Environmental Engineering Reference
In-Depth Information
table 4.5
Fractionation of Methylmercury after Reaction
with Creatine Kinase (CK)
of different isotopes of the same element exhibiting drastic
differences in toxicity.
Source Tracking
Hg isotope composition (%)
A pioneering study illustrated the power of Hg isotope
ratio measurements to trace sources of contamination
(Foucher et al., 2009). The authors investigated the well-
documented Hg mining region near Idrija, Slovenia, and
measured sediments alongside the Idrijca River, the Soˇa/
Isonzo River and in the Gulf of Trieste to determine the
variation in Hg isotopic composition versus distance from
the Hg mine. Samples collected from the rivers Idrijca
and Soˇa/Isonzo and near the shore in the Gulf of Trieste
had very uniform
199
Hg
200
Hg
201
Hg
202
Hg
Starting MeHg
17.0
23.1
13.2
26.6
CK-bound MeHg
34.0
7.4
28.2
10.3
Fractionation (‰)
202/200
Hg
209
202/198
Hg
202
Hg
a
422
0.15‰, which
was not different from Hg ore at the Idrija Mine (
δ
202
Hg values of
0.32
0.26
202/199
Hg
806
0.12‰) suggesting that sediments throughout the water-
shed of the Soˇa/Isonzo River to the Gulf of Trieste are
dominated by Hg exported from the mining area. In con-
trast, the regional background of the Adriatic Sea was
determined to be
202/201
Hg
819
199/198
Hg
199
Hg
b
888
201/198
Hg
201
Hg
b
895
0.46‰. Sediments from the
southern part of the Gulf outside the river plume showed
intermediate
2.53
199
Hg
781
2.00‰. Using
a simple binary mixing model, the authors were able to
demonstrate that all samples investigated in this study
were a result of combinations of Hg originating from the
Idrija Mine and from the Adriatic Sea. They estimated that
the contribution of Hg from the mining area in surfi cial
sediments of the Gulf of Trieste progressively decreases
from 90% or more in the northern part to less than 45%
in the south. These results are a promising fi rst demonstra-
tion of tracking mercury sources in natural systems using
mercury stable isotope ratios.
δ
202
Hg, ranging from
1.19 to
201
Hg
578
199
Hg/
201
Hg
1.35
SOURCE
: Hg isotope composition data are from published data
(Buchachenko et al., 2004), and deviations were obtained as indicated
in footnotes.
a.
202
Hg was obtained using scale factors from Table 4.3 according
202
Hg
202
Hg
to
2.02.
b. To calculate the deviations relative to the
198
Hg ratio (in the
absence of
198
Hg fractions or ratios), the delta notation was converted
from
xxx
Hg
[(
xxx
Hg/
198
Hg)
sample
/(
xxx
Hg/
198
Hg)
NIST 3133
1]
1000
into
(
198
Hg
std
/
198
Hg
sample
.
Since this is true for all Hg isotope pairs, one can set up the equation
(
xxx
Hg/1000
xxx
Hg
std
/
xxx
Hg
sample
1)
Future Directions
202
Hg/1000
202
Hg
std
/
202
Hg
sample
199
Hg/1000
1)
(
1)
199
Hg
std
/
199
Hg
sample
and solve for
The fi eld of mercury isotope geochemistry is rapidly emerg-
ing and the analytical protocols and methods are now well
enough defi ned to measure Hg isotope ratios in many matri-
ces. However, there are still many unresolved questions
that will be answered only through a combination of care-
ful, well-designed laboratory experiments and collection
of fi eld data. Hg is an important environmental pollutant
that continues to be under close scrutiny. It is expected that
Hg isotope analyses will contribute to our understanding
of the Hg cycle and in the future may even be the critical
tool to resolve the “holy grail” of Hg biogeochemistry—that
is, the differentiation between geogenic and anthropogenic
sources of Hg in the environment.
In particular, the following areas will likely receive
increased attention in the future. Our database of well-
constrained Hg isotope fractionation factors is still very
limited and requires expansion to fully understand Hg
isotope variations in nature. We also need to expand our
database on natural Hg isotope variations. Additional work
is needed to extend it to environmental compartments
199
Hg to obtain this value:
199
Hg
(
202
Hg
std
/
199
Hg
std
)/(
202
Hg
sample
/
199
Hg
sample
)
202
Hg
[(
202
Hg
std
/
199
Hg
std
)/(
202
Hg
sample
/
199
Hg
sample
)
1]
1000
or
199
Hg
(
202
Hg
std
/
199
Hg
std
)/(
202
Hg
sample
/
199
Hg
sample
)
202
Hg
202/199
Hg.
201
Hg was calculated in similar fashion.
measurements, which were conducted by gas isotope ratio
mass spectrometry. In addition, the assumption that the
CK-MMHg reaction produces intermediate ion-radical
pairs is not supported in the literature. Hence, indepen-
dent corroboration of this reaction and the correspond-
ing isotope fractionation is urgently needed. If binding of
MMHg to enzymes, proteins, and other cysteine residues
would indeed strongly favor odd Hg isotopes as suggested,
it would have profound consequences for Hg isotope frac-
tionation during bioaccumulation considering that MMHg
in fi sh is thought to be bound to cysteine residues (Harris
et al., 2003). As well, it would constitute a unique example
