PHYTOREMEDIATION TECHNOLOGIES APPLICABLE TO AQUATIC REQUIREMENT (AN OVERVIEW)

The purpose of this paper is to provide a concise discussion of the processes associated with the use of water phytoremediation as a cleanup or containment technique for remediation of contaminated water bodies. The different fonns of water phytoremediation are defined and their applications are discussed. The types of contaminants that are appropriate for phytoremediation are summarized. lnfonnation is provided on the types of vegetation that can be used in water phytoremediation. The advantages and disadvantages of phytoremediation methods are discussed too.


INTRODUCTION
Every year large amounts of waste is released into the environment and a significant number of pollutants remain in the environment causing widespread soil and fresh water pollution.
International organizations (as FAQ, WHO, etc) have always paid great attention to wastewater treatment and its implications when used for sustainable agricultural development with a view to water quality and has studied the use of drainage water for irrigation, wastewater treatment and its reuse in agriculture, and water quality and health.
Since 1975, from all around the world, public and private research organizations and several university institutes have been turning their attention to alternative biotechnologies for the treatment of organic sewage, thus reducing the polluting charge and making it safe for the environment; one of these is phytoremediation.
Today, phytoremediation systems represent the real alternative for the reclamation of wastewater and its reuse in agriculture for safe food production linked to a healthy human nutrition and environmental surface water quality management.
The phytoremediation industry is moving into several treatment markets: municipal, agroidustrial, zootechnical and into the treatment of organic and inorganic pollutants present in surface waters (rivers, lakes, etc).
Phytoremediation is actually a broad class of remediation techniques which include many treatment strategies. Obviously, the common thread through all of phytoremediation techniques is the use of plants to treat a contaminant problem, However, due to the diverse nature of chemical contamination and the diversity of plants with the potential to treat them, remedial project managers must choose between a wide variety of phytoremediation techniques to solve the problem at hand, Despite the diversity of phytoremediation technologies, its application is limited by a number of factors, Phytoremediation can only work at sites that are well suited for plant growth, This means that the concentration of pollutants cannot be toxic to the plants, and the pollution cannot be so deep in the soils or groundwater that plant roots cannot reach it As a result, phytoremediation may be a good strategy for sites conducive to plant growth with shallow contamination, it may be a good secondary or tertiary phase in a treatment train for highly polluted sites, it may form a buffer area for emergency use, or it may not be a viable option for a site, Table I lists the various types of phytoremediation technologies applicable for contaminated  water treatment At a phytoremediation site, combinations of the phytoremediation processes mentioned above may occur simultaneously or in sequence for a particular contaminant, or different processes may act on different contaminants or at different exposure concentrations. For example, TCE in soil can be subject to biodegradation in the root zone (rhizodegradation) and metabolism within the plant (phytodegradation), with loss of some contaminant or metabolite through volatilization from the plant (phytovolatilization), Some metals or radionuclides in water can be accumulated on or within roots (rhizofiltration) while other metals or radionuclides are simultaneously taken up into the aerial portion of the plant (phytoextraction).

Hydraulic
• An engineered pump-and-treat system does not need to • Water uptake by plants is affected by climatic and groundwater, Water-soluble teachable Control be installed; • Costs wi II be lower; • Roots will penetrate into and be in contact with a much greater volume of soil than if a pumping well is used; seasonal conditions; thus, the rate of water uptake will not be constant. Water uptake by deciduous trees will slow considerably during winter; • Groundwater removal is limited by the root depth of the vegetation.  • Contaminants or metabolites released to the atmosphere might be subject to more effective or rapid natural degradation processes such as photodegradation.
Secondary advantages include the stabilization of stream banks and prevention of soil erosion. Aquatic and terrestrial habitats are greatly improved by riparian forest corridors.
Contaminant degradation due to enzymes produced by a plant can occur in an environment free of microorganisms (for example, an environment in which the microorganisms have been killed by high contaminant levels). Plants are able to grow in sterile soil and also in soil that has concentration levels that are toxic to microorganisrns. Thus, phytodegradation potentially could occur in soils where biodegradation cannot.

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• The contaminant or a hazardous metabolite (such as vinyl chloride formed from TCE) might be released into the atmosphere. One study indicated TCE transpiration, but other studies found no transpiration; • The contaminant or a hazardous metabolite might accumulate in vegetation and be passed on in later products such as fruit or lumber. Low levels of metabolites have been found in plant tissue The use of buffer strips might be limited to easily assimilated and metabolized compounds. Land use constraints may restrict application.
• Toxic intermediates or degradation products may form. In a study unrelated to phytoremediation research, PCP was metabolized to the potential mutagen tetrachlorocatechol in wheat plants and cell cultures; • The presence or identity of metabolites within a plant might be difficult to determine; thus contaminant destruction could be difficult to confirm.  The plant biomass containing the extracted contaminant can be a resource. For example, biomass that contains selenium (Se), an essential nutrient, has been transported to areas that are deficient in Se and used for animal feed.

• Contaminant destruction occurs in situ;
• Translocation of the compound to the plant or atmosphere is less likely than with other phytoremediation technologies since degradation occurs at the source of the contamination; • Mineralization of the contaminant can occur; • Low installation and maintenance cost as compared to other remedial options.
• Metal hyperaccumulators are generally slowgrowing with a small biomass and shallow root systems.
• Plant biomass must be harvested and removed, followed by metal reclamation or proper disposal of the biomass;

Phytoextraction, or phytomining, is the process of planting a crop of a species that is known to accumulate contaminants in the shoots and leaves of the plants, and then harvesting the crop and removing the contaminant from the site. Unlike the destructive degradation mechanisms, this technique yields a mass of plant and contaminant (typically metals) that must be transported for disposal or recycling. This is a concentration technology that leaves a much smaller mass to be disposed of when compared to excavation· and landfilling. This technology is being evaluated in a Superfund Innovative Technology Evaluation (SITE) demonstration, and may also be a technology amenable to contaminant recovery and recycling. Rhizofiltration is similar to phytoextraction in that it is also a concentration technology. It differs from phytoextraction in that the mechanism is root accumulation and harvest using hydroponic (soil-less) growing techniques. This is useful for separating metal contaminants from water. Rhizofiltration has been demonstrated on U.S. Department of Energy (DOE) sites for radionuclides.
Volatilization or transpiration through plants into the atmosphere is another possible mechanism for removing a contaminant from the soil or water of a site. It is often raised as a concern in response to a proposed phytoremediation project, but has not been shown to be an actual pathway for many contaminants. Mercury (Hg) has been shown to move through a plant and into the air in a plant that was genetically altered to allow it to do so. The thought behind this media switching is that elemental Hg in the air poses less risk than other Hg forms in the soil. However, the technology or the associated risk has not been evaluated.

PHYTOREMEDIATION TECHNOLOGIES
This chapter presents a literature review and evaluation of the major water phytoremediation processes or technologies. The technologies presented represent the major, significant, or widely studied forms of phytoremediation. It is divided into subsections that present definitions, mechanisms, site characteristics, applicable media, and contaminants amenable to each process, and the associated concentrations where available. The advantages and disadvantages of each process are also discussed.

Rhizofiltration
Rhizofiltration is the adsorption or precipitation onto plant roots, or absorption into the roots of contaminants that are in solution surrounding the root zone, due to biotic or abiotic processes. Plant uptake, concentration, and translocation might occur, depending on the contaminant Exudates from the plant roots might cause precipitation of some metals. Rhizofiltration first results in contaminant containment, in which the contaminants are immobilized or accumulated on or within the plant Contaminants are then removed by physically removing the plant

Hydraulic Control
Hydraulic control is the use of plants to remove groundwater through uptake and consumption in order to contain or control the migration of contaminants. Hydraulic control is also known as phytohydraulics or hydraulic plume control.

Phytovolatilization
Phytovolatilization is the uptake and transpiration of a contaminant by a plant, with release of the contaminant or a modified form of the contaminant to the atmosphere from the plant through contaminant uptake, plant metabolism, and plant transpiration. Phytodegradation is a related phytoremediation process that can occur along with phytovolatilization.

Riparian Corridors/Buffer Strips
Riparian corridors/buffer strips are generally applied along streams and river banks to control and remediate surface runoff and groundwater contamination moving into the river. These systems can also be installed to prevent down gradient migration of a contaminated groundwater plume and to degrade contaminants in the plume. Mechanisms for remediation include water uptake, contaminant uptake, and plant metabolism. Riparian corridors are similar in conception to physical and chemical permeable barriers such as trenches filled with iron filings, in that they treat groundwater without extraction containment Riparian corridors and buffer strips may incorporate certain aspects of hydraulic control, phytodegradation, rhizodegradation, phytovolatilization, and perhaps phytoextraction.

Phytodegradation
Phytodegradation (also known as phytotransformation) is the breakdown of contaminants taken up by plants through metabolic processes within the plant, or the breakdown of contaminants external to the plant through the effect of compounds (such as enzymes) produced by the plants. The main mechanism is plant uptake and metabolism. Additionally, degradation may occur outside the plant, due to the release of compounds that cause transformation. Any degradation caused by microorganisms associated with or affected by the plant root is considered rhizodegradation.

Phytoextraction
Phytoextraction is the uptake of contaminants by plant roots and translocation within the plants. Contaminants are generally removed by harvesting the plants, This concentration technology leaves a much smaller mass to be disposed of than excavation of the soil or other media does. This technology is most often applied to metal-contaminated soil,

Rhizodegradation
Rhizodegradation is the breakdown of an organic contaminant in soil through microbial activity that is enhanced by the presence of the root zone, Rhizodegradation is also known as plant-assisted degradation, plant-assisted bioremediation, plant-aided in situ biodegradation, and enhanced rhizosphere biodegradation. Root-zone biodegradation is the mechanism for implementing rhizodegradation, Root The chemical and physical effects of the exudates and any associated increase in microbial populations might change the soil pH or affect the contaminants in other ways,

Constructed Wetlands
Constructed wetlands or treatment wetlands are artificial wetlands that are used for treating organic, inorganic, and nutrient contaminants in contaminated surface water, municipal waste water, domestic sewage; refinery effluents, acid mine drainage, or landfill leachate, Phytoremediation methods used in treatment wetlands may be different and they can also be combined. A considerable amount of research and applied work has been conducted using constructed wetlands for these applications, Cole (1998) provides an overview of constructed wetlands, and more detailed discussions are provided in Kadlec and Knight (1996), Natural wetlands have also been examined for treatment of these wastes, Ground-water treatment is less common, though conceivable, Except in a few cases, constructed wetlands generally have not been used in remediation of hazardous waste sites; however, constructed and natural wetlands have been investigated for the phytodegradation of munitions-contaminated water, In the future, constructed wetlands might become an option for treatment of water extracted from hazardous waste sites, using rhizofiltration and phytodegradation, Integration of hazardous waste site phytoremediation and constructed wetland technologies might increase in the future, There are a number of different forms of phytoremediation, discussed above, Defining these forms is useful to clarify and understand the different processes that can occur due to vegetation, what happens to a contaminant, where the contaminant remediation occurs, and what should be done for effective phytoremediation, The different forms of phytoremediation may apply to specific types of contaminants or contaminated media, and may require di fferent types of plants,

CONCLUSIONS
The purpose of this paper is to provide a concise di scussion of the processes associated with the use of water phytoremediation as a cleanup or containment technique for remediation of contaminated sites, Introductory materi al on plant processes i s provided, The different forms of water phytoremediation are defined and their applications are di scussed. The types of contaminated media and contaminants that are appropriate for phytoremediation are summarized. Information is provided on the types of vegetation that have been studied or used in phytoremediation, The advantages and disadvantages of phytoremediation are discussed too. Phytoremediation is the use of plants to partially or substantially remediate selected contaminants in contaminated soil, sludge, sediment, ground water, surface water, and waste water, It utilizes a variety of plant biological processes and the physical characteristics of plants to aid in site remediation, Phytoremediation has also been called green remediation, botano-remediation, agroremediation, and vegetative remediation.
Phytoremediation is a integration of processes, with the different processes occurring to differing degrees for different conditions, media, contaminants, and plants. A variety of terms have been used in the literature to refer to these various processes. This discussion defines and uses a number of tenn s as a convenient means of introducing and conceptualizing the processes that occur during phytoremediation, However, it must be realized that the various processes described by these terms all tend to overlap to some degree and occur in varying proportions during phytoremediation. Phytoremediation encompasses a number of different methods that can lead to contaminant degradation, removal (through accumulation or dissipation), or immobilization,