Application of Biotechnology to Remediation of Contaminated Land Sites
What is a Contaminated Land?
Land may be said to be contaminated when there are substances in, on or under it that actually, or potentially, form a hazard to health or the environment. Confirmation that a piece of land is contaminated is based on the actual or potential identification of the source(s) of pollution and a defined link between the source and the land in question.
The major pollutants of land contamination are heavy metals (e.g. arsenic, cadmium and lead), oils and tars (gasoline, diesel, etc), non-degradable chemical substances and preparations (e.g. solid waste. solvents, chemical effluents, etc.), toxic gases, industrial particles (asbestos, quarrying dust) and other radioactive substances.
Sources of land contamination include agricultural production, mining, quarrying, sewage sludge accumulation, dredged spoils, improper disposal of household and other municipal and hospital wastes, demolition and construction waste, oil spill and industrial liquid and solid waste including radioactive waste.
Land may be contaminated by accidents or spills, leaking underground storage tanks, past industrial uses and waste disposal.
|Table: Sources and Methods of Land Pollution|
|Agriculture||accumulation of animal manures excessive input of chemical fertilizers illicit dumping of tainted crops on land|
|MiningandQuarrying||using of explosives to blow up mines using of machineries which emits toxic byproducts and leaks to the ground|
|Sewagesludge||Improper sanitation system causes sludge to leak at surrounding soil|
|Dredgedspoils||improper method of dredging at fertile land causes soil infertility, leaving the soil more prone to external pollution|
|Household||Improper waste disposal system causes waste accumulation improper sanitation system|
|Demolition andconstruction||non-biodegradable rubbles or debris which are not cleared settled in the soil undergo chemical reactions and increase soil toxicity|
|Industrial||poisonous/toxic emissions of gases which are not filtered or neutralised improper discharge of toxic and polluted effluent oil and tar contaminated land contamination by radioactive substances|
There are many methods for remediating contaminated land to restore them to their original state. These include in situ techniques such as bio-sparging, bio-venting and bio augumentation; composting and compost- related methods such as windrows, land farming and bio-pile and phyto-mediation.
Where phyto-remediation is found ineffective as a result of poor growth resulting from poor environmental conditions, soil fertility may be optimised using either microbes or different organic and / chemical fertilizers.
Remediation of Heavy Metal Contaminated Land
Heavy metal contamination is the presence in soil or water of metal especially heavy metals in concentrations higher than recommended values. A heavy metal is any metal with a specific gravity greater than about 5.0, especially one that is toxic and/or poisonous.
They include lead, mercury, zinc, copper, cadmium, mercury, nickel, and iron. The sources of heavy metal contamination of land are variable but can be broadly divided into natural or anthropogenic (i.e. human activity sources).
The sources include a wide variety of anthropogenic sources in the form of metal mine tailings, disposal of high metal wastes in improperly protected landfills, leaded gasoline and lead-based paints, land application of fertilizer, animal manures, bio-solids (sewage sludge), compost, pesticides, coal combustion residues, petrochemicals, and atmospheric deposition.
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The heavy metals essentially become contaminants in the soil environments because
(i) Their rates of generation via man-made cycles are more rapid relative to natural assimilation capacities
(ii) They become transferred from mines to random environmental locations where higher potentials of direct exposure occur,
(iii) The concentrations of the metals in discarded products are relatively high compared to those in the receiving environment, and
(iv) The chemical form (species) in which a metal is found in the receiving environmental system may render it more bio- available.
Heavy metals in the soil from anthropogenic sources tend to be more mobile, hence bio-available than pedogenic, or lithogenic ones. Contaminated soil or land can be remedied by several methods broadly classified as (i) source control or (ii) containment remedies.
Source control involves in situ and ex situ treatment technologies for sources of contamination. In situ or in place means that the contaminated soil is treated in its original place; unmoved, unexcavated; remaining at the site or in the subsurface. In situ treatment technologies treat or remove the contaminant from soil without excavation or removal of the soil.
Ex situ means that the contaminated soil is moved, excavated, or removed from the site or subsurface for treatment at other sites. Implementation of ex situ remedies requires excavation or removal of the contaminated soil. Some of these methods are biological in nature while others are non biological.
Table: Technologies for Remediation of Heavy Metal- Contaminated Soils
|Isolation||(i) Capping (ii) subsurface barriers|
|Immobilization||(i) Solidification/stabilisation (ii) vitrification (iii) chemical treatment|
|Toxicity and/or mobility reduction||(i) Chemical treatment, (ii) permeable treatment walls iii) biological treatment (phyto-remediation), bioleaching, biochemical processes|
|Physical separation||Several engineering based escavations|
|Extraction||(i) Soil washing, pyro-metallurgical extraction, in situ soil flushing, and electro-kinetic treatment|
The biological methods used to decontaminate heavy metal contaminated lands include the in situ methods of bio-sparging, bio- venting, bio-piling and phyto-remediation. Phyto-remediation is particularly useful because some plants have the ability to remove and stabilise metal contaminants.
Some species for instance have the peculiar characteristic to bio-accumulate metals up to 100-fold greater than those typically measured in shoots of the common non-accumulator plants.
Thus, a hyper-accumulator plant will concentrate more than 10 mg kg−1 Hg, 100 mg kg−1 Cd, 1000 mg kg−1 Co, Cr, Cu, and Pb; 10 000 mg kg−1 Zn and Ni. Phyto-remediation is usually followed by soil amendments using different fertilizers (chemical, organic or plants with the capacity to fix nitrogen from the air).
Phyto-remediation is energy eﬃcient, aesthetically pleasing method of remediating sites with low- to-moderate levels of contamination, and it can be used in conjunction with other more traditional remedial methods as a finishing step to the remedial process.
The advantages of phyto-remediation compared with classical remediation are that;
(i) It is more economically viable using the same tools and supplies as agriculture,
(ii) It is less disruptive to the environment and does not involve waiting for new plant communities to recolonise the site,
(iii) Disposal sites are not needed,
(iv) It is more likely to be accepted by the public as it is more aesthetically pleasing than traditional methods,
(v) It avoids excavation and transport of polluted media thus reducing the risk of spreading the contamination, and
(vi) It has the potential to treat sites polluted with more than one type of pollutant.
The disadvantages are:
(i) It is dependent on the growing conditions required by the plant (i.e., climate, geology, altitude, and temperature),
(ii) Large-scale operations require access to agricultural equipment and knowledge,
(iii) Success is dependent on the tolerance of the plant to the pollutant,
(iv) Contaminants collected in senescing tissues may be released back into the environment in autumn,
(v) Contaminants may be collected in woody tissues used as fuel,
(vi) Time taken to remediate sites far exceeds that of other technologies,
(vii) Contaminant solubility may be increased leading to greater environmental damage and the possibility of leaching.
Remediation of Non-Degradable Chemical Substances and Preparations in Contaminated Lands
Non-biodegradable substances are any organic or inorganic substance that cannot be broken down to smaller substances by natural processes. They include:
Substances such as:
Plastics (polyethylene, nylon, rayon, polyester, lexan, PVC (polyvinyl chloride), dacron), Metals (iron, platinum, steel, tin, aluminum, lead, silver, gold, arsenic, bismuth, zinc, chromium…), Ceramics (carbon fiber, fiberglass, kevlar), foams (cups, coolers), glasses, circuit boards/silicon based materials, noble gases and more, Diamond.
And chemical preparations such as: pesticides, Styrofoam, chips bags, plastic bottles, regular shopping bags, detergents, motor oil, paint, varnish, and chemical dyes.
The method applied to decontaminate land contaminated by non-degradable chemicals substances and preparations depend on the substance and preparation involved. However, compost piling and other ex situ compost methods can be used to degrade several chemicals including pesticides, dyes, varnish etc.
The microorganisms usually involved are species of fungi and bacteria usually found in garden compost piles or genetically-engineered for quicker results. Compost used in bioremediation is referred to as tailored or designed compost because it is specially formulated to treat specific contaminants depending upon the site.
A yard-waste compost may work well for soil contaminated with heavy metals, whereas wood chips and well-aged compost can remediate soil contaminated with the herbicides and other pesticides.
Remediation of Oil and Tar Contaminated Land
Oil and tar contamination of land involves any piece of land contaminated with fraction of petroleum listed in the table below;
Table: Carbon Content, Boiling Points and Uses of Petroleum Fractions
|NamesofFractions||Catoms in the molecule||Boilingrange inoC||Uses of the fraction – mainlydependsonits physicalproperties|
|Fuel Gas, LPG, Refinery Gas||1 to 4||-160 to 20oC||Methane gas fuel, C3-4 easily liquefied, portable energy source bottled gas for cooking (butane), higher pressure cylinders (propane)|
|Gasoline, Petrol||5 to 11||20 to 60oC||Easily vaporised, highly flammable, easily ignited, car fuel|
|Naphtha||7 to 13||60 to 180oC||Not good as a fuel, but valuable source of organic molecules to make other things, cracked to make more petrol and alkenes|
|Paraffin, Kerosene||10 to 16||120 to 240oC||Less flammable than petrol, domestic heater fuel, jet fuel|
|Diesel oil, Gas oil||15 to 25||220 to 250oC||Car and larger vehicle fuel|
|Fuel oil, lubricating oils and Waxes||20 to 70||250 to 350oC||Not so easily evaporated, not as flammable, safe to store for central heating oil, quite viscous (sticky) and can also be used for lubricating oils, clear waxes and polishes|
|Bitumen (Tar)||over 70||over 350oC||Forms a thick, black, tough and resistant adhesive on cooling, used as waterproofing material and to sticks rock chips on roofs or road surfaces|
Soil contaminated with petroleum is hazardous to human health, causes organic pollution of ground water which limits its use, causes economic loss, environmental problems and decreases the agricultural productivity of the soil.
Remediation of the oil contaminated soil can be achieved in many ways including physico-chemical and biological methods. Biological methods are more economical and efficient than chemical and physical methods.
Almost all the bioremediation methods such asparging, bio-venting, bio-piles and the various compost related methods can be applied to decontaminate oil contaminated sites.
The constituents of oil differ distinctly in volatility, volubility, and susceptibility to biodegradation. Some compounds are easily degraded, some resist degradation and some are non-biodegradable.
The biodegradation of different petroleum compounds occurs simultaneously but at different rates because different species of microbes preferentially attack different compounds.
This leads to the successive disappearance of individual components of petroleum over time. This is because the various biotechnology methods used work by increasing degradating and/or detoxifying the petroleum products in soil.
Biological methods of bioremediation through microorganisms such as bacteria and fungi are very efficient, but the low solubility and adsorption of high molecular weight hydrocarbons limits their availability to microorganisms.
The microbes present in the soil which first recognize the oil and its constituent are the bio-surfactants and bio emulsifiers. These will eventually attach themselves to the hydrocarbon present in the petroleum and use them as a source of energy and carbon.
Microorganisms produce enzymes in the presence of carbon sources which are responsible for attacking the hydrocarbon molecules. Many different enzymes and metabolic pathways are involved in the degradation of hydrocarbons contained in petroleum.
The bacteria most frequently used in the bioremediation of oil and tar contaminated sites include species of Aeromonas, Moraxella, Beijerinckia, Flavobacteria, Chrobacteria, Nocardia ,Corynebacteria, Atinetobacter, Mycobactena, Modococci, Streptomyces, Bacilli, Arthrobacter, Aeromonas, Cyanobacteria etc.
Most of these are commercially available as frozen dried bacteria. A minimum of 2×108 CFU/ml of bacteria is required to initiate and sustain bioremediation. Where this density does not exist, bio-augumentation by means of nutrient optimisation may be carried out to assist bacteria population growth.
This involves carbon and macronutrient supplementation. The essential micronutrients needed are nitrogen and phosphorous and the optimum nutrient ratio is Carbon: Nitrogen: Phosphorus of 100:10:4.
This ratio may be achieved by adding at least 1 ppm of ammonium nitrogen and 0.4 ppm of orthophosphate to the soil.
Remediation of Land Contaminated with As bestos and Mining Dust
Asbestos is a useful material made of six different fibrous minerals: chrysotile, crocidolite, amosite, tremolite, anthophyllite, and actinolite. These minerals come from mines throughout the world including Nigeria.
Asbestos has a very high heat retardant capacity, for which reason it is used in the manufacture of heat insulating products like roofing shingles, automobile brake pads, floor tiles, assorted gaskets, wraps for insulation of heating ducts and water pipes in homes, offices, and other buildings.
These inert asbestos-containing products are not dangerous and constitute no hazards to health, but once they are damaged or breached, or during manufacturing processes, they release asbestos dust, which people can inhale.
Asbestos dust contains fragmented particles considered hazardous because they can cause lung problems, including the development of mesothelioma, a form of lung cancer.
On its own, mine dusts are dust emissions from mining activities. The mine dust and also the asbestos dust mix with particles in air, becoming part of what is generally called particulate matter (PM).
Particulate matter contain both naturally- occurring particles and emissions from different human activities, including vehicle exhaust, quarrying, wood processing, industrial processes, power stations, farming and biomass burning.
Particulate matter is classified into three major classes on the bases of particulate sizes – greater than 10 µm (PM10+), 10 µm and 2.5 µm (PM 2.5). Each of these is associated with health risks for which reason control of asbestos and mine dust is an important component of environmental health practice.
Asbestos dust contains fibres which are made of pathogenic elements like iron and other metals. Crocidolite, one of the most potently carcinogenic components of asbestos, contains up to 29 % iron, which has the capacity to form highly reactive free radicals that damage DNA and eventually trigger cancers in humans and animals.
Experiments in vitro demonstrate that iron removal makes the asbestos considerably less hazardous by reducing their potential to generate radicals and to damage DNA.
Several fungi species have the capacity to extract iron from crocidolites and are therefore very good candidates for bioremediation of asbestos contaminated land. Fungi species perform this task in several ways.
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First, species such as Fusarium oxysporum, Mortierella hyalina and Oidiodendron maius, a mycorrhizal fungus, extract iron from crocidolites. Second, some such as the fungal hyphae form a web of thin strands that bind asbestos fibres, making them less liable to escape into the air.
Third, fungal chelators modify fibre surfaces in vitro, destroying active sites involved in the triggering of the carcinogenic mechanisms. As a result of these, fungi species, either naturally occurring or genetically-engineered are widely used in the bioremediation of asbestos contaminated sites.
In summary, several pollutants including heavy metals, toxic chemical substances, oil and tar as well as asbestos and mining dust usually cause contamination. Because of this, steps in effective remediation must begin with determining the type and sources of contamination.
Once the type of pollutant is identified and probably the source too, appropriate remediation technologies are chosen. Although, there are several physical and chemical methods of remediating contaminated sites, biotechnology methods are always preferred for several reasons.
They are cheaper, simpler and above all more environmentally friendly. Bioremediation of contaminated sites may involve several methods which are either in situ (treatment at the site of contamination) or ex situ (removal of contaminated soil to another site for treatment).
The most important in situ methods are bio-asparging, bio-venting, bio- augumentation and phyto-remediation, while the ex situ methods include forms of composting such as windrows, land farming and bio-piling.
Each of these methods uses either resident microorganisms at the site or introduced species which may be naturally-occurring or genetically- engineered. Both naturally- occurring and genetically-engineered species are commercially available in different parts of the world.
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