THALLIUM SPECIATION IN THE THIRD SISTER LAKE
Tser-Sheng Lin (Department of Environmental Engineering and
Health, Yuanpei Technical College, Hsih-Chu, Taiwan 300), Jerome Nriagu, Imar
Mansilla-Rivera (Department of Environmental Health Sciences, School of Public
Health, University of Michigan, Ann Arbor, MI 48109)
Abstract
An ion-exchange separation technique followed by analysis with
atomic absorption spectroscopy was employed to investigate the dissolved and
chemical forms of thallium in the Third Sister Lake in Ann Arbor, Michigan. The
average concentration of dissolved thallium in the Third Sister Lake was 87.8
ng/L with a standard deviation of 30.8 ng/L, which was much higher than those
found in Huron River (21 ng/L) and Raisin River (26 ng/L) in Michigan as well
as the Great Lakes (4 ~ 18 ng/L). Tl(III) may comprise 70 % of total dissolved
thallium when dissolved oxygen was saturated but would decrease to 16.8 % if
dissolved oxygen was lower than 2 mg/L. Hence, the dominant thallium form in
water samples could be significantly influenced by the dissolved oxygen in the
samples.
Introduction
Thallium has been attracting more attention
recently, probably because of its intoxication found worldwide (Malbrain et
al., 1997; Chandler et al., 1990; Zhou and Liu, 1985) as well as its
application in high-tech industries (Smith and Carson, 1977). Thallium happens
to be more toxic than mercury, lead and cadmium for mammals (Wallwork-Barber et
al., 1985; Vercruysee, 1984). Although
the USEPA includes thallium in the list of priority pollutants (Keith and
Telliard, 1976), there are few reports on the thallium concentration in natural
waters, especially, in the US. The predominant form of thallium in natural
waters, which affects its toxicity, distribution, mobility and biological
availability, is analytically determined to be Tl(III) (Lin and Nriagu, 1999a;
Lin and Nriagu, 1999b; Batley and Florence, 1975) rather than the theoretically
recognized Tl(I) (Lin and Nriagu, 1998). All of these experimental findings are
obtained by analyzing the oxygen saturated water samples. However, the
predominant Tl(III) only comprises only about 68% of total dissolved Tl in
freshwaters (Lin and Nriagu, 1999a; Lin and Nriagu, 1999b). This phenomenon
implies that the distribution of Tl forms in freshwaters may vary significantly
as the dissolved oxygen changes. Since the complexes of Tl(III) species in
freshwaters are very complicated, it seems unfeasibly to predict the
distribution of Tl species in freshwaters as the dissolved oxygen changing. We
studied the Tl speciation in the Third Sister Lake in Ann Arbor, which
possessed a anoxic hypolimnion during the winter, in order to describe the
possible relationship between Tl species and dissolved oxygen.
Material and Methods
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. All labware and bottles were low-density polyethylene (LDPE), which
were carefully cleaned using an extensive nine-step procedure proposed by
Nriagu et al. (1993). 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.
Five samples at different depths (0, 5, 8,
13 m) were collected from the Third Sister Lake, Ann Arbor in February of 1997;
using an acid-leached 2.5-L Go-Flo bottle (General Oceanics Inc., Miami, FL) on
Nylon rope and tripped with a epoxy-coated messenger. Upon retrieval, the water sample was immediately transferred into
two one-liter aliquot LDPE bottles. These samples then were transported back
the laboratory and separation processes were finished within eight hours of
sample collection.
Speciation Procedure.
Filtered
sample was transferred to a Mariotte reservoir bottle designed to maintain a
constant flow of approximately 8 mL/min through a column of Chelex-100 resin as
described in detail by Beaubien et al.
(1994).
During
the first sample flow-through, Tl(III) was selectively removed from the water
samples by the resin. The trapped
Tl(III) was subsequently eluted using 60 mL of 14% nitric acid. The Tl(I) in
the effluent was oxidized using bromine (100 mL) and the solution
re-circulated through the resin to remove and preconcentrate the Tl(III). The procedure resulted in preconcentration
of thallium in each sample by up to 200-fold. The recovery, selectivity and
detection limit of this method were detailed elsewhere (Lin and Nriagu , 1999b).
Determination of
Dissolved Oxygen
The
dissolved oxygen in water column was determined by utilizing the iodometric
method and carried out by the laboratory of the Department of Biology,
University of Michigan.
Results and
Discussion
The average concentration of total dissolved
thallium (Tl(I) + Tl(III)) in the Third Sister Lake is 87.7 ng/L with a
standard deviation of 30.8 ng/L. This high observed value may be as a result of
that the Third Sister Lake is serving as a sink receiving emissions from
industries upstream, since the Tl level in natural waters can be elevated
significantly by industrial emission (Lin and Nriagu, 1999b). The
concentrations of Tl(III) and Tl(I) in the Third Sister Lake are presented in
Table 1. The average concentrations of Tl(III) and Tl(I) are 32.4 ± 13.7
and 55.4 ±
37.9 ng/L, respectively. However, their measured values vary magnificently as
depth changing. Profile of the dominant redox forms of thallium in water
samples collected from the Third Sister Lake in February of 1997 is shown in
Figure 1. The predominant form of dissolved thallium in water column of the
Third Sister Lake is not consistent at various depths. For example, the
predominant form of Tl is Tl(III) (Tl(III)/(Tl(III)+Tl(I))) = 70.4 %) at the
surface of water column where the dissolved oxygen is equal to 10.4 mg/L; by
contrast, Tl(I) will be dominant (Tl(III)/(Tl(III)+Tl(I))) = 16.8 %) at the
depth of 8 m where the measured dissolved oxygen is 1.6 mg/L. The fluctuation
of Tl(III)/ (Tl(III)+Tl(I)) ratio as depth may reflect the different redox
potential in water column as depth varying. The increasing level of Tl(III) in
the bottom waters (Figure 1) may be explained by the elevation of colloidal
fraction because colloidal form of Tl(III) is suggested to comprise a
significant proportion of Tl(III) species (Lin and Nriagu, 1999b). It seems
impossible to derive the redox behavior of Tl species in natural waters with
the data we have; however, it does reveal that the predominant form of Tl in
lake waters may be very different seasonally, particularly, in lakes with a
seasonally anoxic hypolimnion.
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