PHYTOREMEDIATION: TYPES, ADVANTAGES AND DISADVANTAGES - BIOLOGY - 2025

The phytoremediation is the set of technological practices using living plants and their associated microorganisms to environmental sanitation of soil, water and air.

Phytoremediation technologies make use of the natural capacity of some plants to absorb, concentrate and metabolize elements and chemical compounds that are present in the environment as pollutants. Plants can be used for the extraction, immobilization and stabilization, degradation or volatilization of contaminants.

Figure 1. Phytoremediation in the field. Source.: flickr.com/photos/daniela_naturephotography

The soil, surface and groundwater, and the atmosphere can be contaminated as a consequence of some natural processes -such as geological erosion, volcanic activity, among others-, and also due to the effect of human activities (industrial, agricultural, wastewater, mining, construction, transportation).

Industrial emissions and effluents, waste materials, explosives, agrochemicals (fertilizers, herbicides, pesticides), rain or acid deposition, radioactive materials, among many others, are pollution factors that come from human activities.

Phytoremediation emerges as an inexpensive, effective, publicly accepted technology for borremediation of various types of environmental contamination.

The word "phytoremediation" comes from the Greek "phyto", which means living plant, and from the Latin "remediare" which means to restore balance; that is to say, recover the state of balance through the use of plants.

Types of phytoremediation

Phytoremediation technologies are based on the physiological processes of plants and the microorganisms associated with them, such as nutrition, photosynthesis, metabolism, evapotranspiration, among others.

Depending on the type of pollutant, the degree of contamination of the place and the level of removal or decontamination that is needed, phytoremediation techniques are used as a contaminant containment mechanism (phytostabilization techniques, rhizofiltration), or as a elimination mechanism (techniques of phytoextraction, phytodegradation and phytovolatilization).

Figure 2. Types of phytoremediation. Source: Townie (Arulnangai & Xavier Dengra from the original in.png extension), from Wikimedia Commons

These phytoremediation techniques include:

Phytodegradation

This technique, also called phytotransformation, consists of selecting and using plants that have the ability to degrade the pollutants they have absorbed.

In phytodegradation, special enzymes that some plants have, cause the breakdown of the molecules of the polluting compounds, transforming them into smaller, non-toxic or less toxic molecules.

Plants can also mineralize pollutants to simple, assimilable compounds, such as carbon dioxide (CO ₂) and water (H ₂ O).

Examples of this type of enzyme are dehalogenase and oxygenase; the first one favors the removal of halogens from chemical compounds and the second one oxidizes substances.

Phytodegradation has been used to remove explosives, such as TNT (trinitrotoluene), organochlorine and organophosphate pesticides, halogenated hydrocarbons, among other contaminants.

Rhizoremediation

When the degradation of pollutants is produced by the action of microorganisms that live in the roots of plants, the remediation technique is called rhizoremediation.

Phytostabilization

This type of phytoremediation is based on plants that absorb pollutants and immobilize them inside.

These plants are known to reduce the bioavailability of pollutants through the production and excretion by the roots of chemical compounds that inactivate toxic substances through absorption, adsorption or precipitation-solidification mechanisms.

In this way, pollutants are no longer available in the environment for other living beings, their migration to groundwater and their dispersion to larger areas of soils are prevented.

Some plants that have been used in phytostabilization are: Lupinus albus (to immobilize arsenic, As and cadmium, Cd), Hyparrhenia hirta (immobilization of lead, Pb), Zygophyllum fabago (immobilization of zinc, Zn), Anthyllis Vulneraria (immobilization of zinc, lead and cadmium), Deschampia cespitosa (immobilization of lead, cadmium and zinc) and Cardaminopsis arenosa (immobilization of lead, cadmium and zinc), among others.

Phytostimulation

In this case, plants are used that stimulate the development of microorganisms that degrade pollutants. These microorganisms live in the roots of plants.

Phytoextraction

Phytoextraction, also called phytoaccumulation or phyto-sequestration, uses plants or algae to remove contaminants from the soil or water.

After the plant or algae has absorbed the polluting chemicals from the water or soil and has accumulated them, they are harvested as biomass and generally incinerated.

Figure 3. Phytoremediation in pools, rehabilitation of an abandoned uranium mine. Portugal. Source: flickr.com/photos/daniela_naturephotography

The ashes are deposited in special places or security dumps or are used to recover metals. This last technique is called herbalism.

Hyper-accumulating plants

Organisms that are capable of absorbing extremely high amounts of pollutants from soil and water are called hyperaccumulators.

Hyperaccumulating plants of arsenic (As), lead (Pb), cobalt (Co), copper (Cu), manganese (Mn), nickel (Ni), selenium (Se), and zinc (Zn) have been reported.

Phytoextraction of metals has been carried out with plants such as Thlaspi caerulescens (extraction of cadmium, Cd), Vetiveria zizanoides (extraction of zinc Zn, cadmium Cd, and lead Pb), Brassica juncea (extraction of lead Pb) and Pistia stratiotis (extraction of silver Ag, mercury Hg, nickel Ni, lead Pb and zinc Zn), among others.

Phytofiltration

This type of phytoremediation is used in the decontamination of groundwater and surface waters. Pollutants are absorbed by microorganisms or by roots, or are attached (adsorbed) to the surfaces of both.

Figure 4. Root growth in the laboratory, in liquid medium. Source: pixabay.com

In phytofiltration the plants are grown with hydroponic techniques and when the root is well developed, the plants are transferred to polluted waters.

Some plants used as phytofilters are: Scirpus lacustris, Lemna gibba, Azolla caroliniana, Elatine trianda and Polygonum punctatum.

Phytovolatilization

This technique works when the roots of the plants absorb polluted water and release the pollutants transformed into a gaseous or volatile form into the atmosphere, through the transpiration of the leaves.

The phytovolatilizing action of selenium (Se) of plants, Salicornia bigelovii, Astragalus bisulcatus and Chara canescens and also the ability to transpire mercury (Hg), of the plant species Arabidopsis thaliana is known.

Advantages of phytoremediation

• The application of phytoremediation techniques is much cheaper than the implementation of conventional decontamination methods.
• Phytoremediation technologies are efficient applied in large areas with medium levels of contamination.
• Being in situ decontamination techniques, the contaminated environment does not have to be transported, thus avoiding the dispersion of pollutants by water or air.
• The application of phytoremediation technologies allows the recovery of valuable metals and water.
• To apply these technologies, only conventional agricultural practices are required; The construction of special facilities is not necessary, nor the training of trained personnel for its implementation.
• Phytoremediation technologies do not consume electrical energy, nor do they produce polluting greenhouse gas emissions.
• They are technologies that preserve soil, water and atmosphere.
• They are the decontamination methods with the lowest environmental impact.

Disadvantages and limitations

• Phytoremediation techniques can only have an effect in the zone occupied by the roots of the plants, that is, in a limited area and depth.
• Phytoremediation is not totally efficient in preventing the leaching or percolation of contaminants into groundwater.
• Phytoremediation techniques are slow methods of decontamination, since they require a waiting time for the growth of plants and microorganisms associated with them.
• The growth and survival of the plants used in these techniques are affected by the degree of toxicity of the pollutants.
• The application of phytoremediation techniques can have negative effects on the ecosystems where they are implemented, due to the bioaccumulation of pollutants in plants, which can subsequently pass into the food chains through primary and secondary consumers.

References

1. Carpena RO and Bernal MP. 2007. Keys to phytoremediation: phytotechnologies for soil recovery. Ecosystems 16 (2). May.
2. Environmental Protection Agency (EPA-600-R-99-107). 2000. Introduction to Phytoremediation.
3. Gerhardt KE, Huang XD, Glick BR, Greenberg BM. 2008. Phytoremediation and rhizoremediation of organic soil contaminants: Potential and challenges. Plant Science. MISSING LEAVES
4. Ghosh M and Singh SP. 2005. A review of phytoremediation of heavy metals and utilization of its byproducts. Applied Ecology and Environmental Research. 3 (1): 1-18.
5. Wang, L., Ji, B., Hu, Y., Liu, R., & Sun, W. (2017). A review on in situ phytoremediation of mine tailings. Chemosphere, 184, 594–600. doi: 10.1016 / j.chemosphere.2017.06.025