Vaporize volatile contaminants

A U.S. EPA study (41) showed that soil vapor extraction (SVE) is an effective treatment for removing volatile contaminants from the vadose zone. Sandy soils are more effectively treated than clay or soils with higher organic content because higher air flows are possible in sand and clays—organic soils tend to adsorb or retain more contaminants. Removal of volatiles is rapid in the initial phase of treatment and thereafter decreases rapidly thereafter-an important consideration in the design of air emissions control over the life of the project. 

Soil vapor extraction (SVE) can be used to remove volatile contaminants and, when combined with another technology, to treat nonvolatile contaminants. If contamination has reached the aquifer, it is necessary to use SVE in combination with groundwater pumping and air stripping. 

The partitioning exhibited through the Henry s Law constant can be used to estimate the vaporization of various PCB contaminants from solid surfaces. In the presence of water, organic compounds volatilize more rapidly than would be expected based upon vaporization of the pure compound. This tendency accounts for the presence of low vapor pressure contaminants, such as the PCBs, in the atmosphere at higher concentrations than one would estimate from the chemistry of the pure compounds [403,408,409]... 

An additional comphcating factor with respect to air samphng rate calibration is the uncertainty associated with determinations of vapor-phase and particle sorbed concentrations of analytes by HiVol sampling. These systems suffer from artifacts such as the volatilization of particle-bound contaminants, insufficient retention of small particles, and adsorption of vapor-phase contaminants on the GFFs (Ockenden et al., 1998 Lohman et al., 2001). These artifacts may cause concentrations in the vapor-phase to be overestimated or underestimated, which results in sampling rates that are too low or too high. 

In the past (prior to 1974), exposure of humans to heptachlor and heptachlor epoxide was directly related to the application of heptachlor as an insecticide. However, because of the persistence and bioaccumulation of heptachlor and heptachlor epoxide, exposure of the general population can occur through ingestion of contaminated food (especially cow s or maternal human milk), inhalation of vapors from contaminated soil and water, or direct contact with residual heptachlor from pesticide application. People whose homes have been treated may continue to be exposed to these chemicals in the air over long periods. Occupational exposure can occur in the manufacture of the chemical or from use of heptachlor to control fire ants. The most likely routes of exposure at hazardous waste sites are unknown. Heptachlor has been found infrequently in soil and groundwater at hazardous waste sites. Children who eat contaminated soil or people who obtain tap water from wells located near hazardous waste sites might be exposed to heptachlor. Also, since both compounds can volatilize from soil, people living near hazardous waste sites may be exposed to the compounds in the air. 

The two-phase vacuum extraction (TPVE) technology allows for the in situ remediation of soils and groundwater contaminated with volatile organic compounds (VOCs). Two-phase vacuum extraction is similar to conventional vapor extraction in the equipment required, with the exception that it is designed to actively remove contaminated groundwater from the extraction well along with the vapor-phase contamination. 

Grid injection is a commercially available, in situ technology for the treatment of soils contaminated with organic compounds. The technology injects steam to vaporize volatiles and drive out nonvolatiles in a fashion similar to steam stripping. 

Spartech is a mobile system designed for the removal of volatile contaminants from aquifers. It is a patented air sparging system that works by bubbling air through an aquifer and is essentially an in situ method of air stripping. The volatilized contaminants may be recovered by using a soil vapor extraction (SVE) system or similar device. 

The HD CatOx system treats vapor emissions contaminated with halogenated volatile organic compounds (VOCs). HD CatOx is a trade acronym for the term halohydrocarbon destruction catalytic oxidation system. This system is based on the use of a proprietary catalyst for a... 

The GEiM-lOOO low-temperature thermal desorption unit is an ex situ technology that treats soils contaminated with volatile organic compounds (VOCs). This process involves a countercurrent drum, pulse-jet baghouse, and a catalytic oxidizer mounted on a single portable trailer. As the soil is heated in the GEM-1000 unit, contaminants are vaporized. The contaminants are then directed to the system s catalytic oxidizer, which is designed to convert virtually all of the VOCs to carbon dioxide and water vapor. The oxidizer contains approximately 4.9 ft of noble metal catalyst and can destroy between 95 and 99% of the hydrocarbons when operating between 600 and 1250°F. 

The primary application of the PSVE technology will likely be to complement active soil vapor extraction efforts. PSVE could also be used on the edge of unsaturated zone contaminant plumes where concentrations of volatile contaminants are low or for enhancement of bioremediation activities. The primary advantages of PSVE application are low capital costs and minimal operating costs. One-way valves may also be incorporated so that the system only takes in or lets out air through wells. 

Based on a cost analysis performed at the U.S. Department of Energy s Hanford site, in Richland, Washington, PSVE was found to be a cost-effective method for remediation of soils containing lower concentrations of volatile contaminants. PSVE used on wells that average 10 standard cubic feet per minute (scfm) airflow rates was found to be more cost-effective than active soil vapor extraction for concentrations below 500 parts per million (ppm) by volume of carbon tetrachloride. For wells that average 5 scfm, PSVE is more cost effective below 100 ppm (D14489S, p. iii). For further details of this analysis, refer to Table 1. 

During SIVE applications, traditional soil vapor extraction (SVE) is augmented by steam, which is injected into the subsurface. The steam vaporizes volatile and semivolatile contaminants and displaces liquids in soil pores. Both vapor and liquids are then pumped to the surface via extraction wells. 

The remediation site must be capable of supporting drilling operations where vapor extraction wells are required. Microorganisms used for bioremediation are not effective in toxic soil conditions. Also, because the rate of bioremediation is much slower than vapor extraction, the system must operate for a period of time after volatile contaminants have been removed by vapor... 

TerraTherm Environmental Services, Inc., a subsidiary of Shell Technology Ventures, Inc., has developed the in situ thermal desorption (ISTD) thermal blanket technology to treat or remove volatile and semivolatile contaminants from near-surface soils and pavements. The contaminant removal is accomplished by heating the soil in sim (without excavation) to desorb and treat contaminants. In addition to evaporation and volatilization, contaminants are removed by several mechanisms, including steam distillation, pyrolysis, oxidation, and other chemical reactions. Vaporized contaminants are drawn to the surface by vacuum, collected beneath an impermeable sheet, and routed to a vapor treatment system where contaminants are thermally oxidized or adsorbed. 

In the process, the TDU heats contaminant molecules above their boiling points to desorb the contaminants into the vapor phase in an oxygen-deficient environment, thus preventing oxidization of the contaminants. The volatilized contaminants and the moisture from the soil are condensed and collected within the system. The process is then controlled by using infrared heating and an alloy belt feed system within the TDU. 

The purpose of trip blanks is to assess the collected sample representativeness by determining whether contaminants have been introduced into the samples while they were handled in the field and in transit, i.e. in coolers with ice transported from the site to the analytical laboratory. A possible mechanism of such contamination is the ability of some volatile compounds, such as methylene chloride or chlorofluor-ocarbons (Freons), to penetrate the PTFE-lined septum and dissolve in water. Potential sources of this type of contamination are either ambient volatile contaminants or the VOCs that could be emanating from the samples themselves, causing sample cross-contamination. To eliminate ambient contamination, samples must not be exposed to atmospheres containing organic vapors. Cross-contamination is best controlled by such QA measures as sample segregation and proper packaging. 

Hand-held field PIDs and FIDs are often used for headspace screening of soil vapor for the presence of VOCs and petroleum fuels during soil excavation. Fifty to 100 g of soil are placed into a resealing plastic bag and allowed to equilibrate at the ambient or an elevated temperature. The meter s inlet is then inserted into the bag, and the concentration of the volatilized contaminants in the headspace above the soil is measured. In reality, the instrument is detecting all of the ionizable compounds in the headspace above the soil, including gaseous byproducts of bacterial activity and... 

The vapors of volatile contaminants, such as radon and volatile organic compounds, can be transported through diffusion from the soil pore spaces into buildings. Three principal factors are needed to define the ratio of contaminant concentration in indoor... 

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