INTRODUCTION:

Soil pollution by heavy metals:

Heavy metals are conventionally defined as elements with metallic properties (ductility, conductivity, stability as cations, ligand specificity, etc.) and an atomic number >20. The most common heavy metal contaminants are Cd, Cr, Cu, Hg, Pb and Ni. Metals are natural components in soil with a number of heavy metals being required by plants as micronutrients. However, pollution of biosphere by toxic metals has accelerated dramatically since the beginning of the industrial revolution. As a result of human activities such as mining and smelting of metals, electroplating, gas exhaust, energy and fuel production, fertilizer, sewage and pesticide application, municipal waste generation, etc. (Kabata-Pendias and Pendias, 1989), metal pollution has become one of the most severe environmental problems today.

Excessive accumulation of heavy metals is toxic to most plants. Heavy metals ions, when present at an elevated level in the environment, are excessively absorbed by roots and translocated to shoot, leading to impaired metabolism and reduced growth (Bingham et al., 1986; Foy et al., 1978). Heavy metal contamination to water and soil poses a major environmental and human health problem. In addition, excessive metal concentrations in contaminated soils result in decreased soil microbial activity and soil fertility, and yield losses (McGrath et al., 1995).

Cadmium, as a non-essential, toxic heavy metal to plants, which may well demonstrate the problem, can inhibit root and shoot growth, affect nutrient uptake and homeostasis, and is frequently accumulated by agriculturally important crops (Sanità di Toppi and Gabrielli, 1999). Thus, when Cd-enriched crop products are consumed by animals and humans, it can cause diseases. On condition that soil Cd pollution is cumulative with levels increasing over time, the soil may eventually become unusable for crop production. Similarly, contamination of soil with Cd can negatively affect biodiversity and the activity of soil microbial communities (McGrath, 1994). Burd et al.(1998)’s experiments revealed that canola seeds developed normally in the presence of up to 1 mmol/L nickel chloride, but that plant root and shoot elongation were inhibited at higher levels.

Remediation technologies:

Heavy metals cannot be destroyed biologically (no “degradation”, change in the nuclear structure of the element, occurs) but are only transformed from one oxidation state or organic complex to another (Garbisu and Alkorta, 2001), remediation of heavy metal contamination in soils is more difficult.

Until now, methods used for their remediation such as excavation and land fill, thermal treatment, acid leaching and electro reclamation are not suitable for practical applications, because of their high cost, low efficiency, large destruction of soil structure and fertility and high dependence on the contaminants of concern, soil properties, site conditions, and so on. Thus, the development of phytoremediation strategies for heavy metals contaminated soils is necessary (Chaney et al., 2000; Cheng et al., 2002; Lasat, 2002).

Metal transfer in plants:

Phytoremediation of heavy metals may take one of several forms: phytoextraction, rhizofiltration, phytostabilization, and phytovolatilization. Phytoextraction refers to processes in which plants are used to concentrate metals from the soil into the roots and shoots of the plant; rhizofiltration is the use of plant roots to absorb, concentrate or precipitate metals from effluents; and phytostabilization is the use of plants to reduce the mobility of heavy metals through absorption and precipitation by plants, thus reducing their bioavailability; phytovolatilization is the uptake and release into the atmosphere of volatile materials such as mercury- or arsenic-containing compounds.

The ideal plant for phytoextraction should grow rapidly, produce a high amount of biomass, and be able to tolerate and accumulate high concentrations of metals in shoots. Most of the commonly known heavy metal accumulators belong to the Brassicaceae family (Kumar et al., 1995). Although hyperaccumulator plants have exceptionally high metal accumulating capacity, most of these have a slow growth rate and often produce limited amounts of biomass when the concentration of available metal in the contaminated soil is very high. An alternative is to use species with a lower metal accumulating capacity but higher growth rates, such as Indian mustard (Brassica juncea); another alternative is to provide them with an associated plant growth-promoting rhizobacteria, which also is considered to be an important component of phytoremediation technology (Wenzel et al., 1999; Glick, 2003). Obviously, the rhizosphere contains a large microbial population with high metabolic activity compared to bulk soil (Anderson et al., 1993). Microbial populations are known to affect heavy metals mobility and availability to the plant through release of chelating agents, acidification, phosphate solubilization, and redox changes (Abou-Shanab et al., 2003a; Smith and Read, 1997). Especially, some plant growth-promoting bacteria associated with plant roots also may exert some beneficial effects on plant growth and nutrition through a number of mechanisms such as N2 fixation, production of phytohormones and siderophores, and transformation of nutrient elements when they are either applied to seeds or incorporated into the soil (Kloepper et al., 1989; Glick, 1995; Glick et al., 1999). The use of rhizobacteria in combination with plants is expected to provide high efficiency for phytoremediation (Abou-Shanab et al., 2003a; Whiting et al., 2001). Therefore, the potential and the exact mechanism of rhizobacteria to enhance phytoremediation of soil heavy metals pollution have recently received some attention (de Souza et al., 1999a; Whiting et al., 2001). For example, Burd et al.(1998) observed that both the number of Indian mustard seeds that germinated in a nickel-contaminated soil, and the attainable plant size increased by 50%~100% by the addition of K. ascorbata SUD165/26, an associated plant growth-promoting rhizobacteria, to the soil in preliminary field trials, and de Souza et al.(1999b) investigated phytoremediation of Se and Hg in constructed wetlands and found that accumulation of Se and Hg were enhanced by rhziobacteria in wetland plant tissues.

Selection of plants:

The ability of a plant species to clean up a metal-contaminated site depends upon the amount of metals that can be accumulated by the candidate plant, the growth rate of the plant and the planting density. There are several factors which decide the ideal plant for phytoremediation. One of them is that the plant should have sufficient tolerance to the site conditions to grow well and should be able to accumulate multiple metal contaminants. The most important factor is that the plant species should be fast growing and easy to harvest (McIntyre, 2003). In general, favorable plant properties for phytoremediation are to be fast growing, have high biomass, and are tolerant to pollution. High levels of plant uptake, translocation, and accumulation in harvestable tissues of the plant are important properties for the phytoextraction of inorganics (Pilon-Smits, 2005). There are many naturally occurring metal accumulators. But biotechnology techniques can be used to develop plants with even better characteristics for phytoremediation such as ability to accumulate multiple metals (McIntyre, 2003). Theses advances are promising for improving the effective use of phytoremediation technology for cleaning up the soil of contaminated sites.

Phytoremediation of Heavy Metals in Soil:

Heavy metal contamination of soil is still an unsolved problem. Heavy metal compounds in soil are very hazardous pollutants for the following reasons:

· Non-biodegradable,

· Extremely toxic at low concentrations, and

· Chances of mobilization under changing physical-chemical conditions. Selection of a remediation technique for a site contaminated with metals is complex, time consuming and site specific. Some factors that influence selection of a suitable procedure are size, location and history of site, accessibility to the site, effectiveness of treatment options, soil and contaminant characteristics, availability of technical and financial resources, and degree of contamination (McIntyre, 2003).

Phytoremediation is an emerging technology which can be effectively used for the remediation of metal contaminated sites. The bioavailability of metals to plants is affected by different factors such as soil and plant characteristics, and various environmental factors. The main soil characteristics include pH, presence of hydrous oxides of iron and manganese, organic matter content, clay content, phosphate content, redox potential, soil particle size (surface area of soil particles), and cation exchange capacity. Climatic conditions, irrigation, and soil fertilizing practices are examples of environmental factors. The species of plant, character of plant tissue, and age of vegetation also affect metal uptake (McIntyre, 2003).

The metal uptake by a plant is depends on the concentration of soluble and bioavailable fraction of metals in the soil solution. The bioavailable fraction of metal in the soil can be determined by the Potential Bioavailable Sequential Extraction (PBASE) procedure (Basta and Gradwohl, 2000). Even though chemical extraction won’t extract metal from the soil in a manner identical to that of a plant root system, it can be used as a reliable method for assessing the bioavailability of metals bound to soil particles (Basta and Gradwohl, 2000).

In a polluted soil, the concentration of bioavailable pollutants tends to reduce over time due to physical, chemical and biological processes. Because of this reason, aged soils are more difficult to phytoremediation (Pilon-Smits, 2005). It is known that to enhance metal solubility, plants either excrete organic ligands or lower the soil pH in the rhizosphere. To improve metal solubility in the soil solution, synthetic chelates such as ethylene diamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), pyridine-2-6dicarboxylic acid (PDA), citric acid, nitric acid, hydrochloric acid and fluorosilicic acid can be used in phytoremediation studies (Romkens et al., 2002). The addition of excess chelating agents may increase the chances of leaching the metals from the soil to groundwater (Romkens et al., 2002). If the metal concentration in the soil is near to the phytotoxic levels, then addition of lime or organic matter reduces the metal solubility (Pilon-Smits, 2005).

Heavy Metal Toxicity to Plants:

A major disadvantage of phytoremediation is that high concentrations of heavy metals or certain combinations of heavy metals may adversely affects plant growth and biomass production by disrupting the physiology and morphology of plants. Some plant species have the ability to grow and develop in metalliferous (metal rich soils) soils such as near to mining sites. Such plants can be utilized to clean up heavy metal polluted sites. The general effects of various metals in plant are (Gardea-Torresdey et al., 2005):

Cadmium: Decreases seed germination, lipid content and plant growth, but induce the production of phytochelatins. Phytochelatin is a metal binding peptide and has an important role in cadmium detoxification in plants.

Chromium: Causes decrease in enzyme activity and plant growth, and produces membrane damage, chlorosis and root damage.

Copper: Disrupts photosynthesis, plant growth and reproductive processes, and decreases thylakoid surface area.

Mercury: Helps to accumulate phenol, but decreases the photosynthetic- activity, water uptake and antioxidant enzymes.

Nickel: Reduces seed germination, protein production, chlorophyll and enzyme production, and accumulation of dry mass, but increases the amount of free amino acids. Lead: Reduces chlorophyll production and plant growth, but increases superoxide dismutase (metal containing antioxidant enzyme).

Zinc: Reduces nickel toxicity and seed germination, but increases plant growth and ATP/chlorophyll ratio at moderate concentrations (Gardea-Torresdey et al, 2005).

Fate of Absorbed Metals in Plant:

The metals absorbed in a plant can accumulate in various parts of the plant. For an effective phytoremediation process, the metals should be accumulated in a harvestable part of the plant. Brake fern, one of the major plants for arsenic phytoremediation, accumulated almost 95% of arsenic taken up into the aboveground biomass. The arsenic concentration in the brake fern root was the least when compared to the other parts. The highest concentration was reported in old fronds followed by young fronds, fiddle heads, and rhizomes (Zhang, 2002). Arsenate usually enters the plant root through the phosphate uptake system, and to limit the toxicity the plant chemically reduce As(V) to As(III) in the roots. In the case of Indian mustard, a large portion of absorbed As remains in the root itself and a small amount of arsenic is transported to the shoots, however the addition of water soluble As- chelators can increase this fraction (Salt, 2002). In most of plants, the major portion of absorbed Cd remains in the root of the plant and only some is translocated to the shoots (Salt, 2002). Sunflower accumulates zinc mostly in the stem (437.81 mg Zn/ kg dry weight) and lead in roots (54.53 mg Pb/kg dry weight). In the case of corn, lead and zinc were accumulated more in leaves (84.52 mg Pb/kg dry weight) (1967 mg Zn/kg dry weight) (Spirochova et al., 2003). Hemidesmus indicus accumulates lead in the shoots (Sekhar et al., 2005) and Smilo grass accumulates lead in roots and zinc in shoots (Garcia et al., 2004). Experiments on Thlaspi praecox revealed that Zn and Cd accumulate in the shoots and their concentration in the shoots is linearly correlated with total soil Zn and Cd concentrations, thus confirming that the plant can be used for the phytoremediation of soil contaminated with Zn and Cd. At the same time 80% of the accumulated lead is immobilized in the roots (Mikus et al., 2005).

CONCLUSIONS:

The contamination of heavy metals to the environment that is soil water, plant and air is of great concern due to its potential impact on human and animal health. Cheaper and effective technologies are needed to protect the precious natural resources and biological lives. The mechanism of metal uptake, accumulation, exclusion, translocation, osmoregulation and copartmentation vary with each plant species and determine its specific role in phytoremediation .another advantage of phytoremediation is that it leaves the soil fertile and has less adverse environmental effects as compared to conventional procedures.

The research finding shown that the low doses of heavy metals applied stimulated the root and shoot elongation of sunflower plants.it also shows that the heavy metals were efficiently up taken at all concentration using high biomass producing plant helianthus annuus and the uptake was increased along the increasing concentration in soil. The present technology will help to remediate the higher concentration of metals which can be applicable at the site of remediate the heavy metals .thus, an increase in plant resistance heavy metal toxicity (Zn,Cu,Ni,Pb,Cd)seems possible by phytoremediation using helianthus annuus.

REFERENCES:

1. Alkorta I, Hernandez-Allica J, Becerril J. M, Amezaga I, Albizu I and Garbisu C. Recent findings on the phytoremediation of soils contaminated with environmentally toxic heavy metals and metalloids such as zinc, cadmium, lead, and arsenic. Reviews in Environmental Science and Bio/Technology. 3; 2004: 71-90.

2. Angelova V, Ivanov K and Ivanova R. Effects of chemical forms of Pb, Cd and Zn in polluted soils on their uptake by Tobacco. Journal of Plant Nutrition. 27(5); 2004: 757-773.

3. Axtell NR, Sternberg SPK and Claussen K. Lead and nickel removal using Microspora and Lemna minor’. Bioresource Technology. 89; 2000: 41-48.

4. Basta N, Gradwohl R. Estimation of Cd, Pb, and Zn bioavailability in smelter-contaminated soils by a sequential extraction procedure. Journal of Soil Contamination. 9(2); 2000: 149-164.

Received on 30.08.2013 Accepted on 01.09.2013

Research J. Engineering and Tech. 4(4): Oct.-Dec., 2013 page 242-245