Transport Of Volatile Organic Compounds Research Articles

Evidence of a sewer vapor transport pathway at the USEPA vapor intrusion research duplex

The role of sewer lines as preferential pathways for vapor intrusion is poorly understood. Although the importance of sewer lines for volatile organic compound (VOC) transport has been documented at a small number of sites with vapor intrusion, sewer lines are not routinely sampled during most vapor intrusion investigations. We have used a tracer study and VOC concentration measurements to evaluate the role of the combined sanitary/storm sewer line in VOC transport at the USEPA vapor intrusion research duplex in Indianapolis, Indiana. The results from the tracer study demonstrated gas migration from the sewer main line into the duplex. The migration pathway appears to be complex and may include leakage from the sewer lateral at a location below the building foundation. Vapor samples collected from the sewer line demonstrated the presence of tetrachloroethene (PCE) and chloroform in the sewer main in front of the duplex and at multiple sample locations within the sewer line upstream of the duplex. These test results combined with results from the prior multi-year study of the duplex indicate that the sewer line plays an important role in transport of VOCs from the subsurface source to the immediate vicinity of the duplex building envelope.

Bubble-Facilitated VOC Transport from LNAPL Smear Zones and Its Potential Effect on Vapor Intrusion.

Most conceptual and mathematical models of soil vapor intrusion assume that the transport of volatile organic compounds (VOCs) from a source toward a building is limited by diffusion through the soil gas. Under conditions where advection occurs, transport rates are higher and can lead to higher indoor air concentrations. Advection-dominated conditions can be created by gas bubble flow in the saturated zone. A series of laboratory column experiments were conducted to measure mass flux due to bubble-facilitated VOC transport from light nonaqueous phase liquid (LNAPL) smear zones. Smear zones that contained both LNAPL residual and trapped gas, as well as those that contained only LNAPL residual, were investigated. Results showed that the VOC mass flux due to bubble-facilitated transport was orders-of-magnitude higher than under diffusion-limited conditions. Results also showed that the mass flux due to bubble-facilitated transport was intermittent, and increased with an increased supply of dissolved gases.

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This paper presents the transport of volatile organic compounds (VOC) through contaminated subsoil to groundwater using one dimensional vadose zone transport model, VLEACH. The model is applied using field specific data for a typical Toluene at Fuel pumping station and Transformer dumping yard. Laboratory column leaching studies were conducted to assess the mass flow rate of Toluene flux through subsoil and compared with VLEACH model output. The research also focused on the sensitivity of the model output with the varying input parameters, one at a time, across its maximum range of values identified from the study area. The observed Toluene fluxes are not consistent with the experimentally input concentrations and may not be valid due to its lower density of 0.865 g/ml. Hence, liquid phase transport of Toluene for experimental column leaching studies is not suitable. The sensitivity analysis results reveals that, model output is a function of fractional organic carbon content as well as water content.

VOCs transport with VOCs-adsorbing particles under VOCs emission from carpet

Many kinds of Volatile organic compounds (VOCs) emitted from building materials can deteriorate indoor air quality. Thus, it is essential to investigate how VOCs transport and how the airside VOCs concentration can be reduced. In order to remove airside VOCs, activated carbon particles can be used. This paper investigates the effect of activated carbon particles on the airside concentration of VOCs emitted from a new carpet in a room. A new numerical method for predicting airside VOCs concentration under the presence of activated carbon particles is developed, which includes spatially repeated iteration to get the transient airside VOCs concentration in association with the activated carbon particles’ adsorption of VOCs. The current numerical model is validated over an existing experimental result. Commercial software FLUENT 12.0 is adopted to solve the continuity, momentum, turbulence and concentration equations in two main cases under the situations with and without considering particle deposition. Numerical results show that the transient concentrations of VOCs in the air reach peaks, decay rapidly, drop slowly and remain in the room for a long time. When the particle concentration is at a higher level, the decrease in airside VOCs concentration is more notable. The concept proposed in this study can be utilized, with the use of activated carbon particles, to enhance the emission rate of VOCs in raw building materials before they are furnished in newly built indoor spaces.

Modelling of volatile organic compounds concentrations in rooms due to electronic devices

Abstract The objective of this study is to develop an approach concerning the integration of volatile organic compounds (VOCs) emissions due to office equipment in computational fluid dynamics (CFD) simulations, in order to assess the indoor air quality (IAQ). The transport and diffusion phenomena of VOCs are taking into account in the CFD model by means of conservation equations of the mass fraction, written for each VOC that is intended to be considered in the simulation. These equations include source terms of mass for each VOC, based on VOC generation rates of different sources considered in the numerical model (computers, monitors, and laser printers). On the other hand, these equations are added to the basic equations describing turbulent confined non-isothermal flows (conservation of mass, momentum, energy, and turbulent quantities) in CFD modelling. The numerical model is applied in this study for a small office, taking into account a conventional mixing ventilation system (low air flow rates with different air supply temperatures). Health hazard assessments are accomplished by taking into account in the CFD model the indoor levels of the following five VOCs: benzaldehyde, ethylbenzene, o-xylene, styrene, and toluene. The CFD model proposed in this study allows achieving values of VOCs concentrations throughout the entire indoor environment. Consequently, results are presented in terms of benzaldehyde, ethylbenzene, o-xylene, styrene, and toluene concentration contours in the office, as well as mean and peak values of these VOCs in the occupied zone of the room. The results show that the estimated VOCs concentration levels due to office equipment are far below the set threshold limit values. However, the reported maximum concentrations of VOCs taken into account in the occupied zone tend to approach in some measure levels of concern with respect to odour or sensory irritation. Finally, the numerical description of VOCs sources for CFD modelling developed in this work may be extended for other indoor VOCs sources. As a result, the numerical approach proposed in this study can lead to relevant health hazard analyses, being an appropriate alternative to experimental investigations, challenging to perform in situ.

Assessment of Consolidation-Induced VOC Transport for a GML/GCL Composite Liner System

This paper presents a numerical investigation of the effect of consolidation on the transport of a volatile organic compound (VOC), trichloroethylene (TCE), through a composite liner system comprising a geomembrane liner (GML) overlying and intimate contact with a geosynthetic clay liner (GCL). The numerical simulations were conducted using the model CST2, and considered coupled consolidation and contaminant transport with representative geometry, material properties, and applied stress conditions. The simulation results indicate that, depending on conditions, GCL consolidation can have significant effects on TCE mass flux, cumulative mass outflow, and distribution of contaminant concentration within the GCL, not only during the course of consolidation but also long after consolidation has ceased. Because of the small thickness of the GCL, consolidation-induced advection is insignificant. However, consolidation can significantly impact TCE transport through the GCL via changes in GCL material properties, including decreases in thickness, porosity, and effective diffusion coefficient. In general, the effects of GCL consolidation increase with increasing magnitude of applied stress, decreasing loading period (i.e., increasing loading rate), and increasing variation in the effective diffusion coefficient. The traditional performance assessment approach neglects GCL consolidation and fails to consider associated changes in material properties and, thus, can lead to significantly different results. Simulation results indicate that, in general, when the applied stress is lower than 125 kPa or the exponent for porosity-dependent effective diffusion coefficient is lower than 4, the effect of consolidation on TCE transport through the GML/GCL composite liner is insignificant, with differences in performance of the GML/GCL composite liner resulting from consolidation being less than 15%.

Laboratory experiments and simulations of MTBE transport in layered heterogeneous porous media

Methyl tert-butyl ether (MTBE) has been considered as a typical volatile organic compound (VOC) in the subsurface environment, which can result in a significant risk to subsurface ecosystems and human health. As the awareness on groundwater quality is increasing, it is of great importance to identify the effect of heterogeneity on VOCs transport through subsurface porous media. Thus, experiments on transport through homogeneous and layered heterogeneous porous media were carried out. In this study, NaCl and MTBE were used as a nonreactive tracer and a VOC, respectively. The results of tracer tests in homogeneous sand columns indicate that the test values fit well with simulated values. The breakthrough curves of homogeneous porous media show that the flow velocity of MTBE is slower than that of the NaCl, which means MTBE has been seriously retarded during transport process. In addition, MTBE transport is greatly dependent on the heterogeneity of the porous media with higher retention in the layered heterogeneous media compared to homogeneous media, which refers to non-Fickian transfer. The results imply that the layered heterogeneity could weaken the contamination intensity, thereby retarding the contaminant transport and prolonging the contaminants contact time in the heterogeneous porous media.

Characterizing volatile organic compounds in leachate from Gohagoda municipal solid waste dumpsite, Sri Lanka

Fate and transport of volatile organic compounds (VOCs) in landfill leachate causes environmental pollution and human health concern. These VOCs can directly be emitted to the atmosphere and also can end up in leachate plume from open solid waste dumpsites. However, no data exists on the VOCs in landfill leachate in Sri Lanka. Hence, the purpose of this study was to assess the variation of VOCs in landfill leachate at the Gohagoda Municipal Solid Waste (MSW) dumpsite, Kandy, Sri Lanka in a dry (Spring) and a wet (Autumn) month. A list of the key offensive VOCs covering 13 chemicals in leachate was quantified by using a head space–gas-chromatography–mass-spectrometry (HS-GC–MS). Results indicated that benzene and toluene were most commonly observed in both sampling periods, ranging from 1.78–21.7 and 1.73–20.2µgL−1, respectively. 4-Isopropyltoluene had the highest concentrations of the VOCs investigated in both sampling periods (38.0 and 129µgL−1 in spring and autumn, respectively). Totally, 9 different VOCs out of 13 listed were detected in the landfill leachate. In comparison, the number of VOCs and total VOC concentrations were higher at all sampling locations in spring except GSV1 sampling point. Thereby, it appears that a time series analysis of VOCs during the dry and wet months may provide a better understanding of the degree and risk of pollution of the Mahaweli River water.

Biodegradation of vapor-phase toluene in unsaturated porous media: Column experiments

Biodegradation of organic chemicals in the vapor phase of soils and vertical flow filters has gained attention as promising approach to clean up volatile organic compounds (VOC). The drivers of VOC biodegradation in unsaturated systems however still remain poorly understood. Here, we analyzed the processes controlling aerobic VOC biodegradation in a laboratory setup mimicking the unsaturated zone above a shallow aquifer. The setup allowed for diffusive vapor-phase transport and biodegradation of three VOC: non-deuterated and deuterated toluene as two compounds of highly differing biodegradability but (nearly) identical physical and chemical properties, and MTBE as (at the applied experimental conditions) non-biodegradable tracer and internal control. Our results showed for toluene an effective microbial degradation within centimeter VOC transport distances despite high gas-phase diffusivity. Degradation rates were controlled by the reactivity of the compounds while oxic conditions were found everywhere in the system. This confirms hypotheses that vadose zone biodegradation rates can be extremely high and are able to prevent the outgassing of VOC to the atmosphere within a centimeter range if compound properties and site conditions allow for sufficiently high degradation rates.

FORest Canopy Atmosphere Transfer (FORCAsT) 1.0: a 1-D model of biosphere–atmosphere chemical exchange

Abstract. Biosphere–atmosphere interactions play a critical role in governing atmospheric composition, mediating the concentrations of key species such as ozone and aerosol, thereby influencing air quality and climate. The exchange of reactive trace gases and their oxidation products (both gas and particle phase) is of particular importance in this process. The FORCAsT (FORest Canopy Atmosphere Transfer) 1-D model is developed to study the emission, deposition, chemistry and transport of volatile organic compounds (VOCs) and their oxidation products in the atmosphere within and above the forest canopy. We include an equilibrium partitioning scheme, making FORCAsT one of the few canopy models currently capable of simulating the formation of secondary organic aerosols (SOAs) from VOC oxidation in a forest environment. We evaluate the capability of FORCAsT to reproduce observed concentrations of key gas-phase species and report modeled SOA concentrations within and above a mixed forest at the University of Michigan Biological Station (UMBS) during the Community Atmosphere-Biosphere Interactions Experiment (CABINEX) field campaign in the summer of 2009. We examine the impact of two different gas-phase chemical mechanisms on modelled concentrations of short-lived primary emissions, such as isoprene and monoterpenes, and their oxidation products. While the two chemistry schemes perform similarly under high-NOx conditions, they diverge at the low levels of NOx at UMBS. We identify peroxy radical and alkyl nitrate chemistry as the key causes of the differences, highlighting the importance of this chemistry in understanding the fate of biogenic VOCs (bVOCs) for both the modelling and measurement communities.

Numerical modelling on fate and transport of petroleum hydrocarbons in an unsaturated subsurface system for varying source scenario

The vertical transport of petroleum hydrocarbons from a surface spill through an unsaturated subsurface system is of major concern in assessing the vulnerability of groundwater contamination. A realistic representation on fate and transport of volatile organic compounds at different periods after spill is quite challenging due to the variation in the source behaviour at the surface of spill as well as the variation in the hydrodynamic parameters and the associated inter-phase partitioning coefficients within the subsurface. In the present study, a one dimensional numerical model is developed to simulate the transport of benzene in an unsaturated subsurface system considering the effect of volatilization, dissolution, adsorption and microbial degradation of benzene for (i) constant continuous source, (ii) continuous decaying source, and (iii) residual source. The numerical results suggest that volatilization is the important sink for contaminant removal considering the soil air migration within the unsaturated zone. It is also observed that the coupled effect of dissolution and volatilization is important for the decaying source at the surface immediately after the spill, whereas rate-limited dissolution from residually entrapped source is responsible for the extended contamination towards later period.

A predictive method for crude oil volatile organic compounds emission from soil: evaporation and diffusion behavior investigation of binary gas mixtures.

Due to their mobility and toxicity, crude oil volatile organic compounds (VOCs) are representative components for oil pipeline contaminated sites detection. Therefore, contaminated location risk assessment, with airborne light detection and ranging (LIDAR) survey, in particular, requires ground-based determinative methods for oil VOCs, the interaction between oil VOCs and soil, and information on how they diffuse from underground into atmosphere. First, we developed a method for determination of crude oil VOC binary mixtures (take n-pentane and n-hexane as examples), taking synergistic effects of VOC mixtures on polydimethylsiloxane (PDMS) solid-phase microextraction (SPME) fibers into consideration. Using this method, we further aim to extract VOCs from small volumes, for example, from soil pores, using a custom-made sampling device for nondestructive SPME fiber intrusion, and to study VOC transport through heterogeneous porous media. Second, specific surface Brunauer-Emmett-Teller(BET) analysis was conducted and used for estimation of VOC isotherm parameters in soil. Finally, two models were fitted for VOC emission prediction, and the results were compared to the experimental emission results. It was found that free diffusion mode worked well, and an empirical correction factor seems to be needed for the other model to adapt to our condition for single and binary systems.

Determination of Chlorinated Solvent Sorption by Porous Material-Application to Trichloroethene Vapor on Cement Mortar.

Experiments have been performed to investigate the sorption of trichloroethene (TCE) vapor by concrete material or, more specifically, the cement mortar component. Gas-flow experiments were conducted using columns packed with small pieces of cement mortar obtained from the grinding of typical concrete material. Transport and retardation of TCE at high vapor concentrations (500 mg L-1) was compared to that of a non-reactive gas tracer (Sulfur Hexafluoride, SF6). The results show a large magnitude of retardation (retardation factor = 23) and sorption (sorption coefficient = 10.6 cm3 g-1) for TCE, compared to negligible sorption for SF6. This magnitude of sorption obtained with pollutant vapor is much bigger than the one obtained for aqueous-flow experiments conducted for water-saturated systems. The considerable sorption exhibited for TCE under vapor-flow conditions is attributed to some combination of accumulation at the air-water interface and vapor-phase adsorption, both of which are anticipated to be significant for this system given the large surface area associated with the cement mortar. Transport of both SF6 and TCE was simulated successfully with a two-region physical non-equilibrium model, consistent with the dual-medium structure of the crushed cement mortar. This work emphasizes the importance of taking into account sorption phenomena when modeling transport of volatile organic compounds through concrete material, especially in regard to assessing vapor intrusion.

The Atmosphere can be a Source of Certain Water Soluble Volatile Organic Compounds in Urban Streams

AbstractSurface water and air volatile organic compound (VOC) data from 10 U.S. Geological Survey monitoring sites were used to evaluate the potential for direct transport of VOCs from the atmosphere to urban streams. Analytical results of 87 VOC compounds were screened by evaluating the occurrence and detection levels in both water and air, and equilibrium concentrations in water (Cws) based on the measured air concentrations. Four compounds (acetone, methyl tertiary butyl ether, toluene, and m‐ & p‐xylene) were detected in more than 20% of water samples, in more than 10% of air samples, and more than 10% of detections in air were greater than long‐term method detection levels (LTMDL) in water. Benzene was detected in more than 20% of water samples and in more than 10% of air samples. Two percent of benzene detections in air were greater than one‐half the LTMDL in water. Six compounds (chloroform, p‐isopropyltoluene, methylene chloride, perchloroethene, 1,1,1‐trichloroethane, and trichloroethene) were detected in more than 20% of water samples and in more than 10% of air samples. Five VOCs, toluene, m‐ & p‐xylene, methyl tert‐butyl ether (MTBE), acetone, and benzene were identified as having sufficiently high concentrations in the atmosphere to be a source to urban streams. MTBE, acetone, and benzene exhibited behavior that was consistent with equilibrium concentrations in the atmosphere.

Estimating the Impact of Vadose Zone Sources on Groundwater to Support Performance Assessment of Soil Vapor Extraction

Soil vapor extraction (SVE) is a prevalent remediation remedy for volatile organic compound (VOC) contaminants in the vadose zone. To support selection of an appropriate condition at which SVE may be terminated for site closure or for transition to another remedy, an evaluation is needed to determine whether vadose zone VOC contamination has been diminished sufficiently to keep groundwater concentrations below threshold values. A conceptual model for this evaluation was developed for VOC fate and transport from a vadose zone source to groundwater when vapor-phase diffusive transport is the dominant transport process. A numerical analysis showed that, for these conditions, the groundwater concentration is controlled by a limited set of parameters, including site-specific dimensions, vadose zone properties, and source characteristics. On the basis of these findings, a procedure was then developed for estimating groundwater concentrations using results from the three-dimensional multiphase transport simulations for a matrix of parameter value combinations and covering a range of potential site conditions. Interpolation and scaling processes are applied to estimate groundwater concentrations at compliance (monitoring) wells for specific site conditions of interest using the data from the simulation results. The interpolation and scaling methodology using these simulation results provides a far less computationally intensive alternative to site-specific three-dimensional multiphase site modeling, while still allowing for parameter sensitivity and uncertainty analyses. With iterative application, the approach can be used to consider the effect of a diminishing vadose zone source over time on future groundwater concentrations. This novel approach and related simulation results have been incorporated into a user-friendly Microsoft® Excel®-based spreadsheet tool entitled SVEET (Soil Vapor Extraction Endstate Tool), which has been made available to the public.

Conjugated Polymer/Nanoparticles Nanocomposites for High Efficient and Real-Time Volatile Organic Compounds Sensors

The present work demonstrates a high efficient and low cost volatile organic compounds (VOCs) sensor. Nowadays, VOCs, which are typically toxic, explosive, flammable, and an environmental hazard, are extensively used in R&D laboratories and industrial productions. Real-time and accurately monitoring the presence of harmful VOC during the usage, storage, or transport of VOCs is extremely important which protects humans and the environment from exposure in case of an accident and leakage of VOCs. The present work utilizes conducting polymer/nanoparticles blends to sense various VOCs by detecting the variation of optical properties. The novel sensor features high sensitivity, high accuracy, quick response, and very low cost. Furthermore, it is easy to fabricate into a sensing chip and can be equipped anywhere such as a laboratory or a factory where the VOCs are either used or produced and on each joint between transporting pipes or each switch of VOC storage tanks. Real-time sensing is achievable on the basis of the instant response to VOC concentrations of explosive limits. Therefore, an alarm can be delivered within a few minutes for in time remedies. This research starts from investigating fundamental properties, processing adjustments, and a performance test and finally extends to real device fabrication that practically performs the sensing capability. The demonstrated results significantly advance the current sensor technology and are promising in commercial validity in the near future for human and environmental safety concerns against hazardous VOCs.

Simulation-Based Variance Components Analysis for Characterization of Interaction Effects of Random Factors on Trichloroethylene Vapor Transport in Unsaturated Porous Media

This paper presents a simulation-based random effects model for analyzing the interaction effects of random factors on the gaseous transport of volatile organic compounds (VOCs) in the unsaturated zone. The model has the advantages of addressing parameter uncertainty characterized by inherent randomness in simulating the gaseous transport of VOCs in the unsaturated porous media, as well as of investigating their potential interactions and the nonlinear effects on the VOC vapor concentrations through a variance components analysis. The proposed model is applied to a numerical experiment on the transport behavior of trichloroethylene (TCE) vapors in the unsaturated porous media. The results reveal that variations of random factors can cause a significant difference in the gaseous TCE concentrations, and the temporal and spatial variability in the TCE vapor concentrations can result in different effects of factors on the model response.

Effect of VOCs-adsorbing particles on the VOCs emission from wood

This study presents the effect of the VOCs-adsorbing particles and air velocity on the transport of volatile organic compounds (VOCs) in a wood in a small-scale geometry. This study aims to show that the use of VOCs-adsorbing particle can enhance significantly the VOCs emission from building indoor materials before they are actually installed in a residential place. A numerical simulation based on two-dimensional computational fluid dynamics model has been performed to investigate the influences of air velocity and particle concentration. Together with air, particles come into the channel on which a wood is laid. Numerical results show that particles in the air blown to the channel drastically decrease VOCs concentration in the air. With an increase in the inlet particle concentration, the volume-averaged VOCs concentration in the airside decreases a lot. The air flow rate has significant impacts on the airside VOCs concentration. An increase in the air flow rate decreases the airside VOCs concentration. Under the same air flow rate at the inlet, larger inlet velocity with smaller channel width yields lower airside VOCs concentration when particles are released into the model. Meanwhile, the VOCs concentration in wood of 5cm thickness decreases a little bit under a situation with high inlet velocity and high inlet particle concentration. When wood thickness is thinner, the VOCs concentration in wood significantly decreased, which means larger increase in the rate of VOCs emission from a thin wood.

A membrane bioreactor for the simultaneous treatment of acetone, toluene, limonene and hexane at trace level concentrations

The performance of a flat-membrane biofilm reactor (MBR) for the removal of acetone, toluene, limonene and hexane at concentrations ranging from 1.3 to 3.2 mg m−3 was investigated at different gas residence times (GRT): 60, 30, 15 and 7 s. A preliminary abiotic test was conducted to assess the mass transport of the selected volatile organic compounds (VOCs) through the membrane. A reduced transport of limonene and hexane was observed with water present over the dense side of the membrane. The presence of a biofilm attached on the dense side of the membrane following bioreactor inoculation significantly increased VOC transport. High acetone and toluene removals (>93%) were recorded in the MBR regardless of the GRT. To remediate the low hexane removal performance (RE < 24%) recorded at the initial stages of the process, a re-inoculation of the membrane with a hexane-degrading consortium embedded in silicon oil was performed. Although hexane removal did not exceed 27%, this re-inoculation increased limonene removals up to 90% at a GRT of 7 s. The absence of inhibition of hexane biodegradation by substrate competition confirmed that hexane removal in the MBR was indeed limited by the mass transfer through the membrane. Despite the low carbon source spectrum and load, the microbiological analysis of the communities present in the MBR showed high species richness (Shannon–Wiener indices of 3.2–3.5) and a high pair-wise similarity (84–97%) between the suspended and the attached biomass.

Vapor-phase transport of trichloroethene in an intermediate-scale vadose-zone system: retention processes and tracer-based prediction.

Gas-phase transport experiments were conducted using a large weighing lysimeter to evaluate retention processes for volatile organic compounds (VOCs) in water-unsaturated (vadose-zone) systems, and to test the utility of gas-phase tracers for predicting VOC retardation. Trichloroethene (TCE) served as a model VOC, while trichlorofluoromethane (CFM) and heptane were used as partitioning tracers to independently characterize retention by water and the air-water interface, respectively. Retardation factors for TCE ranged between 1.9 and 3.5, depending on water content. The results indicate that dissolution into the bulk water was the primary retention mechanism for TCE under all conditions studied, contributing approximately two-thirds of the total measured retention. Accumulation at the air-water interface comprised a significant fraction of the observed retention for all experiments, with an average contribution of approximately 24%. Sorption to the solid phase contributed approximately 10% to retention. Water contents and air-water interfacial areas estimated based on the CFM and heptane tracer data, respectively, were similar to independently measured values. Retardation factors for TCE predicted using the partitioning-tracer data were in reasonable agreement with the measured values. These results suggest that gas-phase tracer tests hold promise for characterizing the retention and transport of VOCs in the vadose-zone.