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Response to Comment on “Temperature Dependence of Slow Adsorption and Desorption Kinetics of Organic Compounds in Sediments” SIR: In a comment on our recent paper (1), Opperhuizen and Schrap state that, in their opinion, we used an incorrect equation for the interpretation of our sediment desorption data. This equation is
St/S0 ) Frap exp(-krapt) + Fslow exp(-kslowt)
(1)
in which St/S0 is the amount sorbed at time t; Frap and Fslow are the rapidly and slowly desorbing fractions, respectively; and krap and kslow are the rate constants of rapid and slow desorption, respectively. This equation has been used in the same form by Young and Leong (2) as well as by Langenfeld et al. (3). Opperhuizen and Schrap argue that the exponents in this equation are not equal to the desorption rate constants; instead, the desorption rate constants are relatively complicated functions of these exponents. This is explained in the articles they have previously published on this topic (4, 5). However, we think it is justified to use eq 1 in our case. We will not elaborate on the mathematical deduction of this equation (this is relatively straightforward) but rather focus on the point that is crucial in this discussion. This crucial point is the assumption that readsorption is negligible during our desorption experiments. Readsorption during desorption can occur in three ways: (i) from water to the slowly sorbing sediment compartment, (ii) from the rapidly sorbing to the slowly sorbing sediment compartment, and (iii) from water to the rapidly sorbing sediment compartment. In our paper (ref 1, Experimental Section), we have stated that readsorption is required to be negligible for the validity of eq 1, referring to another paper we recently published (6). In that paper, we discuss at length the issue of readsorption during desorption. There we have shown that readsorption rates (ng/h) are 10-400 times lower than desorption rates (ng/h) during the slow desorption phase. The reason that readsorption is so low lies in the character of our sediment extraction method, using solid Tenax TA beads. These beads adsorb water-dissolved organic compounds at a much higher rate than sediment does; rate constants for adsorption to Tenax are around 15 h-1, whereas rate constants of slow adsorption to sediment are only in the order of 0.000120.006 h-1 at the sediment concentration of 6 g/L that we employed (4). In addition, in our experimental setup the Tenax sorption capacity is much larger than the sediment sorption capacity: Tenax-water partition coefficients and organic carbon (OC)-water partition coefficients are in the same order of magnitude, while we had at least 10 times more Tenax (0.2-0.6 g) than OC (0.02-0.06 g) in our
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experimental system and did 12 or more replacements of Tenax during a desorption experiment. Thus, the assumption of no readsorption appears to be justified for the slow desorption phase because of the large difference in readsorption and desorption rates. During the rapid desorption phase, readsorption rates can be calculated to be a factor of 3-10 lower than desorption rates, so the assumption of negligible readsorption may be questionable in some cases for the rapid desorption phase. In our paper (1), however, we only draw conclusions on slow desorption. On the whole, we believe that the data and conclusions presented in our paper would not be different if Opperhuizen and Schrap’s model (4, 5) were applied instead of the simple model represented by eq 1. The model described by Opperhuizen and Schrap is required for desorption data description when extraction rate constants are low (below approximately 1 h-1), whereas in our case the extraction rate constants caused by the Tenax beads are so high that the use of eq 1 is justified and in our opinion preferable because of its relative simplicity. Finally, we would like to apologize for incorrectly referring to the paper by Schrap et al. (4) when introducing eq 1.
Literature Cited (1) Cornelissen, G.; Van Noort, P. C. M.; Parsons, J. P.; Govers, H. A. J. Environ. Sci. Technol. 1997, 31, 454-460. (2) Young, T. M.; Leong, G. Preprints of papers, 214th ACS National Meeting, Environmental Chemistry Division, Las Vegas, September 7-11, 1997; American Chemical Society: Washington, DC, 1997; Vol. 37, pp 141-143. (3) Langenfeld, J. J.; Hawthorne, S. B.; Miller. D. J.; Pawliszyn, J. J. Anal. Chem. 1995, 67, 1727-1736. (4) Schrap, S. M.; Sleijpen, G. L. G.; Seinen, W.; Opperhuizen, A. Environ. Sci. Pollut. Res. 1994, 1, 81-92. (5) Schrap, S. M.; Sleijpen, G. L. G.; Seinen, W.; Opperhuizen, A. Environ. Sci. Pollut. Res. 1994, 1, 70-81. (6) Cornelissen, G.; Van Noort, P. C. M.; Govers, H. A. J. Environ. Toxicol. Chem. 1997, 16, 1351-1357.
Gerard Cornelissen* and Paul C. M. van Noort Institute for Inland Water Management and Wastewater Treatment RIZA P.O. Box 17 8200 AA Lelystad, The Netherlands
John R. Parsons and Harrie A. J. Govers Department of Environmental and Toxicological Chemistry ARISE University of Amsterdam Nieuwe Achtergracht 166 1018 WV Amsterdam, The Netherlands ES972018O
S0013-936X(97)02018-X CCC: $15.00
1998 American Chemical Society Published on Web 03/15/1998
