120
M. Hempel and J. Thoeming
ly, cement or fly ash is mixed with the suspension. After hardening reaction takes
place, and after some hours, a concrete-like mass with low permeability
(10 -9 cm/s) can be deposited at suitable sites. Leaching tests are used for quality
control of solidified mass.
3.3.4
Chemical Immobilization
In contrast to solidification and stabilization techniques, there are some methods
which aim to immobilize mercury by transforming the existing chemical species
into a form of lower toxicity, lower solubility or lower available form. This can be
achieved by adding a sulphur-containing chemical additive to the soil (Aqua
Control 1992). Hg reacts with the sulphur compounds to form HgS or HgScontaining compounds. The chemical reagent can be added either by spraying
onto the excavated soil or by injecting the sulphur-containing solution into the
undisturbed soil using high pressure.
Immobilization techniques in general have the disadvantage that the pollutants
remain in the environment. Chemical or physical immobilization could be a
sufficient treatment for a period of time. The physical barriers may break down
or have limited barrier function. Immobilized Hg compounds could be
remobilized by changes of pH, redox or by biological activity. There are doubts
that sulphur compounds added to Hg in the soil form pure HgS.
4
Emerging and Potential Alternative Cleanup Methods
4.1
Alternative Thermal Techniques
A large variety of modified thermal techniques are emerging, and have been
demonstrated on a pilot-scale plant.
A steam distillation method was designed by Luther et al. (1995) for highly
contaminated sludges. Mercury is released from the sludge by steam
distillation at temperatures below 300°C and so predrying is not necessary.
The mercury-bearing steam is separated from the soil by cyclones. The gas is
cleaned by a washing technique, followed by condensation and adsorption on
charcoal. The condensed water is cleaned by ultrafiltration and reverse osmosis
before reuse.
Koshinski et al. (1996) describe a pilot-scale facility where the contaminated
material is blended with additives prior to processing. The additives need to
decompose stable mercury compounds, convert mercury-compounds to elemental form, retain the chloride and sulphide components and prevent the release of
chloride and sulphur in gaseous form. The blended material is heated in two
stages up to 540-650 0c. Gaseous mercury is condensed in a heat exchanger.
M. Hempel and J. Thoeming
ly, cement or fly ash is mixed with the suspension. After hardening reaction takes
place, and after some hours, a concrete-like mass with low permeability
(10 -9 cm/s) can be deposited at suitable sites. Leaching tests are used for quality
control of solidified mass.
3.3.4
Chemical Immobilization
In contrast to solidification and stabilization techniques, there are some methods
which aim to immobilize mercury by transforming the existing chemical species
into a form of lower toxicity, lower solubility or lower available form. This can be
achieved by adding a sulphur-containing chemical additive to the soil (Aqua
Control 1992). Hg reacts with the sulphur compounds to form HgS or HgScontaining compounds. The chemical reagent can be added either by spraying
onto the excavated soil or by injecting the sulphur-containing solution into the
undisturbed soil using high pressure.
Immobilization techniques in general have the disadvantage that the pollutants
remain in the environment. Chemical or physical immobilization could be a
sufficient treatment for a period of time. The physical barriers may break down
or have limited barrier function. Immobilized Hg compounds could be
remobilized by changes of pH, redox or by biological activity. There are doubts
that sulphur compounds added to Hg in the soil form pure HgS.
4
Emerging and Potential Alternative Cleanup Methods
4.1
Alternative Thermal Techniques
A large variety of modified thermal techniques are emerging, and have been
demonstrated on a pilot-scale plant.
A steam distillation method was designed by Luther et al. (1995) for highly
contaminated sludges. Mercury is released from the sludge by steam
distillation at temperatures below 300°C and so predrying is not necessary.
The mercury-bearing steam is separated from the soil by cyclones. The gas is
cleaned by a washing technique, followed by condensation and adsorption on
charcoal. The condensed water is cleaned by ultrafiltration and reverse osmosis
before reuse.
Koshinski et al. (1996) describe a pilot-scale facility where the contaminated
material is blended with additives prior to processing. The additives need to
decompose stable mercury compounds, convert mercury-compounds to elemental form, retain the chloride and sulphide components and prevent the release of
chloride and sulphur in gaseous form. The blended material is heated in two
stages up to 540-650 0c. Gaseous mercury is condensed in a heat exchanger.