Remediation and Management of Contaminated Soil - Sustainable Practices in Geoenvironmental Engineering - page 415

that polymerize and cement are injected to encapsulate the soils. Leaching of the contami-
nants must, however, be carefully monitored as is the case for vitriication, the formation
of a glassy solid. Cement- or silicate-based (5% to 10% by weight additives) processes are
useful for soils and sediments and are economical as the mixing equipment and materi-
als are readily available. Other materials containing iron (red mud, sludge from a water
treatment plants, bog iron ore, unused steel shot, and steel shot waste) have been evalu-
ated (Mulligan and kamali, 2003) for immobilizing cadmium and arsenic contaminants in
sediments. All were effective in reducing the bioavailability of the metals to plants, but the
safest was sludge from a drinking water plant with low levels of As. However, if there are
different types of metals present, the treatment may not be as effective.
Water contents greater than 20% or chlorinated hydrocarbons contents greater than 5%
increase the amount of agents required. Variability in the water content, grain size, and
the presence of debris can make handling of the materials dificult and decrease the efi-
ciency of the solidiication process. In addition, since immobilization leads to an increase
in volume, larger areas of land are required for disposal. Thus, smaller volumes for treat-
ment are more appropriate. Costs range from $30 to 250 per tonne (Hazardous Waste
Consultant, 1996).
For organic contaminants such as oil or gasoline, thermoplastic binder and organic
polymerization have been utilized. The most commonly used thermoplastic material is
polyethylene or asphalt. Organic polymerization for immobilization has also been used
for radioactive wastes, organic chlorides, phenols, paint sludge, and cyanides (ASTM,
1985). The organic materials can include polyethylene, polypropylene, urea formaldehyde,
or parafin. The contaminated soil is dried and mixed into the polymer that is cooled to
form a solid. The mix can be extruded into a metal drum for easy transport and disposal.
Full-scale projects have been performed in the United States, Canada, Japan, and Belgium.
If the process is performed ex situ, the soil is usually screened to remove large materials,
mixed with the binder and water (e.g., in a rotary drum), and then transferred to a disposal
area. Off-gases would need to be treated for dust or volatile organic contaminants. The
process can be performed in a mobile unit or at a ixed site.
For in situ applications, the reagents must be prepared, an auger can be used to mix
the binder directly into the soil and off gases should be treated, particularly for organic
contaminants. Dust generation is minimal, and costs related to excavation and transport
are eliminated. Boulders, bedrock, and clay can cause mixing problems. Contamination of
the groundwater must be avoided. Therefore, mobile contaminants such as Cr(VI) should
be converted irst to the less mobile Cr(III) form. After solidiication, reuse of the land for
buildings may be possible. Backill may be required as a cover for revegetation. Durability
testing will be required. In all cases, compatibility testing should be performed to deter-
mine the most appropriate binding material and TCLP tests after the binding will indicate
the leachability of the contaminants.
Halogenated semivolatiles, nonhalogenated semivolatiles, and nonvolatiles, volatile,
and nonvolatile metals, low-level radioactive materials, corrosives, and cyanides have been
treated effectively. In the Netherlands, a rotating drum was used in a full scale experiment
(Rienks, 1998). Six hundred eighty tonnes of dewatered sediment were treated at 600°C for
38.5 h for mineral oil, PAHs, and mercury. Mercury levels decreased by 80% from 1.5 to 0.3
mg/kg while mineral oil and PAHs decreased by greater than 99.8%. Leaching of arsenic,
molybdenum, and luoride increased after thermal treatment, which can have implications
in the reuse of the treated sediments as road or construction materials. At a former wood-
preserving plant in Fresno, California, contaminated groundwater with As, Cr, Cu, and
PCP was found (USEPA, 2005). Various hydrocarbons, PAHs were found in the irst 2 m