bioremediation of metal contaminated soil
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BIOREMEDIATION OF METAL CONTAMINATED SOIL
BIOREMEDIATION OF METAL CONTAMINATED SOILBY: HIMANSHU ARORAROLLNO. 03M.Sc (f) evs1
ContentsIntroductionSources of Metals in the SoilPrinciples of BioremediationTypes of BioremediationMicroorganisms Used in BioremediationMechanisms of BioremediationBiosorptionBiimmobolizationBioleachingBiomineralization2
3PhytoremediationHyperaccumulatorsPlant Microbe Interaction: RhizoremediationDesigner Microbe ApproachAdvantages and Disadvantages of BioremediationCase StudyConclusionsReferences
IntroductionIndiscriminate release of metals into the soil is a major health concern worldwide, as they cannot be broken down to non-toxic forms and therefore have long-lasting effects on the ecosystem. Many of them are toxic even at very low concentrations; As, Cd, Cu, Pb, Hg, Ni, Ag, Zn etc. are not only cytotoxic but also carcinogenic and mutagenic in nature.
There are several techniques to remove these metals, including chemical precipitation, oxidation or reduction, filtration, ion-exchange, reverse osmosis, membrane technology, evaporation and electrochemical treatment. But most of these techniques become ineffective when the concentrations of heavy metals are less than 100 mg/L 4
Alternatively, use of microorganisms and plants for remediation purposes is thus a possible solution for metal pollution since it includes sustainable remediation technologies to rectify and re-establish the natural condition of soil.
Bioremediation is the collective range of clean up methods by using natural microorganism (such as bacteria, Fungi, biopolymers) and plants (phytoremediation) to degrade hazardous organic contaminants or convert hazardous inorganic contaminants to environmentally less toxic or nontoxic compounds of safe levels in soils, subsurface materials, water, sludges, and residues.
Sources of Metals in the SoilMetals occur naturally in the environment from pedogenetic processes of weathering of parent materials and also through anthropogenic sources.
Principles of BioremediationBioremediation is based on the idea that organisms are capable to take in pollutants from the environment and use them to enhance their growth and metabolism. Or convert them from a toxic to a nontoxic or less toxic. Bacteria and fungi are well known for degrading complex molecules and transform the product into part of their metabolism7
Types of BioremediationOn the basis of removal and transportation of wastes for treatment there are basically two methods. These are 1) in-situ bioremediation and 2) ex-situ bioremediation.1) In-situ BioremediationNo need to excavate or remove soils or water in order to accomplish remediation.In-situ biodegradation involves supplying oxygen and nutrients by circulating aqueous solutions through contaminated soils to stimulate naturally occurring bacteria to degrade organic contaminants.It is of two types: Intrinsic bioremediation and Engineered In-situ bioremediation.8
In-situ bioremediation approach deals with stimulation of indigenous or naturally occurring microbial populations by feeding them nutrients and oxygen to increase their metabolic activity.Whereas engineered bioremediation approach involves the introduction of certain microorganisms to the site of contamination. There are different techniques of in-situ bioremediation: Biosparging, Bioaugumentation, Bioventing.Biosparging involves the injection of air under pressure below the water table to increase groundwater oxygen concentrations and enhance the rate of biological degradation of contaminants by naturally occurring bacteria.Bioaugumentation involves practice of adding specialized microbes or their enzyme preparation to the polluted sites to accumulate transformation or stabilization of specific pollutants.Bioventing involves supplying air and nutrients through wells to contaminated soil to stimulate the indigenous bacteria..9
2) Ex-situ BioremediationEx-situ bioremediation techniques involve the excavation or removal of contaminated soil from ground.Depending on the state of the contaminant to be removed, ex-situ bioremediation is classified as a) solid phase system and b) slurry phase systems. a) Solid Phase SystemThe Solid phase treatment includes wastes such as leaves, animal manures and agricultural wastes and problematic wastes like domestic and industrial wastes, sewage sludge and municipal solid wastes. Solid phase soil treatment processes include land farming, soil biopiles, and composting.10
Land farming is a simple technique in which contaminated soil is excavated and spread over a prepared bed and periodically tilled until pollutants are degraded. The goal is to stimulate indigenous biodegradative microorganisms and facilitate their aerobic degradation of contaminants.Composting is a technique that involves combining contaminated soil with nonhazardous organic amendants such as manure or agricultural wastes. The presence of these organic materials supports the development of a rich microbial population and elevated temperature characteristic of composting.Biopiles are a hybrid of land farming and composting. Essentially, engineered cells are constructed as aerated composted piles. Typically used for treatment of surface contamination with petroleum hydrocarbons they are a refined version of land farming that end to control physical losses of the contaminants by leaching and volatilization. Biopiles provide a favorable environment for indigenous aerobic and anaerobic microorganisms 11
b) Slurry Phase SystemContaminated soil is combined with water and other additives in a large tank called a bioreactor and mixed to keep the micro organisms, which are already present in the soil, in contact with the contaminants in the soil. Nutrients and oxygen are added and conditions in the bioreactor are controlled to create the optimum environment for the microorganisms to degrade the contaminants.When the treatment is completed, the water is removed from the solids, which are disposed of or treated further if they still contain pollutants.12
Microorganisms Used in BioremediationThere are a number of microorganisms that can be used to remove metal from environment, such as bacteria, fungi, yeast and algae.Because of the adaptability of microbes and other biological systems, these can be used to degrade or remediate environmental hazards. The organisms that are utilized vary, depending on the chemical nature of the polluting agents, and are to be selected carefully as they only survive within a limited range of chemical contaminants.The microorganisms can be subdivided into following groups:Aerobic: Degrade metals in presence of Oxygen.Anaerobic: Degrade metals in absence of Oxygen.Methylotrophs: Aerobic bacteria that grow utilizing methane for carbon and energy.
Lignolytic Fungi: The ability of white-rot fungi (kind of Lignolytic Fungi) to adsorb and accumulate metals together with the excellent mechanical properties of fungal mycelial provide an opportunity for application of fungal mycelia in selective sorption of individual heavy metal ions.
MicroorganismsElementsBacillus spp. Pseudomonas aeruginosaCu, ZnZooglea spp. Citrobacter spp.U, Cu, NiCo, Ni, CdCitrobacter spp. Chlorella vulgarisCd, U, PbAu, Cu, Ni, U, Pb, Hg, ZnAspergilus nigerCd, Zn, Ag, Th, UPleurotus ostreatusCd, Cu, ZnRhizopus arrhizusAg, Hg, P, Cd, Pb, CaStereum hirsutumCd, Co, Cu, NiPhormidium valderiumCd, PbGanoderma applantusCu, Hg, Pb
Microbes Utilize the Metals
Mechanisms of BioremediationRemediation of environment niches such as soil, sediments and water amended with metals can be achieved through biologically encoded changes in the oxidation state.Microorganisms are omnipresent that dominate in metal-contaminated soil and can easily convert metals into non-toxic forms but microbes are unable to simplify them into harmless compounds, they should be used according to their specialization for the type of contaminants.Microorganisms are capable of two-way defense viz. production of degradative enzymes for the target pollutants as well as resistance to relevant heavy metals.Metal-microbe interactions like biosorption and bioaccumulation, biomineralization, bioleaching and bioimmobilization (enzyme-catalyzed transformations) are used as the mechanisms for the removal of metals from the soil.
BiosorptionIt involves a solid phase(sorbent or biosorbent; Biological material) and a liquid phase (solvent, normally water) containing a dissolved species to be sorbed (sorbate, metal ions). Due to higher affinity of the sorbent for the sorbate species, the later is attracted and removed by different mechanisms.The process continue until the equilibrium is established between the amount of solid-bound sorbate species and its portion remaining in the solution.The metal biosorption process by living cells is a two-step process: 1) Passive biosorption, 2) Active biosorption.In Passive biosorption, metal ions are adsorbed to the surface of cells by interactions between metals and functional groups displayed on the surface of cells. Difference in the cell wall composition of different microbial groups, cause significant difference in the type and amount of metal ion binding to them. E.g. In E. coli K12, peptidoglycan was found to be a potent binder and carboxylate groups were the principal component in metal binding.16
Passive biosorption is metabolically independent and proceeds rapidly by any one or a combination of the metal binding mechanism: Coordination, complexation, ion exchange, electrostatic interaction, metal precipitation, physical adsorption or extracellular interactionIn active biosorption, metal ions penetrate the cell membrane and enters into the cell.E.g. Uptake of Pb by dried mass of Chlorella vulgaris, an alga.E.g.