Page 8 - Plant-Based Remediation Processes

Preface

The idea of cleaning up contaminated environments by using green plants is not
new. About 300 years ago, plants were proposed to be used in the treatment of
wastewater (Hartman 1975). At the end of the nineteenth century, Thlaspi
caerulescens and Viola calaminaria were the first plant species documented to
accumulate high levels of metals in leaves (Baumann 1885). At present, there are
about 420 species belonging to about 45 plant families which have been reported as
hyperaccumulators of heavy metals (Cobbett 2003). Although the identification of
new plant species with this property is still growing from field collections (Kra ¨mer
2003), only a few species have been tested in the laboratory to confirm their
hyperaccumulating behaviors. The urgency to discover hyperaccumulators has
shown several intriguing patterns (Baker and Whiting 2002). First, several plant
families contain an inexplicably high number of hyperaccumulators: among those
are Asteraceae, Brassicaceae, Euphorbiaceae, Fabaceae, Flacourtiaceae, and
Violaceae, suggesting that several families and genera within them may be pre-
adapted/predisposed to deal with high concentrations of metal. Second, there
appears to be a disproportionately high percentage of hyperaccumulators in tropical
regions.
Plant tolerance to heavy metals depends largely on plant efficiency in the uptake,
translocation, and further sequestration of heavy metals in specialized tissues or in
trichomes and organelles such as vacuoles. The uptake of metals depends on their
bioavailability, and plants have evolved mechanisms to make micronutrients bio-
available. Some plants have developed resistance to high metal concentrations,
basically by two mechanisms, avoidance and tolerance. The first mechanism
involved exclusion of metals outside the roots, and the second mechanism consists
basically in complexing the metals to avoid protein and enzyme inactivation. Some
plants can also accumulate metals in their tissues at concentrations higher than
those found in the soil, and these plants as referred as hyperaccumulators (Gupta
and Sandalio 2012).
Given the nature and extent of contamination worldwide and the costs involved
in remediation, recent years have seen a drive toward alternative yet effective
technologies for the remediation of polluted sites. In this regard, bioremediation,

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