Environ. Sci. Technol. 2006, 40, 2489-2490
Comment on “Sorption Kinetics of Organic Contaminants by Sandy Aquifer and Its Kerogen Isolate” This correspondence presents an alternative interpretation of the experiments presented by Ran et al. (1) in their manuscript “Sorption Kinetics of Organic Contaminants by Sandy Aquifer and Its Kerogen Isolate.” The first author and colleagues have published a number of manuscripts describing sorption of hydrophobic compounds to Borden aquifer sand and to non-carbonate carbonaceous material (CM) isolated from a bulk sample of the sand through a combination of demineralization and density separation (termed “isolate”). This most recent manuscript attempts to determine the time required to achieve sorption equilibrium with phenanthrene (PHEN) on bulk Borden aquifer sand and with both PHEN and 1,2-dichlorobenzene (DCB) on the isolate. (DCB sorption to the bulk sand is reported in a prior manuscript.) The authors emphasize a power law to model time-dependent behavior. They use their results to assert that equilibration times are much longer than previously estimated by Ball and Roberts (2) for the same material and that the estimates by Ball and Roberts (2) for tetrachlorobenzene (TeCB) are significantly in error. Ran et al. (1) report that PHEN sorption equilibrium is not attained for either the bulk aquifer solids or the isolate by the completion of their experiments (120 days for the isolate and 365 days for the bulk sand). Yet the Koc data presented in Figure 3a and b of their manuscript (for PHEN with the bulk sand and isolate) suggest that the systems are, in fact, equilibrated. The Koc values have reached an asymptote, represented by relatively consistent values over 3-5 sampling times between ∼2200 and 8800 h (90 and 365 days) for the aquifer solids and at sampling times greater than 1000 h (42 days) for the isolate. The model used by Ran et al. (1) does not include an upper bound or equilibrium sorption value. An alternative model that does incorporate the approach to equilibrium would provide an improved fit of the data. There are several key assumptions in the authors’ data interpretation which are equivocal and yet underpin the authors’ conclusions about the magnitude of equilibrium PHEN sorption and the time to achieve it. The most important of these is that the CM in the isolate is representative of the CM bulk sand. On pages 1653 and 1654, the authors state that “the extracted kerogen accounting for 19% of the organic carbon could represent the NOM [natural organic matter] property in the bulk sand.” (In a related manuscript by some of the same authors (3), the value is reported as 18.7%. We use this value in our calculations.). We counter that their sorption data for dichlorobenzene (DCB) are not consistent with this interpretation. We suggest the alternative hypothesis that the isolate CM represents only that portion of high sorbing CM that is resistant to the isolation procedure (strong acid treatments) and that the CM lost through this procedure comprises an amorphous material that acts largely as a partitioning medium with respect to HOC sorption. In Figure 1, we present the authors’ isotherm results for DCB sorption to the bulk sand in comparison to two models: the author’s original isotherm model and a revised formulation based on the alternative hypothesis described above. We note that Ran et al. interpreted the DCB partition studies to have been at equilibrium for both the isolate ((1), p 1652) and the bulk sand ((3) p 1703). If we assume, as Ran et al. 10.1021/es052472p CCC: $33.50 Published on Web 02/18/2006
2006 American Chemical Society
FIGURE 1. DCB sorption to bulk Borden sand modeled using the two alternative hypotheses and comparison of these to the measured isotherm for the bulk sample. All values are normalized to the original sand mass. The hypotheses are (1) that the isolate CM is representative of the bulk sand CM, shown by the dashed line with circles, as presumed by Ran et al. (1); or (2) that the CM removed during behaves as a partitioning sorbentsthe alternative hypothesis presented here. For the latter model, we sum observed sorption to the isolate (qe,i) and estimated partitioning (qe,p) to the remaining CM, where each contribution to total sorption is normalized to bulk sample mass. The qe,p is estimated as Kocfoc,pCrS where foc,p ) (1 - Ooc,i)foc,bulk, the fraction of the bulk CM retained in the isolate is Ooc,i () 0.187), foc,bulk is the fraction organic carbon content of the bulk sand, and S is the DCB solubility. The qe,i is estimated as [Kf,i(CrS)nOoc,ifoc,bulk]/foc,i where Kf,i and n are the DCB Freundlich sorption coefficients reported for the isolate, and foc,i is the fraction organic carbon content in the isolate () 0.74%). Model (1) is calculated as qe,i/Ooc,i. (1) did, that the equilibrium sorption to the isolate CM is representative of the bulk CM, then the expected total sorption should be obtained as shown in Figure 1. This estimate significantly overpredicts the observed bulk sorption isotherm. Alternatively, the sum of the observed isolate sorption and estimated partitioning to the remaining CM (as described in the figure caption) produces a result that matches the observed isotherm reasonably well. The above comparisons strongly support our alternative hypothesis, i.e., that the isolate CM is not representative of the entire CM with respect to HOC sorption. Because the application of the authors’ rate model depends on an assumption that the isolate CM is representative of the bulk CM, we seriously question the applicability of this model to the bulk sand results. Thus, neither the rate data (described in the prior paragraph) nor the PHEN modeling results (the applicability of which is put in question by the DCB data) can be used to support the authors’ (1) conclusion that very long equilibration times (beyond those previously reported) are needed for TeCB equilibration. We also take exception with the conclusions drawn from comparisons between TeCB sorption to different grain-size separates sieved from the bulk sample (measured by Ball and Roberts (2)) and PHEN sorption to the bulk sample and isolate. The authors use similarities and differences to support interpretations of isolate “representativeness” and, therefore, evaluation of time to equilibrium. While one certainly can compare these various isotherms, we disagree with the authors’ interpretation of the comparative results in several regards. Our first major concern is with the authors’ assertion that because the compounds have similar Kow, the sorption parameters should be the same. There are two difficulties with this. First, correlations of Koc with Kow do not precisely hold, and especially when one is comparing between two different “classes” of chemicals (i.e., a chorinated benzene VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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versus a polycyclic aromatic hydrocarbon). (See Schwarzenbach et al. (5) for additional discussion.) Second, the log Kow values of the two compounds have quite large uncertainties associated with them (e.g., measured values for PHEN range between 4.16 and 4.63 as reported in ref 6). Our second major concern is with the authors’ conclusions from comparisons between the Koc for a particular grain size fraction (as measured and reported by Ball and Roberts (2)) with the new Koc results for either the bulk sample or isolate. Ball and Roberts (2) demonstrated that Koc for a particular compound (either PCE or TeCB) varies with grain size in the Borden sandsthat is, that there is substantial heterogeneity of the sorbent between grain sizes and that no single grain size separate is representative of the bulk. Thus, there is no reason to expect good comparison of bulk material to a grain size separatesin essence, Ran et al. (1) are comparing apples and oranges. In conclusion, we are appreciative of the efforts by these authors and others (1, 4, 7, 8) to isolate the more strongly sorbing fraction of organic and/or elemental carbon that is associated with the Borden aquifer solids, and we agree that the preponderance of evidence strongly suggests that such phases are responsible for the nonconformity of Borden material Koc to previously published relations of Koc to Kow or aqueous solubility (1, 2). We disagree, however, that the CM isolated by these authors is representative of all of the CM in the bulk Borden sample or that the authors’ reported power-law rate model can be gainfully extended beyond the system in which it was calibrated. In addition, we take issue with results that attempt to compare, in simplistic nonmechanistic ways, the sorption results for bulk material or bulk material isolates with sorption results from single examples of separated size fractions that are known to differ in their mineralogic composition as well as their Koc (2).
Literature Cited (1) Ran, Y.; Xing, B.; Rao, P. S. C.; Sheng, G..; Fu, J. Sorption kinetics of organic contaminants by sandy aquifer and its kerogen isolate. Environ. Sci. Technol. 2005, 39, 1649-1657.
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(2) Ball, W. P.; Roberts, P. V. Long-Term Sorption of Halogenated Organic Chemicals by Aquifer Material. 1. Equilibrium. Environ. Sci. Technol. 1991, 25, 1223-1237. (3) Ran, Y.; Xiao, H.; Huang, W. L.; Peng, P. A.; Liu, D. H.; Fu, J. M.; Sheng, G. Y. Kerogen in aquifer material and its strong sorption for nonionic organic pollutants. J. Environ. Qual. 2003, 32, 1701-1709. (4) Ran, Y.; Huang, W. L.; Rao, P. S. C.; Liu, D. H.; Sheng, G. Y.; Fu, J. M. The role of condensed organic matter in the nonlinear sorption of hydrophobic organic contaminants by a peat and sediments. J. Environ. Qual. 2002, 31, 1953-1962. (5) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry, 2nd ed.; John Wiley & Sons: Hoboken, NJ, 2003. (6) Mackay, D.; Shiu, W. Y.; Ma, K. C. Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals; Vol II Polynuclear Aromatic Hydrocarbons, Polychlorinated Dioxins, and Dibenzofurans; Lewis Publishers: Ann Arbor, MI, 1992. (7) Ran, Y.; Xiao, B. H.; Fu, J. M.; Sheng, G. Y.; Sorption and desorption hysteresis of organic contaminants by kerogen in a sandy aquifer material. Chemosphere 2003, 50, 1365-1376. (8) Ran, Y.; Xing, B.; Rao, P. S. C.; Fu, J. Importance of adsorption (hole-filling) mechanism for hydrophobic organic contaminants on an aquifer kerogen isolate. Environ. Sci. Technol. 2004, 38, 4340-4348.
Richelle M. Allen-King Department of Geology 876 Natural Sciences Complex State University of New York at Buffalo Buffalo, New York 14260
William P. Ball Department of Geography and Environmental Engineering The Johns Hopkins University 3400 North Charles Street Baltimore, Maryland 21218 ES052472P
