126
M. Hempel and ). Thoeming
In contrast to this good result, only 6% of the mercury present in the soil
originating from the polluted site could be removed. The authors assumed that
the organic material present in the soil, which consumed oxidant iodine, was
responsible for this low level of removal.
A bench-scale experiment (Lagemann et al. 1989) with soil from a polluted site
(initial cone. 334 mg Hg/kg) removed 67%, which means that no mg/kg
remained in the soil. Hansen et al. (1996) also investigated soil from a polluted
site (initial cone. 685 mg/kg) using a well-described membrane technique called
electrodialysis. The removal rate of mercury was up to 55% (average 23%), but
the real removal potential is not clear since they did not vary the duration of the
experiment. Actually, the sandy character of the soil lowers electro osmotic as
well as electrophoretic transport. This probably leads to the dependence of the
removal rate on the concentration of mercuric ions in the soil which, in its turn,
depends on the concentration of chloride or other complexing agents.
Electrophoretic remediation is claimed in a patent by Doering and
Decristofero (1990) but until now there are no results available. It can be
assumed that elemental mercury can be transported electrophoretically and that
this method may work. However, particulate mercury is usually associated with
ionic and organic species which should not be left in the soil. Consequently
future work should not focus on electrophoresis alone but on all electrokinetic
transport mechanisms simultaneously.
Other Techniques. Until now, no other promlsmg techniques have been
proposed. A technique for the reduction of mercury present in contaminated
soil was in an article by an unknown group. (Anonymous 1991). In this
technique, hydrogen produced electrolytically in an electrolytic cell which
contains the soil and an electrolyte should lead to a complete conversion of all
mercury-containing compounds to the elemental form. This elemental mercury
precipitate would be amalgamated by the copper cathode at the bottom of the
cell. For this process no results are presented, and it seems likely that neither the
reduction nor the amalgamation would work - at least sufficiently well.
Another suggestion (Behrends and Lorenz 1989) is to oxidize contaminated
soil with FeCI 3 solution to produce a calomel layer on the mercury droplets and
to agitate the slurry strongly so as to minimize the size of these particles. The
idea is to obtain a homogeneous distribution of the mercury in the slurry. The
authors call this process decontamination, but actually it is a preparation for
immobilization.
Tittel and Hess (1994) investigated the separation of elemental mercury using
amalgamated metal surfaces such as copper, silver and dental amalgam. On a
laboratory scale he reduced the Hg content of the fine fraction of a soil
« 125 pm) from 3.8-6.8 g Hg/kg to below 1 g Hg/kg. For soil fractions> 125 ~lm,
Hg was reduced down to 94% of the feed concentration. Abrasion was the main
problem during these experiments.
Little work has been done on cleanup methods with biological techniques.
Promising bioremediation techniques are described for water insitu (Brunke et
al. 1993; Barkay et al. 1995). Hg-resistant bacteria are used which enhance the
transformation from methylmercury or ionic mercury to the volatile elemental
M. Hempel and ). Thoeming
In contrast to this good result, only 6% of the mercury present in the soil
originating from the polluted site could be removed. The authors assumed that
the organic material present in the soil, which consumed oxidant iodine, was
responsible for this low level of removal.
A bench-scale experiment (Lagemann et al. 1989) with soil from a polluted site
(initial cone. 334 mg Hg/kg) removed 67%, which means that no mg/kg
remained in the soil. Hansen et al. (1996) also investigated soil from a polluted
site (initial cone. 685 mg/kg) using a well-described membrane technique called
electrodialysis. The removal rate of mercury was up to 55% (average 23%), but
the real removal potential is not clear since they did not vary the duration of the
experiment. Actually, the sandy character of the soil lowers electro osmotic as
well as electrophoretic transport. This probably leads to the dependence of the
removal rate on the concentration of mercuric ions in the soil which, in its turn,
depends on the concentration of chloride or other complexing agents.
Electrophoretic remediation is claimed in a patent by Doering and
Decristofero (1990) but until now there are no results available. It can be
assumed that elemental mercury can be transported electrophoretically and that
this method may work. However, particulate mercury is usually associated with
ionic and organic species which should not be left in the soil. Consequently
future work should not focus on electrophoresis alone but on all electrokinetic
transport mechanisms simultaneously.
Other Techniques. Until now, no other promlsmg techniques have been
proposed. A technique for the reduction of mercury present in contaminated
soil was in an article by an unknown group. (Anonymous 1991). In this
technique, hydrogen produced electrolytically in an electrolytic cell which
contains the soil and an electrolyte should lead to a complete conversion of all
mercury-containing compounds to the elemental form. This elemental mercury
precipitate would be amalgamated by the copper cathode at the bottom of the
cell. For this process no results are presented, and it seems likely that neither the
reduction nor the amalgamation would work - at least sufficiently well.
Another suggestion (Behrends and Lorenz 1989) is to oxidize contaminated
soil with FeCI 3 solution to produce a calomel layer on the mercury droplets and
to agitate the slurry strongly so as to minimize the size of these particles. The
idea is to obtain a homogeneous distribution of the mercury in the slurry. The
authors call this process decontamination, but actually it is a preparation for
immobilization.
Tittel and Hess (1994) investigated the separation of elemental mercury using
amalgamated metal surfaces such as copper, silver and dental amalgam. On a
laboratory scale he reduced the Hg content of the fine fraction of a soil
« 125 pm) from 3.8-6.8 g Hg/kg to below 1 g Hg/kg. For soil fractions> 125 ~lm,
Hg was reduced down to 94% of the feed concentration. Abrasion was the main
problem during these experiments.
Little work has been done on cleanup methods with biological techniques.
Promising bioremediation techniques are described for water insitu (Brunke et
al. 1993; Barkay et al. 1995). Hg-resistant bacteria are used which enhance the
transformation from methylmercury or ionic mercury to the volatile elemental