Keywords

1 Introduction

During the green revolution, to meet the need of food production for increasing human population, fertilizers and pesticides were used to increase crop productivity and prevent pest attacks (Verma et al. 2014). Pesticides are various groups of inorganic and organic chemicals such as insecticides, herbicides, fungicides, rodenticides, nematicides used to control or kill pests such as insects, herbs, weeds, rodents, nematode, and microorganisms (Table 6.1). An increase in the consumption of pesticides, with the introduction of aldrin, benzene hexachloride (BHC), dieldrin, dichlorodiphenyltrichloroethane (DDT), endrin, and 2,4-dichlorophenoxyacetic acid (2,4D) was mainly began after World War II (Ortiz et al. 2013). However, indiscriminate and unregulated use of pesticides has increased its residues in air, water, soil, and food chain causing negative effects to human health, birds, wildlife, domestic animals, fish (Sharma et al. 2016).

Table 6.1 Pesticides, their characteristics and examples
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In addition to this, pesticides can be categorized according to their chemical composition, which comprises four main groups, namely organochlorines, organophosphorus, carbamates and pyrethrin and pyrethroids (Fig. 6.1) with examples (Table 6.2).

Fig. 6.1
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Type of pesticides based on the chemical composition

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Table 6.2 Types of pesticides and their examples
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2 Organochlorine Pesticides

Organochlorine pesticides or chlorinated hydrocarbons are organic compounds consisting of five or more covalently bonded chlorine atoms, mainly used in agriculture for controlling pests, vector control and in public health. These are non-polar, lipophilic, and persistent. Therefore, unregulated and indiscriminate application of organochlorine pesticides leads to a long-term residual effect in the environment which results in environmental pollution and accumulation in mammals. Aldrin, chlordane, dieldrin, DDT, endosulfan, and lindane are the most common examples of organochlorine pesticides (Ahmad and Ahmad 2014).

3 Organophosphorus Pesticides

Organophosphorus pesticides are a broad spectrum of pesticides as they control a wide range of pests. These can be heterocyclic, cyclic, and aliphatic with phosphorus present in the centre. These pesticides are less toxic as compared to organochlorine pesticides. They have multiple functions such as it can be used as stomach and contact poisons as well as fumigants resulting in nerve poisons. They showed toxicity to vertebrates and invertebrates by binding to acetylcholinesterase or cholinesterases leading to interruption of nerve impulses. Common examples of organophosphorus pesticides are parathion, malathion, diazinon and glyphosate (Ortiz-Hernández et al. 2013).

4 Carbamates

Carbamates can be used as a contact poison, stomach poison and fumigant poison. It is similar to organophosphates in the mode of action, such as by interrupting nerve signals transmission leads to poisoning which causes the death of pest. However, their origin is different, as carbamates are obtained from carbamic acid, whereas organophosphates are derived from phosphoric acid. It can also be used as a contact poison, stomach poison and fumigant poison. Carbamates cause less environmental pollution due to their similar molecular structure to that of natural organic substances resulting in easy degradation. Some of the widely used insecticides are bendiocarb, carbaryl, carbofuran, dioxacarb, fenoxycarb, fenobucarb, isoprocarb, methomyl and propoxur (Kaur et al. 2019).

5 Pyrethroids

Synthetic pyrethroid can be synthesized by copying the structure of natural pyrethrins and used against household pests. As compared to natural pyrethrins, synthetic pyrethroid pesticides are non-persistent with longer residual effects. These pesticides are low toxic to mammals and birds while more toxic to insects and fish. These pesticides are less toxic as compared to organophosphates and carbamates. Allethrin, cyfluthrin, cypermethrin, deltamethrin, and permethrin are the most used synthetic pyrethroid pesticides (Ortiz-Hernández et al. 2013).

6 Different Approaches for Pesticide Remediation

Several methods such as physicochemical and biological play major roles in the remediation of contaminated sites as well as decreasing the residual levels to safe and acceptable levels resulting in minimizing the toxic effects of pesticides on the human health and environment.

6.1 Physicochemical Methods

Physicochemical treatments , such as the Fenton process, heterogeneous photocatalysis (HPC), plasma oxidation and ozonation, thermal desorption (at low temperature) and incineration (Table 6.3) have been applied for the removal of contaminants.

Table 6.3 Characteristics and limitations of different physicochemical methods
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6.2 Biological Methods

Several biological systems, mainly bacteria and fungi are used in the degradation of pesticides from contaminated sites. Because of the adaption of several metabolic pathways, wide distribution and diversity, microorganisms can be vitally used for the remediation of pesticides. The degradation efficacy relies on optimization of environmental conditions, for instance, pH of the soil, temperature, moisture contents as well as microbial community (Chishti et al. 2013; Javaid et al. 2016). Various microorganisms that have the potential to degrade pesticides are listed in Table 6.4.

Table 6.4 Pesticide degradation by microorganisms
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In recent years, the use of fungi as an effective tool for the biodegradation process has increased due to relatively easy cultivation and possession of a great enzymatic metabolism (Camacho-Morales and Sánchez 2016). Several studies that reported pesticides degradation by fungi are listed in Table 6.5.

Table 6.5 Pesticide degradation by fungi
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7 Several Enzymes Involved in Pesticide Degradation

Enzymes play an essential role in the bioremediation of individual pesticides. The use of enzymes to degrade or transform pesticides is an emerging technology as it is more effective than physicochemical techniques. Enzymes are involved in the pesticide degradation via evolved metabolic resistance and several intrinsic detoxification mechanisms in the target organism, whereas in the environment through biodegradation by water and soil microorganisms. Pesticide metabolism involves (i) transformation of the parent compound to a more water-soluble and a less toxic product via hydrolysis, reduction, or oxidation, (ii) conjugation of pesticide metabolites to an amino acid or sugar resulting in a decrease in toxicity as well as increase in water solubility, (iii) conversion of pesticide metabolites into non-toxic secondary conjugates. Bacteria and fungi involved extracellular or intracellular enzymes which are involved in pesticide metabolism (Ortiz-Hernández et al. 2013). Enzymes involved in bioremediation were mainly oxidoreductases, monooxygenase, dioxygenases, carboxylesterases, phosphotriesterases, haloalkane dehalogenases, haloalkane dehydrochlorinases, diisopropylfluorophosphatase, Paraoxonase (PON1), organophosphate acid anhydrolase (OPAA), carboxylesterases (Table 6.6). Several enzymes that have been applied for the degradation of pesticides from polluted environments are present in Table 6.7.

Table 6.6 Enzymes and their characteristics
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Table 6.7 Summary of pesticide degrading enzymes of various microbial strains
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8 Conclusion

In addition to controlling or killing pests, pesticides can move off-site resulting in contamination of water, soil and the ecosystem resulting in damage to non-target organisms. The bioremediation process for the detoxification and/or removal of pesticide residues from polluted soil is the best option available in integrated agricultural management practices, due to its eco-friendly, cost-effective and efficacious nature. Various microorganisms (bacteria and fungi) are used in the removal of pesticides from contaminated sites. Bacterial degradation involves specific genes and enzymes for the breakdown of functional groups present in the pesticides. In conclusion, although significant research has been done on the enzymes associated with the biodegradation of pesticides, further research in relevant environmental conditions is needed to confirm the ability of bacteria and fungi for large-scale decontamination. In addition, significant degradation of pollutants will be enhanced using genetically engineered microorganisms that will produce many recombinant enzymes using eco-friendly technology is still unexplored.