Soil vapour extraction

Air sparging Groundwater Air is pumped through groundwater to air-strip volatile contaminants which are then removed by soil vapour extraction... 

Figure 1 Schematic of a soil vapour extraction system...

The at least 50% higher vapour pressure of MTBE, ETBE and diisopropyl ether (DIPE) compared to benzene describes the stronger volatilization tendency of these fuel oxygenates. Therefore these substances are amenable to soil vapour extraction. 

Remediation strategies may involve the use of one or more technologies with an increase in overall efficiency (e.g. multi-phase extraction - a combination of soil vapour extraction and groundwater pump-and-treat). From those presented above, an overview of hydrolysis in acidic conditions and adsorption on different media is presented in the following sections. 

Kawata et al. [ 128] have described the effects of headspace conditions on recoveries of volatile organic compounds from sediments and soils. Hewitt [129] compared three vapour partitioning headspace and three solvent extraction methods for the preparation of soil samples for volatile organic carbon determination in soils. Methanol extraction was the most efficient method of spiked volatile organic carbon recovery, which depended on the soil organic carbon content, the octanol-water partitioning coefficients of analytes and the extraction time. 

Air sparging has been apphed to remediation of CAH (especially tetra-chloroethene and trichloroethene) and BTEX for many years, but it is only conditionally suitable for the treatment of MTBE-contaminated groundwater. The low Henry constant of MTBE has an unfavourable effect since imder in-situ conditions the substance can hardly be transferred from the water into the gas phase. To avoid cross-contamination of the imsaturated zone, air sparging is usually combined with a soil air vapour extraction. However, air sparging can stimulate biological processes, which may lead to a significant MTBE decay. 

Sakamoto et al. [148] have shown that the differential determinations of different forms of mercury in soil can be accomplished by successive extraction and cold vapour atomic absorption spectrometry. 

Bandyopadhyay and Das [151] extracted mercury from soils with the liquid anion exchanger Aliquat-336 prior to determination by cold vapour atomic absorption spectrometry. 

ISO (2001) Soil Quality - Determination of mercury in aqua regia soil extracts with cold-vapour atomic spectrometry or cold vapour atomic fluorescence spectrometry, ISO/DIS 16772. 

With the advent of unleaded gasolines, the oxygenates, such as MTBE which have been used to replace lead additives, are readily soluble in water and have low vapour pressures making purge and trap less of an option especially when the recommended method of extraction of the soil by methanol is adopted. 

The compounds described in this chapter cover a broad range of properties. They can be hydrophobic or hydrophilic, which may also be expressed by different distribution coefficients, Kow and Koc, between octanol and water or soil organic carbon and water. They also have different vapour pressures and solubilities in water. Due to the different properties, the interaction with the soil matrix also differs. Moreover, soil is not a single species it contains mineral compounds of different sizes and properties as well as organic matter from various origins - all in different proportions. All the various compounds to be analysed and the diverse soil properties have implications for the pre-treatment of the sample, extraction or isolation of the analyte from the soil matrix, possible clean-up methods and the analytical procedure which can be used for quantification and qualification. 

Morei ef a/. (1996) [5] Chioroform iiquid and vapour, 75 min thereafter 40 h equilibration prior to extraction Deionized water, 16 h,1 10 0-50 mg P/kg soil during 24 h equilibration and 16 h extraction Phosphate None... 

Traditional methods for the extraction of methylmercury from soil and sediment involve water vapour or vacuum distillation ° besides acid leaching procedures, that were introduced in the late 1960s. Here, microwave-assisted extraction has also been very successful methylmercury is extracted from sediments using acetic acid in less than 3 min. ... 

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