Just another WordPress.com site

Bioremediation

INTRODUCTION

The past decade has shown, in greater or lesser degree, our carelessness and negligence in using our natural resources. The problems associated with contamination of natural resources are prominently increasing in many countries. Contaminated environment generally result from production, use, and disposal of hazardous substances from industrial activities. The problem is worldwide, and the estimated number of contaminated sites is significant. It is now widely recognized that contaminated environment is a potential threat to human health, and its continual discovery over recent years has led to international efforts to remedy many of these sites, to enable the site to be redeveloped for use.

To “bioremediate”, means to use living things to eliminate environmental contamination such as contaminated soil or groundwater. Some microorganisms that live in soil and groundwater naturally eat certain chemicals that are harmful to people and the environment. The microorganisms are able to change these chemicals into water and harmless gases, such as carbon dioxide. Plants can also be used to clean up soil, water or air; this is called phytoremediation

Bioremediation is an option that offers the possibility to destroy or render harmless various con­taminants using natural biological activity. As such, it uses relatively low-cost, low-technology tech­niques, which generally have a high public acceptance and can often be carried out on site. It will not always be suitable, however, as the range of contaminants on which it is effective is limited, the time scales involved are relatively long, and the residual contaminant levels achievable may not always be appropriate. Although the methodologies employed are not technically complex, considerable experi­ence and expertise may be required to design and implement a successful bioremediation program, due to the need to thoroughly assess a site for suitability and to optimize conditions to achieve a satisfacto­ry result.

Bioremediation has been used at a number of sites worldwide… Here, we intended to assist by providing a straightforward, pragmatic view of the processes involved in bioremediation, the pros and cons of the technique, and the issues to be considered when dealing with a proposal for bioremediation.

HISTORY

Bioremediation has been described as “a treatability technology that uses biological activity to reduce the concentration or toxicity of a pollutant. It commonly uses processes by which microorganisms transform or degrade chemicals in the environment” (King 1). This use of microorganisms (mainly bacteria) to destroy or transform hazardous contaminants is not a new idea. Microorganisms have been used since 600 B.C. by the Romans and others to treat their wastewater. Although this same technology is still usedtoday to treat wastewater it has been expanded to treat an array of other contaminants. In

fact, bioremediation has been used commercially for almost 30 years. The first commercial use of a bioremediation system was in 1972 to clean up a Sun Oil pipeline spill in Ambler, Pennsylvania

CONVENTIONAL STRATEGIES OF REMEDIATION

The conventional techniques used for remediation have been to dig up contaminated soil and remove it to a landfill, or to cap and contain the contaminated areas of a site. The methods have some drawbacks. The first method simply moves the contamination elsewhere and may create significant risks in the excavation, handling, and transport of hazardous material.

Additionally, it is very difficult and increasingly expensive to find new landfill sites for the final disposal of the material. The cap and contain method is only an temporary solution since the contamination remains on site, requiring monitor­ing and maintenance of the isolation barriers long into the future, with all the associated costs and potential liability.

A better approach than these traditional methods is to completely destroy the pollutants if possi­ble, or at least to transform them to innocuous substances. Some technologies that have been used are high-temperature incineration and various types of chemical decomposition (e.g., base-catalyzed dechlorination, UV oxidation).

They can be very effective at reducing levels of a range of contaminants, but have several drawbacks, principally their technological complexity, the cost for small-scale appli­cation, and the lack of public acceptance, especially for incineration that may increase the exposure to contaminants for both the workers at the site and nearby residents.

Conventional ways of Bioremediation

Dig up and remove it to a landfill

clip_image001

Better approaches:

  • Destroy them completely, if possible
  • Transform them in to harmless substances

Drawbacks

  • Technological complexity
  • The cost for small scale application – expensive
  • Lack of public acceptance – especially in incineration
  • Incineration generates more toxic compounds
  • Materials released from imperfect incineration – cause undesirable imbalance in the atmosphere. Ex. Ozone depletion
  • Fall back on earth and pollute some other environment
  • Dioxin production due to burning of plastics – leads to cancer
  • May increase the exposure to contaminants, for both workers and nearby residents

PRINCIPLES OF BIOREMEDIATION

clip_image003

Figure 1: Bioremediation Triangle

There are three essential components needed for bioremediation. These three components are microorganisms, food, and nutrients. These three main components shown in Figure 1 are known as the bioremediation triangle. Microorganisms are found almost everywhere on earth with the exception of active volcanoes. So a lack of food and nutrients are usually the missing ingredients that prevent successful bioremediation. Microorganisms find the food they eat in the soil or water where they live. However, if a contaminant is present it can become an additional food source for the microorganisms. The contaminant serves two useful purposes for the microbes. First, the contaminant provides a source of carbon needed for growth. Second,the microbes obtain energy by breaking chemical bonds and transferring electrons away from the contaminant. This is known as an oxidation-reduction reaction. The contaminant that loses electrons is oxidized and the chemical that gains the electrons(electron acceptor) is reduced. The energy gained from the electron transfer is used along with the carbon and some electrons to produce more cells. Microbes generally use oxygenas an electron acceptor but nitrate, sulfate, iron, and CO2 are also commonly used. The use of oxygen as an electron acceptor is called aerobic respiration. The major byproducts of aerobic respiration are carbon dioxide, water, and an increase in the microbe population. Anaerobic respiration uses nitrate, sulfate, iron, or CO2 as the electron acceptor instead of oxygen. Anaerobic respiration can occur after the oxygen has been depleted by aerobic respiration or where there is not sufficient oxygen in the first place. The process of anaerobic degradation has been ignored for many years. However, recently it has been gaining more attention;

There are also several nutrients that must be accessible to the microorganisms for bioremediation to be successful. These include moisture, nitrogen, phosphorus, and other trace elements. Microorganisms like other organisms need moisture to survive and grow.In addition, microbes depend on the moisture to transport food to them since they do not have mouths. The optimal moisture content for microbes in the vadose zone has been determined to be between 10 and 25% (King 16). Besides moisture, nitrogen (ammonia)and phosphorus (orthophosphate) are two major nutrients needed for the microorganisms. The microorganisms also require minor elements such as sulfur, potassium, magnesium,calcium, manganese, iron, cobalt, copper, nickel, and zinc (King 19). However, these minor elements are usually available in the environment in sufficient amounts where nitrogen and phosphorus may be lacking and need to be added. There are many contaminants susceptible to bioremediation. Petroleum hydrocarbons, in particular, benzene, toluene, ethylbenzene, and xylene (BTEX), the major components of gasoline, have been biodegraded using this technology. In addition, alcohols, ketones, and esters are well established as being biodegradable by microorganisms. Many other contaminants are emerging as treatable using bioremediation such as halogenated aliphatics, halogenated aromatics, polychlorinated biphenyls, and nitroaromatics.

FACTORS AFFECTING BIOREMEDIATION

The factors affecting bioremediation can be divided into following categories.

1. Microbial factors

2. Environmental factors

Microbial Factors

Microorganisms can be isolated from almost any environmental conditions. Microbes will adapt and grow at subzero temperatures, as well as extreme heat, desert conditions, in water, with an excess of oxygen, and in anaerobic conditions, with the presence of hazardous compounds or on any waste stream. The main requirements are an energy source and a carbon source. Because of the adaptability of microbes and other biological systems, these can be used to degrade or remediate environmental hazards. We can subdivide these microorganisms into the following groups

1. Aerobic

2. Anaerobic

3. Ligninolytic Fungi

4. Methylotrophs

Aerobic

These microbes have often been reported to degrade pesticides and hydrocarbons, both alkanes and polyaromatic compounds. Many of these bacteria use the contaminant as the sole source of carbon and energy.

Examples of aerobic bacteria recognized for their degradative abilities are Pseudomonas, Alcaligenes, Sphingomonas, Rhodococcus, and Mycobacterium.

Anaerobic

Anaerobic bacteria are not as frequently used as aerobic bacteria. There is an increasing interest in anaerobic bacteria used for bioremediation of polychlorinated biphenyls (PCBs) in river sediments, dechlorination of the solvent trichloroethylene (TCE), and

chloroform.

Ligninolytic fungi

Fungi such as the white rot fungus Phanaerochaete chrysosporium have the

ability to degrade an extremely diverse range of persistent or toxic environmental pollutants. Common substrates used include straw, saw dust, or corn cobs.

Methylotrophs

Aerobic bacteria that grow utilizing methane for carbon and energy. The initial enzyme in the pathway for aerobic degradation, methane monooxygenase, has a broad substrate range and is active against a wide range of compounds, including the chlorinated aliphatics trichloroethylene and 1,2-dichloroethane.

For degradation it is necessary that bacteria and the contaminants be in contact. This is not easily achieved, as neither the microbes nor contaminants are uniformly spread in the soil. Some bacteria are mobile and exhibit a chemotactic response, sensing the contaminant and moving toward it. Other microbes such as fungi grow in a filamentous form toward the contaminant. It is possible to enhance the mobilization of the contaminant utilizing some surfactants such as sodium dodecyl sulphate (SDS)

Microbes are used to degrade gasoline, the most common contaminant of groundwater in the United States. Adding powdered seaweed to DDT-contaminated soil boosts the cleaning activity of DDT-eating microbes. In one test site, 80% of the DDT was removed after six weeks. Microbes and fungi are used in air filters to control odours from sewage treatment plants and in the paint industry. A gene for a protein found in rat livers that binds with toxic metals has been inserted in both tobacco plants and algae. With this gene, the tobacco plant and the algae are able to extract several hundred times more toxic metal compounds from soil or water compared to plants without the gene. One particular microbe degrades polycyclic aromatic hydrocarbons (PAH’s), which are cancer-causing petroleum by-products. The microbes, called simply “sulfate-reducers”, are able to attack PAH’s in the sediment of Boston Harbor where scientists thought the contaminant could not be treated due to lack of oxygen.

Examples of microbes used for bioremediation include:

· Deinococcus radiodurans bacteria have been genetically modified to digest solvents and heavy metals, as well as toluene and ionic mercury from highly radioactive nuclear waste.

· Geobacter sufurreducens bacteria can turn uranium dissolved in groundwater into a non-soluble, collectable form.

· Dehalococcoides ethenogenes bacteria are being used in ten states to clean up chlorinated solvents that have been linked to cancer. The bacteria are naturally found in both soil and water and are able to digest the solvents much faster than using traditional clean-up methods.

· Thermus brockianus, found in Yellowstone National Park, produces an enzyme that breaks down hydrogen peroxide 80,000 times faster than current chemicals in use.

· Alcaligenes eutrophus, naturally degrades 2,4-D, the third most widely used herbicide in the U.S.

Some contaminants potentially suitable for bioremediation.

Class of contaminants

Specific examples

Aerobic

Anaerobic

Potential sources

Chlorinated solvents

Trichloroethylene

 

+

Drycleaners

 

Perchloroethylene

   

Chemical manufacture

Polychlorinated biphenyls

4-Chlorobiphenyl

 

+

Electrical manufacturing

 

4,4

Dichlorobiphenyl

   

Power station

       

Railway yards

Chlorinated phenol

Pentachlorophenol

 

+

Timber treatment

       

Landfills

“BTEX”

Benzene

+

+

Oil production and storage

 

Toluene

   

Gas work sites

 

Ethylbenzene

   

Airports

 

Xylene

   

Paint manufacture

       

Port facilities

       

Railway yards

       

Chemical manufacture

Polyaromatic hydrocarbons

Naphthalene

+

 

Oil production and storage

(PAHs)

Antracene

   

Gas work sites

 

Fluorene

   

Coke plants

 

Pyrene

   

Engine works

 

Benzo(a)pyrene

   

Landfills

       

Tar production and storage

       

Boiler ash dump sites

       

Power stations

Pesticides

Atrazine

+

+

Agriculture

 

Carbaryl

   

Timber treatment

 

Carbofuran

   

Pesticide manufacture

 

Coumphos

   

Recreational areas

         
ENVIRONMENTAL FACTORS
1. Nutrients

Although the microorganisms are present in contaminated soil, they cannot necessarily be there in the numbers required for bioremediation of the site. Their growth and activity must be stimulated. Biostimulation usually involves the addition of nutrients and oxygen to help indigenous microorgan­isms. These nutrients are the basic building blocks of life and allow microbes to create the necessary enzymes to break down the contaminants. All of them will need nitrogen, phosphorous, and carbon (e.g., see Table below).

Element

Percentage

Element

Percentage

Carbon

50

Sodium

1

Nitrogen Oxygen Hydrogen Phosphorous Sulphur Potassium

14

20

8

3

1

1

Calcium Magnesium Chloride

Iron

All others

0.5

0.5

0.5

0.2

0.3

2. Composition of a microbial cell

Carbon is the most basic element of living forms and is needed in greater quantities than other elements. In addition to hydrogen, oxygen, and nitrogen it constitutes about 95% of the weight of cells.Phosphorous and sulphur contribute with 70% of the remainders. The nutritional requirement of carbon to nitrogen ratio is 10:1, and carbon to phosphorous is 30:1.

3. Environmental requirements

Optimum environmental conditions for the degradation of contaminants are reported in Table below:

Parameters

Condition required for microbial activity

Optimum value for an oil degradation

Soil moisture

25–28% of water holding capacity

30–90%

Soil pH

5.5–8.8

6.5–8.0

Oxygen content

Aerobic, minimum air-filled pore space of 10%

10–40%

Nutrient content

N and p for microbial growth

C:N:P = 100:10:1

Temperature (°C)

15–45

20–30

Contaminants

Not too toxic

Hydrocarbon 5–10% of dry weight of soil

Heavy metals

Total content 2000 ppm

700 ppm

Type of soil

Low clay or silt content

 

4. Environmental conditions affecting degradation

Microbial growth and activity are readily affected by pH, temperature, and moisture. Although microorganisms have been also isolated in extreme conditions, most of them grow optimally over a nar­row range, so that it is important to achieve optimal conditions.

If the soil has too much acid it is possible to rinse the pH by adding lime. Temperature affects bio­chemical reactions rates, and the rates of many of them double for each 10 °C rise in temperature. Above a certain temperature, however, the cells die. Plastic covering can be used to enhance solar warming in late spring, summer, and autumn. Available water is essential for all the living organisms, and irrigation is needed to achieve the optimal moisture level. The amount of available oxygen will determine whether the system is aerobic or anaerobic. Hydrocarbons are readily degraded under aerobic conditions, whereas chlorurate compounds are degraded only in anaerobic ones. To increase the oxygen amount in the soil it is possible to till or sparge air. In some cases, hydrogen peroxide or magnesium peroxide can be introduced in the environment. Soil structure controls the effective delivery of air, water, and nutrients. To improve soil structure, materials such as gypsum or organic matter can be applied. Low soil permeability can impede move­ment of water, nutrients, and oxygen; hence, soils with low permeability may not be appropriate for in situ clean-up techniques.

STRATEGIES AND TECHNIQUES INVOLVED IN BIOREMEDIATION

Basically two types of techniques are involved in Bioremediation

· In situ Bioremediation (at the site)

· Ex situ Bioremediation (away from the site)

In situ Bioremediation

In situ techniques are defined as those that are applied to soil and groundwater at the site with minimal disturbance. These techniques are generally the most desirable options due to lower cost and fewer disturbances since they provide the treatment in place avoiding excavation and transport of contaminants. In situ treatment is limited by the depth of the soil that can be effectively treated. In many soils effective oxygen diffusion for desirable rates of bioremediation extend to a range of only a few centimetres to about 30 cm into the soil, although depths of 60 cm and greater have been effectively treated in some cases.

In situ Bioremediation types:

Bioventing is the most common in situ treatment and involves supplying air and nutrients through wells to contaminated soil to stimulate the indigenous bacteria. Bioventing employs low air flow rates and provides only the amount of oxygen necessary for the biodegradation while minimizing volatiliza­tion and release of contaminants to the atmosphere. It works for simple hydrocarbons and can be used where the contamination is deep under the surface.

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 can be used for soil and groundwater. Generally, this technique includes conditions such as the infil­tration of water-containing nutrients and oxygen or other electron acceptors for groundwater treatment.

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. Biosparging increases the mixing in the saturated zone and there­by increases the contact between soil and groundwater. The ease and low cost of installing small-diam­eter air injection points allows considerable flexibility in the design and construction of the system

Bioaugmentation. Bioremediation frequently involves the addition of microorganisms indigenous or exogenous to the contaminated sites. Two factors limit the use of added microbial cultures in a land treatment unit: 1) nonindigenous cultures rarely compete well enough with an indigenous population to develop and sustain useful population levels and 2) most soils with long-term exposure to biodegrad­able waste have indigenous microorganisms that are effective degrades if the land treatment unit is well managed.

Ex situ bioremediation

Ex situ techniques are those that are applied to soil and groundwater at the site which has been removed from the site via excavation (soil) or pumping (water). These techniques involve the excavation or removal of contaminated soil from ground.

Ex situ Bioremediation types:

These techniques involve the excavation or removal of contaminated soil from ground.

Landfarming 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. In general, the practice is limited to the treatment of superficial 10–35 cm of soil. Since landfarming has the potential to reduce monitoring and maintenance costs, as well as clean-up liabilities, it has received much atten­tion as a disposal alternative.

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 landfarming 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 landfarming that tend to control physical losses of the contaminants by leaching and volatilization. Biopiles provide a favorable environment for indigenous aerobic and anaerobic microorganisms.

Bioreactors Slurry reactors or aqueous reactors are used for ex situ treatment of contaminated soil and water pumped up from a contaminated plume. Bioremediation in reactors involves the pro­cessing of contaminated solid material (soil, sediment, sludge) or water through an engineered con­tainment system. A slurry bioreactor may be defined as a containment vessel and apparatus used to cre­ate a three-phase (solid, liquid, and gas) mixing condition to increase the bioremediation rate of soil-bound and water-soluble pollutants as a water slurry of the contaminated soil and biomass (usually indigenous microorganisms) capable of degrading target contaminants. In general, the rate and extent of biodegradation are greater in a bioreactor system than in situ or in solid-phase systems because the contained environment is more manageable and hence more controllable and predictable. Despite the advantages of reactor systems, there are some disadvantages. The contaminated soil requires pre-treatment (e.g., excavation) or alternatively the contaminant can be stripped from the soil via soil washing or physical extraction (e.g., vacuum extraction) before being placed in a bioreactor.

Monitoring bioremediation

The process of bioremediation can be monitored indirectly by measuring the Oxidation Reduction Potential or redox in soil and groundwater, together with pH, temperature, oxygen content, electron acceptor/donor concentrations, and concentration of breakdown products (e.g. carbon dioxide). This table shows the (decreasing) biological breakdown rate as function of the redox potential.

Process

Reaction

Redox potential (Eh in mV)

Aerobic:

O2 + 4e + 4H+ → 2H2O

600 ~ 400

Anaerobic:

   

Denitrification

2NO3 + 10e + 12H+ → N2 + 6H2O

500 ~ 200

Manganese IV reduction

  MnO2 + 2e + 4H+ → Mn2+ + 2H2O   

400 ~ 200

Iron III reduction

Fe(OH)3 + e + 3H+ → Fe2+ + 3H2O

300 ~ 100

Sulfate reduction

SO42− + 8e +10 H+ → H2S + 4H2O

0 ~ −150

Fermentation

2CH2O → CO2 + CH4

−150 ~ −220

Types of Bioremediation

Bioremediation techniques can be subdivided into various based on following factors

Based on type of atmosphere in which Bioremediation takes place it can be divided into two types

A) Engineered Bioremediation

B) Intrinsic Bioremediation

Based on Type of organism being used for Bioremediation

C) Mycoremediation

D) Phytoremediation

ENGINEERED BIOREMEDIATION

Factors effecting engineered bioremediation

· Contact between the microbes and the substrate

· Proper physical environment

· Nutrients

· Oxygen

· Absence of toxic compounds

Sources of microorganisms

· From contaminated field sites(with varying environmental conditions subzero temperatures or extreme heat, desert conditions or in water, with excess of oxygen or in anaerobic conditions, with presence of hazardous compounds or on any waste stream)

· From culture collections

· Genetically Engineered Microorganisms (GEMs)

Electro kinetically enhanced bioremediation (EEB) is a method of engineered bioremediation of soil contaminated by such organic compounds as solvents and petroleum products. As depicted schematically in the figure, EEB involves the utilization of controlled flows of liquids and gases into and out of the ground via wells, in conjunction with electrokinetic transport of matter through pores in the soil, to provide reagents and nutrients that enhance the natural degradation of contaminants by indigenous and/or introduced microorganisms.

The operational parameters of an EEB setup can be tailored to obtain the desired flows of reagents and nutrients in variably textured and layered soils of variable hydraulic permeability and of moisture content that can range from saturation down to as little as about 7 percent. A major attractive feature of EEB is the ability to control the movements of charged anionic and cationic as well as noncharged chemical species.

The basic components of electrokinetic enhancement of bioremediation are the following:

* Ions are transported by electromigration; that is, with minimum transport of liquid through the soil. The ions of interest include nutrient agents, electron donors (e.g., lactate) or electron acceptors (e.g., nitrate or sulfate) added to the soil. Electromigration is utilized as an efficient mode of electrokinetic transport in vadosezone soils.

* Water in soil is pumped (horizontally or vertically, depending on the positions of electrode wells) by induced electro-osmotic flow. Whereas the hydraulic flow used in older methods decreases with decreasing pore size and is thus not effective for treating tightly packed soil, electro-osmotic flow is less restricted by tight packing. Electro-osmosis is utilized to enhance the transport of both ions and such noncharged particles as micro-organisms, by moving water from anodes (positive electrodes) toward cathodes (negative electrodes).

* Electrophoresis induced in soil under an applied electric field is used to control the transport and/or distribution of micro-organisms throughout the treated soil volume. The beneficial effect of electrophoresis can be augmented or otherwise modified by use of electro-osmotic flushing of the soil.

* The applied electric current can be utilized to heat the soil to the optimum temperature for bioremediation.

* The gaseous and liquid products of electrolysis of water in the soil are removed from electrode wells and mixed and reinjected into the ground as needed to maintain the pH of the soil within a range favorable for bioremediation.

Disadvantages

Mostly GEMs do not work the way we expect:

  1. Lab strains become food source for soil protozoa
  2. Inability of GEMs to contact the compounds to be degraded
  3. Failure of GEMs to survive/compete indigenous microorganisms. Mostly due to lack / decreased activity of House Keeping Genes.

INTRINSIC BIOREMEDIATION

It is a natural attenuation process that leads to the decrease in contaminant levels in a particular environment due to unmanaged physical, chemical and biological processes.

Conversion of environmental pollutants into the harmless forms through the innate capabilities of naturally occurring microbial population is called intrinsic bioremediation. However, there is increasing interest on intrinsic bioremediation for control of all or some of the contamination at waste sites. The intrinsic i.e. inherent capacity of microorganism, to metabolize the contaminants should be tested at laboratory and field levels before use for intrinsic bioremediation. Through site monitoring programmes progress of intrinsic bioremediation should be recorded time to time. The conditions of site that favours intrinsic bioremediation are ground water flow throughout the year, carbonate minerals to buffer acidity produced during biodegradation supply of electron acceptors and nutrients for microbial growth and absence of toxic compounds. The other environmental factors such as pH concentration, temperature and nutrient availability determine whether or not biotransformation takes place. Bioremediation of waste mixtures containing metals such as Hg, Pb, As and cyanide at toxic concentration can create problem (Madsen, l99l).
The ability of surface bacteria to degrade a given mixture of pollutants in ground water is dependent on the type and concentration of compounds, electron acceptor and duration of bacteria exposed to contaminants. Therefore, ability of indigenous bacteria degrading contaminants can be determined in laboratory by plate count and macrocosm studies

Example: Microbes in Hudson River mud developed an ability to partially degrade PCB (Poly Chlorinated Biphenyls)

 

Process occurs in two steps

 

Partial dehalogenation of PCBs occurs naturally under anaerobic conditions

clip_image004

Less chlorinated residues

clip_image005

Then mud is aerated to promote the complete degradation

of these less chlorinated residues

MYCOREMEDIATION

Mycoremediation is a form of bioremediation, the process of using fungi to return an environment (usually soil) contaminated by pollutants to a less contaminated state. The term Mycoremediation was coined by Paul Stamets and refers specifically to the use of fungal mycelia in bioremediation.

One of the primary roles of fungi in the ecosystem is decomposition, which is performed by the mycelium. The mycelium secretes extracellular enzymes and acids that break down lignin and cellulose, the two main building blocks of plant fiber. These are organic compounds composed of long chains of carbon and hydrogen, structurally similar to many organic pollutants. The key to Mycoremediation is determining the right fungal species to target a specific pollutant. Certain strains have been reported to successfully degrade the nerve gases VX and sarin.

In an experiment conducted in conjunction with Thomas, a major contributor in the bioremediation industry, a plot of soil contaminated with diesel oil was inoculated with mycelia of oyster mushrooms; traditional bioremediation techniques (bacteria) were used on control plots. After four weeks, more than 95% of many of the PAH (polycyclic aromatic hydrocarbons) had been reduced to non-toxic components in the mycelial-inoculated plots. It appears that the natural microbial community participates with the fungi to break down contaminants, eventually into carbon dioxide and water. Wood-degrading fungi are particularly effective in breaking down aromatic pollutants (toxic components of petroleum), as well as chlorinated compounds.

Mycofiltration is a similar or same process, using fungal mycelia to filter toxic waste and microorganisms from water in soil

PHYTOREMEDIATION

Although the application of microbe biotechnology has been successful with petroleum-based con­stituents, microbial digestion has met limited success for widespread residual organic and metals pol­lutants. Vegetation- based remediation shows potential for accumulating, immobilizing, and transform­ing a low level of persistent contaminants. In natural ecosystems, plants act as filters and metabolize substances generated by nature. Phytoremediation is an emerging technology that uses plants to remove contaminants from soil and water [14–16]. The term “phytoremediation” is relatively new, coined in 1991. Its potential for encouraging the biodegradation of organic contaminants requires further research, although it may be a promising area for the future.

clip_image007

Phytoextraction or phytoaccumulation is the process used by the plants to accumulate contami­nants into the roots and aboveground shoots or leaves. This technique saves tremendous remediation cost by accumulating low levels of contaminants from a widespread area. Unlike the degradation mech­anisms, this process produces a mass of plants and contaminants (usually metals) that can be transport­ed for disposal or recycling.

Phytotransformation or phytodegradation refers to the uptake of organic contaminants from soil, sediments, or water and, subsequently, their transformation to more stable, less toxic, or less mobile form. Metal chromium can be reduced from hexavalent to trivalent chromium, which is a less mobile and noncarcinogenic form.

Phytostabilization is a technique in which plants reduce the mobility and migration of contami­nated soil. Leachable constituents are adsorbed and bound into the plant structure so that they form a stable mass of plant from which the contaminants will not reenter the environment.

Phytodegradation or rhizodegradation is the breakdown of contaminants through the activity existing in the rhizosphere. This activity is due to the presence of proteins and enzymes produced by the plants or by soil organisms such as bacteria, yeast, and fungi. Rhizodegradation is a symbiotic rela­tionship that has evolved between plants and microbes. Plants provide nutrients necessary for the microbes to thrive, while microbes provide a healthier soil environment.

Rhizofiltration is a water remediation technique that involves the uptake of contaminants by plant roots. Rhizofiltration is used to reduce contamination in natural wetlands and estuary areas. In Table 5, we can see an overview of phytoremediation applications.

Applications

A significant amount of work has been conducted to examine the ability of plants to remediate heavy metal contaminated soils. Plants are often capable of the uptake and storage of significant concentrations of some heavy metals and other compounds in their roots shoots and leaves, referred to as phytoextraction. The plants are then harvested and disposed of in an approved manner, such as in a hazardous waste landfill. This technique results in up to a 95% reduction in waste volume over the equivalent concentration of contaminated soil. The plants that are capable of this type of remediation are referred to as hyperaccumulators. Types of plants that appear promising for this form of remediation include the mustard plant, alpine pennycress, broccoli and cabbage.

Phytotransformation occurs when plants transform organic contaminants into less toxic, less mobile or more stable form. This process includes phytodegradation, which is the metabolism of the organic contaminant by the plant enzymes and phytovolatilization, which is the volatilization of organic contaminants as they pass through the plant leaves. The release of these pollutants into the air results in the exchange of one form of pollution for another.

Phytostabilization immobilizes the contaminants and reduces their migration through the soil by absorbing and binding leachable constituents to the plant structure. This process effectively reduces the bioavailability of the harmful contaminants. Almost any vegetation present at contaminated sites will contribute to phytostabilization.

At the soil-root interface, known as the rhizosphere, there is a very large and very active microbial population. Often the plant and microbial populations provide needed organic and inorganic compounds for one another. The rhizosphere environment is high in microbial abundance and rich in microbial metabolic activity, which has the potential to enhance the rate of biodegradation of contaminants by the microorganisms. Generally, the plant is not directly involved in the biodegradation process. It serves as a catalyst for increasing microbial growth and activity, which subsequently increases the biodegradation potential. However, the

rhizosphere can be limited in its remediation potential because it does not extend far from the root. This process is often referred to as phytostimulation or plant-assisted bioremediation.

Phytoremediation Designs

clip_image008clip_image010

Contaminants and Contamination Levels

Phytoremediation is effective in the remediation of all types of soil contaminants, however it is more effective on lower concentrations of contaminants. The previous page listed the metals. Some of the organic compounds are:

· Anthracene

· Atrazine

· TCE

· Pyrene

· Toluene

· Phenol

· Other contaminants from TNT, pesticides, chlorinated solvents, and fuel/oil

Types of Vegetation Used

· Some of the plants used in phytoremediation are:

· Alfalfa

· Hybrid Poplar Trees

· Blue-green Algae

· Duck Weed

· Arrowroot

· Sudan Grass

· Rye Grass

· Bermuda Grass

· Alpine Bluegrass

· Yellow or White Water Lillies

Pros and Cons of Phytoremediation

Advantages:

· Phytoremediation is cost effective

· It is suited to remediation of large areas of soil

· It is environmentally friendly

· Phytoremediation sites are more aesthetically pleasing

· Phytoremediation sites are low maintenance

· It involves no noisy and expensive equipment

Disadvantages

· Not as effective for sites with high contaminant concentrations

· Phytoremediation is slower than conventional methods

· It does not work through the winter (Seasonally effective)

EXAMPLE OF A BIOREMEDIATION SYTEM

Ground Water Bioremediation

There are two main types of bioremediation systems for treating ground water: water circulation and air injection systems. Water circulation systems work by circulating water that contains nutrients and other substances needed to help the microorganisms grow between the injection and recovery wells. Generally as much of the free product as possible is removed before this process begins. This system injects nutrients such as nitrogen and phosphorus and an electron acceptor (often hydrogen peroxide, H2O2) into the contaminated soil and ground water. The microorganisms biodegrade the contaminants and then the water is removed using a recovery well. The recovered water is then treated with an air stripper to remove any remaining volatile contaminants. In addition, this method has the option of providing an additional above ground treatment facility. The recovered water can be injected into the system again or it can be placed somewhere else and uncontaminated water can be used for injection. This type of system

is shown in Figure

clip_image011

Figure: Water Circulation System

Air injection systems or air sparging is another way to treat contaminated ground water.

One of the greatest advantages of this ground water treatment technique is that water does not have to be pumped. This process involves the injection of air directly into the ground water below the contaminant plume. The air displaces the water in the ground providing the microorganisms with an electron acceptor needed for bioremediation. The air also helps to remove the volatile contaminants that can be captured by using a soil vapour recovery system. If nutrients or water are not present in sufficient quantities they can be provided using an injection well. This system works because air movement helps to mix and distribute the nutrients to the microorganisms as shown in Figure 3.

clip_image012

Figure : Air Injection System

BIOREMEDIATION AT HOME

Look for household, carpet, bathroom or drain cleaners with “enzyme action”. Those enzymes are bacterial enzymes, made by friendly bacteria in those products. Once the pollutant in your carpet or bathroom surface is gone, the bacteria run out of food, and die off. Some plants are known for their ability to improve air quality by absorbing indoor air pollutants such as formaldehyde, benzene and trichloroethylene. Formaldehyde is found in tobacco smoke and burning wood, and emitted by curtains, carpets, furniture, glues and household cleaning products. Benzene is a common solvent found in glues, oils, furniture wax, detergents and paints. Trichloroethylene is found in paints, adhesives, inks and varnishes.

Some of the best houseplants for filtering indoor air (and the pollutants they filter) include:

Green spider plant – Chlorophytum comosum (formaldehyde)

Mother-in-law’s tongue or snake plant – Sansevieria trifasciata (formaldehyde and benzene)

Chrysanthemums – Chrysanthemum sp. (benzene)

Gerbera daisies – Gerbera sp. (benzene and trichloroethylene)

Varieties of Philodendron sp.: P. scandens, P. domesticum and P. selloum (formaldehyde)

Varieties of Dracaena sp.: D. fragrans,D. deremensis, D. warneckii and D. marginata (formaldehyde)

English ivy, Hedera helix (benzene)

Golden pothos – Epipiremnum aureum (formaldehyde)

Peace lily Spathiphyllum (formaldehyde, benzene and trichloroethylene)

Aloe vera – Aloe vera (formaldehyde)

For an average home of under 2,000 square feet, it is recommended to use at least fifteen samples of a good variety of these common houseplants to help improve air quality. The plants should also be grown in six-inch containersn or larger.

BIOREMEDIATION – ADVANTAGES & DISADVANTAGES

Advantages

• Bioremediation is a natural process and is therefore perceived by the public as an acceptable waste treatment process for contaminated material such as soil. Microbes able to degrade the contaminant increase in numbers when the contaminant is present; when the contaminant is degraded, the biodegradative population declines. The residues for the treatment are usually harmless products and include carbon dioxide, water, and cell biomass.

• Theoretically, bioremediation is useful for the complete destruction of a wide variety of contaminants. Many compounds that are legally considered to be hazardous can be transformed to harmless products. This eliminates the chance of future liability associated with treatment and disposal of contaminated material.

• Instead of transferring contaminants from one environmental medium to another, for example, from land to water or air, the complete destruction of target pollutants is possible.

• Bioremediation can often be carried out on site, often without causing a major disruption of normal activities. This also eliminates the need to transport quantities of waste off site and the potential threats to human health and the environment that can arise during transportation.

• Bioremediation can prove less expensive than other technologies that are used for clean-up of hazardous waste.

Disadvantages

• Bioremediation is limited to those compounds that are biodegradable. Not all compounds are susceptible to rapid and complete degradation.

• There are some concerns that the products of biodegradation may be more persistent or toxic than the parent compound.

• Biological processes are often highly specific. Important site factors required for success include the presence of metabolically capable microbial populations, suitable environmental growth conditions, and appropriate levels of nutrients and contaminants.

• It is difficult to extrapolate from bench and pilot-scale studies to full-scale field operations.

• Research is needed to develop and engineer bioremediation technologies that are appropriate for sites with complex mixtures of contaminants that are not evenly dispersed in the environment. Contaminants may be present as solids, liquids, and gases.

• Bioremediation often takes longer than other treatment options, such as excavation and removal of soil or incineration.

• Regulatory uncertainty remains regarding acceptable performance criteria for bioremediation. There is no accepted definition of “clean”, evaluating performance of bioremediation is difficult, and there are no acceptable endpoints for bioremediation treatments.

REFERENCES

1. Bioremediation. An overview M. Vidali Dipartimento di Chimica Inorganica, Metallorganica, e Analitica, Università di Padova Via Loredan, 4 35128 Padova, Italy

2. T. Cairney. Contaminated Land, p. 4, Blackie, London (1993).

3. R. B. King, G. M. Long, J. K. Sheldon. Practical Environmental Bioremediation: The Field Guide, 2nd ed., Lewis, Boca Raton, FL (1997).

4. National Research Council. In Situ Bioremediation: When Does It Work?, National Academy Press, Washington, DC (1993).

5. R. D. Norris, R. E. Hinchee, R. Brown, P. L. McCarty, L. Semprini, J. T. Wilson, D. H. Kampbell,

6. M. Reinhard, E. J. Bouwer, P. C. Borden, T. M. Vogel, J. M. Thomas, C. H. Ward. Handbook of Bioremediation. Lewis, Boca Raton, FL (1993).

7. R. E. Hinchee, J. L. Means, D. R. Burrisl. Bioremediation of Inorganics. Battelle Press, Columbus, OH (1995).

8. 6 P. E. Flathman, D. Jerger, J. E. Exner. Bioremediation: Field Experience, Lewis, Boca Raton, FL (1993).

9. J. G. Mueller, C. E. Cerniglia, P. H. Pritchard. Bioremediation of Environments Contaminated by Polycyclic Aromatic Hydrocarbons. In Bioremediation: Principles and Applications, pp. 125–194, Cambridge University Press, Cambridge (1996).

10. P. J. S. Colberg and L. Y. Young. Anaerobic Degradation of Nonhalogenated Homocyclic Aromatic Compounds Coupled with Nitrate, Iron, or Sulfate Reduction. In Microbial Transformation and Degradation of Toxic Organic Chemicals, pp. 307–330, Wiley-Liss, New York (1995).

11. A. S. Allard and A. H. Neilson. Int. Biodeterioration Biodegradation 39, 253–285 (1997).

12. http://www.clu-in.org. Online manual: Technology Practices Manual for Surfactants and Cosolvents, CH2MHILL.

13. U.S. EPA Seminars. Bioremediation of Hazardous Waste Sites: Practical Approach to Implementation, EPA/625/K-96/001.

14. U.S. EPA. Handbook on In Situ Treatment of Hazardous Waste Contaminated Soils, EPA/540/2- 90/002.

15. F. M. von Fahnestock, G. B. Wickramanayake, K. J. Kratzke, W. R. Major. Biopile Design,

16. Operation, and Maintenance Handbook for Treating Hydrocarbon Contaminated Soil, Battelle Press, Columbus, OH (1998).

17. U.S. EPA. Phytoremediation Resource Guide. EPA/542/B-99/003 (1999), available online at http://www.epa.gov/tio.

18. U.S. EPA. Introduction to Phytoremediation. EPA/600/R-99/107 (February 2000).

19. I. Raskin and B. D. Ensley. Phytoremediation of Toxic Metals: Using Plants to Clean Up The Environment, Wiley, New York (2000)

20. David J. Glass, Prospects for use and regulation of transgenic plants in Phytoremediation

Advertisements

Comments on: "Bioremediation" (3)

  1. thank u….hope im going to pass my exam 2morow using the above knowledge

  2. Have you ever considered creating an e-book or guest authoring
    on other sites? I have a blog based upon on
    the same ideas you discuss and would love to have you share some stories/information.
    I know my audience would enjoy your work. If you are even remotely interested, feel free to send me an email.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

Tag Cloud

%d bloggers like this: