Correspondence Comment on “Critical Evaluation of Desorption Phenomena of Heavy Metals from Natural Sediments” History shows that scientists often go to great lengths to demonstrate that anomalous observations can be accommodated by an established theory or, conversely, that they are erroneous. This process can perdure until eventually a “paradigm” or perspective shift occurs, whereby a new conceptual approach that satisfactorily accounts for the contentious observations emerges and supplants the earlier one (1). Upon reading the recent article by Gao et al. (2), it is hard not to wonder whether research on metal desorption from soils and sediments is not in the pre-shift phase and whether a slight change of perspective would not provide a more satisfactory conceptual framework than the current viewpoint, which is focused almost entirely on the abiotic sorbent phase (i.e., clays, oxides, humified organic matter). In their interesting article, Gao et al. (2) show how lead (Pb) and cadmium (Cd) sorbed to Utica sediments from the Hudson River can be increasingly desorbed or redistributed from the sediment to solution by repeated washings with an electrolyte solution, by sequestration with EDTA, or by lowering the pH with a mineral acid. The authors observed significant desorption hysteresis when the replaced-supernatant method was used. The hysteresis appeared to increase with aging time, but it was not manifested when desorption was initiated by lowering the solution pH. Their results suggest that desorption of metal ions from the solid phase of natural sediments depends not only on the surface chemistry of the solids and on the properties of the desorbing metals but also on the strength of “sinks” that are present in the system. Before Gao et al. (2), other authors made similar observations. Working with a simple, model system, Strawn et al. (3) showed that 98% of Pb, held to the surface of γ-Al2O3 by an inner-sphere bidentate bonding mechanism, could be desorbed within 3 days at pH 6.5 using a cation-exchange resin. Jensen-Spaulding et al. (4) showed that within 350 h extracellular polymers induced a 2-4-fold increase in copper (Cu) and Pb release from long-contaminated (>20 yr) soils even though Pb, particularly “aged” Pb, is generally considered immobile in soils. Furthermore, as Jensen-Spaulding et al. (4) point out, at the microscale of a bacterial cell, local polymer concentrations would probably be much higher than those used in their experiments and the desorption of metals into biofilm coatings is likely to be significantly increased, even in cases where the contaminants may have been aged for over a decade. The traditional view of metal release focuses on desorption and considers that the key actors are the abiotic solid sorbents + metal couple (with other sorbents and complexing agents in solution playing only secondary support roles). The experimental results of Gao et al. (2) and others suggest that one should instead view the release of metals from abiotic solid phases as part of a broader repartitioning process (Figure 1), which involves on equal footage the metal, the abiotic solid sorbents, and whatever sink is present in the system. Along with the characteristics of the metal and the sorbent, the strength of the sink determines the extent and rate of the repartitioning. Natural systems include a wide range of sinks beyond those considered by Gao et al. (2), such as dispersed colloidal 10.1021/es0493594 CCC: $27.50 Published on Web 07/24/2004
2004 American Chemical Society
FIGURE 1. Metal release is generally viewed as a two-compartment process in which only the sorbent and metal in solution (i.e., freeion or total dissolved, which includes complexed and/or colloidal species) are considered (A). However, mounting evidence suggests that at least three components should be considered: sorbent, metal, and sink. Sinks may be numerous and varied and include complexing agents, microbes, and plant roots, among others (B). particles that have dissociated from the matrix and move with the soil solution, dissolved organic matter, and labile complexes. They also include micro- and macro-flora and fauna that influence metal repartitioning either passively or actively. For example, the “aging effect” in which the release of metals appears to be imperceptively slow is thought to be due in part to physical or diffusional limitations. However, plants and microbes have evolved mechanisms that circumvent these limitations or enhance abiotic reactions, such as the release of phytosiderophores by plant roots to obtain iron under Fe deficiency stress. In addition to increasing Fe uptake, this behavior is also associated, in graminaceous VOL. 38, NO. 17, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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species, with uptakes of Zn, Ni, and Cd up to 200% higher than in controls (5). Similarly, bacteria in soils and sediments can themselves actively or passively influence metal partitioning. The cell walls of bacteria are (predominantly) electronegative and situated in such a way as to allow intimate interaction with soluble components, such as metals, in the bacterium’s environment. These cells behave as surface nucleation sites and can passively accumulate large quantities of metals (6). In actively respiring bacteria, the metabolically influenced microenvironment surrounding the cell can lead to the development of mineral phases, which would not have been predicted based on the bulk geochemistry. For example, the sediments in Green Lake (Fayetteville, NY) consist of calcite even though the natural geochemistry of the bulk lake water should favor the precipitation of gypsum (7). Soils and sediments are multicomponent systems with properties that are continually being influenced by physical, chemical, hydrological, geological, and biological processes (6). In these environments, biological sinks exhibit a wide range of responses; under given conditions, one organism may act as a sink, whereas another may not be able to do so. For instance, earthworms (Eisenia veneta) have been observed to accumulate Cd under conditions where leafy Swiss chard did not (8). The presence of multiple sinks in some cases may cause the behavior of one to alter the activity of others, as observed by Sayer et al. (9), who found that rye grass (Lolium perenne) grown in a blend of sand and pyromorphite (one of the least soluble and most stable lead minerals) took up 30 times more Pb when the fungus Aspergillus niger was present. A key feature of many sinks involved in metal repartitioning in soils and sediments is that they are susceptible to transport with the liquid phase. This transport is often passive (as in the case of colloids or dissolved organic matter, remnants of plant roots, fungal hyphae, and lysed bacterial cells that are carried away by the moving liquid phase), but it may be active as well (as in the case of motile bacteria or macrofauna). Regardless of its nature, the movement of sinks
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through soils and sediments may lead to significant facilitated transport of the metals and ultimately to the contamination of groundwater reservoirs or surface water bodies. In particular, the often overlooked role of biological sinks in this context may be vital to characterizing otherwise unexplained losses of metals from soil and sediment profiles (10, 11). Further research on this issue appears necessary to fully understand metal contamination in these ecosystems and to manage it appropriately.
Literature Cited (1) Kuhn, T. S. The Structure of Scientific Revolutions; The University of Chicago Press: Chicago, IL, 1962. (2) Gao, Y.; Kan, A. T.; Tomson, M. B. Environ. Sci. Technol. 2003, 37, 5566-5573. (3) Strawn, D. G.; Scheidegger, A. M.; Sparks, D. L. Environ. Sci. Technol. 1998, 32, 2596-2601. (4) Jensen-Spaulding, A.; Shuler, M. L.; Lion, L. W. Water Res. 2004, 38, 1121-1128. (5) Romheld, V.; Awad, F. J. Plant Nutr. 2000, 23, 1857-1866. (6) Ledin, M. Earth-Sci. Rev. 2000, 51, 1-31. (7) Schultze-Lam, S.; Fortin, D.; Davis, B. S.; Beveridge, T. J. Chem. Geol. 1996, 132, 171-181. (8) Oste, L. A.; Dolfing, J.; Ma, W.-C.; Lexmond, T. M. Environ. Toxicol. Chem. 2001, 20, 1339-1345. (9) Sayer, J. A.; Cotter-Howells, J. D.; Watson, C.; Hillier, S.; Gadd, G. M. Curr. Biol. 1999, 9, 691-694. (10) Baveye, P.; McBride, M. B.; Bouldin, D.; Hinesly, T. D.; Dahdoh, M. S. A.; Abdel-Sabour, M. F. Sci. Total Environ. 1999, 227, 13-28. (11) Qureshi, S.; Richards, B. K.; Hay, A. G.; Tsai, C. C.; McBride, M. B.; Baveye, P.; Steenhuis, T. S. Environ. Sci. Technol. 2003, 37, 3361-3366.
Astrid R. Jacobson* and Philippe Baveye Department of Crop and Soil Science Cornell University 1002 Bradfield Hall Ithaca, New York 14853-1901 ES0493594
