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Vol. 5 No. 4 - October 1999

Toxic Metals and Phytoremediation

By: U. N. Rai And Amit Pal

Phytoremediation, popularly known as 'green clean' is a novel strategy for the removal of toxic contaminants from the environment by using plants. This concept is increasingly being adopted as it is a cost effective and user-friendly alternative to traditional methods of treatment. Toxic metal pollution in water and soil is a major environmental problem and most conventional remediation approaches do not provide acceptable solutions.

Rapid growth in population and massive industrialisation in recent years has resulted in pollution of the biosphere with toxic metals.

Plants possess some characteristic features which enable them to absorb from soil and water, such heavy metals which are essential for their growth and development. These metals include iron (Fe), manganese (Mn), copper (Cu), molybdenum (Mo) and nickel (Ni). Plants also accumulate toxic metals which may not have any biological function, these include : silver (Ag), cadmium (Cd), chromium (Cr), cobalt (Co), mercury (Hg), lead (Pb), and selenium (Se) etc.

However, excessive accumulation of these metals can be toxic to most plants. The use of terrestrial plants for environmental remediation through metal accumulation has been gaining acceptance in recent years and accumulation of Ni, Co, Cu, Mn, Pb, Zn, Se and Hg in high concentrations has been extensively reported. Phytoremediation, which essentially involves the use of plants for environmental cleanup can be divided into the following:

         Phytoextraction in which metal accumulating plants are used to transport and concentrate metals from the soil into the harvestable roots and above-ground shoots,

         Rhizofiltration in which plant roots absorb, precipitate and concentrate toxic metals from polluted effluents,

         Phytostabilization in which heavy metal-tolerant plants are used to reduce the mobility of heavy metals, thereby reducing the risk of further environmental degradation by leaching into the ground water or by airborne spread.

History of phytoremediation

The basic idea that plants can be used for environmental remediation is quite old. However, it was not untill 1948, that some Italian researchers first reported nickel hyperaccumulation in the Italian serpentine plant Alyssum bertolonii. This finding was all but forgotten untill 1977, when researcher Robert Brooks, of Massey University in New Zealand, made similar observations. This time, the concept caught on. Extensive researches on the use of semi-aquatic plants (or ecosystems as a whole), for treating radionuclide contaminated waters were initiated in Russia at the dawn of the nuclear era. The knowledge that aquatic or semi-aquatic vascular plants such as, water hyacinth (Eichhornia crassipes), pennywarth (Hydrocotyle umbellata), duckweed (Lemna minor), and water velvet (Azolla pinnata), can take up Pb, Cu, Cd, Fe and Hg from contaminated solutions existed for a long time. This ability is currently utilized in many wetlands, which may be effective in removing some heavy metals as well as organics from water.

While initially researchers in the United Kingdom showed interest in these plants, Chaney was the first US scientist to publish his report on 'hyperaccumulator plants potential as toxic site cleaners'. Scott Cunningham a scientist at DuPont began the study of lead removal. Raskin and his colleagues founded 'Phytotech' a New Jersey remediation company, which filed two patents, one for phytoextraction (the use of plants to remove heavy metals from soil) and another for rhizofiltration, in which plants remove heavy metals, from streams. Federal agencies have only recently begun funding phytoremediation projects. Now, dozens of labs in academia and industry are undertaking phytoremediation jobs on a large scale.

From laboratory to market place

Phytoremediation has already been successfully implemented by the US Air Force to clean trichloroethylene from ground water using poplar trees, and by the US army to clean 2,4,6-trinitrotolune (TNT) and hexahydro 1,3,5-trinitro-1,3,5-triazine (RDX) from contaminated wetlands, using a variety of plants.

In India, aquatic vascular plants like Hydrilla verticillata, Spirodela polyrrhiza, Bacopa monnierii, Phragmites karka and Scirpus lacustris have been used to treat chromium contaminated effluent and sludge from leather tanning industries.

Application of phytoremediation for the clean up of industrial wastes contaminated with toxic metals is another important area that has blossomed in recent years. Certain plants translocate metals from their surroundings and accumulate them in their above ground parts at high concentrations. Thus, they can be harvested and removed from the site. Two routes are currently being explored to develop metal accumulating plants: genetic engineering and, the selective breeding of naturally occurring hyperaccumulator-plants.

Recently, Richard Meagher, at the University of Georgia (Athens) by inserting an altered mercuric ion reductase gene (mer A) into Arabidopsis thaliana, produced mercury resistant transgenic merA plants which are soon to be tested in soil. Results to date suggest that the cost of phytoremediation of mercury contaminated soils will be one-tenth to one-hundredth the cost of other traditional engineering methods, including land filling, thermal treatments and chemical extraction.

Future

Currently phytoremediation technology is being used to clear off various polluting elements such as heavy metals, insecticides, petro products, explosives, chlorinated solvents, and industrial by-products. Many of the plants used in this programme, are slow growing and, although their metal accumulation ability is high, they produced low biomass. Therefore, the clean-up. rates may prove slower than conventional methods. This is particularly problematic as the phytoextraction success is often based on the amount of contaminant removed/hectare/year. In addition, frame limitations also apply. Despite these short comings, many forms of phytoremediation have emerged from the laboratories and are currently in practice and public acceptance is very encouraging.

The challenge facing phytoremediation in the market place should be faced jointly by scientists, environmental engineers and regulators to prove the technology's efficacy at pilot sites. It is incumbent upon phytoremediation researchers to conduct basic laboratory work, before attempting field application. By employing this technology as part of living cultural, ecological systems, we can bring our understanding and appreciation of green clean into sharper focus for increased societal benefits.

Dr. U.N. Rai is a Scientist and Dr. Amit Pal is Research Associate in Environmental Sciences Division at National Botanical Research Institute, Lucknow, India.


This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.


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