Environ. Sci. Technol. 2004, 38, 4703
Response to Comment on “Critical Evaluation of Desorption Phenomena of Heavy Metals from Natural Sediments” First, we thank Jacobson and Baveye for their interest in our paper on the desorption of heavy metals from sediment and for their kind remarks. We would be honored to be at even the “pre-shift phase” of a true Kuhnsian paradigm shift in such an important environmental topic, as would most scientists. Similarly, many scientists who wish that they were a part of a paradigm shift see new fanciful mechanistic interpretations in what might well otherwise be described using “standard” explanations. Anyway, Jacobson and Baveye seem to suggest that we should have considered a range of “sinks” such as colloids, organisms and organism exoproducts, and facilitated transport. We agree completely and enthusiastically but do not see any of this as a “new paradigm shift” in environmental science. Throughout the paper, we emphasized the importance of dissolved colloids and organic complexing agents. We explicitly used sodium azide to inhibit microbial growth to limit the number of variables being studied, because we considered microbial processes to be potentially important; possibly we should have been more explicit in our acknowledgment of the importance of these and many other processes. For colloids, we explicitly used a washing procedure as shown in Figures 5a-d and 6a-d in our original paper. The importance of facilitated transport of heavy metals by mobile sinks is of considerable interest to the authorssarguably, a graduate student (Candita Cook, now Candita West) in one of the authors’ (M.B.T.) laboratories ca. 1979 actually coined the phrase “facilitated transport”, as used in environmental chemistry (see Ph.D. dissertation (1) and other papers by the authors (2, 3) for interesting functional results of this mechanism.) A main thrust of the suggestions of Jacobson and Baveye seems to be related to the importance of microorganisms and their selectively excreted complexing molecules that have evolved to acquire needed heavy metals as implied in their Figure 1. We enthusiastically join them in this interest and pursuit. Implicit in their comments is the notion that microorganisms produce “inordinately” effective biosequestrants and that these sequestrants, or sinks, should be considered “equally ...” in heavy metal desorption. This whole field of bio-geochemistry is re-gaining importance as illustrated by some of the papers of Beveye, Benifield, and Luttge, to mention a few. Much earlier, similar bio-mineralogy concepts have been brought to bear in such diverse fields as (i) in-situ mining alternative for heavy metals; (ii) explanation of caries formation; and (iii) corrosion in various industries. Although physical-chemical mechanisms have been the primary focus of much the authors’ previous research, we have recently explicitly examined the impact of microbial sinks on the kinetics and thermodynamics of heavy metal release. In parallel with the abiotic studies on heavy metal hysteresis in the present paper being discussed, we conducted a large series of experiments on heavy metal release from sediments in the presence and absence of microorganisms (4, 5), and more experiments are in progress. When the sediments were aerated without sodium azide, the solution pH decreased from about 8 to 4.5 pH; the redox potential increased from about -350 to +400 mV, as expected (5). The extensive release of many naturally occurring heavy metals was followed versus pH and redox using ICP/MS. 10.1021/es040071o CCC: $27.50 Published on Web 07/24/2004
2004 American Chemical Society
FIGURE 1. Plot of aqueous Mn vs pH. Solid triangels are aerated, and open triangles are sparged with nitrogen and pH adjusted by adding mineral acid. Three groups of heavy metal behavior were identified: (i) Mn, Ni, Co, and Zn; (ii) Fe, Pb, and As; and (iii) Cu. Among the many experiments we have run, one series tested the hypothesis that the heavy metal release, in the presence of microorganisms, was a result of lowered pH alone. We prepared an identical sediment slurry, except we sparged it with nitrogen gas and we produced the same “pH versus time” profile by adding mineral acid. Numerous other parameters in the presence and absence of biological sinks, etc. have been tested (see Ref. 5 and will be published separately). During the second experiment (sparged with nitrogen), the Eh remained at about -350 mV. For the group 1 elements, the release appeared to be the same, and the release was simply due to a pH effect, see Figure 1 for illustration with Mn. For the other two groups of heavy metals patterns were more complicated and may suggest specific microbial involvement in the heavy metal release, but we are still experimenting and testing specific hypotheses. Finally, we applaud the suggestions and commentary because we have had similar concerns with our research on the fate and transport of hydrophobic organic compounds and of heavy metals in groundwater and sediments. We would further suggest the monograph Perspectives in Environmental Chemistry (6) as a fertile source of additional possible paradigm shifts and suggestions.
Literature Cited (1) West, C. C. Dissolved organic carbon facilitated transport of netural organic compounds in subsurface systems. Ph.D. Thesis, Rice University, 1984. (2) Hutchins, S. R.; Tomson, M. B.; Bedient, P. B.; Ward, C. H. Fate of trace organics during land application of municipal wastewater. CRC Crit. Rev. Environ. Control 1985, 15, 355. (3) Kan, A. T.; Tomson, M. B. Ground water transport of hydrophobic organic compounds in the presence of dissolved organic matter. Environ. Toxicol. Chem. 1990, 9, 253. (4) Gao, Y.; et al. Mobilization of heavy metals and inorganic compounds during resuspension of anoxic sediments from Trepangier Bayou, Louisiana. Environ. Sci. Technol. (submitted for publication). (5) Tomson, M. B.; Kan, A. T.; Gao, Y. Hazardous Substance Research Center, 2002-2003 Annual Report; Louisiana State University; http://www.hsrc.org/hsrc/html/ssw/sswar03.pdf. (6) Macalady, D. L. Perspectives in Environmental Chemistry; Oxford University Press: New York, 1998; p 512.
Yan Gao, Amy T. Kan,* and Mason B. Tomson Department of Civil & Environmental Engineering Rice University, MS 519 6100 South Main Street Houston Texas 77005-1892 ES040071O VOL. 38, NO. 17, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4703
