Biotechnological Aspects of Soil Decontamination - Biotechnologies and Biomimetics for Civil Engineering - page 375

17.3.2 Immobilization

Immobilization approaches are favored where pollution covers an extensive area,
and where the main targets to be protected are water resources and plants.
Immobilization can be achieved by complexing the contaminants or through
increasing the soil pH by various amendments including addition of liming
materials, phosphate compounds, and biosoilds (Alloway and Jackson 1991 ).
Increased pH decreases the solubility of heavy metals like Cd, Cu, Ni, and Zn in
soil. Although the risk of potential exposure to plants is reduced, their concen-
tration remains unchanged. Mechanisms include increasing metal adsorption
through higher surface charge, formation of insoluble metal complexes, precipi-
tation, and redox reactions leading to immobile valency form. Basta et al. ( 2001 )
found that a range of soil amendments, especially alkaline biosolids, reduced the
extractability and phytotoxicity of smelter waste polluted with Cd, Pb, or Zn.

17.3.3 Soil Washing (with Solvents)

Soil contaminated with metal pollutants can be decontaminated by two treatment
methodologies:

• That leaves the metal in the soil such as solidification/stabilization and vitri-
fication, which immobilizes the contaminants, thus limiting their movement.
• That removes the heavy metals from the soils. Technology such as soil
washing, in situ soil flushing transfer the contaminants to liquid phase by
desorption and solubilization.

Soil washing for metals after physical treatment processes is then washed with
solvents on the basis of their ability to solubilize specific contaminants and to
transform them into nonhazardous material and also on their environmental
impacts (Feng et al. 2001 ; Chu and Chan 2003 ; Khan et al. 2004 ). Soil washing
usually employs wash solutions, such as acids, bases, chelating agents, reducing
agents, or other additives as the extracting agents. Thus, heavy metals can be
removed from soils using various agents added to the soil. This can be done in
reactors or as heap leaching. These agents are inorganic acids such as sulfuric and
hydrochloric acids (pH less than 2), organic acids including acetic and citric acids
(pH not less than 4), chelating agents such as ethylenediaminetetracetic acid
(EDTA) and nitrilotriacetic acid (NTA), and the various combinations of the
abovementioned .The cleaned soil can then be returned to the original site. Both
organics and metals are removed. The effectiveness of this treatment approach can
be high for hydrophilic pollutants such as aniline and phenols (Rajput et al. 1994 ).
Metal removing efficiencies during soil washing depends on the soil particle size,
metal characteristics, extractant chemistry, and processing conditions. pH plays a
very important role in metal extraction from soils (Peters 1999 ). Limited