A persistent compound is slow to degrade in the environment , which often results in its accumulation and deleterious effects on human and environmental health if the compound is toxic. Toxic metals such as lead and cadmium , organochlorides such as polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs) are persistent compounds.
Persistent molecules are termed recalcitrant if they fail to degrade, metabolize, or mineralize at significant rates. Their compounds can be transported through the environment over long periods of time and over long distances, resulting in long-term exposure and possible changes in organisms and ecosystems. However, organisms and ecosystems may adapt to the compounds, and deleterious effects may weaken or even disappear.
Compounds may be persistent for several reasons. A compound can be persistent due to its chemical structure. For example, in a molecule, the number and arrangement of chlorine ions or hydroxyl groups can make a compound recalcitrant. It can also persist due to unfavorable environmental conditions such as pH , temperature, ionic strength, potential for oxidation reduction reactions , unavailability of nutrients, and absence of organisms that can degrade the compound.
The period of persistence can be expressed as the time required for half of the compound to be lost, the half-life . It is also often expressed as the time for detectable levels of the compound to disappear entirely. Compounds are classified for environmental persistence in the following categories:(1) Not degradable, compound half-life of several centuries; (2) Strong persistence, compound half-life of several years; (3) Medium persistence, compound half-life of several months; and (4) Low persistence, compound half-life less than several months.
Non-degradable compounds include metals and many radioisotopes, while semi-degradable compounds (of medium to strong persistence) include PAHs and chlorinated compounds. Compounds with low persistence include most organic compounds based on nitrogen , sulfur, and phosphorus .
In general, the greater the persistence of a compound, the more it will accumulate in the environment and the food chain/web . Non-degradable and strongly persistent compounds will accumulate in the environment and/or organisms. For example, because of bioaccumulation through the food chain, dieldrin can reduce populations of birds of prey. Compounds with intermediate persistence may or may not accumulate, while non-persistent pollutants generally do not accumulate. However, even compounds with low persistence can have long-term deleterious effects on the environment. 2,4-D and 2,4,5-T can cause defoliation , which may result in soil erosion and long-term effects on the ecosystem .
Chemical properties of a compound can be used to assess persistence in the environment. Important properties include: (1) rate of biodegradation (both aerobic and anaerobic ); (2) rate of hydrolysis; (3) rate of oxidation or reduction; and (4) rate of photolysis in air, soil, and water. In addition, the effects of key parameters, such as temperature, concentration, and pH, on the rate constants and the identity and persistence of transformation products should also be investigated. However, measured values for these properties for many persistent compounds are not available, since there are thousands of chemicals , and the time and resources required to measure the desired properties for all the chemicals is unrealistic. In addition, the data that are available are often of variable quality.
Persistence of a compound in the environment is dependent on many interacting environmental and compound-specific factors, which makes understanding the causes of persistence of a specific compound complex and in many cases incomplete.
[Judith Sims ]
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