Kinetic Studies of Biodegradation PDF

MIC-644 Dr. Sadiah Muhammad Saleemullah Khan Kinetic studies of biodegradation Thousands of chemicals and materials with varied properties and functionalities are manufactured and used for commercial and day-to-day applications, whose ultimate fate...

MIC-644 Dr. Sadiah Muhammad Saleemullah Khan Kinetic studies of biodegradation Thousands of chemicals and materials with varied properties and functionalities are manufactured and used for commercial and day-to-day applications, whose ultimate fate in the environment may not be known. During their manufacture and use, these substances are often discharged into the environment through different routes in air, water and land. Creation of tremendous quantities of solid waste of all kind and its effective disposal has posed innumerable problems that need technological breakthroughs. Many of these substances degrade slowly and exert toxic effects on plants and animals, thus causing large scale environmental degradation. Biodegradation, either aerobic or anaerobic, can be an approach to cleave big molecules through a series of steps in to smaller molecules from a mosaic of chemicals and materials and some of them can be valorised as pollution abatement strategy and source of energy through biogas generation. Biogas can be produced from nearly all kind of biomass, among which the primary agricultural sectors and various organic waste streams can be properly tapped as renewable source of energy. Untreated or poorly managed animal manure is a major source of air and water pollution. Mechanisms of biodegradation Cellulose, lignocellulose and lignin are major sources of plant biomass and are polymeric substances; therefore, their recycling is indispensable for the carbon cycle. Each of these polymer is degraded by a variety of microorganisms which produce scores of enzymes that work in tandem. The diversity of cellulosic and lignocellulosic substrates has contributed to the difficulties found in enzymatic treatment. Fungi are the best-known microorganisms capable of degrading these three polymers. Because the substrates are insoluble, both bacterial and fungal degradation occur exo-cellularly, either in association with the outer cell envelope layer or extra-cellularly. Microorganisms have two types of extracellular enzymatic systems, namely, the hydrolytic system, which produces hydrolases and is responsible for cellulose and hemicellulose degradation; and a unique oxidative and extracellular ligninolytic system, which depolymerizes lignin. The man- made chemicals and materials are comprised of different entities and functional groups which need to be degraded effectively by microorganisms and no single microorganism is obviously capable of doing it. Growth and co-metabolism are the two mechanisms of biodegradation. In the case of growth, organic substance is used as the sole source of carbon and energy, which leads to complete degradation (mineralization). Archaebacteria, prokaryotes and eukaryotes (like fungi, algae, yeasts, protozoa) play dominant role in mineralization. On the contrary, co-metabolism encompasses the metabolism of an organic compound in the presence of a growth substrate which is used as the primary carbon and energy source. Thus, biodegradation processes and their rates differ greatly depending on the type of substrate and conditions such as temperature, pH, and aqueous phase solubility, but frequently the major final products of the degradation are carbon dioxide and methane. Growth-associated degradation of aliphatic compounds Growth-associated degradation produces CO2, H2O, and cell biomass. The cells act as the complex biocatalysts of degradation. Further, cell biomass may be mineralized after exhaustion of the degradable pollutants in a contaminated site. Bulk chemicals like aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylenes, and naphthalene are widely used as fuels, industrial solvents and feedstock for petrochemical industry. Phenols and chlorophenols are another class of chemicals, employed in a variety of industries. Since all micro-organisms make aromatic compounds such as aromatic amino acids, phenols, or quinines, in large amounts, many microorganisms have evolved catabolic pathways to degrade aromatic compounds. In general, man-made organic chemicals (xenobiotics) can be degraded by microorganisms, when the respective molecules are similar to natural compounds. In general, benzene, condensed ring and related compounds are characterized by a higher thermodynamic stability than aliphatic compounds. Benzene oxidation begins with hydroxylation catalyzed by a dioxygenase leading to a diol which is then converted to catechol by a dehydrogenase. Mechanism Monooxygenase and dioxygenase reactions: In this mechanism, monooxygenase initially incorporates one O atom from O2 into the xenobiotic substrate whereas the other is reduced to H2O. On the contrary, dioxygenase incorporates both atoms into the substrate. Hydroxylation and dehydrogenation are also common in degradation routes of other aromatic hydrocarbons. The introduction of a substituent group onto the benzene ring renders alternative mechanisms possible to attack side chains or to oxidize the aromatic ring. Many aromatic substrates are degraded by a limited number of reactions such as hydroxylation, oxygenolytic ring cleavage, isomerization, and hydrolysis. The inducible nature of the enzymes and their substrate specificity enable bacteria such as pseudomonads and rhodococci with a high degradation activity, to acclimatize their metabolism to the effective utilization of substrate mixtures in polluted soils and also to grow at a high rate. Co-metabolic degradation of organo-pollutants Co-metabolism is a common phenomenon of microbial activities and the basis of biotransformation used in biotechnology to convert molecules in to useful modified forms. Microorganisms growing on a particular substrate also oxidize a second substrate. The co- substrate is not incorporated, but the product may be available as substrate for other organisms of a mixed culture. The rudiments of co-metabolic transformation are the enzymes of the growing cells and the synthesis of cofactors necessary for enzymatic reactions; for instance, of hydrogen donors (reducing equivalents, NADH) for oxygenases. Several aromatic substrates can be converted enzymatically to natural intermediates of degradation such as catechol and protecatechuate Mechanism Degradation of aromatic natural and xenobiotic compounds into two central intermediates, catechol and protocatechuate. Co- metabolism of chloroaromatics is a general activity of bacteria in mixtures of industrial pollutants, which may be auto-oxidized or polymerized in soil to humic-like structures. Substitution of halogen as well as nitro and sulfo groups at the aromatic ring is accomplished by an increasing electrophilicity of the molecule. These compounds resist the electrophilic attack by oxygenases of aerobic bacteria. Compounds that persist under oxic condition are polychlorinated biphenyls (PCBs), chlorinated dioxins and some pesticides like DDT. To overcome the relatively high persistence of halogenated xenobiotics, reductive attack of anaerobic bacteria is of great value. The process reduces the degree of chlorination and, therefore, makes the product more accessible to mineralization by aerobic bacteria. Reductive dehalogenation which is the first step of degradation of PCBs requires anaerobic conditions wherein organic substrates act as electron donors. PCBs accept electrons to allow the anaerobic bacteria to transfer electrons to these compounds. Most dechlorinating cultures are a mixed consortia. Anaerobic dechlorination is always incomplete and the products are di- and monochlorinated biphenyls. These products can be metabolized further by aerobic microorganisms. Aerobic biodegradation Many microorganisms grow under aerobic conditions. The cellular respiration process (CSP) begins with aerobes which employ oxygen to oxidize substrates such as sugars and fats to derive energy. Before the onset of CSP, glucose molecules are degraded into smaller molecules in the cytoplasm of the aerobes. The smaller molecules then enter a mitochondrion, where aerobic respiration takes place. Oxygen is used to break down small entities into water and carbon dioxide, accompanied by release of energy. Aerobic degradation does not produce foul gases, unlike anaerobic process. The aerobic process leads to a more complete digestion of solid waste reducing build-up by more than 50% in most cases. The major enzymatic reactions of aerobic biodegradation are oxidations catalyzed by oxygenases and peroxidases. Oxygenases are oxido-reductases that incorporate oxygen into the substrate. Degradative organisms need oxygen at two metabolic sites, namely, at the initial attack of the substrate and at the end of the respiratory chain. Higher fungi possess a unique oxidative system for the degradation of lignin based on extracellular ligninolytic peroxidases and laccases. This enzymatic system is important for the co-metabolic degradation of persistent organic pollutants. The predominant bacteria of polluted soils belong to a spectrum of genera and species (Table 1). The most important classes of organic pollutants in the environment are mineral oil constituents and halogenated petrochemicals, for the biodegradation of which the capacities of aerobic microorganisms are of great consequence. The most rapid and complete degradation of the majority of pollutants is brought about under aerobic conditions and these include petroleum hydrocarbons, chlorinated aliphatics, benzene, toluene, phenol, naphthalene, fluorine, pyrene, chloroanilines, pentachlorophenol and dichlorobenzenes. Many cultures of bacteria grow on these chemicals and are capable of producing enzymes which degrade them into non-toxic species. Gram negative bacteria Gram positive bacteria Pseudomonas species Nocardia species Xanthomonas species Mycobacteria species Alcaligenes species Corynebacterium species Flavobacterium species Arthobacter species Cytophaga group Bacillus species The chemicals must be accessible to the degrading organisms. For example, hydrocarbons are immiscible in water and their degradation requires the production of biosurfactants in order to have effective biodegradation. The initial intracellular attack of organic pollutants is an oxidative process and therefore, the activation and incorporation of oxygen is the main enzymatic reaction catalyzed by oxygenases and peroxidases. Peripheral degradation pathways convert organic pollutants step by step into intermediates of the central intermediary metabolism, such as the tricarboxylic acid cycle. Biosynthesis of cell biomass from the central precursor metabolites (acetyl-CoA, succinate, pyruvate) is required. Sugars needed for various biosyntheses and growth must be synthesized by gluconeogenesis. The predominant degraders of organo-pollutants in the oxic zone of contaminated areas are chemo-organotropic species that are able to use a large number of natural and xenobiotic compounds as carbon sources and electron donors for the generation of energy. Although many bacteria are able to metabolize organic pollutants, a single bacterium does not possess the enzymatic capability to degrade all or even most of the organic pollutants from a heterogeneous mixture originating from particular industries. Thus, mixed microbial communities have the most powerful biodegradative potential. The genetic potential and certain environmental factors such as temperature, pH, and available nitrogen and phosphorus sources govern the rate and the extent of degradation. Anaerobic biodegradation Among biological treatments, anaerobic digestion is frequently the most economical process, due to the high energy recovery linked to the process and its limited environmental impact. Anaerobic biodegradation results when the anaerobic microbes are predominant over the aerobic microbes. Here oxygen does not serve as the final electron acceptor or reactant. Manganese and iron ions, and substances like sulfur, sulfate, nitrate, carbon dioxide, some organic intermediates and pollutants are reduced by electrons originating from oxidation of organic compounds. The common example of anaerobic process is the biodegradable waste in landfill. Paper and other materials degrade more slowly over longer periods of time. Biogas, coming from anaerobic digestion, mainly consists of methane and can be collected efficiently and used for eco‐friendly power generation as has been demonstrated on larger scale. Anaerobic digestion is widely used, as part of an integrated waste management system, to treat wastewater sludge and biodegradable waste because it provides volume and mass reduction of the input material. It reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is a renewable energy source because the process produces methane and CO2-rich biogas suitable for energy production helping to replace fossil fuel requirement. Also, the nutrient‐rich solids left after digestion can be used as fertilizer. There are four major biological and chemical steps of anaerobic digestion: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The mechanism commences with bacterial hydrolysis of the organic matter to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acetogenic bacteria convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acid. Methanogens then ultimately transform these products in to methane and carbon dioxide. Hydrolysis The first phase is the hydrolysis of polymers (carbohydrates, fats, and proteins), which yields soluble sugars, amino acids, long-chain carboxylic acids, and glycerol. Equation 1. shows an example of a hydrolysis reaction where organic waste is broken down into a simple sugar, in this case, glucose. Equation 1 : C6H10O4 + 2H2O → C6H12O6 + 2H2 Fermentation/Acidogenesis In the second phase, acidogenic bacteria transform the products of the first reaction into short chain volatile acids, ketones, alcohols, hydrogen and carbon dioxide. The principal acidogenesis stage products are propionic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH), acetic acid (CH3COOH), formic acid (HCOOH), lactic acid (C3H6O3), ethanol (C2H5OH) and methanol (CH3OH), among other. From these products, the hydrogen, carbon dioxide and acetic acid will be utilized directly by the methanogenic bacteria in the final stage Acetogenesis In the third phase, known as acetogenesis, the rest of the acidogenesis products, i.e. the propionic acid, butyric acid and alcohols are transformed by acetogenic or fatty acid oxidizing bacteria into hydrogen, carbon dioxide and acetic acid. Hydrogen plays an important intermediary role in this process, as the reaction will only occur if the hydrogen partial pressure is low enough to thermodynamically allow the conversion of all the acids. Such lowering of the partial pressure is carried out by hydrogen scavenging bacteria. Methanogenesis The fourth and final phase is called methanogenesis. During this stage, microorganisms convert the hydrogen and acetic acid formed by the acid formers to methane gas and carbon dioxide. The most common methanogenic substrates are acetate and CO2 plus H2. The bacteria responsible for this conversion are called methanogens and are strict anaerobes. Most methanogens have a pH optimum around 7. Should the activity of the fermentative organisms exceed that of the carboxylic acid degraders and methanogens, there will be an imbalance in the ecosystem. Carboxylic acids and H2 will accumulate and the pH of the system will fall, thus inhibiting methanogenesis. Waste stabilization is accomplished when methane gas and carbon dioxide are produced. The general scheme of anaerobic substrate biodegradation and microbial community relationships is illustrated in figure given below: Biodegradation of industrial organic pollutants Knowledge of fate of chemicals discharged in the environment, the life cycle analysis and the mechanisms by which they degrade are of great importance in designing biodegradation systems since many of the industrial chemicals are toxic, recalcitrant and bioaccumulating in organisms. Volatile Organic Compounds (VOCs) There are two classes of VOCs that are responsible for a large number of land and groundwater contamination: (i) petroleum hydrocarbons (PHCs) such as gasoline, diesel, and jet fuel, and (ii) chlorinated hydrocarbon (CHC) solvents such as the dry cleaning agents such as tetrachloroethylene, perchloroethylene (PCE) and the degreasing solvents such as trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and PCE. PHCs biodegrade readily under aerobic medium, whereas CHCs characteristically biodegrade much more slowly and under anaerobic conditions. Because PHC biodegradation is relatively rapid when oxygen is present, aerobic biodegradation can usually limit the concentration and subsurface migration of petroleum vapours in unsaturated soils. Further, CHC biodegradation can produce toxic moieties, such as dichloroethylene and vinyl chloride, while petroleum degradation usually produces carbon dioxide, water, and sometimes methane or other simple hydrocarbons. A second primary difference is density of pollutant. PHC liquids are lighter than water and immiscible. PHCs can float on the groundwater surface (water table), whereas chlorinated solvents being heavier than water sink through the groundwater column to the bottom of the aquifer. These major differences in biodegradability and density lead to very different subsurface behaviour that often reduces the potential for human exposure. Petroleum Hydrocarbons (PHCs) It is known that microorganisms capable of aerobically degrading PHCs are present in nearly all subsurface soil environments. Effective aerobic biodegradation of PHCs hinges on the soil having adequate oxygen and water content to provide a habitat for sufficient populations of active microorganisms. If oxygen is present, these organisms will generally consume available PHCs. Furthermore, aerobic biodegradation of petroleum compounds can occur relatively quickly, with degradation half lives as short as hours or days under some conditions. Some petroleum compounds can also biodegrade under anaerobic conditions; however, above the water table, where oxygen is usually available in the soil zone, this process is insignificant and often much slower than aerobic biodegradation. Aerobic biodegradation consumes oxygen and generates carbon dioxide and water. This leads to a characteristic vertical concentration profile in the unsaturated zone in which oxygen concentrations decrease with depth and VOCs including PHCs and methane from anaerobic biodegradation and carbon dioxide concentrations increase with depth. Chlorinated Hydrocarbon (CHC) Solvents Chlorinated solvents such as tetrachloroethylene (TCE), 1,1,2,2-tetrachloroethane, carbon tetrachloride, and chloroform are released as waste products by spills, land-filling, and discharge to sewers during manufacture and their use as solvents in a variety of cleaning processes or as vehicles for solid slurries. TCE is a major pollutant of the industry. It is biodegraded under anaerobic conditions through hydrogenolysis that sequentially produces isomers of 1,2-dichloroethylene (1,2-DCE), vinyl chloride (VC), and ethylene. Ethane, methane, and carbon dioxide have also been reported as degradation products. Methanotrophs are microorganisms that primarily oxidize methane for energy and growth using methane monooxygenase (MMO) enzymes and are a group of aerobic bacteria transform TCE through co-metabolic oxidation. Quinoline Quinoline occurs commonly in coal tar, oil shale, and petroleum, and is used as an intermediate and solvent in many industries. Due to its toxicity and repulsive odor, quinoline-containing waste is detrimental to human health and environmental quality. Microorganism transform quinoline into 2-hydroxyquinoline in the first step. Therefore, quinoline pollution can be eliminated by applying bio-augmentation strategies. Phenols Phenols are harmful to organisms at low concentrations and classified as hazardous pollutants because of their potential to harm human health. They exist in different concentrations in wastewaters originated from coking, synthetic rubber, plastics, paper, oil, gasoline, etc. Biological treatment, activated carbon adsorption and solvent extraction are some of the most widely used methods for removing phenol and family compounds from wastewaters. Many aerobic bacteria are capable of using phenol as the sole source of carbon and energy. In recent years, the strain of Pseudomonas putida has been the most widely used to degrade phenol. Under aerobic conditions, phenol may be converted by the bacterial biomass to CO2; other intermediates such as benzoate, catechol, cis-cis-muconate, β-ketoadipate, succinate and acetate are formed during the biodegradation process. Fluoro benzenes Toluene degrading enzymes can transform many 3-fluoro-substituted benzenes to the corresponding 2,3-catechols with the concomitant release of inorganic fluoride. Polycyclic Aromatic Hydrocarbons (PCAHs) PCAHs are toxic, mutagenic and resist biodegradation. Many strategies have been developed to treat them, including volatilization, photo-oxidation, chemical oxidation, bioaccumulation, and adsorption on soil particles. Soil clean-up may also be achieved by bioremediation. Two processes have been found to increase the activity of microorganisms during bioremediation: bio-stimulation and bio-augmentation. Bio-stimulation involves the addition of nutrients and/or a terminal electron acceptor to increase the meager activity of indigenous microbial populations. Bio-augmentation involves the addition of external microbial strains (indigenous or exogenous) which have the ability to degrade the desired toxic compounds. The ability to degrade PCAHs depends on the complexity of their structure and the extent of enzymatic adaptation by bacteria. Naphthalene It is carcinogenic and persistent organic pollutant. Bacteria such as Pseudomonas putida, Rhodococcus opacus, Mycobacterium sp., Nocardia otitidiscaviarum, and Bacillus pumilus are known to biodegrade naphthalene and support the cell growth. Plasticizers Plasticizers are polymeric additives, used to impart flexibility to polymer materials. The biodegradation of some plasticizers can lead to the formation of metabolites with increased persistence and toxicity relative to the original compounds. Use of plasticizers has grown considerably, both with respect to product variety and production volume. Phthalates are the most widely used plasticizers. Presence of phthalates and their metabolites in rats, mice, human plasma and liver are related to adverse health effects such as endocrine disruption and peroxisome proliferation. The high production volumes of phthalates and their incomplete biodegradation have led to the presence of these compounds and a number of toxic and stable metabolites in surface waters, groundwater, air, soil and tissue of living organisms. Nowadays dibenzoates have been approved by the European Chemical Agency as alternatives to phthalates. Dibenzoate containing compounds can be degraded by common soil microorganisms such as Rhodotorula rubra and Rhodococcus rhodochrous into monobenzoate metabolites. Plastics Over the years, plastics have brought economic, environmental and social advantages. Today’s material world uses tremendous quantities of plastics of all hue and origins. However, their wide spread use has also increased plastic waste, which brings its own economic, environmental and social problems. The redesign of plastic products, whether individual polymer or product structure, could help alleviate some of the problems associated with plastic waste. Redesign could have an impact at all levels of the hierarchy established by the European Waste Framework Directive: prevention, re-use, recycle, recovery and disposal. Polyethylene, polypropylene and polystyrene, and water-soluble polymers, such as polyacrylamide, polyvinyl alcohol and polyacrylic acid are bulk polymers used in a variety of industries and products. Some of the plastics are not biodegradable and deleterious to the environment due to their accumulation. Plastics can be disposed of by incineration or recycling, but incineration is very difficult, dangerous and expensive whereas recycling is a long process and not very efficient. Some plastics still cannot be recycled or incinerated due to pigments, coatings and other additives during manufacture of materials. Making biodegradable and ecofriendly plastics will avoid accumulation, recycling and incineration Polyvinyl alcohol Polyvinyl alcohol (PVA) is water-soluble but also has thermoplasticity. In addition to its use as a water-soluble polymer, for instance, as a substituent for starch in industrial processes, it can also be molded in various shapes, such as containers and films. PVA can therefore be used to make water-soluble and biodegradable carriers, which may be useful in the manufacture of delivery systems for chemicals such as fertilizers, pesticides, and herbicides. Among the vinyl polymers produced industrially, PVA is the only one known to be mineralized by microorganisms. Extensive use of PVA, in textile and paper industries generates considerable amount of contaminated wastewaters. In aqueous solution, the biodegradation mechanism of PVA involves the random endocleavage of the polymer chains. The initial step is associated with the specific oxidation of methane-carbon bearing the hydroxyl group, as mediated by oxidase and dehydrogenase type enzymes, to give β-hydroxyketone as well as 1,3-diketone moieties. The latter groups are able to facilitate the carbon-carbon bond cleavage as promoted by specific β-diketone hydrolase, leading to the formation of carboxylic and methyl ketone end groups. Most of the PVA-degrading microorganisms are aerobic bacteria belonging to Pseudomonas, Alcaligenes, and Bacillus genus. Polyhydroxyalkanoates Polyhydroxyalkanoates (PHAs) are degraded to CO2 and water in aerobic conditions and methane in anaerobic conditions by microbes found in soil, water and other various natural habitats. PHAs are the only proposed replacement polymers that are completely biodegradable. The structures of these polymers have a very similar structure of the petroleum-derived thermoplastics. PHAs also possess similar physical properties as plastics including the ability to be molded, made into films, and also into fibers. Prospective of anaerobic digestion and biogas energy The foregoing analysis shows that anaerobic digestion technologies have matured so far to treat several organic micro-pollutants, halogenated compounds, substituted aromatics, azo-linkages, nitro-aromatics and the like in industrial effluents and also for municipal effluents containing industrial loads. Anaerobic digestion of sewage sludge followed by recycling on agricultural land is currently the largest world-wide application of anaerobic processes. Treatment of sludge and slurries targeted at the production of safe end products can be tackled with niche anaerobic technologies. There is an emphasis worldwide on renewable energy system among which biogas produced from any biological feedstocks including primary agricultural sectors and from various organic waste streams will come in to prominence in near future. It is estimated that at least 25% of all bioenergy in the future can originate from biogas, produced from wet organic materials like animal manure, slurries from cattle and pig production units as well as from poultry, fish and fur, whole crop silages, wet food and feed wastes, etc. Anaerobic digestion of animal manure offers several environmental, agricultural and socio-economic benefits throughout such as improved fertilizer quality of manure, considerable reduction of odors and inactivation of pathogens and more importantly production of biogas production, as clean, renewable fuel, for multiple utilizations. This biogas can be upgraded to natural gas to mix with the existing natural gas grid which will be cost effective. The potential development of biogas from manure co- digestion includes the use of new feedstock types such as by-products from food processing industries, bio-slurries from biofuels processing industries as well as the biological degradation of toxic organic wastes from pharmaceutical industries, etc. Bioplastics Since disposal is one of the important aspect, bioplastics are being favoured. There are three main categories of bio-based plastics: (i) Natural polymers from renewable sources, such as cellulose, starch and plant-based proteins, (ii) Polymers synthesised from monomers derived from renewable resources. For example, polylactic acid (PLA) is produced by the fermentation of starch, corn or sugar, (iii) Polymers produced by microorganisms. For example, PHA (polyhydroxyalkanoate) is produced by bacteria through fermentation of sugar or lipids. Biodegradable plastics are not by definition bio-based and bio-based plastics are not always biodegradable, although some fall into both categories, such as PHAs. The term bioplastics is often used to refer to both bio-based and biodegradable plastics. The main applications of bioplastics are disposable plastic bags, packaging and loose fill packaging (beads and chips), dishes and cutlery, electronic casings and car components. However, bioplastics cannot substitute all types of plastic; particularly certain types of food packaging that require gas permeability. Development of novel biodegradable plastic is a solution for the plastic disposal problem since plastics are immiscible in water and are thermo-elastic polymeric materials. Biodegradability of plastics is governed by both their chemical and physical properties. Other factors affecting degradability are the forces associated with covalent bonds of polymer molecules, hydrogen bonds, van der Waals forces, coulombic forces, etc. Enzymatic degradation is an effective way. Lipase and esterase can hydrolyze fatty acid esters, triglycerides and aliphatic polyesters. These lipolytic enzymes have an important role in the degradation of natural aliphatic polyesters such as cutin, suberin and esteroid in the natural environment and animal digestive tract. Biodegradable plastics decompose in the natural environment from the action of bacteria. Biodegradation of plastics can be achieved through the action of micro-bacteria and fungi in the environment to metabolize the molecular structure of plastic films to produce an inert humus‐like material that is less harmful to the environment, along with water, carbon dioxide and/or methane. The Pest problem All living things strive to survive, but unfortunately the needs of other creatures sometimes conflict with our needs. An example of this conflict is the struggle between pests and humans. Pests are creatures that injure or kill plants or domestic animals, transmit disease, cause economic damage, or are a nuisance in some other way. They eat our food crops or ornamental plants, infect plants that are useful to us, make us sick by transmitting infectious organisms, infest our livestock and pets, and destroy property. An effective form of pest control is essential if we're going to win the battle with pest organisms. Many different chemicals are used to kill pests. These pesticides often work well, but since they're designed to kill living things they may cause serious problems in humans or pets. Pesticides contaminate the environment and the food that we eat and may enter our bodies when we're applying them to our plants or animals. They sometimes harm other organisms in addition to their target. Another problem with using chemicals to control pests is that a pest may become resistant to a pesticide. Biological pest control involves the use of another living organism to kill a pest. No chemicals are needed, there is no environmental contamination with pesticides, and the pests don’t become resistant to the control method. However, introducing a plant or animal to an area where it doesn’t normally occur can create new problems. Types of Biological Pest Control There are three types of biological pest control. In classical biological control, natural predators, parasites, or pathogens of a pest are imported into an area to protect a crop or livestock. A "pathogen" is an organism that causes disease. Importation can be a useful strategy when the pest has been introduced from another region and has no predators in its new habitat. In conservation biological control no new plants or animals are introduced to an area, but the environment is manipulated to favor the survival of local enemies of the pest. For example, a farmer or gardener may provide additional food sources or suitable habitats for a pest's enemies. In augmentation biological control, plants and animals that control a particular pest and are already present in an area are increased in number by inoculation or inundation. Inoculation is the introduction of relatively few organisms. Inundation involves the introduction of a very large number of organisms. The Environmental Working Group or EWG publishes a yearly "Dirty Dozen" list of the produce with the most pesticide residues. In the 2019 version of the list, strawberries have the most pesticides and spinach has the second highest level. The organization recommends that we eat these items in an organic form. Disadvantages Despite the appealing advantages of biological pest control there may be important disadvantages. Artificially increasing the population of a certain predator may have unforeseen consequences. In addition, an organism that has been introduced from another area to destroy a pest may become a pest itself, especially if it has no natural predators in its new habitat. A famous example of this effect is the introduction of the cane toad into Australia. In 1935, cane toads were transported from Hawaii to North Queensland. The goal was for the toads to catch and eat the beetles that were attacking the sugar cane crops. Not only was this plan unsuccessful (the toads couldn't jump high enough to reach the beetles on the sugar cane stalks), but the cane toad has now become an invasive species. The toads have spread to new areas and have a thriving population. They feed on native animals and the toxin in their bodies often kills their potential predators. Fortunately, previous experiences have taught researchers how to better assess the likelihood that an introduced predator, parasite, or pathogen will cause a problem. Nature's behavior can't be completely predicted, however, and scientists never know for certain what will happen when they introduce a plant or animal to an area. Biological pest control often takes longer to work than chemical pest control and frequently reduces a pest population to a low level rather than eliminating it completely. These facts may be considered a disadvantage by some people. Once a predator population is established, however, biological pest control will operate on its own without the need for further human input (as long as the predator survives). Biopesticides Biopesticides are produced from or by living things and are considered to be safer for humans than chemical pesticides. There are three types of biopesticides—microbial pesticides, plant- incorporated protectants (or PIPs), and biochemical pesticides. Microbial Pesticides Microbial pesticides are made from microorganisms, such as bacteria or fungi, which are used to infect and kill pests. Although the microbes are said to form a pesticide, their use is actually an example of biological pest control. A popular microbial pesticide is the bacterium called Bacillus thuringiensis, also known as Bt. Different strains of Bt exist, each producing a distinct mix of proteins. Some of these proteins kill insect larvae. Different proteins kill different species of insects. Plant-Incorporated Protectants or PIPs PIPs are chemicals made by plants that have been genetically altered in order to produce a particular pesticide. For example, the Bt genes that make pesticide proteins can be inserted into plants. The genes become active and the plants produce their own pesticide, which kills insects that try to eat the plant. The pesticide proteins appear to be harmless to humans. The effects of PIPs are tested before farmers can use them. Biochemical Pesticides Biochemical pesticides are non-toxic chemicals made by living creatures. They are usually the only kind of pesticide that organic food producers are allowed to use. A biochemical pesticide's job is to control a pest, but it may not kill the pest directly. Semiochemicals are chemicals released by living things that influence the behavior of other organisms. A pheromone is a semiochemical that affects a member of the same species as the organism that made the pheromone. Insect pheromones attract other insects, which may be insects of the opposite gender or insects of both genders, depending on the pheromone. Pheromones can be used by farmers to lure insects into a trap. Pyrethrins Pyrethrins are another type of biochemical pesticide. They are made in the seed cases of a type of chrysanthemum and kill insects by damaging their nervous systems. Unlike some chemical pesticides, pyrethrins quickly break down in the environment and are said to be non-residual chemicals. They have low toxicity to humans and other mammals but should still be treated with respect. It's important to realize that just because a chemical is natural doesn't automatically mean that it's completely safe for humans. Still, pyrethrins are considered to be some of the safest chemicals to use as pesticides. They are toxic to fish and bees, however. A substance called piperonyl butoxide is often added to pyrethrin insecticides. Piperonyl butoxide has no ability to kill insects by itself but is still a helpful substance. It makes the insecticidal ability of pyrethrins stronger by stopping an insect's body from breaking the chemicals down. Kitchen Ingredients That May Remove Pests Some common kitchen substances may be useful for getting rid of garden pests and are worth trying before another method of pest control is used. For example, a canola oil spray is sometimes used as an insecticide yet is nontoxic to humans. It shouldn't be sprayed near water, however. Garlic is said to repel birds and insects and also degrades quickly. Black pepper oil is used to repel mammals. Although it may not be a common household product in some countries, neem seeds and the oil from the seeds are used to create a natural pesticide that kills many insects. Chili peppers are chopped and then soaked in water for a day to make an insecticide. Some people add a small amount of soapy water to the chili water to make a spray that will stick to plants. Try to use a soap or detergent that is safe for the environment if you do this. Be careful if you use chili peppers, since they can burn and irritate skin and mucous membranes. Types of Chemical Pesticides Chemical pesticides are synthetic substances that are created to kill or injure pests. They can be classified in several different ways. For example, pesticides may be categorized based on when they begin to work after they are applied to a pest. Contact pesticides kill a pest shortly after touching the surface of its body. Systemic pesticides are absorbed by plants or animals and must spread through the inside of their bodies to untreated areas before they can kill the pests. The pesticides may travel through the whole body or just to one particular area in the body. Pesticides may also be classified according to how they affect pests. Desiccants remove water from the bodies of plants or animals, for example, and defoliants cause plants to drop their leaves. Insect growth regulators kill insect larvae by interfering with the process in which juvenile insects molt and turn into adults. Although most pesticides kill the pests that they attack, not all of them do. Repellents simply repel pests, as their name implies. An example of this type of pesticide is DEET, a common substance in personal insect repellents. Sterilizing agents interfere with the ability of the pest to reproduce, but they don't kill the creature that they affect. Additional ways to classify pesticides are by the type of organism that they are designed to kill, as the table below shows, or by their chemical structure. Classifying Pesticides According to Their Target Pesticide Type Target Pest nematicides nematodes (roundworms) molluscicides slugs and snails insecticides Insects acaricides (or miticides) fleas, ticks, and mites piscicides Fish avicides Birds rodenticides rodents bactericides bacteria algicides Algae fungicides Fungi herbicides plants Potential Problems for Human Health Pesticides are potent chemicals designed to destroy pests. They may harm us, too. This harm is generally reduced because farmers often have to follow strict laws about pesticide use. These laws include rules about allowable pesticide levels on crops and about pesticide storage, transport, and application. Despite all the regulations, however, we do ingest pesticides in our foods and drinks, inhale pesticides from the air that we breathe, and absorb pesticides through our skin. The agencies that regulate pesticides usually admit that chemical pesticide use does involve safety risks, but they say that these risks are acceptable considering our need to protect agricultural crops and feed people. Many individuals disagree with the idea that the risk is "acceptable", however. The agencies also claim that most people are exposed to only small amounts of pesticides in their daily lives. However, if a pesticide is very toxic a small amount can be dangerous. Possible Health Effects The effects of a pesticide on the human body depend on several factors, including the nature of the pesticide, the amount of chemical involved, the length and frequency of exposure, and the age of the person being exposed to the pesticide. Children are especially susceptible to the effects of chemicals because of their small size and the fact that their bodies and nervous systems are still developing. Symptoms of acute pesticide poisoning develop immediately or shortly after exposure to a dangerous dose of the chemical. The symptoms may be relatively minor, such as a headache, dizziness, nausea, and diarrhea. More serious symptoms include vomiting, abdominal pain, a rapid pulse, lack of muscle coordination, mental confusion, inability to breathe, burns, loss of consciousness, and even death. Other possible effects of pesticide exposure may take longer to develop. It's hard to definitely prove that a pesticide is responsible for a human disease, but certain pesticides are suspected of causing nervous system damage or cancer. Common Types of Insecticides and Their Dangers Many pests are insects and so most pesticides are insecticides. Important types of insecticides, which are classified based on their chemical structure, are organophosphates, carbamates, organochlorines, pyrethroids, and neonicotinoids. Organophosphates Organophosphates kill insects by interfering with the activity of their brains and nervous systems. Unfortunately, they can also affect the nervous systems of humans and other animals. They do this by altering a normal process involving acetylcholine, a common neurotransmitter. Neurotransmitters control the transmission of a nerve impulse from one nerve cell to the next. They are normally broken down or removed once they've done their job. Organophosphates interfere with the action of acetylcholinesterase, the enzyme that breaks down acetylcholine. Carbamates Carbamates are also used as insecticides and work in a similar way to organophosphates. They break down quicker and are less dangerous to humans, however. Organochlorines The most famous organochlorine is DDT (Dichlorodiphenyltrichloroethane). It has been banned in several countries for decades, except for very specialized use, but it is a very persistent pesticide. "Persistent" pesticides stay in the environment for a long time and don't break down. DDT is still found in soil and in the bodies of animals and humans. DDT thins the shells of bird eggs, causing the developing babies to die. It also disrupts our endocrine systems (which produce the hormones that we need) and is thought to damage genes and increase the risk of cancer. Pyrethroids Pyrethroids are synthetic chemicals derived from pyrethrins. Like pyrethrins, their use is increasing because they are considered to be less toxic than the other categories of insecticides. Neonicotinoids Neonicotinoids are derived from nicotine, a plant chemical. They interfere with a pathway that is common in the insect nervous system and are suspected of playing a role in honeybee colony collapse disorder. Integrated Pest Management or IPM Due to the concerns about chemical pesticide safety, some communities are now using integrated pest management techniques to control pest problems. Integrated pest management, or IPM, involves the use of multiple techniques to solve a pest problem as safely as possible. These techniques include physical or mechanical methods, such as picking pests off plants, creating barriers to block pests from entering an area, removing clutter, and changing the conditions that attract pests. They also include choosing an appropriate garden or field design. Companion plants that protect a desired crop may be grown beside the crop, for example. The soil composition or growing conditions may be changed to discourage pests. Biological pest control methods and biopesticide applications are also used in IPM. Even chemical pesticides are used, if they are absolutely necessary.