Correspondence Comment on “Estimation of Nonaqueous Phase Liquid-Water Interfacial Areas in Porous Media following Mobilization by Chemical Flooding” SIR: We read with interest the recent paper by Saripalli et al. (1) that further develops the concept of using anionic surfactants to measure nonaqueous phase liquid (NAPL)water interfacial areas in porous media, which was originally presented by Saripalli et al. (2). We agree with the authors that quantitative information on NAPL-water interfacial areas is needed to better understand the dissolution of residual NAPL into groundwater, surfactant-enhanced NAPL solubilization and mobilization, and other related processes. The authors are to be commended for addressing this important problem in a novel way. In Saripalli et al. (2) the authors evaluate the feasibility of the method in small-scale column experiments using glass beads, decane as a model NAPL, and sodium dodecyl benzenesulfonate (SDBS) surfactant. Unfortunately, there are several important issues that potentially limit the utility of this method under field conditions. First, it is worthy to note that although labels on the sodium dodecylbenzene sulfonate (SDBS) surfactant used in these studies (1, 2) and others indicate a high level of purity (e.g. > 99%), the surfactant is not pure in the sense that it contains only a single component. The name SDBS is used because the average alkyl-chain length of the surfactant is 12 carbons. However, all commercial products labeled SDBS, also known as linear alkylbenzene sulfonate (LAS), actually are mixtures of C10 to C14 alkyl-chain homologues, each of which also has phenyl-position isomers. The fact that LAS surfactants are actually mixtures has important implications for their use as tracers of NAPL-water interfacial areas. Although the authors (2) assume that adsorption at the solid-water interface is negligible, sorption of LAS to natural sediments has been demonstrated in both laboratory (3-5) and field experiments (6, 7). For example, sorption of LAS during groundwater transport is seen clearly by comparing breakthrough curves for LAS and a bromide tracer (Figure 1a) from natural gradient tracer tests performed at the Cape Cod Toxic Waste Research Site (8) where the aquifer contains < 0.1% organic carbon and no residual NAPL. The retardation factor (Rf) of 1.4 reported for LAS in a field tracer test (8) is larger than the Rf of approximately 1.2, from which an interfacial area of 338 cm2/cm3 is inferred for a glass bead column containing 20% residual decane (1). Identifying a surfactant with the tendency to accumulate only at NAPLwater interfaces is difficult since the amphiphilic character of the surfactant also causes its sorption to aquifer sediments. The inability of the method to distinguish between retardation caused by sorption to aquifer sediment and accumulation at the NAPL-water interface seriously limits the applicability of the method in the field. In addition, the sorption of LAS homologues to naturally occurring sediment organic matter increases with increasing alkyl-chain length, which results in the “chromatographic” separation of LAS components during transport. An example from the natural gradient tracer tests performed at the Cape Cod site illustrates this behavior; retardation factors for individual LAS homologues ranged from 1.0 to 2.9 (Figure 3836
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FIGURE 1. Breakthrough curves for (a) conservative tracer (bromide) and individual linear alkylbenzene sulfonate (LAS) homologues in a natural gradient tracer test and (b) in a column containing clean glass beads and no residual decane. 1a) (8). We also observed chromatographic separation in a column experiment similar to that described in Sarapalli et al. (1) in which a 220 mg/L solution of LAS and 100 mg/L sodium bromide was passed through a 1.1 × 30 cm column filled with 250 µm clean glass beads without any residual decane. The breakthrough curves for LAS homologues indicate that even for a column of clean glass beads, chromatographic separation of LAS occurs with retardation factors ranging from 1.0 for C10-LAS to 1.3 for C13-LAS (Figure 1b). In the absence of residual decane, the retardation factor for the total LAS mixture is 1.07, which is essentially identical to the retardation factor that can be inferred from Figure 1 of Saripalli et al. (1) that was used to calculate a decanewater interfacial area of 93 cm2/cm3. Chromatograms obtained by high performance liquid chromatography at two points on the breakthrough curve (data not shown) indicated that only toward the end of the breakthrough curve did the LAS composition approach that of the injected LAS solution. This finding has implications for the use of a single value of Ko, the secant of the isotherm slope (dΓ/dC) for the adsorption of LAS at the fluid-fluid interface, which is used to calculate interfacial area from experimental retardation factors. First, the relation between surface tension and concentration is a function of the LAS mixture composition (9) and typically is determined from serial dilutions of a LAS mixture of fixed composition. Second, because the relation between surface tension and concentration is not linear (2), Ko is a function of LAS 10.1021/es9803355 CCC: $15.00
1998 American Chemical Society Published on Web 10/24/1998
concentration. Therefore, the single value of Ko derived for a single concentration of known composition is of questionable applicability because the composition and concentration of LAS varies continuously during field (and column) experiments.
(7) Robertson, W. D.; Sudicky, E. A.; Cherry, J. A.; Rapapport, R. A.; Shimp, R. J. In International Symposium on Contaminant Transport in Groundwater; Stuggart, Germany, 1989. (8) Krueger, C. J.; Barber, L. B.; Metge, D. W.; Field, J. A. Environ. Sci. Technol. Accepted for publication. (9) Smith, D. L. J. Am. Oil Chem. Soc. 1997, 74, 837-845.
Literature Cited (1) Saripalli, K. P.; Annable, M. D.; Rao, P. S. C. Environ. Sci. Technol. 1997, 31, 3384-3388. (2) Saripalli, K. P.; Kim, H.; Rao, P. S. C.; Annable, M. D. Environ. Sci. Technol. 1997, 31, 932-936. (3) Hand, V. C.; Williams, G. K. Environ. Sci. Technol. 1987, 21, 370-373. (4) Brownawell, B. J.; Chen, H.; Zhang, W.; Westall, J. C. In Organic Substances and Sediments in Water; Baker, R. A., Ed.; Lewis Publishers: Chelsea, MI, 1991. (5) Matthijs, E.; Henau, H. D. Tenside Detergents 1985, 22, 299304. (6) Barber, L. B.; Krueger, C.; Metge, D. W.; Harvey, R. W.; Field, J. A. In Surfactant Enhanced Surfactant Remediation: Emerging Technologies; Sabatini, D. A., Ed.; ACS Symposium Series 594; American Chemical Society: Washington DC, 1995.
Jennifer A. Field* Department of Agricultural Chemistry Oregon State University Corvallis, Oegon 97331
Jonathan D. Istok Department of Civil, Construction, and Environmental Engineering Corvallis, Oregon 97331 ES9803355
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