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CHAPTER ONE

1.0       INTRODUCTION

1.1       Phytoremediation

Phytoremediation is defined as the combined use of plants, soil amendments and agronomic practices to remove pollutants from the environment or to reduce its toxicity (Clemente et al., 2005). Depending upon the process by which plants are removing or reducing the toxic effect of contaminants from the soil.

1.2     Mechanism of Phytoremediation Technologies

Phytoremediation technology can be broadly classified as follows:

i.          Phytoextraction: This is the process of using pollutant-accumulating plants to remove metals or organics from soil by concentrating them in harvestable plant parts.

ii.         Phytotransformation: This is the partial or total degradation of complex organic molecules by their incorporation into plant tissues.

iii.        Phytostimulation: In this process the release of plant exudates or enzymes into the root zone stimulates the microbial and fungal degradation of organic pollutants.

iv.        Phytostabilization: This is a method that uses plants to reduce mobility of contaminants (both organic and metallic contaminants) by preventing erosion, leaching, or runoff and to reduce bioavailability of pollutants in the environment, thereby preventing their migration to groundwater or their entry into the food chain (Pilon-Smits, 2005).

v.         Phytovolatilisation: This is the technique of using plants to volatilize pollutants or metabolites. This technology can be used for volatile organic carbons (VOCs) and for the few inorganics that can exist in volatile forms such as selenium and mercury (Pilon- Smits, 2005).

vi.        Rhizo-filtration: This is the use of plant roots to absorb or adsorb pollutants, mainly metals, but also organic pollutants, from water and aqueous waste streams.

vii.       Pump and tree: This method is the use of trees to evaporate water and simultaneously to extract pollutants from the soil.

viii.      Hydraulic control: It is the controlling of water table and soil field capacity by plant canopies. (Schwitzguebel, 2004)














Figure I:





1.3     Phytoremediation Process Basics

The discovery of metal accumulating properties in certain plants leads to the development of phytoremediation technology. Research in the field of phytoremediation is aiming to develop innovative, economical and environmentally compatible approaches to remove heavy metals from the environment. Even apart from the metal hyper accumulating property of the plants, the presence of ground cover with plants helps to shield people from direct contact with the soil and prevents the blowing of contaminated dust around the neighborhood (Raskin and Ensley, 2000).

This particular review gives a detailed overview about the state of the art of phytoremediation and explanation of different technologies of phytoremediation such as phytoextraction, rhizofiltration etc. The article raised a concern about disposal of plants those used for phytoremediation, especially for phytoextraction, since the plant tissue may be enriched with contaminants. One of the important thoughts included in the article is the limited applicability of this method to a heavily contaminated soil, since the time required for cleaning up the contaminated site will be very long. The article says that phytoremediation may also be limited by the bioavailable fraction of pollutant in the soil. The author recommended that combinations of different technologies will be the most cost-effective and efficient remediation solution. In addition to the above mentioned details on phytoremediation, other topics such as plant processes involved in uptake, translocation, sequestration, and degradation of organic and inorganic pollutants, and new 14 developments such as use of genetic engineering in the field of phytoremediation are also reviewed (Raskin and Ensley, 2000).

Table 1: General Advantages and Disadvantages of Phytoremediation (Raskin and Ensley, 2000)

Advantages

Disadvantages

Cost

-        Low capital and operating cost

-        Metal recycling provides further economic advantages

Time

-        Slower compared to other techniques andseasonally dependent

-        Most of the hyper accumulators are slow growers

Performance

-        Permanent treatment solution

-        In situ application avoids excavation

-        Capable of remediating bioavailablefraction of contaminants

-        Capable of mineralizing organics

-        Applicable to variety of contaminants

-        Eliminate secondary air or water borne

Performance

-        Not capable of 100% reduction

-        May not be functional for all mixed wastes

-        High contaminant concentration may betoxic to plants

-        Soil phytoremediation is applicable only tosurface soils

Other

-        Public acceptance due to aesthetic reasons

-        Compatible with risk-based remediation

-        Can be used for site investigation or afterclosure

Other


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