Semivolatile contaminants

In the thermal desorption technique excavated soil is heated to around 200 to 1000°F (93 to 538°C). Volatile and some semivolatile contaminants are vaporized and carried off by air, combustion gas, or inert gas. Off-gas is typically processed to remove particulates. Volatiles in the off-gas may be burned in an afterburner, collected on activated carbon, or recovered in condensation equipment. Thermal desorption systems are physical separation processes that are not designed to provide high levels of organic destruction, although some systems will result in localized oxidation or pyrolysis. 

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. 

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. 

Thermal desorption is a technology that physically separates volatile and some semivolatile contaminants from contaminated media. In thermal desorption, heated air is used to volatilize contaminants at temperatures below those used for incineration. There are both in situ and ex situ applications of the technology. Ex situ treatments typically are used to remediate soil, sediments, sludges, and filter cakes. In situ applications of the technology use injected steam, thermal blankets, or heat supplied by electrodes to volatilize contaminants, which are then removed using extraction wells. 

To accomplish the thermal desorption, contaminated media are heated, generally between 300 and 1000°F, thus driving off the water, volatile contaminants, and some semivolatile contaminants from the contaminated media and into the off-gas stream. The removed contaminants are then treated by thermal oxidation in an afterburner, condensed in a single- or multiple-stage condenser, or captured by carbon adsorption beds. 

Semmens, M. J., Method of Removing Organic Volatile and Semivolatile Contaminants from Water, U. S. Patent 4960520,1990. 

Hauser, B., Popp, P., and Kleine-Benne, E., Membrane-assisted solvent extraction of triazines and other semivolatile contaminants directly coupled to large-volume injection-gas chromatography-mass spectrometric detection, J. Chromatogr. A, 963, 27-36, 2002. 

While the gas chromatographic technique is used for most of the analysis of volatile and semivolatile contaminants in workplace air for compotmds with relative low volatility, or compounds that are deriva-tized for better stability or increased sensitivity, HPLC is the analytical method of choice. UV - visible and fluorescence detectors are the two common types of detectors that are used. 

However, the membrane was found not to be very efficient at removing phenolics. Rejections were in the range of 18% for phenolics. Overall, based on a comparison of total concentratitHis of a pre-designated list of creosote-derived PAH and phenolic semivolatile contaminants in the permeate versus the feed water, the system did not meet the claimed rejectirai efficiency of 90%. 

Frequently, column problems are caused by the samples that are being analyzed. This type of problem is more likely to occur on capillary columns because of their low capacity for contamination. Contamination results when the sample contains nonvolatile or even semivolatile materials such as salts, sugars, proteins, and so on. Column contamination is more frequently observed with splitless injection because larger amounts of material are being injected on the column. 

The symptoms of column contamination include irregular peak shape, loss of resolution, loss of retention, irregular or noisy baseline, and ghost peaks from semivolatile materials of a previous run or from sample decomposition. Some of these problems can be the result of a contaminated injector. 

Complex mixtures of contaminants in the soil, such as a mixture of metals, nonvolatile organics, semivolatile organics, and so on, make it difficult to formulate a single suitable washing fluid that will remove all the different types of contaminants from the soil. Sequential washing steps, using different additives, may be needed. In fact, each type of contaminated soil requires a special treatment procedure, which is determined through laboratory or pre-industrial tests, so that system modifications and optimum operative conditions are specified. 

Malina, G., Grotenhuis, J.T.C. and Rulkens, W.H., Vapor extraction/bioventing sequential treatment of soil contaminated with volatile and semivolatile hydrocarbon mixtures, Bioremed. J., 6, 159-176, 2002. 

Raising the temperature of the soil increases the vapor pressure of the contaminants, improving their ability to volatilize. Many semivolatile compounds will eventually be released as the temperature rises, although these compounds tend to need longer residence times. 

Sites suitable for conventional SVE have certain typical characteristics. The contaminating chemicals are volatile or semivolatile (vapor pressure of 0.5 mm Hg or greater). Removal of metals, most pesticides, and PCBs by vacuum is not possible because their vapor pressures are too low. The chemicals must be slightly soluble in water, or the soil moisture content must be relatively low. Soluble chemicals such as acetone or alcohols are not readily strippable because their vapor pressure in moist soils is too low. Chemicals to be removed must be sorbed on the soils above the water table or floating on it (LNAPL). Volatile dense nonaqueous liquids (DNAPLs) trapped between the soil grains can also be readily removed. The soil must also have sufficiendy high effective porosity (permeability) to allow free flow of air through the impacted zone. 

Bristol DW, Howard LC, Lewis RG, et al. 1982. Chemical analysis of human blood for assessment of environmental exposure to semivolatile organochlorine chemical contaminants. J Anal Toxicol 6 269- 275. 

Samples containing heavy oil, along with the volatile components can severely contaminate pnrge-and-trap instrumentation, and caution is advised when interpreting the data. For such samples it may be advisable to use a separatory funnel for the water extraction method for semivolatiles (EPA 3520). In this method, the sample is ponred into a funnel-shaped piece of glassware, solvent is added, and the mixtnre is shaken vigorously. After layer separation, the extract (i.e., the solvent layer) is removed. Altered, dried with a desiccant, and concentrated. Multiple extractions on the same sample may increase overall recovery. 

For the separation of samples from contaminated soil, there are also several possible methods, depending on whether the contaminant is volatile or semivolatile ... 

Applicable to a wide variety of VOCs and certain semivolatile organic contaminants (SVOCs) under specific conditions... 

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