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Application and Techniques For Phytoremediation

INTRODUCTION

Phytoremediation (‘phyto’ means plant) is a generic term for the group of technologies that use plants for remediating soils, sludges, sediments and water contaminated with organic and inorganic contaminants. Phytoremediation can be defined as “the efficient use of plants to remove, detoxify or immobilise environmental contaminants in a growth matrix (soil, water or sediments) through the natural biological, chemical or physical activities and processes of the plants”.

Plants are unique organisms equipped with remarkable metabolic and absorption capabilities, as well as transport systems that can take up nutrients or contaminants selectively from the growth matrix, soil or water. Phytoremediation involves growing plants in a contaminated matrix, for a required growth period, to remove contaminants from the matrix, or facilitate immobilisation (binding/containment) or degradation (detoxification) of the pollutants. The plants can be subsequently harvested, processed and disposed.

Plants have evolved a great diversity of genetic adaptations to handle the accumulated pollutants that occur in the environment. Growing and, in some cases, harvesting plants on a contaminated site as a remediation method is a passive technique that can be used to clean up sites with shallow, low to moderate levels of contamination. Phytoremediation can be used to clean up metals, pesticides, solvents, explosives, crude oil, polyaromatic hydrocarbons, and landfill leachates. It can also be used for river basin management through the hydraulic control of contaminants. Phytoremediation has been studied extensively in research and small-scale demonstrations, but full-scale applications are currently limited to a small number of projects. Further research and development will lead to wider acceptance and use of phytoremediation

There are several ways in which plants are used to clean up, or remediate, contaminated sites. To remove pollutants from soil, sediment and/or water, plants can break down, or degrade, organic pollutants or contain and stabilise metal contaminants by acting as filters or traps.

Application And Techniques For PhytoremediationThe uptake of contaminants in plants occurs primarily through the root system, in which the principal mechanisms for preventing contaminant toxicity are found. The root system provides an enormous surface area that absorbs and accumulates the water and nutrients essential for growth, as well as other non-essential contaminants. Researchers are finding that the use of trees (rather than smaller plants) is effective in treating deeper contamination because tree roots penetrate more deeply into the ground. In addition, deep-lying contaminated ground water can be treated by pumping the water out of the ground and using plants to treat the contamination.

Since the dawn of the Industrial Revolution, mankind has been introducing numerous hazardous compounds into the environment at an exponential rate. These hazardous pollutants consist of a variety of organic compounds and heavy metals, which pose serious risks to human health. The problem of environmental pollution has assumed an unprecedented proportion in many parts of the world. Many methods and processes of preventing, removing and correcting the negative effects of pollutants released into the environments exist, but their application for this purpose has either been poorly implemented or not at all, a situation that is worsening owing probably to claims of lack of virile regulatory bodies. The use of plants to reduce contaminant levels in soil is a cost effective method of reducing the risk to human and ecosystem health posed by contaminated soil sites. The objective of this review is to discuss the different phytoremediation mechanisms and their potentials as remediation techniques that utilize the age long inherent abilities of living plants to remove pollutants from the environment but which are yet to become a commercially available technology in many parts of the world especially the developing countries.

The mechanisms and efficiency of phytoremediation depend on the type of contaminant, bioavailability and soil properties (Cunningham and Ow, 1996). There are several ways by which plants clean up or remediate contaminated sites. The uptake of contaminants in plants occurs primarily through the root system, in which the principal mechanisms for preventing toxicity are found. The root system provides an enormous surface area that absorbs and accumulates water and nutrients essential for growth along with other non-essential contaminants (Raskin and Ensley, 2000). This review has identified seven mechanisms by which plants can affect contaminant mass in soil, sediments, and water. Although overlap or similarities can be observed between some of these mechanisms, and the nomenclature varies, this report makes reference to seven phytoremediation mechanisms, each explained in detail below. Each of these mechanisms will have an effect on the volume, mobility, or toxicity of contaminants,

Plant roots also cause changes at the soil-root interface as they release inorganic and organic compounds (root exudates) in the rhizosphere. These root exudates affect the number and activity of the microorganisms, the aggregation and stability of the soil particles around the root, and the availability of the contaminants. Root exudates, by themselves can increase (mobilise) or decrease (immobilise) directly or indirectly the availability of the contaminants in the root zone (rhizosphere) of the plant through changes in soil characteristics, release of organic substances, changes in chemical composition, and/or increase in plant-assisted microbial activity.

Phytoremediation is an alternative or complimentary technology that can be used along with or, in some cases in place of mechanical conventional clean-up technologies that often require high capital inputs and are labor and energy intensive. Phytoremediation is an in situ remediation technology that utilizes the inherent abilities of living plants. It is also an ecologically friendly, solar-energy driven clean-up technology, based on the concept of using nature to cleanse nature.

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 waste waters. 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 Nutrition Lab,New York, an expert on plant responses to stress and use of plants to clean up or re-mediate 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.

A major environmental concern due to dispersal of industrial and urban wastes generated by human activities is the contamination of soil. Controlled and uncontrolled disposal of waste, accidental and process spillage, mining and smelting of metalliferousores, sewage sludge application to agricultural soils are responsible for the migration of contaminants into non-contaminated sites as dust or leachate and contribute towards contamination of our ecosystem. A wide range of inorganic and organic compounds cause contamination, these include heavy metals, combustible and putriscible substances, hazardous wastes, explosives and petroleum products. Major component of inorganic contaminates are heavy metals

They present a different problem than organic contaminants. Soil microorganisms can degrade organic contaminants, while metals need immobilisation or physical removal. Although many metals are essential, all metals are toxic at higher concentrations, because they cause oxidative stress by formation of free radicals. Another reason why metals may be toxic is that they can replace essential metals in pigments or enzymes disrupting their function

. hus, metals render the land unsuitable for plant growth and destroy the biodiversity. Though several regulatory steps have been implemented to reduce or restrict the release of pollutants in the soil,they are not sufficient for checking the contamination.There are a number of conventional remediation technologies which are employed to remediate environmental contamination with heavy metals such as solidification, soil washing and permeable barriers.But a majority of these technologies are costly to implement and cause further disturbance to the already damaged environment. Phytoremediation is evolving as a cost-effective alternative to high-energy,high-cost conventional methods. It is considered to be a “Green Revolution” in the field of innovative cleanup technologies. Bioremediation by use of plants constitutes phytoremediation. Specific plants are cultivated at the sites of polluted soil. These plants are capable of stimulating the bio-degradation of pollutants in the soil adjacent to roots (rhizosphere), although phytoremediation is a cheap and environment friendly clean-up process for the bio degradation of soil pollutants, it takes several years

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Johnathan April 17, 2018 - 6:41 pm

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admin April 17, 2018 - 10:28 pm

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