BIOREMEDIATION: CHARACTERISTICS, TYPES, ADVANTAGES AND DISADVANTAGES - BIOLOGY - 2025

The bioremediation is a set of biotechnologies sanitation using metabolic capabilities of bacterial microorganisms, fungi, plants and / or isolated enzymes, to remove contaminants in soil and water.

Microorganisms (bacteria and fungi) and some plants can biotransform a wide variety of polluting and toxic organic compounds, until they are not harmful or harmless. They can even biodegrade some organic compounds to their simplest forms, such as methane (CH ₄) and carbon dioxide (CO ₂).

Figure 1. Environmental contamination by oil spill, later treated with bioremediation. Source: commons.wikimedia.org

Some microorganisms and plants can also extract or immobilize toxic chemical elements, such as heavy metals, in the environment (in situ). By immobilizing the toxic substance in the environment, it is no longer available to living organisms and therefore does not affect them.

Therefore, reducing the bioavailability of a toxic substance is also a form of bioremediation, although it does not imply the removal of the substance from the environment.

There is currently a growing scientific and commercial interest in developing economical and low environmental impact (or “environmentally friendly”) technologies, such as the bioremediation of surface and groundwater, sludge and contaminated soils.

Characteristics of bioremediation

Contaminants that can be bioremediated

Among the pollutants that have been bioremediated, are heavy metals, radioactive substances, toxic organic pollutants, explosive substances, organic compounds derived from oil (polyaromatic hydrocarbons or HPAs), phenols, among others.

Physicochemical conditions during bioremediation

Because bioremediation processes depend on the activity of microorganisms and living plants or their isolated enzymes, the appropriate physicochemical conditions must be maintained for each organism or enzyme system, in order to optimize their metabolic activity in the bioremediation process.

Factors to be optimized and maintained throughout the bioremediation process

-The concentration and bioavailability of the pollutant under environmental conditions: since if it is too high it can be harmful to the same microorganisms that have the ability to biotransform them.

-Humidity: the availability of water is essential for living organisms, as well as for the enzymatic activity of cell-free biological catalysts. Generally, 12 to 25% relative humidity should be maintained in soils undergoing bioremediation.

-Temperature: it must be in the range that allows the survival of the applied organisms and / or the required enzymatic activity.

-The bioavailable nutrients: essential for the growth and multiplication of the microorganisms of interest. Mainly, carbon, phosphorus and nitrogen must be controlled, as well as some essential minerals.

-The acidity or alkalinity of the aqueous medium or pH (measurement of H ⁺ ions in the medium).

-The availability of oxygen: in most bioremediation techniques, aerobic microorganisms are used (for example in composting, biopiles and “landfarming”), and aeration of the substrate is necessary. However, anaerobic microorganisms can be used in bioremediation processes, under very controlled conditions in the laboratory (using bioreactors).

Types of bioremediation

Among the applied bioremediation biotechnologies are the following:

Biostimulation

Biostimulation consists of the in situ stimulation of those microorganisms already present in the environment that was contaminated (autochthonous microorganisms), capable of bioremediating the contaminating substance.

In situ biostimulation is achieved by optimizing the physicochemical conditions for the desired process to occur, that is; pH, oxygen, humidity, temperature, among others, and adding the necessary nutrients.

Bioaugmentation

Bioaugmentation involves increasing the amount of microorganisms of interest (preferably autochthonous), thanks to the addition of their inocula grown in the laboratory.

Subsequently, once the microorganisms of interest have been inoculated in situ, the physicochemical conditions must be optimized (such as in biostimulation), to promote the degrading activity of the microorganisms.

For the application of bioaugmentation, the costs of microbial culture in bioreactors in the laboratory must be considered.

Both biostimulation and bioaugmentation can be combined with all the other biotechnologies described below.

Composting

Composting consists of mixing the contaminated material with uncontaminated soil supplemented with plant or animal breeding agents and nutrients. This mixture forms cones up to 3 m high, spaced apart.

The oxygenation of the lower layers of the cones must be controlled, through their regular removal from one site to another with machinery. Optimal conditions of humidity, temperature, pH, nutrients, among others, must also be maintained.

Biopiles

The bioremediation technique with biopiles is the same as the composting technique described above, except for:

• The absence of breeding agents of plant or animal origin.
• Elimination of aeration by movement from one site to another.

The biopiles remain fixed in the same place, being aerated in their internal layers through a system of pipes, whose installation, operation and maintenance costs must be considered from the design phase of the system.

Landfarming

The biotechnology called “landfarming” (translated from English: tilling the land), consists of mixing the contaminated material (mud or sediment) with the first 30 cm of uncontaminated soil of a large area.

In those first centimeters of soil, the degradation of polluting substances is favored thanks to its aeration and mixing. Agricultural machinery is used for these tasks, such as plow tractors.

The main disadvantage of landfarming is that it necessarily requires large tracts of land, which could be used for food production.

Phytoremediation

Phytoremediation, also called micro-organism and plant-assisted bioremediation, is a set of biotechnologies based on the use of plants and micro-organisms to remove, confine or reduce the toxicity of polluting substances in surface or underground waters, sludge and soils.

During phytoremediation, degradation, extraction and / or stabilization (decrease in bioavailability) of the contaminant can occur. These processes depend on the interactions between plants and microorganisms that live very close to their roots, in an area called the rhizosphere.

Figure 2. Bioremediation of water contaminated with plants and microorganisms. Source: Wikyhelper, from Wikimedia Commons

Phytoremediation has been especially successful in the removal of heavy metals and radioactive substances from soils and surface or groundwater (or rhizofiltration of contaminated water).

In this case, the plants accumulate the metals from the environment in their tissues and then they are harvested and incinerated under controlled conditions, so that the pollutant goes from being dispersed in the environment, to being concentrated in the form of ashes.

The ashes obtained can be treated to recover the metal (if it is of economic interest), or can be abandoned in places of final disposal of waste.

A disadvantage of phytoremediation is the lack of in-depth knowledge of the interactions that occur between the organisms involved (plants, bacteria, and possibly mycorrhizal fungi).

On the other hand, environmental conditions that satisfy the needs of all applied organisms must be maintained.

Bioreactors

Bioreactors are containers of considerable size, which allow to maintain very controlled physicochemical conditions in aqueous culture media, with the aim of favoring a biological process of interest.

Bacterial microorganisms and fungi can be cultured on a large scale in the laboratory in bioreactors and then applied in in situ bioaugmentation processes. Micro-organisms can also be cultured in the interest of obtaining their pollutant-degrading enzymes.

Bioreactors are used in ex situ bioremediation processes, by mixing the contaminated substrate with the microbial culture medium, favoring the degradation of the contaminant.

Microorganisms grown in bioreactors can even be anaerobic, in which case, the aqueous culture medium must be devoid of dissolved oxygen.

Figure 3. Bioreactor. Source: es.m.wikipedia.org

Among bioremediation biotechnologies, the use of bioreactors is relatively expensive, due to equipment maintenance and requirements for microbial culture.

Microremediation

The use of fungal microorganisms (microscopic fungi) in bioremediation processes of a toxic pollutant is called mycorremediation.

It should be considered that the cultivation of microscopic fungi is usually more complex than that of bacteria and therefore implies higher costs. Furthermore, fungi grow and reproduce more slowly than bacteria, with fungal-assisted bioremediation being a slower process.

Bioremediation versus conventional physical and chemical technologies

-Advantage

Bioremediation biotechnologies are much more economical and environmentally friendly than conventionally applied chemical and physical environmental sanitation technologies.

This means that the application of bioremediation has a lower environmental impact than conventional physicochemical practices.

On the other hand, among the microorganisms applied in bioremediation processes, some can even mineralize the polluting compounds, ensuring their disappearance from the environment, something difficult to achieve in a single step with conventional physicochemical processes.

-Disadvantages and aspects to consider

Microbial metabolic capacities existing in nature

Given that only 1% of the microorganisms existing in nature have been isolated, a limitation of bioremediation is precisely the identification of microorganisms capable of biodegrading a specific contaminating substance.

Lack of knowledge of the applied system

On the other hand, bioremediation works with a complex system of two or more living organisms, which is generally not fully understood.

Some microorganisms studied have biotransformed polluting compounds into even more toxic by-products. For this reason, it is necessary to previously study bioremediation organisms and their interactions in depth in the laboratory.

In addition, small-scale pilot tests (in the field) must be carried out before applying them en masse, and finally the bioremediation processes must be monitored in situ, to ensure that environmental sanitation occurs correctly.

Extrapolation of results obtained in the laboratory

Due to the high complexity of biological systems, results obtained on a small scale in the laboratory cannot always be extrapolated to field processes.

Particularities of each bioremediation process

Each bioremediation process involves a specific experimental design, according to the particular conditions of the contaminated site, the type of contaminant to be treated and the organisms to be applied.

It is then necessary that these processes be directed by interdisciplinary groups of specialists, among whom must be biologists, chemists, engineers, among others.

The maintenance of the environmental physicochemical conditions to favor the growth and metabolic activity of interest, implies a permanent work during the bioremediation process.

Time required

Finally, bioremediation processes can take longer than conventional physicochemical processes.

References

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