Environ. Sci. Technol. 2003, 37, 2720-2726
Oxidation of Thallium by Freshwater Plankton Communities BENJAMIN S. TWINING,† MICHAEL R. T W I S S , ‡,§ A N D N I C H O L A S S . F I S H E R * ,† Marine Sciences Research Center, Stony Brook University, Stony Brook, New York 11794-5000, and Department of Chemistry, Biology, and Chemical Engineering, Ryerson Polytechnic University, 350 Victoria Street, Toronto, Ontario M5B 2K3, Canada
Thallium is a toxic metal that is of emerging concern in the Great Lakes. It can exist as either Tl(I) or Tl(III), and its oxidation state affects its complexation and subsequent bioavailability and toxicity. We conducted lab and field incubations with 204Tl(I) and natural plankton assemblages to study the occurrence and mechanism of Tl oxidation. We observed that Tl(III) comprised 74% of total dissolved Tl after a 60 h incubation in surface waters from Lake Ontario, revealing a maximum specific oxidation rate of 0.014 h-1. No Tl(I) oxidation was observed in sterile-filtered control treatments, indicating that solar radiation alone does not oxidize Tl(I) to Tl(III). Additional incubations with pond water revealed that Tl(I) oxidation is mediated by microbial activity and is not related to the presence of abiotic particles or phytoplankton or protozoa. We also identified a minor fraction (5-13%) of nonion-exchangeable (Chelex100 resin; pH 1.5) Tl that may represent dimethylthallium or complexed thallium. This study demonstrates that planktonic bacteria are responsible for oxidizing the thermodynamically stable Tl(I) to the more abundant Tl(III).
Introduction Thallium is a highly toxic element listed by the U.S. EPA as a priority pollutant (1). Formerly used as a pesticide and rodenticide until being banned in the mid-1960s, Tl is now used primarily as a component of industrial alloys, optical glass, and electrical components (2). In addition to industrial sources, the mining and combustion of coal are major sources of Tl to the environment (3), and Tl concentrations are elevated in areas of the Great Lakes impacted by human activities and urban development (4, 5). Although total dissolved Tl concentrations in the Great Lakes remain in the subnanomolar range (6, 7), Tl is highly bioconcentrated in lake trout from these waters, with dry weight concentrations averaging 1.4 µg g-1 in Lake Michigan (8). Food is an important uptake route of metals for many aquatic organisms (9), and plankton are the base of the aquatic food web in these waters (10). Phytoplankton are known to concentrate some metals very appreciably out of ambient water (11) and may serve to introduce Tl into aquatic food webs. However, the incorporation of Tl into the plankton communities of the Great Lakes has only recently received attention (12). * Corresponding author phone: (631)632-8649; fax: (631)632-3072; e-mail:
[email protected]. † Stony Brook University. ‡ Ryerson Polytechnic University. § Present address: Department of Biology, Clarkson University, Potsdam, New York 13699-5805. 2720
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 12, 2003
Thallium’s two redox species differ greatly in their aqueous behavior and reactivity; therefore, any consideration of Tl cycling and bioaccumulation in aquatic ecosystems must consider redox transformations. Thallium likely enters the Great Lakes primarily as Tl2O through atmospheric deposition of coal fly ash (13); it then rapidly dissociates to thermodynamically stable Tl(I) (14). Although the conservative vertical distribution of Tl in seawater has been used to support the theory that the less-reactive Tl(I) predominates in oxic waters (15), direct measurements of Tl speciation in natural waters have provided evidence to the contrary. Using an anion-exchange resin, Batley and Florence (16) estimated that up to 80% of Tl in seawater was present as anionic Tl(III) complexes, presumably Tl(OH)4-. More recently, Lin and Nriagu (7, 17) have used a cation-exchange resin to separate Tl(I) and Tl(III) in samples from the Great Lakes and tributaries. Both studies revealed Tl(III) as the dominant redox species, comprising 66 and 68% of total dissolved Tl in these lentic and lotic environments. These results have been questioned by Cheam (18), who noted that the Tl concentrations in the standards used to validate the method were 104 times higher than natural Tl concentrations in the Great Lakes. Cheam also suggested that acidifying the samples prior to analysis may enhance Tl oxidation in the sample, although this was refuted by Lin and Nriagu (19). In addition to the current debate regarding Tl speciation, the mechanism by which Tl is oxidized in natural waters against thermodynamic gradients is not known, nor is the rate of this transformation. The redox state of Tl is of particular interest because the uptake and toxicity of this metal to aquatic organisms will depend on it. Since Tl(I) acts as an analogue for K+ in Na,KATPase, this form of the metal can be actively transported across cell membranes (20), and several studies have confirmed this uptake route for Tl(I) in macroalgae, phytoplankton, and invertebrates (21-23). Unlike Tl(I), Tl(III) is strongly hydrolyzed, with the predominant form being Tl(OH)3 (14); this greatly reduces the bioavailability of the thallic species. While most studies of Tl toxicity have considered only the Tl(I) species, Ralph and Twiss (24) compared the toxicity of both Tl species (and Cd) to Chlorella sp. isolated from Lake Erie. Tl(III) was shown to be 50 000 times more toxic than Tl(I) (and 43 000 times more toxic than Cd) on a free-ion basis. However, since Tl3+ occurs at such vanishingly low concentrations (5 min. The oxidized effluent was passed through the same column at 6 mL min-1 followed by a 10 mL rinse of deionized water, and the combined effluent was sampled. Two milliliters of 3% H2SO3 was then added to the column, to reduce the retained Tl(III) back to Tl(I), and allowed to soak for >1 h. After soaking, the resin was stirred with a pasteur pipet, and the Tl was eluted with 58 mL of 0.05 M HNO3 followed by 60 mL of 3.2 M HNO3. The performance of the columns was also tested with radiolabeled DMT synthesized in the laboratory (23). To do this, 50 µL of DMT (11 × 109 Bq 204Tl‚mol Tl-1) was added to both 100 mL of deionized water and 100 mL of Lake Erie water filtered through a 0.2-µm pore size polycarbonate filter (Millipore) contained in a Teflon filtration apparatus (Savillex; Minnetonka, MN). Each water sample was then acidified and sent through the columns as described above, and the effluents were sampled for activity. No effort was made to elute the trace amounts of DMT retained by the columns. All samples were counted in a Packard Tri-Carb scintillation counter following the addition of 10 mL of Ultima Gold XR scintillation cocktail. Counting times were generally 5 min and were adjusted to ensure propagated counting errors
