Oxygenated fuel induced cosolvent effects on the dissolution of polynuclear aromatic hydrocarbons from contaminated soil
Introduction
Oxygenated gasoline has been used in many areas under the mandate of the Clean Air Act Amendments of 1990. These Amendments require fuel oxygenates, such as methyl tert-butyl ether (MTBE) or ethanol, to be added to gasoline in specific metropolitan areas to reduce atmospheric concentrations of carbon monoxide and ozone. Also, oxygenated fuels have been studied by car manufacturers under the alternative fuel vehicle program for the purpose of reducing emissions. The alternative fuels, M85 and E85 (15% gasoline and 85% methanol or ethanol, respectively), were commonly selected for evaluation under the original guidelines of the Alternative Motor Fuels Act of 1988 and the Energy Policy Act of 1992.
Releases of oxygenated fuels to the subsurface from underground storage tanks, pipelines, and refueling facilities provide point source for entry of oxygenated compounds as well as gasoline hydrocarbons into the hydrological cycle. The fuel additive has brought a lot of attention in groundwater and surface water contamination issue in the past decade. In particular, MTBE contamination is widespread and currently considered a major threat to drinking water resources (Johnson et al., 2000; Powers et al., 2001). As the nation moves toward phasing out MTBE in gasoline, ethanol is emerging as a potential replacement in reformulated gasoline. The use of oxygenated fuels has brought increased interest to the transport and fate of miscible organic liquids in the environment and the effect that these liquids can have on the transport and fate of other contaminants (Squillace et al., 1996, Squillace et al., 1997; Chen and Delfino, 1997; Pankow et al., 1997; Franklin et al., 2000; Shih et al., 2004).
The addition of polar organic solvents that are completely-miscible or highly-soluble in water to a mixture of hydrocarbons and water initiates a cosolvent effect (Fu and Luthy, 1986; Munz and Roberts, 1986; Groves, 1988; Pinal et al., 1990, Pinal et al., 1991). Water-miscible cosolvents such as alcohols (e.g., methanol, ethanol), and ketones (e.g., acetone) may reduce the net polarity of the oxygenate-water solvent. When a miscible constituent is added to water, it increases the quantity of nonionic organic compounds that can dissolve in the mixed solvent. For instance, the presence of the cosolvent in an oxygenated fuel (e.g., M85, which is 85% methanol and 15% gasoline) could enhance the transport of the fuel constituents. The effect of cosolvents on the solution-phase activity of organic compounds depends on the nature of the solute and solvent–cosolvent system. A simple relationship describing the influence of cosolvent on the solubility of a solute in the mixed-solvent system is the log-linear cosolvency model (Yalkowsky et al., 1972, Yalkowsky et al., 1976; Yalkowsky and Rubino, 1985; Fu and Luthy, 1986; Kimble and Chin, 1994; Chen and Delfino, 1997):where S is the solubility of the solute in mixed-solvent (m), and water (w), β is an empirical coefficient that accounts for water–cosolvent interactions, fc is the volume fraction of organic cosolvent in the aqueous phase, and σ represents the cosolvency power of the cosolvent.
Cosolvency is defined as the effect of organic solvents on the solubility and sorption of hydrophobic organic compounds (HOCs) (Rao et al., 1990). The cosolvency power (σ) can be viewed as the hypothetical partition coefficient for the HOCs between a cosolvent and water. The logarithm of the ratio of HOC solubilities in pure cosolvent and in pure water (Sw) is equal to the cosolvency power (σ):where Sc is the solubility of the solute in pure cosolvent (Pinal et al., 1990).
Miscible organic liquids affect the fate and transport of organic contaminants in the subsurface by altering the sorption characteristics of hydrophobic compounds. The addition of cosolvents has been shown to increase mass transfer rate in sorption (Nkedi-Kizza et al., 1989; Brusseau et al., 1991; Augustijn et al., 1994). Cosolvent will result in a decrease in retardation and sorption coefficient. For the sorption of HOCs from aqueous-organic binary solvent mixtures, the sorption coefficient is predicted to decrease exponentially as the fraction of organic solvent increases. A log-linear cosolvency model has been established to relate the equilibrium sorption coefficient (Kp) to the volume fraction of cosolvent from binary mixed solvent (Rao et al., 1985, Yalkowsky and Rubino, 1985, Augustijn et al., 1994, Kimble and Chin, 1994). The equation is expressed as:where Kp,b and Kp,w are the equilibrium sorption constants for the binary solvent and aqueous systems, respectively, α is a nonideality coefficient that accounts for cosolvent–sorbent interactions, β accounts for water–cosolvent interaction as defined in Eq. (1), and σ is the cosolvency power of the cosolvent.
The decrease in Kp caused by addition of cosolvent results in a reduction in retardation. In contaminant transport, equilibrium sorption is commonly expressed in terms of a retardation factor (R) which represents the residence time of a chemical in pore volumes:where θ is the volumetric water content (ml/cm3), and ρ is the dry bulk density (g/cm3).
When spilled oxygenated fuels encounter contaminated soil, sediment or aquifer material, the aqueous concentrations of HOCs that previously had sorbed onto soil/sediment may increase by orders of magnitude due to the enhancement of solubility and decreased sorption induced by the miscible oxygenated compounds from oxygenated fuels. Given the theory discussed above, the use of oxygenated fuels indicates that cosolvency could be an important phenomenon in the release and transport of contaminants that are pre-deposited in soils. The objective of this study was to investigate the cosolvent-induced dissolution of PAHs from a coal tar contaminated soil due to oxygenated fuel spills. Specifically, the phase redistribution of PAHs from contaminated soil upon the addition of oxygenated fuels was studied using packed columns to simulate the subsurface environment. The results are expected to provide fundamental information in terms of fuel spill remediation and allow further assessment of the effects of spills and leaks of oxygenated fuels in the subsurface environment.
Section snippets
Contaminated soil
Coal tar contaminated soil from Waterloo, Iowa was used to study the redistribution of polynuclear aromatic hydrocarbons (PAHs) in soil and aqueous phases in a packed soil column system. The contaminated soil used in this study was the subsample of surface soil sample collected from a former manufactured gas plants (MGP) site. Upon receipt, samples were wet-sieved through a 2 mm sieve, homogenized, and stored at 4 °C in glass bottles with Teflon® lined caps. The physical and chemical properties
Recovery of surrogates
Surrogate compounds were added to each aqueous sample prior to extraction. Recovery of surrogate compounds ranged from 76% to 87% with an average recovery of approximately 83%. Spiking contaminated soil samples with a solution of surrogate compounds prior to extraction allowed for the assessment of the impacts of the sample matrix and analyte type on the relative extraction efficiencies of the batch extraction method. Recovery of surrogate compounds ranged from 67% to 93% with an average
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