Persistence and Degradation of Pesticides in Composting
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Organics Persistence and Degradation of Pesticides in Composting Introduction objectionable compounds, even recalcitrant Composting is a process in which microorganisms xenobiotics such as DDT, PCB and TCE. rapidly consume organic matter, using it as an · Compost feedstock contains numerous and energy source and converting it into carbon diverse active microorganisms, all with their dioxide, water, microbial biomass, heat and own characteristics and capabilities. This compost. Feedstock used to fuel the composting diversity means a greater chance that a process may originate from a number of different pesticide will encounter a microbe that can sources, including yard trimmings, manure, degrade it. biosolids, and agricultural residues. These materials may contain a number of synthetic Mechanisms Affecting Pesticides organic compounds or xenobiotics, including During Composting pesticides. During composting, a pesticide may undergo the Many different physical and chemical factors help following physical and/or biological changes: determine the overall persistence of a pesticide. In Biological Degradation general, composting provides an optimal Microorganisms have developed many enzymes environment for pesticide destruction. This that can break down natural compounds. Modern publication discusses the potential for a pesticide scientists, though, have created pesticides with to become inactivated and degraded during chemical structures not found in nature. These composting. unique structures are often responsible for a The Composting Process pesticide's effectiveness and also explain why pesticides can persist in the environment. Compost is well-suited for pesticide degradation because: A pesticide's environmental persistence largely depends on its chemical structure and on the · The elevated or thermophilic temperatures presence of unusual functional groups, which are achieved during composting permit faster large sub-structures within the pesticide molecule. biochemical reactions than possible under The chemical structure helps determine its water ambient temperatures, accelerating pesticide solubility and consequently, its bioavailability, degradation. The high temperatures can also since microbes more readily assimilate water- make pesticides more bioavailable, increasing soluble compounds. the chance of microbial degradation. When a pesticide’s functional groups are attached · Some microorganisms may co-metabolize with weak or labile bonds, it can degrade more pesticides, where the microbes rely on the rapidly. Many modern pesticides have such bonds feedstock for food and energy while breaking designed into them to avoid problems of extended down an adjacent pesticide. Co-metabolism persistence. Adding water may break many labile means that the microorganism does not bonds. This process is called hydrolysis and the receive any energy or potential food from the enzymes that promote hydrolysis are termed secondary reaction (in this case, from hydrolytic. Malathion is an example of an breaking down the pesticide). The many insecticide containing many such labile bonds that different organic matter structures in compost may be broken using hydrolytic enzymes (for help to promote co-metabolism of numerous example, esterase and phosphatase). Note: Italicized words other than publications or scientific nomenclature are defined in the glossary. 2 Other pesticides capable of hydrolytic degradation The hyphae release extracellular enzymes, which are: carbamate pesticides, urea derivatives, break down the pesticide and allow it to pass into pyrethroids, diazinon, dicamba, dichloropicolinic the cells. This allows the production of additional acid, dimethoate, phenylalkanoic ester, hyphae and/or energy. Although fungi are present dimethoate, phenylalkanoic pyrazon, atrazine, in compost feedstock, they contribute more to linuron, propanil, chlorpyrifos, and 2,4-D.Two other composting in its later stages. As bacteria exhaust classes of enzymes, mono- and di-oxygenases, the easily degraded organic matter from the are also commonly associated with pesticide feedstock, fungi then begin to degrade the more degradation. These enzymes introduce one or two recalcitrant polymeric organic matter. oxygen atoms, respectively, into the structure of a Intracellular Decomposition pesticide. This oxidation process often makes the After extracellular enzymes begin breaking down a pesticide more amenable to further degradation by pesticide or if it is otherwise bioavailable, a increasing its water solubility, thereby increasing pesticide may enter the cell of a microorganism. its bioavailability. Degradation may begin at the To pass into a cell efficiently, the pesticide must extracellular level and then proceed further at the be dissolved in water. Generally pesticides intracellular level. containing more oxygen, nitrogen, and sulfur tend Extracellular Decomposition to be more water soluble due to hydrogen Many of the same enzymes microorganisms use bonding. to break down cellulose, hemicellulose, and Once inside a cell, a pesticide may undergo lignin—the primary natural compounds in most varying degrees of degradation. Mineralization plant material—may also degrade pesticides reduces the pesticide to carbon dioxide, water, during composting. The large polymeric structure and other inorganic components. Typically, it of these natural compounds prevents their accounts for only a small portion of the passage into the microorganism for consumption. “disappearance” of a pesticide through To deal with this problem, microorganisms begin composting. breaking down chemicals outside their “body,” or Adsorption extracellularly. They excrete enzymes out of their Water-insoluble pesticides tend to adsorb onto cells that react with the bonds in cellulose, and within organic matter, making them even less hemicellulose, and/or lignin, breaking them down bioavailable. The chemistry of the functional into smaller components. The shortened polymers groups in the pesticide and the organic matter can then be subjected to further degradation. dictates the strength of this pesticide-organic Extracellular enzymes can have very low matter interaction. “specificity,” working like a key that fits different Adsorbed pesticides are generally much more locks. They can, therefore, react with many resistant to breakdown than water-soluble different chemicals. If the enzyme finds a pesticide pesticides. This is because the latter have a much before reaching its “intended” substrate (for greater chance of contact with pesticide-degrading example, cellulose, hemicellulose, lignin), it may microorganisms as described above. react with it, changing the pesticide into a possibly Consequently, highly adsorbed pesticides are not less toxic and less hazardous form. Such co- considered bioavailable, enabling them to persist metabolism appears to play a significant role in for months or even years. However, when a degrading pesticides found in compost and soil. pesticide is adsorbed to organic matter that Fungi are the source of most extracellular eventually decomposes, it may once again enzymes. Some fungi often associated with become bioavailable. compost and soil organic matter are in the genera Additional factors can make adsorption a likely Trichoderma, Gliocladium, Penicillium, and outcome for even water-soluble pesticides. For Phanerochaete. Fungi grow through the example, many pesticides contain acidic and development of hyphae (long strings of cells) that nitrogen-containing functional groups that can extend throughout compost or soil organic matter. adsorb due to the presence of a negative or 3 positive charge, respectively. A negatively charged percentage of pesticide is typically lost to pesticide will adsorb to positively charged mineralization. functional groups on organic matter, while Besides mineralization, the “disappearance” of positively charged pesticides will adsorb to pesticides may occur by volatilization, adsorption, negatively charged functional groups on organic leaching, or other methods noted earlier. A matter and clays. pesticide adsorbed to a compost molecule, while Volatilization technically present, may also be inactivated and Volatilization occurs when a pesticide partitions could permanently lose its pest control function. from the solid or aqueous phase to the gas phase. Many of the studies showed that concentrations of Once volatilized, a pesticide may diffuse into the organophosphate and carbamate pesticides were atmosphere and either be destroyed or continue lower after composting. However, recalcitrant as an environmental risk. When mixing disturbs a organochlorine insecticides (for example, DDT) soil contaminated by a pesticide or other organic and pyridine carboxylic acid herbicides (for compound, a 30 percent or greater loss of the soil example, clopyralid and picloram) are more contaminant through volatilization is not unusual. resistant to degradation. Volatilization of a pesticide is highly temperature Lemmon and Pylypiw (1992) studied the dependent; thermophilic temperatures typically persistence of chlorpyrifos, diazinon, isofenphos, increase pesticide losses. The tendency for a and pendimethalin after composting with grass pesticide to volatilize also depends upon its size, clippings. The authors found the pesticides structure, and function. Moisture also affects undetectable shortly after application, and they volatilization rates. Water may physically impede disappeared quickly after composting. the flow of a gas phase pesticide by obstructing