Evidence for Very Tight Sequestration of BTEX Compounds in Manufactured Gas Plant Soils Based on Selective Supercritical Fluid Extraction and Soil/Water Partitioning

Abstract

Benzene, toluene, ethylbenzene, o-, m-, and p-xylenes (BTEX), and polycyclic aromatic hydrocarbons (PAHs) were extracted from eight manufactured gas plant (MGP) soils from sites that had been abandoned for several decades. Supercritical fluid extraction (SFE) with pure carbon dioxide demonstrated the presence of BTEX compounds that were highly sequestered in both coal gas and oil gas MGP soils and soots. Benzene was generally the slowest compound to extract from all samples and was even more difficult to extract than most two- to five-ring PAHs found on the same samples. Since the solubility of benzene in carbon dioxide is 2-5 orders of magnitude higher than the solubilities of PAHs, these results demonstrate that benzene was more tightly sequestered than toluene, ethylbenzene, xylenes, or the multi-ring PAHs. Additional evidence for very tight binding was based on the fact that BTEX concentrations determined using either SFE or with methylene chloride sonication were much higher than those obtained by the U.S. EPA purge-and-trap method, especially for benzene (whose concentration was underestimated by as much as 1000-fold by the EPA method). However, soil/water desorption showed little benzene mobility, and Kd values for benzene were 1-2 orders of magnitude higher than those calculated based on literature sorption K(OC) values. These results indicate that environmentally relevant concentrations of benzene may be better represented by mild extraction methods than by methods capable of extracting tightly bound benzene.

Modeling Studies on the Transport of Benzene and H2S in CO2-Water Systems

“…Poling (2007). Other data are from (Hawthorne and Miller, 2003), adapted from Bartle et al (1991), Miller et al (1996), and Mackay and Shiu (1977). …”

“…Francis (1954) reported that benzene and near-critical CO 2 are fully miscible at room temperature and 6.6 MPa. Hawthorne and Miller (2003), in a study related to supercritical fluid extraction of benzene from soils, indicate that benzene and SCC are fully miscible.…”

“…The distribution of benzene between soil and water has been studied in the field of environmental remediation for cleaning organic contaminated soils (for example, Hawthorne and Miller, 2003). Usually a K d (distribution coefficient) approach is applied.…”

Factors affecting the phytoaccumulation of weathered, soil-borne organic contaminants: analyses at the ex Planta and in Planta sides of the plant root

“…The extreme hydrophobicity of POPs, many of which have K ow values near or above 10 6 , purportedly resulted in their preferential adsorption to soil organic matter. Sequestration or irreversible adsorption of POPs in the soil matrix over time (Hawthorne and Miller 2003) was also assumed to reduce their bioavailability to soil microbes, fauna such as earthworms (Morrison et al 2000) and, by extension, to flora. Consequently, half-lives of the chemicals in soils range from years (Handbook of Environmental Degradation Rate 1991), to decades (Mattina et al 1999) and even centuries (Kurt-Karakus et al 2006).…”

Impact of soil microstructure on the molecular transport dynamics of 1,2-dichloroethane

“…In a subsequent study, Hawthorne and Miller (2003) used increasingly vigorous supercritical fluid extraction (SFE) conditions to characterize the degree of sequestration for benzene, toluene, ethylbenzene, o-, m-, p-xylenes in addition to the PAHs from 8 of the samples investigated in the previous study. For all the samples, the largest slowly desorbed fractions of total chemical present requiring the strongest SFE conditions were found to be benzeneNm,pxyleneNnaphthalene.…”