The following points highlight the five mechanisms of metal phytoremediation by plants. The mechanisms are: 1. Phytoextraction 2. Phytostabilization 3. Rhizofiltration 4. Phytovolatization 5. Phytodegradation.
Mechanism # 1. Phytoextraction:
This approach is primarily used for the treatment of contaminated soils. This process uses plants to absorb, concentrate, and precipitate toxic metal from contaminate soil and loaded into shoots, leaves etc. Number of plants have been identified which can accumulate metal at unprecedented rate and are potential to remove metal from contaminated soils.
Hyper accumulator plant species capable of accumulating two times concentration of more metal than common non-accumulating plant. It is, however, no plant can accumulate heavy metals like lead, and phytoextraction facilitate permanent removal of contaminate from the soil. Once this process is completed, hyper accumulator plant biomass must be harvested and disposed properly.
There are several plants being considered as hyper accumulator found in the families. Brassicaceae, Euphorbiaceae, Asteraceae and Lamiaceae. Phytoextraction approach is limited by certain constraints such as rate of metal uptake by roots, rate of xylem loading and cellular tolerance to toxic metals.
Therefore, to enhance the process plant must extract large concentration of heavy metals and production of large quantities of biomass. In addition, remediated plants must have mechanism to detoxify and/or tolerate high metal concentration. The phytoextraction of heavy metals represents one of the largest economic opportunities for phytoremediation because of the significance of environmental problems associated with metal.
Contaminated soils are very competitive outcome of plant based remediation technology.
In view of plant based phytoextraction technology following character required for an ideal plant involved in remediation technology:
(a) Plants must exhibit tolerant to high levels and the metal
(b) Accumulate high levels of the metal in its harvestable parts,
(c) Exhibit rapid growth and
(d) Potential to produce a high biomass in the field.
Chelate-assisted or Induced Phytoextraction:
Several chelating agents have been known to assist phytoextraction of metals (Table 23.1). The strategy is based on the fact that application of metal chelates to the soil potentially enhances metal accumulation by plants. This is due to the fact that under many circumstances, in the soil, it is common to find cases of low bioavailability, preventing the extraction process.
Table 23.1 Chelating agents used in phytoremediation
Due to low bioavailability of metals in soil, it is observed that metal accumulation in the shoots of hydroponically grown plants shows better accumulation than the metal accumulation studied in soil grown plants. The formation of metal-chelate complexes prevents precipitation and absorption of the metals thereby maintaining their availability for plant uptake.
The supply or addition of chelating agents to the soil can bring metals into the solution through desorption of sorbed species, dissolution of Fe and Mn oxides, and dissolution of precipitated compounds. Therefore addition of chelates to the soil increases the translocation of heavy metals from soil into the shoots has opened a new vistas in phytoextraction of metals.
Additionally, synthetic chelating agents like EDTA for lead, EGTA for cadmium, possibly citrate for uranium facilitates absorption process. Application of metal chelates for phytomining of gold has been accomplished by using ammonium thiocyanate and ammonium thiosulfate. The 10 day grown plants were measured for gold concentration. The highest mean gold concentrations were found in carrot roots (48.3 mg/kg) dry wt) among other root crops.
Thiacyanate induced gold uptake was first reported in the dried leaves of Brassica juncea upto 57 mg/kg.
Yin et al., (2003) developed genetically improved hyperaccumulator crop with certain distinct characters for commercial phytoextraction of nickel. The ecotypes of Alyssum shoot accumulate 4200 to 20400 mg kg‑1 in the field-grown plants.
Rice land contaminated with cadmium is a serious health risk. Recent studies show that paddy rice in Japan, Korea and China has been shown to be contaminated by cadmium and zinc. Phytoextraction of Cd by Thlaspi caerulscence has been accomplished as a short cleanup process.
EDTA in Induced Phytoextraction:
The most commonly used chelating agent, ethylene diamine tetraacetic acid (EDTA) is a very versatile mobilizing agent that can form four or six bonds with a metal ion, including both transitional metal ions and main group ions. EDTA has been extensively used to form soluble complexes with lead, since it can solubilize pb from soil.
With the help of EDTA, plants can remove between 180 and 530 kg ha-1 of lead every year, making remediation of sites contaminated with upto 2500 mg kg-1 pb possible under 10 years. Certain high biomass crop plants such as Indian mustard (Brassica juncea) and sunflower (Helianthus annus) accumulate significant amount of pb when induced through the addition of chelating agents such as EDTA.
Since EDTA is known to be toxic for plants and create adverse environmental effect due to metal mobilization during extended periods of time, minimize the phytotoxicity environmental probes, research is being carried out on the gradual application of small doses of the chelating agent during the growth period.
EDTA has been proved to be very effective in facilitating uptake of various other metals like cadmium (Cd), Copper (Cu), Nickel (Ni), and Zinc (Zn). EDTA has been shown to enhance phytoextraction of Zn, Cd by Chinese cabbage (B. rapa).
Other chelating agents have been so far tested are HEDTA, DTPA, CDTA, EGTA, EDDHA, HEIDA, EDDS, NTA, HBED citric acid and malic acid etc.
Mechanism # 2. Phytostabilization:
Phytostabilization, also known as place-inactivation. Phytostabilization process involves plant root to prevent movement of contaminants and helps in bioavailability in soils. The utility of plants is to decrease percolation of water through the soil matrix, which may otherwise from hazardous leech out.
Growing of plants in these site prevent soil erosion and the distribution of toxic metal to other areas. This process can overcome the problems associated with disposal of hazardous waste. However, left out contaminants in the soil, applications of extensive fertilizers are some of the main constraints with this process.
These are mainly used for the remediation of soil, sediment of sludges are also used to treat contaminated land areas affected by mining activities. Some of the suitable plants used for phytostabilization are three species of grasses; they are Agrostis, Fistuca and Rubra.
Mechanism # 3. Rhizofiltration:
Rhizofiltration approach is mainly used to remediate extracted ground water and waste water containing low concentration of contaminants. Rhizofiltration is processes in which both terrestrial and aquatic plants are utilise to absorb, concentrate and precipitate contaminant from polluted aqueous sources in their roots.
This technique can be used for Pb, Cd, Cu, Zn and Cr, which are primarily retained within the roots. Certain plants like Indian mustard, sunflower, rye, spinach and corn have been investigated for the ability to remove lead from water. Comparision of above plants shows that sunflower reduced lead concentration significantly.
Recently, test at Chernobyl in the Ukraine shows the ability of sunflower to remove uranium contamination from the water in the near nuclear power station accident site. This approach can be used either insitu or exsitu application.
Another advantage in those contaminants does not have to translocate to the shoots. A terrestrial plant are useful when compared to aquatic plants due to presence of fibrous and much longer root system increases surface areas of root.
Mechanism # 4. Phytovolatization:
Phytovolatization is primarily uses the plants to take up contaminants like mercury and solenium from the soil convert them into volatile forms and finally release them into the atmosphere through transpiration as detoxified vapour.
Mercury contaminants can be reduced by this method, however, released mercury into the environment is likely to be recycled by precipitate and then re-enters into water system. Transpiration pull of fast growing trees are useful in rapid uptake of contaminants in ground water.
Experimental success have been achieved by genetically modified variety of yellow poplar, Liriodendron tulipifera, confirms the ability to tolerance higher mercury concentration (Fig. 23.1). In another case study, trichloroethylene (TCE) is mobile pollutant, have been volatilise by poplar plants upto 90% they take up. Encouraged by this, tree species with their enormous transpiration pull are useful choice to clean up contaminants.
Mechanism # 5. Phytodegradation:
It involves biological breakdown of contaminants, either internally after entered inside or externally, using secretary enzymes. In phytodegradation process organic molecule pollutants are subjected to biodegradation into simple substances, which is then enters plant system. This novel approach is useful for biodegradation of pollutants like herbicide, explosives and chlorinated solvents.
The effect of rhizosphere process or in the phytoremediation of polychlorinated biphenyl (PCB) has been reported. Presence of plants significantly increased the biological activity like microbes and enzyme activity resulting in remediation of PCB contaminated soil.
In addition, all plants modify the surrounding soil (rhizosphere) through the root exudates like organic and inorganic substrates. The exudates cause rhizosphere-inhabiting microbial populations to increase well beyond the limit and attract motile bacteria and fungal hyphaae that stimulate an array of positive, neutral or negative interactions with plants.
Many plants derived chemicals, including those generated from root turnover, stimulates microorganisms to biodegrade xenobiotics. Certain root exudates like salicylate, which induces systemic acquired resistance (SAR) in plants has been linked to the microbial degradation of napthale. In addition, certain plant derived flavanoids, could support the growth of some PCB- degrading microorganisms and enhance PCB metabolism.
Mercury is a toxic hazardous metal contaminate soil in the form of mercuric form (Hg2+), mercurous form (Hg22+) etc. Methyl-mercury is highly toxic than ionic mercury and can be biomagnified upto several fold (160 fold) when introduced into food chain (Table 23.2). Phytoextraction of mercury has been inspired by discovering some plant species produce specific peptides, termed phytochelates that bind and detoxify hazardous metals such as cadmium. Phytochelatin peptides were discovered in Arabidopsis and other mustard species. However, more refined strategy for mercury phytoremediation could be seen by transgenic plants.
