use of dead or dried aquatic plants, for metal removal as a simple biosorbent material has advantages in its high efficiency including minimization of the volume of chemical and/or biological sludge, no nutrient requirements, low cost, conservation, transport, handling, and metal recovery [113–116].
2.5.2 Phytovolatilization
Phytovolatization is the process in which pollutants are up taken by the plants from the soil and then converted into a volatile form and then released in the atmosphere. Phytostabilization can be used for organic pollutants and other heavy metals like Se and Hg. But, as explained earlier, phytostabilization transfers the pollutants into the atmosphere, from one medium to another, and does not remove the pollutants permanently. The pollutants in the atmosphere can also be redeposited into the soil at a later time. In phytoremediation of organics, plant metabolism contributes to the contaminant reduction by transformation breakdown, stabilization, or volatilizing contaminant compounds from soil and groundwater. Phytodegradation is the breakdown of organics, taken up by the plant to simpler molecules that are incorporated into plant tissues. Plants contain enzymes that can breakdown and convert ammunition wastes, chlorinated solvents such as trichloroethylene, and other herbicides. The enzymes are usually dehalogenases, oxygenases, and reductases. Rhizodegradation is the breakdown of organics in the soil through microbial activity of the root zone and is a much slower process than phytodegradation. Yeast, fungi, bacteria, and other microorganisms consume and digest organic substances like fuels and solvents. All phytoremediation technologies are not exclusive and may be used concurrently, but the metal extraction depends on its bioavailable fraction in soil [117, 118].
2.5.3 Phytodegradation
Phytodegradation, also known as phytotransformation, is the use of plants and microorganisms to uptake, metabolize, and degrade the organic contaminant. In this method, plant roots are used in association with microorganisms to detoxify soil contaminated with organic compounds [119]. Some plants can decontaminate soil, sludge, sediment, and ground and surface water by producing enzymes. It involves organic compounds, including herbicides, insecticides, chlorinated solvents, and inorganic contaminants [120].
2.6 Adaptive Mechanism of Bioremediation for Heavy Metals, Pesticides, and Herbicides
Plants adopt different strategies and complex mechanisms for survival under critical conditions, which are mostly caused by biotic or abiotic stresses. Plant stress occurs through damage by certain living or nonliving species. Living organisms such as viruses, bacteria, parasites, fungi, beneficial or harmful insects, and native or cultivated plants mostly initiate biotic stresses. Abiotic stresses such as, mineral toxicity, excessive watering, drought, extreme temperatures, salinity negatively impact development, growth, yield, and seed quality of plants and other crops. Differentiation between the damage caused by living or nonliving agents is a very difficult task even with the help of accurate diagnosis and close examinations. Relative oxygen species generated in the plants that accumulate in cells under environmental stress results in the oxidation of carbohydrates, proteins, lipid, chlorophyll, nucleic acids, etc. [121–123].
2.6.1 Defense System
Plant cells have evolved intricate defense systems including: (i) Plant hormones (phytohormones), such as, ethylene, jasmonic acid [JA], salicylic acid [SA], abscisic acid [ABA], and brassino‐steroids. These phytohormones are mostly required by plants for their growth and development and occasionally act as defense mechanisms. (ii) Enzymatic systems that comprise superoxide dismutase [SOD], ascorbate peroxidase [APX], catalase [CAT], glutathione reductases [GR], dehydro‐ascorbate reductases [DHAR], mono dehydro‐ascorbate reductases [MSHAR], glutathione peroxidase [GPX], glutathione‐S‐transferase [GST] and guaicol peroxidase [GOPX]. (iii) Nonenzymatic systems include glutathione [GSH], ascorbic acid, nonprotein amino acids, α‐tocopherol, phenolic compounds and alkaloids, which can hunt primordial generated ROS [124, 125].
Different research has documented different mechanisms for the degradation of HMs and pesticides as follows.
2.6.1.1 Adsorption
The adsorption of orthophosphate‐pesticides in soil and the resulting decrease in fluidity are important factors that influence their behavior in nature. The degree of adsorption as well as the rate and extent of final degradation are influenced by several factors including solubility, volatility, charge, polarity, molecular structure, and pesticide size. The process of soil particle adsorption can prevent degradation of pesticides by separating the pesticide from the enzyme that degrades it or by enhancing the degradation process. Abiotic hydrolysis degradation improves the adsorption process. Conversely, volatilization or leaching following adsorption leads to a reduction in the loss of pesticides. Various physical and chemical forces in the process of soil particle adsorption include van der Waals forces, dipole–dipole interactions, hydrogen bonds, and ions replacement. However, there is less information available for the adsorption of ionizable pesticides and extensive studies are needed to analyze the background mechanisms to predict the nature of pesticide interactions with the soil, as these phenomena may affect other processes [126, 127].
2.6.1.2 Photodegradation
Photolysis of organophosphorus pesticides can be a very important degradation pathway in aqueous environments as well as in the gaseous phase. The degradation of chlorpyrifos under environmental conditions has been studied and around 200 American crabs were killed by about 20 μg/l in the Ebre Delta Irrigation ditch in Spain. The content of chlorpyrifos and its conversion products was recorded four days after application. The chlorpyrifos transformation product is 3‐methyl‐4‐nitrophenol, Acaricion, and S‐methyl isomer. The half‐life of chlorpyrifos is 13 hours and the degradation rate constant 0.053/hour, mainly by photolysis. The degradation of chlorpyrifos and the formation of its transformation products are closely related to environmental factors such as wind [126–128].
2.6.1.3 Hydrolysis
Hydrolysis is the most thoroughly studied degradation pathway for organophosphorus pesticides. The organophosphorus pesticide can be diverse and usually involves the cleavage of bonds, which produce the best product. A good example of bond cleavage can be found in the hydrolysis of diazinon, where the oxygen attached to the pyrimidine ring can most effectively stabilize the negative charge and similar behavior can be found for other phosphorothioates as well. During the hydrolysis of dichlorvos, the possible initial hydrolytic cleavage lies between the P and O atoms attached to the carbon atom of the double bond. Alkaline hydrolysis is the major pathway for malathion and is consistent with previous laboratory studies. It has been observed that only the biological and photochemical degradation of malathion is slow, further the biodegradation is important for parathion. Alkaline hydrolysis and photolysis are only minor ways of degrading parathion. Digestion mechanisms include copper‐catalyzed chlorpyrifos hydrolysis. P. putida can use parathionmethyl as the sole source for C and/or P. Bacteria producing enzyme, the organophosphoric anhydrase, which hydrolyzes parathionmethyl to p‐nitrophenol, further degrades to hydroquinone and 1, 2, 4‐benzenetriol and then cleaves to acetic acid by glycerol oxygenase [126, 129, 130].
2.6.1.3.1 Enzymatic Degradation
Enzymes known to hydrolyze many organophosphorus pesticides are made from a variety of aquatic species. These enzymes are known as organophosphoric anhydrases, although they are also known as paraoxonase, esterase, phosphotriesterase, diisopropylfluorophosphatase, and parathion hydrolase. However, these enzymes can hydrolyze a variety of organophosphorus acetylcholinesterase
