Page 225 - Soil Degradation, Conservation and Remediation

214 6 Soil Pollution

carcinogen and DTPA is identified as toxic and a potential carcinogen. EDTA
continues to be explored extensively for soil remediation because of its ability to

mobilize metal cations efficiently coupled with only a minor impact on the physical
and chemical properties of the soil matrix (Lee and Marshall 2003 ). The extraction
kinetics of copper, zinc, iron, and manganese from the contaminated sediment of
the Clark Fork River in western Montana, USA, with disodium ethylenediaminetet-
raacetate (Na2EDTA) as the extraction agent, were investigated. The results showed

the extraction process consisted of rapid extraction in the first minutes followed by
much slower extraction for the remainder of the experiment. The rate of extraction,
particularly in the rapid phase, demonstrated clear pH dependence: the lower the
pH, the faster the extraction rate.

Bioremediation

In bioremediation, organisms are employed in extraction and removal of metals
from the contaminated soil. Organisms include microorganisms and higher plants.

Microbial Remediation
Natural organisms, either indigenous or extraneous are the prime agents used for
bioremediation of heavy metals in soil (Prescott et al. 2002 ). The organisms that are
utilized vary, depending on the chemical nature of the polluting agents and are to
be selected carefully as they only survive within a limited range of chemical con-
taminants (Dubey 2004 ). Since numerous types of pollutants are to be encountered
in a contaminated site, diverse types of microorganisms are likely to be required for
effective mediation (Watanabe et al. 2001 ). Bioremediation can occur naturally or
through intervention processes (Agarwal 1998 ). Natural degradation of pollutants

relies on indigenous microflora that is effective against specific contaminants and it
usually occurs at a slow rate. With intervention processes, the rate of biodegradation
is aided by encouraging growth of microorganisms, under optimized physicochemical
conditions (Smith et al. 1998a , b ).

Phytoremediation
Phytoremediation uses the ability of plants to adsorb, degrade, volatilize, or accumulate
contaminants in soil, sediments, surface, or groundwater. According to Ghosh and
Singh ( 2005 ), the chief processes of phytoremediation are phytostabilization,
phytodegradation, phytoaccumulation, phytovolatilization, rhizodegradation, and
evapotranspiration. Phytoaccumulation or phytoextraction is the most commonly
and popularly used process of phytoremediation of heavy metals from contaminated
soils. Phytoextraction utilizes the metal hyperaccumulating ability of some plants
in their aboveground parts. Selected hyperaccumulating plants are grown in contami-
nated soils and harvested aboveground parts are treated in different ways (reduction
in volume and weight, composting, compaction, burial, thermal treatment).