Environmental Engineering Reference
In-Depth Information
Xin et al., 2007), temperature (Dudas and Cannon, 1983;
Schroeder and Munthe, 1998; Ericksen et al., 2006), and
wind velocity and other meteorologic conditions (Landa,
1978; Lindberg et al., 1979; Kim et al., 1995; Gustin et al.,
1997; Zhang et al., 2001). Given the extreme spatial and
temporal variability of these parameters, Hg emission from
soils is also highly variable, and as such, global estimates
are poorly constrained (Gustin et al., 2008).
Schlüter (2000) reviewed processes that lead to soil Hg(0)
production and emission. In general, an increase in soil
Hg(0) emission rate has been reported with decreasing
SoM and clay content. This can be attributed to the favor-
able binding of Hg(II) to these soil constituents. A lower
sorption affinity and capacity renders Hg(II) more avail-
able to biotic or abiotic reduction (Zhang and Lindberg,
1999). relatively fast (on the order of several hours)
desorption of inorganic Hg(II) from soil has been shown
by Yin et al. (1997b), suggesting that Hg(II) may be read-
ily available for reduction. However, the Hg(II) concentra-
tions used by Yin et al. were in the parts per million range,
as mentioned above. In most soil environments, since Hg
concentrations are considerably lower than the parts per
million range and Hg(II) is largely associated with the
reduced sulfur groups of SoM, the Hg(II) desorption rate,
and consequently its availability for reduction, is expected
to be lower.
DoM, mainly in the form of humic and fulvic acids, has
been shown to reduce Hg(II) to Hg(0) in the dark (Alberts
et al., 1974; Allard and Arsine, 1991). Alberts et al. used
a 5:1 DoM:Hg(II) ratio (on a gram-to-gram basis) at pH
6.7-8.2 and observed a first-order reduction of Hg(II). The
reduction was attributed to the organic free radicals of the
quinonelike moieties associated with the DoM. Based on
previously published work, Schlüter (2000) proposed that
the DoM-mediated reduction of Hg(II) depends on the
DoM:Hg(II) ratio. At high ratios (
.
1000:1), Hg(II) would be
stabilized via complexation with the reduced sulfur groups,
and at low ratios (
0.5:1), Hg(II) reduction would be lim-
ited perhaps because of the consumption of the reductive
potential of the DoM. A rapid initial Hg(II) reduction rate
followed by a much slower rate by DoM has been observed
and attributed to the consumption of the reductive poten-
tial and the effective binding by DoM at low Hg(II) con-
centrations (Alberts et al., 1974; Allard and Arsine, 1991;
Schlüter et al., 1995a). The presence of quinonelike moi-
eties in DoM (Cory and McKnight, 2005) and the redox-
active character of hydroquinone-quinone couples with
a wide range of redox potentials in DoM (Scott et al.,
1998; Fimmen et al., 2007) have been shown using spec-
troscopic techniques. DoM has also been shown to reduce
other metals, such as Cr(vI) and Fe(III) abiotically, and to
enhance the microbially catalyzed reduction of Cr(vI) and
U(vI) potentially via an electron shuttling mechanism
(Nevin and Lovley, 2000; Gu and Chen, 2003). A similar
mechanism may also be expected for reduction of Hg(II) in
the presence of DoM.
Allard and Arsine (1991) observed a decrease in the pro-
duction rate of Hg(0) with increasing Cl
2
and Eu concen-
trations, prompting them to propose that Hg(II)-DoM
complexes should be formed before electron transfer can
take place. Presence of strong ligands for Hg(II), such as
Cl
2
, inhibits reduction by forming stable complexes such
as HgC1
4
2
2
(Amyot et al., 1997).
Provided that solar radiation results in the reduction
of Hg(II) to Hg(0) in surface waters, Zhang and Lindberg
(1999) suggested that it might also play a role in Hg(0) pro-
duction in near-surface soils. They presented several sce-
narios by which sunlight could affect Hg(0) production.
These include direct photolysis in the presence of oH
2
and organic acids, production of superoxide that leads to
Hg(II) reduction, photolysis of Fe(III)-organic acid com-
plexes that produce reducing organic radicals, and surface-
catalyzed reduction of adsorbed Hg(II). DoM-catalyzed
Hg(0) reduction can be especially enhanced in the presence
of sunlight, as observed by Allard and Arsine (1991). Xiao
et al. (1995) suggested that Hg(0) production in the near-
surface soil environment that is subject to Uv irradiation
can be enhanced because of the presence of SoM. Sunlight-
mediated soil Hg(II) reduction and emission is perhaps
least likely in closed-canopy forests, but more likely in
open fields (Grigal, 2002).
redox cycling of Hg is complicated by reduction and oxi-
dation reactions in the same system (Zhang and Lindberg,
1999). For example, Hg(0) production in sunlight has been
followed by its oxidation in the dark in organic-rich Florida
Everglades water (Lindberg et al., 1999). Soil Hg(0) emission/
uptake processes are also controlled by the ambient Hg(0)
and soil Hg concentrations, as well as the presence of sun-
light. Xin and Gustin (2007) observed that at low air Hg(0)
concentrations (2.8
6
0.8 ng m
2
3
), soils emitted Hg(0) in
sunlight, but took up Hg(0) in the dark. At higher air Hg(0)
concentrations, however, they observed soil Hg(0) uptake
regardless of the light condition. It is generally accepted
that soils with naturally high Hg concentrations emit Hg(0)
(Gustin, 2003), whereas soils with generally
100 ng g
2
1
of
Hg can act as sources or sinks depending on the physical
environment (Xin and Gustin, 2007).
Increases in Hg emission from unsaturated soils follow-
ing wetting events have been reported (Carpi and Lindberg,
1998; Frescholtz and Gustin, 2004; Gustin and Stamenkovic,
2005). Lindberg et al. (1999) attributed the enhancement of
Hg emission by an increase in soil moisture to: (a) physical
displacement of Hg(0) by water filling the pores, (b) Hg(0)
desorption from the soil surface by a wetting fluid with a
higher affinity to the soil surface, and (c) reduction of Hg(II)
following Hg(II) mobilization in solution. Studies involving
soil Hg(II) amendments have observed an increase in Hg(0)
production with an increase in pH (Landa, 1978; Schlüter
et al., 1995b). In the forest and pasture oxisols of the Amazon,
Lacerda et al. (2004) observed a significant negative
correlation between soil Hg and pH, and proposed that a
higher soil pH favors Hg(II) reduction followed by Hg(0)
