Transport Of Volatile Organic Compounds Research Articles

Design and laboratory testing of a chamber device to measure total flux of volatile organic compounds from the unsaturated zone under natural conditions

To determine if an aquifer contaminated with volatile organic compounds (VOCs) has potential for natural remediation, all natural processes affecting the fate and transport of VOCs in the subsurface must be identified and quantified. This research addresses the quantification of air-phase volatile organic compounds (VOCs) leaving the unsaturated zone soil gas and entering the atmosphere—including the additional flux provided by advective soil-gas movement induced by barometric pumping. A simple and easy-to-use device for measuring VOC flux under natural conditions is presented. The vertical flux chamber (VFC) was designed using numerical simulations and evaluated in the laboratory. Mass-balance numerical simulations based on continuously stirred tank reactor equations (CSTR) provided information on flux measurement performance of several sampling configurations with the final chamber configuration measuring greater than 96% of model-simulated fluxes. A laboratory device was constructed to evaluate the flux chamber under both diffusion-only and advection-plus-diffusion transport conditions. The flux chamber measured an average of 82% of 15 diffusion-only fluxes and an average of 95% of 15 additional advection-plus-diffusion flux experiments. The vertical flux chamber has the capability of providing reliable measurement of VOC flux from the unsaturated zone under both diffusion and advection transport conditions.

Effectiveness of Scrap Tire Chips as Sorptive Drainage Material

Scrap tire disposal is a problem of growing concern. One solution to this problem is innovative methods for the reuse and recycling of scrap tires. Based on batch isotherm tests, scrap tire chips have been identified to be good sorbents of volatile organic compounds (VOCs) and could be used as leachate drainage layer material in solid waste landfills and in other similar applications. To demonstrate the effects of tire chips on the leachate they come in contact with in a drainage layer over a liner, large-scale tank tests simulating the drainage layer and the clay liner and also field tests were performed. Two cells were constructed in a landfill: one with scrap tire chips and the other with gravel leachate collection layer. According to the results of the large-scale tank tests and field tests, shredded tire chips have a significantly positive impact on the quality of the leachate with which they come in contact. The use of scrap tires in landfills would reduce the magnitude of the current tire disposal problem (a 1 ha landfill requires approximately 300,000 tires to fill 0.3 m of a leachate collection layer) and convert one waste into a beneficial construction material and simultaneously mitigate the problem of VOC transport from through landfill liners.

Effects of daily precipitation and evapotranspiration patterns on flow and VOC transport to groundwater along a watershed flow path.

MTBE and other volatile organic compounds (VOCs) are widely observed in shallow groundwater in the United States, especially in urban areas. Previous studies suggest that the atmosphere and/or nonpoint surficial sources could be responsible for some of those VOCs, especially in areas where there is net recharge to groundwater. However, in semiarid locations where annual potential evapotranspiration can exceed annual precipitation, VOC detections in groundwater can be frequent. VOC transport to groundwater under net discharge conditions has not previously been examined. A numerical model is used here to demonstrate that daily precipitation and evapotranspiration (ET) patterns can have a significant effect on recharge to groundwater, watertable elevations, and VOC transport. Ten-year precipitation/ET scenarios from six sites in the United States are examined using both actual daily observed values and "average" pulsed precipitation. MTBE and tetrachloroethylene transport, including gas-phase diffusion, are considered. The effects of the precipitation/ET scenarios on net recharge and groundwater flow are significant and complicated, especially under low-precipitation conditions when pulsed precipitation can significantly underestimate transport to groundwater. In addition to precipitation and evapotranspiration effects, location of VOC entry into the subsurface within the watershed is important for transport in groundwater. This is caused by groundwater hydraulics at the watershed scale as well as variations in ET within the watershed. The model results indicate that it is important to consider both daily precipitation/ET patterns and location within the watershed in order to interpret VOC occurrence in groundwater, especially in low-precipitation settings.

Theoretical development and analytical solutions for transport of volatile organic compounds in dual-porosity soils

Predicting the behavior of volatile organic compounds in soils or sediments is necessary for managing their use and designing appropriate remedial systems to eliminate potential threats to the environment, particularly the air and groundwater resources. In this effort, based on continuity of mass flux, we derive a mass flux boundary condition of the third type in terms of physically based mass transfer rate coefficients, describing the resistance to mass inflow of the soil–air interface, and obtain one-dimensional analytical solutions for transport and degradation of volatile organic compounds in semi-infinite structured soils under steady, unsaturated flow conditions. The advective–dispersive mass balance formulation allows for mobile–immobile liquid phase and vapor diffusive mass transfer, with linear equilibrium adsorption and liquid–vapor phase partitioning in the dynamic and stagnant soil regions. The mass transfer rate coefficients of volatile organic chemicals across the soil–air interface are expressed in terms of solute properties and hydrodynamic characteristics of resistive soil and air-boundary layers. The solutions estimate solute vapor flux from soil surface and describe mobile-phase solute concentration as a function of depth in the soil and time. In particular, solutions were derived for: (1) zero-initial concentration in the soil profile subject to a continuous and pulsed source at the soil surface; and (2) depletion from the soil following an initially contaminated soil profile. Sensitivity analysis with respect to different dimensionless parameters is conducted and the effect on solute concentration and vapor flux of such parameters as volatilization mass transfer velocity relative to infiltration, soil Peclet number, biochemical decay, and diffusive mass transfer into the immobile phase, is plotted and the results are discussed. The mass transfer rate coefficients and the analytical solutions are applied to simulate transport of an example volatile organic compound in an aggregated soil. The simulated results indicate that macropore-aggregate vapor phase diffusion may profoundly impact transport of volatile compounds in aggregated soils.

Use of chloroflurocarbons as internal standards for the measurement of atmospheric non-methane volatile organic compounds sampled onto solid adsorbent cartridges.

Solid adsorbents have proven useful for determining the vertical profiles of volatile organic compounds (VOCs) using sampling platforms such as balloons, kites, and light aircraft, and those profiles provide valuable information about the sources, sinks, transformations, and transport of atmospheric VOCs. One of the largest contributions to error in VOC concentrations is the estimation of the volume of air sampled on the adsorbent cartridge. These errors arise from different sources, such as variations in pumping flow rates from changes in ambient temperature and pressure with altitude, and decrease in the sampling pump battery power. Another significant source for sampling rate variations are differences in the flow resistance of individual sampling cartridges. To improve the accuracy and precision of VOC measurements, the use of ambient chlorofluorocarbons (CFCs) as internal standards was investigated. A multibed solid adsorbent, AirToxic (Supelco), was chosen for its wide sampling range (C3-C12). Analysis was accomplished by thermal desorption and dual detection GC/FID/ECD, resulting in sensitive and selective detection of both VOCs and CFCs in the same sample. Long-lived chlorinated compounds (CFC-11, CFC-12, CFC-113, CCl4 and CH3CCl3) banned by the Montreal Protocol and subsequent amendments were studied for their ability to predict sample volumes using both ground-based and vertical profiling platforms through the boundary layer and free troposphere. Of these compounds, CFC-113 and CCl4 were found to yield the greatest accuracy and precision for sampling volume determination. Use of ambient CFC-113 and CCl4 as internal standards resulted in accuracy and precision of generally better than 10% for the prediction of sample volumes in ground-, balloon-, and aircraft-based measurements. Consequently, use of CFCs as reference compounds can yield a significant improvement of accuracy and precision for ambient VOC measurements in situations where accurate flow control is troublesome.

A review of aqueous-phase VOC transport in modern landfill liners

Leachates from municipal solid waste (MSW) and hazardous waste landfills contain a wide range of volatile organic compounds (VOCs) in addition to inorganic compounds. VOCs have been shown to migrate and contaminate the surrounding environment and impair the use of groundwater. Therefore, the effectiveness of modern landfill liner systems to minimize migration of VOCs is of concern. Most modern landfills employ a composite liner consisting of a geomembrane overlying a compacted clay liner or a geosynthetic clay liner. The geomembrane is often believed to be the primary barrier to contaminant transport. However, for VOCs, the clay component usually controls the rate of transport since VOCs are shown to diffuse through geomembrane at appreciable rates. Additionally, analyses have shown that transport of volatile organic compounds (VOCs) generally is more critical than transport of inorganic compounds (e.g., toxic heavy metals), even though VOCs are often found at lower concentrations in leachates. Therefore, the effectiveness of modern landfill liner systems to minimize migration of VOCs and transport of VOCs through clay liners and modeling of transport through composite liners merit scrutiny. This paper presents a review of recent research by the author and others on these topics. A systematic and comprehensive approach to determine mass transport parameters for transport of VOCs in liquid phase through compacted clay liners, geosynthetic clay liners (GCLs), and geomembranes has enabled to develop realistic models to predict mass flux of VOCs through modern composite liners and provide a quantitative basis to evaluate potential for transport of dissolved VOCs and the equivalency of different composite liners.

Mass transport in the membrane air-stripping process using microporous polypropylene hollow fibers: effect of toluene in aqueous feed

Membrane air-stripping (MAS), using microporous polypropylene hollow fiber membrane modules, is one of the most promising processes for removal and recovery of volatile organic compounds (VOCs) from water/wastewater. In this work, aqueous feed containing VOCs was allowed to cross-flow on the shell side, whereas air flowed through the lumen of fibers. Chloroform, toluene and their mixture were used as model VOCs. The effects of presence of toluene alone and in mixture with chloroform in aqueous feed on the mass transport of VOCs through the membrane are reported. It was found that Henry’s law constants (HLCs) for toluene as well as chloroform did not change significantly in mixtures. The tests showed that higher toluene adsorption than that of chloroform on the fibers. It appeared that toluene blocked the pores partially, due to its strong affinity for the membrane material, resulting in substantially reduced mass transport.

Semianalytical Simulation of Soil Vapor Extraction

Soil vapor extraction (SVE) is a proven effective in situ technology for the removal of volatile organic compounds (VOCs) from the subsurface. However, most systems designed to date are based on empirical methods, rules-of-thumb, or results obtained from time-consuming and expensive pilot-scale field tests. Thus, it is very difficult to extrapolate laboratory results to field applications, to test hypotheses, to evaluate different subsurface scenarios, to accumulate experience, to optimize operation parameters, to predict field performance, and to modify operation parameters in response to unanticipated subsurface conditions. In this paper, the development of a three-dimensional semianalytical model for the simulation of transport and fate of VOCs in the subsurface during a multiple-well soil vapor extraction operation is presented. The model provides the necessary but simple-to-use engineering design tools for the process. Illustrative examples on the use of the model developed are also presented.

Modeling effective diffusivity of volatile organic compounds in activated carbon fiber.

Volatile organic compounds (VOCs) comprise 67% of total hazardous air pollutants (HAPs) that are emitted by major industrial point sources into the U.S. atmosphere (1). Adsorption by activated carbon fiber (ACF) has been recognized as one of the feasible regenerative control processes to separate and recover VOCs for reuse. Characteristics of VOCs transport in ACFs are required to efficiently design ACF sorption systems. However, extensive resources are spent experimentally obtaining transient sorption data to design adsorption systems. As an alternative, this work develops a new model that predicts effective diffusivities of VOCs into ACFs. The diffusion process is modeled as Knudsen transport into the ACF open pore spaces coupled with activated surface diffusion on the ACF's internal surface area. Temperature and Darken's factors are included in the surface diffusion model to provide corrections for thermodynamic state and deviation from Fick's Law, respectively. Depth of the adsorption potential well is considered as the product of the heat of adsorption of a reference VOC, an adsorption similarity factor, and a surface diffusion energy factor. Introduction of the adsorption similarity factor in the effective diffusivity model is a new concept providing a means to predict effective diffusivity of similar adsorption systems from a reference system. Experimental data from a short length column are used to determine effective diffusivity of acetone in ACF. Results from this diffusivity model are compared to experimental values for the acetone/ACF system to describe the degree of closure between modeled and experimental results.

Transport, Behavior, and Fate of Volatile Organic Compounds in Streams

Volatile organic compounds (VOCs) are compounds with chemical and physical properties that allow the compounds to move freely between the water and air phases of the environment. VOCs are widespread in the environment because of this mobility. Many VOCs have properties that make them suspected or known hazards to the health of humans and aquatic organisms. Consequently, understanding the processes affecting the concentration and distribution of VOCs in the environment is necessary.The transport, behavior, and fate of VOCs in streams are determined by combinations of chemical, physical, and biological processes. These processes are volatilization, absorption, wet and dry deposition, microbial degradation, sorption, hydrolysis, aquatic photolysis, oxidation, chemical reaction, biocon-centration, advection, and dispersion. The relative importance of each of these processes depends on the characteristics of the VOC and the stream.The U.S. Geological Survey National Water-Quality Assessment Program selected 55 VOCs for study. This article reviews the characteristics of the various processes that could affect the transport, behavior, and fate of these VOCs in streams.

Interaction of volatile organic compounds with indoor materials—a small-scale screening method

Indoor air pollution caused by volatile organic compounds (VOCs) may affect the health and well-being of inhabitants. Uptake and release of these compounds by and from indoor materials alter their concentrations in indoor air: uptake will lower peak concentrations, whereas subsequent (slow) release at lower concentration levels will prolong the presence of VOCs in indoor air. An experimental set-up has been implemented where indoor materials are placed as a “membrane” separating two air compartments. Both compartments – consisting of Field and Laboratory Emission Cells FLECs – are constantly flushed with air, one air stream containing a mixture of 20 VOCs, and concentrations in both compartments are measured after 1 h. Ten materials usually covering extensive surfaces indoors were consecutively exposed to the vapour mixture at concentration levels typically found in indoor environments. Under the chosen experimental conditions, five of these materials exhibited a permeability high enough that VOCs could be detected on the other side. Mass transport of VOCs into and through indoor materials has therefore been confirmed by experiment. The set-up allows for a quick screening of indoor materials with respect to their sorption capacity and permeability.

Estimation of volatile organic compounds (VOC) concentration within inner void volume of vented waste drums

A test program has been conducted to demonstrate that the concentration of volatile organic compounds (VOC)s within the innermost layer of confinement in a vented waste drum can be estimated using a model incorporating diffusion and permeation transport principles as well as limited waste drum sampling data. A waste drum is assumed to contain a vented rigid polymer liner that holds waste packaged in multiple layers of polymer bags. The model consists of a series of material balance equations describing steady-state VOC transport from each distinct void volume in the drum. The primary model input is the measured drum headspace VOC concentration. Model parameters are determined or estimated based on available process knowledge. This paper summarizes the experimental measurements and model predictions of 13 VOCs in three different drum configurations in vented waste drums.

Theoretical study of multicomponent soil vapor extraction: propagation of evaporation–condensation fronts

Theoretical analysis of the multicomponent soil vapor extraction (SVE) process is presented. It is based on general principles of transport in porous medium and the Raoult's law describing the equilibrium between multicomponent nonaqueous phase liquid (NAPL) and aqueous/gaseous phases. The principal simplifying assumptions of the theory include: isothermal conditions, homogeneous porous medium, radial advective transport of volatile organic compounds, steady-state radial soil gas flow through the contaminated zone, stagnant NAPL and aqueous phases, constant solid–water distribution and gas–water partitioning coefficients. The main emphasis of the study is on the description of propagating composition fronts under local equilibrium conditions. Construction of multicomponent front solutions is carried out by applying the concept of coherent waves, developed by Helfferich and Klein for multicomponent chromatography. It is shown that under typical conditions component volatilities control the sequence of propagating fronts. However, this sequence may change for very small NAPL saturation values (∼10 −3–10 −4 or less) and/or relatively high retardation factors (>10 2). Tailing effect of nonequilibrium (kinetically controlled) gas–water partitioning behind the NAPL zone is also addressed in the presented study. This effect is shown to diminish with the increase of NAPL saturation.

Modeling the Reduction of Vapor Phase Emissions from Surface Soils Due to Soil Matrix Effects: Porosity/Tortuosity Concepts

ABSTRACT A major route for transport of volatile organic compounds within porous media is vapor phase diffusion. The diffusion rate through a porous medium is less than that through free-air due to the decreased cross-sectional area available for gas movement and the increased path length due to pore tortuosity. Numerous empirical expressions have been published that relate the diffusion coefficient in porous media to the diffusion coefficient in free-air (unobstructed gas phase). Published measurements of relative diffusivity and air-filled porosity were combined into a database. Empirical expressions available in the literature, including the popular Millington-Quirk equation, were evaluated along with a fourth-degree polynomial expression developed by the authors to determine the best type of equation to predict relative diffusivity as a function of air-filled porosity over the domain of values for porosity ranging from 0.071 to 1 for different types of materials. Mean square deviations were used as the statistical test to compare equations. The polynomial expression developed in this project produced a significantly different effective diffusion coefficient (1.3 x 10-6 m2/sec) compared to values of 9.2 x 10-6 m2/sec and 3.1 x 10-6 m2/ sec predicted by forms of the Millington-Quirk equation for a specific case.

Analytical Solutions for Partitioned Diffusion in Laminates: II. Harmonic Forcing Conditions

The migration of organic compounds in stratified media is of fundamental concern in environmental and chemical engineering research. The diffusive transport of volatile organic compounds through laminate systems is characterized by partitioning, i.e., the development of concentration discontinuities at the interfaces between the individual laminae. If the transport is governed by cyclic transients, the relevant equations can be written in terms of coupled systems of diffusion equations subject to sinusoidal boundary conditions. This paper solves these systems of equations to present new algebraic solutions for propagation of the sinusoidal modes through arbitrary (finite) numbers of contiguous one-dimensional laminae. Both Cartesian and radial coordinate systems are considered. Two independent formulations of the lamina interface matching conditions are considered, corresponding to (1) an instantaneous partitioning model and to (2) a mass-limited partitioning model. It is shown that sinusoidal components of the concentration solutions propagate dispersively throughout the laminates. This is manifested, for example, in changes in shape of concentration pulses as measured at different points in the laminate system. Algorithms for generating exact dispersion relations for partitioning laminates are given, and an experimental technique for studying interfacial dynamics via frequency domain measurements is proposed.

Design, Installation, and Uses of Combination Ground Water and Gas Sampling Wells

AbstractWaste disposal sites with volatile organic compounds (VOCs) frequently contain contaminants that are present in both the ground water and vadose zone. Vertical sampling is useful where transport of VOCs in the vadose zone may effect ground water and where steep vertical gradients in chemical concentrations are anticipated. Designs for combination ground water and gas sampling wells place the tubing inside the casing with the sample port penetrating the casing for sampling. This physically interferes with pump or sampler placement. This paper describes a well design that combines a ground water well with gas sampling ports by attaching the gas sampling tubing and ports to the exterior of the casing. Placement of the tubing on the exterior of the casing allows exact definition of gas port depth, reduces physical interference between the various monitoring equipment, and allows simultaneous remediation and monitoring in a single well. The usefulness and versatility of this design was demonstrated at the Idaho National Engineering and Environmental Laboratory (INEEL) with the installation of seven wells with 53 gas ports, in a geologic formation consisting of deep basalt with sedimentary interbeds at depths from 7.2 to 178 m below land surface. The INEEL combination well design is easy to construct, install, and operate.

Parameter-Induced Uncertainty in Modeling Vadose Zone Transport of VOCs

This paper evaluates varying results produced by parameter uncertainty and variability in a one-dimensional vadose zone transport model, VLEACH, predicting transport of volatile organic compounds (VOCs) from a contaminated near-surface vadose zone downward to the water table. The model is applied using field data for trichloroethylene (TCE) and tetrachloroethylene (PCE) at a Superfund site in southern California. The research first evaluates sensitivity of the model results to variations in input parameters taken one at a time. Results demonstrate that parameter variations within the ranges seen in field data or derived from the literature may produce variability in model results of more than an order of magnitude. Second, the research evaluates variability in the results generated by uncertainty in parameters when varying jointly, using Monte Carlo simulation techniques. Monte Carlo results demonstrate that model outcomes are strongly affected for the selected site by uncertainty in several parameters produced by a variety of factors, including field measurements of subsurface lithography, forecast mean annual precipitation, and chemical properties of VOCs.

UNDERGROUND CONTAMINATION MECHANIZEM OF VOCs UNDER OSCILLATING WATER TABLE

Groundwater contaminations by VOCs (Volatile Organic compounds) are searched in many sites. These contamination are spread in both saturated zone and unsaturated zone. In these cases, mass transport is very important between these two zones, because the volatilization of VOCs from groundwater and the subsequence process are same mechanism on groundwater contamination.In this paper, the mechanism of VOCs transport in subsurface and these processes are discussed. Two types of experiments are carried out. One is the evaluation of the transport of VOCs in saturatedunsaturated zone under a soil moisture profile. Other is the estimation of the transport of VOCs under the oscillating water table condition. And a numerical model, considered with mass transfer on the interface, is applied to both saturated zone and unsaturated zone. This simulation agrees qualitatively with experimental results.

Distribution of volatile organic compounds in porewater of the seasonally unsaturated Chalk aquifer

Abstract The Chalk aquifer is contaminated at many sites at outcrop by volatile organic compounds (VOCs). Most studies of VOCs concentrate on the saturated zone, ignoring potentially significant accumulations of dissolved contaminants within porewaters of the unsaturated zone. Of particular interest in the Chalk is the large zone of the aquifer over which seasonal fluctuation of the water table occurs. VOCs accumulated in this zone (resulting from vertical migration and diffusion from contaminated ground water at seasonal high water levels) can form a reservoir of contaminants which will be released into the aquifer over a long period. Better understanding of the VOC behaviour in the seasonally unsaturated zone is needed to improve risk assessment and optimize remediation response. Determination of VOC occurrence and concentration in the seasonally unsaturated zone at a contaminated site on the Chalk outcrop is described. Some 500 samples of chalk have been taken from cores drilled in the unsaturated zone over a depth interval of 20 m. Headspace analysis has been used to determine porewater chemistry for a range of VOCs. Vertical profiles of contaminant con- centration are presented and interpreted in terms of VOC transport mechanisms. Hydrocarbon contamination at the site poses an additional element of complexity. The presence of chloride associated with VOC contamination provides a conservative comparison for the distribution behaviour of the VOCs.

Laboratory-scale analysis of aquifer remediation by in-well vapor stripping 2. Modeling results

The removal of volatile organic compounds (VOCs) from groundwater through in-well vapor stripping has been demonstrated by Gonen and Gvirtzman (1997, J. Contam. Hydrol., 00: 000-000) at the laboratory scale. The present study compares experimental breakthrough curves with those predicted by three-dimensional numerical simulation of VOC transport, volatilization, and removal. We are able to sufficiently model the behavior of the laboratory system by assuming an isotropic, homogeneous hydraulic conductivity field and uniform linear retardation of the VOCs. The exponential reductions in concentrations of trichlorethylene, chloroform, and toluene were well represented by the simulation model. Local disparities between experimental and simulated breakthrough curves appear to result primarily from differences between the actual and estimated initial concentrations, and secondarily from differences in the actual and modeled flow field. Our analysis suggests that the in-well vapor stripping process is understood at the laboratory scale. The model developed for this work provides a sound basis for current analysis at the field scale.