Distribution, Bioavailability and Speciation of Mercury
193
6
Mercury Compounds in Soil
6.1
Results and Discussion
The dominating Hg fraction in soil as found by solid-phase pyrolysis is the group
of matrix-bound mercury forms which cannot be specified (Fig. 6). These forms
may sorb on Fe/Mn-oxyhydroxide coatings covering grain surfaces, they may be
fixed to carbonate incrustions or bound to humic matter, i.e. organically bound
(e.g. Hodel et al. 1995). Identifiable organic Hg species (organomercurials) such
as phenyl-, methyl-, diphenyl-, dimethyl- and diethyl-Hg were determined only
at percentages of less than 0.5% of total Hg.
The formation of Hgo in the subsoil could be documented by means of the
pyrolysis method and soil gas measurements. Hg droplets could indeed be
recognized macroscopically in the soil of the source area in the kyanization
building. The soil gas measurements served as a means for rapid delineation on
site and for an aimed selection of soil samples for laboratory analysis. Figures 4
and 6 illustrate the practical tracing of the maximum soil contamination by
means of direct soil gas measurements on site.
In the former release areas, spatial soil gas contaminations were determined at
a depth of 3 m which exceeded 1000 ~lg of Hgo /m 3 in the centres of
contamination. The Hg (total) concentrations of the soil samples from the insitu loess layer corresponded in general to this pattern of distribution.
The secondary transformation of initially low volatile compounds of cationic
Hg(II) may lead to the formation of elementary Hgo (Hg2+ + 2e - =} HgO) either
abiotically where reducible reagents are present, such as humic matter or iron
and also by microbial mediation (Brosset 1987; Schuster 1991). Due to the high
steam pressure of elementary mercury (HgO), horizontal propagation of mercury
may also occur in the vapour phase, as indicated by the results of the soil gas
measurements. Considering the results of Fang (1978) on the sorption behaviour
of gaseous Hgo on soil, excess of the natural background concentration of Hg in
soil (ca. ~0.1 mg/kg) up to concentrations in the lower mg/kg range in loess or
fine grains of the gravel sediments may, in this case, also result from the
horizontal diffusion of Hgo in the gas phase ..
The soil gas measurements, which can easily be carried out, have to be
regarded as very useful for several reasons: on the one hand, they may serve as a
basis for the risk assessment with regard to the important exposure path via the
air, the secondary formation and the propagation behaviour of Hgo, on the other
hand, they minimize the investigation efforts during delineation and, finally, they
are an important means for interpretation of Hg distribution pattern in the
subsoil.
In some of the soil samples, the water-soluble Hg fractions of HgIIa, HgIIb and
Hgo were determined. Here, the question was, in which form mercury is
preferentially transported to deeper soil layers. Figure 7 illustrates the results of
four different sample groups.
193
6
Mercury Compounds in Soil
6.1
Results and Discussion
The dominating Hg fraction in soil as found by solid-phase pyrolysis is the group
of matrix-bound mercury forms which cannot be specified (Fig. 6). These forms
may sorb on Fe/Mn-oxyhydroxide coatings covering grain surfaces, they may be
fixed to carbonate incrustions or bound to humic matter, i.e. organically bound
(e.g. Hodel et al. 1995). Identifiable organic Hg species (organomercurials) such
as phenyl-, methyl-, diphenyl-, dimethyl- and diethyl-Hg were determined only
at percentages of less than 0.5% of total Hg.
The formation of Hgo in the subsoil could be documented by means of the
pyrolysis method and soil gas measurements. Hg droplets could indeed be
recognized macroscopically in the soil of the source area in the kyanization
building. The soil gas measurements served as a means for rapid delineation on
site and for an aimed selection of soil samples for laboratory analysis. Figures 4
and 6 illustrate the practical tracing of the maximum soil contamination by
means of direct soil gas measurements on site.
In the former release areas, spatial soil gas contaminations were determined at
a depth of 3 m which exceeded 1000 ~lg of Hgo /m 3 in the centres of
contamination. The Hg (total) concentrations of the soil samples from the insitu loess layer corresponded in general to this pattern of distribution.
The secondary transformation of initially low volatile compounds of cationic
Hg(II) may lead to the formation of elementary Hgo (Hg2+ + 2e - =} HgO) either
abiotically where reducible reagents are present, such as humic matter or iron
and also by microbial mediation (Brosset 1987; Schuster 1991). Due to the high
steam pressure of elementary mercury (HgO), horizontal propagation of mercury
may also occur in the vapour phase, as indicated by the results of the soil gas
measurements. Considering the results of Fang (1978) on the sorption behaviour
of gaseous Hgo on soil, excess of the natural background concentration of Hg in
soil (ca. ~0.1 mg/kg) up to concentrations in the lower mg/kg range in loess or
fine grains of the gravel sediments may, in this case, also result from the
horizontal diffusion of Hgo in the gas phase ..
The soil gas measurements, which can easily be carried out, have to be
regarded as very useful for several reasons: on the one hand, they may serve as a
basis for the risk assessment with regard to the important exposure path via the
air, the secondary formation and the propagation behaviour of Hgo, on the other
hand, they minimize the investigation efforts during delineation and, finally, they
are an important means for interpretation of Hg distribution pattern in the
subsoil.
In some of the soil samples, the water-soluble Hg fractions of HgIIa, HgIIb and
Hgo were determined. Here, the question was, in which form mercury is
preferentially transported to deeper soil layers. Figure 7 illustrates the results of
four different sample groups.