneous rate-limited dissolution and uniform/heterogeneous rate-limited sorption/desorption are evaluated. The results show that both heterogeneous rate-limited sorption/desorption and heterogeneous rate-limited dissolution can significantly increase the time or pore volumes required to elute immiscible-liquid constituents from a contaminated porous medium. However, sorption/desorption has minimal influence on elution behavior until essentially all of the immiscible liquid has been removed. For typical immiscible-liquid constituents that have relatively low sorption, the asymptotic elution tailing produced by heterogeneous rate-limited sorption/desorption begins at effluent concentrations that are several orders of magnitude below the initial steady-state concentrations associated with dissolution of the immiscible liquid. Conversely, the enhanced elution tailing associated with heterogeneous rate-limited dissolution begins at concentrations that are approximately one-tenth of the initial steady-state concentrations. Hence, dissolution may generally control elution behavior of immiscible-liquid constituents in cases wherein grain/pore-scale heterogeneity significantly influences both dissolution and sorption/desorption.     

Enhanced desorption of phenanthrene from soils using hydroxypropyl-beta-cyclodextrin: experimental results and model predictions

Objective of this study was to evaluate the effects of hydroxypropyl-beta-cyclodextrin (HPCD) on the removal of phenanthrene from solid phase. Batch tests for the phenanthrene distribution between aqueous and solid phase were conducted in the presence of HPCD. Column tests and numerical simulations were conducted to evaluate the roles of HPCD cavities and interaction rates between water, HPCD, and solid phase in the enhanced removal of phenanthrene. Experimental results showed that HPCD was effective in removing sorbed phenanthrene from subsurface environment, primarily due to its negligible sorption to the solid phase and the partitioning of phenanthrene into HPCD cavities. From the numerical simulations, it was found that rate-limited partitioning of phenanthrene into HPCD cavities was most influential factor in the enhanced elution of phenanthrene. Sorption and desorption rate of phenanthrene between aqueous and solid phase was very fast or near equilibrium state. Interaction rates of contaminant between water, HPCD, and solid phase could be affected by other factors such as soil types and organic matter contents. Results from this study implied that HPCD flushing could be effectively applied for the removal of hydrophobic organic pollutants existing in the soils as sorbed or NAPL state.     

Multiple microbial activities for volatile organic compounds reduction by biofiltration

In the northeast of Italy, high volatile organic carbon (VOC) emissions originate from small-medium companies producing furniture. In these conditions it is difficult to propose a single, efficient, and economic system to reduce pollution. Among the various choices, the biofiltration method could be a good solution, because microbial populations possess multiple VOC degradation potentials used to oxidize these compounds to CO2. Starting from the air emissions of a typical industrial wood-painting plant, a series of experiments studied in vitro microbial degradation of each individual VOC. Isolated strains were then added to a laboratory-scale biofiltration apparatus filled with an organic matrix, and the different VOC behavior demonstrated the potential of single and/or synergic microbial removal actions. When a single substrate was fed, the removal efficiency of a Pseudomonas aeruginosa inoculated reactor was 1.1, 1.17, and 0.33 g m(-3) hr(-1), respectively, for xylene, toluene, and ethoxy propyl acetate. A VOC mixture composed of butyl acetate, ethyl acetate, diacetin alcohol, ethoxy propanol acetate, methyl ethyl ketone, methyl isobutyl ketone, toluene, and xylene was then fed into a 2-m(3) reactor treating 100 m3 hr(-1) of contaminated air. The reactor was filled with the same mixture of organic matrix, enriched with all of the isolated strains together. During reactor study, different VOC loading rates were used, and the behavior was evaluated continuously. After a short acclimation period, the removal efficiency was > 65% at VOC load of 150-200 g m(-3) hr(-1). Quantification of removal efficiencies and VOC speciation confirmed the relationship among removal efficiencies, compound biodegradability, and the dynamic transport of each mixture component within the organic matrix. Samples of the fixed bed were withdrawn at different intervals and the heterogeneous microbial community evaluated for both total and differential compound counts.     

Freezing of a clayey silt contaminated with an organic solvent

In a study of water migration characteristics and organic contaminant transfer mechanisms in a freezing fine-grained saturated soil, a series of one-dimensional freezing tests were conducted on a clayey silt contaminated with a miscible, non-reactive organic compound, 1-propanol, at various concentrations. The experimental results indicate that the frost heave behaviour and solute rejection mechanisms of a soil contaminated with 1-propanol is similar to that of the same soil contaminated with sodium-chloride salt. It was found that 1-propanol is rejected from the pore for rates of cooling smaller than 4 ± 1°C/day. Diffusion appears to control contaminant redistribution in the unfrozen soil. Finally, there has been no contaminant redistribution in the frozen soil for periods up to 245 hours.     

Demonstration of In Situ Steam and Hot-Air Stripping Technology

Between September 5 and October 5,1989 a field demonstration of the NovaTerra, Inc. Detoxifier [formerly called Toxic Treatment (USA)] was performed by the U.S. EPA under the Superfund Innovative Technology Evaluation (SITE) program. The NovaTerra Detoxifier process injects steam and hot air directly into the ground to vaporize and strip volatile and semivolatile organics. Two augers loosen and homogenize the soil during the stripping process. The steam, hot air and organics are carried to the soil surface and collected for treatment.

The field demonstration was performed at the GATX Annex Terminal located at the Port of Los Angeles, San Pedro, California. Approximately 17 percent of the 5.2 acre site is contaminated with chlorinated solvents, plasticizers, coatings, adhesives and paint additives, and other miscellaneous chemicals from aboveground storage tanks and transfer operations to railroad cars.

The objectives of this SITE Demonstration were to determine the in situ soil concentrations before and after treatment, quantify process stream emissions (fugitive and sidestreams), determine process operating conditions, and determine if vertical migration of contaminants is occurring. Results from the SITE demonstration showed that a substantial amount of the VOCs were removed, about half the SVOCs were removed, there was very little fugitive air emissions from the operation, and what little downward migration occured (if any) was inconsequential.     

Removal and Destruction of Organic Contaminants in Water Using Adsorption,Steam Regeneration,and Photocatalytic Oxidation: A Pilot-Scale Study

ABSTRACT

The overall objective of this pilot-scale study is to investigate the technical feasibility of the removal and destruction of organic contaminants in water using adsorption and photocatalytic oxidation. The process consists of two consecutive operational steps: (1) removal of organic contaminants using fixed-bed adsorption; and (2) regeneration of spent adsorbent using photocatalysis or steam, followed by decontamination of steam condensate using photocatalysis. The pilot-scale study was conducted to evaluate these options at a water treatment plant in Wausau (Wisconsin) for treatment of groundwater contaminated with tetrachloroethene (PCE), trichloroethene (TCE), cis-dichloroethene (cis-DCE), toluene, ethylbenzene (EB), and xylenes. The adsorbents used were F-400 GAC and Ambersorb 563.

In the first treatment strategy, the adsorbents were impregnated with photocatalyst and used for the removal of aqueous organics. The spent adsorbents were then exposed to ultraviolet light to achieve photocatalytic regeneration. Regeneration of adsorbents using photocatalysis was observed to be not effective, probably because the impregnated photocatalyst was fouled by background organic matter present in the groundwater matrix.

In the second treatment strategy, the spent adsorbents were regenerated using steam, followed by cleanup of steam condensate using photocatalysis. Four cycles of adsorption and three cycles of steam regeneration were performed. Ambersorb 563 adsorbent was successfully regenerated using saturated steam at 160 °C within 20 hours. The steam condensate was treated using fixed-bed photo-catalysis using 1% Pt-TiO₂ photocatalyst supported on silica gel. After 35 minutes of empty bed contact time, more than 95% removal of TCE, cis-DCE, toluene, EB, and xylenes was achieved, and more than 75% removal of PCE was observed.

In the case of activated carbon adsorbent, steam regeneration was not effective, and a significant loss in adsorbent capacity was observed.     

Cosolvent flushing for the remediation of PAHs from former manufactured gas plants

Cosolvent flushing is a technique that has been proposed for the removal of hydrophobic organic contaminants in the subsurface. Cosolvents have been shown to dramatically increase the solubility of such compounds compared to the aqueous solubility; however, limited data are available on the effectiveness of cosolvents for field-contaminated media. In this work, we examine cosolvent flushing for the removal of polycyclic aromatic hydrocarbons (PAHs) in soil from a former manufactured gas plant (FMGP). Batch studies confirmed that the relationship between the soil-cosolvent partitioning coefficient (K(i)) and the volume fraction of cosolvent (f(c)) followed a standard log-linear equation. Using methanol at an fc of 0.95, column studies were conducted at varying length scales, ranging from 11.9 to 110 cm. Removal of PAH compounds was determined as a function of pore volumes (PVs) of cosolvent flushed. Despite using a high f(c), rate and chromatographic effects were observed in all the columns. PAH effluent concentrations were modeled using a common two-site sorption model. Model fits were improved by using MeOH breakthrough curves to determine fitted dispersion coefficients. Fitted mass-transfer rates were two to three orders of magnitude lower than predicted values based on published data using artificially contaminated sands.     

Remediation of NAPL below the water table by steam-induced heat conduction

Previous experimental studies have shown that NAPL will be removed when it is contacted by steam. However, in full-scale operations, steam may not contact the NAPL directly and this is the situation addressed in this study. A two-dimensional intermediate scale sand box experiment was performed where an organic contaminant was emplaced below the water table at the interface between a coarse and a fine sand layer. Steam was injected above the water table and after an initial heating period the contaminant was recovered at the outlet. The experiment was successfully modeled using the numerical code T2VOC and the dominant removal mechanism was identified to be heat conduction induced boiling of the separate phase contaminant. Subsequent numerical modeling showed that this mechanism was insensitive to the porous medium properties and that it could be evaluated by considering only one-dimensional heat conduction.     

Investigation of polychlorinated biphenyl removal from contaminated soil using microwave-generated steam

A feasibility study of polychlorinated biphenyl (PCB) removal from contaminated soils using microwave-generated steam (MGS) was performed. Initial experimental results show that MGS effectively removed PCBs from contaminated soil with an overall removal efficiency of greater than 98% at a steam-to-soil mass ratio of 3:1. Removal efficiency was found to be dependent upon the amount of steam employed, expressed as a mass ratio of steam applied to soil mass. Evaporation was identified as a major mechanism in removing PCBs from the soil. Rapid expansion and evaporation of pore water by microwave dielectric heating accelerated evaporation rates of PCB molecules. Increased solubility of PCBs into the heated aqueous phase is also hypothesized. Together these effects increase mass-transfer rates, thus enhancing removal of PCBs from the soil.     

In Memoriam

ABSTRACT

A feasibility study of polychlorinated biphenyl (PCB) removal from contaminated soils using microwave-generated steam (MGS) was performed. Initial experimental results show that MGS effectively removed PCBs from contaminated soil with an overall removal efficiency of greater than 98% at a steam-to-soil mass ratio of 3:1. Removal efficiency was found to be dependent upon the amount of steam employed, expressed as a mass ratio of steam applied to soil mass. Evaporation was identified as a major mechanism in removing PCBs from the soil. Rapid expansion and evaporation of pore water by microwave dielectric heating accelerated evaporation rates of PCB molecules. Increased solubility of PCBs into the heated aqueous phase is also hypothesized. Together these effects increase mass-transfer rates, thus enhancing removal of PCBs from the soil.     

重金属污染土壤淋洗技术的基础研究与工程应用进展

随着城市结构调整,工业企业易地搬迁后遗留下大量污染场地,严重威胁人居环境,亟待开展土壤修复.土壤淋洗技术具有工艺简单、处理范围广、修复效率高和治理费用相对低廉等优点,是目前修复重金属污染土壤最有效的技术之一,同时对于有机物污染土壤也具有显著修复效果.经过大量资料、文献调研,系统梳理土壤淋洗技术在国内的研究现状,结合实际...     

Microbial growth with vapor-phase substrate

Limited information exists on influences of the diffusive transport of volatile organic contaminants (VOC) on bacterial activity in the unsaturated zone of the terrestrial subsurface. Diffusion of VOC in the vapor-phase is much more efficient than in water and results in effective VOC transport and high bioavailability despite restricted mobility of bacteria in the vadose zone. Since many bacteria tend to accumulate at solid-water, solid-air and air-water interfaces, such phase boundaries are of a special interest for VOC-biodegradation. In an attempt to evaluate microbial activity toward air-borne substrates, this study investigated the spatio-temporal interplay between growth of Pseudomonas putida (NAH7) on vapor-phase naphthalene (NAPH) and its repercussion on vapor-phase NAPH concentrations. Our data demonstrate that growth rates of strain PpG7 were inversely correlated to the distance from the source of vapor-phase NAPH. Despite the high gas phase diffusivity of NAPH, microbial growth was absent at distances above 5 cm from the source when sufficient biomass was located in between. This indicates a high efficiency of suspended bacteria to acquire vapor-phase compounds and influence headspace concentration gradients at the centimeter-scale. It further suggests a crucial role of microorganisms as biofilters for gas-phase VOC emanating from contaminated groundwater or soil.     

Assessment of volatile organic compound removal by indoor plants—a novel experimental setup

Indoor plants can remove volatile organic compounds (VOCs) from the air. The majority of knowledge comes from laboratory studies where results cannot directly be transferred to real-life settings. The aim of this study was to develop an experimental test system to assess VOC removal by indoor plants which allows for an improved real-life simulation. Parameters such as relative humidity, air exchange rate and VOC concentration are controlled and can be varied to simulate different real-life settings. For example, toluene diffusion through a needle gave concentrations in the range of 0.10–2.35 μg/L with deviations from theoretical values of 3.2–10.5 %. Overall, the system proved to be functional for the assessment of VOC removal by indoor plants with Hedera helix reaching a toluene removal rate of up to 66.5 μg/m²/h. The mode of toluene exposure (semi-dynamic or dynamic) had a significant influence on the removal rate obtained by H. helix.