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62 O.P. Abioye et al.

literature (Salt et al. 1998). Hyperaccumulation of metals has been found in
temperate as well as tropical regions throughout the plant kingdom, but is generally
restricted to endemic plant species growing on mineralized soil and related rock
types (Baker et al. 1989).
Heavy metal contamination of the soil has become serious and continuous
problem of the world, which has attracted a great deal of attention from government
and regulatory authorities in the past few decades to prevent further heavy metals’
addition and soil deterioration and to implement possible methods of remediation
(Ahmad et al. 2011). Humans and ecosystem may be exposed to chemical hazards
such as heavy metals (lead, chromium, arsenic, zinc, cadmium, copper, mercury,
and nickel) through the direct ingestion of contaminated soils, consumption of
crops and vegetables grown on the contaminated lands, or drinking water that has
percolated through such soils (McLaughlin et al. 2000). For example, in their
assessment, Chaney et al. (2005) indicated that subsistence farmers eating rice
grain grown on contaminated sites throughout their lifetime are at risk from dietary
exposure to cadmium. With greater awareness by the governments and the public of
the implications of degraded environment on human and animal health, there has
been increasing interest amongst the scientific community in the development of
technologies to remediate contaminated sites (Bolan et al. 2008). In developing
countries with great population density and scarce funds available for environmen-
tal restoration, low-cost and ecologically sustainable technologies are required to
remediate contaminated lands so as to reduce the associated risks, make the land
resource available for agricultural production, enhance food security, and scale
down land tenure problems. Remediation of heavy metal-contaminated sites is
particularly challenging because unlike organic contaminants which are oxidized
to carbon (IV) oxide by microbial action, most metals do not undergo microbial or
chemical degradation and are toxic and their total concentration in soils persists for
a long time after their introduction (Adriano 2003; Kirpichtchikova et al. 2006).
Remediation techniques include (1) ex situ (excavation) or in situ (on-site) soil
washing/leaching/flushing with chemical agents, (2) chemical immobilization/sta-
bilization method to reduce the solubility of heavy metals by adding some nontoxic
materials into the soils, (3) electro kinetics (electro migration), (4) covering the
original polluted soil surface with clean soils, and (5) dilution method (mixing
polluted soils with surface and subsurface clean soils to reduce the concentration of
heavy metals).
2+
Forms of lead include ionic lead (Pb ), lead oxides, and hydroxides, and
lead–metal oxyanion complexes are the general forms of lead that are released
into the soil, groundwater, and surface waters. The most stable forms of lead are
Pb 2+ and lead-hydroxy complexes. Pb 2+ is the most common and reactive form of
lead, forming mononuclear and polynuclear oxides and hydroxides (Ground-Water
Remediation Technologies Analysis Center 1997). The predominant insoluble lead
compounds are lead phosphates, lead carbonates (form when the pH is above 6),
and lead hydroxides (Raskin and Ensley 2000). Lead sulfide (PbS) is the most stable
solid form within the soil matrix and forms under reducing conditions when