Degradation of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 70

The distribution of microbes in soil is not homogeneous. The population of microbes in
the proximity of the surface is high, which may be due to the fact that microbes naturally
live on the material excreted from a plant's roots. Pesticides are generally degraded by
microbes along with material excreted by the roots of plants.
Bacterial degradation dominates in soils and water at pH > 5.5, while in acidic medium,
soil fungal degradation dominates.
Microbial degradation is of two types, aerobic and anaerobic, and depends on the sur-
rounding conditions. Aerobic metabolism is a process in which oxygen is utilized to oxidize
the pesticide and occurs in soil, not in water, as less oxygen is available. While in anaerobic
degradation oxygen is not present, degradation occurs via some other pathway. In soils, the
degradation of pesticides in the first layer is aerobic, while below this layer, it is anaerobic.
During microbial degradation reaction of pesticides, a large number of reactions have
been observed viz. reduction, oxidation, hydrolysis, dealkylation, hydroxylation, ring
cleavage, and conjugation.

3.4.1 Factors Influencing Microbial Degradation

1. Water depth : At different water depths, different microbes exist, and a chemi-
cal may or may not be metabolized depending on the microbes present at that
depth. At lower depths, the rate of metabolism will be slower because of the lower
temperature.

2. Mobility : If a pesticide is strongly bound to the soil, the biodegradation process can-
not occur. The organic content in soil strongly influences the binding of nonpolar
pesticides in soil and their biodegradation. As microbes move more freely in water
than in soil, pesticides in water will be degraded faster in water than in soil.

3. Primary/secondary metabolism : Primary metabolism occurs when microbes are able
to derive energy from the metabolism of a pesticide. The microbial population is
directly proportional to the pesticide concentration.

If the microbial population does not use pesticide for energy to live, secondary
metabolism occurs. The pesticide is degraded by the microbes as it degrades other
material in the soil.

4. Temperature : High temperature increases the degradation rate, as with the rise in
temperature sorption decreases and metabolic activity of microbes' increases.

5. pH : The microbial degradation increases with pH.

6. Soil moisture : Waterlogged conditions in combination with a high number of nutri-
ents promote growth of anaerobic microbial species. Bacteria, in general, require a
high level of moisture for degradation.

7. Organic matter : In soils rich with organic matter, usually degradation of pesticides
increases due to cometabolism.

8. Pesticide : The microbial degradation of pesticides depends on the structure of the
pesticide as:

a. The microbial degradation of polar, water-soluble pesticides is more than

nonpolar, water-insoluble pesticides. As cations are more easily adsorbed on soil
particles, microbial degradation of anionic pesticides is faster than cationic ones.

b. Degradation of aliphatic moiety is faster than aromatic ones.