**7. Phytoremediation of heavy metal polluted sewage and wastewater**

Pesticides, oils, colors, phenol, cyanides, hazardous organics, phosphorus, suspended particles, and heavy metals can all be found in untreated industrial and home wastewater. Among these harmful compounds, heavy metals are easily collected in the environment. To reduce the risk to the environment and human health, it is crucial to remove harmful contaminants from the environment. The fact that heavy metals can take on various chemical forms makes it challenging to remove them from wastewater. Most metals are not biodegradable, and they can easily move between trophic levels to accumulate chronically in the biota. High pollutant concentrations in wastewater are extremely harmful to both human health and aquatic ecosystems.

The development of several heavy metals, including zinc, cadmium, lead, nickel, and so on, is a result of wastewater irrigation. Some of these metals, for instance Ni, Zn, Cd, Cu and Pb, are frequently noticed in the soil's subsurface when untreated wastewater is used to irrigate the soil. Long-term wastewater irrigation raises the level of hazardous heavy metal content in the soil [13].

Phytoremediation has become a significant experimental and practical strategy to use plants to remove heavy metals from sludges, sewage waters, spillage sites, and polluted places. When compared to other remediation methods, phytoremediation offers several benefits:


The idea of using metal accumulator plants in phytoremediation for the removal of heavy metals and several more-than toxins was originally proposed in 1983, but it has been around for 300 years. Numerous terms, including agro-remediation, green remediation, vegetative remediation, botanic remedy, and organic (green) technology, are used to refer to phytoremediation. For the elimination of potentially hazardous metals from the environment, phytoremediation is regarded as an efficient incredibly attractive, economically advantageous, and environmentally beneficial technology. In phytoremediation, plants collect pollutants through their roots and then move them to their aboveground parts of the body [13].

To degrade, remove, or immobilize the pollutants, phytoremediation uses a variety of mechanisms, including accumulation (phytoextraction, rhizofiltration,), degradation (rhizo-degradation, phytodegradation,), immobilization (Phyto stabilization and hydraulic control), and dissipation (phytovolatilization). Plants use a variety of these methods to lower the amounts of pollutants in soil and water, depending on the contaminants. For instance, plants absorb and store HMs in their tissues, and they breakdown organic contaminants to lessen their toxicity in the soil and water. Depending on the types, forms, and mediums of the contaminants, various plants use various strategies or combinations of them to remediate soil and water. Rhizo-filtration, phytodegradation, phytovolatilization, rhizo-degradation, and phytodegradation are all methods for cleaning up contaminated groundwater. Rhizo-filtration, phytodegradation, and rhizo-degradation are three treatment options for surface and wastewater contamination. Through phytodegradation, phytovolatilization, phytoextraction, and rhizodegradation, contamination caused by soil, sediments, or sludge is remedied [15].

#### **7.1 Phytoextraction**

Phytoextraction can be used to eliminate heavy metals [15]. By translocation in the sections of roots that can be harvested, pollutants are retained in the shoot tissue [16]. One of the environmentally safe, long-lasting alternatives for soil cleanup is phytoextraction, which also offers the potential for reusing the metals that are extracted through phytomining. The hyperaccumulating plants' tolerance levels and development restrictions frequently place a limit on phytoextraction. Through the introduction of moderate stress cues that lead to acclimatization in the plant, priming can affect the tolerance of plants to stress. Utilizing various types of chemicals that are added exogenously to plant organs (such as roots, leaves, etc.), plant priming's capacity to increase abiotic stress tolerance has been thoroughly studied [9].

#### **7.2 Phytodegradation**

Enzymatic activity breaks down organic pollutants [16]. Organic substances can undergo phytodegradation either inside the plant or in the rhizosphere. This technique can be used to remove a wide variety of substances and classes of compounds from the environment, including as solvents in groundwater, aromatic compounds in soils, volatile compounds in the air, and petroleum [8]. Petroleum is a pollutant that can remain in the environment for a very long time before vegetation fully recovers, and its persistence can be attributed to the hydrocarbons' slow biodegradation. The effects of petroleum on plants might be direct when they encounter oil or indirect when biotic and abiotic changes related to plant development occur. Because of their potential to degrade contaminants, several species of Caesalpiniacae, Mimosaceae, Fabaceae, and Poaceae have been investigated. When compared to soils with no vegetation, the decomposition of petroleum and its derivatives in soils with Juncus roemerianus Scheele, Sorghum bicolor L. Moench, Vigna sinensis (L.) Endl. ex Hassk., Panicum maximum Jacq, Medicago sativa L., Brachiaria brizantha. Stapf., and Festuca arundinacea Vill. The Poaceae family of plants are most important as compared to other plant families because they encourage the removal of pollutants. By altering the physical and chemical properties of the soil, plants and their roots have a direct impact on the breakdown of contaminants [10].
