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Soil remediation techniques

In order to restore the natural ecosystem functions, to improve the quality of human and animal life dependent on the polluted land area, the search for effective and feasible remedial measures to address land pollution has become important. Location/site specific remediation management has been advocated by government organizations, environmentalists, policy makers and land owners for rational utilization of polluted soil. Remediation of large rural areas with marginally polluted soils, and agricultural fields should be approached differently than the remediation of heavily contaminated areas such as those around mining and smelting sites. In general, remediation technologies, whether in place or ex situ can be achieved either by removal of the heavy metals (“site decontamination or clean-up techniques”) or by preventing then- spread to surrounding soil and groundwater (isolation and/or immobilization), thereby reducing exposure risk.

Removal of contaminants and risk minimization are the major approaches for heavy metal polluted soil. Several technologies have been developed for their remediation based on clean-up, detoxification and risk minimization approaches. All of these technologies have both advantages and disadvantages in respect of the extent of applicability, side-effects on other components of environment, cost and ease of adoption, speed and effectiveness of remediation. Remediation of large areas of heavy metal contaminated soil by conventional methods (e.g., excavation, physicochemical treatment) is too expensive for large sites (Willscher et al. 2013). The use of plants and associated microorganisms to stabilize inactivate, remove, or degrade toxic environmental pollutants, which is generally termed as phytoremediation, is gaming more and more attention. Phytoremediation represents an emerging and sustainable technology for the remediation of slightly to moderately contaminated sites.

Phytoremediation

The term phytoremediation is derived from the Greek prefix phyto, meaning “plant”, and the Latin suffix remedium, “able to cure” or “restore”. Phytoremediation technology uses metal accumulation and exclusion abilities of plants to clean up heavy metal polluted areas (Baker and Walker 1990, Sclmoor 2002). It actually refers to a diverse collection of plant-based technologies that use either naturally occurring or genetically engineered plants for cleaning contaminated environments (Flathman and Lanza 1998). It is also called as “Green remediation” and “Botanical bioremediation” and is complementary to classical bioremediation techniques, which are based on the use of microorganisms. It can be applied to a wide range of organic (Sclmoor et al. 1995) and inorganic contaminants. Phytoremediation is a general term which includes several processes, among which phytoextraction and phytostabilisation are the most reliable for heavy metals.

The research of phytoremediation has its origin in the nuclear disaster at Chernobyl in the year 1986 which caused severe radioactive contamination. The scientists were hopeful that plants could play a key role in cleansing some of the contamination. Three years after the explosion, the government requested the International Atomic Energy Agency (IAEA) to assess the radiological and health situation in the area surrounding the nuclear power plant. The study found radioactive emissions and toxic metals-including iodine, cesium-13 7, strontium, and plutonium-concentrated within the soil, plants, and annuals which entered the food chain via grazers, such as cows and other livestock, which fed on plants grown in contaminated soils. These elements finally accumulated in the meat and milk products eventually consumed by humans. Therefore, a soil clean-up technique was employed using green plants to get rid of toxins from the soil. This technique is known as phytoremediation, a term coined by Ilya Raskin who was one of the members of the original task force sent by the IAEA to examine food safety at the Chernobyl site. According to him, using plants to alter the environment “has been around forever, since the time plants were used to chain swamps.” What is new, he asserts, is the systematic and scientific investigation of how plants can be used to decontaminate soil and water. It takes advantage of the plant’s ability to remove pollutants from the environment or to make them harmless or less dangerous.

The term phytoremediation, like the term itself, consists of two major processes that decontaminate the matrix (remove, extract, degrade, volatalise, etc.) and processes that stabilize the contaminant in the soil to reduce or prevent further environmental damage (sequester, solidify, precipiatate, etc.) (Cuuningham et al. 1996). The former process is known as phytodecontamination and the latter is called phytostabilization. The choice of which of these alternative techniques should be implemented at a site is not solely a matter of economics, for they have different constraints and applications and are sensitive to different site parameters such as concentration of the contaminant, soil chemistry, contamination depth or the time frame required for remediation. If an immediate reduction in risk is required, phytostabilisation would be chosen because of the length required for plants to remove the contaminant for extraction. However, as sites where decontamination is desired and feasible (phytodecontamination), phytoextraction is more appropriate technique despite the higher cost (Cuuningham and Berti 1999).

 
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