Phytoremediation of Hazardous Lead from Environment

By: Sudhakar Srivastava, Seema Mishra and R.D.Tripathi

Lead (Pb) pollution of the environment is a major problem today. It causes health hazards to livestock and human beings, children being most sensitive. Recently it has been established as a potential carcinogen. Lead enters in the environment through air, water and soil and finally enters the food chain through contaminated water, edibles and other foodstuffs. Besides, human beings can be directly exposed through occupational and environmental exposures.

Most of the heavy metal contaminated sites are lead affected. Lead affects many physiological parameters in plants and causes sharp decrease in crop productivity. Currently lead contaminated sites are being remediated by a variety of rather costly engineering technologies. Phytoremediation, popularly known as green clean, is an emerging technology for the clean up of contaminated sites by the use of plants, and is ecofriendly and low cost technology compared to traditional engineering remediation methods.

There are many ways in which plants may be utilized for remediation and these constitute different subcategories of phytoremediation viz. phytoextraction, rhizofiltration, phytostablization and phytovolatilization. Of these phytoextraction is best suited for remediation of lead contaminated sites and is much studied area. It is defined as accumulation of metal in above ground plant parts and those plants which accumulate more than 0.1% of lead as dry weight of shoot are known as "hyperaccumulators".

Lead persists in soil for very long time. Its remediaion is problematic due to its low availability to plants as it forms complexes and gets precipitated and also due to its low traslocation from root to shoot.

A few plants are known to hyperaccumulate lead such as Thlaspi rotundifolium, T. alpestre, Alyssum wulfenianum, Polycarpaea synandra, Armeria martima, few bryophytes and lichens etc. Various aquatic plants like Hydrilla, Vallisneria, Marsilea, Cyperus, Polygonum etc., algae like Phaeodactylum, Stichococcus, Stigeoclonium etc. have also been found to be potential accumulators of lead. However, many of these plants named above are not good for phytoremediation due to their very low biomass and slow growth rates, thus will take very long time for effective remediation of a site. It is speculated that even the best accumulators will take 13-14 years to clean up a site.

To achieve lead remediation in reasonable time there is need of plants which has short lifetime, could accumulate greater than 1% of lead of shoot dry weight and produce more than 20 tonnes of shoot biomass ha^-1 year^-1. To achieve this goal availability of lead in soil needs to be increased. Chelate mobilization has been studied in much detail in this context.

Chelates bind lead removing it from complexes and thus increase its availability to plants. They also enhance its translocation from root to shoot and cause more accumulatiom. Many chelates e.g. ethylenediaminetetraaceticacid (EDTA),N-hydroxyethylenediamine-triaceticacid (HEDTA), ethyleneglycol-bis (β-aminoethylether) (EGTA) etc. has been tested. Of all these EDTA has been proved to best chelate. It considerably increases availability of lead, enhances its mobility from root to shoot to even more than 100 times and increases lead accumulation in shoot to more than 400 fold. Research is going on to find suitable amount and number of doses of chelate and timing of its application. It has been found that multiple doses in small quantities are better than single time application of high dose. Application after planting gives good results as compared to application before plantation. Soil condition like fertilizer applied, soil temperature, salinity etc. also affect effectiveness of remediation.

Researchers have studied accumulation of lead by high biomass plants like Brassica juncea, B. rapa, Helianthus annuus, Vicia faba, Pisum sativum, Phaseolus vulgaris etc. on application of lead. Brassica juncea could accumulate more than 1.5% of lead of its shoot dry weight on EDTA application. It has been designated as best lead phytoremediator plant as it is non-edible plant and poses no health hazards for public. It is also a very high biomass plant and has short life cycle. However there are concerns about side effects associated with chelate application. Pb-EDTA easily percolates through soil profile and causes ground water pollution. Studies are thus going on to find a better biodegradable organic chelate.

Once inside the plant metal needs to be detoxified. Detoxification of lead occurs by its binding to some chemical groups e.g. to cell wall, to polyphosphates bodies in some cyanobacteria and in most cases by binding to specific peptides called phytochelatins.

In case of cadmium (Cd), S^2- incorporation stabilizes the PC-Cd complexes. The PC-metal complexes are finally transported to vacuole via ATP-binding cassette (ABC) type transporter where metal causes no harm. Though mechanism has not yet been elucidated for Pb, however, as PC-Pb complexes are formed, it is likely that these are sequestrated in vacuole in a similar way.

A future prospective of the problem is to genetically engineer high biomass non-edible plants for better hyperaccumulation of lead. There is one report of transfer of PC synthase gene (TaPCS1) of wheat to Nicotiana glauca (Shrub tobacco), where it doubled the accumulation of Pb.

Biosorption is another emerging potentially economic technology for metal removal and recovery. It consists of several mechanisms including ion exchange, chelation, adsorption, and ion entrapment in structural polysaccharide network. This also uses dead or inactive microbial biomass for sorption of metal by chemically active surface groups. Due to high surface area, microbial biomass eg. algae like Oscillatoria, Anabaena, Eudorina etc. fungi e.g. Aspergillus and bacteria like Pseudomonas can adsorb high amount of lead on their surfaces and can achieve quick remediation.

Thus phytoremediation is best-suited technology in present context to clean up Pb contaminated sites and it is cost effective as well. Since there are no known high biomass plants which could hyperaccumulate lead, research is going on to genetically engineer the plants, which produce more biomass in short time and then incorporating genes for hyperaccumulation and detoxification of lead into them to achieve the goal. Genetic engineering efforts thus may strengthen phytoremediation as a much better applied technology in recent future and remediation will be achieved in quicker time at low cost.