Introduction

Growing environmental pollution due to the increase in industrialization and urban waste by human activities such as mining, urbanization, smelting, and accidental spillage destroy the quality of water, air, and soil by polluting them. Such human activities release various contaminants or pollutants like organic or inorganic that are not good for the soil, water, and air. Contaminants like leachate reach the non-contaminated site and contaminate our resources and ecosystem. Various organic compounds like PCBs, petroleum products, high molecular weight hydrocarbons, and inorganic like heavy metals can cause contamination. Various processes are involved in remediating the contaminated site; one of them is phytoremediation. Phytoremediation is a process of remediation of a contaminated site like soil and water with the help of plants. In this process, the plants used can withstand polluted sites can degrade the pollutant or convert them into less toxic substances from more toxic substances and accumulate inside the body or precipitate in the root zone.

Phytoremediation Techniques

Phytoremediation is further divided into different types of techniques; they are:

 https://en.wikipedia.org/wiki/Phytoremediation

Phytoextraction

It is one of the types of the phytoremediation process, which is the most suitable approach for polluted soil to remediate it, in which pollutants are extracted from the soil and then precipitated or stored into the plant biomass. This technique is suitable for that polluted areas where concentration is low or superficial. This process is categorized into two different techniques: chelated phytoextraction and continuous phytoextraction. The chelated phytoextraction process uses some artificial chelates, which are added to the polluted site, which increases the contaminant mobility and uptake, whereas, in the case of continuous phytoextraction, the natural hyperaccumulator plant is used to the remediation process, the hyperaccumulator plant natural accumulates a higher amount of contaminant as compared to other plants, for example:- Mustard, sunflower.

Phytostabilization

It is a technique in which hyper-accumulator plants are used for toxic metals to stabilize them by accumulation, adsorption, precipitation, complexation process through roots and immobilized them and exclude the bioavailability of those toxic metals present in the soil and prevent the entry into groundwater or the food chain.

Phytodegradation/Transformation

It is a technique used to decontaminate the polluted site by using plants. It is also referred to as phytotransformation, in which the organic contaminants are degraded or transformed into different or less harmful simpler forms of contaminants and picked up by the plants and incorporated into their body tissues. Plants have some enzymes that are used to degrade chlorinated solvents like trichloroethylene and ammunition wastes.

Phytostimulation/ Rhizodegradation

The phytostimulation or rhizodegradation technique occurs with the help of microbes like fungi and bacteria, in which microbes help the degradation of the pollutants present in the soil or rhizosphere region and transform into less harmful form, and it is referred to as rhizodegradation. It is a slower process than phytodegradation.

Phytovolatilization

In this technique, plants uptake the soil contaminants, convert them into volatile form, and release them back into the atmosphere by transpiration in low concentrations. It is majorly used for treating the soil, which is contaminated by mercury. The mercury ion is released back as elemental mercury, and this process is successfully reported in treating the tritium (3H). The re-emitting of mercury causes problems, which is a drawback of this method.

Rhizofiltration

It is a phytoremediation technique used by both aquatic and terrestrial plants. Plants absorb the organic and inorganic contaminants by the plants’ roots from contaminated water, then concentrate and precipitate into roots. This method can partially treat industrial discharge, acid mine drainage, or agricultural runoff and can be used to absorb heavy metals like lead, copper, cadmium, mercury, zinc, chromium. This method is used in both in-situ and ex-situ conditions and non-hyperaccumulator plants.

Plant Selection

The plant selection for the phytoremediation process requires those plants that can survive in a polluted area and uptake the concerned pollutant and accumulate in their upper body part or degrade it. Fast growth with high biomass. Easily harvestable and non-consumable by animals and humans. Usually, the native plant species are best suited for this process. Some plants have proven quality for the Phytoremediation process—for example, Indian mustard, white willow, poplar tree, Indian grass, sunflower.

Advantages & Disadvantages

1. The technique is eco-friendly and does not cause any pollution.
2. It does not require any heavy machinery equipment to employ this method.
3. It can be used for a wide range of both organic and inorganic pollutants.
4. This technique can be applied in situ and ex-situ with effluent and soil substrate, respectively.
5. It can reduce waste by about 95% in landfills.
6. It is cheap and aesthetic.
7. It is a slow process that takes time to remediate the polluted site.
8. It cannot be used for the heavily polluted site.
9. The use of non-native plants can create biodiversity imbalance.
10. It is restricted to the shallow contaminated site within the root zone.
11. Climate acts as the limiting factor that challenges the degradation process of pollutants.