Heavy metals beyond the threshold concentration in nature are often considered as phytotoxic. They cause damage to environment by affecting the soil fertility, biomass, crop yield and the human health. There are various approaches put forth by researches, but the most cost-efficient emerging remedy is been seen as phytoremediation; where the plants are itself involved partially or substantially in remediating heavy metals such as Copper, Arsenic, Nickel, etc from the soil, ground water, surface water. This review article comprehensively discusses the application, concepts and future trends in phytoremediation of heavy metals.
Introduction
Environmental pollution by heavy metals has become a serious problem in the world. The mobilization of heavy metals through extraction from ores and subsequent processing for different applications has led to the release of these elements into the environment. The problem of heavy metals’ pollution is becoming more and more serious with increasing industrialization and disturbance of natural biogeochemical cycles. Unlike organic substances, heavy metals are essentially nonbiodegradable and therefore accumulate in the environment. The accumulation of heavy metals in soils and waters poses a risk to the environmental and human health. These elements accumulate in the body tissues of living organisms (bioaccumulation) and their concentrations increase as they pass from lower trophic levels to higher trophic levels (a phenomenon known as biomagnification). In soil, heavy metals cause toxicological effects on soil microbes, which may lead to a decrease in their numbers and activities (Khan et. al, 2010).
Regarding their role in biological systems, heavy metals are classified as essential and non-essential. Essential heavy metals are those, which are needed by living organisms in minute quantities for vital physiological and biochemical functions. Examples of essential heavy metals are Fe, Mn, Cu, Zn, and Ni (Cempel and Nikel, 2006; Gohre and Paszkowski, 2006). Non-essential heavy metals are those, which are not needed by living organisms for any physiological and biochemical functions. Examples of nonessential heavy metals are Cd, Pb, As, Hg, and Cr (Mertz, 1981; Karenlampi et. al, 2000; Suzuki et. al, 2001; Cobbett, 2003; Sanchez-Chardi et. al, 2009; Dabonne et. al, 2010).
Objective
The objective of this review to give an account about how heavy metals pollution are causing problems in the environment
1.Sources of heavy metals in the environment
[image: ]Heavy metal pollution enters the environmental ecosystem through natural and anthropogenic sources. The most natural sources are weathering of minerals, erosion and volcanic activity while anthropogenic sources include mining, smelting, electroplating, use of pesticides and (phosphate) fertilizers as well as biosolids in agriculture, sludge dumping, industrial discharge, atmospheric deposition, etc. (Modaihsh et. al, 2004; Chehregani and Malayeri, 2007; Fulekar et. al, 2009; Wuana and Okieimen, 2011).
2. A green solution to the problem of heavy metal pollution
The term ‘‘phytoremediation’’ is a combination of two words: Greek Phyto (meaning plant) and Latin Remedium (meaning to correct or remove an evil). Phytoremediation is an emerging approach that offers ecological benefits and a cost-efficient alternative to earlier remediation methods. It involves use of plants to partially or substantially remediate selected contaminants in contaminated soil, sludge, sediment, ground water, surface water, and wastewater (Vishnoi and Srivastava 2008).
Considering soil contamination with heavy metals like (Copper, Arsenic, Chromium, Nickel, Lead, Zinc, Iron) and need for its remediation, researchers have tried to find plant species which have the capability of accumulating all this heavy metal. Macrophytes have been used during the last two decades for metal removal (Denny and Wilkins 1987).
Plants generally handle the contaminants without affecting topsoil, thus conserving its utility and fertility. They may improve soil fertility with inputs of organic matter (Mench et. al, 2009). Green plants have an enormous ability to uptake pollutants from the environment and accomplish their detoxification by various mechanisms. Phytoremediation technology is a relatively recent technology with research studies conducted mostly during the last two decades (1990 onwards). The concept of phytoremediation (as phytoextraction) was suggested by Chaney (1983). The idea is aesthetically pleasant and has good public acceptance. It is suitable for application at very large field sites where other remediation methods are not cost effective or practicable (Garbisu and Alkorta, 2003). Phytoremediation method is a low installation and maintenance costs compared to other remediation options (Van Aken, 2009).
Regarding cost, phytoremediation can cost as less as 5% of alternative clean-up methods (Prasad, 2003). The establishment of vegetation on polluted soils also helps prevent erosion and metal leaching. From an economic point of view, the purpose of phytoremediation of polluted land can be threefold: (1) risk containment (phytostabilization); (2) phytoextraction of metals with market value such as Ni, Tl and Au; (3) durable land management where phytoextraction gradually improves soil quality for subsequent cultivation of value (Vangronsveld et.al, 2009).
Phytoremediation Application at Global Scenario
Chernobyl, Ukraine
The Chernobyl disaster, also referred to as the Chernobyl accident, was a catastrophic nuclear accident that occurred on 25–26 April 1986 in the No. 4 nuclear reactor of the Chernobyl Nuclear Power Plant, near the now-abandoned town of Pripyat, in northern Soviet Ukraine. large quantities of radioactive compounds were released in the area. Thirty-two years after the reactor ruptured, the site still leeches radioactive contaminants into its surrounding environment. Researchers sunflowers to clean a toxic pond located a kilometre from the Chernobyl reactor. A New Jersey-based company Phytotech made floated rafts of sunflowers on the pond, their roots dangling into the water and absorbing contaminants in a macabre hydroponics setup. The sunflowers were loaded with fungi and microbes to aid in the breakdown of the absorbed materials, worked so well that sunflowers are still being used in Chernobyl to this day.
Cannabis plant were also used because they have expansive root system that made it a particularly good candidate for the task at Chernobyl. Since roots are the part of the plant most effective at absorbing one of the most abundant radionuclides in the area, strontium-90 (short grasses, though shown effective at reducing cesium-137 in Chernobyl, would leave strontium-90 relatively untouched). Hemp is also hardy enough to not only withstand radiation and maintain biomass in spite of such conditions, but to also produce seeds that have levels of metals “within normal limits”. Research into hemp’s promising remedial potential has been still ongoing on Chernobyl.
North Carolina, USA
A study was conducted at a former fuel storage facility at the US Coast Guard Support Centre, Elizabeth City, NC. From 1942–1991, above ground and underground storage tanks for aircraft refuelling were located at these two hectares site 150 meters south of the Pasquotank River. Leaks from these storage tanks have contaminated the soil and shallow groundwater. An estimated 567,000–756,000 liters (150,000–200,000 U.S. gallons) of free-product, i.e., gasoline, diesel, and aviation jet fuel remain at the site based on monitoring well measurements. Benzene and MTBE concentrations in the groundwater were found as high as 2,100 μg/L and 2,500 μg/L, respectively. Wells in the most contaminated areas of the site contain up to 85 cm of free product floating on the water table. Depth to water table range from 61–335 cm across the site depending upon well location and season.
Phreatophytic trees, such as willows and poplars were selected because they can reach and extract water from the capillary fringe or saturated zone and are beneficial for groundwater contaminated with biodegradable organics. Hybrid poplars (Populus deltoides Bartram ex Marsh. x nigra L.) were selected for their deep root systems, rapid growth, high water uptake rates, tolerance to contaminants. Black, coyote, and sandbar willows (Salix nigra Marsh., Salix interior Rowlee, and Salix exigua Nutt., respectively) were selected for general tolerance for saturated conditions, ability to rapidly produce adventitious roots and success in removing dissolved-phase gasoline and PAHs from ground and surface waters.
Phytoremediation Application in Indian Scenario
Jaipur, Orissa
The study was undertaken at South Kaliapani chromite mine area, Sukinda valley of Jajpur district Orissa. 5000 sq. ft. of experimental land was selected for cultivation practice in South Kaliapani, Sukinda area near the mining activity. Cr (VI) contaminated mine wastewater, i.e. the untreated mine wastewaters from South Kaliapani Mines, Orissa Mining Corporation (OMC)was passed in the field. Cr contaminated irrigated water was used all the time for cultivation of plant. On 100 days after transplantation, there was about reduction of Chromium level.
Rayagada, Orissa
The investigation was carried out to reduce the levels of heavy metals and other toxicity loads in paper mill effluent of JK Paper mill of Rayagada District, Orissa, India. The study was conducted using six aquatic macrophytes, Eichhornia crassipes, Hydrilla verticillata, Jussiaea repens, Lemna minor, Pistia stratiotes and Trapa natans grown in paper mill effluent of JK Paper mill of Rayagada, Orissa, for remediation of heavy metals. Phytoremediation ability of these aquatic macrophytic species for copper (Cu) and mercury (Hg) was indicated by assessing the decrease in the levels of heavy metals from effluent water. Maximum reduction (66.5 %) in Hg content of untreated paper mill effluent was observed using L. minor followed by T. natans (64.8%). L. minor showed highest reduction (71.4 %) of Cu content from effluent water followed by E. crassipes (63.6 %). The present phytoremediation approach was considered more effective than conventional chemical treatment method for removing toxic contaminants from paper mill effluent.
Phytoremediation is a time-consuming method. It takes months and years to clean up the surrounding the pollution. Limited by the depth of the root systems and the solubility and availability of the pollutants. Depends on the climatic and seasonal conditions. Ineffective upon the disturbance of plants by diseases and pests. Amendments and agronomic practice may adversely affect the mobility of pollutants.
Conclusion
The present review has presented the comprehensive details of phytoremediation of heavy metals using different types of plants. The techniques can be used at large scale. Phytomining can be emerging technique in future. Phytomining is the production of a `crop' of a metal by growing high-biomass plants that accumulate high metal concentrations.
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