Environmental Engineering Reference
In-Depth Information
using composite isotope ratio data. Jackson et al. (2007)
also reported the extraction of MMHg from fi sh samples for
subsequent compound-specifi c isotope ratio measurement.
Unfortunately, their data are inclusive. Although
anoxic water. Considering the postulated strong magnetic
isotope effect (MIE) in the interaction of MMHg with cys-
teine residues (Buchachenko, 2001), a (bio)accumulation
process involving MMHg binding to cysteine in proteins
may also introduce a signifi cant MIF. It remains to be seen
whether the similarity of the
δ
202
Hg for
lake trout was reported with
0.62 to
0.83‰ (Shipiskan
Lake) and
0.74 to
0.57‰ (Cli Lake), the corresponding
201
Hg ratio in photo-
demethylation reactions and fi sh tissue is directly related
or merely a coincidence. The determination of
199
Hg:
values for
δ
Me
202
Hg (
1.04 and
0.35‰ in Shipiskan and
Cli Lake, respectively) and
δ
202
Hgi (
0.11 and
0.33‰) do
Me
199
Hg
not match up.
In Lake Michigan,
and
Me
201
Hg in water would greatly aid in answering
this question. Unfortunately, MMHg in surface water is
extremely low, making these measurements nearly impos-
sible. Regardless, the combined use of MDF and MIF in
food web studies offers a new powerful tool to unravel the
complexities of Hg sources and accumulation in food webs.
δ
202
Hg for burbot fi sh varied between
-0.29 and
1.2‰ and values for New England (mainly yel-
low perch and chain pickerel) ranged from
0.30 to -2.33‰
(Bergquist and Blum, 2007). While
202
Hg varied greatly
within and among locations and fi sh species, in general, a
strong correlation between
δ
202
Hg and the Hg concentration
in fi sh muscle was again observed. The authors speculated,
therefore, that
δ
Mercury Isotope Systematics and Processes
Causing Mercury Isotope Fractionation
202
Hg values might indicate fi sh age and tro-
phic status. The enrichment of
202
Hg in fi sh hav ing higher Hg
concentrations is noteworthy. A preferential elimination of
lighter isotopes by fi sh leading to the observed fractionation
in older and more contaminated fi sh was suggested. This idea
may well be supported by a study by Van Walleghem et al.
(2007), which showed that fi sh are able to eliminate MMHg,
creating an opportunity for signifi cant isotope fractionation.
The most striking result of both studies, however, is the
observation of mass independent fractionation (MIF) in
fi sh and food-web samples.
δ
Clearly, to fully understand the observed Hg isotope ratio
deviations in nature, a comprehensive theoretical framework
describing the processes leading to Hg isotope fractionation
is necessary. Many studies have investigated individual reac-
tions and processes and reveal deeper insight into the Hg
isotope system. Table 4.4 summarizes those investigations
and the following section provides additional insight.
201
Hg between
0.59‰ and
Reduction Processes
5.51‰ were
observed (Bergquist and Blum, 2007). The more com-
prehensive food-web data in Figure 4.2 show frequently
large
4.21‰ and
199
Hg between
0.29 and
Owing to its rich redox chemistry, many mercury species
are subject to photoreduction, microbial reduction, and
chemical reduction processes. All three reduction path-
ways have in common that they supply Hg(0) to the pool
of atmospheric Hg. Consequently, any fractionation dur-
ing reduction processes has the potential to regulate or
alter the Hg isotope signature of the atmosphere on a local,
regional, and maybe even global scale.
5.19‰ in biota. It is striking that
sediments in all three lakes were the only samples with
undetectable MIF. Despite the large variation among dif-
ferent samples, the ratio of
199
Hg—up to
201
Hg is very con-
sistent and was, on average, 1.28 for all fi sh samples in
both studies. Interestingly, almost identical MIF was mea-
sured in the laboratory when subjecting MMHg in water
to photodemethylation (Bergquist and Blum, 2007). The
MMHg remaining in solution had a similar
199
Hg go
PHOTOREDUCTION
201
Hg
ratio as that observed in fi sh in the wild. The authors inter-
preted this fi nding as an imprinting of the aqueous MMHg
MIF onto the food web. This hypothesis would imply that
the original MMHg MIF remains unperturbed regardless
of potential (mass-dependent or mass-independent) frac-
tionation during bioaccumulation. Jackson et al. (2007)
observed an increase of
199
Hg:
Using a solar simulator, our laboratory systematically inves-
tigated photoreduction of ionic Hg in natural waters to
identify, which environmental parameter affects Hg isotope
fractionation (Zheng and Hintelmann, 2009). Filtered lake
water from Harp Lake (Haliburton, Ontario, Canada) with
a residual dissolved organic carbon (DOC) content of 12
mg/L was amended with 10 µg/L of Hg and irradiated under
a solar simulator. Reduced Hg(0) was continuously purged
from the reactor and trapped in acidic permanganate solu-
tion. Every 2 hours, subsamples were collected, for a total
reaction time of 10 hours, after which 30% of the initial
Hg was reduced and lost from the reactor. The Hg isotope
composition was determined for the residual aqueous Hg,
and for the fi rst time also for the reduced Hg(0) product of
the reaction. A mass balance confi rmed that all the experi-
mental Hg could be accounted for in either the remaining
or the trapping solution. An exemplary plot is shown in
199
Hg with Hg concentration (and
trophic level) in their food-web study (Figure 4.2), which
would be in contrast with the theory that the MIF is unaf-
fected by bioaccumulation. However,
199
Hg measured
in lower-food-web organisms might be biased by
199
Hgi,
Me
199
Hg measured in fi sh.
In addition, the effects of biotic methylation and demeth-
ylation as well as bioaccumulation on Hg isotope fraction-
ation are mostly unknown to date. Only a small fraction
of MMHg in water is subject to photoreduction, while
the majority of MMHg is produced in sediments or deep
which is likely different from
