Bioremediation

Alfred R. Conklin, Ph.D.

Bioremediation can be defined in many different ways. It could be the active degradation or passive removal or concentration of environmental pollutants by a biological system. Or it could be degradation by a specific enzyme in a microbial or plant cell. A plant system might increase the rate of degradation due to the unique characteristics of the rhizosphere. Bioremediation might involve the concentration, or deposition of inorganic pollutants in the rhizosphere. It could also involve plants as the agents of passive movement of pollutants out of soil and into the atmosphere. There are undoubtedly still other ways of looking at or defining bioremediation.

Since its first description bioremediation has been subdivided into more specific and descriptive sub-areas which have separate names. The flow chart below indicates these various “branches” of the bioremediation tree. The most common division is between microbial and phyto remediation. In the former case microorganisms, bacteria, actinomcetes and fungi are the active organisms in the remediation process. In phytoremediation typically trees are the active organism. However, from the flow chart you will note that other plants and microorganisms have been used in bioremediation.

Plants and trees interact with soil to produce an area around the roots called the rhizosphere. This is an area of high microbial activity, lower pH, increased organic matter, higher carbon dioxide and lower water and nutrient content. Particularly because of increased microbial activity this area also contributes a unique environment, conducive to the break down of organic compounds. Because plants are constantly taking up water the rhizosphere is an area where concentration of metals and organic compounds occurs. Once in the rhizosphere inorganic and organic compounds, including metals, may be deposited, taken up or broken down by plants.

Active degradation of contaminants involves enzymes, or metabolic pathways, which accommodate the pollutant. The plant or microorganism may use the pollutant as a source of nutrients, carbon, nitrogen, phosphorus etc., as a source of energy or both. Common short-lived insecticides and herbicides are broken down and used in such a manner. Nitrates can be removed from water under anaerobic conditions by a process called denitrification. During this process nitrate is used as an electron acceptor and thus the organism is using the nitrate in energy production.

Passive removal could be uptake along with water and release into the atmosphere by transpiration. Gases dissolved in water may be taken up by plants, transported up the plant system and released into the atmosphere during transpiration. On the other hand one might simply wish to have trees remove water from soil to prevent it and associated pollutants from moving into or out of the area.

Passive removal can also take place in the rhizosphere. This could be by concentration or by concentration and precipitation. If a plant can be found which takes up the contaminant then the plant can be grown and the pollutant “harvested” along with the plant. If the pollutant is a metal the plant material can be burned and the metal recovered. If the pollutant is a heavy metal and it is precipitated in the rhizosphere then the plant, roots and surrounding soil may need to be “harvested”.

Versatility is the rule. It was once thought that microorganisms were omnipotent. That is that they could and would break down any organic compound. This was particularly true of soil microorganisms since there are a tremendous variety of different types of microorganism, aerobes, anaerobes, heterotrophes, autotropes, bacteria, actinomycetes, fungi etc. in soil. However, some complex organic compounds are highly resistant to degradation. Of most concern are manmade compounds, which are toxic. There are also some natural organic compounds, such as humus, which are also resistant to decomposition. These compounds are generally seen to be beneficial.

In addition to complexity, concentration is a problem. If the pollutant is too concentrated it may interfere with biological activity. This could be direct interference as in toxicity or indirect as in the creation of a high osmotic potential. On the other hand if the concentration is vary low there may not be enough of the pollutant present to sustain the population of degraders.

For situations where the pollutant is highly concentrated and thus interfering with degradation then dilution may be necessary. If the pollutant is soluble in water then adding water may be sufficient. This would also work if the problem is osmotic. If the pollutant is insoluble then soil or some inert additive might be needed as a diluent. This of course presents the additional problem of creating more polluted material.

It might be noted that water-soluble compounds are usually more easily decomposed than are water insoluble materials. In some cases the addition of a surfactant will bring the pollutant into solution and result in an increased rate of decomposition.

In situations where the pollutant concentration is low the decomposition process may be slow. Addition of organic matter to maintain a high microbial population may help in these situations. On the other hand it may be better to allow additional time to permit the natural decomposition process to proceed.  This would only be a viable process where the pollutant was not able to move out of the polluted site.

For microorganisms surviving the competition with other microorganisms is problematic. If the microorganism needs the pollutant to live and it is at low concentration then maintaining a large population of the microorganism so that rapid degradation takes place will be hard. If the microorganism degrades the pollutant along with other needed substrates the question then becomes will there be enough of the needed substrate present to allow for degradation of the pollutant?

Survival is the name of the game. If a plant or microbial species is not adapted to a location or an environment will it survive to do its remediation work? If the answer is yes, the question then becomes for how long? For plants, particularly trees, adaptability to a variety of environmental conditions may be great.  However, if trees are to be used to control water movement in an area where the water contains some salt or is brackish will the trees live? One might also ask which variety of tree will grow best.

One approach, which has been tried, is to find or develop a microorganism capable of degrading the pollutant in question. This microorganism can be grown in large numbers and added to the soil to effect remediation. This will only work if the organism survives. Often these organisms do not survive meaning that new batches must be added to soil frequently.

Bioremediation using any type of organism is a valuable remediation tool. It has many advantages including the fact that often it is carried out in situ and thus involves minimal disturbance to the site. In addition microorganisms and plant roots can explore every recess of soil and thus carry out a more complete remediation.

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