Remediation and Management of Contaminated Soil - Sustainable Practices in Geoenvironmental Engineering - page 416

of the soil. The Silicate Technology Corporation (STC) immobilization with proprietary
organophilic materials was used. The soil was removed from the unlined disposal pond
and transported to the processing area for mixing, addition of reagents, and discharge
into concrete forms. Neutral conditions were required for As stabilization as mobility
increased at lower pH values. Arsenite could not be converted to arsenate by the process.
PCP and other organic contaminants were below TCLP threshold levels before and after
treatment. Mixing had to be thorough and dust was a problem that had to be minimized.
11.4.5 Vitrification
Another immobilization technique is vitriication, which involves the insertion of elec-
trodes into the soil, which must be able to carry a current to heat the soil up to 1400°C to
2000°C and then to solidify as it cools. Toxic gases from the organic contaminants can also
be produced during vitriication and must be collected and treated. Volume reduction of
20% to 50% occurs leading to subsidence above the melt and thus backill is required to
ill the volume (Weston, 1988). The heavy metal and radionuclide contaminants remain
in the glass-like substance (USEPA, 2006). Costs can be high since fuel values are low and
moisture contents are high (above 20%). Vitriication operation costs are directly related to
water and electricity consumption and treatment depth and area. Estimates of vitriication
costs are in the order of $375-425 per tonne of soil treated.
A technology developed for the remediation of organic contaminants and immobiliza-
tion of metals in a glassy matrix has been evaluated for treatment of dredged sediments
from NewYork/New Jersey harbor (Institute of Gas Technology [IGT], 1996). The tech-
nique uses a plasma torch to heat the sediments. Feeding of the wet sediments into the
plasma reactor and adjustment of residence times can be dificult, however. Cadmium,
mercury, and lead levels were reduced eficiently (97%, 95%, and 82%). Glass tiles and iber
glass materials were produced and could be used as valuable end products.
Temperatures higher than 1200°C could possibly degrade organic compounds and vola-
tilize heavy metals. Solids, such as minerals, will melt at this temperature. The technique
that utilizes this temperature range is called vitriication technique or GeoMelt process . The
materials can be burned, electrically melted or other means. According to Kurion, in the
United States, Japan, and Australia, the GeoMelt process has been in commercial use since
the 1990s and has treated more than 26,000 metric tons of waste, including remediating
sites contaminated with heavy metals, radioisotopes, pesticides, herbicides, solvents, PCBs,
dioxins, and furans.
The GeoMelt processes are designed to be an in situ or mobile container thermal treat-
ment process that involves the electric melting of contaminated soils, sludges, or other
earthen materials and debris either in situ or ex situ for the purpose of permanently
destroying, removing, and/or immobilizing hazardous and radioactive contaminants. Ex
situ technology for vitriication is illustrated as shown in Figure 11.6.
Excavated materials are irst dried and transported into a forge. The materials are
melted at a temperature higher than 1200°C. The produced gas is cooled down and treated
with activated carbon. After contaminants are removed by the activated carbon, the gas
is released into the air. Since hazardous materials, such as organic compounds and heavy
metals, in the materials are vaporized, the solids after vitriication are usually clean. This
technique is recommended as one of treatment techniques for sediments contaminated
with dioxin, in the Japanese technical guideline of sediments contaminated with dioxin.
The process low is shown in Figure 11.7.