and agricultural purposes. Plants growing on HM‐polluted sites take in this contaminated water and accumulate HM in them and when these plants are used as food, these HM are transferred to the organism's bodies accumulating again [43]. Once fresh water organisms are exposed to HMs, like Cr, Ni, Cd, and Pb, accumulation is increased with increased exposure time, harming the consumer's kidneys and other body organs [47]. Food crops consumption that are irrigated with HM‐contaminated water is also a major route of human exposure to HM [48, 49]. Metals are being utilized in a range of ways in industries and agriculture; particularly heavy metals such as Hg, Cd, Pb, and As. These constitute a significant potential threat to human health via contaminated water [50]. Water pollution remains a major source of morbidity and mortality in different parts of the world. Water environment treatment has led to improved health outcomes among Chinese people. Reduced water pollution mediated the associations between water environment treatment and health outcomes [51].
2.3.5 Humans
HMs are among one of the widespread pollutants in the world, which cause severe threats to almost every organism including humans. In agricultural soils, HMs like Cd, Cu, Pb, and Zn concentrations are 0.097, 22.6, 26.0, and 74.2 mg/kg, respectively and can even reach 3.16, 99.3, 84.1, and 147 mg/kg, respectively. In water, Cd, Cu, Pb, and Zn concentrations are 0.080, 7.91, 15.7, and 18.7 ug/l respectively and cause contamination in drinking water and food that threatens human health [52]. Occupational exposures to Mn, Cu, Fe, Hg, Zn, and Al pose a risk factor for Parkinson's disease [53, 54]. Rice grown on Cd contaminated soils causes Cd loaded grains and poses kidney failure in humans consuming the contaminated rice [55–57]. Table 2.1 defines HM effects on plants and humans.
Table 2.1 Effect of different heavy metals on plants and humans.
| Heavy metals | Effects on plants | Citations | Effects on Humans | Citations |
| As | Growth, leaf gas‐exchange, water potential, protein content, and biomass. | [58–61] | Carcinogen, cyto, and genotoxic, cause diabetes, cardiovascular diseases, and disrupts DNA repair. | [62–66] |
| Al | Decreases in biomass, photosynthesis, protein contents, disrupts NPK, Ca, Mg uptake | [67–70] | Neuro, geno, and cytotoxic, damages membrane and DNA, cause chromosomal and lymphocyte aberrations. | [71, 72] |
| Cr | Damages rhizobia, production of ROS and oxidative stress, low seed germination and reduced nutrient uptake | [73–76] | Carcinogen, damages DNA and reproductive system with birth and growth defects. | [73–77] |
| Ni | Reduces growth and photosynthetic pigment contents, low crop growth, increases imbalance between K and Ca | [78–80] | Cardiovascular diseases, haemo, immuno, neuro, geno, nephron, and hepatotoxic, carcinogen and cause reproductive system illness. | [81, 82] |
| Cu | Low yield, failure in making seed, less photosynthetic rate, ROS production, cell damage, disturbs photosynthesis and electron transport. | [83, 84] | Cardiovascular system and disrupts enzymes activity. | [85, 88] |
| Pb | Stunted growth, chlorosis and blackening of root system, inhibit photosynthesis, upset water balance, changes affects membrane structure. | [89, 90] | Mental retardation, growth impairment, comma, convulsions, anemia, hypertension, and immunotoxicity. | [91, 92] |
| Cd | Reduction in soil biota, stomata opening, transpiration, and photosynthesis. | [[93–95] | Carcinogen, negative impact on cardiovascular, immune and reproductive system, nephrotoxic. | [96–98] |
| Zn | Inhibition of several enzymes lowers photosynthesis rate. | [99, 100] | Nausea, vomiting, diarrhea, kidney, and stomach damage, burning, stinging, itching | [86, 101, 102] |
| Hg | Reduces soil microbes, poor plant growth, reduced nutrient uptake, and reduced seed germination, decreased biomass. | [103–105] | Neurobehavioral defects, walking, vision, hearing, and speech is affected | [65, 91, 106] |
2.4 Fate of Pesticides and Its Biodegradation in Soil
Organophosphate pesticide primarily catalyzes the degradation of the neurotransmitter acetylcholine in the synapse. This enzyme rapidly hydrolyzes the acetylcholine, a neurotransmitter into choline by stopping the stimulation of nerves. These compounds block the normal activity of acetylcholinesterase by binding covalently to the enzyme, thereby changing the activity and function. The accumulation of neurotransmitter acetylcholine (Ach) in the synapse leads to blockage of nerves because regeneration of acetylcholine esterase is a slow process and takes hours or even days. This blockage of nerves causes permanent paralysis and finally the death of insects and pests. Solanum lycopersicum L. being the important human diet constituent is grown throughout the world. It is commonly used in a salad, consumed as a sauce and juice. Different types of pesticides like cypermethrin, deltamethrin, profenofos, and chlorpyriphos, are used to protect the tomato crop from damage caused by these insects and pests [87].
Bioaccumulation is the primary cause of toxicity of pesticides as their higher concentrations in biological systems leads to major health problems. The persistence of organophosphate pesticides in soil has also been related to the organic matter, clay content, and iron or aluminum oxy content of the soil. These have a higher affinity to absorbing the pesticides and act as a sink for such hydrophobic compounds affecting plants, humans, and animals. Pesticides undergo various changes in the environment comprising their adsorption transmission and degradation, which depends on the nature of soil, pesticide type, and its physico‐chemical properties. The predominant process involved in transformation of such molecules is facilitated by microbes [78] followed by photolysis or photo‐degradation and chemical transformations [43]. Chemical and microbial degradation are difficult to distinguish as both processes go side by side. Further the physical properties of the soil also play an integral part, as the clay content in soil leads to an increase in surface area, which enhances hydrolytic conversion [55].
Bioremediation is considered an environment friendly, green, and economical technology for degradation of persistent organic pollutants such as pesticides. Pseudomonas putida, Bacillus subtilis, Burkholderia gladioli, and Pseudomonas aeruginosa have been reported as efficient microbial species for the degradation of profenofos pesticide. These microbial species degrade profenofos by hydrolysis to yield 4‐bromo‐2‐chlorophenol as metabolite [73]. Hydrolysis being the most significant step plays an important role in detoxifying the organophosphate compound that makes it vulnerable to further degradation. Esterase or phosphortriesterase enzymes are responsible