11-04-2014, 03:50 PM
CURRENT ADVANCES IN PHYTOREMEDIATION TECHNOLOGIES
Introduction
Phytoremediation, the use of plants and their associated microbes to remedy contaminated soils, sediments, and groundwater, is emerging as a cost-effective and environmentally friendly technology. Due in large part to its aesthetic appeal, this technology has gained increasing attention over the past 10 years. Phytoremediation uses different plant processes and mechanisms normally involved in the accumulation, complexation, volatilization, and degradation of organic and inorganic pollutants. Certain plants, called hyperaccumulators, are good candidates in phytoremediation, particularly for the removal of heavy metals. Phytoremediation efficiency of plants can be substantially improved using genetic engineering technologies(31). Because it is at the interface between the atmosphere and the earth’s crust, as well as being the substrate for natural and agricultural ecosystems, the soil is open to inputs of heavy metals from many sources (78). Due to their immutable nature, metals are a group of pollutants of much concern . As a result of their sedentary nature, plants have evolved diverse abilities for dealing with toxic compounds in their environment. Plants act as solar-driven pumping and filtering systems as they take up contaminants (mainly water soluble) through their roots and transport/translocate them through various plant tissues where they can be metabolized, sequestered, or volatilized (46,49,52,58). Phytoremediation is a low-cost option, particularly suited to large sites that have relatively low levels of contamination.
Types of Pollutants
There are two major classes of contaminants: organic and inorganic. Organic contaminants include different compounds such as petroleum hydrocarbons, chlorinated solvents, halogenated hydrocarbons such as trichloroethylene (TCE), and explosives such as trinitrotoluene (TNT). When compared to inorganics, the organic pollutants are relatively less toxic to plants because they are less reactive and do not accumulate readily. Inorganic compounds include heavy metals such as mercury, lead, and cadmium, and nonmetallic compounds such as arsenic and radionuclides like uranium. Some metals are essential for the normal growth and development of life forms. However, at high concentrations, metals become toxic and lead to oxidative stress with production of reactive oxygen species and free radicals, which are highly damaging to cells (105, 104). Some metal ions are particularly reactive and can interfere with the structure and function of proteins in living systems. High concentration of inorganic pollutantsmayalso lead to replacement of other essential nutrients (103). Elements/compounds such as mercury and cyanide may be introduced during extraction process. During wet season, 141 acidic leachate which contains heavy metals finds its way to groundwater sources as point pollution source.
Phytoremediation processes
Phytoremediation is generally referred to as the use of vegetation for in situ treatment of contaminated soils, sediments, and water. The phytoremediation processes work sequentially or simultaneously, depending on the type of contaminants and treatment goals. Different processes may act on different contaminants or at different (81)
Phytofiltration / Rhizofiltration
It is defined as the use of plants, both terrestrial and aquatic; to absorb, concentrate, and precipitate contaminants from polluted aqueous sources with low contaminant concentration in their roots. Rhizofiltration can partially treat industrial discharge, agricultural runoff, or acid mine drainage. It can be used for lead, cadmium, copper, nickel, zinc and chromium, which are primarily retained with in the roots [19,20]. Aquatic and semiaquatic plants, as well as dried plant materials, have often been evaluated in various water purification systems [68]. Commercial applications of these methods are hampered by the relatively slow growth rate and/or metal-binding capacities of tested plant material. Hydroponically cultivated roots of terrestrial plants were recently found to be more effective in removing heavy metals from water than earlier developed plant-based systems. An ideal plant for rhizofiltration should have rapidly growing roots with the ability to remove toxic metals from solution over extended periods of time(69). Screening roots of hydroponically cultivated plants for
their ability to remove and concentrate heavy metals from the solution resulted in the identification of certain varieties of sunflower as the most efficient plants for rhizofiltration [68].