Effect of Sediment Redox-pH Conditions on Degradation

Considerable evidence exists that microbial activities contribute to the formation of reactive products and also to the alteration of environmental parameters such as pH, redox potential, or other factors that are conducive to the secondary or nonenzymatic transformation of pesticidal molecules. Incorporation of pesticide molecules or their intermediates into soil humus often takes place by interaction between enzymatic and nonenzymatic processes. 

Through microbial activities, a reducing environment can be created, particularly in a flooded soil. Many pesticides are degraded by reductive reactions that proceed nonenzymatically under anaerobic conditions. DDT, methoxychlor, and heptachlor readily break down in an anaerobic flooded ecosystem (Sethunathan, 1973). 

FIGURE 13.11 Effect of nitrate and sulfate on the anaerobic degradation of TNT in sediments. 

FIGURE 13.12 Degradation rates of C-labeled pentachlorophenol at pH 6.5 under controlled redox potentials. (From DeLaune et al., 1983.) 

FIGURE 13.13 Microbial mineralization rates of C-labeled polychlorinated biphenyls at pH 6.5 under controlled redox potential showing degradation was greater under intermediate redox potential +250 mV. 

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