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Phytoremediation


Published on Aug 15, 2016

Abstract

Phytoremediation is best applied at sites with shallow contamination by organic, nutrient, or metal pollutants. Phytoremediation is well-suited for use at very large field sites where other methods of remediation are not cost-effective or practicable; at sites with low concentrations of contaminants where only "polishing treatment" is required over long periods of time; and in conjunction with other technologies where vegetation is used as a final cap and closure of the site. There are limitations to the technology that need to be considered carefully before it is selected for site remediation.

These include: limited regulatory acceptance, long duration of time sometimes required for clean-up to below action levels, potential contamination of the vegetation and food chain, and difficulty establishing and maintaining vegetation at some toxic waste sites. This detailed report discusses the current status of phytoremediation to treat soils and ground water. Several field demonstration summaries are presented, with such information as: participants, compounds treated, site characteristics, results, and contacts.

An emerging technology for cleaning contaminated soils and shallow ground water is phytoremediation, an environmentally friendly, low-cost, and low-tech process. Phytoremediation encompasses all plant-influenced biological, chemical, and physical processes that aid in the uptake, degradation, and metabolism of contaminants by either plants or free-living organisms in the plant's rhizosphere. Aphytoremediation system can be viewed as a biological, solar-driven, pump-and-treat system with an extensive, self-extending uptake network (the root system) that enhances the soil and below-ground ecosystem for subsequent productive use.

Mechanisms of Phytoremediation

Plants and bacteria can form specific associations in which the plant provides the bacteria with a specific carbon source that induces the bacteria to reduce the toxicity of the contaminated soil. Alternatively, plants and bacteria can form nonspecific associations in which normal plant processes stimulate the microbial community, which in the course of normal metabolic activity degrades contaminants in soil. Plants can provide carbon substrates and nutrients, as well as increase contaminant solubility. These biochemical mechanisms increase the degradative activity of bacteria associated with plant roots. In return, bacteria can augment the degradative capacity of plants or reduce the toxicity of the contaminated soil.

During phytoremediation, PAHs that are resistant to degradation may adsorb to the surfaces of plant roots, making the roots an important sink for specific PAHs. Tall fescue and alfalfa were grown in a greenhouse under controlled conditions, and roots were harvested at three growth stages: vegetative, flowering, and mature. Naphthalene adsorption to the various plant roots was then evaluated. Results show that the mass of naphthalene volatilized was the largest component of the mass balance (32-45%). The mass in solution was usually greater than that adsorbed to the roots. The affinity of naphthalene for alfalfa roots was greater thanthat for tall fescue roots, but fescue roots were present in much greater quantities in the soil compared with alfalfa. Naphthalene adsorption on the roots of both plant species increased with plant age.

Plants as Phytoremediators

The principal application of phytoremediation is for lightly contaminated soils and waters where the material to be treated is at a shallow or medium depth and the area to be treated is large. This will make agronomic tech-niques economical and applicable for both planting and harvesting. In addition, the site owner must be prepared to accept a longer remediation period. Plants that are able to decontaminate soils does one or more of the following: 1) plant uptake of contaminant from soil particles or soil liquid into their roots; 2) bind the contaminant into their root tissue, physically or chemically; and 3) transport the contaminant from their roots into growing shoots and prevent or inhibit the contaminant from leaching out of the soil.

Moreover, the plants should not only accumulate, de-grade or volatilize the contaminants, but should also grow quickly in a range of different conditions and lend themselves to easy harvesting. If the plants are left to die in situ, the contaminants will return to the soil. So, for complete removal of contaminants from an area, the plants must be cut and disposed of elsewhere in a non-polluting way. Some examples of plants used in phyore-mediation practices are the following: water hyacinths (Eichornia crassipes); poplar trees (Populus spp.); forage kochia (Kochia spp); alfalfa (Medicago sativa); Ken-tucky bluegrass (Poa pratensis); Scirpus spp, coontail (Ceratophyllum demersum L.); American pondweed (Potamogeton nodosus); and the emergent common ar-rowhead (Sagittaria latifolia) amongst others

Phytoremediation as a Cleansing Tool

An Overview Phytoremediation is described as a natural process car-ried out by plants and trees in the cleaning up and stabi-lization of contaminated soils and ground water. It is actually a generic term for several ways in which plants can be used for these purposes. It is characterized by the use of vegetative species for in situ treatment of land areas polluted by a variety of hazardous substances [15]. Garbisu [16] defined phytoremediation as an emerging cost effective, non-intrusive, aesthetically pleasing, and low cost technology using the remarkable ability of plants to metabolize various elements and compounds from the environment in their tissues.

Phytoremediation technology is applicable to a broad range of contami-nants, including metals and radionuclides, as well as or-ganic compounds like chlorinated solvents, polychlori- biphenyls, polycyclic aromatic hydrocarbons, pesti-cides/insecticides, explosives and surfactants. According to Macek [17], phytoremediation is the direct use of green plants to degrade, contain, or render harmless various environmental contaminants, including recalcitrant or- ganic compounds or heavy metals. Plants are especially useful in the process of bioremediation because they prevent erosion and leaching that can spread the toxic substances to surrounding areas. Several types of phytoremediation are being used to-day.

One is phytoextraction, which relies on a plant’s natural ability to take up certain substances (such as heavy metals) from the environment and sequester them in their cells until the plant can be harvested. Another is phytodegredation in which plants convert organic pol-lutants into a non-toxic form. Next is phytostabilization, which makes plants release certain chemicals that bind with the contaminant to make it less bioavailable and less mobile in the surrounding environment. Last is phyto-volitization, a process through which plants extract pol-lutants from the soil and then convert them into a gas that can be safely released into the atmosphere [19]. Rhizofil-tration is a similar concept to phytoextraction, but mainly use with the remediation of contaminated groundwater rather than the remediation of polluted soils.

Phytoremediation Species in Coastal Water

The Philippines is blessed to have relatively high man-grove diversity having 35 species [26] including five major families, namely: Avicenniaceae; Arecaceae; Com- bretaceae; Lythraceae; and Rhizophoraceae [27]. Though Philippines has high mangrove diversity, it was reported that there was a drastic decline of mangrove resources from 450,000 hectares in 1918 to 120,000 hectares in 1995. The decrease of the mangrove forests was due to human activities, such as fish pond conversion, human settlement, and salt production [28]. However, with the alarming rate of mangrove forest degradation, Philip-pines strived to continue greater conservation of man-groves and reforestation of the coastal areas

Mangroves are higher plants, which are found mostly in the intertidal areas of tropical and subtropical shore-lines and show remarkable tolerance to high amounts of salt and oxygen poor soil. The mechanisms of mangrove to keep the salt away from the cytoplasm of the cell were through the excretion of salt in their salt glands found in the leaves and roots and through storage of salts in the mature leaves, bark and wood.












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