Plant and Microbe based remediation of contaminated sites

Plant and Microbe based remediation of contaminated sites
By Dr. Yashpal Singh
Chairman,The Wealthy Waste School India

Hazardous waste contaminated sites are frequently encountered and present severe environmental problems, through the pollutants finding their way into the various components of the ecosystem. Being of a persistent nature, recalcitrant, highly toxic and bio accumulative they have to be judiciously managed. Remediation of Hazardous waste sites is therefore very important. Both physicochemical and biological pathways have been utilized for remediation. Physicochemical pathways are associated with high expenses, higher labour cost, alteration in soil quality, alteration in water quality, disruption of native soil microflora and the generation of toxic byproducts. Bio remediation on the other hand takes care of hazardous waste contaminated sites in a more efficient and cost-effective way.

Bioremediation includes the processes through which plants and/or indigenous or inoculated micro-organisms degrade or sequester organic contaminants in soil or ground water. It attempts to accelerate the natural bio degradation process by providing nutrients, electron acceptors and competent degrading microorganisms. The techniques are typically implemented at low costs. Contaminants are destroyed and no residual treatment is required. Some compounds however can be transformed into more toxic compounds (TCE to Vinyl Chloride) which may be mobilised in ground water if proper steps are not taken.

Bioremediation technology includes phytoremediation and rhizo-remediation. It is generally considered to include natural attenuation, bio assimilation or bio augmentation.

Tree species in association with mycorrhizae have shown promising prospects for Phyto remediation of Chromium contaminated lands. Organic acids have been used to enhance extractions of immobile metals from soils due to its ability to complex with metals and increase its availability.

Phyto remediation can be used to clean up organic contaminants from surface water, ground water, leachate and municipal and industrial waste water. It can also be successfully used for the removal of Arsenic, Mercury, Fluorides, Cyanides, reclamation of abandoned mine lands and fly ash disposal sites.

It has its limitations in that high concentrations of hazardous wastes can be toxic to plants, it can transfer contaminants from one media to another (Water to air), it is not effective for strongly sorbed or weakly sorbed contaminants, needs large areas of land, the toxicity and bio availability of biodegradation products is not always known. Products may bioaccumulate in water, plants or animals.

A. Plant based bio remediation could be achieved by:

1. ‘Phyto sequestration’ in the root zone where either the contaminant can complex with the root zone exudates leading to the precipitation or immobilisation of contaminants in the root zone or the transport proteins associated with the exterior root membrane can irreversibly bind and stabilise contaminants on the root surface and prevent the contaminants from reaching the plant. Contaminants can also be sequestered into the vacuoles of root cells preventing further translocation into the Xylem.
2. Phyto degradation Where the contaminant is taken up inside the plant, broken down, mineralised or metabolised by the plant itself through various internal enzymatic reactions and metabolic processes. Mineralisation could also be done by endophytic organisms. The endophytic symbiotic bacterium Methylobacterium populi that lives within poplar trees can mineralise RDX and HMX. The oxidation and reduction cycles operating during photosynthesis offer additional contaminant break down potential.
3. Phyto Volatilisation is the volatilisation of contaminants or its break down products either from the leaf stomata or plant stems. Organic substances like TCE and inorganic contaminants like mercury can also be volatilised. Tobacco plants have been modified to take up the highly toxic methyl mercury, alter the chemical speciation and Phyto- volatilise relatively safe levels of the less toxic elemental mercury into the environment. Selenium can be taken up by Brassica and other wetland plants and converted (by methylation to the volatile dimethyl selenium), into non-toxic forms which are volatilised by plants.
4. Phyto stabilization Phyto stabilisation refers to the holding of soils and sediments in place by vegetation and to immobilising toxic substances in soils. Hydraulic control is possible in some ways because of transpiration which prevents migration of leachate towards ground water. Phyto stabilisation is especially applicable for metal contaminants at waste sites where the best alternative is to hold the contaminants at one place. Phyto stabilisation covers are simply soils or sediments planted with selected plant species to prevent bulk soil migration or contaminant migration. Phyto stabilisation covers are also used for infiltration control by utilising the capacity of plants to intercept rains and prevent infiltration. They also maximise the evaporation from the soil and evapotranspiration from plants. Sometimes an anaerobic zone is created beneath the Phyto cover or the subsurface which is driven to methanogenic conditions. This results in gas formation which has to be controlled before it restricts the oxygen transport needed for cell respiration in the root system. The typical range of effectiveness for phyto-remediation ground covers is 30 to 60 cms. below ground surface. Depths up to 1.5 meters have also been reported. Covers can be used for the remediation of PAH’s, PCB’s and other persistent organic pollutants that are typically less mobile, soluble, biodegradable and bio available. Phyto remediation ground covers have also been used to extract specific organic contaminants such as metals, salts and radio nuclides in concentrations higher than what exist in soils.
5. Phyto extraction: Is the ability of the plants to take up contaminants into the root and then translocate them to the aerial parts. Transfer to roots usually occurs through the transpiration route when contaminants in solution are taken up or through vapour adsorption onto the organic root membrane in the vadose zone. Once adsorbed, the contaminant is taken up either through transpiration or translocation. The contaminant may be stored in the plant biomass, sequestered into the cell vacuoles of the aerial parts, metabolised through phyto-degradation or phyto-volatilised. BTEX, Chlorinated solvents, and short chain aliphatic chemicals are generally susceptible to phyto extraction. For inorganic constituents such as salts, metals and radio nuclides, the uptake in plants depends on the redox state, chemical speciation in the soil and the plant species. As a general rule readily bio available inorganics include As, Cd, Cu, Ni, Se and Zn, moderately available are Co, Fe and Mn where Cr, Pb and V are not easily available. Hyper accumulators are plants which accumulate huge quantities of contaminant metals and metalloids generally more than 1000 mg/kg dry weight.
6. Rhizo filtration is the use of plant roots to absorb, concentrate and/or precipitate hazardous compounds, particularly heavy metals or radionuclides from aqueous solutions. Rhizo- filtration is effective if wetlands can be created and all of the contaminated water allowed to come into contact with the root zone. Contaminants should have the capacity to sorb strongly to the roots such as Pb, Cr⁺³, Uranium and As⁺⁵. Harvested plants containing heavy metals could be disposed of or treated to recycle the metal. Rhizo filtration is believed to be effective for dilute concentrations of contaminants in large volumes of water.
7. Rhizo remediation Rhizo degradation, sometimes called Phyto stimulation, rhizosphere bio degradation or plant assisted bio remediation/degradation is the enhanced break down of a contaminant by increasing the bioactivity using the plant rhizosphere environment to stimulate the microbial populations. The presence of a contaminant in a soil tends to naturally select organisms such as bacteria, yeast and fungi that prefer that contaminant as a source of food and energy.
8. Phyto hydraulics Phyto hydraulics is the ability of vegetation to evapo transpire sources of surface water and ground water. The vertical migration of water from the surface down ward can be limited by the water interception capacity of the above ground canopy and evapo-trans piration through the root system. The horizontal migration of ground water can be restricted by using deep rooted plants to intercept, take up and transpire the water. Phreatophytes which are deep rooted, highly transpiring, water loving plants and can service in conditions of temporary saturation have been used. Populars, Willows, Prosopis, Eucalyptus are typical phreatophytes used in bio remediation.
9. Tree hydraulic Barriers Most tree hydraulic barrier applications concentrate the plantings above and at the down gradient end of the plume. In general, the deep rooted, high transpiring trees must be activity tapping into the ground water to create the barrier. A relatively large number of trees and area may be required to extract the volumes necessary to achieve contaminant. In addition to the capacity of deep-rooted plants to take up contaminants by transpiration plants can also be used to phyto remediate deeper soils and contaminated plumes that are located near the top of the water table. Phyto hydraulics can be used to bring the contaminants into the root zone.
10. Riparian Buffers Riparian buffers are vegetated areas that protect adjacent water sources from Nonpoint source pollution. In addition, these buffers provide bank stabilisation and habitat for aquatic and other wild life. Placement of a riparian buffer would be along and upgradient of the ground water surface water interface.

B. Microbe based remediation

1. Microbial Bio remediation utilises microorganisms to degrade organic contaminants in soils, sludge and solids either ex-situ or in-situ. The contaminants are utilised as the food source. Aerobic remediation requires an oxygen source and the end products are CO2 and Water. Anaerobic Processes are carried out in the absence of oxygen and the end products include methane, hydrogen gas, H2S, Sulphides, Elemental sulphur and Nitrogen gas.
2. Ex-situ bioremediation includes slurry phase bio remediation in which the soils are mixed in water to form a slurry to keep solids suspended and microorganisms in contact with the soil contaminants. In solid phase bio remediation the soils are placed in a cell or building and tilled with adding water and nutrients. Land farming, bio-piles and composting are examples of ex-situ solid phase bioremediation.
3. In-situ remediation allows ground water to be treated without being brought up to the surface. This results in cost savings but on the other hand it generally requires longer treatment periods because of the uncertainty of the reactants reacting uniformly owing to the difference in aquifer characteristics. 68,99 Enhanced bio remediation, a form of in-situ soil remediation technology is a viable and potentially cost-effective method. It has been implemented effectively at sites to reduce contaminants mass, shorten the life span of the source area and reduce ground water concentrations of chlorinated ethenes. Enhanced In-situ remediation is not appropriate for all sites. Conditions that prevent microbial growth areas containing large pools of DNAPL and low hydraulic conductivity all limit the effectiveness of enhanced ISB. The use of treatment trans or bio remediation as a polishing technique is a promising application to reach site closures.
4. Bioremediation techniques have been successfully utilised to remediate ground waters contaminated with petroleum hydrocar bons, solvents, pesticides, wood preservatives and other organic chemicals.
5. Biological permeable barriers Here, Biosorption can be applied in-situ without the expense of pumping out the contaminated water or excavating the soil. This technique provides low cost, easy operating and safe treatment. It is particularly useful for high volume low concentration of waste water. The immobilised microbial stratum may be placed in an engineered trench across the flow path of a contaminated plume to create a biological permeable barrier. Contaminated ground water enters the biological permeable barrier to which electron donor and nutrients may be supplied through the ground water gradient while the remediated ground water exits the barrier. The bio barrier can be applied in the field by injecting starved bacteria and then nutrients in a services of injection wells. The pore space is sealed by bacterial growth and external polysaccharide production and then the bio barrier is formed in soil. Bio barrier has applicability as an alternative liner material in land fill. It is able to immobilise heavy metals in situ protecting the environment from the hazardous leachate.
6. Electrokinetic bio remediation technology is designed to activate dormant microbial populations by use of selected nutrients to promote growth, reproduction and metabolism of the microorganisms capable of transforming organic contaminants. The bio electric technology facilitates the distribution and dispersal of nutrients over the contaminated volume of soil or directed at a particular location to save costs. Normally there is no requirement to add microorganisms.

Electrokinetic bio remediation or bio electric remediation technology for continuous treatment of ground water or soil in-situ utilises either electro osmosis or electro chemical migration to initiate or enhance in-situ bio remediation. Electro-osmosis can be used to accelerate the natural ground water movement and increase the efficiency of the bio degradation by the addition of the biological agent into the coarse soils.   

Electrochemical or ion migration is the dominant process in stiff clays and mixed clays. Under these conditions, electro osmosis has limited or no effect on ground water movement. With electrochemical migration the electric field will move uniformly through the soil and ions will readily pass through the small pores in the day. Natural biological population tend to exist around organic pollutant spills in soils. Complex organic compounds, however, are not primary energy sources for microbial populations. Energy sources for microbial population and bio degradation will not flourish unless sufficient nutrients and electron acceptors are available to initiate growth and cause remediation. The process of electrochemical immigration may be limited by

• • The concentration of contaminants being above the toxic threshold for the bacterium.
  • The bio remediation of mixed organic pollutants may produce by products which are toxic to the microorganisms thereby inhibiting the bio degradation process.

7. Bio Stimulation and Bio augmentation of Ground Water.

Oxygen is quite often limiting in bio stimulation since the contaminant can be used as a carbon and energy source by the organism and the contaminant concentration greatly exceeds the oxygen input needed by the organism.

Introduction of air, oxygen or hydrogen peroxide via infiltration galleries, tilling, sparging or venting have proved to be extremity effective in bio remediating petroleum contaminants and a variety of other organic compounds that are not particularly recalcitrant.

Nitrogen and Phosphates have also been successfully used for bio stimulation especially in grossly anaerobic environments where denitrification has reduced the nitrogen content or where phosphorous is in biologically unavailable form. Where the contaminant is not a good source of carbon or energy and other sources of carbon or energy are unavailable it would be necessary to add other sources of carbon. An additional source of organic carbon will also be required if the total organic carbon content in the environment falls below 1 ppm, Methane, Methanol, acetate, molasses, sugars, agricultural compost, phenol and toluene have often been added as secondary carbon supplements to the terrestrial sub-surface to stimulate bio remediation.

Bio augmentation may provide significant advantages over bio stimulation for:

• Environments where the indigenous bacteria have not had time to adopt to the contaminants.
• Particularly recalcitrant contaminants that only a very limited number of organisms are capable of degrading.

• Environments that do not allow a critical biomass to establish.

• Applications where the desired goal is to plug the formation for contaminant e.g. bio curtain.
• Controlled environments where specific mocula of high rate degraders will greatly enhance the process e.p. permeable reactive barriers. The hydraulic conductivity for bio augmentation has to be much higher than that of the 10^-4 per sec. for bio stimulation.

For many bio augmentation applications it becomes difficult to distinguish if the added organisms have provided a significant advantage over nutrient stimulation alone. Given the problems and high cost of producing the organisms for inoculation and delivery problems, bio augmentation applications will probably remain limited. Though bio augmentation provides designer degraders, it has not proven to be better than bio stimulation in repeated field trials. Indeed, there is only one bacterium reported to have the capacity to perform better than bio stimulation, Dehalococcoides ethenogenes for dehalo-respiration of chlorinated solvents.

Where unmediated risk to health and environment do not demand an emergency response, nearly all engineered bio remediation projects could substantially reduce costs by stopping the bio stimulation or bio augmentation process early and allowing intrinsic bio remediation to furnish the clean-up process. Intrinsic bio remediation, a strategy of natural attenuation is a favourable alternative to the high cost engineered bio remediation technologies and could provide the best overall solution.

8. Enhanced biodegradation

Enhanced bioremediation is the process of enhancing the rate of biodegradation by increasing the concentration of electron acceptors and nutrients in ground water, surface water and leachate. Oxygen is the main electron acceptor for aerobic bioremediation. Nitrate serves as an alternative electron acceptor under anoxic conditions.

a. Oxygenation

Heating the bioremediation site such as use of warm water injection may speed up the remediation process. Too high temperatures may be detrimental. Temperature also affects the abiotic loss of contaminants through evaporation. Oxygen enhanced bioremediation is achieved through Air sparging below the water table. It enhances the rate of oxidative degradation and promotes mixing. It is usually used in conjunction with soil vapour evaporation or bioventing to enhance removal of volatile compounds. Dilute H₂O₂ is also used by circulating through the contaminated zone. It also enhances the rate of oxidative degradation by naturally occurring microbes. Higher concentrations may be toxic to microbes.

Solubilized nitrates may also be circulated to enhance the rate of degradation of organic contaminants. This technology enhances the anaerobic degradation through addition of nitrates.

b. Bio venting

Bio venting is the process of aerating soils to stimulate in situ biological activity and promote bioremediation. Bioventing typically is applied in-situ to the vadose zone (i.e. unsaturated soils) by injecting oxygen in the form of air. Bioventing systems are designed to maximise biodegradation while minimising volatilisation. This technology is applicable to any chemical that can be aerobically degraded but has been used largely in petroleum contaminated sites. Bioventing zones are designed to biologically degrade the contaminant source within the vadose zone, thereby preventing future and/or contained contamination of ground water. A typical bio venting system injects air at a low rate into the vadose zone. It is related to the process of soil vapour extraction but differs from it in that while bio venting is designed to maximise biodegradation of aerobically degradable organic compounds with some volatilisation, soil vapour extraction on the other hand is designed to achieve maximum degradation through volatilisation and only some biodegradation.

Bioventing reduces vapour treatment costs which typically range from 50 to 80% of total remediation costs.

Bioventing is most effective in quickly treating the most toxic, soluble and mobile constituents in fuels (benzine, toluene, ethyl benzenes and xylenes) Results are typically seen in less than a year. (97% in the first year).

Bioventing is best applicable if the contaminants of concern are aerobically biodegradable like crude oil, waste oil, heating oil, petroleum based hydraulic fluid, diesel, jet fuel, gasolene, aviation fuel alcohols, acetone etc. and the majority of the contaminant mass is above the ground water level (i.e. vadose zone), the depth to the average ground water table is more than 3 feet and contaminated soil volumes are more than 500 cubic yards, soil gas displays low O₂ (less than 5% vol/vol) and high CO₂ (<2% v/v) profile compared to nearby uncontaminated area. Bio venting systems work better in soils that are more permeable.

C. Specific Contaminant removal

1. 1. Biological Metal Reduction

Microorganisms can transform metal and metalloid species by oxidation, reduction, methylation and decarboxylation. Metal reduction can be mediated by actively metabolising bacteria or through a passive reaction whereby the metal is reduced when it interacts with binding sites both within and outside the organism. The electron transport systems of the relevant microorganism play a very important role in partial reduction, producing a metal that is less mobile or less toxic in the environment. Hexavalent to trivalent chromium is an important example. The trivalent chromium is precipitated as chromium hydroxide. The reaction may occur by both aerobic and anaerobic mechanism.

Several heterotrophic bacteria such as Pseudomonas putida, Pseudomonas chromatophyla, Bacterium dechromaticans, Aeromonas dechromatica etc. are capable of performing this process.

It has been found that microbial consortia are more effective than individual pure cultures. The utilisation of the immobilised bacteria markedly increases the reduction rate.

Cell suspensions of Pyrobaculum islandicum reduced U (VI), TC(VII), Cr (VII), Co (VII), Mn (IV). The same ability has been mentioned for Deinococcus radiodurans, the most radioactive resistant organism discovered.

A number of bacteria and fungi transform metals by methylation and demethylation reactions. Bio methylated derivative although toxic, are often volatile and may be eliminated from a system by evaporation. However, without adequate measures of capture of volatile metals, the contaminant is merely repartitioned from water/soil to the atmosphere. Bio methylation caused problems at Minamata.

Biosorption is the sequestering of metal from soil or water by the intervention of microbial mass (living or dead) through a passive metabolism independent system. The sequestering of metals via an active metabolism dependent mechanism is called bio accumulation. Where living biomass is used, both systems (biosorption and bioaccumulation) have equal role to play.

Metal binding by cell wall and extra cellular polymers is the most important mechanism in the bioremediation of metal contaminated waters. Both living and dead microorganisms can participate in this form of bioremediation. Dead biomass has better biosorptive properties than living biomass, does not require nutrients and can be exposed to environments which cannot support life. Dead biomass can be used in a manner similar to ion exchange resins. Several methods like heat treatment and chemical treatment are available for obtaining dead biomass with a high metal binding capacity.

The microbial cell walls and polymers which bind metals can be stripped of metals and reactivated for reuse. Now maintenance systems in which the immobilised biomass is packaged in process bags and placed in toughs have also been developed.

Bio sorption is most effective in the treatment of surface and ground waters containing low concentrations (Lower than about 20 mg/L) of cationic heavy metals.

A series of trials have been conducted using dealignated seaweed to absorb metals from acidic, iron rich waters from the Libiola mine in Italy. The results confirmed that dealginated sea weed act as a good metal absorber in the treatment of acid mine waters as well as circum neutral mine drainage. Dealginated seaweeds, a waste product from the alginate industry has been used as a bio adsorber to remove potentially toxic elements from mine waters during the BIOMAN project. An absorption of 1.5% to 2.4% Zn (w/w) has been achieved. The dealginated sea weed required removal of the fines and then acidification to neutralise the very alkaline solution it produces prior to drying to produce granules of the delignated sea weed, which can be used in treatment plants.

During intracellular accumulation, the metal ions may be conveyed across the cell membrane and accumulate within the cell. A number of factors such as external pH, the presence of other metal ions and the presence of organic matter that complexes the metals, can affect the intra cellular accumulation of metal ions. The accumulated metals may precipitate or bind to cell organelles and diminish the toxicity of the metals to the living cell. Algal biomass is especially effective in this respect Many eukaryotic microorganisms and specially yeasts are also suitable.

The MERE SAFIN process (Metal Removal by Sand filter inoculation) utilises bacteria capable of bio-sorbing or bio precipitating by growing them as films on sand as a supporting medium. The biofilm adsorbs the metals during contact with the water. Metal loaded biomass is removed from the supporting material by a sand filter uplift. This is subject to settling/thickening and the thickened sludge put through a filter press. The filter cake has about 70% water and 10%, metals w/w (dry basis). The cake can be burnt in a shaft furnace. A rotating bio contactor containing biofilm of Pseudomonas has been used for removal of free and metal complexed cyanide and thiocyanate and for removal of heavy metals by bio sorption in an aerobic process.

Escherischia coli and Schizophyllum commune, a bacterial fungal mixture has been successfully used to adsorb uranium. Mercury contaminated waters have been remediated by using several strains of bacteria.

2. Organic matter decomposition

Organic matter decomposition for organic matter decontamination through phyto remediation, uptake by plants is the first crucial step. The Octanol water partition coefficient of organic contaminants (log Kow which is a measure of the lipophilicity and hydrophilicity of a substance in a two-phase system of n-octanol and water and used to estimate the environmental fate of a substance) is very important in permitting uptake. Log Kow values below 1 indicate highly soluble contaminants which will not be accumulated in the root zone while the contaminants with a log Kow of 3.5 or more show high sorption to the roots but slow or no translocation to the stems and leaves. Plants, however take up contaminants with a log Kow of between 0.5 to 3.5, as well as weak electrolytes. Plant endophyte relationships and the microbial communities play a key role in degrading the hazardous contaminants in rhizospheres to various extent. Plant uptake is frequently associated with the inhibition of plant growth and an increasing tendency to oxidant stress. Phenols, Benzene, Chlorophenol, Trichloro ethene and Toluene are some organic compounds with a log Kow of less than 3.5.

Mycor rhizosphere bacteria and fungi may play a crucial role in establishing plants in degraded systems. Within the rhizosphere, microbial degradative activities prevail in order to extract energy and carbon from the contaminants for microbial cell growth.   Bio remediation is the most ecofriendly and economically viable amongst all available methods of sludge management. TERI has developed a consortium of bacteria which could degrade oil sludge in soil and in laboratories. This is called the Oil Zapper.

3. Pesticides

Pesticides In view of the wide range of catabolic reactions mediated by bacterial enzymes, the capacity of bacteria and fungi to degrade Xenobiotics (substances present but not naturally produced in cells) is impressive. Pesticides can be decontaminated through Chemical treatment and incineration (both are very expensive processes), Landfills (which may not be a permanent solution) and through bio remediation (which is a low-cost feasible option). HCH degradation by Sphingomonas paucimibilis and endosulfan degrading microbes have been reported. Organo phosphorous pesticides, a highly toxic group, have been observed to be degraded by naturally occurring soil bacteria Klebsiella oxytoca, Bacillus spp., Pandoraea sp. and Micrococcus sp. are the bacteria reported to degrade endosulfan in solution and soils. Many fungi like Aspergillus niger, A. Terrens, Cladosporium oxysporum, Mucor thermo-hyalospora, Fusarium ventricosum, Phanerochaete chrysosporium, Trichoderma harzianum and algae such as Anabaena sp. Chlorococcum sp. and Scenedesmus sp. can also degrade endosulfan.

4. Dense Nonaqueous Phase Liquids -DNAPL

At remediation sites for chlorinated ethenes, hydrogen producing substrates are frequently used for bio-stimulation. The addition of electron donors and nutrients encourage cell growth thereby increasing the number of microorganisms to degrade contaminants. Substrates provide a carbon source for microbial growth and electron donors for dichlorination. Electron donors can be slow release like oils (Olive oil, soybean oil, vegetable oil) and commercially produced Hydrogen release compound or high release compounds such as Lactic Acid, Molasses and Lactate etc.

The best conditions to support reductive dechlorination are methanogenic conditions Although PCE and TCE degradation occur within most anaerobic conditions, DCE and VC dechlorination occur almost only under sulphate reducing and methanogenic conditions.

Reductive dichlorination through bio stimulation may indicate a lack of reductive de-chlorinators and the site may require bio augmentation.

Dehalo coccoides, Dehalobacter, Sulfuro spirillum, De sulfuromonas, De sulfito bacterium and Clostridum spcies have all shown the capability of reductive chlorination of chlorethenes. Dehalococcoides species are the only isolates capable of complete de-chlorination. To enhance reductive dechlorination through bio augmentation, mixed cultures containing strains of Dehalococcoides are usually used. Although the presence of Dehalo coccoides is generally essential for reductive dechlorination, not all dehalococcoides have the capability for complete dechlorination of highly chlorinated ethenes. For bio augmentations a consortium of microorganisms (not only Dehalococcoides) is used to promote complete reductive dechlorination.

Bio remediation may enhance the DNAPL dissolution rates which would result in the reduction in longevity of the source zone by facilitating mass transfers from the NAPL to the aqueous phase and making more contaminants available for bioremediation. Enhanced dissolution occurs when reductive dechlorination occurs close to the DNAPL interface. Dissolution increases many times in biotic systems as compared to abiotic systems (03 to 04 times) Enhanced dissolution rates increase the amounts of contaminants that are dissolved in the aqueous phase and is therefore bio available to be reduced by micro-organisms. Bio remediation also enhances the solubilisation of NAPL contaminants.

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Also See:

1. Remediation of Contaminated Sites- Permeable Reactive Barriers
2. Remediation of Contaminated Sites-The Pump and Treat System
3. Remediation of Contaminated Sites
4. Soil Vapour Extraction and Air Sparging