Phytoextraction of Trace Metals: Principles and Applications - Environmental Sustainability: Role of Green Technologies - page 206

Phytoextraction of Trace Metals:
Principles and Applications

Tiziana Centofanti

1 Introduction

but at high concentration is toxic. In the western
side of the San Joaquin Valley in California,
soils contain signifi cant quantities of soluble
mineral salts and trace elements such as Se and
boron (B) that have been leached into shallow
groundwater and/or drainage waters because of
irrigation practices. Soluble Se bioaccumulated
in the avian food chain and resulted in an envi-
ronmental disaster with high mortality and
reproduction failure of migratory birds (Letey
et al. 2002 ; Ohlendorf et al. 1986 ).
According to the German Advisory Council
on Global Change, 22 million hectares of land
are contaminated with TEs worldwide (GACGC
1994 ). Specifi cally, a comprehensive inventory of
global soil contamination is lacking ( www.
globalsoilweek.org ). For example, at the
European level, it is unclear what threshold val-
ues should be used to classify a soil as polluted
and with regard to TEs at which locations can
high natural background values be expected
(Morvan et al. 2008 ).
For the government (i.e. USA and EU) to act,
usually the contamination must be severe enough
to cause barren soils, limit crop production, and
cause groundwater contamination and unsafe
living conditions. Lack of global inventory of soil
contamination and of consequent public aware-
ness of the health risks is one of the reasons why
governments fail to require and fund remediation
in areas that are of low economic value such as
marginal agricultural land, former mining areas,
landfi lls, and postindustrial sites. Knowledge of

Trace elements (TEs) occur at minor concentra-
tion (>1 g kg −1 ) in the organisms, and some are
essential nutrients (Cr, Mn, Fe, Co, Ni, Cu, Zn,
Mo, B, and Cl) for animals and plants. As a con-
sequence of human activities such as industrial
production, mining, transport, and agriculture,
they are released in the environment at high
concentrations. TEs can accumulate over time
under specifi c environmental conditions, thus
becoming environmental contaminants (Cs, Cr,
W, U, Cd, Hg, Tl, Pb, Sn, As, Sb, Se). The envi-
ronmental risk of TEs is associated with the
mobility and bioavailability of the metals more
than their total concentration. When they
become environmentally mobile and move
between media (i.e. soil to water), they can enter
the food chain by being taken up by plants and
animals. TEs cannot be degraded or broken
down and at high concentration are toxic to
organisms and tend to bioaccumulate in the
environment. For example, selenium (Se) is a
naturally occurring element with a wide distri-
bution in almost all parent materials on Earth.
At low concentration, Se is an essential nutrient