Correspondence Comment on “Thallium Speciation in the Great Lakes” SIR: Lin and Nriagu (1) reported a very interesting and important finding that 66-68% of total dissolved thallium in Lake Michigan and two rivers (Raisin and Huron) are in the trivalent form. This is probably the best paper on quantitative Tl speciation. I agree with the authors that Tl3+ forms stronger complexes with ligands than Tl+, and this helps stabilize the thallic species. It follows that some detailed discussions on the effects of acidification on the formation of these species would seem highly necessary but were lacking. If I may, here is such a discussion that points to an overestimation of the thallic fraction. Some background information is first given to make the discussion clearer. Given the high standard reduction potential of 1.28 V and a huge equilibrium constant of 1043.3 for the reaction Tl3+(aq) + 2e- f Tl+(aq) (2), the Tl3+ species should be readily reduced to Tl+ in aqueous systems, i.e., Tl+ species should be thermodynamically much more stable than Tl3+ species. Furthermore, the solubility of thallium(I) hydroxide is high (350 g/L) as opposed to thallium(III) hydroxide, which is insoluble (Ksp ) 1045.2) (3). Thus, in the pH range of most natural waters, pH 5-9, the concentration of dissolved Tl+ far exceeds that of Tl3+ in an aqueous system containing mainly Tl species. In a dilute natural water matrix containing a very small amount of ligands, the concentration of monovalent thallium should also prevail. Indeed the majority of publications indicate that the monovalent species predominate in natural waters (4-13). Lee (10) indicated that only in the presence of extremely strong oxidizing agents (such as MnO4- and Cl2) and high acidity would Tl(III) be expected to exist. Since the Great Lakes waters are far from being highly oxidizing media or highly acidic, the thallic species are expected to be small relative to the thallous species. However, in natural aqueous environments such as seawaters or lake waters, numerous complexing agents, particulates, and colloids exist to an extent that could alter the equilibrium of the dilute system to a different one. In fact, the thermodynamic considerations made by Batley and Florence (14) predicted that, both in seawater and freshwater, thallium exists primarily in the trivalent state and that the primary species in seawater was TlCl63-. These authors also claimed to have confirmed the trivalent state predominance by experiment. However, there have been some concerns expressed vis-a`-vis this claim (9). Also more recently, Lin and Nriagu (15) pointed out that Batley and Florence’s speciation model results were not confirmed by experimental results. Lin (16) and Lin and Nriagu (1, 15), using Chelex-100 to separate and preconcentrate thallium species followed by GFAAS analyses, reported that 66-68% of total dissolved Tl was in trivalent form in the Raisin and Huron Rivers and in Lake Michigan waters. They however suspected that at least 35% of the trivalent Tl may be in colloidal form, which corresponds to 25% of the total dissolved form. In essence, they reported that the total dissolved Tl was made up of at least 25% colloidal Tl, at most 43% Tl(III) and 32% Tl(I). It is unclear if the colloidal fraction contains only Tl(III) species. Some concerns may be raised about Lin and Nriagu’s speciation model. First, they acidified lake samples before the speciation-determining step of separating Tl(I) from Tl(III) species using Chelex-100 resin. The acidification to ∼0.2% 10.1021/es0009267 CCC: $19.00 Published on Web 04/29/2000
2000 American Chemical Society
HNO3 rendered the sample medium more oxidizing and brought the sample pH from ∼9 to ∼1.5-1.8. This alteration of lake water matrix to high acidity matrix favors the formation of Tl3+ species (10, 16); therefore, the formation of Tl(III) complexes with various ligands such as Cl- to form TlCl2+, TlCl2+, TlCl4-, and TlCl63- (8, 14, 17). Since Cl- ions are present to a much greater extent than the total dissolved thallium concentration in the Great Lake waters (10-4 M Cl- vs 10-10 M Tl), the formation of chloride complexes is certain to occur and forces thermodynamically the formation of Tl3+ species at the expenses of Tl+ species, whose concentration subsequently decreases. According to the calculated predominance diagram for the Tl3+-OH-Cl- species (16), the predominant species would be TlCl2+ at the chloride concentration of 10-4 M. Furthermore, at low pH, the formation of the complex TlSO4+ is very favorable as this complex is more stable than Tl3+ ions (18), particularly when the concentration of SO42ions (∼3 × 10-4 M) is much higher than dissolved Tl. More importantly, the acidification with nitric acid that resulted in adding a huge molar concentration of NO3- (∼3.3 × 10-2 M) much favored the formation of the TlNO32+ complex since log K for this complexation reaction is fairly large (7.2) as compared to a log K of 8.1 for the formation of complex TlCl2+, or a log K of 9.02 for TlSO4+. Thermodynamically, Tl3+ species would be used up and replaced at the expense of Tl+ species. Secondly, in the Chelex-100 recovery experiment of monoand trivalent species (1), an amount of 60-165 µg/L Tl mixed standard was used. This was about 10 000 times higher than the TI concentration in the Great Lakes waters and therefore was not at all representative of the Great Lake environment in terms of concentration and matrix. (Lake waters contain numerous ligands including Cl- and SO4- ions as indicated above.) Also, the 16 Tl(I) recoveries averaged to only 92%, which indicates a loss of 8% of Tl(I); this loss corresponds to about 8 µg/L, which is 200 times the concentration of total thallium in the Great Lakes waters. Furthermore, had they used a mixed standard of 40 ng/L or less (∼10-10 M), which would be a much more representative concentration, the loss would be greater than 8% because there would be more Tl3+ species present based on their pE-pH predominance diagrams of Tl at 10-5, 10-10, and 10-l5 M Tl (16, 18). At a concentration of 10-5 M thallium and at pH 1.5-1.8, there is no apparent presence of Tl3+, whereas at 10-10 M thallium and pH 1.5 or even pH 2 there is a substantial amount of Tl3+ formed, which would, within the frame of the Great Lakes water matrix, readily complex the Cl-, SO42-, and added NO3ions as mentioned above in the first concern. This again induces the formation of Tl3+ species and decreases the concentration of Tl+ species. The above concerns strongly suggest that the reported thallic fraction of 66-68% (1, 15) is higher, due to sample acidification, than that in the discussed natural unacidified waters. It would seem prudent, in any speciation-determining step, to process a natural water as is or as close to the natural matrix as possible.
Literature Cited (1) Lin, T.-S.; Nriagu, J. Environ. Sci. Technol. 1999, 33, 3394. (2) Kotvly, S.; Sucha, L Handbook of chemical equilibrium in analytical chemistry; Wiley: New York, 1985. (3) Dean, J. A. Lange’s Handbook of Chemistry, 13th ed.; McGrawHill: New York, 1985. (4) Kaplan, D. I.; Mattigod, S. V. In Thallium in the Environment; Nriagu, J. O., Ed.; John Wiley & Sons: New York, 1998; Chapter 2. VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(5) Flegal, A. R.; Patterson, C. C. Mar. Chem. 1985, 15, 327. (6) Bodek, I.; Lyman, W. J.; Reehl, W. F.; Rosenblatt, D. H. Environmental Inorganic Chemistry; Pergamon: New York, 1988. (7) Vink, B. W. Chem. Geol. 1993, 19, 119. (8) Smith, I. C.; Carson, B. L. Trace Metals in the Environment V(I); Ann Arbor Science: Ann Arbor, MI, 1977. (9) Schoer, J. In The handbook of environmental chemistry, Vol. 3, Part C; Hutzinger, O., Ed.; Springer-Verlag: New York, 1984. (10) Lee, A. G. The chemistry of thallium; Elsevier Publishing Co.: New York, 1971. (11) Shaw, D. M. Geochim. Cosmochim. Acta 1952, 2, 118. (12) Landford, A. Effect of trace metals on-stream ecology. Presented at the 1969 Cooling Tower Institute Annual Meeting, Anaheim, CA, January 20, 1969. (13) Wedepohl, K. H. Handbook of geochemistry, Element 81, Vol. 2, No. 3; Springer-Verlag: Berlin, 1972. (14) Batley, G. E.; Florence, T. M. J. Electroanal. Chem. Interfacial Electrochem. 1975, 61, 205.
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(15) Lin, T.-S.; Nriagu, J. Anal. Chim. Acta 1999, 395, 301. (16) Lin, T-S. Ph.D. Thesis, School of Public Health, University of Michigan, Ann Arbor, 1997. (17) Wade, K.; Banister, A. J. The chemistry of aluminum, gallium, indium and thallium, Vol. 12; Pergamon texts in inorganic chemistry; Pergamon Press: New York, 1973. (18) Lin, T. S.; Nriagu, N. O. In Thallium in the Environment; Nriagu, J. O., Ed.; John Wiley & Sons: New York, 1998; Chapter 3.
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