Environ. Sci. Technol. 2000, 34, 4250-4251
Response to Comment on “Irreversible Adsorption of Chlorinated Benzenes to Natural Sediments: Implications for Sediment Quality Criteria” SIR: First, we would like to thank Griffiths and Allison for their interest in our recent paper (1) and for their cooperation and discussions in preparing this response. They provided us with copies of the unpublished computer code, COMPETE, as well as preliminary documentation. The documents were helpful in our understanding of their comments and concerns. Essentially, they proposed that our desorption data could be explained with a two-compartment kinetic model. We will demonstrate our reasoning based on our experimental results and field observations. In the above-mentioned paper (1), we proposed that sorption and desorption of chlorinated benzene compounds could be modeled using an irreversible isotherm that we previously developed (2). This is a two-compartment sorption model:
qtotal ) qreversible + qirreversible (µg/g of sediment) wherein the total sorption, qtotal, is the sum of sorption in the reversible and the irreversible compartments. We have found that desorption from the first compartment is generally complete within 1 d, or less, for suspended sediments, as we used in these experiments (2); this is in reasonable agreement with radial diffusion model predictions (3). Desorption from the second compartment is generally complete within 3-8 d. The model discussed in the paper is an equilibrium model and, as such, would only be expected to be applicable to time periods in excess of about 1 week. For shorter time periods, we would recommend the use of a mass transport and kinetic model (4) with the two-compartment irreversible equilibrium model as a limiting case for longer time periods. Griffiths and Allison (10) propose that a slow-fast kinetic model can be used to fit the 1,2-dichlorobenzene (DCB) desorption data in our paper. Of course, this type of twocompartment kinetic model has been of considerable interest to us, and we have systematically tested desorption times of various compounds and sediments from minutes to 180 d. We had performed several years of similar time-consuming desorption studies on naphthalene (Naph) with essentially identical Kow and Koc values (5) as DCB. In these and other preliminary experiments (2, 6), there appeared to be no discernible differences between DCB and Naph desorption. This is illustrated in the Figure 1 for Naph, replotted from Kan et al. (7). The data shown in Figure 1 were discussed in detail in a previous publication (7). To summarize, the open circles are the adsorption data presented in ref 7, and the open triangles are desorption data from experiment 3 of ref 7. Note that the desorption time for the last desorption data point (point A) is 74 d. The solid triangle (point B) is the expected concentration if the desorption is assumed to be kinetically controlled with a characteristic desorption time of 116 d as proposed by Griffith and Allison (10). Note that Griffith and Allison calculated a set of desorption data that matched the expected reversible isotherm by assuming 12 60-d time steps (10). At this point, we cannot test their predictions experi* Corresponding author phone: (713)285-5224; e-mail: atk@ rice.edu. 4250
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 19, 2000
FIGURE 1. Plot of the adsorption and desorption isotherms for naphthalene with Lula sediment. mentally; it would take approximately 2 yr. However, the slow desorption is not observed to be the limiting factor in our experiment with naphthalene where desorption time is 74 d (point A). Using their kinetic model, we should have observed a significant increase in aqueous phase concentration after 74 d (point B), but we did not. The adsorption isotherm follows the reversible sorption path quantitatively, but the desorption deviates as predicted by our irreversible model. Several of the desorption data points were monitored for time periods that would have been expected to show some systematic trend toward the reversible linear isotherm if a kinetic effect similar to that suggested by Griffiths and Allison (10) were operative. In fact, no such kinetic effect was observed in the experiments using naphthalene nor in any similar experiment using phenanthrene for 137 d (8), or PCBs for 180 d (7), to mention a few. After over 10 yr of weathering in a Louisiana bayou sediments, chlorinated benzene concentrations in all cases are better predicted using the irreversible isotherm than the SQC-type linear isotherm (2, 9). Similarly, when the sediments were collected and returned to our laboratory for desorption studies, the desorption was quantitatively modeled using our irreversible isotherm. In all cases examined (6), for field concentrations that span several orders of magnitude, the concentrations are in better agreement with our predictions than with a reversible isotherm. With respect to the implications for the EPA-proposed sediment quality criteria (SQC), clearly it is of paramount importance to know if, in the long term, sorbed contaminants can be expected to desorb from sediments into the overlying waters and become an ecological or a human health risk. This is where we started this research over 10 yr ago, funded by the EPA Hazardous Substances Research Center, South and Southwest and previously by the National Center for Groundwater Research. At this time, it appears that there is a preponderance of evidence to suggest that a second, albeit smaller, fraction of sorbed contaminants desorbs differently (hence the term “irreversible”) from the bulk of the initially sorbed mass. It appears that, after about 3-8 d, this second fraction is controlled by equilibrium considerations that are consistent with our proposed two-compartment model. For most organic compounds, equilibrium desorption from this second compartment is orders of magnitude below that from the first compartment. The potential cost savings for remediation could be enormous if it is established with confidence that this second compartment is controlled by equilibrium considerations, as proposed by the present authors, and not by slow kinetics, as proposed by Griffiths and Allison (10). 10.1021/es002013i CCC: $19.00
2000 American Chemical Society Published on Web 08/23/2000
Literature Cited (1) Chen, W.; Kan, A. T.; Tomson, M. B. Environ. Sci. Technol. 2000, 34, 385-392. (2) Kan, A. T.; Fu, G.; Hunter, M.; Chen, W.; Ward, C. H.; Tomson, M. B. Environ. Sci. Technol. 1998, 32, 892-902. (3) Wu, S.-C.; Gschwend, P. M. Environ. Sci. Technol. 1986, 20, 717-725. (4) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry; John Wiley & Sons: New York, 1993. (5) Mackay, D.; Shiu, W. Y.; Ma, K. C. Illustrated Handbok of Physical-Chemical Properties and Environmental Fate for Organic Chemicals; Lewis Puslishers: Ann Arbor, MI, 1992; Vol. 1. (6) Chen, W.; Kan, A. T.; Fu, G.; Vignona, L. C.; Tomson, M. B. Environ. Toxicol. Chem. 1999, 18, 1610-1616. (7) Kan, A. T.; Fu, G.; Hunter, M. A.; Tomson, M. B. Environ. Sci. Technol. 1997, 31, 2176-2185. (8) Fu, G.; Kan, A. T.; Tomson, M. B. Environ. Toxicol. Chem. 1994, 13, 1559-1567.
(9) Pereira, W. E.; Rostad, C. E.; Chiou, C. T.; Brinton, T. I.; Barbar, L. B., II. Environ. Sci. Technol. 1988, 22, 772-778. (10) Griffiths, R. A.; Allison, C. R. Environ. Sci. Technol. 2000, 34, 4249.
W. Chen Brown and Caldwell, Inc. 1415 Louisianna, Suite 2500 Houston, Texas 77002
A. T. Kan* and M. B. Tomson Department of Environmental Science and Engineering Rice University, MS-519 6100 South Main Street Houston, Texas 77005 ES002013I
VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4251
