Absorption, the stage in which the absorption of photons of light gives excited molecules 2. Primary photochemical process 3. Secondary photochemical processes. These groups all contain double bonds, in some form, aromatic rings, or series of double bonds separated by a single bond. For such groups, or chemicals containing only these functional groups, phototransformation is likely to be unimportant.
In addition, phototransformation can only occur when the chemical is likely to be exposed to solar radiation. For example, the atmosphere, upper layers of water bodies, and the surface of soil are likely locations in which chemicals would be exposed to solar radiation. The Beer—Lambert law relates absorbance to concentration at a given wavelength for a chemical in solution Figure 3.
This law can be expressed by the following equations:. It is also available in substantial quantities within all biota and in the vapor form in the atmosphere. It is one of the most important chemical processes that can act upon the many types of organic compounds occurring in the environment arising from both natural and man-made sources.
In this reaction, an organic molecule R—X reacts with water, forming a new C—O bond and cleaving a C—X bond in the original molecule. The overall effect is a displacement of the X group by a hydroxyl group. A wide variety of functional groups and compounds are potentially susceptible to hydrolysis, including peptides, the glycosidic linkage in polysaccharides, and the ester group in fats.
These reactions are shown in a generalized form in Table 3. Hydrolysis also occurs with a wide range of synthetic compounds, including many pesticides and other substances. These reactions can be mediated by biota or can occur without the need for biological assistance. However, when they occur abiotically, the rates of reaction can be very slow.
Some chemical structures and groups tend to be resistant to hydrolysis, includ- ing alkanes, polycyclic aromatic hydrocarbons, alcohols, aldehydes, and ketones. The importance of hydrolysis from an environmental fate view is that the reaction introduces a hydroxyl group into the parent molecule and may fragment the molecule into smaller groups. The hydroxyl group is a polar group see Chapter 2 and usually tends to increase the polarity of the molecule, but this depends on the nature of the group that is removed.
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The products of hydrolysis are usually more susceptible to biotransformation, and the hydroxyl group makes the chemical more water soluble, as does the smaller size of molecular fragments that may be produced. With com- pounds having high KOW values see Chapter 2 , the KOW values of the products will be less and the biological activity altered. Furthermore, the product is usually less toxic than the initial starting material, but there are some exceptions to this. This is illustrated in Figure 3. This results in a strong dependence of the rate of hydrolysis on the pH of the water in which the reaction occurs.
These processes can be divided into two broad categories: 1 microbial transformations and 2 transformation by higher organisms. These two different groups are described below. Generally, as the amount of organic matter increases in a water body, the microbial population also increases.
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Microorganisms play a major role in the biogeochemical cycles of various elements that occur in the environment. Microorganisms include bacteria, small, single-celled organisms; fungi, nonphotosynthetic organisms; algae, photosynthetic organisms; protozoans, unicellular, eukaryotic microorganisms; and viruses, para- sitic microorganisms unable to multiply outside the host tissues.
Most bacteria fall into the size range 0. The majority of bacteria in aquatic environments are nutritionally het- erotrophic. Fungi are aerobic organisms require atmospheric oxygen that can be uni- or multicellular and generally can thrive in more acidic media than bacteria. Perhaps the most important function of fungi in the environment is the breakdown of cellulose in wood and other plant materials. To accomplish this, fungal cells secrete an enzyme cellulase that breaks insoluble cellulose down to soluble car- bohydrates that can be absorbed by the fungal cell.
A particularly important example of these decomposition products is the humic substances discussed in Chapter 17 that occur in soil and runoff water entering natural water bodies. Algae are photosynthetic and are abundant in both fresh and saline waters, soil, and other sectors of the environment. Het- erotrophic bacteria, which are capable of using complex carbon compounds as their principal source of energy, can degrade organic compounds to provide the energy and carbon required for growth.
Many toxic and synthetic substances function as growth substrates for bacteria in a manner similar to naturally occurring organic compounds. Metab- olism of growth substances usually results in relatively complete degradation or mineralization to carbon dioxide, water, and inorganic salts.
This production of enzymes is referred to as enzyme induction. These factors are outlined below:. Prior exposure to the organic compound: Prior exposure to the organic compound substrate reduces the adaption or lag time. Thus, lag times in pristine environments should generally be much longer than in locations that have been previously exposed. Initial numbers of suitable species: Areas with larger microbial com- munities should require relatively short lag times to develop a viable population of degrading microorganisms Table 3.
The presence of more easily degraded carbon sources: The presence of more easily degraded carbon sources may delay the adaptation of the microbial community to more persistent contaminants. For example, it has been found that microorganisms degraded added glucose completely before degrading hydrocarbons in lake water. Concentration of the organic compound: There may be concentration thresholds below which adaptation does not occur.
On the other hand, too high a concentration of the organic compound may be toxic to the micro- organisms.
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When the lag phase is completed, population growth occurs rapidly and usually increases at an exponential rate. At the completion of this phase, the microorganism population effectively establishes an equilibrium with growth substances available, and a stationary phase in terms of population occurs. Finally, the growth substances are exhausted and wastes accumulate, leading to a decline in the population number, as illustrated in Figure 3.
This is in contrast to growth substances that are able to serve as the sole carbon source for a microbial community. Microorganisms can degrade com- pounds that they apparently cannot use for growth or energy via co-metabolism. Often, this is an important mechanism for the degradation of pesticides in soil. Often, no lag period occurs before co-metabolism begins, and accumulation of inter- mediate products resulting from partial degradation is likely.
Generally, slower rates of degradation are observed compared to metabolism of growth substances. Usually, the dissolved oxygen concentration in natural waters ranges from 0 to about 10 mg l—1 due to the low solubility of oxygen in water. This process is a redox process with each molecule of oxygen having an oxidation state of zero, being reduced by addition of hydrogen and two electrons to an oxidation state of —2, as it is in water. Thus, the availability of electrons can be seen as a controlling factor for redox processes in aquatic systems.
If the organic compound contains nitrogen, the normal products of carbon dioxide and water are formed, but the nitrogen present is converted into ammonia NH3 and nitrate ion NO3—. From this general type of reaction, bacteria and other microorganisms extract the energy needed to carry out their metabolic processes, to synthesize new cell material, for reproduction and for movement. When DO is removed, anaerobic degradation without atmospheric oxygen occurs, having generally lower energy yields and microbial growth rates.
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Most organic substances are biodegraded more slowly under anaerobic conditions, although there are a number of exceptions to this general rule. Rate constants derived for degradation in oxygenated systems do not apply to anaerobic systems. The dissolved oxygen content of waters in various parts of the environment can vary considerably.
In zones such as sediments and bottom waters that lack a mechanism for aeration, DO can be in short supply and organic compounds are typically microbially degraded utilizing a sequence of avail- able oxidizing agents. Initially, aerobic degradation occurs, but when the DO falls to 0. This process can be simply expressed by the following equation:.
If nitrogen was present in the original organic compound undergoing oxidation, then ammonia could also be produced. Generally, different microorganisms are responsible for, or associated with, the use of the various oxidizing agents. In the complete absence of dissolved oxygen, the degradation of organic com- pounds can still proceed. This can be represented for carbohydrates by the following equations:.
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These processes release a relatively low amount of energy, as can be seen by the production of methane and ethanol, which are capable of further oxidation with the release of relatively large amounts of energy. Microorganisms require nutrients, such as nitrogen and phosphorus, in order to metabolize organic substances. The availability of nutrients is one of the most important factors controlling the activity of heterotrophic microorganisms in aquatic environments. As well as the major nutrients C, N, and P , microbial growth and activity can be affected by essential micronutrients growth factors, trace metals.
For example, the degradation of petroleum oil in seawater can be enhanced by the addition of iron, which is consistent with the role of iron as a cofactor in some of the enzymes responsible for hydrocarbon oxidation. The pH scale, previously described in Chapter 2, is used to conveniently char- acterize acidity as the concentration of the hydrogen ions in a system. It can be seen from the discussion above that reduction-oxidation redox conditions also lie on a somewhat similar continuum in aquatic systems.
Redox can be characterized as the effective concentration of electrons; even though electrons do not exist in solution, they are available from substances in the water. Thus, the pE scale is the negative of the log of the effective concentration of electrons in water. Low pE values indicate that there are relatively high concentrations of electrons available and that reducing conditions exist, whereas high pE values indicate that few electrons are available and oxidizing conditions are prevalent.
Air and water occupy the remaining volume of the soil in a complex system of pores and channels. Well-drained soils are generally aerobic in nature, with a fairly ready supply of oxygen from the atmosphere. Microorganisms are generally considered to be aquatic in nature, but soil micro- organisms tend to exist in a sorbed state attached to the soil solid matter.
Decreasing soil moisture content usually decreases the rate of degradation of organic compounds such as pesticides.
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This is particularly true at lower soil moisture levels approaching air-dried soil conditions. At excessive soil moisture levels, the soil may change from an aerobic to an anaerobic condition with generally reduced microbial degradation rates. One of the major enzyme systems for facilitating degradation of xenobiotic lipophilic compounds is the mixed- function oxidase MFO system. The name mixed-function oxidase is used because the system acts as an oxygenase and an oxidase. The key enzyme in the system is cytochrome P, which is based on a heme structure. Cytochrome P is not one or two enzymes, but a family of closely related enzymes called isozymes.
This enzyme system facilitates the insertion of oxygen into C—H bonds of substrates containing aliphatic groups. This is illustrated by the reaction below:.