Page 223 - Soil Degradation, Conservation and Remediation

212 6 Soil Pollution

(Shively et al. 1986 ). Solidification/stabilization technologies are not useful for some
2−
forms of metal contamination, such as species that exist as oxyanions (e.g., Cr 2 O 7 ,
−
AsO 3 ) or metals that do not have low-solubility hydroxides (e.g., Hg). Solidifi cation/
stabilization may not be applicable soils contaminated with organic pollutants.
Cement-based binders and stabilizers are common materials used for implementation

of solidification/stabilization technologies (Conner 1990 ). Portland cement, a
mixture of Ca silicates, aluminates, aluminoferrites, and sulfates, is an important
cement-based material. Pozzolanic materials, which consist of small spherical

particles formed by coal combustion (such as fly ash) and in lime and cement kilns,
are also commonly used for the purpose. Organic binders may also be used to treat
metals through polymer microencapsulation. This process uses organic materials

such as bitumen, polyethylene, paraffins, waxes, and other polyolefins as thermoplastic
or thermosetting resins.

Vitrification

Vitrification involves the conversion of contaminated soil materials into glass-like
substances. This is achieved by high-temperature treatment of the contaminated
area that results in the formation of vitreous material. Most soils can be treated by
vitrification, and a wide variety of inorganic and organic contaminants can be targeted.

Vitrification may be performed ex situ or in situ although in situ processes are preferred

due to the lower energy requirements and cost (USEPA 1992b ). Typical stages in ex

situ vitrification processes may include excavation, pretreatment, mixing, feeding,

melting and vitrification, off-gas collection and treatment, and forming or casting
of the melted product. Some additives such as sand, clay, and/or native soil may
be used for vitrifi cation. The vitrified waste may be recycled and used as clean fi ll,

aggregate, or other reusable materials. In situ vitrification involves passing electric

current through the soil using an array of electrodes inserted vertically into the
contaminated region. Resistance heating in the starter path melts the soil. A single
melt can treat up to 1,000 t of contaminated soil to depths of 20 ft, at a typical
−1
treatment rate of 3–6 t h . Larger areas are treated by fusing together multiple

individual vitrification zones (Wuana and Okieimen 2011 ).
Electrokinesis
Electrokinetic remediation offers a great potential for the remediation of hazardous
waste sites, especially those containing fine-grained soils contaminated with heavy

metals. It can be used either in situ or ex situ involving the application of a low level
DC current or voltage gradient across electrodes that encompass the contaminated
soil. As a result, the contaminants are transported toward either the cathode well/
reservoir or the anode well/reservoir, depending on their charge. Numerous studies
dealing with the electrokinetic remediation of soils contaminated with cationic
metallic contaminants such as lead, copper, and cadmium have been reported (Hicks
and Tondorf 1994 ). These studies have demonstrated migration of signifi cant
amounts of cationic metallic contaminants toward the cathode region where they are