Environ. Sci. Technol. 1997, 31, 3734-3735
Response to Comment on “Accumulation of Organochlorine Pesticides and PCBs by Semipermeable Membrane Devices and Mytilus edulis in New Bedford Harbor” SIR: We thank Huckins et al. for their interest and comments on our paper (1). Their previous paper in this Journal (2) provided the basis and inspiration for our work. From their correspondence, however, we cannot decipher any substantial scientific disagreement. It appears that any differences we have on the design and use of SPMDs is mostly philosophical. We agree on the theoretical description of SPMD behavior as described by Huckins et al. (2). We agree that there are some unresolved issues related to practical implementation; for example, the importance of diffusion layer controlled uptake versus membrane controlled uptake and the use of permeability reference compounds to correct for changes in uptake rates due to changes in temperature, biofouling, and/or flow conditions. We also agree that further research will allow us to resolve these issues and that SPMDs offer tremendous potential for both research and monitoring studies. It is our position that to resolve these uncertainties and better understand the behavior of SPMDs one needs to experiment with both the SPMD design and environmental conditions. Our paper was simply an attempt to study SPMD behavior at one design extremesthat of maximal accumulation rates and rapid equilibration. The conclusions from our paper were as follows. (a) One can modify the standard SPMD design to increase the rate of uptake so that equilibrium between the dissolved phase and the lipid phase is reached much faster than with the standard design. (b) This modified design appears to be a better mimic of accumulation in mussels than the standard design. (c) Average dissolved concentrations in New Bedford Harbor that were estimated using SPMD residues and an equilibrium model were in excellent agreement with measured concentrations. Thus, it appears that these or perhaps other modifications could offer advantages over the standard SPMD design for certain applications. Huckins et al. also mention in their correspondence that they are modifying the standard SPMD so that even moderately hydrophobic compounds such as naphthalene will remain in a linear uptake phase over a 30-60-day deployment. This is in response to data indicating that the standard SPMD deployed for 30-60 days will approach equilibrium (>75% of the equilibrium concentration in the SPMD) for many moderately hydrophobic compounds. Thus, the use of a linear uptake model for the standard SPMD and for these compounds may be less appropriate than an equilibrium model. Huckins et al. are modifying the standard SPMD to force a linear uptake of most compounds of interest. In our paper, we modified the standard design to force rapid equilibrium for most compounds of interest. We view the two approaches as complimentary and have never suggested that our SPMD design using an equilibrium model is inherently better than the standard SPMD design using a linear model. We do suggest that regardless of design, one must know where (approximately) each analyte is on the uptake curve at the time of SPMD retrieval and use the model (linear or equilibrium) that best represents conditions at the site of deployment. To help illustrate the differences between our SPMD design and the standard SPMD we have modeled the behavior of several alternative SPMD designs during numerous plausible scenarios in nature, such as during constant exposure conditions or when 10-fold spikes in exposure concentrations
3734
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997
FIGURE 1. Simulation of PCB 52 accumulation into two different SPMD designs: the standard SPMD (SPMD-std, 2) designed by Huckins et al. (2) and a more rapidly equilibrating SPMD (SPMDlite, 9) designed by Hofelt and Shea (1). Two different scenarios are shown: (a) where exposure concentration (Cw, b) is constant and (b) where Cw increases 10-fold for three separate 3-day periods. occur at various times, durations, and periodicities during deployment. These model results and some experimental results during these changing exposure conditions will be presented in a subsequent publication. An example is shown in Figure 1 for a model compound, PCB congener 52, where actual exposure concentrations (Cw) and predicted SPMD residues (CSPMD) are plotted in arbitrary units as a function of time. When Cw is constant (Figure 1a), the standard SPMD (SPMD-std) accumulates the compound in a linear manner over 30 days, while the more rapidly equilibrating SPMD (SPMD-lite) approaches equilibrium. Using the final SPMD residue values and the appropriate model (linear uptake model for the standard SPMD and equilibrium model for the rapidly equilibrating SPMD), we estimate nearly identical values for the average Cw (1.00). An alternative case is shown in Figure 1b, where Cw increases 10-fold for three separate 3-day periods during a 30-day deployment. The standard SPMD accumulates the compound in a linear manner over the course of the deployment and does not lose a significant fraction during periods of lower exposure. Thus, it is still integrating exposure over time. The more rapidly equilibrating SPMD accumulates a much higher concentration of the compound in a nearly linear manner during high exposure periods, but actually loses the compound during the lower exposure period. In this particular case, the average Cw estimated from the final SPMD residue concentrations are nearly identical for both designs (3.20 vs 3.22). Other scenarios such as having a large concentration spike at the beginning or end of the deployment period would cause the rapidly equilibrating SPMD to underestimate and overestimate, respectively, the actual average Cw. However, in most of our
S0013-936X(97)02014-2 CCC: $14.00
1997 American Chemical Society
model simulations, the deviations using the SPMD-lite are within a factor of 2 of the actual Cw. The good agreement between predicted and actual Cw in these model simulations and in the field (see our original paper), along with the simplicity and convenience of using an equilibrium model and published octanol-water partition coefficients, provide justification to consider our SPMD design for some applications. In addition, we have found that the more rapidly accumulating SPMD provides better limits of detection, particularly when shorter deployment times are used to reduce the effect of biofouling (compare the CSPMD values at 100 h in Figure 1b). Our point is not that an equilibrium model is better than a linear model, but it can lead to comparable results without the need to measure additional sampling rates in the laboratory and then develop a means for adjusting those rates to represent specific conditions at the site of deployment. Design modifications such as ours also can lead to a better understanding of the behavior and limitations of SPMDs. Huckins et al. suggest that modifications to the standard SPMD design, such as those advocated by us, be undertaken only after careful consideration. We agree with this and further suggest that this cautionary note be applied to any passive sampler design including the standard, commercially available SPMD. We understand the desire to develop a universal passive sampling device based on a standardized SPMD design that is well-characterized, has sampling rates that are known to be reproducible in nature, and has a broad range of
applications. Our modified SPMD design does not meet these criteria, nor was that our intention. As currently designed, the standard SPMD does not meet these criteria either. Ultimately, a truly integrative sampler that has well-known sampling rates that are applicable to a broad range of analytes and environmental conditions should be preferable to an equilibrium sampler for many applications, particularly routine monitoring of contaminated waters. However, it is highly unlikely that a single passive sampler design would be universally optimum for all applications. Thus, the design and use of the SPMD must match the application, and the limitations and potential problems during a particular deployment must be understood. Our understanding of the behavior and limitations of SPMDs will become more complete only after experimentation such as that reported by us in our original paper (1).
Literature Cited (1) Hofelt, C. S.; Shea, D. Environ. Sci. Technol. 1997, 31, 154-159. (2) Huckins, J. N.; Manuweera, G. K.; Mackay, D.; Lebo, J. A.; Petty, J. D. Environ. Sci. Technol. 1993, 27, 2489-2496.
Damian Shea* and Chris S. Hofelt Department of Toxicology North Carolina State University 3709 Hillsborough Street Raleigh, North Carolina 27607 ES972014J
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3735
