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Phytoremediation: A Green Approach for the Restoration of Heavy Metal Contaminated Soils

Introduction

Soil is the uppermost layer of Earth’s crust on which every person’s life, well-being and fulfilment depend. It takes over thousands of years or even longer to develop one inch of soil. Agriculture productivity and production of good quality food is dependent on good quality soil, necessitating protection of fertile land area. In recent years, soil degr adation is a major global concern as a result of increasing demand on the land for food production and waste disposal. Every year in every country, soil resources are impaired and in some cases lost for productive use because of misuse, application of toxic materials, or poor land management systems. In India, about 60% of the geographical area is occupied by agricultural laud, most of which is facing one or more kind degradation stresses (NAAS 2010).

Among the various soil degradation types, soil pollution is one of the most sensitive environmental issues faced today across the globe. Soil pollution can occur either by natural processes or man-made (anthropogenic) by the introduction of chemicals to the natural soil environment through industrial activity (industrial sector), chemicals used in agriculture (agricultural sector), or improper disposal of waste (urban sector). It brings undesirable change in the physical, chemical or biological properties of the soil, which adversely affects crop production, soil quality, human nutrition, surrounding environment and thereby causes huge disturbance in the ecological balance (Abrahams 2002).

Soil contaminants’ types and status

Based on the nature of the soil contaminants, they can be broadly classified as organic (those that contain carbon) and inorganic (metals, inorganic ions and salts) contaminants. Both organic and inorganic contaminants cause potential hazard to soil, plants, annuals and human beings. The most prominent organic contaminants are petroleum hydrocarbons (benzene, xylene, toluene, ethylene, and alkanes), polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), trichloroethylene (TCE), chlorinated aromatic compounds, detergents, pesticides (including their chemical impurities such as dioxins), dyes and antibiotics. Inorganic contaminants in soil includes nitrates, phosphates, and heavy metals such as lead, cadmium, chromium, cobalt, nickel, arsenic, zinc, non, copper and mercury, inorganic acids, and radionuclides (radioactive substances).

Among different kinds of contaminants, the problem of heavy metal pollution has emerged as a matter of concern at local, regional and global scales. Heavy metals are often referred to as trace or toxic elements because of then toxic effect on living things (Jamp 2003). The level of toxicity and the bioavailability to the plant species depends on several factors, i.e., chemical form (species) and the amount that is presented to the plant in the environment, as well as certain external factors like soil pH, oxidation-reduction potential, presence of other cations and anions in the system and clay and organic matter content in soil (Loska and Wiechula 2000, Wyszkovvska and Wyszkovvski 2003). Because of the persistent and non-biodegradable nature of metals in soil, remediation of heavy metal polluted soil is a greater task (Gao 1986).

In India, heavy metal contamination in soil due to anthropogenic activity has been reported from different areas (Sachan 2007, Shanker et al. 2005, Deka and Bhattacharyya 2009). The Central Pollution Control Board (CPCB) in India has identified critically polluted industrial areas and clusters or potential impact zone based on its Comprehensive Environmental Pollution Index (CEPI) rating. Forty three critically polluted zones were reported in the 16 states which have CEPI rating more than 70. Among the 43 sites, 21 sites exist in only four states namely Gujarat, Uttar Pradesh, Maharashtra and Tamil Nadu. Further, the Central Pollution Control Board (CPCB) has identified 17 industries as highly polluting industries, the majority of which are manufacturing industries like aluminum, cement, chlor-alkali, copper, distillery, dyes and intermediates, fertilizers, iron and steel, oil refineries, pesticides, petrochemicals, pharmaceuticals, pulp and paper, sugar, tannery, power plant and zinc smelters.

Impact of soil contaminants on ecosystem and soil quality

Arange of chemicals introduced to the soil system remain in the soil for many years and their negative impact on soil health are quite alarming because it can cause huge imbalance to the ecosystem and affects health of the living creatures on earth. Irrespective of their sources in the soil, accumulation of contaminants can degrade soil quality, reduce crop yield and the quality of agricultural products, and thus negatively impact the health of human, animals, and the ecosystem (Nagajyoti et al. 2010). Different soil contaminants interact differently with soil components and microorganisms. Based on the type of contaminants, the possible impact on various ecosystems also varies.

Plants that grow on polluted soil may have lower yields because the hazardous chemicals in the soil interfere with then growth. Besides, when plants take up the soil contaminants, they pass them up the food chain, endangering the health of annuals and humans. In some parts of the world, heavily polluted soils with metals and chemicals such as lead, asbestos, and sulfur are considered unfavorable for crop production and cannot be used to grow crops. Further, soil pollution can also lead to scarcity of food if the soil is rendered infertile and crops fail to grow on it, thereby leading to acute food shortage or famines. The most prominent effect of soil pollution on soil chemical properties includes development of salinity, sodicity and acidity, changes in nutrient uptake pattern, decrease in nutrient use efficiency, etc. In addition, soil contaminants (inorganic salts, ions, organic contaminants, heavy metals, dyes and antibiotics) can have negative impact on the lives of the living organisms which can result in the gradual death of many organisms. Soil contaminants also affect several microbially mediated nutrient transformation processes like nitrogen fixation, nutrient mineralization, etc. (Bondarenko et al. 2010, Arora et al. 2010, Bianucci et al. 2011). Thus, the effects of pollution on soil are quite alarming and can cause huge disturbances in the ecological balance and health of living creatures on Earth.

 
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