Environ. Sci. Technol. 1999, 33, 3394-3397
Thallium Speciation in the Great Lakes T S E R - S H E N G L I N * ,† A N D JEROME NRIAGU‡ Department of Environmental Engineering and Health, Yuanpei Technical College, Hsinchu, Taiwan 300, and Department of Environmental and Industrial Health, School of Public Health, University of Michigan, Ann Arbor, Michigan 48109
cally, Tl+ seems to be favored in natural waters in most instances; however, the formation of Tl3+ complexes may lead to the persistence of the trivalent state (22-26). Predictions based on equilibrium thermodynamics have been published but have not been confirmed by actual measurements (22, 27). As an analogy, the predominant form of Cr in natural waters is usually reported to be Cr(VI), whereas Cr(III) is thermodynamically favored since E0 ) +1.33 V for the Cr(VI)/Cr(III) redox couple (28, 29). This report presents the first data on the forms and distribution of thallium in Lakes Michigan, Huron, and Erie obtained using the ultraclean laboratory method.
Materials and Methods An ion-exchange separation technique followed by analysis with atomic absorption spectroscopy was used to study the chemical forms and distribution of thallium in Lakes Michigan, Huron, and Erie. The dominant thallium form found in water samples analyzed was the oxidized Tl(III) which comprised 68 ( 6% of the total dissolved thallium, contrary to thermodynamic prediction that Tl(I) is favored in natural waters. A significant proportion of Tl(III) may be in colloidal form. No definite spatial (horizontal or vertical) pattern was found in the distribution of total dissolved thallium in the water columns of Lakes Michigan, Huron, and Erie. An overall decline of thallium concentration from Lake Michigan to Lake Erie was observed which may be related to rapid scavenging removal from the water column.
Introduction The accumulation of thallium in the Great Lakes must be viewed with some concern. Recent studies show that thallium concentrations in Great Lakes waters are generally higher than those of cadmium and occasionally exceed the lead levels in some contaminated areas (1). Reported average levels of thallium in natural waters include 1-410 ng/L in lakes, 13-1350 ng/L in rivers, 1-550 ng/L in groundwater, and 10-20 ng/L in seawater (1-15). The published data indicate that thallium concentration in natural waters can become elevated because of human activities. Thallium compounds are remarkably toxic, their lethal dose for human beings ranging from 0.5 to 1.0 g after a single ingestion (16-18). The toxicity of thallium to mammals is similar to that of mercury but higher than those of lead and cadmium, copper, and zinc (17-21). Over 40 million people in the United States and Canada get their water supply from the Great Lakes, and the risks associated with the elevated levels of thallium have yet to be properly assessed. Thallium is classified a priority pollutant in the Great Lakes Water Agreement between United States and Canada, and thallium salts are designated a hazardous substance under U.S. Federal Water Pollution Control Act. A search of the literature, however, revealed few measurements of thallium in the Great Lakes, and no previous study of the forms of thallium in any of these lakes. In fact, little is currently known about the chemistry of pollutant thallium in freshwater environments, and attempts to predict its behavior from thermodynamic principles have yielded contradictory results (9, 10, 22, 23). Thermodynami* Corresponding author phone: 886-35-381183; fax: 886-35385353; e-mail:
[email protected]. † Yuanpei Technical College. ‡ University of Michigan. 3394
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Reagents and Equipment. Ultrapure water used in the experiments was generated by a Milli-Q Plus water system (Millipore Corp., Bedford, MA). Trace metal grade nitric and hydrochloric acids used for sample acidification and resin cleaning were purchased from Fisher Scientific. To minimize the contamination risk, all labware and bottles used in this study were low-density polyethylene (LDPE), which were carefully cleaned using an extensive nine-step procedure which included soaking in acetone, cleaning with soap, and four-step leaching with different acids. Details of the process have been described in elsewhere (32). The decontaminated bottles were filled with dilute nitric acid, triple bagged, and stored in ice box for subsequent shipment to the field. Processing of all samples was conducted under the Class 100 clean laboratory environment. Thallium concentration in each sample was determined by means of a Perkin-Elmer 4100ZL graphite furnace atomic absorption spectrometer (GFAAS) equipped with a Zeeman background corrector. Sampling Procedure. Samples were collected from six stations on Lake Erie (see Figure 1) in July 1995; these samples were analyzed only for total dissolved thallium. Samples obtained from nine stations on Lakes Michigan and Huron (Figure 1, except for Station 3 and 4) in July 1997 were analyzed for total dissolved and particulate thallium. Dissolved thallium species were determined on samples collected in May 1997 from two stations (Stations 3 and 4) on Lake Michigan (Figure 1). At each station, two field blanks were prepared to estimate the amount of each element that was added into the samples from the reagent, walls of the sample containers, the filtration apparatus, and the air in the clean laboratory as well as during instrumental measurement. Surface samples were collected from a rubber raft rowed at least 100 m upwind of the mother ship, if possible. Otherwise, surface samples were obtained 1 m away from the ship upwind with a raft rod sampler. All sample bottles were opened, filled, and closed under water. Subsurface samples were obtained upwind beside the research vessel using an acid-leached 5-L Go-Flo bottle (General Oceanics Inc., Miami, FL) attached to a Kevlar rope and tripped with a Teflon messenger. Upon retrieval, the filled Go-Flo bottle was immediately transferred to a portable Class 100 lamina flow fume hood. The Go-Flo bottle was connected to an online filtering assembly (made of Teflon; see ref 30) and two 1-L aliquots of the sample filtered through acid-leached 0.4 µm polycarbonate membrane (Nuclepore), which was rinsed first with dilute acid and then ultrapure water just before the filtration process. For speciation experiments, 2-L aliquots of filtered samples were collected. Each filtered sample was acidified to pH of -1.8 with nitric acid and stored in a refrigerator until analyzed. 10.1021/es981096o CCC: $18.00
1999 American Chemical Society Published on Web 08/26/1999
TABLE 1. Average Recovery and Standard Deviations for Four Replicates of Multiple Species Standards Tl(I)
Tl(III)
replicate
input (µg/L)
recovery (%)
input (µg/L)
recovery (%)
4 4 4 4
20 40 150 15
95.3 ( 5.6 90.4 ( 2.5 90.2 ( 2.6 92.6 ( 3.2
40 40 15 150
97.6 ( 5.7 102.5 ( 7.7 102.4 ( 2.3 91.7 ( 2.3
TABLE 2. Dissolved Thallium (ng/L) Lake Erie station
depth (m)
dissolved Tl (ng/L)
23
1 6 20 40 50 1 6 12 16 20 surface 4
4.7 4.1 7.7 4.4 2.8 9.5 4.0 9.4 7.7 9.9 9.8 8.5
84
338
FIGURE 1. Average concentration of dissolved and particulate thallium at different stations in Lake Michigan, Huron, and Eire. No data available for particulate thallium in Lake Eire. Preconcentration of Dissolved Thallium. For determination of dissolved thallium in lake water, 2.0 L of filtered sample was transferred to a Mariotte reservoir bottle designed to maintain a constant flow of approximately 5 mL/min through the Dowex 50-8X resin column as described in detail by Beaubien et al. (28). This resin retains both Tl(I) and Tl(III) quantitatively (30). The dissolved thallium trapped by the resin was then recovered with 10 mL of 14% nitric acid and analyzed. The procedure resulted in preconcentration of thallium in each sample by up to 200-fold. This process was evaluated with the analysis of six spiked samples containing 40 µg/L of mixtures of Tl(I) and Tl(III) in different ratios and at varying pH values. A recovery efficiency of 99.8 ( 3.9% was achieved for the spiked samples. The detection limit defined as 3σ (standard deviation) of six blanks was 0.7 ng/L. This method was validated by analysis of real sample using a different analytical technique. Five samples obtained from the Great Lakes were chosen randomly and analyzed by the proposed method, and their duplicates were analyzed by means of ICP-MS (Perkin-Elmer Sciex Elan 5000 equipped with an ultrasonic nebulizer). The results obtained by the two methods (i.e., GFAAS and ICP-MS) were consistent, and the difference was always within 5%. Speciation Procedure. For chemical speciation experiment, all samples were transported back to the laboratory immediately after sampling and processed within 8 h of collection. Actual separation of Tl(I) and Tl(III) was carried out by means of Chelex-100 technique. The pH of the filtered sample was initially adjusted to ∼1.8 with nitric acid; our previous study showed that the Tl(III)/Tl(I) ratio did not change by adjustment of pH to this value (32). The sample was passed through a column of Chelex-100 resin at a constant flow of approximately 8 mL/min controlled by a Mariotte reservoir bottle. During the first sample flowthrough, Tl(III) was selectively removed from the water samples by the resin. The trapped Tl(III) was subsequently
station
depth (m)
974
surface 4
957
946
6 12 16 18 surface 6 10 15 20
dissolved Tl (ng/L) 10.5 8.5 10.7 10.4 10.8 7.6 4.3 10.6 7.9 8.7 7.7 10.2
eluted using 60 mL of 14% nitric acid. The Tl(I) in the effluent was oxidized using bromine (100 µL), and the solution recirculated through the resin to remove and preconcentrate the Tl(III). The selectivity of this technique was evaluated by testing four spiked mixtures of Tl(I) and Tl(III) at different concentrations ranging from 15 to 150 µg/L of Tl(I) and Tl(III). For each test, four replicates were prepared and examined. The results (Table 1) showed that the overall recoveries for Tl(I) and Tl(III) were 92.3 ( 2.4% and 98.6 ( 4.4%, respectively. The detection limit defined as 3σ (standard deviation) of six blanks was 1 ng/L for both Tl(I) and Tl(III). Details of the validation procedure for the methodology are presented elsewhere (31, 32). The interference of other metals was expected to be negligible since most of the other metals would not be trapped by Chelex-100 resin at pH < 2 (33-37). Particulate Thallium. Particulate thallium concentration was obtained by dissolving the filtered residue in 20 mL of 4 N nitric acid solution heated (about 50 °C) by a 500 W infrared lamp for 48 h.
Results and Discussion Dissolved Thallium. The thallium concentration in field and reagent blanks was lower than the method detection limit (0.7 ng/L). Concentrations of dissolved thallium in Lake Erie range from 2.8 to 11 ng/L and average 8.0 ( 2.5 ng/L (n ) 24) (Table 2). These values are comparable to, but somehow lower than, the data reported by Cheam et al. (1) which averaged 9.1 ( 1.5 ng/L. This disparity is due primarily to the lower concentrations (4.7 ( 1.8 ng/L) we found in the eastern basin of this lake. Average concentration of thallium in surface samples is 9.4 ng/L and is higher than that of subsurface samples (7.5 ng/L) especially in nearshore stations (Table 2). Higher concentrations of thallium in surface waters of nearshore areas may be attributed to (a) increased atmospheric deposition from local sources, (b) surface skimming of contaminated and warmer river waters before the plume is mixed into the lake water, and (c) higher concentration of suspended particulates in the hypolimnion which engender faster removal of adsorbed thallium from the water column. Concentrations of thallium in Lake Michigan range from 8.8 to 18 ng/L (Table 3) and average 14.2 ( 2.5 ng/L (n ) 22). Higher average concentration in Lake Michigan compared VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Dissolved, Particulate, and Total Thallium (ng/L) and Dissolved/Total Ratio of Thallium in Lake Michigan station depth (m) 1
surface 6 12 16 4 14 30 50 70 90 surface 5 10 15 4 14 30 60 90 6 16 36
2
5
7
9
dissolved particulate total % dissolved/ Tl (ng/L) Tl (ng/L) Tl (ng/L) total 15.7 14.2 12.6 8.8 11.3 13.3 13.3 15 14.2 12.4 11.7 18.3 16.7 18 13.8 17.3 14.1 10.3 16.5 16.5 15.1 13
0.9 0.9 1.0 0.8 0.5 0.8 0.9 0.8 0.8 0.8 0.7 0.9 0.9 1.0 0.9 0.9 0.8 0.7 1.0 0.9 0.9 0.9
16.6 15.1 13.6 9.6 11.8 14.1 14.2 15.8 15.0 13.2 12.4 19.2 17.6 19.0 14.7 18.2 14.9 11.0 17.5 17.4 16.0 13.9 mean SD n
94.6 94.2 92.6 91.6 95.6 94.3 93.9 94.9 94.6 93.8 94.2 95.1 95.1 94.8 94.1 94.9 94.7 93.6 94.4 95.0 94.2 93.4 94 0.9 22
TABLE 4. Dissolved, Particulate, and Total Thallium (ng/L) and Dissolved/Total Ratio of Thallium in Lake Huron station
depth (m)
dissolved Tl (ng/L)
particulate Tl (ng/L)
total Tl (ng/L)
% dissolved/ total
7
surface 6 16 surface 5 30 60 90 6 16 26 36 surface 8 25 50 70 90
10.5 15.8 7 16.2 14.6 18 15.5 15.4 8 3.5 7.5 11.1 7.9 6.1 6.7 2.6 9.5 8.6
0.4 0.6 0.3 0.4 0.8 0.5 0.5 0.4 0.4 0.2 0.4 0.6 0.4 0.5 0.4 0.4 0.6 0.8
10.9 16.4 7.3 16.6 15.4 18.5 16.0 15.8 8.4 3.7 7.9 11.7 8.3 6.6 7.1 3.0 10.1 9.4 mean SD n
96.2 96.1 96.5 97.5 94.5 97.1 96.7 97.4 95.2 95.4 95.3 94.8 94.9 92.2 93.9 87.2 94.1 91.8 95 2 19
8
12
to Lake Erie may be related to the fact that the majority of sampling stations are located near populated areas of Green Bay, WI, Grand Traverse Bay, MI, and Grand Haven, MI (Figure 1). The similarity of dissolve thallium levels in the three areass16 ng/L in Green Bay, 15 ng/L in Grand Traverse Bay, and 13 ng/L in Grand Havensshould be noted, however. Only two surface samples were collected during the cruise, making it impossible to discern any vertical trends in July 1997. Dissolved thallium concentrations in Lake Huron range from 2.6 to 18 ng/L (Table 4) and average 10.0 ( 4.6 ng/L (n ) 19). Lower concentrations in Lake Huron (especially at Stations 11 and 12) compared to Lake Michigan suggest that a substantial portion of dissolved thallium is removed from Lake Michigan water before draining into Lake Huron. How 3396
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and where the removal occurs are unknown at this time. The lower thallium level in Lake Huron than Lake Michigan may also be attributed to the diluting effect of water being exported from Lake Superior. However, lower concentrations at Stations 11 and 12, which are located upstream of the connecting point between Lakes Superior and Huron, suggest that the dilution effect may not be an over-riding factor affecting thallium levels in Lake Huron. Relatively high concentrations of thallium (15.9 ( 1.3 ng/L) observed at Station 8 may reflect the influence of being located in a major recreational area. The temperature of water column at Station 8 was constant at about 4 °C (31), which may result in an unusual circulation and resuspension of sediments. The regional distribution pattern for dissolved thallium concentrations in Great Lakes drainage basin is rather interesting. The trend in average concentration is observed to be Superior (1.2 ng/L) < Michigan (14.2 ng/L) > Huron (10 ng/L) > Erie (8 ng/L); data for Lake Superior are from ref 1. For a contaminant with conservative geochemical properties, one would expect the concentration to increase progressively from Lake Superior to Lake Erie, in accordance with increasing industrial density in the Great Lakes watershed. The pattern displayed by thallium suggests that it occurs in a form that is geochemically reactive. Rapid removal of thallium is common in riverine ecosystemssmore than 85% of thalliumswas found to be removed from the polluted river water within the distance of 9 kilometers (38). It may be noted in this connection that thallium is strongly adsorbed by clays and ferrohydroxides natural waters (39-42). Total and Particulate Thallium. In this paper, total thallium concentration is defined as the sum of measured dissolved and particulate thallium, and the values for Lakes Michigan and Huron are shown in Tables 3 and 4. In general, average dissolved thallium fraction makes up 95% of total thallium in Lake Huron and 94% in Lake Michigan. Our results are higher than those reported by Cheam et al. (1) who found that the proportion of dissolved thallium to total thallium in the Great Lakes to be 80-90%. The disparity may be attributed to the different methods used to estimate the particulate fraction of thallium. Cheam et al. (1) derived the particulate fraction by subtracting dissolved thallium from total thallium concentrations in water samples. We quantified the fraction using nitric acid to extract thallium from particulates. Although non-HNO3 extractable thallium would not be included, we believe that most of the thallium is associated with the labile phase (41) and that non-HNO3 extractable fraction is less than 5% of total particulate thallium. Chemical Speciation. Profiles of the dominant redox forms of thallium in water samples collected from Lake Michigan in May of 1997 are shown in Figure 1. The predominant form of dissolved thallium in water column of Lake Michigan is determined to be Tl(III) and accounts for 68 ( 6% of the dissolved phase. Flegal and Patterson (9) suggested that Tl(I) should be the predominant form based on their observation of a relatively constant distribution of thallium in seawater and by an analogy of the estimated residence times of thallium and alkali metals in seawater; they did not have any direct evidence for their suggestion. By contrast, Batley and Florence (10) used an anion-exchange resin and isotope spike method to show that Tl(III) (in the form of TlCl63-) is the dominant redox species in seawater. The fraction of the spiked Tl(I) that was oxidized to Tl(III) was not considered in the experiment by Batley and Florence (10). It would seem unrealistic to conclude that the forms of thallium in the Great Lakes are similar to those of seawater. The amount and nature of chelating agents in the Great Lake differ from those of seawater, and photoinduced reactions may play a complicating role in the behavior of thallium in freshwater ecosystems (42-47). We are not aware of any previous measurements of thallium redox species in fresh-
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)
(24) (25) (26) (27) (28) (29) (30)
FIGURE 2. Vertical concentration profiles for the thallium species measured at Lake Michigan. water ecosystems, and our results clearly point to a need for additional work on the redox chemistry of this element. According to common practice in environmental chemistry, ”dissolved” thallium is operationally defined as the fraction that passes through 0.45 um filter membrane. A previous study (48) showed that Tl3+ and its complexes such as TlCl63- trapped by resins can be easily reduced to Tl(I) by sulfurous acid. In our study, we found that about 35% of the Tl(III) fraction or (25% of dissolved thallium) retained by Chelex-100 resin could not be reduced using sulfurous acid (32). This nonreducible fraction may be regarded as the colloidal form of thallium in the water samples; this proposition needs to be confirmed using other analytical methods. It is conceivable, however, that the high levels of Tl(III) in the bottom waters of the Great Lakes (Figure 2) may be related to increased formation of the colloidal fraction.
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Received for review October 23, 1998. Revised manuscript received July 7, 1999. Accepted July 13, 1999. ES981096O
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