Figure 13.7 Structures of DDT (dichlorodiphenyltrichloroethane), DDE (dichlorodiphenyldi-chloroethene, and DDD (dichlorodiphenyldichloroethane).
inappropriate for use in composting operations. Similarly, the pesticide DDT was found to disappear fairly rapidly under some conditions; unfortunately, it was simply converted to DDD or DDE, which were at least as problematic as the parent compound (Figure 13.7). Thus, this biotransformation (Section 18.5) did not represent substantial biodegradation of the compound.
It is often desirable to be more specific when discussing biodegradation, and to consider both the rate and the extent of the transformation. Mineralization, or ultimate biodegradation, is the complete conversion of an organic molecule to inorganic products (mainly CO2 and H2O under aerobic conditions, CO2 and CH4 or H2S under anaerobic ones). Readily biodegradable organics can be rapidly mineralized. Recalcitrant compounds (also called refractory), on the other hand, are difficult to degrade, and are mineralized slowly if at all. However, biodegradability refers to an inherent property of the material; whether it actually will be biodegraded depends on the environment in which it is present. For example, readable newspapers and intact pieces of fruit buried for decades have been recovered from landfills, particularly under dry conditions. Even more dramatically (Figure 13.8), the corpses of murder victims have been recovered from northern bogs (with cold, acid, and anaerobic conditions) after 2000 years with much of their clothing, skin, hair, and even stomach contents intact!
In general, conditions that promote growth are likely to speed biodegradation. Thus, warm, moist conditions often lead to higher rates than cold, dry ones. Also, most—but
not all—materials are more readily biodegradable under aerobic conditions than under anaerobic ones.
Most biodegradation occurs as a result of microbial activity in which the compound is utilized as a carbon and energy source. Typically, complex organics are broken down enzymatically into simpler molecules, which then enter the cell's basic biochemical pathways, such as glycolysis and the Krebs cycle (Section 5.4.3). Compounds containing nitrogen, sulfur, or other essential elements may also be utilized as sources of these nutrients.
All naturally occurring and most human-made organic compounds are biodegradable under appropriate conditions. However, some xenobiotic ("foreign" to biology; i.e., synthetic) compounds are extremely recalcitrant. In fact, many, such as polychlorinated biphenyls (PCBs) and most plastics, were designed to resist degradation. Long hydrocarbon chain molecules, such as polyethylene, are slow to degrade because microbial attack is mainly from the ends. Branching also makes biodegradation difficult, as it can hinder or prevent the molecule from fitting into the active site of appropriate enzymes. The early alkyl benzenesulfonate (ABS) detergents, used widely in the 1950s, for example, were highly branched and thus recalcitrant, leading to dramatic foaming in wastewater treatment plants and receiving waters (Figure 13.9). This problem was eliminated when the linear alkyl sulfonate (LAS) detergents were introduced as a replacement.
ch3 ch3 /ch3 h3c—c - ch2 - ch- ch2 4 ch- ch2 j- ch3
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