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Comment on “Accumulation of Organochlorine Pesticides and PCBs by Semipermeable Membrane Devices and Mytilus edulis in New Bedford Harbor” SIR: Hofelt and Shea (1) recently conducted a side-by-side comparison of lipid-containing semipermeable membrane devices (SPMDs) and blue mussels, and they suggested changes to optimize the standard SPMD design (2-4) for a specific application. Given the growth of SPMD use, Hofelt and Shea raise several timely issues related to SPMD design, performance, application, and comparability to biota. The commercially available, standard SPMD consists of 1 mL of triolein (20% triolein by SPMD weight) sealed in low-density polyethylene (LDPE) tubing with 450 cm 2 of surface area and a wall thickness of 75-90 µm. In contrast, Hofelt and Shea’s SPMD configuration consisted of 0.5 mL of triolein (15% triolein by SPMD weight) sealed in LDPE tubing with 900 cm2 of surface area and a wall thickness of 25 µm. These modifications of the standard SPMD design were intended to increase SPMD sampling rates (i.e., reduce response times) and to enable the use of an equilibrium partition modeling approach, thereby more closely simulating bivalve uptake of chemicals. Bivalves often approach or reach steady-state concentrations of organic chemicals in exposures of e40 days (5-7), and thus, inferences of environmental chemical levels from contaminant concentrations in tissues are necessarily based on equilibrium partition models. The obvious advantages conferred by a universal or standardized SPMD suggest that modifications such as those advocated by Hofelt and Shea (1) should be undertaken only after careful consideration. This does not mean that important research results into the effects of configuration changes on SPMD performance (1) should be ignored or that new developments should not be utilized in certain applications. It is instructive to examine the relationship between equilibrium or steady-state concentrations in a passive insitu sampling device or organism and the rates of uptake and elimination. Assuming a simple, one-compartment model:
Css/Cw ) K ) k1/k2
(1)
where Css is the chemical concentration in the whole SPMD or organism at equilibrium or steady state, Cw is the water concentration, K is the partition coefficient, k1 is the uptake rate constant (L d-1 g-1), and k2 is the elimination rate constant (t-1). For SPMDs, the magnitude of K is fixed for a specific compound and SPMD configuration. Thus, when k1 is increased by increasing the area of the exchanging surface and not the mass and/or by thinning the membrane (assuming membrane rate control), k2 will rise proportionally. Both overall uptake and elimination are governed by firstorder kinetics, and the response time (8) and halflife are 1/k2 and t1/2 (ln 2/k2), respectively. When the lipid content is known, the fractional amount (Fr) of water cleared relative to the equilibrium clearance capacity of the lipid in an SPMD or fish is given by
Fr ) VwcL/(KLwVL)
(2)
where VwcL is the volume of water cleared of chemical at time t or Rst (Rs is the SPMD sampling rate in L d-1 or k1 g), KLw is the equilibrium lipid-water partition coefficient, and VL is the volume of lipid. Because Chiou (9) has shown by direct
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liquid-liquid partitioning that Kow ≈ KLw, and Huckins et al. (4) found the KLw values of Chiou were similar to those measured in SPMDs at g90% of steady state:
t1/2u ) -ln 0.5 (KowVL/Rs)
(3)
the t1/2u or halftime of chemical uptake (note that t1/2u ≈ t1/2) does not account for the significant contribution of the SPMD membrane, and thus is an underestimation. Since it requires about four t1/2u values to reach >90% of the steady-state chemical concentration, t1/2u or t1/2 values must be small (e.g., 100 L d -1 would be required for PCBs with log Kow values > 6.5. Achieving a 100 L d -1 sampling rate with the Hofelt and Shea design seems improbable based on potential encounter volume (volume of water encountering a surface in which chemical exchange can occur) limitations (8, 13). Also, we found only a 2-fold increase in the rate of loss from naphthalene-spiked SPMDs placed in clean water with a 4-fold decrease (from 153 to 38 µm) in wall thickness (unpublished data, Midwest Science Center, Columbia, MO). This may be due to the switching of rate control to the aqueous diffusion layer as the membrane is thinned or to thickness-related changes in membrane structure. Huckins et al. (14) found that the aqueous diffusion layer may play a significant role in the uptake of some chemicals by standard SPMDs, especially in quiescent systems and for SPMDs with thin membranes. Lipid normalization of residue concentrations were performed by Hofelt and Shea (1). Although the correlation between lipid-normalized BCFs and lipid-normalized SPMD accumulation factors appeared to be good, non-normalized data should be examined as well to ensure that variance is actually reduced. When SPMDs or biota are in the linear or curvilinear phase of uptake, lipid weighting of data is inappropriate. For example, if two environmental monitors (fish and SPMDs) are in the linear or curvilinear phase of chemical uptake and their sampling rates are identical, both matrices would contain about the same amount of chemical at the end of an exposure. However, if one monitor contains 5% lipid and the other contains 15%, normalization of their chemical concentrations to lipid weight during the linear uptake phase would result in values that are 3-fold greater in the 5% lipid sample. This procedural artifact is in marked
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1997 American Chemical Society
FIGURE 1. Comparison of alkylated naphthalene residence time in SPMDs and oysters (Crassotrea gigas). Note that bivalves, such as oysters, are known to have very low or no detectable concentrations of mixed function oxygenase enzymes. contrast to the equivalent lipid-weighted values (assuming ideal case) that would be obtained if both sampling matrices were at equilibrium. Overall, we believe that the work of Hofelt and Shea (1) is a valuable contribution but changes in SPMD design and the use of lipid normalization should be cautiously considered in light of our comments.
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Literature Cited (1) Hofelt, C. S.; Shea, D. Environ. Sci. Technol. 1997, 31, 154-159. (2) Huckins, J. N.; Tubergen, M. W.; Manuweera, G. K. Chemosphere 1990, 20, 533-552. (3) Huckins, J. N.; Manuweera, G. K.; Petty, J. D.; MacKay, D.; Lebo, J. A. Environ. Sci. Technol. 1993, 27, 2489-2496. (4) Huckins, J. N.; Petty J. D.; Lebo, J. A.; Orazio, C. E.; Prest, H. F.; Tillitt, D. E.; Ellis, G. S.; Johnson, B. T.; Manuweera, G. K. In Techniques in Aquatic Toxicology; Ostrander, G. K., Ed.; CRCLewis Publishers: Boca Raton, FL, 1996; pp 625-655. (5) Goldberg, E. D. Mar. Pollut. Bull. 1975, 6, 111. (6) Langston, W. J. Mar. Biol. 1978, 45, 265-272. (7) Phillips, D. H., Ed. Quantitative Aquatic Biological Indicators; Applied Science Publishers LTD: London, England, 1980; p 488. (8) Mackay, D. In Bioavailability: Physical, Chemical and Biological Interactions; Hamelink, J. L., Landrum, P. F., Bergman, H. L., Benson, W. H., Eds.; SETAC Special Publications Series; Society for Environmental Toxicology and Chemistry: Pensacola, FL, 1994; pp 171-177. (9) Chiou, C. T. Environ. Sci. Technol. 1985, 19, 57. (10) Prest, H. F.; Hodgins, M. M.; Petty, J. D.; Huckins, J. N.; Lebo, J. A.; Orazio, C. E.; Gibson, V. L. Laboratory comparison of oysters and semipermeable membrane devices (SPMDs); Report to the
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American Petroleum Institute, Washington, DC; Midwest Science Center: Columbia, MO, 1997; p 102. Petty, J. D.; Orazio, C. E. Application of Semipermeable Membrane Devices (SPMDs) as Passive Monitors of the Environment of Antarctica; Final Report to the National Science Foundation, Project No. OPP-931435; Midwest Science Center: Columbia, MO, 1996; p 95. Huckins, J. N.; Petty, J. D.; Orazio, C. E.; Lebo, J. A.; Prest, H. F.; Zajicek, J. L.; Tillitt, D. E.; Meadows, J. C.; Johnson, B. T.; Gibson, V. L.; Clark, R. C. Proceedings of 3rd International SPMD Workshop, Midwest Science Center, Columbia, MO, 1994. Mackay, D.; Bennett, E.; Metcalfe, T. 17th Meeting of the Society of Environmental Toxicology and Chemistry, Washington, DC, 1996; Platform 158. Huckins, J. N.; Petty, J. D.; Meadows, J. C.; Echols, K. R.; Gale, R. W.; Lebo, J. A.; Orazio, C. E.; Tillitt, D. E. 17th Meeting of the Society of Environmental Toxicology and Chemistry, Washington, DC, 1996; Poster PO 521.
James N. Huckins,* Jimmie D. Petty, Jon A. Lebo, and Carl E. Orazio Midwest Science Center U.S. Geological Survey Columbia, Missouri 65201
Harry F. Prest Long Marine Laboratory University of California-Santa Cruz Santa Cruz, California 95060 ES9704287
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