Degradation of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 76

• Chemical degradation is influenced by organic matter content, pH, temperature,
nature of pesticide, crop residue, and effluent irrigation.
• Generally, hydrolysis of pesticides in alkaline medium is faster.
• Photolysis of pesticides is influenced by the intensity of sunlight, exposure time,
properties of the application site and pesticides, pH of the medium, water depth,
and presence of other commonly occurring ions.
• Pesticides having organic chromophore or metal-organic complexes can absorb
light easily and undergo photodegradation.
• Photodegradation of pesticides in soils increases in the presence of soil organic
matter such as fulvic acid and humic acid, dyes such as rose Bengal, pigments
such as chlorophyll and xanthone, secondary plant metabolites such as riboflavin,
tyrosine, etc.
• During photodegradation of pesticides, ˙CH 3 , ˙R, RO˙, ROO˙, NO˙, ˙OH, ˙NO 3 , etc.
are formed. The energy required for this process is approximately 400 kJ/mole.
• Due to the heterogeneity of soil, together with soil properties varying with meteo-
rological conditions, mechanisms of photolysis of pesticides in soil are difficult to
understand.
• Photo-induced generation of reactive species under limited mobility modifies
degradation mechanisms.
• Microbial degradation is of two types, aerobic and anaerobic, and carried out by
bacteria and fungi.
• Bacterial microbial degradation dominates in soils and water at pH > 5.5, while in
acidic medium, soil fungal degradation dominates.
• Water depth, mobility, temperature, pH, soil moisture, organic matter, primary
and secondary metabolism, and structure of pesticides are the major factors that
influences biotic degradation.
• The most important degraders in soil are within the genuses, Arthrobacter ,
Aspergillus , Alcaligenes , Bacillus , Corynebacterium , Flavobacterium , Fusarium , Nocardia ,
Penicillium , Pseudomonas , and Trichoderma .
• Pesticide degradation generally follows first-order kinetics, and t 1/2 ranges from
0.02 to 4 years (Tables 3.2 and 3.3).

TABLE 3.2
Half-Life Period for Different Groups of Pesticides in
Soil
Types of Pesticides Approximate Half-Life (years)

Dieldrin, BHC, DDT 2-4

2,4-D and 2,4,5-T 0.1-0.4

Organophosphate 0.02-0.2

Carbamate 0.02-0.1

Pyrethroids 0.005-0.01