Introduction
Phytoremediation is a type of bioremediation in which plants are used to degrade or absorb contaminants to render soil, water, or air harmless. It also involves microbial activities to assist plants in remediating the pollutants. It has many advantages i.e. cost effective, involves little manpower, is aesthetically pleasing, accepted by surrounding residents. In this article, mechanisms of phytoremediation are discussed.
Mechanisms of Phytoremediation
To Treat Metal Contaminants
- Phytoextraction
- Rhizofiltration
- Phytostabilisation
To Treat Organic Contaminants:
- Phytodegradation
- Phytovolatilisation
1: Phytoextraction
Phytoextraction, also known as phytoaccumulation, is a process in which plants absorb the contaminants through roots and accumulate them in above-ground portions i.e. shoots and leaves. The desirable plants for phytoextraction are hyper-accumulator species.
However the plants. After phytoextraction, the plants are harvested and incinerated. The ash produced due to the incineration is disposed of by sending it to landfills. In this way, contaminants present in soil are removed.
Some examples of phytoextraction from soils
1: Arsenic using sunflower or Chinese brake
2: Cadmium using Willow
3:Cadmium and zinc using Alpine pennycress,
4: Lead, using Indian Mustard, Ragweed, Hemp, or Poplar trees
5: Uranium, using sunflowers
6: Mercury, selenium by transgenic plants
Advantages
1: Environmental Friendly
2: Less Expensive
3: don’t cause any harm to the soil
Disadvantage
1: Takes a long time
2: Phytostabilization
In this process, contaminants absorb on roots or precipitate in the rhizosphere. This process limits the mobility of contaminants in soil. Also, the plants excrete special redox enzymes which convert contaminants(heavy metals) to less toxic form. This method is commonly used at the sites which got contaminated due to metal mining. However, this is just a management strategy as it can’t remove pollutants from the soil.
3: Phytovolatalization
It is a process in which plants absorb contaminants through roots from the soil and then transport them through the xylem, converting them into less toxic as well as volatile forms. And then transpire them into the atmosphere. This method has been mostly used to remove mercury and selenium because these possess high volatility.
Examples
1: Astragalus racemosus converts selenium to dimethyl diselenide
2: Arabidopsis thaliana converts Hg2+ into Hg0
Drawbacks:
- Edible parts of the plant may have contaminants
- Contaminants can’t be fully removed.
4: Rhizofilteration
This method is used to treat contaminated waste water, groundwater and surface water. In this process, contaminants absorb and precipitate on roots to clean water. Plants that are metal tolerant, hypoxia tolerant, and have large surface areas are suitable for rhizofilteration. Terrestrial plants posses more fibrous systems and develop roots than aquatic plants. That’s why terrestrial plants are more desirable for rhizofilteration when compared to aquatic plants.
Examples
Helianthus annus, Phragmites australis, Brassica juncea, Populus Fontinalis antipyretica, and Lemna
5: Rhizodegradation
This a process in which microorganisms i.e. bacteria, fungi, and yeast, present in the rhizosphere degrade the organic contaminants present in the soil. Plants secrete nutrient-containing exudes which provides microorganisms with nitrogen and carbon sources. Due to this, microbial activities increase by 10-100 times. Enzymes secreted by plants also help in degrading the organic contaminants present in soil.
Limitations of Phytoremediation
Phytoremediation is an effective alternative method for cleaning up environmental contaminants, but it has several limitations. Research is often conducted in controlled environments, which may not yield representative results. The success of phytoremediation depends on the successful growth of plant species, which can vary depending on soil and climate. Plants’ physiology may be affected by various living organisms and microbes, making them more vulnerable to diseases. Additionally, plants can thrive only within certain levels of contaminant concentration, which can affect their efficiency. Using mixed and local species or genetically engineered species can minimize the risks of diseases and pests. Contaminants must be within the reach of plants’ roots, so screening for plants with long and deep roots and higher root biomass can improve efficiency. Phytoremediation is slow and may take three to five years to show full results. Proper handling and disposal of plants with toxic contaminants are also concerns. It may not work for all pollutants, such as lower molecular weight chlorinated PCBs.
Conclusion
The plants use different mechanisms i.e. phytodegradation, rhizofilteration, Rhizodegradation etc, to make the environment clean and healthy. Phytoremediation is an easy, green, and environmental friendly technique to get rid of pollutants. In the era of industrial, temperature is also increase along with pollution. The plants not only help us in decreasing pollution but also lower the temperature.