Envlron. Sci. Technol. 1999, 27, 759-760
SIR: We thank Acree for his discussions of our fluorescence quenching data interpretation. We agree with his derivation and explanation of a system with two static quenchers (his eq 5); however, we suggest that our original eq 2 (I)is more appropriate for a case of one static quencher plus one dynamic quencher. A derivation of this result can be deduced from a modeling analysis of the following reactions: Koc
+ OC + PAH-OC PAH + hv, PAH* kf PAH* PAH + hv2 kQ PAH* + Q PAH + Q* PAH
-
(1)
ka
-
-.
-
(2) (3)
kPa[PAHl total
3, F
--
kf + k a k f k[PAHI total
(4)
kn
PAH* PAH (5) where reaction 1 reflects the "static" association of the PAH probe with organic colloids (OC) resulting in a nonfluorescent complex. (Note, here we assume this associationis completely effectivein preventing those PAH molecules from fluorescing. See ref 2); reaction 2 reflects light absorption by the PAH probe at rate ka, creating an electronically excited PAH species, PAH*; reaction 3 reflects light emission by the excited PAH species at rate kf, which is the fluorescence we detect; reaction 4 reflects the dynamic quenching of the PAH* with intrinsic bimolecular constant rate k~ through collisions with a dissolved species, Q; and reaction 5 reflects all other nonradiative pathways at rate k, by which PAH* might return to its ground state. Since we are interested in the fluorescence
F = kf[PAH*] (6) we need to deduce the steady-state level of this excited PAH species. A kinetic expression for this species would appear: d[PAH*l = ka[PAH] - ~Q[$][PAH*]dt kf[PAH*] - k,[PAH*] (7) which at steady-state (d[PAH*l/dt = 0) yields
Since reaction 1 reflects an equilibrium, we also have
Substituting this result in eq 8 and using this expression for [PAH*l in eq 6, we arrive at the general result
Using eq 10, we can now see that, after ultrafiltration and removal of OC, a probe solution titration should appear: 0013-936X/93/0927-0759$04.00/0
This result is equivalent to eq 3 in our initial publication, where Kuf = kQ/(kf+ kn). Similarly, using eq 10 above, we can deduce the system quenching when whole porewater containing both Q and OC is added:
0 1993 American Chemical Society
(1 + IOCIKoc)( &[Ql+ f
1) (12)
n
which we showed in our original publication as eq 2, and which we based on discussion in ref 3. Expanding this result
one can see that we will only be able to distinguish this combination of quenching behaviors (i.e., two static quenchers as derived by Acree versus one static and one dynamic quencher as suggested by us) when the last term on the right of eq 13 becomes significant relative to 1. We were unable to carry out experiments to observe this nonlinear effect due to the insufficient amounts of porewater available to use. Nonetheless, we stress that this quenching system as reflected by eq 2 in our original report is fundamentally different from that described by Acree; more data are needed to distinguish which equation more accurately reflects these processes in sedimentary porewater. Finally, we note that if Acree's model is the correct one for our system, we could deduce the binding coefficient (his KPAH.Q2)from data such as we show in Figure 1 of ref 1 (i.e., using the quenching seen for the ultrafiltered case). These particular data suggest a log K P A H -for Q ~pyrene of about 4.4, which, when corrected for salting, might decrease to about 4.2 under freshwater conditions. This result is somewhat higher than what we would predict for organic macromoleculeslike fulvic acids ( 4 )that could have passed our ultrafilters. [Interpolated log Koc(pyrene) for Sanhedron Soil fulvic acids is 3.1 and for Suwanee River fulvic acids is 2.8.1 The alternative explanation is that dynamic quenching explains ultrafiltrate's behavior. Utilizing the arguments of Gauthier et al. (51,and assuming the dissolved organic carbon we are adding to the pyrene solution i s acting as a dynamic quencher, we find unreasonably high values for the bimolecular quenching rate constant. Thus, other components of the pore water in much higher abundance than the dissolved organic carbon may be the active quenchers, though they would have to present at least 10-100 times the dissolved organic carbon abundance. We suspect that dynamic quenching is the operative quenching mechanism in our ultrafiltrated porewater; if it is not, then our estimates of hydrophobic chemical mobilities in sediment need to be revised upward. Environ. Scl. Technoi.. Vol. 27, No. 4, 1993
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Literature Cited (1) Chin, Y.-P.; Gschwend, P. M. Environ. Sci. Technol. 1992, 26, 1621.
(2) Backhus. D. A.: Gschwend. P. M. Environ. Sci. Technol. 1990, 24,' 1214.' (3) Lakowicz, J. Principles of Fluorescence Spectroscopy; Plenum: New York, 1983. ( 4 ) Chiou, C . T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Environ. Sci. Technol. 1986, 20, 502.
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(5) Gauthier, T. D.; Shane, E. C.; Guerin, W. F.; Seitz, W. R.; Grant, C. L. Environ. Sci. Technol. 1986, 20, 1162. Yu-Ping Chin and Phlllp M. Gschwend' Ralph M. Parsons Laboratory Department of Civil and Environmental Engineering Massachusetts Institute of Technology Cambridge, Massachusetts 02 139
