Effect of compositional heterogeneity on dissolution of non-ideal LNAPL mixtures.
The extent of dissolution of petroleum hydrocarbon fuels into groundwater depends greatly on fuel composition. Petroleum fuels can consist of thousands of compounds creating different interactions within the non-aqueous phase liquid (NAPL), thereby affecting the relative dissolution of the components and hence a groundwater plume's composition over long periods. Laboratory experiments were conducted to study the variability in the effective solubilities and activity coefficients for common constituents of gasoline fuels (benzene, toluene, p-xylene and 1,2,4-trimethylbenzene) (BTX) in matrices with an extreme range of molar volumes and chemical affinities. Four synthetic mixtures were investigated comprising BTX with the bulk of the NAPL mixtures made up of either, ethylbenzene (an aromatic like BTX with similar molar volume); 1,3,5-trimethylbenzene (an aromatic with a greater molar volume); n-hexane (an aliphatic with a low molar volume); and n-decane (an aliphatic with a high molar volume). Equilibrium solubility values for the constituents were under-predicted by Raoult's law by up to 30% (higher experimental concentrations) for the mixture with n-hexane as a filler and over-predicted by up to 12% (lower experimental concentrations) for the aromatic mixtures with ethylbenzene and 1,3,5-trimethylbenzene as fillers. Application of PP-LFER (poly-parameter linear free energy relationship) model for non-ideal mixtures also resulted in poor correlation between experimentally measured and predicted concentrations, indicating that differences in chemical affinities can be the major cause of deviation from ideal behavior. Synthetic mixtures were compared with the dissolution behavior of fresh and naturally weathered unleaded gasoline. The presence of lighter aliphatic components in the gasoline had a profound effect on estimating effective solubility due to chemical affinity differences (estimated at 0.0055 per percentage increase in the molar proportion of aliphatic) as well as reduced molar volumes (estimated at -0.0091 in the activity coefficient per unit increase in molar volume, mL/mol). Previously measured changes in activity coefficients due to natural weathering of 0.25 compares well to 0.27 calculated here based on changes in the chemical affinity and molar volumes. The study suggests that the initial estimation of the composition of a fuel is crucial in evaluating dissolution processes due to ideal and non-ideal dissolution, and in predicting long term dissolution trends and the longevity of NAPL petroleum plume risks.
Department of Civil Engineering, Bannari Amman Institute of Technology, Sathyamangalam, Erode, India. Electronic address: devamv@gmail.com.
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Vasudevan
CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia; School of Earth and Environment, The University of Western Australia, Nedlands, WA, Australia.
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Johnston
CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia.
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Bastow
CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia.
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Lekmine
CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia.
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Rayner
Environmental and Water Resources Engineering Division, Department of Civil Engineering, Indian Institute of Technology, Madras, India. Electronic address: indunambi@iitm.ac.in.
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Nambi
Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology, Madras, India. Electronic address: gskumar@iitm.ac.in.
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Suresh Kumar
Department of Chemical Engineering, Indian Institute of Technology, Madras, India. Electronic address: rrk@iitm.ac.in.
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Ravi Krishna
CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia; School of Earth and Environment, The University of Western Australia, Nedlands, WA, Australia. Electronic address: Greg.Davis@csiro.au.
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Davis
D001554:Benzene / Q000737:chemistry; D001555:Benzene Derivatives / Q000737:chemistry; D005742:Gasoline / Q000032:analysis; D060587:Groundwater; D006586:Hexanes / Q000737:chemistry; D012995:Solubility; D014050:Toluene / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000032:analysis / Q000737:chemistry*; D014992:Xylenes / Q000737:chemistry
C482494:1,3,5-trimethylenebenzene; D001555:Benzene Derivatives; D005742:Gasoline; D006586:Hexanes; D014874:Water Pollutants, Chemical; D014992:Xylenes; C026385:n-hexane; C010313:pseudocumene; D014050:Toluene; C031286:4-xylene; D001554:Benzene; C004912:ethylbenzene
Activity coefficient; Effective solubility; Multicomponent dissolution; Non-aqueous phase liquid; Non-ideal; UNIFAC