Environmental Engineering Reference
In-Depth Information
mercury, and even if they can, it is unclear whether this
process would produce substantial amounts of MMHg.
The existence of suboxic microenvironments in larger
aggregates or biofi lm-coated particles (Kühl and Jørgensen,
1992; Ploug et al., 1997) may allow for limited microbial
sulfate reduction, which is linked to MMHg production. In
addition, MMHg production rates are substantially higher
in biofi lms than in planktonic sulfate-reducing cells (Lin
and Jay, 2007). Any MMHg produced in particle micro-
environments would have to diffuse out of particles or be
released as the aggregates are remineralized in order to be
a direct source of MMHg to the water column. Any MMHg
retained in sinking particles will be exported to surface
sediments, although this could be subsequently remobi-
lized to overlying deep waters.
The importance of sinking particles as sites of MMHg
production may be limited because the impact of fl uid fl ow
on the diffusion and exchange of gases and solutes between
aggregates and the surrounding solution (Kiorboe et al.,
2001) makes substantial sulfate reduction unlikely in small
or rapidly sinking, loosely consolidated particles (Ploug
et al.,
1997). Exceptions to this could be in oxygen minimum
zones and other intermediate waters where the reminer-
alization of particulate organic carbon results in low oxy-
gen concentrations that may allow for sulfate reduction in
particle microenvironments as particles pass through these
waters. Elevated levels of DMHg and methylated Hg have
been measured in such intermediate waters at depths asso-
ciated with organic matter mineralization and low oxygen
concentrations (Cossa and Coquery, 2005; Cossa et al., 1994,
1997, 2009; Mason and Fitzgerald, 1990, 1993; Mason and
Sullivan, 1999; Mason et al., 1995; Sunderland et al., 2009).
It is not known how much of this methylated Hg is MMHg,
nor whether the Hg methylation at these depths occurs pri-
marily in sinking particles or in the bulk water column.
those in freshwater by a factor of 2-3. Photodegradation
of MMHg would be limited to the upper 50 m of oligotro-
phic oceanic waters because of the attenuation of sunlight
with water depth (Smith and Baker, 1981), and this would
be considerably shallower in coastal waters and other areas
with appreciable dissolved and particulate organic matter that
absorb light, especially in the UV region (Morris et al., 1995).
We estimate that 0.7 Mmol of MMHg is photodegraded in
the photic zone of the ocean annually, assuming a MMHg
concentration of 0.05 pM in surface waters, a MMHg photo-
deg radation rate constant of 0.12 d
1
for the uppermost layer
of the water column during a cloudless summer day at a lat-
itude of 38°N (Monperrus et al., 2007; Whalin et al., 2007),
and extrapolating to the rest of the ocean by accounting for
typical DOM concentrations and light-absorbing proper-
ties for neritic and open ocean waters (Hansell and Carlson,
2002), the attenuation of light at depth from absorption
and scattering by water and DOM (Smith and Baker, 1981;
Morris et al., 1995), and latitudinal differences and seasonal
changes in sunlight irradiance and cloud cover. Although
there are many potential sources of error in such a calcula-
tion, one of the more conspicuous is the value chosen for
the concentration of MMHg in the mixed layer, which is
poorly known and often below detection limits.
VOLATILIZATION TO THE ATMOSPHERE
The volatilization of MMHg from surface waters and its
presence in the atmosphere has been reported (Iverfeldt
and Lindqvist, 1982; Lee et al., 2002; Mester and Sturgeon,
2002). However, this topic has been little studied, and it is
not clear whether all of the analytical methods used to date
have actually measured MMHg rather than gaseous form(s)
of inorganic or elemental mercury.
Any volatilization of MMHg that does occur from seawa-
ter would likely involve the species CH
3
HgCl
0
. CH
3
HgCl
0
is predicted to be the dominant inorganic complex under
the oxic saline conditions found in the ocean. However,
organic material, sulfi de, or other ligands in seawater are
capable of binding MMHg and dominating its complex-
ation (Amirbahman et al., 2002; Dyrssen and Wedborg,
1991). Such complexation would diminish the importance
of chloride in binding MMHg, and thus decrease rates of
MMHg volatilization because of lower levels of CH
3
HgCl
0
.
The exception to this would be if sulfi de were present at low
nanomolar concentrations, as has been reported (Cutter
et al., 1999; Luther and Tsamakis, 1989), then the dominant
complex could be CH
3
HgSH
0
(Dyrssen and Wedborg, 1991).
This neutrally charged complex could be suffi ciently volatile
to result in the loss of MMHg to the atmosphere as reported
for MMHgCl
0
, although this has yet to be demonstrated.
Sinks for Monomethylmercury in the
Marine Environment
MONOMETHYLMERCURY PHOTODECOMPOSITION
Studies indicate that the photodecomposition of MMHg is
likely the dominant mechanism of MMHg degradation or
loss in the photic zone of aquatic environments (Bergquist
and Blum, 2007; Sellers et al., 1996; Monperrus et al., 2007).
This may be an indirect process mediated by reactive oxy-
gen species or other reactive intermediates generated by
photochemistry, such as singlet oxygen and the hydroxyl
radical (Chen et al., 2003; Suda et al., 1993).
It has been proposed that rates of MMHg photodemethyl-
ation are substantially lower in seawater as compared with
freshwater (Whalin et al., 2007). However, MMHg photo-
degradation rates as high as 25% d
1
have been reported for
marine surface waters (Monperrus et al., 2007), and when
normalized to light levels and exposure duration, photo-
demethylation rates in seawater are likely to be less than
BIOTIC DEMETHYLATION OF MONOMETHYLMERCURY
The biotic demethylation of MMHg has been reviewed else-
where (Barkay et al., 2003; Misra, 1992); therefore, it is not
