Correspondence Comment on “Addition of Carbon Sorbents to Reduce PCB and PAH Bioavailability in Marine Sediments: Physicochemical Tests” The underlying concepts of PCB and PAH sorption by activated carbon have been extensively documented and broadly articulated in the literature. So too has knowledge regarding the transfer of such compounds from modestly sorbing solids (e.g., soils and sediments) to others having greater sorption affinities (e.g., Tenax beads and activated carbons). There have been many thoughtful and generally successful applications of these two pillars of common knowledge to a variety of environmental systems. In the sense that this common knowledge is central to the concept of the proposed scheme, it would seem on first consideration to support the authors’ novel notion of dispersing activated carbon to fresh and marine water bodies to attenuate sediment- associated PCB and PAH contamination (1). While the fundamental processes involved are in fact well understood and widely accepted, the particular use of carbon proposed by the authors is indeed novel (1) (i.e., no one, to the best of my knowledge, has attempted to apply these process concepts in either a situation similar to or at a scale approaching that proposed). This may relate to the fact that several concerns regarding costs, implementation feasibility, and potentially adverse environmental impacts accrue to the notion as proposed. I inquire therefore as to what new scientific facts and/or relevant applications information the authors (1) have brought forward to abate said concerns. The adsorption science presented in this and the companion papers cited by the authors appears relatively routine, and the conclusions relating thereto are obvious from earlier literature, as evident, for example, by examination of a related 1997 critical review paper by Luthy et al. (2). As for information regarding applications feasibility, no readily apparent attempt has been made to address the most critical and least well understood aspects of what is proposed (i.e., costs, implementation feasibility, and potential environmental risks associated with the release of large quantities of activated carbon to open water bodies). From an implementation perspective, the dispersal of tons of activated carbon (3.4% by sediment dry weight) in a controlled and cost-effective manner into harbors and other open bodies of water seems a non-trivial issue. It is common knowledge that the particles of such fine (e.g., 75-300 µm particle diameter) and highly porous activated carbon are both light (low bulk density) and friable (easily fractured or ground by friction into yet smaller and lighter particles). To thoroughly wet such particles and increase their bulk density to allow settling, it is necessary, as done in the present study, to boil them in water to drive out the air contained in their pores. After cooling these particles gradually resorb air and/ or other gases from water and return slowly to their low bulk density state. Thus, even if it is somehow feasible to deposit tons of these small and light particles in a reasonably controlled manner under open water conditions, it can reasonably be expected that they will be re-suspended via hydrodynamic bottom scour and bioturbation. These normal 10.1021/es048209q CCC: $30.25 Published on Web 01/12/2005
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system processes can in turn be expected to cause both attrition in particle size and wide-scale dispersal throughout most natural aquatic environments The significant effects of bioturbation alone on releases of contaminants from sediments to the water column under even relatively quiescent conditions were stressed recently by Thibodeaux and Bierman (3). More bothersome and problematic are issues that may in fact render the ultimate consequences of carbon dumping directly opposite those intended and claimed by the authors (1). For example, will activated carbon when contacted with contaminated sediments become essentially a hazardous material? As the authors properly claim (1), and their data confirm, it will be laden with enriched masses of PCBs, PAHs, and other noxious contaminants extracted from the sediments. Personal observations of slurry-like nephelometric layers swirling above freshwater and marine sediment beds lead me to think the authors’ suggestion of placing a layer of sand on top of the carbon to prevent its dispersal may involve elements of wishful thinking. It is my concern that the effects referenced above could ultimately result in continuous, extensive, and unpredictable transport of small, light, and heavily contaminated carbon particles throughout natural open water systems. These dispersed particles are likely then to be ingested, along with their associated masses of contaminants, by fish and other aquatic species. The potential reality that these particles might thus constitute an entirely new, widely dispersed, particularly enriched, and readily available route for entry of potent contaminants to aquatic food chains is somewhat chilling. Under the Implications header on page 5463 of the article (1), the authors tepidly qualify their conclusions by noting that additional considerations such as ... “deployment and particle dynamics under various hydrodynamic conditions need to be assessed”. Given the potential environmental risks involved with what they propose, this caution seems vapid. An acknowledgment that the studies described simply do not address similitude requirements sufficiently to permit conclusions regarding direct relevance to natural systems would be more appropriate. The work falls short of satisfying similitude requirements for the scale and situation assumptions and conclusions presented, both from the fundamental perspective of relevant dimensionless group correspondence (4) and from the reality perspective of applicable process correspondence (i.e., incorporation of such relevant processes as bioturbation, sediment scour, nonNewtonian flow and mixing, particle abrasion, resuspension, etc.). Substantially different questions must be asked and answered, more relevant work conducted, and more convincing evidence presented before the broadcasting of activated carbon upon earth’s open waters should be attempted. I refrain from rather obvious and similiar comment on the authors’ alternative notion of using coke for the purposes stated.
Literature Cited (1) Zimmerman, J. R.; Ghosh, U.; Millward, R. N.; Bridges, T. S.; Luthy, R. G. Addition of carbon sorbents to reduce PCB and PAH bioavailability in marine sediments: physicochemical tests. Environ. Sci. Technol. 2004, 38 (20), 5458-5464. VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(2) Luthy, R. G.; Aiken, G. R.; Brusseau, M. L.; Cunningham, S. D.; Gschwend, P. M.; Pignatello, J. J.; Reinhard, M.; Traina, S. J.; Weber, W. J., Jr.; Westall, J. C. Sequestration of hydrophobic organic contaminants by geosorbents. Environ. Sci. Technol. 1997, 31, 3341-3347. (3) Thibodeaux, L. J.; Bierman, V. J. The bioturbation driven chemical release process. Environ. Sci. Technol. 2003, 37, 252A258A. (4) Weber, W. J., Jr.; DiGiano, F. A. Process Dynamics in Environmental Systems; Wiley-Interscience: New York, 1995.
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Walter J. Weber, Jr. Department of Chemical Engineering 4103 ERB The University of Michigan Ann Arbor, Michigan 48109-2099 ES048209Q
