Prakasam, V:Decontamination of the environment using biota

Prakasam, V, Decontamination of the environment using biota, (Ed.) S.Jisha, B.Hari & T.K.Remesan, Proc. Nat. Sem. on Env. Biotech. Chall. and Oppor.,  ENVIROTECH-2008, P.G. Dept. of Zoology, S.N.C. Natiika, pp1-6.

(Dept. of Environmental Sciences,  University of Kerala, Thiruvananthapuram-695581, e.mail:prakasamvr@gmail.com)

Abstract

By the activities of man a large number of pollutants are released into the soil and water.  These pollutants include numerous organic and inorganic substances.  They cause water contamination, ecological disturbances and risks to human health.  Therefore steps are on way for clean up of these contaminants.  This paper deals with the use of biota in the removal of such pollutants from the environment, undertaken through a process called remediation.  Remediation includes broadly Phytoremediation, Bioremediation and Zooremediation.

Phytoremediation is the biological treatment process that utilizes plants to enhance degradation and removal of contaminants from soil or ground water.  It includes phytoextraction, rhizofiltration, phytodegradation, phytovolatilization, rhizoremediation and phytostabilization.  By using rhizoremediation technology in constructed wetlands sewage treatment is also carried out.  It is also termed as root-zone technology.  Another important method is called bioremediation.  It is restoration of contaminated environment using microbes.  Based on the location where bioremediation process occurs it can be classified as ex-situ and in- situ bioremediation.  The two major approaches in bioremediation consist of biostimulation and bioaugmentation.  Interestingly GMOS have been recently employed in bioremediation process.  Zooremediation\vermi remediation is another process, though not widely practiced, where the ability of animals in clean up of environmental pollutants is made use of, especially in management of wastes.  All these technologies which are used in treatment water, waste water and soil are detailed in this paper.

Introduction

By man’s developmental activities a large number of pollutants are introduced into the soil, water and air. The pollutants include numerous organic and inorganic substances such as municipal sewage, petroleum products, hazardous wastes from industries, chemicals, agricultural pesticides, fertilizers, heavy metals and radio active materials. Rising environmental contamination day-by-day is causing ecological disturbances and risks to human health. Therefore efforts are on the way for decontamination or clean up of the environment. This paper deals with the use of biota (living organisms) in the removal of pollutants generated from various sources.

Remediation is the transformation of toxic chemicals or substances to less toxic or unharmful substances. Bioremediation is an accepted and important technology for restoration of contaminated environment using microbes. The goal of bioremediation is to stimulate microbes with nutrients and other chemicals that enable them to destroy the contaminants. The approach is to make advantage of the microbial metabolic potential for eliminating environmental pollutants by transformation of organic pollutants through processes such as degradation or mineralization, co metabolism, polymerization or conjugation, accumulation etc. Microbial degradation can also be due to the indirect effect of microbes on chemical or physical environment resulting in a secondary transformation reaction. The efficiency of the microbes in bioremediation depends on different metabolic pathways followed by them to degrade the hydrocarbons. It is also dependent on the genes controlling the enzymes in the degradation process. It leaves less toxic, stable chemical forms of contaminants along with the byproducts like water, CO2 CH4 and H. The process minimizes environmental damage by removing toxic chemicals.  Remediation using plants and animals is respectively phytoremediation and zooremediation.

Phytoremediation: Phytoremediation is a biological treatment process that utilizes natural processes harbored in (or stimulated by) plants to enhance degradation and removal of contaminants from contaminated soil or ground water. It utilizes physical, chemical and biological processes to remove, degrade, transform or stabilize contaminants within soil and ground water.

Phytoextraction: Refers to the extraction and translocation of heavy metals (eg. Cd, Ni, Hg, Se and radionuclides) from shallow contaminated soil to plant tissue. Metal Hyperaccumulators are plants known to extract and selectively absorb large quantities of heavy metals, resulting in their accumulation in plant tissue at greater concentrations than in the contaminated soil. eg.: Sun flower (Helianthus agnus), Indian mustard (Brassica juncea), Crucifers (Thlaspi caerulescens. T. elegans), Violets (Viola calaminaria), Serpentines (Alyssum bertolonii), Corn, nettles and dandelion. Phytoextraction procedure includes (1) Plant selected species in the contaminated area (2) allow the plants to grow, and harvest (3) incinerate the plant tissue and (4) extract heavy metals from the plant ashes for recycling purposes.

Rhizofiltration: This application refers to the use of aquatic plants in wetlands or hydroponic reactors. The submerged roots of such plants act as filters for the absorption of a wide variety of contaminants. When the sorption capacity of the submerged roots is saturated, the plants are removed and replaced.

Phytodegradation: Certain organic pollutants can be removed from soil and ground water through plant uptake. Once the organic xenobiotic enters the plant system, it is partitioned to different plant parts through translocation. Then the plants use detoxification mechanisms that transform parent chemicals to non phytotoxic metabolites. Any number of reactions within the following series may occur.

Phase I             Conversion (includes oxidation, reduction, hydrolysis)

Phase II            Conjugation (chemically link the phase I product to glutathione, sugars or amino acids and thus alters the solubility and toxicity of the contaminant)

Phase III          Compartmentation (Chemicals are conjugated and segregated into vacuoles on bound to the cell wall material- hemi cellulose or lignin)

Some contaminants such as RDX (Hexahydro-1,3,5- trinitro-1,3,5-triazine) may accumulate in leaves. This is of concern, become leaves could fall to the ground potentially reintroducing the contaminant to the environment or to be eaten by animals (potentially impacting food web)

Hydraulic control: Plants are used to prevent off-site migration and /or decrease downward migration of contaminants. Trees and grasses act as solar “pumps”, removing water from soils and aquifers through transpiration. Deep rooted plants are most often used, eg. Poplar (Populus), Willow (Salix). When planted densely (more than 600 acres per acre), poplar and willows usually reach optimum working conditions after 3-4 years during canopy closure when almost all the direct sunlight is intercepted.

Phytovolatalization: In this process the natural ability of a plant to volatize a contaminant that has been taken up through its roots is made use of. It is a potentially viable remediation strategy for many volatile organic compounds such as BTEX (Benzene, Toluene, Ethyl benzene and Xylenes), TCE (Trichloroethylene), Vinyl chloride or Carbon tetrachloride.

Rhizoremediation:This application refers to bioremediation in the root zone. Microbial degradation in the Rhizosphere might be the most significant mechanism for removal of hydrophobic compounds such as PAHS and PCBs. (Polynuclear Aromatic hydro carbon such as Naphthalene, Anthracene etc. Polychlorinated biphenyls which are common dielectric fluids in transformer oil). The strong sorption of such compounds to soil decreases their bioavailabity for plant uptake and phytotransportation but increases their retention in the root zone, which facilities the participation of micro-organisms in the cleanup process. The rhizosphere of most plants promotes a wealth of microorganisms  that can contribute significantly to the degradation of petroleum hydrocarbons during phytoremediation (Deposition of plant- derived carbon sources through root exudation and / or root turn over provides rhizosphere bacteria with numerous organic substrates). Thus the plant can influence the microbial community within its root zone though not directly act on these contaminants.

Phytostabilization: This application aims to prevent the dispersion of the contaminated sediment and soil by using plants (mainly grass) to minimize erosion by wind or rain action.

Root- zone technology: It is a low cost ecotechnology used for treating a variety of municipal, industrial and urban run off waste water. Natural wetlands or constructed wetlands with aquatic plants are made use of in this technology. Three integrated components of the system are the reeds/macrophytes, the gravel bed and the microorganisms.

Reeds (Phragmites karka) absorb oxygen through stomatal openings behind the leaves and transfer it to hollow roots. It then enters the root-zone and promotes the growth of bacteria and fungi. These microorganisms oxidize impurities in the waste water. Macrophytic plants such as water hyacinth and Typha spp. have also shown successful results. Aquatic weeds can be harvested easily. The technology has been applied to different industries like food processing, petroleum refining, chemical industries, breweries and distilleries, plastic, metal, pulp and paper industry.

Vetiveria zizanioides (Ramacham) could be used for Phytoremediation. It is a wonder grass vetiver, globally used for remediation of contaminated land fill sites, stabilization and rehabilitation of eroded mined lands and for waste water treatment. It has been found to withstand extreme temperatures from 10 oC to 48oC and grow in annual rainfall regions from 200-3000mm. It can tolerate very high soil salinity. It can tolerate to very high levels of heavy metals, Al, Mn, As, Cd, Cr, Ni, Cu, Pb, Hg, Se, and Zn. Herbicides and pesticides are very effectively removed by it from the contaminated soil or water (Sinhaet.al., 2005).

Phytoremediation of landfill leachate: Land fills are still the most widely used solid waste disposal method used across the world. Leachate generated from land fill areas exerts environmental risks mostly on surface and ground water, with its high pollutant content, most notably metals, which cause an unbearable lower water quality. During dumping or after the capacity of the landfill has been reached a decontamination and remediation programme should be taken for the area. A recent study (Sogut et.al., (2005) has shown that the perennial plant / Pennisteum clandestinum was quite tolerant to pollution and accumulated heavy metals such as Cr, Ni, Zn and Pb from leachate. This plant was thus found suitable for decontamination and remediation of land fill site.

Phycoremediation: It may be defined in a broad sense as the use of macroalgae and microalgae for the removal or biotransformation of pollutants including nutrients and xenobiotics from waste water and CO2 from waste air. eg: Cyanobacteria used in bioremediation of metals. It is applied to the removal of nutrients, heavy metals and other toxic chemicals from waste and industrial effluents.

Defluoridation: Drinking water from different parts of India is reported to contain excess fluoride. The removal of fluoride from water is called defluoridation. It is possible with indigenous plant materials or aquatic plants. The barks of Moringa indica (drum stick) and Emblica officinalis (Indian gooseberry), the roots of Vetiveria zizanioides(Khuskus) and the leaves of Cynadon dactylon (Bermuda grass) are reported to have appreciable defluoridation capacity (Kartikayan et.al., 2006).   Phytoremediation of Fluoride contaminated ground water using Hydrilla verticellata has also been reported recently (Ambika and Sumalatha, 2005).

Zooremediation: The solid waste from municipal waste (sewage sludge) contains several microbial pathogens such as Salmonella, Shigella and Escherichia. Vermicomposting using the earthworm, Lampito mauritii indicated that these pathogens are removed during this process (Vermiremediation). The midgut analysis also proved the removal of these pathogens by earthworm (Kumar and Sekaran, 2004).   Some animals have the ability to clean up excess nutrients from the waste water. For example, a large scale mussel culture can lead to nutrient decrease in eutrophicated water (Ghosh et.al., 2005).  Marine mussels are also known to concentrate heavy metals in their tissues.

References

Mc. Cutcheon., J and L, Schnoor (2003).  Phytoremediation-transportation and control of contaminants, John Wiley & Sons. Inc., New Jersey.

Agarwal, S. K (2005). Wealth and waste, APH publishing corporation, New Delhi.

Ghosh, T. K (2005). Biotechnology in environmental Management Vol. I & II. APH publishing corporation, New Delhi.

Alvarez, P. J. J and W. A, Illman (2006). Bioremediation and Natural Attenuation, Wiley Interscience A. John Wiley & Sons. Inc. New Jersey.