Cd
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 for this reaction. Numerous microorganisms can hydrolyze the organophosphate pesticide. Illustrations include Pseudomonas diminuta MG and Flavobacterium ATCC 27551 possessing the organophosphate hydrolase enzyme [51]. Bacterial strains isolated from pesticide‐contaminated soil and these strains were tested for their degradation capability and it was found that JCp4 and FCp1 degraded 84.4 and 78.6% the chlorpyrifos pesticide respectively. Moreover, these strains also showed plant growth promoting traits, which includes phytohormone production, phosphate solubilization, and N2 fixation, etc. [49]. In Pakistan, pesticide residues were found above permissible limits in different vegetables analyzed for pesticide residues. It was found that all samples contained pesticide residues of carbofuran and chlorpyrifos and limits with concentrations ranging from 0.01–0.39 and 0.05–0.96 mg/kg, respectively were observed [81].
2.5 Strategies of Bioremediation
Bioremediation is the process of using organisms to neutralize or remove contaminants from waste. It is very important to understand that this form of waste remediation uses no toxic chemicals, although it may use an organism that can be detrimental under certain circumstances. A gory, but simple description of bioremediation is the use of maggots in wound care. Wounds that have contamination can have maggots introduced to them. The maggots then eat the contamination allowing the wound to heal correctly – a form of medical bioremediation. There are many other types that are used to control different waste contamination, which are now described.
2.5.1 Microbial Remediation
The use of microbes such as bacteria and fungi for soil rejuvenation is a form of environmental remediation. The objective of microbial remediation is to remove soil contaminants and pollutants [107]. Though the industrial use of microbes for removing contaminants goes back only three decades, microbes, aerobes, anaerobes, and facultative anaerobes have been contributing to soil improvement for billions of years. They help with N‐fixation, limiting growth of plant pathogens, and the decomposition of heavy metals, pesticides, and hydrocarbons in the soil. The microbial flora is nourished by the contaminants, degrading them for energy and reproduction. Microbial remediation can be divided into three grades:
Natural attenuation: The process takes place naturally with indigenous soil microorganisms.
Biostimulation: The natural process receives external help in the form of nutrients, moisture, and an ideal pH for the microorganisms.
Bioaugmentation: This involves the use of externally introduced microorganisms, which is the case in situations such as oil spills where the naturally occurring microbes may die out because of the intensity of the contamination [108].
Rhizofiltration: Phytofiltration is used to inhibit organic pollutants in wastewater and surface water from mixing with water streams or groundwater using plants for filtration purpose, as they can absorb or adsorb the pollutants [109]. Phytofiltration can also be: rhizofiltration in which plant roots are used; blastofiltration in which seedlings are utilized; and caulofiltration that uses excised plant shoots) [110]. Due to phytofiltration, the movement of contaminants in the soil is minimized [111]. Phytofiltration is defined as the use of plants, both terrestrial and aquatic to absorb, concentrate, and precipitate contaminants from polluted aqueous sources with low contaminant concentration in their roots. Rhizofiltration can partially treat industrial discharge, agricultural runoff, or acid mine drainage. It can be used for lead, cadmium, copper, nickel, zinc, and chromium that are primarily retained within the roots [4]. The advantages of rhizofiltration include its ability to be used in in‐situ or ex‐situ applications and species other than hyperaccumulators can also be used. Plants like sunflower, Indian mustard, tobacco, rye, spinach, and corn have been studied for their ability to remove lead from effluent, with sunflowers having the greatest ability. Indian mustard has proven to be effective in removing a wide concentration range of lead (4–500 mg/l). This technology has been tested in the field with Uranium contaminated water at concentrations of 21–874 μg/l; the treated U‐concentration reported by studies was <20 μg/l before discharge into the environment. The use of some metal accumulator aquatic plants species, both living and dead, and constructed wetlands for the removal of heavy metals from industrial wastewater has gained considerable interest [112]. Aquatic plants and microorganisms can remove metals from water through processes of biosorption and
