Fate Of Volatile Organic Compounds Research Articles

A novel laboratory system for determining fate of volatile organic compounds in planted systems

AbstractContradictory observations regarding the uptake and translocation of volatile organic compounds (VOCs) by plants have been reported, most notably for trichloroethylene (TCE). Experimental artifacts resulting from the use of semistatic or low‐flow laboratory systems may account for part of the discrepancy. Innovative plant growth chambers are required to rigorously quantify the movement of VOCs through higher plants while maintaining a natural plant environment. The plant must be sealed in a chamber that allows rapid exchange of air to remove the water vapor lost in transpiration, to resupply the CO2 consumed in photosynthesis, and to resupply the O2 consumed in root‐zone respiration. Inadequate airflow through the foliar region results in high humidity, which dramatically reduces transpiration and may reduce contaminant flux. Oxygen depletion in static root zones induces root stress, which can increase root membrane permeability. The root zone must be separated from the shoots to differentiate between plant uptake and foliar deposition. Here we describe the design, construction, and testing of a dual‐vacuum, continuous high‐flow chamber system for accurately determining the fate of VOCs in plants. The system provides a natural plant environment, complete root/shoot separation, the ability to quantify phytovolatilization and mineralization in both root and shoot compartments, continuous root‐zone aeration, and high mass recovery.

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.

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.

Effects of humidity and soil organic matter on the sorption of chlorinated methanes in synthetic humic-clay complexes

Vapor-phase sorption is the most influential process governing the transport and the fate of volatile organic compounds in soil. To understand the influence of both soil organic content and the humidity of soil on the vapor sorption is an important process for degradation processes. The single-pellet moment technique was used to investigate sorption and diffusion of trichloromethane (TCM) and carbon tetrachloride (CTC) at varying relative humidities (0–80%) of synthetic humic-clay complex pellets consisting of clay (montmorillonite) and different amounts of organic matter (humic acid). The effective diffusivities of TCM and CTC did not show a noticeable change with moisture and humic acid content. On the other hand, with increasing humic acid content of clay at 0% relative humidity, an appreciable decrease of the equilibrium sorption constants of the tracers (TCM, CTC) was found because of the blockage of some sites of the mineral surfaces and especially micropores by the humic acid. The presence of water also reduced dramatically the sorption of TCM and CTC on synthetic humic-clay complexes. Above 20% relative humidity, the sorption coefficient of TCM and CTC varied only slightly with humic acid content. It was concluded that the sorption of TCM and CTC in synthetic humic-clay complexes was strongly effected by the moisture and humic acid content.

Air/Water Exchange Dynamics of 13 Volatile Chlorinated C1- and C2-Hydrocarbons and Monocyclic Aromatic Hydrocarbons in the Southern North Sea and the Scheldt Estuary

Whereas a lot of marine research work has been done on heavy metals and semivolatile organic compounds, far less attention has been paid to the fate of volatile organic compounds in this area. This work focuses on 13 volatile organic compounds in marine waters and the marine atmosphere. During six campaigns in September 1994−December 1995, simultaneous air and water sampling was carried out in both the southern North Sea area and the Scheldt Estuary. Mean water concentrations for individual compounds were in the 2.2−72.9 ppt range (n = 38), whereas atmospheric concentrations varied between 2.3 and 854 pptv. Samples from the Scheldt Estuary proved to have elevated concentrations of chlorinated compounds. A number of water and air samples showed enhanced concentrations of C2-substituted monocyclic aromatic hydrocarbons, which could not clearly be linked to anthropogenic activities. Back-trajectory calculations showed that a number of over sea atmospheric samples were affected by atmospheric transport from i...

Volatile organic compounds in the surface waters of a British estuary. Part 2. Fate processes

The fate of volatile organic compounds (VOC) in the Southampton Water estuary, presented in Part 1 of this paper, are described. The principal processes responsible for removing VOC from the water column include dilution and net transport, volatilisation and adsorption onto sediments. Volatilisation is the most important removal mechanism, and is significantly influenced by temporal and spatial changes in aqueous concentration and by the proximity of sources. Using surface renewal models, volatilisation rates ranging from 300 kg day −1 in summer months to 2000 kg day −1 in winter months were estimated. Within the water column, net transport rates were 3 to 5 times higher in winter relative to summer, reflecting the influence of enhanced input rates and increased dilution flows. Adsorption processes varied widely in their efficiency, and were dependent upon the availability of adsorbing solid surfaces within the water column and VOC affinity for sorption processes. The highest levels of adsorption were identified for chloroform in wastewater effluents where the organic matter content of suspended solids approached 100%. Within the estuary as a whole, not more than 5% of VOC such as aromatics and organohalogens was predicted to be adsorbed under the most favourable environmental conditions.

Fate of Volatile Organic Compounds and Pesticides In Composted Municipal Solid Waste

Composting of municipal solid waste (MSW) has many advantages including volume reduction, reduced atmospheric emissions of methane from landfills, and the potential for beneficial use of the end product. Information about the fate of volatile organic chemicals (VOCs) which are commonly present in MSW is needed to protect the environment from undesirable emissions. To achieve this goal, the present study was undertaken to evaluate the fate of four VOCs (benzene, carbon tetrachloride, dichlorobenzene, and xylene) and two pesticides (Captan and Lindane) added to simulated MSW. Volatile organic chemicals were added at three concentrations while pesticides were tested at a single concentration. All of the treatments were replicated three times and composted in 200 L aerated static pile composters. The composters were specially equipped with pumps and flow meters to aerate the feedstock at 1.12-1.68 m3/hr, sorption filters to collect volatile organic chemicals from the exiting air stream, and leachate collection vessels. The feedstock was selected to be representative of MSW after the majority of recyclable metal and glass had been removed and was spiked with organic chemicals known to commonly occur in MSW.The results showed that the majority of VOCs present in the feedstock were lost via volatilization in the initial 48 hours of composting. Concentrations of VOCs were below detection limits after one week of composting in both the leachate and compost. Thus, while aerated pile composting is effective in removing VOCs from the feedstock, the exiting air stream will need to be monitored for approximately one week and possibly treated to meet environmental quality standards.Aerobic composting reduced the initial concentrations of 275 mg/kg each of Captan and Lindane to final concentrations of 53.8 and 158.9 mg/kg, respectively, after five weeks of composting. This corresponded to half lives of 1.5 and four weeks during composting for Captan and Lindane respectively. All concentrations of both pesticides were below detection limits in all samples of air, condensate and leachate, in dicating that none of the pesticides were volatilized and all were retained by the MSW until they biodegraded.

A Multi-Parameter Sensitivity Analysis of a Model Describing the Fate of Volatile Organic Compounds in Trickling Filters

ABSTRACT A frequency array technique was employed to assess the impact of process operating and design conditions, biofilm, and compound-specific properties on the fate of volatile organic compounds (VOCs) in trickling filter processes. The objective of the study was to identify the ranges of parameters over which model predictions of biodégradation, volatilization, and overall removal are particularly sensitive. The model was found to be most sensitive to liquid loading rates less than 20 m3/m2-d and gas loading rates less than 200 m3/m2-d. The model predictions indicate that an effective method for controlling VOC emissions is through control of trickling filter ventilation rates. Effluent recycle had only marginal impact on model predictions. Model predictions were most sensitive to biofilm thickness less than 1 x 10-4 m, liquid-gas mass transfer coefficients less than 0.10 m/d, Henry's law coefficients less than 0.10, and biodégradation rate coefficients less than 20 m3/kg-d. When employing the model for predictive purposes, greater care should be taken in defining parameters for compounds that lie within these ranges.

Oxygen uptake and VOC emissions at enclosed sewer drop structures

Municipal wastewater is currently receiving regulatory scrutiny as an emissions source for volatile organic compounds (VOCs). In response, the fate of VOCs at drop structures has been studied. The study described herein involved two major components. First, 64 pilot experiments were conducted to study the effects of drop height, liquid flow rate, and tail water depth on oxygen transfer. Second, 15 additional experiments were completed to determine the effects of the same parameters on the stripping of a cocktail comprising 10 volatile tracers. Headspace ventilation rates, chemical properties, and the differences between clean water and raw wastewater on VOC stripping were studied. Results suggest that oxygen is a useful surrogate for VOC measurements as both oxygen and VOC mass transfer were observed to be highly dependent on drop height and less on liquid flow rate. Stripping efficiencies for VOCs were observed to vary between 0.5 and 32% depending upon specific compounds and experimental conditions. VOC stripping was also observed to be greater from clean water as compared to wastewater. Gas‐liquid mass transfer of VOCs was seen to be a strong function of Henry's law coefficient, suggesting the Occurrence of gas‐phase mass transfer resistance.

The Effect of Moisture and Soil Texture on the Adsorption of Organic Vapors

AbstractThe fate of volatile organic compounds (VOCs) moving as vapors in the subsurface is dependent on their interaction with the soil. Adsorption of VOC vapors is greatly influenced by soil texture and soil‐water content. The effects of differences in texture and soil‐water content on vapor partition coefficients for trichloroethylene (TCE) were examined. Batch experiments were conducted for a variety of soils and at different soil‐water contents, w, to determine the relationship between the vapor/solid partition coefficient, KD′, and w. In dry soils, KD′ was nonlinearly related to soil‐water content because, in that range, water molecules compete with VOC molecules for adsorption sites on the soil surface. Under wet conditions, KD′ became linearly related to water content according to Henry's law, indicating that adsorbed water molecules were acting as a solvent for VOC molecules. In general, KD′ under oven‐dry conditions did not relate well to total specific surface area of soils, most likely because VOC molecules adsorb only on the outside surfaces of soil particles (due to their nonpolarity), rather than the total surface area present. In the dry range, adsorption was dominated by soils with high specific areas (i.e., high clay content), while soils with higher organic carbon content manifested higher adsorption amounts in the wet moisture range. A one‐parameter, exponential model well described the log KD′‐w curve in the nonlinear region. The model parameter, a, was found to be highly dependent on the specific surface area of the soil. The proposed KD′‐(w) model incorporated in conventional VOC transport models seems promising for analyzing the effects of VOC vapor adsorption on VOC subsurface transport.

Fate of volatile organic compounds in municipal activated sludge plants

ABSTRACT: The fate of volatile organic compounds (VOCs) in municipal activated sludge plants was investigated at pilot and full scale. More than 80% of the mass flow of nonchlorinated compounds was observed to be biodegraded while less than 20% was removed by stripping. Conversely, 46% of the mass flow of chlorinated compounds was biodegraded and 47% was removed by stripping. Adsorption of VOCs onto waste sludge was not a significant removal mechanism. The pilot plant employed was found to effectively emulate the liquid‐gas phase partitioning and overall compound removals observed in a full‐scale plant. Little difference in liquid‐gas phase partitioning was observed for coarse and fine pore diffusers in spite of differing mass‐transfer characteristics as demonstrated by oxygen‐transfer measurements. This implies that equilibrium between the liquid and gas‐phase concentrations was achieved with both diffuser types. Stripping of VOCs was observed to increase with airflow; however, the extent of stripping increased at a declining rate.

Predicting the Fate of Volatile Organic Compounds in Municipal Wastewater Treatment Plants

Pilot plant and full scale investigations were carried out to determine the fate of selected volatile organic compounds (VOCs) in activated sludge aeration basins. Treatability parameters for each VOC were estimated from these investigations and used to calibrate TOXCHEM, computer-based steady state and dynamic models developed to predict the fate of VOCs in municipal activated sludge systems. The pilot plant was fed with wastewater from two different municipal sources. It was operated in parallel with a municipal treatment plant and was found to adequately simulate the performance of the full scale plant. Data suggest that the current models, calibrated with pilot plant data, may produce useful predictions of the fate of VOCs in full scale plants.

Degradation of organic vapors in unsaturated soils

To predict the fate of volatile organic compounds (VOC s) in unsaturated soil, it is necessary to understand the physical and chemical processes that occur in the soil system. Two important removal mechanisms of organic vapors in soil are sorption and biodegradation. Modeling efforts to describe the removal of organic vapors in soil require constitutive relationships that are developed from laboratory data. This paper reports measurements of removal coefficients for three VOC s in a fine sandy loam soil. Sorption coefficients and degradation removal rates were determined from batch reactors for three compounds: benzene, trichloroethylene (TCE) and o-xylene. Results indicate organic vapors are sorbed and then removed by biodegradation in the unsaturated soil system. This information has potential for use in the bioremediation of soils contaminated with VOC s and for impacting decisions on air emissions, regulatory limits and on-site controls.

Marina developments as sources of hydrocarbon inputs to estuaries

The intense development of marinas, especially within estuaries on the coastline of southern England, has led to major inputs of potentially toxic organic compounds. During a research programme on the occurrence, transport and fate of volatile organic compounds within the estuarine water and sediments of Southampton Water, we discovered massive inputs of hydrocarbons, both during and after construction phases. In one instance, measurements made before development and at 3-monthly intervals over a 3-year period showed a 10-fold increase of a range of aromatics, alkanes cycloalkanes and organohalogens. Our investigations show that, during construction phases, the local aquatic environment becomes a major sink for anthropogenic wastes relating to construction activities. At post-construction phases, yachts and small boats continue to act as major sources of hydrocarbons to the estuary.

Soil-gas surveying techniques

Delineation and remediation of subsurface contamination have become a major focus of environmental science during the past five years. Conventional technologies available for subsurface investigations always will be required to confirm and monitor subsurface contamination; however, quicker and less expensive techniques are useful for preliminary site evaluations. Soil-gas surveying is a technique that is applicable to a wide range of volatile organic compounds (VOCs) under a variety of geologic and hydrologic settings. The most common uses of soil-gas data include planning monitoring well networks and defining plume boundaries for remedial action. Preliminary screening techniques are effective in selecting locations for detailed sampling and analysis. Site investigators can use results from a preliminary soil-gas survey to drill monitoring wells at locations within the boundaries of a VOC volume. Soil-gas investigations also can be used to identify sources of VOCs and to distinguish between soil and groundwater contamination. Chemical analysis of soil gases has recently been used to monitor solvent and fuel leaks from underground storage tanks. In order to effectively design soil-gas surveys and interpret their results, the subsurface transport and fate of VOCs must be understood. These phenomena can have a profound impact on the presence and concentrations of VOCs more » in the soil atmosphere. Physical, chemical, and microbiological processes can be important in determining VOC concentrations in soil gas. « less

Distributions and Fate of Volatile Organic Compounds in Narragansett Bay, Rhode Island

A wide range of volatile organic compounds (VOC) were measured in the water column along a north–south transect in Narragansett Bay, Rhode Island, during two summers and two winters. The dominant VOC are chlorinated C2-hydrocarbons, chlorobenzenes, and aromatic hydrocarbons. Major sources for these anthropogenic compounds are municipal and industrial wastewaters discharged into the upper bay. Concentration trends along the transect are variable depending on the compound class and the season of sampling, indicating that different processes control the distributions of different compounds. Volatilization apparently is a major removal process for all VOC. Calculations suggest water column residence times with respect to volatilization of 150–300 h in Narragansett Bay. Biodegradation, particularly in summer, is important for aromatic hydrocarbons, which are degraded in a few days. Sorption onto particulate matter and eventual sedimentation is minor, except for the higher molecular weight alkanes.