[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 inhibitors. Among the aquatic species, enzymes have been identified that are partially characterized by salmon, invertebrates such as Rangia cuneata, protozoa Tetrahymena thermophila, and various thermophilic and other bacteria. These enzymes evolved in response to the metabolism of naturally occurring organophosphates and halogenated organic compounds. Several researchers have highlighted the presence of organophosphorus hydrolase (Aryldialkylphosphatase) genes in microbial cells that degrade organic phosphorus compounds and hydrolases as the main enzyme behind the process. An enzyme derived from an over‐produced P. diminuta undergoes hydrolysis of a phosphate bond in an organophosphorus pesticide molecule resulting in an up‐to 100‐fold reduction in toxicity. The use of hydrolases and related genes to understand the complex interactions between microorganisms and pesticides can significantly improve understanding of biodegradation processes and facilitate bioremediation [126, 129, 131].
2.7 Behavior of Inorganic and Organic Pollutants in Soil
Translocation, mobility, uptake, and accumulation of contaminants in plants are noticeably caused by main two factors: (i) environmental factors, and (ii) genetic makeup between plant species to uptake contaminants, their translocation and storage to different organs and plant resistance against specific contaminants. These mechanisms of contaminants in different parts of plants are known to differ and are also imbalanced. The uptake mechanisms for organic or inorganic complexes are also diverse. The movement of inorganic compounds within plants for example, nutrients, metals, and metalloids is generally carried out by through active transport and passive diffusion. An example, using Ni, showed that passive diffusion was influenced at a high concentration of nickel whereas active transport of Ni2+ was revealed to play a vital role in its uptake from medium to low concentrations of Ni. Inorganic contaminants, usually transportation of metals, are carried from root membranes with the support of membrane transport proteins that belong to the CDF proteins family (cation diffusion facilitators). It is remarkable that a binding domain of protein identifies only specific ions and is responsible for its transportation [132]. In contrast to the inorganic translocation system, organic contaminants don't have any specific carrier to carry them through the plant. They simply move into the symplast and xylem apoplast due to their hydrophobicity and also pass into the leaves by simple diffusion [133]. Moreover, translocation of both organic and inorganic pollutants is influenced by soil rhizosphere microorganisms, which are in symbiosis with the roots [129, 134]. Microorganisms also have diverse functions, for example, they excrete organic compounds to the soil, which increases the bioavailability in the soil, and the transit of metals to the plant from the roots, they may also alter their chemical properties, which could make them harmless, harmful, mobile, or immobile [117].
Due to the distinct ability of HM accumulation in plants, they are divided into three classes: accumulators – obtain high levels of metals on the surface of the ground, easy to reap organs, and self‐reliance of metal concentrations in the soil; excluders – they have very confined transit of metals to the shoots from the roots even if the soil is enrich with the metal contaminants; indicators – they manifest the levels of metal contamination in the rhizospheric soil [135]. Furthermost the plants belong to the class of excluder, as in the case of Pb, its translocation to the shoots is very confined but it accumulates in the cell walls of the cortex due to the weak transportation of metal ions to the shoots [136], or due to the prolonged distance between the shots/roots [125]. The degree of metal uptake and its translocation varies from plant to plant spices and it is one of the most notable features of plant resistance [137]. Whereas the ability of HM accumulation to one or another level, they are recognized as hyperaccumulators, accumulators, and nonaccumulators [138]. Hyperaccumulators have 100 times higher concentrations in their shoots than nonaccumulators. Until now, about 400 species have been characterized as metal hyperaccumulators as they genetically have great capacity to accumulate high levels of HM in their shoots. At the time of uptake from the roots, the major fraction of metal is present in the plant rhizodermis and cortex [134, 139, 140]. The distribution of metals in hyperaccumulator plants is truly systemized through proficiency of many detoxification mechanisms and is determined by the features of metal transport. Ni is easily transported to the tissues