1.1 Background of study
Pollutants are contaminants that gets introduced into the natural environment beyond permitted limits and adversely affect wildlife and impact human health. This may include Land, Water, Air, Radioactive, Thermal, Light and Sound pollution. Even low levels of pollutants in the environment pose a risk due to potential accumulation at higher trophic levels, a process called biomagnification. Organic contaminants are usually xenobiotic to plants, but inorganic contaminants such as metals are commonly found in low concentration in the soil. Metals such as cobalt (Co), copper (Cu), iron (Fe), molybdenum (Mo), manganese (Mn) and zinc (Zn) are critical for plant growth, and are classified as essential micronutrients.Other metals that are commonly found as contaminants, and are non-essential for plants, include arsenic (As),Cadmium (Cd), chromium (Cr), mercury (Hg), nickel (Ni),Lead (Pb), selenium (Se), uranium (U), vanadium (V), andWolfram (W). Metals can have toxic effects on plants, even at low concentrations. Metal uptake byPlants generally takes place by specific transporters (channel proteins) or H+ coupled carrier proteins located on the cell membrane of the root. For example, the Fe regulated transporter (IRT1) allows the uptake of Fe.Uptake of other metals also occurs via IRT1 transporters, especially if very low concentrations of Fe exist in the soil. Inadvertent uptake of non-essential metals also takes place via other cell membrane transporters.For example, uptake of as (as AsO3) occurs inadvertently via phosphorus-transporters. Organic molecules enter plant roots via simple diffusion.
Phytoremediation basically refers to the use of plants and associated soil microbes to reduce the concentrations or toxic effects of contaminants in the environment. Phytoremediation is widely accepted as a cost-effective environmental restoration technology. Phytoremediation is an alternative to engineering procedures that are usually more destructive to the soil. Phytoremediation of contaminated sites should ideally not exceed one decade to reach acceptable levels of contaminants in the environment. Phytoremediation is, however, limited to the root-zone of plants. Also, this technology has limited application where the concentrations of contaminants are toxic to plants. Phytoremediation technologies are available for various environments and types of contaminants. These involve different processes such as in situ stabilization or degradation and removal (i.e., Volatilization or extraction) of contaminants.