Phytoremediation of soil and water in CKD affected areas


by Piyasiri Amilasith Yapa

It is becoming increasingly clear that cadmium toxicity plays a major role in chronic kidney disease (CKD) in NCP. Kelaniya University researchers have reported that arsenic toxicity is also associated with CKD. Despite so many studies and reports etc for so many years, strangely no action has been taken to rectify this situation.

If the soil and water are polluted with heavy metals as claimed they will remain polluted until some action is taken to clean up the soil and water. People will continue to ingest toxic metals through water and food crops grown in contaminated soil and continue to become fatally ill. Are we to allow this to go on for generations to come?

Phytoremediation is a method used in other countries to clean up soil and water polluted by toxic substances including heavy metals. Phytoremediation has been defined as the use of plants to remove or inactivate pollutants from soils and wastewaters. It all started in 1948 when Pichi Sermolli, an Italian scientist observed an unusual accumulation of nickel in some plants. Today scientists know of hundreds of plant species capable of selectively absorbing and accumulating specific elements and substances without showing toxicity symptoms – they are known as hyper accumulators. For instance our tea plant (Camellia sinensis) is a well known aluminum accumulator.

Depending on the plant species , the actual mechanism of phytoaccumulation can be phytofiltration, phytostabilization, phytovolatilization, or phytodegradation.

As far as cadmium is concerned scientists have already identified through research quite a number of plant species that can accumulate cadmium. Some examples are – Athyrium yokoscense, Avena strigosa, Bacopa monnieri (lunuwila), Brassica juncea, Valisnaria americana, Crotalaria juncea (andana hiriya), Eichhornia crassipes (water hyacinth), Helianthus annus (sunflower), Hydrilla verticillata, Lemna minor (duck weed), Pistia stratiotes (water lettuce –diya gowa), Salix viminalis, Spirolelea polyrhiza (giant duck weed), Tagetes erecta, Thlaspi caerulescenes (alpine pennycress) and Valisnaria spiralis (Eel grass).

Of all these species, Thlaspi caerulescenes has been reported to be the best hyper accumulator of cadmium, unfortunately it is not available in Sri Lanka. It is a small weedy member of cabbage family and thrives on soils wth high levels of zinc and cadmium. It possess genes to accumulate excessive amounts of heavy metals in other parts of the plant such as leaves and shoots. A typical Thlaspi plant can accumulate about 30,000 ppm of zinc and 1500 ppm cadmium without showing any toxicity symptoms where as a normal plant can tolerate as little as 1000 ppm zinc and 20-50 ppm cadmium. Plant shoots can be harvested and the heavy metals can be extracted.

Thlaspi can be used even for cleaning up radioisotopes says Dr Kochian at the US Plant, Soil and Nutrtion Lab,New York, an expert on plant responses to stress and use of plants to clean up or remediate soils contaminated with heavy metals It can accumulate about 20,000 ppm uranium, 100 times higher than the control.

Rorippa globosa is another plant species that can accumulate high levels of cadmium, 107 mg /kg by stem and 150 mg/kg by leaves when the soil cadmium level is 25 mg/kg.

Several workers have studied cadmium accumulation potential of food crops. One interesting observation is the cadmium accumulation potential of red beet. Dr Poniedzia et al of Agricultural University of Krakow, Poland have reported a10.3% reduction in cadmium in soil by red beet. Li et al of China have reported that red beet has removed 14.46 mg/m2 cadmium in one growing season. This is something that we have to be careful since red beet is a popular vegetable and consumption of beet grown in contaminated soil can be harmful. Red beet looks a good candidate for phytoremediation work.

Chara australis is another cadmium hyper accumulator (Clabeaux, 2011). It can withstand over 100 mg /kg cadmium. Dr Subashini and Swamy of Nagarjuna University, India have reported that Physalis minima can remove 63.11 mg/kg cadmium in 60 days. This is a plant species found in Sri Lanka too where it is known as Heen mottu, lin mottu,or nalal batu.

Ji et al of Institute of Applied Ecology, China have described Solanum nigrum (Sinh: Kalu kan weriya) as a cadmium hyper accumulator. Amaranthus sp and sunflower have also been investigated. Zedeh et al (2008) of University of Teheran, Iran say Amaranthus sp is a better cadmium accumulator.

Nicotiana tabacum has been mentioned as a cadmium accumulator by several workers. Dr Scholar and his co-workers of Bharathiar University, Tamil Nadu have confirmed this (2011).

Jatropa curcus or weta endaru or rata endaru is another plant reported to be a cadmium accumulator. This is a species commonly found in Sri Lanka too.

Mentioned above are some of the cadmium hyper accumulators scientifically investigated and reported in other countries for cleaning up of soil. We can make use of such information already available or screen our rich biodiversity and look for better plants than Thalspi caerulescens which is currently regarded as an outstanding cadmium hyper accumulator.

For phytoremediation of arsenic contaminated soil there is that amazing plant called Chinese brake (Pteris vittata). It is a fern with a high potential for arsenic accumulation. In one of his trials, Dr Tu and his co-workers of North Carolina State University, grew young ferns in soil containing 98 mg/ As kg-1 for 20 weeks. At the end of the 20 week trial period, fronds had accumulated a staggering 13,000 mg AS kg -1. About 26% of the initial soil arsenic was removed by the plant after 20 weeks of planting. Several rounds of growing P.vittata will see that arsenic is fully gone ! Pityrogramma callomelanos is another fern that can hyper accumulate arsenic.

For phytoremediation of water bodies water hyacinth (Eichhornia crassipes) is the obvious choice. Its ability to absorb and accumulate high levels of heavy metals including cadmium and arsenic is well known. Extensively studied world over for many decades by scientists, it can absorb other toxic elements such as mercury, lead, nickel, chromium and zinc too. In 1980s I myself have studied its use in treatment of rubber and textile factory effluents.

Whilst cleaning up the existing contaminated soil and water, steps also should be taken to protect them from any future pollution. It is clear that large scale use of low quality phosphate fertilizer containing cadmium as impurities , in the recent past would have contributed to present status of soil and water in the NCP. Distribution of low quality fertilizer under the subsidy scheme must come to an end. Farmers should be encouraged to use organic fertilizer. Chemical fertilizer if used must be of high quality though comparatively more expensive.

It is important to create an awareness of new technologies such as phytoremediation among public so that restoration activities can be implemented as participatory projects.

It is also proposed to introduce phytoremediation into the curriculum of Plant Science Departments of all universities, if they have not done so already. In 2005, I had the privilege of introducing phytoremediation as a course unit for the Plant Biotechnology stream at University of Sri Jayawardenepura. However, unfortunately it has not been continued since my premature retirement in 2007. I urge my former colleagues at USJP to restart this course unit immediately.

Research funding bodies such as National Science Foundation, National Research Council and CARP are requested to award research grants to studies on phytoremediation on a priority basis particularly to find out more efficient local plant species.

Phytoremediation is not something entirely new and it has been there for years in other countries and they have made use of it clean up different types of contaminated situations. It is our turn now though little overdue. Let us remember that phytoremediation is an attractive alternative to current clean up methods that are energy intensive and very expensive.

( The writer is a Life Member of International Society for Environmental Botanists, India and also a Fellow of the National Academy of Sciences, Sri Lanka)

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