Phytoremediation is a bioremediation process that uses various types of plants to remove, transfer, stabilise, extract or destroy contaminants in the soil and groundwater.
Concept of Phytoremediation:
There are different types of phytoremediation mechanisms used to remove or detoxify contaminants from soil and water is discussed as follows.
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In this process, the plant secretes natural substances through its roots and these are nutrients required for growth of micro-organisms in the soil. The microorganisms grow rapidly and accelerate biological degradation of contaminants present in soil.
In this process, chemical compounds produced by the plant immobilise contaminants, rather than degrade them.
Phytoaccumulation (Phytoextraction):
In phytoaccumulation process, rhizosphere part of the plant roots absorb the contaminants along with other nutrients and water. The contaminant is not degraded but stored in the part of plant such as shoots and leaves. This method is mostly used for wastes containing metals.
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It has been seen that water-soluble metals are taken up by plant species selected for their ability to take up large quantities of lead (Pb). The metals are stored in the plants aerial shoots, which are harvested and either smelted for potential metal recovery or are disposed of as a hazardous waste. Generally bioavailable metals for plant uptake include cadmium, nickel, zinc, arsenic, selenium, and copper.
Moderately bioavailable metals are cobalt, manganese, and iron. Lead, chromium, and uranium are not very bioavailable. Chelating agent can play a major role to get metal bioavailable, for example, lead can be made much more bioavailable by the addition of chelating agents to soils. Similarly, the availability of uranium and radio-cesium 137 can be enhanced using citric acid and ammonium nitrate, as chelating agents.
Hydroponic Systems for Treating Water Streams (Rhizofiltration):
Rhizofiltration is similar to phyto-accumulation, but the plants used for this purpose are grown in greenhouses with their roots in water not in soil. This system can be used for ex situ groundwater treatment. Groundwater is pumped to the surface to irrigate these plants and at that duration these plants capture contaminants in different part of plants.
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Typically hydroponic systems utilise an artificial soil medium, such as sand mixed with perlite or vermiculite. As the roots become saturated with contaminants, they are harvested and disposed of.
This is the process in which plants take up water containing organic contaminants and release the contaminants into the air through their leaves as volatile components.
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This process is based on the degradation capability of specific plant species for a particular contaminant. In this process, plants actually metabolise and degrade contaminants within plant tissues.
In hydraulic control process, we can use trees as they have capability to carry water from very deep compare to plants. They indirectly remediate by controlling groundwater movement. Trees act as natural pumps when their roots reach down towards the water table and establish a dense root mass that takes up large quantities of water. For example- A poplar tree, pulls out of the ground 30 to 35 gallons of water per day, and a cottonwood can absorb up to 340 gallons per day.
Limitations and Concerns of Phytoremediation:
The toxicity and bioavailability of products after biodegradation, is not always known. This degradation by-products may be mobilised in groundwater or bio-accumulated in animals or other aquatic life. It is needed to determine the fate of various compounds produced during degradation of contaminants in the plant metabolic cycle to ensure that plant parts/droppings and products do not contribute toxicity or harmful chemicals into the food chain.
It is also needed to understand whether contaminants that collect in the leaves and wood of trees are released when the leaves fall in the autumn or when firewood or mulch from the trees is used. Disposal of harvested plants containing contaminants can be a problem if they contain high concentration of heavy metals as contaminants.
The location of contaminants inside soil in a limiting factor for it remediation. The treatment zone is determined by plant capacity to reach root up to which depth, it is limited to shallow soils, streams, and groundwater.
If the plant root is not capable to reach up to that depth where contaminants are present in water then pumping the water out of the ground and using it to irrigate plantations of trees may be the option to treat contaminated groundwater in such case. Generally, the use of phytoremediation is limited to sites with lower contaminant concentrations and contamination in shallow soils, streams, and groundwater.
However, researchers are finding that the use of trees (rather than smaller plants) allows them to treat deeper contamination because tree roots have capability to reach up to more depth into the ground.
The success of phytoremediation may be seasonal, depending on location and climatic conditions of the area where the plant are to be grown. These climatic factors will also influence its effectiveness. The success of remediation also depends upon the selection of plant species from plant community. Bioremediation using plants is time taking process as the establishment of the plants may require several seasons of irrigation.
It is important to consider extra mobilisation of contaminants in the soil and groundwater during bioremediation if possible. High concentration of contaminant also limits this process as contaminant concentration is too high, plants may die. Phytoremediation is not effective for strongly absorbed contaminants such as polychlorinated biphenyls (PCBs). Phytoremediation also requires a large surface area of land for remediation.
Application of Phytoremediation:
Phytoremediation is used for the removal/treatment of metals, radionuclides, pesticides, explosives, fuels, Volatile Organic Compounds (VOCs) and Semi Volatile Organic Compounds (SVOCs). Researchers are also trying to find out the role of phytoremediation to remediate perchlorate, a contaminant that has been shown to be persistent in surface and groundwater systems. It may be used to cleanup contaminants found in soil and groundwater. For radioactive substances, chelating agents are sometimes used to make the contaminants available to plant uptake.
Mechanisms of Heavy Metal Uptake by Plant:
Contaminant uptake by plants and its mechanisms have been being explored in different ways. Understanding of this mechanism could be used to optimise the factors to improve the performance of plant uptake.
During the remediation process, plants act both as ‘accumulators’ and ‘excluders’. Accumulators survive despite concentrating contaminants in their aerial tissues. They biodegrade or biotransform the contaminants into inert forms in their tissues. The excluders restrict contaminant uptake into their biomass.
Plants have developed highly specific and very efficient mechanisms to obtain essential micronutrients from the environment, even when present at low concentration. Plant roots, aided by plant based chelating agents, plant induced pH changes and redox reactions are able to solubilise and take up micronutrients from very low levels in the soil.
Plants have also developed highly specific mechanisms to translocate and store micronutrients. These mechanisms are also used in the uptake, translocation, and storage of toxic elements, whose chemical properties simulate those of essential elements. Therefore, mechanisms of micronutrient uptake having great interest to phytoremediation.
Classification of Phytoremediation:
Phytoextraction is the uptake/absorption and translocation of contaminants by plant roots into the plants shoots that can be harvested and burned to obtain energy and recycling the metal from the ash.
Phytostabilisation is the process of remediation in which certain plant species are used to immobilise the contaminants in the soil and groundwater. This occurs through absorption and accumulation in plant tissues, adsorption onto roots, or precipitation within the root zone preventing their migration in soil, as well as their movement by erosion and deforestation.
Rhizofiltration is the process in which adsorption or precipitation of contaminants takes place onto plant roots or absorption and sequesterisation in the roots. Contaminants that are present in solution form surrounds the root zone by constructing wetland for cleaning up contaminated wastewater.
Phytovolatilisation is the uptake and removal of a contaminant by a plant, with release of the contaminant or a modified form of the contaminant to the atmosphere from the plant as transpiration occurs. Phytovolatilisation occurs when growing trees and other plants take up water along with the contaminants present in water. These contaminants pass through the plants to the leaves and volatilise into the atmosphere at comparatively low concentrations.
Plants also play an important role in physically stabilising the soil with their root system. This is also helpful for preventing erosion, protecting the soil surface, and reducing the impact of rain. At the same time, plant roots release nutrients that help to improve the growth of microbes to convert in to a rich microbial community in the rhizosphere.
Presence of bacterial community and its composition in the rhizosphere region is affected by complex interactions between soil type, plant species, and root zone location. Population of microorganisms is generally higher in the rhizosphere compare to the root-free soil.
This is due to availability of nutrients nearby this rhizosphere part of soil and also due to a symbiotic relationship between soil micro-organisms and plants. Due to this symbiotic relationship, bioremediation processes can be enhanced. Plant roots also acts as surfaces provider for absorption or precipitation of metal contaminants. In this remediation process the root zone acts as focus of interest.
The contaminants can be absorbed by the root to be subsequently stored or metabolised by the plant. Degradation of contaminants in the soil by plant enzymes released from the roots is also an important phytoremediation mechanism. Many contaminants follow route in which passive uptake takes place, via., micropores in the root cell wall and finally into the root, where degradation can take place.
Factors Affecting the Uptake Mechanisms:
There are several factors which can affect the uptake mechanism of heavy metals. By having better understanding and knowledge about these factors, the uptake performance by plant can be greatly improved.
1. Plant Species:
Selection of potential plants species or varieties is very important as plant species having better remediation power remediate contaminant very fast. The uptake of a compound is affected by natural characteristics of plant species. The effectiveness of the phytoextraction technique depends upon the identification and selections of suitable plant species that hyper accumulate heavy metals.
2. Properties of Medium:
Soil conditioning is most important agricultural practice in which pH adjustment, addition of chelators and use of appropriate fertilisers takes place. This practice also help to improve the growth and to enhance remediation. For example, the amount of lead absorbed by plants is affected by the pH, organic matter, and the phosphorus content of the soil.
3. Root Zone:
The root zone is major area of action in phytoremediation. Through this part of plant contaminants can be absorbed, stored and metabolise in to the plant tissue. Degradation of contaminants in the soil by plant enzymes secreted from the roots is another phytoremediation mechanism. A morphological adaptation due to drought stress condition enhance root diameter and reduced root elongation as a response to less permeability of the dried soil.
4. Vegetative Uptake:
Vegetative uptake by plant is affected by the environmental conditions like temperature which affects growth and root length. Morphology of root structure vary in natural soil conditions and that under greenhouse condition. The success of phytoremediation, depends on a contaminant specific hyper accumulator. Metal uptake by plants also depends on the bioavailability of the metal in the liquid phase.
Bioavailability depends on the retention time of the metal, as well as the interaction with other elements and substances in the water. Factors such as pH, redox potential, and organic matter content affect the tendency of the metal to exist in different ionic and plant-available form.
Plants will affect the soil by lowering the pH and oxidise the sediment. This affects the availability of the metals in bioavailable form of heavy metals by the addition of physicoehemical factors, such as chelating agents and micronutrients.
5. Addition of Chelating Agent:
The remediation process can be improved by increasing the uptake of heavy metals by the plants can be influenced by increasing the bioavailability of heavy metals. This availability can be increased by addition of biodegradable physicochemical factors such as chelating agents, and micronutrients. This can also be improved by stimulating the heavy metal uptake capacity of the microbial community around rhizosphere part of the plant.
The application of chelating agents in heavy metal contaminated soils may promote leaching of the contaminants into the soil. Bioavailability of heavy metals in soils decreases above pH 5.5-6. Therefore, use of a chelating agent is beneficial in alkaline soils. It has been observed that plant exposure to chemicals such as EDTA for a longer period up to 15 days, could improve metal translocation in plant tissue as well as the overall phytoextraction activity.
Selection of appropriate concentration of chelating agents plays crucial role. For example, application of a synthetic chelating agent (EDTA) at 5mmol/kg yielded positive results. Plant roots release citric and oxalic acids, which affect the bioavailability of metals.
In chelate assisted phytoremediation, synthetic chelating agents such as NTA and EDTA are added to enhance the phytoextraction of soil polluting heavy metals. The leaching of metals can be improved for bioavailability of metal to plant by presence of a ligand. This ligand accelerate the uptake of heavy metals through the formation of metal- ligand complexes to leach metals below the root zone.