Environmental Engineering Reference
In-Depth Information
Association of Mercury Species with Soils and
Soil Constituents
HgOCl and HgCl
2
0
, would be dominant in the soil-water
environment. In light of fi ndings regarding the high Hg
concentrations in road dust (Liang et al., 2009; Lu et al.,
2009), it is expected that the presence of high Cl
con-
centrations in road salt facilitate Hg mobilization in road
dust. In the presence of
Solid-Solution Interface
10 ppm concentrations of Cl
,
Hg(OH)
2
0
, and Hg(OH)
would be the dominant species
in most soil-water environments. The presence of even
low concentrations of DOM, however, changes Hg specia-
tion in terrestrial and freshwater ecosystems. For example,
Amirbahman et al. (2002) estimated that in the presence
of 1 ppm of dissolved humic acid and 35 ppm NaCl, 20 ppt
of MeHg is predominantly complexed by the humic acid at
pH
ASSOCIATION OF MERCURY SPECIES WITH SOIL
ORGANIC MATTER
The association of Hg species with soils and individual soil
const it uents has been rev iewed prev iously by several l aut hors
(Andersson, 1979; Schuster, 1991; Grigal, 2003; Gabriel and
Williamson, 2004). It has been shown that under envi-
ronmentally relevant concentrations, Hg and MeHg have
a higher binding strength with organic matter than with
soil minerals (Hintelmann et al., 1995; Amirbahman et al.,
2002; Drexel et al., 2002; Haitzer et al., 2002; Karlsson and
Skyllberg, 2003). In natural ecosystems, saturation of the
soil Hg binding capacity is seldom reached.
In the natural environment, the Hg retention mecha-
nism by organic matter is through the association of Hg
with reduced sulfur functional groups of the organic mat-
ter. Spectroscopic evidence reported by Xia et al. (1999) and
Qian et al. (2002) suggested the involvement of thiol (RSH)
and disulfane (RSSH) functional groups as binding sites
for inorganic Hg and MeHg. The disulfi de group (RSSR
5, whereas below this pH, Cl
dominates MeHg com-
plexation. Higher DOM concentrations shift this transition
to lower pH values. Using the inorganic Hg-DOM stability
constants published by Haitzer et al. (2003), similar con-
clusions can be obtained for the dominance of inorganic
Hg-DOM complexes. Higher organic matter and lower Cl
concentrations in soil and soil water environments favor
Hg and MeHg complexation by organic matter. One would
also expect that the quality of SOM would infl uence Hg
sorption, especially in view of the fact that landscape
conditions lead to differential SOM accumulation (Grigal,
2003).
However, Skyllberg et al. (2000) observed no signifi -
cant differences in the binding constants of Hg to differ-
ent types of SOM with varying reduced sulfur content col-
lected along a transect from upland to wetland. Differences
in Hg binding constants due to differences in SOM quality
might be detectable at higher Hg concentrations, which in
most cases are not environmentally relevant.
In soils, metal adsorption is complicated by the presence
of SOM, whose solubility depends on pH. Yin et al. (1996)
observed maximum inorganic Hg removal from solution
by several sandy and loamy soils at pH values between
3 and 5. At higher pH values, the extent of Hg removal
decreased because of the enhanced dissolution of SOM.
Yin et al. (1997a) observed an S-type isotherm for the asso-
ciation of inorganic Hg with soils with SOM
)
was also implicated as a possible binding site for inor-
ganic Hg. Oxygen- and nitrogen- containing functional
groups are also involved in the binding of inorganic Hg to
SOM. The fraction of total sulfur present as reduced sul-
fur groups depends on the source of organic matter, with
values ranging from 10% for a mineral soil sample to 46%
of total sulfur for a stream humic acid sample (Xia et al.,
1998). A reduced sulfur atom in organosulfi des has been
defi ned as sulfur with an electronic oxidation state rang-
ing between 0 and 0.5 (Morra et al. 1997; Xia et al., 1998).
Reduced sulfur in soils can also be found in amino acids
that are either associated with, or part of, organic matter.
In a humic acid sample isolated from forest soil, cysteine
and methionine were measured at concentrations of 2.74
and 1.71 nmol mg
1
, respectively (Fan et al., 2000). In most
soil environments, reduced sulfur groups with electronic
oxidation states
11 g C kg
1
,
and an L-type isotherm with SOM
7.5 g C kg
1
. A simi-
lar behavior was also observed in other studies, in which
inorganic Hg adsorption isotherms with O-horizon soils
were of S-type (Schlüter, 1997; Amirbahman et al., 2004).
This could be due to the presence of higher DOM concen-
trations in organic soils, and the fact that DOM can pos-
sess a higher binding constant than solid organic matter
for inorganic Hg (Drexel et al., 2002; Amirbahman et al.,
2004). Both of these studies observed a higher inorganic
Hg binding to DOM than to the solids and explained the
lower adsorption to the solid at low total Hg concentrations
as the reactive sites on DOM out-competing those on the
solid. Schlüter (1996) studied transport of labeled inor-
ganic Hg and MeHg in intact soil columns collected from
a well-drained Fe-humus podzol in a coniferous forest. The
applied Hg concentrations ranged approximately between
50 and 500 ng g
1
and were typical of those found in most
1 exist at higher concentrations than Hg
and MeHg (Morra et al., 1997; Xia et al., 1998). In the pres-
ence of exceedingly high concentrations of MeHg, oxygen
and nitrogen atoms, instead of reduced sulfur, have been
detected in the fi rst metal coordination shell (Xia et al.,
1999; Qian et al., 2002).
The extent of inorganic Hg and MeHg association with
organic matter depends on the concentration of organic
matter, pH, and the presence of other competing ligands
such as dissolved sulfi de [S(-II)] and Cl
. Hg speciation
in solution infl uences its adsorption, which is in turn
controlled by pH and the presence of dissolved ligands.
Equilibrium speciation calculations show that in the
absence of DOM and in the presence of
10 ppm concen-
trations of the Cl
ligand, Hg-Cl complexes, specifi cally
